ORIGINAL PAPER

Very low temperature wet-chemistry colloidal routesfor mono- and polymetallic nanosized crystallineinorganic compounds

Paolo Dolcet • Stefano Diodati • Maurizio Casarin •

Silvia Gross

Received: 13 June 2014 / Accepted: 14 October 2014 / Published online: 26 October 2014

� Springer Science+Business Media New York 2014

Abstract The use of low temperature, sustainable pro-

cesses based on cheap and safe chemicals as well as non-

toxic, easy to handle chemicals and solvents is a

challenging issue in modern inorganic chemistry; moreover

the obtainment of crystalline functional nanostructures at

low or even room temperature is the goal of many synthetic

efforts. Within this framework, in these last years, we have

developed in our group different room or low temperature

(T \ 150 �C) wet chemistry colloidal routes to prepare

different inorganic functional nanomaterials. These com-

pounds range from (1) ferrites to (2) pure and doped metal

oxides, sulphides and halogenides, to (3) metal/metal oxide

nanocomposites. The adopted wet chemistry routes

encompassed (1) miniemulsions, (2) coprecipitation com-

bined with hydrothermal route and (3) more classical col-

loidal routes, though revised in some critical aspects. This

mini-review provides an overview of the main features as

well as the pros and cons of the proposed routes for the

obtainment of targeted inorganic systems for applications

in optical bioimaging or in energy applications. It not only

summarises already published work, but also presents some

exciting perspectives disclosed by performed studies and

past experience as well as comparisons with state-of-the-art

research.

Keywords Wet-chemistry � Colloids � Inorganic

compounds � Low temperature � Room temperature �Colloidal routes

1 Introduction

In the preparation of inorganic nanostructures, the opti-

misation of experimental routes which are based on the use

of mild synthesis conditions, accomplished by most solu-

tion processing methods, is currently an intensively

explored field of research. This field complies with the

precepts of both green chemistry [1, 2] as well as of

resources and energy constraints [1]. In addition, it satisfies

the demands and requirements of the manufacturing

industry. The achievement of nanocrystalline and compo-

sitionally pure materials by using mild conditions (e.g. low

operating temperature and pressure), easy to implement

procedures, safe chemicals and solvents, as well as possibly

avoiding the use of critical raw materials [3, 4] is of par-

amount importance.

In this context, the resort to colloidal-based method

appears one of the most viable choices since, when suitably

optimised from the experimental point of view, this class of

methodologies enables, in the above mentioned mild con-

ditions, a fine control on the very early stages of nano-

structure formation (i.e. nucleation and growth). In this

way, materials with the desired compositional, morpho-

logical and structural features are afforded. These manifold

approaches have already been extensively used for the

synthesis of nanostructured inorganic colloids [5]. Indeed,

controlling the size and shape of inorganic nanostructures

Electronic supplementary material The online version of thisarticle (doi:10.1007/s10971-014-3549-4) contains supplementarymaterial, which is available to authorized users.

P. Dolcet � S. Diodati � M. Casarin � S. Gross (&)

Dipartimento di Scienze Chimiche, Universita degli Studi di

Padova and INSTM, UdR di Padova, Via Marzolo, 1,

35131 Padua, Italy

e-mail: silvia.gross@unipd.it

P. Dolcet � S. Gross

Istituto per l’Energetica e le Interfasi, IENI-CNR and INSTM,

UdR di Padova, Via Marzolo, 1, 35131 Padua, Italy

123

J Sol-Gel Sci Technol (2015) 73:591–604

DOI 10.1007/s10971-014-3549-4

in a precise way is a highly fascinating challenge. This

struggle is justified by the improved functional properties

arising when particle size is decreased (to the order of

nanometres) and by the particular nanostructures, shapes

and morphologies which can be obtained. Many different

wet-chemistry methodologies such as (1) sol–gel [6], (2)

solvo/hydrothermal (see Sect. 3) (3) nucleation and growth

in suspension [1, 2, 7] (see Sect. 2) have been developed in

order to achieve these goals. Control over size, shape and

morphology, combined with mild processing parameters

(mainly in terms of temperature and pressure) are favour-

able conditions to be implemented for sustainable and cost-

efficient production of functional inorganic materials.

Further requirements to devise and implement environ-

mentally and economically sustainable synthesis routes are

(1) reproducibility, (2) ease of processing, (3) use of safe,

common chemicals, (4) easy purification steps, (5) high

yields.

In the last years, we have approached different metal

oxides and sulphide systems (pure, mixed and/or doped) by

using classical sol–gel routes [8–12] or colloidal approa-

ches involving the nucleation/growth of the target struc-

tures in suitably optimised solutions/suspensions [13–15].

The former approach involved a post-synthesis thermal

treatment at temperature typically higher than those

addressed in this contribution, and will therefore not be

dealt with further. The latter strategy instead afforded the

formation of crystalline ZnO [15], CuS [13] or ZnS [14]

already at room temperature, without the need of post-

processing, and will therefore be described in greater detail

in the following paragraphs.

2 Nucleation and growth of crystalline colloidal

nanostructures in solution/suspension

2.1 Room temperature synthesis of nanocrystalline

ZnO in different dispersing media: effect of solvent

properties on the morphology of the resulting

nanostructures

ZnO is a very intensively investigated material: its prop-

erties, applications [16–28] and synthesis route have been

extensively reviewed by several authors [29–42], to which

interested readers are referred.

In our case, by implementing and adapting the well-

known procedure reported by Spanhel et al. and Bahne-

mann et al. [32, 33, 42], we developed [15], an easy and

highly reproducible route to nanostructured colloidal ZnO

nanoparticles based on the controlled hydrolysis and con-

densation of zinc acetylacetonate (acac) in alkaline con-

ditions. In detail, by reacting a Zn(acac)2 ethanol solution

with NaOH in a 1:2 molar ratio, ZnO spherical crystalline

nanostructures were obtained after reflux at 80 �C. The

nanostructures displayed a homogeneous size distribution

with particle diameter ranging from 6 to 10 nm, as revealed

by transmission electron microscopy (TEM) micrographs.

Aside from the synthetic purposes, our study was however

also devoted to investigating the influence of the chemistry

of the colloidal system on the final features of the resulting

zinc oxide. In this regard, in order to assess the effect of the

solvent viscosity and dielectric constant on the features of

the obtained material, the same reaction was carried out in

water, glycerol or 1,2-propanediol, characterised by the

chemico-physical properties reported in Table 1 in the ESI,

and yielding nanostructures displaying different size and

morphologies. In all cases, the room temperature crystal-

lisation of hexagonal ZnO was proven by X-ray diffraction

(XRD) analysis, from which, by the Scherrer formula, an

average crystallite size could be calculated (Table 1 in the

ESI). Interestingly, a relevant effect of the chemico-phys-

ical properties of the dispersing media on the morphology

of the resulting nanostructures was highlighted and indeed

different particles sizes and very different morphologies

were obtained. The reflux step was also shown to be a key

factor to avoid the fast precipitation of the oxide and to

achieve a pure compound, since the conversion to the

targeted oxide was complete, and no traces of residual

unreacted Zn(acac)2 could be pointed out, as opposed to

syntheses carried out without heating, where a mixture of

wurtzite, other zinc-based products and organic residue

was obtained [15].

The differences outlined both in size and shape (see

reference 15 for details) could be traced back to the dif-

ferent chemical nature of the different dispersing media.

However, it was not possible to identify a definite corre-

lation between the relevant chemico-physical properties

(pKa, dielectric constant and viscosity at room and final

temperatures) of the four media (Table 1 in the ESI), the

mechanism of hydrolysis and condensation and the

resulting morphologies. In particular, no relationship

between viscosity, acidity, dielectric constant and crystal-

lite size could be rationalised, which would be expected in

the case of an oversimplified growth model based only on

diffusion. The investigation of the kinetics of the process

by time-resolved EXAFS was also very challenging due to

the coexistence of different species and different equilibria

in solution, delivering only an averaged picture of the

whole system. In conclusion, nanocrystalline ZnO nano-

structures displaying different size and shape were

obtained at room temperature by a very easy colloidal route

in four different media, and the effect pKa, dielectric

constant and viscosity on morphology was evidenced,

although not fully rationalised [15].

592 J Sol-Gel Sci Technol (2015) 73:591–604

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2.2 Room temperature synthesis of nanocrystalline

CuS in carboxylic acids as different dispersing

media: effect of solvent properties

In this study, we addressed the synthesis of colloidal CuS

nanostructures by taking advantage of the peculiar features

of the dispersing media, in this case carboxylic acids [13].

The crystalline hexagonal phase (covellite) of CuS is

often described as a p-type semiconductor (energy gap

1.2–2.0 eV) [43, 44], which shows superconductivity at

1.6 K [45]. Cupric sulphide [46] raises interest as a starting

material for transparent conductive films [47, 48] capable

of transmitting only the visible part of the solar spectrum,

as a catalyst [43] and as a filler to enhance conductivity

[49–51] or wear resistance of polymeric materials. In the

nanosized range, CuS has been used as a cathode for Li-ion

batteries [52], as a photocatalyst for the degradation of

organic dyes [53] and as an additive to improve the tri-

bology properties of polymer composites [54]. Because of

their broad absorption in the near IR range [55], CuS

nanoparticles find also use as IR sensors [56], and, in the

last few years, CuS nanostructures were also tested for

nanomedicine applications [57].

Concerning wet-chemistry approaches to CuS [46], this

system has been prepared via: (1) hydro/solvothermal

methods [58–62], (2) sonochemical methods [63, 64], (3)

colloidal synthesis, (4) microemulsion [65–68], (5)

microwave assisted synthesis [69, 70], (6) surfactant-

assisted templating [71], (7) by using chelating agents [72]

or (8) by organogel-assisted synthesis [73]. The most

generally used precursors are copper (II) inorganic com-

pounds (CuO, CuSO4, CuCl2, etc.) and sulphur sources like

gaseous H2S, thioacetamide, thiourea, dimethylthiourea

carbon disulphide, sodium thiosulfate, sodium sulphide and

elemental sulphur (for further details see [13] and refer-

ences therein).

In our case, we addressed the synthesis by promoting the

controlled nucleation and growth on the CuS nanoparticles

in a carboxylic acid suspension (for synthesis details, see

Scheme 1 in ESI and Ref. 13). Specifically, the optimised

methodology was based on the fast nucleation of the sul-

phide triggered by the reaction of thioacetic acid (acting as

a slowly releasing sulphide source) with water and copper

carboxylates (acetate, propionate) in the corresponding

carboxylic acid (acetic, propionic) solvent. In this regard,

the choice of carboxylic acids as dispersing media was

motivated by several chemical considerations and advan-

tages: (1) hydrolysis of the C–S bond (in thioacetic acid) is

favoured thus producing a fast CuS supersaturation and a

high nucleation rate; (2) the mobility of the precursor

molecules is limited by the high viscosity of the medium,

thus favouring nucleation events with respect to particle

growth; (3) the low dielectric constant of the medium

stabilises the nanoparticles dispersion by reducing the

critical coagulation concentration.

As far as the first and second points are concerned, the

acid environment enhances the nucleation rate and, at the

same time, slows down the growth rate. The former event

arises from the fast CuS supersaturation produced by the

acid catalysed hydrolysis of carbon–sulphur bonds of

thioacetate groups. The growth rate is likely dependent on

the nucleophilic power and acidity of copper hydrosulphide

species that condense with each other to produce the sul-

phide: the nucleophilic power and growth rate would be

enhanced by deprotonation of these molecules, but the

acidity of solvent lowers this effect. Furthermore, the rate

of particle growth is reduced due to the relatively high

viscosities of carboxylic acids with respect to many other

common solvents and the low mobility of the precursor

molecules diminishing the effect of collision between

particles and therefore the supply of material to the

growing particles. As far as the third factor is concerned, as

already mentioned, the low dielectric constant of pure

carboxylic acids stabilises the dispersion by reducing the

critical coagulation concentration.

A mechanism describing the different steps leading to

the formation of CuS, and explaining the role of the pre-

cursors, was proposed and assessed by vibrational spec-

troscopy, which is reported in Ref. [13].

The choice of the precursors was also motivated by

considerations regarding the chemistry of the resulting

system. In fact, one main advantage of using copper

acetate and thioacetic acid as precursors is the formation

of acetic acid as a reaction product, i.e. solvent molecules.

Consequently, the low dielectric constant of the reaction

environment is fully preserved. The prepared nanoparti-

cles were investigated by UV–V is spectroscopy, X-ray

photoelectron spectroscopy (XPS), atomic force micros-

copy and dynamic light scattering. The nanoparticle

suspensions are clear and characterised by a blue-

shifted adsorption edge with respect to bulk CuS. Light

scattering measurements performed on acetic acid sus-

pensions highlighted the formation of monodispersed

nanoparticles with an average diameter of about 5 nm,

thus proving the effectiveness of the adopted procedure in

achieving controlled nucleation and growth of the targeted

nanostructures.

In conclusion, as was the case for ZnO (2.1), the suc-

cessful synthesis of inorganic nanostructures was afforded

by a facile, reproducible, in this case room temperature

wet-chemistry colloidal route, in which the proper choice

of precursors and reaction conditions were proven to play a

major role in directing the systems towards the targeted

materials.

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3 Hydrothermal synthesis

3.1 Basics of hydrothermal synthesis

and crystallisation under hydrothermal conditions

The term hydrothermal synthesis typically refers to wet

chemistry routes based on an aqueous medium which are

carried out at relatively high pressures and temperatures

[74], either over or under critical conditions. Originally,

this class of synthesis protocols was developed with the

objective of mimicking the conditions in which crystals

grow within the Earth crust [74, 75]; indeed through this

route the rapid growth of large, pure crystals could be

achieved [74–80].

More recently, this method has been applied to the

synthesis of more complex systems, such as binary or

ternary metal compounds, in nanosized crystalline form.

Several works have been published on many different

compounds such as zinc oxide [81, 82], titania [83–85],

ceria [86, 87], metal sulphides [88, 89] as well as several

more complex compounds [90] such as titanates [91, 92],

phosphates [93] and ferrites [94–99]. All these synthetic

protocols take advantage of the particular physical condi-

tions involved. In general, ionic product of the solvent

tends to increase with rising temperature and pressure,

whereas viscosity decreases. In the case of water, the

dielectric constant depends on both pressure and tempera-

ture as it is directly proportional with the former, but

inversely proportional with the latter [74, 100] (vide infra,

Table 1).

Considering that the focus of the present work is on

crystalline inorganic compounds, special consideration

must be given to the processes which may lead to crys-

tallisation in hydrothermal conditions. In this context, if

nanocrystals are to form from an aqueous supersaturated

solution, the precipitation process must occur slowly [101],

in order to avoid the formation of amorphous solids. Due to

the nonstandard conditions in which hydrothermal syn-

thesis takes place [102], it is possible for normally less

soluble compounds to be solvated with greater efficiency

and therefore to react more easily [74, 100]. More specif-

ically, concerning the formation of oxide particles through

hydrothermal conditions, the main topic of our recent

research, many different process mechanisms can be

hypothesised (also depending on the specific synthetic

protocol adopted). In the most widely accepted theory [74,

90, 103, 104], the process takes place in two steps: (1)

in situ transformation followed by (2) precipitation and

growth. During the former phase the precursors are dis-

solved and tiny amounts of the target oxide are able to form

as solvated species (owing to the conditions in which the

process occurs); in the latter phase, the low solubility of the

aforementioned products causes them to nucleate and form

crystals. Depending on the conditions involved (tempera-

ture, pressure, precursors etc.) different products (such as

core–shell intermediates) [90] and/or crystalline forms may

form throughout the synthesis and be progressively con-

verted into more stable forms.

In general, hydrothermal synthesis is attractive, com-

pared to more traditional synthesis protocols, due to the

fact that crystalline products with high purity and compo-

sitional uniformity may be obtained under relatively mild

conditions. Furthermore, due to the fact that by altering the

temperature and pressure of the reaction mixture, several

solvent properties change [100], non-classical crystallisa-

tion pathways may be explored [74, 77]. The changes

underwent by viscosity, density and dielectric constant in

the case of water at different temperatures and pressures

are reported in Table 1.

Recently we have applied hydrothermal synthesis to the

preparation of nanostructured crystalline spinel ferrites, as

outlined in the following paragraphs. The synthesis here-

after presented (Scheme 1) involves the combination of

coprecipitation of metal oxalates from an aqueous solution

with hydrothermal treatment of the resulting suspension at

mild temperatures. In this protocol, hydrothermal condi-

tions, though sub-critical, were achieved through autoge-

nous pressure (by heating the reaction mixture in a closed

vessel); additionally the possible decomposition of oxalate

species to carbon dioxide may have caused a further

increase in pressure. The observed small size of the

nanocrystalline particles obtained (Table 2 in the ESI)

suggests that nucleation occurred relatively rapidly within

the reaction mixture, with particle nucleation being kinet-

ically favoured over growth. The choice of oxalate as a

Table 1 Properties of water and vapour as a function of temperature

and pressure [102, 105]

Temperature/�C State Density/

kg�m-3Viscosity/

lPa�sDielectric

constant

Pressure equal to 0.1 MPa

27 Liquid 996.56 853.82 80.20

52 Liquid 987.19 530.32 69.32

77 Liquid 973.73 368.80 61.79

102 Vapour 0.590 12.339 1.006

135 Vapour 0.543 13.285 1.005

177 Vapour 0.484 15.426 1.004

Pressure equal to 1.0 MPa

20 Liquid 996.96 853.67 77.78

52 Liquid 987.58 530.48 69.36

75 Liquid 974.13 369.02 61.82

102 Liquid 957.43 276.59 55.09

135 Liquid 937.87 218.79 49.06

177 Liquid 890.39 153.00 38.81

594 J Sol-Gel Sci Technol (2015) 73:591–604

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precipitating counter-ion, as opposed to the more common

hydroxide, was motivated by two reasons: (1) compared to

hydroxides, oxalates of transition metals tend to be less

soluble and have much closer solubility constants [102];

(2) oxalates tend to decompose cleanly, without leaving

undesired organic residue, and yielding formation of

crystalline oxide at temperature much lower than in the

case of hydroxides.

By using this approach, we succeeded in synthesising

four different ferrite spinels (CoFe2O4, MnFe2O4, NiFe2O4

and ZnFe2O4) as pure nanocrystalline compounds at very

low temperatures (75–135 �C) [106], as described more in

detail in the following paragraphs.

3.2 Low temperature hydrothermal synthesis

of nanocrystalline ferrites MFe2O4

The most commonly synthesised ferrites are perovskite fer-

rites, with general formula MFeO3, and spinel ferrites with

general formula MFe2O4. When the hydrothermal method is

taken into consideration, the bismuth ferrite BiFeO3 is the

most commonly synthesised perovskite. Several low tem-

perature routes have been developed for this compound

ranging from the preparation of the pure oxide in crystalline

form [94, 107], to the lanthanum doped species [108]. Low

temperature hydrothermal protocols (60–250 �C) have also

been applied to the synthesis of spinel ferrites, among which

CoFe2O4, NiFe2O4 and ZnFe2O4 [95–99].

As far as our research group is concerned, nanocrystalline

spinel ferrites MFe2O4 (M = Co, Mn, Ni, Zn) were syn-

thesised through a simple route combining coprecipitation of

oxalates and hydrothermal synthesis (Scheme 1) [106, 109].

Previous syntheses, involving the isolation and subsequent

calcination in air of the coprecipitated intermediates at high

temperatures (900–1,300 �C), afforded the nickel, cobalt

and zinc spinels (CoFe2O4, NiFe2O4 and ZnFe2O4) [109–

111] as well as the manganese and strontium perovskites

MnFeO3 [109–111] and SrFeO3-d [109, 112].

However, within the framework of implementing a

greener, more sustainable and milder process, the combi-

nation of coprecipitation with hydrothermal synthesis

appeared as particularly attractive due to several factors:

(1) excellent reproducibility, (2) ease of procedure and

implementation (e.g. scale-up), (3) nontoxic precursors, (4)

mild temperatures, (5) use of water as greenest solvent, (6)

easy and quick purification of products (without the need

for other solvents), (7) high yields, (8) high product purity,

(9) high versatility (numerous different compounds were

synthesised through this method, since oxalates of many

metals can be easily prepared).

Several different syntheses were carried out to explore

the effect of the involved synthetic parameters (nature of

the precursors, nominal ratio between precursors, amount

of peptising agent (tetraethylammonium hydroxide—TE-

NOH), treatment temperature, treatment time, purification

protocol) on the products, which are reported in Supple-

mentary Materials (Table 2 in the ESI). Details on the

synthesis and the protocol specifics are reported in Ref.

[106]. Optimisation of the synthesis protocol led to con-

clude that treatment time and amount of peptising agent

had very limited effect on the characteristics of the

resulting compounds, whereas treatment temperature was

much more influential. Syntheses involving ammonium

hydroxide as a base were unsuccessful and led to impure

products. Finally, suitable choice of precursors and ratio

between them was found to be paramount: excessive oxalic

acid proved to lead to the formation of impure phases.

Concerning this last point, it should be noted that, though

pure samples were obtained for cobalt and manganese

spinels with a nominal M/Fe/acid molar ratio of 1/2/4.5

(Table 2), later syntheses were uniformed for all spinels to

a 1/2/4 nominal molar ratio, to avoid the precipitation of

unidentified impure phases which were occasionally

observed in samples prepared with higher oxalic acid

content. Synthetic attempts were also carried aimed at the

synthesis of other oxides (such as MnFeO3), but they were

unsuccessful (Table 3 in the ESI).

The synthesised oxides were thoroughly characterised

by several different techniques from the structural (X-ray

diffraction—XRD), morphological (transmission electron

microscopy—TEM), compositional (microanalysis, micro-

Raman, X-ray photoelectron spectroscopy—XPS, induc-

tively coupled plasma-atomic emission spectroscopy—

ICP-AES) and functional (superconducting quantum

interference device—SQUID) point of view. In particular

the exact stoichiometry of the samples was assessed by

Scheme 1 Reaction scheme for the synthesis

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ICP-AES, confirming and completing the information

gained through XRD.

XRD analyses confirmed that the powders were

obtained as nanocrystalline, single phase products, and

allowed to calculate, through Rietveld refinement [113],

the crystallite average sizes for the spinels, which were in

the 17–50 nm range (Table 2).

As can be seen from Table 2, a significant variance in

average crystallite size can be observed between the dif-

ferent spinels in dependence of the M metal, as well as the

other synthesis parameters (Table 2 in the ESI). TEM

analyses further confirmed this data, showing that the

particles were indeed nanosized (Fig. 1).

XPS (Fig. 2) and ICP-AES analyses (Table 1) allowed

to gain insight into the composition of the samples

(respectively on the surface and in bulk).

The gathered data further confirmed sample purity as

well as expected stoichiometry and revealed a slight ten-

dency in the zinc ferrite for iron to segregate on the surface

[106].

Micro Raman analyses carried out on different points in

each sample revealed the obtained products to be compo-

sitionally uniform throughout. Microanalysis detected only

negligible amounts of carbon residue on the samples,

confirming that the decomposition of the oxalate precursor

is complete and clean, as well as that the adopted purifi-

cation protocol is appropriate for the removal of organic

residues.

Finally, from a functional point of view, magnetic

measurements were performed through SQUID on two

spinels (namely the cobalt and the manganese ferrites). In

both cases the compounds behaved as predicted, acting as

Table 2 Synthesis conditions

and experimental results for the

obtained compounds

Compound Nominal Fe/

M/acid ratio

Treatment

temperature

and time

Found

structure

Yield % Crystallite

average

size/nm

M/Fe ratio

(from

ICP-AES)

CoFe2O4 2/1/4.5 135 �C; 24 h CoFe2O4 9,990 172 ± 2 2.01

MnFe2O4 2/1/4.5 135 �C; 24 h MnFe2O4 100 2,049 ± 4 2.05

NiFe2O4 2/1/4 135 �C; 24 h NiFe2O4 95 47 ± 2 2.09

ZnFe2O4 2/1/4 135 �C; 24 h ZnFe2O4 92 6 ± 1 /

Fig. 1 TEM micrographs of the four compounds (a) CoFe2O4, (b) MnFe2O4, (c) NiFe2O4 and (d) ZnFe2O4

596 J Sol-Gel Sci Technol (2015) 73:591–604

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hard and soft ferrimagnetic materials respectively and

reaching magnetisation values of 67.7 emu/g (CoFe2O4)

and 61.0 emu/g (MnFe2O4) [106].

In conclusion, hydrothermal synthesis was successfully

used to prepare four spinel ferrites (namely CoFe2O4,

MnFe2O4, NiFe2O4 and ZnFe2O4) with extremely high

purity and yields. The nature and nominal ratios between

precursors have shown to be heavily influential on the final

products, as excessive amounts of either metal or oxalic

acid led to the formation of impure phases. The effect of

other parameters (such as temperature, employed base and

treatment time (Table 2 in the ESI)) on the purity, size and

crystallinity of the resulting compounds has also been

investigated. Treatment temperature and time influenced

the products to a lesser degree, though it has to be pointed

out that shorter treatment times and lower temperatures led

to lower crystallinity and yields. As far as the synthetic

protocol is concerned, it combines reasonable speed, high

quality and purity of products, environmentally friendly

conditions and low costs with a good versatility. The actual

effect of pressure was furthermore explored, as syntheses

carried out under reflux, under identical thermal conditions

but at normal pressure, afforded products with lower purity

and yields [106].

4 Miniemulsions: chemistry in nanoreactors

4.1 Principles of the miniemulsion approach

Amidst colloidal methods, that of miniemulsions represents

a very appealing option. Miniemulsions are heterogeneous

systems critically stabilised against diffusion and collision-

induced degradation processes, characterised by an average

droplet size between 30 and 500 nm [114–121]. From a

practical point of view this means that, even though the

system is metastable, the droplets reach the smallest pos-

sible particle size under the applied conditions, which also

means that miniemulsions exploit the surfactant in the most

efficient way.

Unlike the cases of other emulsions (i.e., macro- and

microemulsions), which are usually homogenised by

employing mechanical stirring, the peculiarity of miniemul-

sions arises from the fact that they exploit high intensity

ultrasounds [118]. These generate intense shear forces that

are able to overcome the viscous resistance, which would

otherwise dissipate much of the provided energy as heat.

The highly turbulent flow created by the ultrasounds

induces many simultaneous droplet formation and disrup-

tion events, leading to a strong decrease in the overall

polydispersity. During sonication, therefore, the mean

size of the droplets changes quite rapidly, until a steady

state is achieved, with no further variation in the average

dimensions (Fig. 3). Simultaneously, interfacial tension is

found to increase with decreasing droplet size, since a

constant amount of surfactant has to be distributed

throughout an increasingly larger total interface, until a

plateau is reached. Overall, the droplets forming mini-

emulsions reach the minimal particle size under the applied

conditions, and are at the critical borderline between sta-

bility and instability. On the other hand, the narrow poly-

dispersity achieved ensures that diffusion degradation (the

process known as Ostwald ripening [7]) is strongly hin-

dered, and hence, with a careful choice of surfactant,

miniemulsion stability can reach a timescale of weeks or

even months [114].

Fig. 2 XPS survey spectra of

the four compounds

(a) CoFe2O4, (b) MnFe2O4,

(c) NiFe2O4 and (d) ZnFe2O4

(binding energy values

corrected for surface charge)

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It is worth highlighting that, together with Ostwald

ripening, chemical exchange processes between mini-

emulsions droplets are also prevented. This means that

each single droplet is independent from the others, thus

acting as a nanometric reactor, able to confine a reaction of

interest, which therefore takes place in a highly parallel

fashion, in 1018–1020 independent nanoreactors. It readily

follows that these systems are extremely attractive for the

opportunity they disclose in achieving a precise control on

the products particle size, since the volume of reaction is

limited to the droplet’s core.

4.2 Application of the miniemulsions approach

to the room temperature synthesis of inorganic

nanostructures

The possibility of generating small, homogenous and sta-

ble monomer droplets that maintain their particular identity

Fig. 3 Homogenisation process in the miniemulsion approach (adapted from [119])

Fig. 4 Schematic

representation of the chemical

processes that can be used for

the synthesis of inorganic

materials via miniemulsion

(adapted from [117])

598 J Sol-Gel Sci Technol (2015) 73:591–604

123

throughout the polymerisation process without any

exchange kinetics involved represents the Holy Grail in

polymer synthesis, and it is indeed in this field that mini-

emulsions were originally developed [118, 119, 122, 123]

and were they find their main applications.

The use of miniemulsion for the direct synthesis of

inorganic materials, on the other hand, is less intensively

explored; nevertheless, it is gaining a growing interest in

the last years [5, 117, 124]. In this regard, a further

advantage in using miniemulsions is their flexibility, since

a wide variety of chemical processes can be exploited in

order to induce the formation of the desired material, as

depicted in Fig. 4.

Some of the first examples found in the literature for the

synthesis of inorganic compounds took advantage of low

melting metals, alloys or salts in order to produce stable

metallic (Wood’s and Rose’s alloys) and inorganic (Fe2O3,

Fe3O4, ZrO2 and CaCO3) nanoparticles. This was achieved

by using the molten precursors as polar phase and inducing

crystallisation by decreasing the temperature below the

melting point, thus exploiting phase transitions [125].

Another widely exploited process, especially for the

preparation of oxide materials, is the sol–gel approach, as

also reviewed by Munoz-Espı et al. and Cao et al. [117,

124]. The popularity of this approach can be traced back to

the high tunability it allows since, by controlling hydrolysis

and condensation rates, the morphology of the nanostruc-

tures can be easily modified, with also the possibility of

introducing further structure-directing agents [126–129].

Through this approach, a variety of different crystalline

materials were prepared, such as mesoporous silica nano-

particles [126], titania and mixed titania/silica and titania/

zirconia nano- [127, 128] and microparticles [129], and

ceria [130] nanoparticles.

In addition, due to its characteristics, the sol–gel process

can be induced to occur at the liquid–liquid interface in the

miniemulsion formulation, generating hollow nanostruc-

tures. This approach was for example exploited for the

synthesis of silica nanocapsules [131], as well as for the

interfacial precipitation of hafnium and zirconium

hydroxides which, upon calcination, maintained their hol-

low morphology and converted in the corresponding oxides

[132].

A further way to influence the morphology of the inor-

ganic materials is using miniemulsion generated polymer

latexes as templates. By tuning not only the dimension of

the latexes, but also their surface functionalities, the

polymer/inorganic interaction can be modified, influencing

the crystallisation process. For further details see refer-

ences [5, 117, 133].

Miniemulsions can also be exploited to generate hybrid

polymer/inorganic nanoparticles that act as precursor and

convert into the desired inorganic material after removal of

the organic component. Through this kind of approach,

either coatings [134] or ordered arrays of metallic nano-

particles [135, 136] might be obtained.

Another common approach for the preparation of inor-

ganic nanostructures exploits precipitation reactions of

metal salts within the droplets core. Among the first

examples in this framework, there is the preparation of

highly stable lanthanide-based phosphors [(Y0.94Eu0.06)2O3,

La0.5Ce0.3Tb0.2PO4, Ba0.9Eu0.1MgAl10O17], which could

also be easily converted to corresponding films [137].

4.3 Synthesis of pure and doped ZnO nanostructures

and of Au/TiO2 nanocomposites by miniemulsion

In this latter framework, i.e. precipitation in confined

space, in recent years we took advantage of these systems

for the controlled preparation of different inorganic nano-

structures, ranging from doped zinc oxide platelets [138,

139] to doped zinc sulphide [140], calcium fluoride [141],

calcium and magnesium hydroxides nanostructures (Table

4 in the ESI) [142], as well as Au/TiO2 nanocomposites

[143].

With the objective of developing materials for optical

bioimaging, the synthesis of ZnO nanostructures was car-

ried out using several surfactants, in order to assess their

influence on miniemulsion stability and final features of the

obtained materials. As expected, non-ionic bulky surfac-

tants performed better in stabilising an inverse miniemul-

sion, thus leading to the formation of nanocrystalline

wurtzite already at room temperature [138, 139]. The

approach used to induce the precipitation of the crystalline

materials was based on the preparation and successive

mixing of two different miniemulsion formulations con-

taining, as the aqueous phase, aqueous solution of the zinc

salt and of a precipitating agent respectively, as depicted in

Scheme 2. The method was then extended to the synthesis,

by coprecipitation, of doped ZnO nanoplatelets (Zn/

M = 50:1 or 20:1; M = AgI, CoII, CuII, EuIII, MgII, MnII)

(Table 5 in the ESI), achieving a very good reproducibility

[138]. The obtained anisotropic nanostructures presented

an average length of ca. 70 nm and thickness of 15 nm; in

particular those with EuIII, showed interesting luminescent

properties (Fig. 5a). Due to the aforementioned lumines-

cence, coupled with the low cytotoxicity of the host matrix,

also assessed in this work, these materials are appealing for

possible future applications in optical bioimaging. Corre-

spondingly, Lactate Dehydrogenase assays were per-

formed, proving that the nanostructures did not induce any

cytotoxic effect on human fibroblast MRC5 cell line. When

the wurtzite platelets were doped with silver ions, the

dopant easily underwent photoreduction, generating an Ag/

ZnO nanocomposite (Fig. 5b). XPS and ICP-AES analyses

allowed to gain insight into the composition of the

J Sol-Gel Sci Technol (2015) 73:591–604 599

123

samples’ surface and assessing the efficiency of the doping

process, highlighting a successful embedding of the ions in

the host matrix.

Due to the high versatility of miniemulsions in terms of

the chemical processes that can take place within their core

(Fig. 4), we also exploited such systems to achieve con-

trolled photodecomposition of a suitable tailored precursor

[144]. Specifically, we used the single-source precursor

AuCl4(NH4)7[Ti2(O2)2(Hcit)(cit)]2�12H2O (Fig. 6), which,

upon UV irradiation, is known to generate the nanocom-

posite Au/TiO2 [143].

This nanocomposite was investigated as photocatalyst in

the oxidation of 2-propanol. By taking advantage of its

properties in combination with the features of miniemul-

sion, a heterogeneous system of nanocrystalline gold

nanoparticles finely dispersed on titania could be easily

obtained (Fig. 7). Due to the chosen photo-triggered

decomposition method [144], gold formed metallic nano-

particles with an average size of 25–40 nm, whereas titania

formed as an amorphous support. XPS analyses evidenced

that almost complete conversion of the precursor into the

nanocomposite could be accomplished with appropriate

irradiation time.

The strong influence of the confinement process induced

by miniemulsion on the final materials was also evidenced

by the catalytic studies. A nanocomposite prepared in

miniemulsion was compared to a sample prepared in sim-

ple water suspension with regard to the catalysis of the

oxidation of 2-propanol. Tests in both gaseous and liquid

phase oxidation evidenced that the former sample is both

mixing A + B

US US

separation, centrifugation

post-functionalisation

characterisation of the suspension

chemical-physical characterisation

cytoxicity assessment

in vivo in vitro testing

metal precursor solution base solution

miniemulsion A miniemulsion B

microwaves

oxide miniemulsion

isolated nanoparticles

bioconjugated nanoparticles

US = ultrasound

US

Scheme 2 Flow chart for the

preparation of inorganic

bioconjugated nanoparticles via

miniemulsion

100 nm

(a) (b)

Fig. 5 (a) PL spectra of Eu doped ZnO nanostructures obtained via miniemulsion; (b) TEM micrograph of Ag/ZnO nanocomposite (adapted

from [138])

600 J Sol-Gel Sci Technol (2015) 73:591–604

123

more active and selective than its counterpart, as evidenced

in Table 3.

We have recently implemented the miniemulsion

approach for the preparation of pure and doped Mg(OH)2,

Ca(OH)2 [142], CaF2 [141], CuS and ZnS [140] nanopar-

ticles and nanostructures. In the case of the halogenide, the

precipitating agent was NaF whereas, in the case of the

sulphides, different sulphide sources were tested in order to

optimise the purity and crystallinity of the obtained

products.

Fig. 6 Crystal structure of the molecular Au/TiO2 single-source precursor AuCl4(NH4)7[Ti2(O2)2(Hcit)(cit)]2�12H2O

Fig. 7 SEM micrograph of Au/TiO2 nanocomposites obtained by

miniemulsion

Table 3 Catalytic tests (oxidation of 2-propanol) of Au/TiO2 nano-

composites prepared in miniemulsion or in suspension

Synthetic route Gaseous phase oxidation Liquid phase

oxidation

Conversion

(%)

Selectivity

(%)

TOF

(mol mg(Cat)-1 h-1)

Miniemulsion 98 80 0.012

Suspension 53 50 0.06

Table 4 Summary of the prepared systems

Compound Synthesis approach References

ZnO Colloidal route [15]

CuS Colloidal route [13]

CoFe2O4 Hydrothermal [106]

MnFe2O4 Hydrothermal [106]

NiFe2O4 Hydrothermal [106]

ZnFe2O4 Hydrothermal [106]

ZnO (pure and doped) Miniemulsion [138, 139]

Au/TiO2 Miniemulsion [143]

ZnS Miniemulsion [140]

CuS Miniemulsion [140]

CaF2 Miniemulsion [141]

Mg(OH)2 Miniemulsion [142]

Ca(OH)2 Miniemulsion [142]

J Sol-Gel Sci Technol (2015) 73:591–604 601

123

In conclusion, droplets formed by miniemulsion showed

to be effective nanoreactors for the controlled preparation

of several inorganic compounds, affording in all cases

crystallisation at room temperature and the formation of

nanosized structures.

5 Conclusions and perspectives

In this paper we have collected the most relevant examples

of our recent activity in the field of low or room temper-

ature synthesis of inorganic colloids. Different wet-chem-

istry approaches were successful in delivering the targeted

systems with desired microstructural and compositional

features. In all cases, the crystallisation of the targeted

nanostructures was pursued at low or room temperature,

and the experimental conditions proved to be determinant

in these cases.

For instance, in the case of miniemulsion, Landfester

and coworkers [5, 114–117, 120, 121] have already pointed

out interesting effects of the confined space on the fate and

structural evolution of the contained inorganic systems. In

particular, the morphosynthetical control of the crystalline

habitus of the formed structures was highlighted: physical

properties of liquids in nanodroplets can be substantially

different from those of the bulk phase and the dynamics of

crystallisation.

In one of our cases, for instance, [139] the confinement

of the precursor formulation in a droplet was proven to be a

key issue in determining the formation of wurtzite instead

of the expected (in the adopted pH range) zinc hydroxide.

In fact, in basic or neutral media (in our case the pH of the

final miniemulsion was determined to be 13.4), ZnII ions

form the amphoteric hydroxide species, ZnOx(OH)g(H2O)z

in a colloidal state, which forms as amorphous or crystal-

line species. This species spontaneously evolves to crys-

talline wurtzite ZnO not only under the action of heating or

irradiation, through a dehydration process, but also upon

prolonged storing and aging in its mother liquor, which in

this case was pursued by their confinement.

Table 4 summarises these efforts by including also the

relevant references.

The very encouraging results achieved have disclosed

several exciting perspectives, and may stimulate many

developments in this field, deriving mainly from:

1. optimisation of the reaction conditions and/or of the

stability of the obtained colloidal systems;

2. implementation of the optimised procedures to differ-

ent chemical systems and complex hierarchical

nanostructures;

3. combination of synthesis and processing methods (e.g.

solvothermal synthesis combined with microwaves

processing, microwaves-triggered decomposition of

single-source precursors in colloidal suspension etc.).

A very fascinating and challenging development in this

field we are now exploring is to establish clear correlations

between the chemico-physical parameters determining the

composition and stability of the colloidal systems and the

observed features (structural, compositional and morpho-

logical) as well as the crystallisation kinetics and

mechanisms.

Colloidal chemistry represents a really bewitching

playground for an inorganic chemist, and surely deserves

further and focussed experimental and theoretical efforts,

for better understanding of structure-properties relation-

ships and of non-classical crystallisation paths.

Acknowledgments The Italian National Research Council (CNR),

the University of Padua, Italy, and the Sabic company are acknowl-

edged for equipment and financial support. S.G. would like to warmly

thank her former master and PhD students Erika Butturini, Daniele

Camozzo, Alessia Famengo, Francesca Latini, Giulia Morgese and

the guest students Julia Migenda, Benjamin Kruner, Antonin Mam-

brini for their valuable contributions to the hereby presented work and

all the colleagues cited in the given references for their appreciated

support in the characterisation of the obtained nanostructures. P.D.

and S.D. acknowledge the PhD School in Molecular Sciences of the

University of Padova for financial support.

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