Sol–gel synthesis and morphological control of nanocrystalline TiO 2 viaurea treatment Institute of Materials Science and Engineering, National Sun Yat-Sen University, Kaohsiung 80424, T
Trang 1Sol–gel synthesis and morphological control of nanocrystalline TiO 2 via
urea treatment
Institute of Materials Science and Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan
Received 5 January 2007; accepted 18 July 2007 Available online 1 August 2007
Abstract
Nanocrystalline TiO2rods and hollow tubes with an engraved pattern on the surface have been prepared by a novel anionic template-assisted sol–gel synthesis via urea treatment and under hydrothermal condition X-ray diffractometry (XRD) results indicate that these nanocrystallines consist predominantly of anatase TiO2, with minor amounts of rutile and brookite Scanning and transmission electron microscopy (SEM and TEM) analyses reveal these rods and hollow tubes may result from the aggregates of nanorods of∼10 nm in diameter The crystallographic faceting found from TEM further reveals the polymorphic nature of the nanocrystalline TiO2thus prepared A “reverse micelle” formation mechanism taking into account the hydrothermal temperature, the pH effect of the sol–gel system, the isoelectric point, the formation of micelles, and the electrostatic interaction between the anionic surfactant and the growing TiO2particulates is proposed to illustrate the competition between the physical micelle assembly of the ionic surfactants and the chemical hydrolysis and condensation reactions of the Ti precursors
©2007 Elsevier Inc All rights reserved
Keywords: Nanocrystalline TiO2; Anionic template-assisted; Sol–gel; Urea; Hydrothermal; Reverse micelle; Isoelectric point; pH effect
1 Introduction
Recently, nanocrystalline titania (TiO2) has attracted much
attention because of its potential applications in
environmen-tal purification [1,2], catalysis and photocatalysis [3–5], gas
sensors[6], dielectric ceramics, pigments, and high efficiency
dye-sensitized solar cells [7–10] These nanocrystalline TiO2
have been prepared with various forms covering from
nanopar-ticles[11,12], thin films[13,14], nanotubes[15]to mesoporous
structure[16]and others[17–20] In particular, nanoscale TiO2
tubes and wires have attracted considerable attention because
they have large surface area and high photocatalytic activity
[20,21] Nanotubular TiO2 materials with diameters∼10 nm
have received intensive attention due to its potential
applica-tions in the fields which are traditionally dominated by carbon
nanotubes On the other hand, hollow fibers of TiO2with larger
outer diameter of 150–600 nm and aspect ratio of∼30 prepared
with an amphiphilic supramolecular organogelator as the
struc-* Corresponding author Fax: +886 7 5250179.
E-mail address:tjhsu@facmail.nsysu.edu.tw (T.-C Hsu).
ture directing agent has been reported By self-assembling into
a fibrous structure, this agent acted as a template later in the sol– gel polymerization of the titanium precursors to become hol-low tubular titania[22] On the other hand, use of membranes
in template synthesis such as the porous alumina membranes and the nanoporous track-etch polymeric membranes in the presence of certain additives has also received intensive atten-tion[23] In this approach, the pores within these nanoporous membranes play the same role as the templated surfactants in controlling nanostructures of the desired materials
Various crystalline phases have been identified for these reported nanocrystalline titania Among the major five poly-morphs of titania, the rutile phase is the most stable one at all temperatures under ambient pressure; the anatase phase is meta-stable; the brookite phase is the least stable one; whereas TiO2
-II and TiO2-III can be derived from the anatase or brookite phase under pressure TiO2-B is the monoclinic form of tita-nium dioxide This mineral can be found in weathering rims
on tektite and perovskite and appears as lamellae in anatase from hydrothermal veins; its density is much lower than that
of the other three polymorphs Polymorphic transformations of
0021-9797/$ – see front matter © 2007 Elsevier Inc All rights reserved.
doi:10.1016/j.jcis.2007.07.062
Trang 2anatase to rutile and of brookite to rutile do not take place
re-versibly[24]
It is generally claimed that the sol–gel synthesis of ceramic
oxides offers advantages such as high purity, good
homogene-ity, low processing temperature, and also the possibility of
mak-ing new nanocrystalline solids outside the range of normal glass
formation[25,26] Factors affecting the ultimate properties of
ceramic materials in a typical sol–gel process may include the
type of solvent, the reactivity of metal precursor, pH of the
re-action medium, and rere-action temperature, among many factors
[27,28] By controlling these material or processing parameters,
different surface chemistry and microstructure of ceramic
ox-ides can be obtained However, the precipitates derived by sol–
gel synthesis are typically amorphous; this requires a high
tem-perature heat treatment to induce the crystallization during the
post-gel stage On the other hand, hydrothermal synthesis has
been carried out at a relatively low temperature (<250◦C) to
produce sufficiently crystalline ceramic solids as compared to
calcination[26,29,30] In the hydrothermal synthesis,
morphol-ogy, composition, structure, grain size, and crystalline phase
can be controlled by changing the hydrothermal parameters
such as reaction temperature and pressure, pH values, sol
com-position, type of the solvent and additive, and the aging time
[11,31,32] Among these processing parameters, the
hydrother-mal temperature is regarded as the most crucial one since it
controls the steam pressure of this closed aqueous system
It has been reported that the addition of liquid ammonia in
the sol–gel synthesis of some inorganic oxides may result in
precipitation Yada et al.[33] obtained a hexagonal structure
alumina by a homogeneous precipitation method using urea,
from which the ammonia was generated at an elevated
tem-perature Banerjee et al.[34]employed urea as the
hydrolyz-ing agent to control the hydrolysis rate in the preparation of a
hexagonal mesoporous nickel oxide To our best
understand-ing, the urea approach has not been applied to the preparation
of TiO2 nanotubes However, a different surfactant-mediated
template laurylamine hydrochloride was adopted by Peng et
al.[35], which resulted in a TiO2tubules with mesostructural
walls, the outer diameter and the wall thickness of the
tita-nia microtubules being 2–8 and 0.2–2 µm We report in this
study a novel anionic template-assisted sol–gel approach via
urea treatment and under hydrothermal condition for
synthe-sizing the nanocrystalline TiO2 rods and hollow tubes,
con-sisting of aggregates of nanorods of∼10 nm in diameter The
nanocrystalline TiO2thus prepared appears to have an engraved
surface morphology and consists mainly of the anatase and
mi-nor amount of rutile and brookite It is also demonstrated that
microstructure and phase content of the nanocrystalline TiO2
can be tailor-made by properly manipulating those
parame-ters during the combined sol–gel and hydrothermal
process-ing
2 Experimental
Titanium tetrachloride (TiCl4; 99.9%, Acros) was used as
the titanium source; sodium dodecyl sulfate (SDS, CH3(CH2)11
OSO Na; 99%, Aldrich) and urea (H NCONH ; 98%, Acros)
were used as received Urea was employed as a hydrolysis agent
to control the hydrolysis rate, it was also used in this work to control the pH value of sol–gel system by a hydrolysis reaction (Eq (1)) at a temperature greater than 80◦C, from which the
ammonia could be generated[34]: (NH2)2CO+ 3H2O→ 2NH+4 + 2OH−+ CO2 (1)
A specific amount of SDS was first dissolved in deionized (DI) water and mixed with urea TiCl4was then added dropwise
to this highly viscous aqueous solution with rigorous stirring Caution on the quick release of HCl should be taken when TiCl4
was added, due primarily to its extreme sensitivity to the mois-ture TiCl4, SDS, urea, and DI water were mixed in a molar ratio of 1:2:30:60 The mixture was stirred at 40◦C for 1 h to
yield a transparent solution The solution was then heated un-til the pH had increased to 2.2; it was immediately transferred into a sealed Teflon-coated autoclave and was further heated at
an elevated hydrothermal temperature for 48 h The pH value
of the final white slurry thus prepared was 5.6 (90◦C), 8.52
(120◦C), 9.12 (150◦C), and 10.6 (180◦C) Since the autoclave
is a closed system under high temperature and high pressure, it
is not feasible to dynamically measure the pH values in situ
dur-ing the sol–gel process Instead of estimatdur-ing the dynamics of the pH during hydrothermal treatment, an alternative approach was adopted The sol–gel reaction was terminated after 24 h and the pH value was measured It was found for all the four sam-ples the pH values at 24 h had reached a value as high as that of
at 48 h, indicating that effect of ammonia on the microstructure was mostly profound in the first half period of the hydrothermal treatment
The measured autogenous pressures inside the autoclave un-der different hydrothermal temperatures were 0.92 bar (90◦C),
2.1 bar (120◦C), 5.13 bar (150◦C), and 10.97 bar (180◦C),
which are very close to the values estimated from ASME Steam Tables (0.70 bar (90◦C), 1.99 bar (120◦C), 4.76 bar
(150◦C), and 10.03 bar (180◦C)) [36] The product was
re-peatedly washed with DI water to remove the organic moieties, filtered with a 0.45 µm filter paper, and then dried in air It was then calcined at 800◦C (at a heating rate of 4◦C/min) for
4 h in air to further remove the residual organic moieties The calcined product was then re-washed with DI water to remove water-soluble impurities such as sodium sulfate formed during calcination
Crystalline phases were determined by the X-ray
diffractom-etry (XRD, Siemens D-5000, Karshrule, Germany) with CuKα radiation and Ni filter operating at 40 kV/30 mA
Microstruc-ture was analyzed by the field-emission scanning electron mi-croscopy (FE-SEM, JEOL™ 6330, Tokyo, Japan) operating at
20 kV Sample for SEM was first mounted onto a carbon adhe-sive pad which was attached to an aluminum stub, it was then air-dried and gold-coated (Pelco SC-6 sputter-coater) Samples ultrasonicated and filtered on holey carbon grids were exam-ined by the transmission electron microscopy (TEM, JEOL™ AEM 3010, Tokyo, Japan) operating at 200 kV
Trang 33 Results and discussion
3.1 Phase identification
The XRD patterns of the as-prepared nanocrystalline TiO2
samples synthesized under different hydrothermal conditions
shown inFig 1reveal diffraction peaks of (101), (004), (200),
and (211), which are characteristic of the anatase phase The
relative broad peaks suggest low crystallinity among the four
samples measured; the crystallinity increases with increasing
hydrothermal temperatures The result is quite different from
the traditional (or typical) sol–gel process in which only
amor-phous phase can be obtained from the precipitates derived by
sol–gel process before calcinations; and further higher
temper-ature heat treatment is normally required to induce
crystalliza-tion Thus, the hydrothermal treatment may be regarded as an
alternative to calcination for promoting the crystallization[11]
A minor diffraction peak assigned as the rutile (110) found in
sample SDS-90 can also discernibly be identified fromFig 1
XRD patterns of calcined nanocrystalline TiO2prepared
un-der different hydrothermal conditions are shown inFig 2
Dif-fraction peaks of anatase (tetragonal, I 41/amd (No 141)),
ru-tile (tetragonal, P 4/mnm (No 136)) and brookite
(orthorhom-bic, Pcab (No 61)) (corresponding to JCPDS No 21-1272,
21-1276 and 29-1360, respectively) are identified
unambigu-ously The fact that no discernible peak was identified in the
low range of 2θ = 1–10◦ has ruled out the existence of the
amorphous mesoporous structure[37,38] This is contradictory
to what was expected in our initial design of experiments and
is also inconsistent with those reported in the literature[34,39]
An amorphous mesoporous structure would usually be formed
in the sol–gel-derived ceramic oxides prepared by the assistance
of ionic template without any hydrothermal treatment It can be
postulated that this hydrothermal treatment in this study has a
profound effect on the ultimate microstructure of the nanocrys-talline TiO2
Summarized inTable 1are the phase contents determined from the integrated XRD peak intensities of anatase (101), ru-tile (110), and brookite (121) by a numerical deconvolution method[40]; also listed are the crystallite sizes calculated by Scherrer equation It is obvious that the crystallite sizes un-der various hydrothermal conditions change consiun-derably; but most are within the range of 30–50 nm, except for SDS-150 (∼13 nm).Table 1also indicates that the meta-stable anatase is the dominant phase for all the samples; other minor phases in-clude the more stable rutile (for the low-temperature samples) and the least stable brookite (for the high-temperature samples) Thus, increasing hydrothermal temperature seems to encour-age a phase transformation from rutile to brookite, while phase content of anatase remains roughly the same The results are consistent with the reported findings[25]; it was argued that the dissolution of an acidified titania sol at low temperatures was slow, resulting in a slow crystallization This crystallization process would be governed by the thermodynamics, not the ki-netics; therefore, the most stable rutile phase should be formed
at low hydrothermal temperatures On the other hand, the meta-stable anatase or brookite should be the favorable phases under higher hydrothermal temperatures, due kinetically to a faster dissolution and a more rapid precipitation
3.2 Morphological characterization
Under a lower magnification, FE-SEM image for SDS-90 shown inFig 3a reveals a long fibrous structure with an as-pect ratio∼30, a value much higher than those reported data which are typically less than 10[18,19]; an engraved pattern
on the surface of SDS-90 sample is also discernible inFig 3b, not found in the other samples prepared at higher hydrothermal
Fig 1 XRD patterns of nanocrystalline TiO2rods and hollow tubes prepared at four hydrothermal temperatures indicated; samples measured before calcination Patterns were compared to the JCPDS data (F: rutile); “s” and “u” denote peaks corresponding to SDS and urea, respectively.
Trang 4Fig 2 XRD patterns of nanocrystalline TiO2 rods and hollow tubes prepared at four hydrothermal temperatures indicated; samples measured after calcination Patterns were compared to the JCPDS data (F: rutile; ": brookite).
Table 1
Summary of phase contents and crystallite sizes
Crystallite size (nm)a Content (%)b Crystallite size (nm)a Content (%)b Crystallite size (nm)a Content (%)b
a Calculated by Scherrer equation.
b Calculated using the equation in Ref [38]
Fig 3 FE-SEM images of nanocrystalline TiO rods and hollow tubes (a) and the engraved surface (b) for SDS-90.
Trang 5Fig 4 FE-SEM images of nanocrystalline TiO2 rods, hollow tubes, and platelets prepared at different hydrothermal temperatures: (a) SDS-90, (b) SDS-120, (c) SDS-150, and (d) SDS-180.
temperatures Note that SDS-90 maintained a pH value under
the isoelectric point (IEP) of TiO2 (∼5.8) during its sol–gel
synthesis A closer examination on SDS-90 is given inFig 4a,
from which a well defined rod-like or hollow-tube-like
mor-phology can be identified with the rod diameter of 50–300 nm
and the hollow tube diameter of 200–600 nm (all have an aspect
ratio ∼30); note that some nanorods of ∼10 nm and with the
same aspect ratio in diameter can also be discernibly identified
The morphology found here is very similar to those reported
TiO2 hollow fibers which adopted an amphiphilic compound
and without hydrothermal treatment[22,40,41] In our case, an
anionic surfactant SDS was adopted as the liquid crystal
tem-plate; then the base-catalyzed sol–gel synthesis was followed
under hydrothermal conditions Due to the high temperature
and high pressure in the autoclave under the hydrothermal
con-dition, a completely different reaction mechanism and
forma-tion sequence of liquid crystal template and sol–gel synthesis
may be rationalized The steam is generated under high
temper-atures to produce a hydrostatic pressure which in turn imposes
a profound effect on the ultimate microstructure of the ceramic oxides thus prepared This autogenous hydrostatic pressure can
be as high as 11 bars under a hydrothermal temperature of
180◦C (exact values can be found in Section2) Therefore, it
can be said that in this aqueous system in the sealed autoclave, the temperature has much higher impact on the sol–gel reaction rate, the morphology, as well as the reaction mechanism When the hydrothermal temperature is increased, it is ob-served fromFig 4that the rods and hollow tubes are gradually transformed into a tabular or platelet structure, possibly due
to the autogenous pressure generated under hydrothermal con-ditions Note that the pH value for SDS-120, SDS-150, and SDS-180 during their sol–gel processing was above the IEP
of TiO2 These tabular or platelet structures remain almost the same physical size as the rods and tubes (same aspect ratio) The straight and parallel stripes (referred toFigs 4b and 4c) on the surface of the rods and hollow tubes may result from the aggregates of the smaller and finer nanorods of∼10 nm in di-ameter[31,32]
Trang 6Fig 5 TEM images of nanocrystalline TiO2hollow tubes and platelets prepared at different hydrothermal temperatures: (a) SDS-90, (b) SDS-120, (c) SDS-150, and (d) SDS-180.
Shown inFig 5a is a TEM image of hollow tube with an
outer diameter of∼600 nm and an inner diameter of ∼200 nm
for SDS-90, in addition to the rods found from SEM (Fig 4a)
The yield of the TiO2nanotubes and nanorods synthesized from
their precursors is estimated from stoichiometric calculation to
be 81%; meanwhile the ratio between the hollow tubes to the
rods is estimated from the SEM image to be about 1/5 The
clear parallel and straight stripes appeared on the surface of the
rod, tube, and platelet inFigs 5a–5care in fact the
crystallo-graphic faceting that occurs due to anisotropic (solid-to-vapor)
surface energy This faceting exists because atoms are packed
in different density along crystal planes, e.g., fcc close-packed
plane on {111} along110 Likewise for TiO2, regardless of
its polymorphs (the anatase is the dominant phase from XRD
results in this study), would have different atomic packing
den-sity, and so faceting appears Crystals with different structure
usually appear faceted On the other hand, these stripes may be the aggregates of nanorods as suggested from SEM observa-tions
With increasing hydrothermal temperatures, the rod or the tube gradually transform into a tabular or platelet structure (Figs 5c, 5d) These TEM images show consistent results as the SEM inFig 3 When proper chemicals are inserted inside, these types of TiO2rods and hollow tubes may possess potential applications due to their unusual catalytic, electric, and optical properties
3.3 Formation mechanism
In sol–gel synthesis using transition metal chlorides as the precursors, hydrolysis and condensation reactions take place very rapidly[37,42,43] In preparing TiO using SDS as the
Trang 7an-Fig 6 Formation of nanocrystalline TiO2rods and hallow tubes is favorable at lower temperatures when the pH value is below IEP of TiO2(path I); while higher temperatures encourage the formation of a tabular structure of TiO2(path II).
ionic surfactant under basic conditions with urea, the formation
of SDS micelles as the templates may have to compete with the
hydrolysis and condensation of Ti precursors At a
hydrother-mal temperature of 90◦C in which the basic urea has not been
able to completely release its hydroxyl ions, the reaction system
appears to have a pH value below IEP of TiO2, thus allowing
a positively charged TiO2 surface due to rapid hydrolysis and
condensation The negatively charged SDS micelles originally
present in the system are now forced to break down and are
attached to the positive TiO2surface, forming the so-called
“re-versed micelle” (path I inFig 6) These SDS-coated TiO2
parti-cles then aggregate into stacked TiO2nanorods; further
calcina-tion at higher temperatures results in the nanocrystalline TiO2
rods and hollow tubes Another possible mechanism of
chemi-cally induced self-transformation of amorphous solid particles
to account for the fabrication of hollow inorganic microspheres
has been mentioned [17] This approach was based on
mor-phologically confined processes of Oswald ripening where no
sacrificial surfactant template was involved The profound
ef-fect found in this study on the microstructure due to the
pres-ence of urea, from which ammonia is chemically produced,
can also be regarded as a result of chemically induced phase
transformation On the other hand, the “reverse micelle”
mech-anism was associated with certain anionic surfactant-templated
sol–gel processes such as dodecyl sulfate (SDS) and sodium
bis(2-ethylhexyl) sulfosuccinate; note that no urea was involved
in the morphological control of TiO2materials in these studies
and that the particle sizes thus prepared are within the
approxi-mately same range as in this study[44–46]
It can now be rationalized that the engraved pattern inFig 3b
may result from the complete destruction of the anionic organic
surfactants SDS on the surface of the nanocrystalline TiO2
dur-ing calcination Initially, SDS was designed to self-assemble
into a template upon which sol–gel reaction was expected to
take place subsequently, an approach similar to the process of the commercially available MCM-41 silica Obviously, the ob-served morphology is not consistent with the generally accepted formation mechanism of the typical liquid crystal template-assisted sol–gel synthesis of ceramic powders The anionic sur-factants appear to have altered the reaction kinetics of hydroly-sis and condensation of Ti precursors
On the other hand, at higher hydrothermal temperatures, a complete urea reaction encourages a basic sol–gel reaction sys-tem; this allows the surface of TiO2to be negatively charged because the pH of the sol–gel system is now above the IEP of TiO2 Therefore, the also negatively charged SDS micelles are incompatible to the TiO2particles and are excluded from the sol–gel processing of TiO2upon further condensation (path II
inFig 6) Without the constraint of the reverse micelles by the rejected SDS molecules, formation of rods and hollow tubes becomes unfavorable and a plate-like morphology with a rough surface may form after the hydrothermal treatment Further cal-cination results in the nanocrystalline TiO2platelets as shown
inFigs 4d and 5d
4 Conclusions
A novel approach adopting the anionic template-assisted sol–gel synthesis via urea treatment and under hydrothermal processing to synthesize the nanocrystalline TiO2rods and low tubes has been demonstrated in this study Similar hol-low tubes with same range of diameters have been reported but using a completely different amphiphilic surfactant In-stead of an amorphous as is reported in most sol–gel-derived ceramic oxides before calcinations, the as-prepared nanocrys-talline TiO2has an anatase phase, although with a low degree
of crystallinity The calcined nanocrystalline TiO2is found to
be mostly anatase, with minor amount of rutile and brookite
Trang 8Low hydrothermal temperatures are found to favor the
forma-tion of TiO2rods and hollow tubes, both may result from the
aggregates of TiO2 nanorods The pH value of the reacting
sol–gel system plays a crucial role in determining the charge
on the surface of the reacting TiO2 particles The pH value
of the sol–gel system under lower hydrothermal temperature is
found to below IEP of TiO2such that the surface charge of the
anatase TiO2 particles becomes positive This forces the SDS
micelles to form a coating on the TiO2particles due to
elec-trostatic attraction This “reverse micelle” mechanism results in
the formation of the nanocrystalline TiO2rods or hollow tubes
Upon calcinations at higher temperatures as the surfactant SDS
is incinerated, an engraved TiO2surface is found Since higher
hydrothermal temperatures encourage the urea reaction to
re-lease the hydroxyl ions, the sol–gel system becomes more basic
and its pH value is far exceeding the IEP of TiO2 The
neg-atively charged SDS micelles are thus rejected from the also
negatively charged TiO2 particle, resulting in a tabular TiO2
structure These types of TiO2 rods and hollow tubes, when
proper chemicals are inserted inside, may possess potential
ap-plications due to their unusual catalytic, electric, and optical
properties
Acknowledgments
This work was funded by the National Science Council of
Taiwan through contract NSC-93-2216-E-110-012 One of the
authors (L.H.K.) wishes to thank C.L Chang for his help with
FE-SEM, and Dr Y.C Wu of National Taipei University of
Technology for her help with TEM
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