In situ synthesis of Pt nanoparticles in SBA-15 by encapsulating in modified template micelles: Size restricted growth within the mesochannels

Article abstract of DOI:10.1039/b717991j

A truly in situ and simple method is developed for nanoparticle incorporation within the mesochannels of SBA-15 involving dispersion of metal precursors in surfactant-modified polymer micelles. The diffusion of the precursor within the micellar structure is aided by interaction with the cationic head group of the surfactant leading to a unique method to facilitate the formation of highly disperse, uniform nanoparticles molded by the walls of the mesochannels. The nanoparticle incorporated mesoporous material has a highly enhanced surface area and adsorption capabilities in comparison to its parent materials without any pore blockage which makes this method ideal for the preparation of nanocatalysts. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b717991j

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
In situ synthesis of Pt nanoparticles in SBA-15 by encapsulating in modified
template micelles: size restricted growth within the mesochannels†
a
Atul K. Prashar, Robert P. Hodgkins,‡ Rajiv Kumar and R. Nandini Devi
b
a
a
*
Received 21st November 2007, Accepted 7th February 2008
First published as an Advance Article on the web 28th February 2008
DOI: 10.1039/b717991j
A truly in situ and simple method is developed for nanoparticle incorporation within the mesochannels
of SBA-15 involving dispersion of metal precursors in surfactant-modified polymer micelles. The
diffusion of the precursor within the micellar structure is aided by interaction with the cationic head
group of the surfactant leading to a unique method to facilitate the formation of highly disperse,
uniform nanoparticles molded by the walls of the mesochannels. The nanoparticle incorporated
mesoporous material has a highly enhanced surface area and adsorption capabilities in comparison to
its parent materials without any pore blockage which makes this method ideal for the preparation of
nanocatalysts.
and even modified surface properties which can be exploited
Introduction
for various applications even beyond catalysis.14 Incorporation
of metal particles into mesoporous compounds is usually
achieved by simply adding metal salt precursors to the oxide
precursor sol–gel mixture15 or wet impregnation of metal salt
on already formed mesoporous compounds.16 These methods
have the disadvantage of uncontrolled growth of metal particles
on the walls since no preventive measures are taken to ensure
that the metal precursors are solely isolated within the channels.
Further, grafting of the inner walls of the siliceous materials with
functional groups which would preferentially attach metal
precursors is also used,17 but complicated procedures need to
be followed to prevent external surface adsorption of the
functional group and hence uncontrolled growth of metal
particles. Recently, pre-synthesized metal nanoparticles capped
by protective agents have been successfully incorporated into
the mesochannels exclusively, through wet impregnation or
capillary inclusion.18,19 However, careful surface modification
and pore engineering of the host material is necessary to facilitate
improved guest uptake. In addition there is a significant reduc-
tion in pore volume and surface area in the resulting hybrid
material reducing the efficiency.20 Somarjai and co-workers
reported another elegant method in which Pt colloidal solution
is prepared and stabilized in a capping agent polyvinylpyrroli-
done (PVP) and encapsulated within the SBA-15 silica frame-
work.21 However, in such a method, highly acidic synthetic
precursors of SBA-15 can be detrimental to the stability of the
metal colloid and hence careful alterations need to be carried
out to the mesoporous materials’ synthesis so that the nano-
particles are stable. At this juncture, it would be worthwhile
exploring better and simpler methodologies for localizing metal
nanoparticles exclusively within the mesochannels while retain-
ing high surface areas and pore volumes—ideal for catalysis
amongst other applications.
In catalysis, dispersion and hence size and morphology of active
metals play an important role in overall performance, activity
and selectivity of the catalysts.1–3 Here the ability to stabilize
active metals in nano form as well as fine tune the size is
paramount to achieving desired performance markers for the
catalyst. Even though engineering metal nanoparticles with
different sizes and morphologies is very well established
now,4–6 it has been difficult to translate these advances into
successful application in catalysis. The main drawback is the
possibility of agglomeration and sintering of the nanoparticles
under harsh catalytic reaction conditions since they are usually
stabilized in organic moieties7,8 like capping agents and protec-
tive polymers. Hence it is imperative to support the nanoparticles
effectively on solid and stable materials which would aid to
minimize such deactivation processes. In conventional catalyst
preparation methods, metal particles are stabilized on the surface
and within the macropores of oxide supports just by adsorption
of precursor ions and subsequent reduction. In spite of poor
control of particle size, these methods are widely followed due
to the ease of preparation in large scale syntheses.
However, the advent of mesoporous compounds like SBA-15,
MCM-419–11 etc. has led to exciting forays into their utilization
as supports for metal nanoparticles.12,13 The advantage is the
highly porous interpenetrating channel systems without any
capillary effect diffusion limitations, thereby increasing the
surface area, within which nanoparticles can be located. This is
expected to give better control of particle size, morphology
aCatalysis Division, National Chemical Laboratory, Pune 411008, India.
E-mail: nr.devi@ncl.res.in
bMaterials Chemistry Research Group, Department of Physical, Inorganic
and Structural Chemistry, Arrhenius Laboratory, Stockholm University,
SE-106 91 Stockholm, Sweden
In our attempts to have better control over the size, amount
and location of Pt particles in SBA-15, we have developed
a simple and convenient in situ method of dispersing the metal
salt precursors within template polymers modified with ionic
surfactants in a controlled manner. The ionic surfactants are
† Electronic supplementary information (ESI) available: Additional
XRD and TEM images, pore size and pore volume calculations,
catalytic conversion–selectivity data, chemical analysis details. See
DOI: 10.1039/b717991j
‡ Present Address: Nanologica AB, Uppsala Innovation Centre,
Uppsala Science Park, SE-751 83 Uppsala, Sweden
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J. Mater. Chem., 2008, 18, 1765–1770 | 1765

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expected to be dispersed within the polymer micelle structure
with the ionic head groups decorating the corona and the
hydrophobic tail groups penetrating the core. Added to this
polymer–surfactant composite, metal precursor ions diffuse
into the corona depending on various factors viz. ageing time,
surfactant and precursor concentration etc. Later calcination
of this material leads to a highly homogeneous dispersion of
metal nanoparticles of fairly uniform size distribution. This
method provides an easy and efficient way to have enhanced
control over fine tuning of the particle sizes at the same time
ensuring that all nanoparticles are localized within the meso-
channels. Further, the textural properties like pore size/volume,
surface area etc. are observed to be highly enhanced when
compared to metal incorporation without the surfactant modifi-
cation. We explain in this manuscript the methodology adopted
in the synthesis of Pt incorporated SBA-15, its characterization
and correlation with catalytic activity.
both an adsorption and desorption branch and typically ran at
ꢁ196 ꢀC after samples were degassed at 120 ꢀC for 2 h once
the final temperature had been maintained. Specific surface areas
were calculated via the BET model at relative pressures of P/P0 ¼
0.06–0.3. The total pore volume was estimated from the uptake
of adsorbate at a relative pressure of P/P0 ¼ 0.99. Pore size distri-
bution curves were obtained via the NLDFT model assuming
cylindrical pore geometry and the micropore volume calculated
via t-plot analyses as a function of relative pressure using the
Broekhoff de Boer model for thickness curves measured between
˚
3.5–5.0 A. Chemical anaysis was carried out in a LabTam 8440
Plasmalab sequential mode ICP-AES spectrometer. The samples
were dissolved in aqua regia and the undissolved siliceous matter
filtered off prior to analysis. Catalytic activity of the Pt incorpo-
rated sample was tested for hydrogenation of cinnamaldehyde. A
100 mL stainless steel reactor equipped with a pressure gauge,
stirrer and thermocouple was used as the reaction vessel.
1.25 g of cinnamaldehyde in 20 mL of isopropanol was charged
in the reactor and heated to 80 ꢀC under a H2 pressure of
2.96 MPa. At intervals, aliquots of the reaction mixture were
taken from the reactor and GC analysis performed on Agilent
Technologies 6890 N Network GC system using a HP5 column.
Experimental section
Synthesis
SBA-15 was first prepared using polymer–surfactant composite
with varying concentrations of cetyl trimethyl ammonium
bromide (CTAB). Typically, 0.0364 g CTAB (LobaChemie)
was added to 100 g of 1 wt% solution of block copolymer
Pluronic P123 (Aldrich) so that the final composition was
Results
Modifying polymer micelles with surfactants is expected to
facilitate metal precursor diffusion by interaction with the ionic
head groups.22 However, polymer–surfactant interaction can
affect the micelle structure in many ways.23–25 These changes,
in turn, will be reflected in the final mesoporous structure.26
SBA-15 was synthesized with varying amounts of the surfactant,
CTAB in the polymer P123 and powder XRD studies were
carried out to follow the effects as shown in Fig. 1. At 1 mM
CTAB, the mesoporous structure with long range order remains
intact as shown by the higher angle peaks, albeit with a negligible
contraction. Thus it becomes evident that the presence of CTAB
at this concentration within the polymer micelle has no effect on
the overall micellar structure. However, as CTAB concentration
is increased to 4 mM, there is a contraction of the unit cell and
ꢀ
1 mM of CTAB and this mixture was stirred for 12 h at 25 C.
This solution was made acidic (pH < 2) by 6.25 mL conc. HCl
(37 %) and 2.08 g tetraethylorthosilicate (TEOS; Aldrich) added
ꢀ
under stirring. Stirring was continued for another 24 h at 40 C
and then autoclaved at 90 ꢀC for 48 h. The product was filtered,
washed, dried and calcined at 500 ꢀC enabling the template
removal. The CTAB concentration was progressively increased
to 32 mM in 1 wt% P123 for other samples. For Pt incorpora-
tion, 2.4 mL of 1 wt% solution of H2PtCl6 (Aldrich) was added
to the polymer–surfactant composite solution containing 1 mM
CTAB and 1 wt% P123 and stirred for 24 h prior to the addition
of conc. HCl and TEOS. The final molar composition of the gel
was 1 TEOS : 585 H2O : 6.3 HCl : 0.017 P123 : 0.01 CTAB : 0.006
H2PtCl6. The same procedure as above was followed.
Characterization
Powder X-ray diffraction of all the samples was carried out in
a PANalytical X’pert Pro duel goniometer diffractometer. A
proportional counter detector was used for low angle experi-
ments and an X’celerator solid state detector was employed in
wide angle experiments. The radiation used was Cu Ka
˚
(1.5418 A) with a Ni filter and the data collection was carried
out using a flat holder in Bragg–Brentano geometry (0.5 to 5ꢀ;
0.2ꢀ minꢁ1). Care was taken to avoid sample displacement
effects. A JEOL JEM-3010 operating at 300 kV (Cs ¼ 0.6 mm,
˚
resolution 1.7 A) was used for HRTEM sample observation. A
Gatan digital camera (model 794, Gatan 1024 ꢂ 1024 pixels,
pixel size 24 ꢂ 24 mm) at 15 000–80 000ꢂ magnifications was
used to record micrographs. Samples were crushed, dispersed
in ethanol before depositing onto a holey carbon grid. Nitrogen
adsorption–desorption isotherms were acquired using a Micro-
meritics ASAP 2020 instrument. The program consisted of
Fig. 1 Comparison of XRD patterns of SBA-15 prepared using 1 wt%
P123 and varying amounts of CTAB: (a) no CTAB, (b) 1 mM CTAB, (c)
4 mM CTAB, (d) 16 mM CTAB and (e) 32 mM CTAB.
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the long range order starts decreasing. Any further increase in
CTAB concentration results in a collapse of the structure since
the micellar structure is completely destroyed.
oxide (EO)–propylene oxide (PO) interface inside the micelle.
The hydrophobic hydrocarbon of the surfactant will reside in
the PO core of the micelle while the cationic head group will
reside at the EO–PO interface. The concentration of platinum
was fixed at 0.06 mM so that the Pt loading will be 1–2 wt% in
the final mesoporous compound. Pt incorporation did not have
any effect on the formation of SBA-15, retaining the long range
order. The XRD studies further showed that a discernible
expansion of the mesoporous unit cell occurred by the inclusion
of Pt when compared to the mesoporous material prepared
without platinum dispersion (Fig. 2). Scanning at high angles
revealed the presence of Pt nanoparticles of ꢃ8 nm calculated
by the Scherrer equation. A small swelling of the material with
Pt before calcination was also observed (see ESI†) pointing to
a possible effect by the mere dispersion of the Pt precursor within
the template on the unit cell.
Our intention was to optimize the concentration of the surfac-
tant CTAB which can be dispersed within the polymer micelle
without altering the micellar structure, under which conditions
the ionic head group of the surfactant will decorate the corona
(Scheme 1). Hence an optimum final concentration of 1 mM
CTAB was selected for further studies. When added to this
composite, [PtCl6]2ꢁ will interact with the cationic head groups
of CTAB and disperse effectively in the corona at the ethylene
Very interestingly, N2 adsorption studies showed that the Pt
incorporation has led to an unusual increase in surface area
and overall adsorption capability when compared to pure and
CTAB modified SBA-15 (Table 1). The shape of the isotherm
of Pt/SBA-15 also remained unchanged indicating the absence
of any blockage of the channels by the Pt nanoparticles
(Fig. 3). The adsorption properties of Pt/SBA-15 are enhanced
compared to pure SBA-15. The surface area increases by 60%,
pore volume by 56%, adsorbate uptake by 57% and the mode
pore diameter by 22%. This is consistent with the unit cell expan-
sion as seen by XRD studies and further suggests that the Pt
Scheme 1 Ensuring the formation of nanoparticles within the meso-
channels by dispersing metal precursors in surfactant-modified polymer
micelles
Fig. 2 XRD patterns of calcined samples: (a) pure SBA-15 using 1 wt%
P123 as template, (b) SBA-15 with 1 mM CTAB incorporated into 1 wt%
P123 and (c) Pt incorporated SBA-15 prepared with 1 wt% polymer
template P123 and 1 mM CTAB. Inset: Wide angle XRD showing
the presence of Pt nanoparticles of average size ꢃ8 nm calculated by
the Scherrer equation.
Fig. 3 N2 adsorption isotherms of (a) pure, (b) CTAB added and (c)
Pt/CTAB incorporated SBA-15.
Table 1 Unit cell and adsorption measurements of calcined samples: pure SBA-15, SBA-15 (with CTAB) and Pt-SBA-15 (with CTAB)
Total pore
volumed/cm3 gꢁ1
Micropore
volumee/cm3 gꢁ1
a
Unit cell /A
b
Pore size /A
BETc/m2 gꢁ1
N2 uptake/cm3 gꢁ1 STP
˚
˚
Material
Pure SBA-15
SBA-15 (with CTAB)
Pt-SBA-15 (with CTAB)
99
98
87
83
106
550 ꢄ 5.0
621 ꢄ 5.2
870 ꢄ 7.4
451
489
707
0.70
0.75
1.09
0.03
0.02
0.04
108
pffiffiffiffiffiffiffi
a
b
Hexagonal lattice parameter: a ¼ 4=3 d10
. Pore size distribution calculated on the adsorption branch of the isotherm via the DFT model assuming
cylindrical pore geometry. c BET specific surface area calculated within relative pressures of P/P0 ¼ 0.06–0.3. d Total pore volume measured at a relative
pressure of P/P0 ¼ 0.99. e Calculated through t-plot analysis as a function of relative pressure using the Broekhoff de Boer model for thickness curve
2
˚
measured between 3.5 and 5 A: log(P/P0 ¼ ꢁ16.11/t + 0.1682exp(ꢁ0.1137t).
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Fig. 4 TEM images of Pt incorporated SBA-15 showing a uniform
dispersion of nanoparticles. The particles are localized within the pores,
grown within and molded by the channel walls. The average particle size
is measured to be 6–8 nm.
Fig. 5 Cinnamaldehyde hydrogenation activity of Pt/SBA-15 (- cinna-
maldehyde; d cinnamyl alcohol; : hydrocinnamaldehyde; ; phenyl-
propanol).
precursors reside within the corona (EO–PO interface) of the
micelles during the synthesis.
TEM images of the final calcined sample show a very uniform
dispersion of Pt particles within the SBA-15 framework. A
general scan of the sample did not show any Pt particles on
the surface of the silica matrix, as expected (Fig. 4). A closer
look at the mesochannels evidenced the presence of nano-
particles of an average size 6–8 nm, located well within the
mesopores molded by the walls of the channels. It also becomes
clear from TEM that some of the nanoparticles may lead to
localized swelling of the mesopore wall structure probably
because the local concentration of the precursor exceeded the
optimum for restricted growth within the channel (ESI†). It is
noteworthy that most of the particles have grown within the
channels and no sign of agglomeration is seen. This shows that
the Pt precursor could diffuse well into the micelle structure
aided by the attractive force towards the ionic head group in
the corona, preventing the formation of clusters of Pt precursor
which would later lead to metal particle agglomeration. To
confirm the role of CTAB in enhancing precursor dispersion,
SBA-15 was prepared by using [PtCl6]2ꢁ/P123 composite as
a template without CTAB addition and the final calcined
material observed under TEM. In this case, agglomeration of
Pt over the surface of the mesoporous material was seen (see
ESI†). The retention of Pt was also highly improved (40%) in
CTAB incorporated polymer when compared to naked micelle
(7%). TEM images of the uncalcined sample showed only traces
of Pt particles (see ESI†).
Fig. 6 TEM images of the Pt sample after catalytic testing. The particle
size remains on average 6–8 nm without agglomeration.
increased initially but decreased rapidly after 1 h as complete
hydrogenation to phenylpropanol started to occur. Hence the
hydrogenation of the terminal aldehyde group was preferred
over the alkene group forming the desired product. It is
interesting to note here that the particles remained stable without
sintering retaining the well-defined mesostructure of the catalyst
after the reaction (Fig. 6).
Discussion
Further understanding of the surface structure and
morphology of the Pt nanoparticles, however, could not be
achieved by TEM since they are situated in the channels encap-
sulated by the silica wall. Under such conditions, probe reactions
utilizing multifunctional molecules are proposed to be ideal tools
to give insight into the selectivity and in turn surface properties
of active metal nanoparticles. Selective hydrogenation of a,b-
unsaturated aldehydes is well studied and structure–selectivity
correlations well established. Hence catalytic activity of the
Pt/SBA-15 was tested for hydrogenation of cinnamaldehyde
(Fig. 5) and selectivity studied by following the formation of
cinnamyl alcohol, hydrocinnamaldehyde and phenyl propanol
by gas chromatography (see ESI†). The selectivity towards
cinnamyl alcohol increased with reaction duration and reached
a maximum at 3 h. Hydrocinnamaldehyde selectivity also
XRD studies of SBA-15 prepared from composites with different
concentrations of CTAB in polymer P123 indicated a clear effect
of the surfactant on the micelle and hence the mesoporous
structure. At lower concentrations of surfactant, the interaction
is cooperative whereby the hydrophobic tail of the surfactant is
anchored to the core and the ionic head group dispersed in the
corona of the polymer micelle. However, it is possible that at
higher concentrations, the polymer micelle gradually mono-
merises to a ‘‘necklace’’ structure in which the surfactant groups
decorate the individual strands of the polymer.27 Selection of
the cooperative composite structure is crucial for two reasons:
(1) at low concentrations, addition of the surfactant does not
disturb the overall micelle structure and hence ensures the forma-
tion of the desired mesoporous material with continued long
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range order; (2) modifying polymers with surfactants in this way
is ideal for metal precursor dispersion since the positively charged
head group of CTAB facilitates the diffusion of negatively
charged [PtCl6]2ꢁ ions. During the formation of the mesoporous
compound, the silica precursor species assemble around the
micelle template leading to the interconnecting channels with
siliceous walls. Thus, the cooperative surfactant polymer
composite with Pt precursor well dispersed in the corona ensures
the encapsulation of the metal species within the channels of the
final mesoporous compound. Without the surfactant additive to
aid proper dispersion of the precursor, agglomeration of the Pt
species occurs which later leads to bigger particles on calcination.
XRD, TEM, adsorption and catalytic studies have thrown light
upon the possible mechanisms of formation of the Pt nano-
particles within the mesochannels and the effect on the overall
physical properties of the metal incorporated SBA-15. We
observe a small swelling of the mesoporous material upon
dispersion of the Pt precursor in the surfactant (as shown by
the XRD of the uncalcined sample). This can probably be attri-
buted to an overall swelling of the corona due to the repulsion
among the cationic head groups, now enhanced by the presence
of the [PtCl6]2ꢁ ions. This in turn increases the micelle volume
and leads to a subsequent increase in the unit cell and pore size
upon calcination. This expansion in the unit cell and pore size
gives strong evidence that the silica walls expand to incorporate
the Pt salts/nanoparticles within the micellar structure. Incorpo-
ration of Pt apparently results in a decrease in the siliceous wall
thickness which could be due to either a dampened interaction
of the micelle with the silica precursor in the presence of Pt salt
or an effect of nanoparticle formation during calcination.
However, it is worth noting that this decrease did not have any
effect on the long range of the mesostructure even after the hydro-
genation reaction. Moreover, TEM images show a widening of
the channels, albeit locally, during the formation of nano-
particles. Since only traces of Pt nanoparticles were observed in
the uncalcined sample, we can deduce that the temperature and
duration of the synthetic procedure is enough for partial forma-
tion of the particles but calcination can be concluded as the final
step of size restricted growth of the nanoparticles.
increased from 2 to 5 nm. The title compound, Pt-SBA-15
showed a maximum selectivity of 53% for cinnamyl alcohol at
a conversion of 65%. This high selectivity towards C]O
hydrogenation is in accordance with the reported observations
indicating an abundance of exposed (111) planes in the Pt
nanoparticles with sizes of the order of 6–8 nm.
The stability of nanoparticles is a well debated requirement in
relation to catalytic processes due to deactivation caused by
metal sintering. Localization of the nanoparticles exclusively
within the channels of SBA-15 clearly deters the agglomeration
of nanoparticles when exposed to aggressive conditions. Along
with this advantage, the ease of incorporation of metal nano-
particles into the mesostructured matrix makes the reported
method ideal for nanocatalyst synthesis. Furthermore, this
method for Pt nanoparticle incorporation using a simple in situ
method from metal precursors makes the material ideal for
host–guest relationships with minimal diffusion limitations
over a large surface when compared to the methods previously
described.
Conclusion
A novel yet convenient method of synthesis is developed for
incorporating Pt nanoparticles into mesoporous SBA-15. The
methodology exploits the electrostatic interaction between metal
precursor salts and the ionic surfactant head groups incorpo-
rated into the polymer micelle in a controlled manner. The
cooperative interaction of the hydrophobic tail group of the
surfactant with the hydrophobic core of the micelle so that
the ionic head groups decorate the corona without altering the
micellar structure is paramount for the formation of well-
ordered SBA-15 as well as uniform dispersion of the Pt
precursor. Structural characterization by diffraction, microscopy
and adsorption studies point to an exclusive localization of
Pt nanoparticles within the mesochannels leading to a material
with enhanced physical properties like adsorption capacity,
surface area and pore volume. Pt nanoparticles of size 6–8 nm
were observed uniformly dispersed and located inside the
mesochannels. Furthermore, incorporation of CTAB and later
Pt precursor seems to alter the relative pore characteristics
resulting in a material superior to the parent compounds. More-
over, most advantageously, the nanoparticles are unaltered and
not sintered after catalytic reactions. Hence this method can be
utilized in nanocatalysis more effectively: added advantages
being the ease of preparation as well as the localization of the
nanoparticles within the channels. Since proper diffusion of the
metal precursor is crucial to nanoparticle formation, we believe
that fine tuning of the particle size is quite plausible with control
in synthesis conditions. A concerted study of these materials in
different stages and under various conditions of preparation is
underway to further understand the exact mechanism of
formation of the nanoparticles.
Cinnamaldehyde hydrogenation is used as a probe for better
understanding the surface morphology of the nanoparticles.
This reaction proceeds through a parallel and consecutive mech-
anism by reduction of C]C and C]O bonds. Particle size is
reported to play a commanding role in deciding selectivity
towards unsaturated alcohol or saturated aldehyde;28 as particle
size increases (5–9 nm), C]O hydrogenation is more favoured.
This is due to the extent of exposure of different crystallographic
planes with increasing particle size. Selectivity arises here due to
differences in adsorption energies of C]O and C]C on
different crystallographic planes of Pt. Hydrogenation of C]O
to give unsaturated alcohol is proposed to be faster on the
Pt(111) plane whereas Pt(110) facilitates C]C adsorption and
hence formation of saturated aldehyde selectively.29,30 We
utilized this fact for further probing the characteristics of the
nanoparticle. Change in activity and selectivity depending on
particle sizes has been reported for Pt based catalysts on different
supports.31,32 3 wt% Pt supported on carbon nanotubes, for
instance, showed ꢃ33% increase in selectivity of cinnamyl
alcohol over hydrocinnamaldehyde when the particle size
Acknowledgements
AKP acknowledges CSIR, New Delhi, India for fellowship and
¨
RPH through Prof. Lennart Bergstrom, Stockholm University,
thanks the Swedish Research Council (VR) for funding. We
thank RSIC, IIT Mumbai, India for ICP analysis.
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17 Z. Zhang, S. Dai, X. Fan, D. A. Blom, S. J. Pennycook and Y. Wei,
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