Characteristics and properties of a novel in situ method of synthesizing mesoporous TiO2 nanopowders by a simple coprecipitation process without adding surfactant

Article abstract of DOI:10.1016/j.jallcom.2014.05.227

In situ synthesis of mesoporous TiO₂ nanopowders using titanium tetrachloride (TiCl₄) and NH₄OH as initial materials has been successfully fabricated by a coprecipitation process without the addition of surfactant. Characteristics and properties of the mesoporous TiO₂ nanopowders were investigated using differential scanning calorimetry/ thermogravimetry (DSC/TG), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) and Barrent-Joyner-Halenda (BJH) analyses, transmission electron microscopy (TEM), selected area electron diffraction (SAED) and high resolution TEM (HRTEM). The results of TG and XRD showed that the NH₄Cl decomposed between 513 and 673 K. XRD results showed that the anatase TiO₂ only contained a single phase when the calcination temperature of the precursor powder was less than 673 K. Whereas phases of anatase and rutile TiO₂ coexist after calcining at 773 K for 2 h. The crystalline size of the anatase and rutile TiO₂ was 14.3 and 26.6 nm, respectively, when the precursor powder was calcined at 773 K for 2 h. The BET and BJH results showed a significant increase in surface area and pore volumes when the NH₄Cl was completely decomposed. The maximum values of BET specific surface area and volume were 172.8 m²/g and 0.392 cm³/g, respectively. The average pore sizes when calcination was at 473 and 773 K for 2 h were 3.8 and 14.0 nm, respectively.

Full text of DOI:10.1016/j.jallcom.2014.05.227

Journal of Alloys and Compounds 613 (2014) 107–116
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Characteristics and properties of a novel in situ method of synthesizing
mesoporous TiO2 nanopowders by a simple coprecipitation process
without adding surfactant
d
Shang-Wei Yeh a,b, Horng-Huey Ko a, Hsiu-Mei Chiang c, Yen-Ling Chen a, , Jian-Hong Lee ,
⇑
Chiu-Ming Wen b, Moo-Chin Wang a,
⇑⇑
a Department of Fragrance and Cosmetic Science, Kaohsiung Medical University, 100 Shih-Chuan 1st Road, Kaohsiung 80782, Taiwan
b Department of Life Science, National University of Kaohsiung, 700 Kaohsiung University Road, Kaohsiung 811, Taiwan
c Department of Cosmeceutics, China Medical University, 91 Hsueh-Shih Road, Taichung 40402, Taiwan
d Clean Energy and Eco-Technology Center, Industrial Technology Research Institute, 8 Gongyan Road, Tainan 734, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
In situ synthesis of mesoporous TiO2 nanopowders using titanium tetrachloride (TiCl4) and NH4OH as
initial materials has been successfully fabricated by a coprecipitation process without the addition of
surfactant. Characteristics and properties of the mesoporous TiO2 nanopowders were investigated using
differential scanning calorimetry/thermogravimetry (DSC/TG), X-ray diffraction (XRD), Brunauer–
Emmett–Teller (BET) and Barrent–Joyner–Halenda (BJH) analyses, transmission electron microscopy
(TEM), selected area electron diffraction (SAED) and high resolution TEM (HRTEM). The results of TG
and XRD showed that the NH4Cl decomposed between 513 and 673 K. XRD results showed that the ana-
tase TiO2 only contained a single phase when the calcination temperature of the precursor powder was
less than 673 K. Whereas phases of anatase and rutile TiO2 coexist after calcining at 773 K for 2 h. The
crystalline size of the anatase and rutile TiO2 was 14.3 and 26.6 nm, respectively, when the precursor
powder was calcined at 773 K for 2 h. The BET and BJH results showed a significant increase in surface
area and pore volumes when the NH4Cl was completely decomposed. The maximum values of BET spe-
cific surface area and volume were 172.8 m2/g and 0.392 cm3/g, respectively. The average pore sizes
when calcination was at 473 and 773 K for 2 h were 3.8 and 14.0 nm, respectively.
Received 23 March 2014
Accepted 31 May 2014
Available online 11 June 2014
Keywords:
Mesoporous TiO2 nanopowders
In situ synthesis
Coprecipitation process
Anatase
Rutile
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
powders create an opaque layer in the skin. To solve the cosmetic
drawback of TiO2 opaque sunscreen, micron-sized TiO2 particles
Ultraviolet A (UVA, 320–400 nm) is a major culprit in photoag-
ing and causing skin cancers. Even UVB (280–320 nm) radiation,
which primarily reaches the top-most layer of the skin, is respon-
sible for acute photodamage including sunburn and some non-
melanoma skin cancers [1]. Therefore, protecting humans against
UVA and UVB radiation is a matter of great important. Certain min-
erals can be used as sun-attenuating or sunscreen agents for pro-
tection against the adverse effects of UVA and UVB radiation.
Titanium dioxide (TiO2) is an important powder frequently used
in sunscreens as an inorganic physical sun blocker due to its being
more effective in the UVB region [2]. But micron-sized TiO2
must be replaced by TiO2 nanoparticles.
The use of TiO2 in cosmetics as a sunscreen agent requires high
purity with controlled crystalline size, surface properties, definite
phase content and morphology. The metastable anatase and stable
rutile phases are the two most important polymorphs of TiO2 [3].
These polymorphs exhibit various properties and, consequently,
different sunscreen performances.
Recently, several techniques such sol–gel [4,5], non-hydrolytic
sol–gel [6], hydrothermal [7,8] and coprecipitation [9–11] processes
have been widely used to prepare nanosized TiO2 powders in a way
that offers great control over their properties such as crystalline
phase, crystallite size and morphology and surface area. Moreover,
mesoporous TiO2 prepared using the sol–gel route was first reported
by Antonelli and Ying [12]. These mesoporous TiO2 powders are of
particular interest because their channels offer a large surface area
and their pore wall framework is nanocrystalline. On the other hand,
⇑
Corresponding author. Tel.: +886 7 3121101x2584; fax: +886 7 3210683.
Corresponding author. Tel.: +886 7 3121101x2366; fax: +886 7 3210683.
E-mail addresses: yelichen@kmu.edu.tw (Y.-L. Chen), mcwang@kmu.edu.tw
⇑⇑
(M.-C. Wang).
http://dx.doi.org/10.1016/j.jallcom.2014.05.227
0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

108
S.-W. Yeh et al. / Journal of Alloys and Compounds 613 (2014) 107–116
The crystalline phases of the TiO2 precursor powder after calcining at various
temperatures for 2 h were identified by X-ray diffraction (XRD, Model Rad IIA,
the thermal stability of mesoporous TiO2 is greater than that of tra-
ditional TiO2 nanoparticles [13].
Rigaku, Tokyo) with Cu Ka radiation and an Ni filter, operating at 40 kV, 30 mA
In recent years, mesoporous TiO2 nanoparticles possessing var-
ious pore size distributions have been successfully fabricated using
diblock copolymers via a sol–gel process in aqueous solution, as
reported by Kim et al. [14]. They demonstrated that their mesopor-
ous TiO2 nanoparticles could have multiple specific surface areas of
186, 210 or 192 m2/g with average pore sizes of 5.1, 6.1 and
6.4 nm, respectively depending on the type of diblock copolymer,
and crystallite sizes of 8.1, 8.3 and 8.8 nm when the precursor
powder was first calcined at 673 K. Hung et al. [13] synthesized
ordered hexagonal mesoporous TiO2 nanoparticles using an evap-
oration-induced self-assembly (EISA) route with Pluronic P123
and tetrabutyl orthotitanate (Ti(OBU)4) as the templating agent
respectively. Further, Herregods et al. [15] also reported that mes-
oporous TiO2 nanosized powders with a controlled uniform pore
size could be synthesized by the EISA route used the diblock
copolymer Brij 58 as surfactant. They indicated that the pore struc-
ture of a mesostructured TiO2 nanopowder was dependent on the
thermal stabilization and template removal. In addition, various
surfactants such as cetyl-trimethyl ammonium bromide (CTAB),
cetyl-trimethylammonium chloride (CTAC), benzalkonium chlo-
ride (BC) or octadecyl-trimethyl ammonium bromide (C18TAB)
could assist the sol–gel reaction at different calcination tempera-
tures and affect the formation of mesoporous TiO2 nanocrystalline
powders, as also pointed out by Casino et al. [16].
and a scanning rate (2h) of 1°/min. The crystallite sizes (D) of the anatase and rutile
TiO2 were calculated using Scherrer’s equation [17].
D ¼ 0:9k=ðb cos hÞ
ð1Þ
where k = 1.5405 Å is the wavelength of the Cu K
a
radiation, and b is the full-width
at half-maximum (FWHM) intensity in radians. The (101) and (110) reflections at
2h = 25.32° and 27.48° were used to define the FWHM intensity of anatase and rutile
TiO2, respectively, within their various crystalline phases, and h is the Bragg’s angle.
Spurr and Myers [18] have also used reflection intensities in the XRD pattern to
determine the anatase TiO2 content. The integrated intensity ratio of the anatase
TiO2 content was defined as follows:
IA
fA
¼
ð2Þ
1:265IR þ IA
where fA is the fraction of anatase TiO2, and IR and IA are the intensities of the (101)
and (110) reflections for the rutile and anatase TiO2, respectively.
The surface areas, pore volumes and average pore sizes of the calcined powders
were obtained by the conventional nitrogen absorption of the Brunauer–Emmett–
Teller (BET) and Barrent–Joyner–Halenda (BJH) analysis methods (Gemini 2360,
Micromeritics, USA).
After the TiO2 precursor powder was calcined at various temperatures for 2 h,
the morphology of the product powder was examined by transmission electron
microscopy (TEM, JEM-2100F, JEOL, Japan) operated at 200 kV. Each TEM sample
was prepared by dispersing the product powder in an ultrasonic bath and then col-
lecting it on a copper grid. Selected area electron diffraction (SAED) was utilized to
confirm the phases of the calcined TiO2 nanocrystallite powder. A high resolution
TEM (HRTEM) examination was also performed on the product powder samples.
As mentioned above, mesoporous TiO2 nanosized powders are
usually synthesized using a surfactant. The synthesis of mesopor-
ous TiO2 nanopowders without a surfactant added has not been
discussed in detail. Therefore, in the present study we attempted
to use residual ammonium chloride (NH4Cl) as the agent of pore
formation for in situ synthesis of mesoporous TiO2 nanosized pow-
ders prepared by a simple coprecipitation process. The characteris-
tics and properties of the mesoporous TiO2 nanopowders were
investigated by calorimetry/thermogravimetry (DSC/TG), X-ray
diffraction (XRD), Brunauer–Emmett–Teller (BET) and Barrent–
Joyner–Halenda (BJH) analyses, transmission electron microscopy
(TEM), selected area electron diffraction (SAED) and high resolu-
tion TEM (HRTEM).
The scope of this work is focused on: (i) studying the thermal
behavior of the TiO2 precursor powder with residual NH4Cl, (ii)
determining the phase transition of the TiO2 precursor powder with
NH4Cl after calcination, (iii) evaluating the surface areas and pore
sizes of the mesoporous TiO2 nanopowders and (iv) observing the
microstructures of the mesoporous TiO2 nanopowders by TEM.
3. Results and discussion
3.1. Thermal behavior of TiO2 precursor powder with residual NH4Cl
Fig. 1 shows the DSC/TG curves of the TiO2 precursor powder
with residual NH4Cl heated in air at a rate of 10 K/min from
303 K to 1173 K. The TG curve reveals the weight loss in the TiO2
precursor powder with residual NH4Cl occurring in a temperature
range between room temperature and 860 K, and shows that the
weight loss approached an exponential decay from 366 to 721 K.
The TG curve of weight loss can be divided into three stages: (i)
from room temperature to 366 K, (ii) from 366 to 721 K and (iii)
from 721 to 860 K. The weight was maintained as nearly constant
when the heating temperature was higher than 860 K.
In the first stage, a weight loss of about 4.1% was attributed to
the free water of the physisorbed [19]. In the second stage, about
37.4% of the sample weight was lost. This result was due to deox-
olation and NH4Cl decomposing from the TiO2 precursor powder.
When the TiO2 was synthesized using TiCl4 as an initial material
prepared by a coprecipitation process, then the precipitates were
2. Experimental procedure
2.1. Sample preparation
The TiO2 precursor powder with residual NH4Cl was prepared using a simple
coprecipitation route. The initial materials included titanium tetrachloride (TiCl4,
purity ꢀ98.5%, supplied by Nihon Shiyaku Reagent, Japan) and aqueous NH4OH
solution (purity ꢀ28%, supplied by Nihon Shiyaku Reagent, Japan). TiCl4 was dis-
solved in deionized water with a volume ratio of 1:20 and the mixed solution
was stirred using a magnetic stirrer and heated to 348 K for 2 h. Under these con-
ditions Ti4+ formed, but due to the very low pH value of the Ti4+ solution, no precip-
itation occurred. In order to increase the pH and promote precipitation, aqueous
NH4OH solution must be added to the mixture as a precipitating agent. When the
NH4OH was slowly added to the water–TiCl4 mixture until the pH adjusted to 7,
then precipitation took place. Subsequently, the precipitates were freeze dried at
218 K in a vacuum without washing.
2.2. Sample characterization
Differential scanning calorimetry/thermogravimetry (DSC/TG, SDT Q600, TA)
analysis was conducted on 200 mg of TiO2 precursor powder at a heating rate of
10 K/min in static air up to 1173 K. Al2O3 powder was used as the reference
material.
Fig. 1. The DSC/TG curves of TiO2 precursor powder with residual NH4Cl content
heated at a rate of 10 K/min in static air from 303 K to 1173 K.

S.-W. Yeh et al. / Journal of Alloys and Compounds 613 (2014) 107–116
109
washed five times with a large amount of deionized water. The TG
curve of this precursor powder had a main weight loss of 17.9%,
expressed in the following reaction as reported by Yeh et al. [10].
peaks at (100), (110), (200) and (211) (JCPDS Card No. 77-
2352). It can also be seen that reflection peaks of (101), (004)
and (116) appeared in the anatase TiO2 (JCPDS Card No. 89-
4921). This phenomenon is attributed to crystallines of type I
NH4Cl and anatase TiO2 coexisting in the precursor powder. How-
ever, the crystallinity of anatase TiO2 was very weak.
TiOðOHÞ2 ! TiO2 þ H2O
ð3Þ
In the present study, the TiO2 precipitate was without washing
and directly freeze dried; therefore, the precursor powder with
residual NH4Cl content. The precursor obtained in the present
study also used TiCl4 as the initial material and was prepared by
the same route as in Yeh et al. [10]. It can be assumed that the reac-
tions in Eq. (3) also hold true in the present study. Therefore, the
TiO2 precursor powder contained about 19.5% residual NH4Cl.
The third stage of weight loss, about 0.8%, can be assigned to
dehydration of surface water molecules most likely existing as a
wide set of energetically nonequivalent surface hydrogen groups
[20]. In the present work the hydrolysis of the TiCl4 occurred at
low pH. The first step in the hydrolysis caused formation of the
[Ti(OH)(OH2)5]3+ species, which was stable under very low pH.
Condensation of these species was not able to occur because of
the positive charge on the hydroxyl group [21]. When the activity
of the solution was high enough to promote deprotonation,
[Ti(OH)3(OH2)3]+ formation led to the condensation of both the
anatase and rutile phases, depending on the exact pH [21]. During
the deoxolation taking place at high pH, condensation can proceed
along the apical direction, causing formation of the skewed chains
of the anatase structure [22,23].
Fig. 2(b) shows the TiO2 precursor powder with residual NH4Cl
content after calcining at 473 K for 2 h. It can be seen that (200)
and (105) reflection peaks of anatase TiO2 initially appear. The
(100), (110), (200) and (211) reflection peak intensities of type
I NH4Cl rapidly decrease. However, type II NH4Cl reflection peaks
at (200) and (220) also first appear (JCPDS Card No. 89-2787). This
result indicates that the phase transition from type I NH4Cl into
type II NH4Cl occurred at 437 K.
The XRD pattern of the TiO2 precursor powder after calcining at
573 K for 2 h is shown in Fig. 2(c), revealing that the anatase TiO2
becomes the major phase and the type I NH4Cl has been fully trans-
formed to type II NH4Cl as the second phase. In addition, the weak
intensity and broad appearance of the anatase TiO2 reflection peaks
reveal its poor crystallinity and/or a composition of finer crystal-
lites in the range from the nanometer to the submicron scales
[24,25]. Fig. 2(d) shows the XRD pattern of the TiO2 precursor pow-
der after calcining at 673 K for 2 h, which reveals that the reflection
peaks of type II NH4Cl have disappeared was due to the complete
decomposition of type II NH4Cl. Moreover, the calcined powder
only contained a single phase of anatase TiO2 and the intensities
of all the reflection peaks were greater in that anatase in
Fig. 2(c). This result occurred because the crystallinity improved
and the crystallite size increased as the calcination temperature
increased.
Moreover, the DSC curve also shows that one endothermic peak
at 366 K was due to dehydration from the surface of the precursor
powder. In addition, one small endothermic peak at 459 K was
attributed to partial type I NH4Cl decomposition (according to
Fig. 2). One larger endothermic peak at 577 K was due to complete
type I NH4Cl decomposition. On the other hand, the first broad exo-
thermic peak at 697 K represents the phase transformation from
anatase to rutile TiO2.
Fig. 2(e) shows the precursor powder after calcining at 773 K for
2 h. It can be seen that the intensities of the anatase TiO2 reflection
peaks are continuously improving and sharpening as the calcina-
tion temperature increases. The (110) and (101) rutile reflections
also first appear (JCPDS Card No. 89-4920), but their crystallinity is
very weak.
3.2. The phase transition of the TiO2 precursor powder with residual
NH4Cl after calcination
The average crystallite sizes and phase content of anatase and
rutile TiO2 in the precursor powder after calcining at various tem-
peratures for 2 h can be calculated using Eqs. (1) and (2). The cal-
culated results are listed in Table 1. It can be seen that the
crystallite of anatase TiO2 was only about 2.4 nm before calcining
the precursor powder. The crystallite size of anatase TiO2 increased
slightly to 3.3 nm after calcining at 473 K for 2 h because sufficient
energy for anatase TiO2 crystallites was not available. The crystal-
lite size of anatase TiO2 increased from 6.5 to 14.3 nm when the
calcination temperature increased from 573 to 773 K. On the other
hand, the crystallite size of rutile TiO2 was 26.6 nm when the pre-
cursor powder was calcined at 773 K for 2 h. When anatase and
rutile TiO2 coexisted in the powder, the crystallite size of the rutile
phase was larger than that of anatase TiO2. This result was attrib-
uted to a critical nuclei size being achieved in the anatase TiO2
crystallite during the calcination process [26].
Fig. 2 shows the XRD patterns of the TiO2 precursor powder
with residual NH4Cl content after calcining at various tempera-
tures for 2 h. The XRD pattern of the precursor powder before
calcination is shown in Fig. 2(a). It reveals the presence of type I
NH4Cl in the precursor powder by the appearance of reflection
On the other hand, before and after calcining between 473 and
673 K for 2 h the precursor powder only contained a single phase
of anatase TiO2. The phase contents of anatase and rutile TiO2 were
80.5% and 19.5%, respectively, when the precursor powder was cal-
cined at 773 K for 2 h.
He et al. [27] reported using a hydrolytic TiCl4 solution with
urea for the synthesis of a TiO2 precursor powder prepared by a
membrane process at pH 8 and 368 K. They pointed out the TiO2
precursor powder was obtained from TiO2 hydrates without micro-
filtration fractionation, and in the precursor powder obtained after
calcining at 773 K for 2 h, the phase contents of anatase and rutile
TiO2 were 55.82% and 44.18%, respectively. In addition, when the
TiO2 precursor powder was obtained from TiO2 hydrates using per-
Fig. 2. XRD patterns of TiO2 precursor powder with residual NH4Cl content was
calcining at various temperatures for 2 h: (a) before, (b) 473 K, (c) 573 K, (d) 673 K
and (e) 773 K. (‘‘N1’’ denotes type I NH4Cl, ‘‘N2’’ denotes type II NH4Cl, ‘‘A’’ denotes
anatase TiO2 and ‘‘R’’ denotes rutile TiO2).
meated 5 lm of cellulose acetate membrane fractionation and the

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S.-W. Yeh et al. / Journal of Alloys and Compounds 613 (2014) 107–116
Table 1
calcining at 473 K. It can be seen that the increase in the volume
of adsorption at a relative pressure of P/P0, less than 0.3, can be
attributed to monolayer N2 converge of the surface. Whereas, the
increase in the relative pressure of P/P0 by 0.4–0.8 was due to N2
filling of the mesopores. The absorbed volume was little changed
when the relative pressure of P/P0 was further increased [13].
Moreover, Fig. 4(a) also reveals classical type IV isotherms with
an H2 hysteresis loop, which is related to the mesoporous structure
[29]. When the desorption branch occurs to located at a relative
pressure of P/P0 around 0.4 for materials with relatively uniform
channel-like pores, then an H2 hysteresis loop is observed [30].
On the other hand, the N2 adsorption/desorption curves show the
Phase contents and crystalline size of TiO2 precursor powder with residual NH4Cl
content was calcining at various temperatures for 2 h.
Calcination temperatures (K) Crystalline size
(nm)
Ratio of phase content
(%)
Anatase
Rutile
Anatase
Rutile
Before calcined
2.4
3.3
6.5
10.5
14.3
–
–
–
–
100
100
100
100
80.5
0
0
0
0
473
573
673
773
26.6
19.5
precursor powder was also calcined at 773 K for 2 h, then the cal-
cination product contained just a single phase of anatase TiO2.
Moreover, Liu et al. [28] also described the hydrolysis of titanium
tetraisopropoxide at 343 K in pure water with subsequent drying
at 373 K for about 100 h, from which the TiO2 precursor powder
was obtained. The phase content of this powder was a mix of ana-
tase and brookite TiO2. The difference between the previous stud-
ies [27,28] and the present work can therefore be attributed to the
various initial materials and different synthesis processes used.
The characteristics of the TiO2 precursor powder with residual
NH4Cl content after calcining at 573 K for various times are shown
in Fig. 3, revealing that the calcination products consist of anatase
TiO2 and type II NH4Cl. It can also be seen that the intensities of the
type II NH4Cl reflection peaks have not changed significantly. Fig. 3
also shows a broad and low intensity anatase (101) reflection
peak. This is because the TiO2 precursor powder calcination at
573 K could not yield complete crystallization, giving a mixture
of amorphous and nanocrystalline titania [26]. Moreover, it can
also be seen that the diffraction intensity of the anatase (101)
reflection peak increases slightly and becomes sharper with pro-
longed calcination time. This phenomenon can be attributed to
the fact that complete crystallization could not occur with calcina-
tion at 573 K, but the crystallinity continued to improve and crys-
talline growth also occurred as the duration increased.
3.3. The surface areas and pore sizes of TiO2 precursor powder with
residual NH4Cl content after calcination
The results of the N2 adsorption/desorption analysis of TiO2 pre-
cursor powder with residual NH4Cl after calcining at various tem-
peratures for 2 h are shown in Fig. 4. Fig. 4(a) shows the N2
adsorption/desorption curves of TiO2 precursor powder after
Fig. 3. XRD patterns of TiO2 precursor powder with residual NH4Cl content was
calcining at 573 K for various times: (a) 10 min, (b) 30 min, and (c) 120 min. (‘‘N2’’
denotes type II NH4Cl and ‘‘A’’ denotes anatase TiO2).
Fig. 4. N2 adsorption/desorption isotherms curves of TiO2 precursor powder with
residual NH4Cl content was calcining at various temperatures for 2 h.

S.-W. Yeh et al. / Journal of Alloys and Compounds 613 (2014) 107–116
111
Table 2
classical type IV with an H2 hysteresis loop, indicating the pres-
ence of an ink-bottle-type pore structure with a narrow entrance
and a large cavity [14,31].
BET surface area, pore volume and BJH adsorption pore sizes of TiO2 precursor powder
with residual NH4Cl content was calcining at various temperatures for 2 h.
Calcination
temperatures (K)
SBET
Pore volume
(cm3/g)
BJH adsorption pore
size (nm)
Fig. 4(b) shows the N2 adsorption/desorption curves for the TiO2
precursor powder with residual NH4Cl content after calcining at
573 K, which also reveals classical type IV isotherms with an H2
hysteresis loop with a relative pressure (P/P0) range of 0.6–0.9.
The relative pressure range of the hysteresis loop for the precursor
powder after calcining at 573 K is higher than at 473 K. Kim et al.
[14] also reported the hysteresis loop of a given material shifts to
a high relative pressure range and steeper when the calcination
temperature is increased.
(m2/g)
473
573
673
99.83
172.8
95.8
0.081
0.388
0.392
3.80
8.00
14.0
The N2 adsorption/desorption curves of TiO2 precursor powder
with residual NH4Cl content after calcining at 673 K are shown in
Fig. 4(c). It can be seen that the curves show a classical type IV iso-
therm with an H1 hysteresis loop in a relative pressure range of
0.75–0.95, which is characteristic of cylindrical pores [13].
The BET surface areas of the TiO2 precursor powder after calcin-
ing at various temperatures for 2 h are listed in Table 1. It can be
seen that the surface area was 99.83 m2/g after calcining at
473 K. This value is less than that reported by Kim et al. [14] and
´
ˇ
Yan et al. [29], but higher than the result of Grujic-Brojcin et al.
[4]. The BET surface area increased from 99.83 to 172.8 m2/g when
the calcination temperature rose from 473 to 573 K. This value is
smaller than the result of Hung et al. [13] but higher than reports
in other previous studies [6,15,27,29,32].
It is well known that the BET surface area and pore volume were
mainly affected by the growth of the TiO2 crystallites. However, the
Fig. 5. BJH adsorption pore size distributions of TiO2 precursor powder with
residual NH4Cl content was calcining at various temperatures for 2 h: (a) 473 K, (b)
573 K and (c) 673 K.
Fig. 6. The TiO2 precursor powder with residual NH4Cl content was calcining at
573 K for various times: (a) BET specific surface area and pore volumes, and (b) BJH
average pore size.

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S.-W. Yeh et al. / Journal of Alloys and Compounds 613 (2014) 107–116
Fig. 7. TEM micrographs, SAED pattern and HRTEM image of TiO2 precursor powder with residual NH4Cl content calcined at 573 K for 1 h: (a) BF image, (b) DF image of the
circle denoted by 06df in (c), (c) SAED pattern of the location area denoted by 03sadp in (a), which is indexed as corresponding to anatase TiO2, and (d) HRTEM of the location
area denoted by 10hrtem in (a), shows the d-spacings of anatase TiO2 (101), (004) and (112) reflections are 3.419, 2.358 and 2.288 Å, respectively, and indicates the
mesoporous structure by arrows, and (e) an enlarged view of location area denoted by em1 in (d), which shows that the d-spacing of anatase TiO2 (112) is 2.288 Å and
indicates the mesoporous structure by arrows.
BET surface area underwent a large increase as the calcination tem-
perature rose from 473 to 573 K. This happened because although
the anatase TiO2 crystallite size only increased from 3.3 to 6.5 nm
and accompanied the small amount of type II NH4Cl that appeared.
However, the type I NH4Cl completely decomposed and created the
surface area increased becomes so accelerated.
for 2 h. Therefore, the BET surface area decreased due to the crys-
tallization of anatase TiO2, subsequent crystallite growth, and the
collapse of the mesoporous structure [33,34].
The pore volumes of mesoporous anatase TiO2 powers obtained
by the BET method are also listed in Table 2. It can be seen that the
values for pore volume increased with increasing calcination tem-
perature. The pore volume in the mesoporous TiO2 powder was
mainly affected by TiO2 crystallite growth [29], i.e. influenced by
calcination temperature. For the TiO2 precursor powder with resid-
ual NH4Cl content after calcining at 473 K for 2 h, the anchoring of
The BET surface area decreased rapidly from 172.8 to 95.8 m2/g
when the calcination temperature rose from 573 to 673 K. The
crystallite size of anatase TiO2 was 10.5 nm and the crystallinity
also increased after the precursor powder was calcined at 673 K

S.-W. Yeh et al. / Journal of Alloys and Compounds 613 (2014) 107–116
113
Fig. 8. TEM micrographs, SAED pattern and HRTEM image of TiO2 precursor powder with residual NH4Cl content calcined at 673 K for 2 h: (a) BF image, (b) DF image of the
circle denoted by 08df in (c), (c) SAED pattern of the location area denoted by 06sadp in (a), which is indexed as corresponding to anatase TiO2, and (d) HRTEM of the location
area denoted by 12hrtem in (a), shows the d-spacings of the anatase TiO2 (101) and (004) reflections are 3.472 and 2.395 Å, respectively, and indicates the mesoporous
structure by arrows.
type I NH4Cl in the framework of the calcination product efficiently
restrained the growth of anatase TiO2 crystallites. Therefore, the
calcination product only had a very smaller pore volume of
0.081 cm3/g. Nevertheless, when the calcination temperature rose
from 473 to 573 K, the decomposition of type I NH4Cl was com-
pleted and accompanied by a minor phase of type II NH4Cl forma-
tion, creating the accelerated increase in pore volume. When the
TiO2 precursor powder was calcined at 673 K for 2 h, the pore vol-
ume was only slightly increased. This result is attributed to the
anatase crystallite growth from 6.5 to 10.5 nm and only a minor
amount of decomposition of type II NH4Cl.
pores appeared when calcined at 673 K [9]. A bimodal pore size
distribution was observed for calcination at 473 K. This result
was due to uncontrollable pore collapse during the calcination
stage [6].
The average pore size increases with increasing anatase TiO2
crystallite size, as confirmed by the XRD analysis (see Table 1),
and the pore size distribution also becomes broader with rising cal-
cination temperature, revealing that the pores from the decompo-
sition of type I and type II NH4Cl have many pores. This result is not
in agreement with the pores formed from the assembly of TiO2
nanoparticles [36].
However, Kim et al. [34] synthesized monodisperse spherical
mesoporous TiO2 particles with controlled size by a simple sol–
gel method and pointed out that the pore volume decreased with
increasing calcination temperature. This result is not in agreement
with the present study.
The average pore size of TiO2 precursor powder with residual
NH4Cl after calcining at various temperatures for 2 h was deter-
mined by the BJH method [35] and the results are shown in
Fig. 5. The BJH adsorption pore sizes obtained are also listed
in Table 2. In fact, Fig. 5(a) shows that TiO2 precursor powder
has a narrower pore size distribution after calcining at 473 K
compared to calcining at other temperatures with an evident
peak at 2.57 nm. In addition, the pore size distribution also
revealed that TiO2 precursor powder calcination at 573 K yielded
a smaller pore size (8.0 nm) than at 673 K (14.0 nm). According
to the results in Fig. 4, the observed hysteresis loops shifts to a
higher relative pressure of P/P0 = 1 can be suggesting the large
The dependence of BET specific surface area and pore volume of
TiO2 precursor powder after calcining at 573 K for various times
are shown in Fig. 6(a). It can be seen that the specific surface area
and pore volume increase from 118.2 to 172.8 m2/g and 0.217 to
0.388 cm3/g, respectively, as the calcination time increased from
10 to 120 min. These results were attributed to a slight increase
in the decomposition of type II NH4Cl with increasing calcination
time. In addition, the crystallite size of anatase TiO2 gradually
grew, also influencing the specific surface area and pore volume
and leading to their increases.
Fig. 6(b) shows the BJH adsorption pore size distribution of TiO2
precursor powder after calcining at 573 K for several different peri-
ods of time. It reveals that the average pore size of mesoporous
anatase TiO2 powder increased from 6.4 to 8.0 nm with increasing
the duration at 573 K. In fact, the factors influencing the average
pore size were similar to those affecting the specific surface area
and pore volume of anatase TiO2 mesoporous powders.

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S.-W. Yeh et al. / Journal of Alloys and Compounds 613 (2014) 107–116
Fig. 9. TEM micrographs, SAED pattern and HRTEM image of TiO2 precursor powder with residual NH4Cl content calcined at 773 K for 2 h: (a) BF image, (b) and (c) are DF
images using the circle denoted by 14df and 15df in (d), respectively, (d) SAED pattern of the location area denoted by 09sadp in (a), which is indexed as corresponding to
coexisting phases of anatase and rutile TiO2, (e) and (f) are HRTEM images of the location area denoted by 24hrtem and 27hrtem in (a), respectively. (e) Shows the d-spacing
of the anatase TiO2 (100) reflection is 3.670 Å, and those of the rutile TiO2 (110) and (111) reflections are 3.236 and 2.195 Å, respectively. (f) Shows the d-spacings of anatase
TiO2 (101), (004) and (200) reflections are 3.488, 2.353 and 1.871 Å, respectively, and that of the rutile TiO2 (110) reflection is 3.227 Å.
As mentioned above, using residual NH4Cl in the TiO2 precursor
powder and calcining at 673 K for 2 h, a single phase of anatase
TiO2 mesoporous powder with suitable pore size was obtained
and the residual NH4Cl was fully removed. Therefore, a single
phase of anatase TiO2 mesoporous powder was successfully
obtained by an in situ synthesis using a coprecipitation process.
Fig. 7 shows TEM bright field (BF), dark field (DF) micrographs,
SAED pattern and high resolution TEM (HRTEM) images of TiO2
precursor powder with residual NH4Cl content calcined at 573 K
for 1 h. Fig. 7(a) illustrates the BF micrograph, which indicates that
many particles had been incorporated and were difficult to resolve
given their small size. This result was due to the TiO2 precursor
powder being synthesized using the coprecipitation method, dur-
ing which, after drying and/or subsequent processing, agglomera-
tion can occur [37]. A DF micrograph of the calcined powder
using the circle denoted by 06df in Fig. 7(c) is shown in Fig. 7(b).
It is seen that the nanopowder was crystallized and the crystallite
size was about 6.0 1.7 nm. Fig. 7(c) shows the SAED pattern of the
location area denoted by 03sadp in Fig. 7(a), which shows the
(101), (103), (200), (105), (213), (116) and (107) reflections of

S.-W. Yeh et al. / Journal of Alloys and Compounds 613 (2014) 107–116
115
anatase TiO2. The Debye rings reveal that the calcined powder was
polycrystalline. The diffraction rings in the SAED pattern corre-
spond well to the XRD reflection peaks. Fig. 7(d) shows the HRTEM
image of the location area denoted by 10hrtem in Fig. 7(a). It can be
seen that the mesopores in Fig. 7(d) denoted by circle. In addition,
the HRTEM image also shows d-spacings of 3.419, 2.288 and
2.358 Å for the anatase TiO2 (101), (112) and (104) reflection
planes, respectively. Fig. 7(e) shows an enlarged view of location
area denoted by em1 in Fig. 7(d). It reveals that the d-spacing of
anatase TiO2 (112) is 2.288 Å. On the other hand, Fig. 7(e) also
indicates that the mesoporous size is about 1.0–2.0 nm.
TEM BF and DF micrographs, SAED pattern and HRTEM images of
the TiO2 precursor powder with residual NH4Cl content calcined at
673 K for 2 h are shown in Fig. 8. Fig. 8(a) shows a BF micrograph
of the calcined powder, which reveals that the crystallites were still
agglomerated so it is too difficult to discriminate individual crystal-
lites. However, the crystallinity of the crystallites continuously
improved as the calcination temperature increased. Fig. 8(b) shows
a DF image of the calcined product in the circle denoted by 08df in
Fig. 8(c), which reveals the crystalline size to be about
12.5 2.5 nm. The SAED pattern of the location area denoted by
03sadp in Fig. 8(a) is shown in Fig. 8(c), which is indexed as corre-
sponding to anatase TiO2. The SAED pattern also provides that after
the precursor powder was calcined at 673 K for 2 h the anatase TiO2
was still maintained. Fig. 8(d) shows a HRTEM image of the location
area denoted by 12hrtem in Fig. 8(a). It shows the mesoporous
structure in the anatase TiO2 crystallites as denoted by circles and
indicated by arrows. In addition, the HRTEM image also reveals that
the calcined product exhibited clear lattice fringes. The HRTEM
image also shows that the d-spacings of anatase TiO2 (101) and
(004) are 3.472 and 2.395 Å, respectively.
Fig. 9 shows the TEM BF and DF micrographs, an SAED pattern
and HRTEM image of the TiO2 precursor powder with residual NH4Cl
content calcined at 773 K for 2 h. Fig. 9(a) shows the BF image, which
reveals that fine crystallites were gradually incorporated into larger
particles with rising calcination temperature. Fig. 9(b) and (c) shows
DF micrographs of the calcined product using the circle denoted by
14df and 15df in Fig. 9(d). Fig. 9(d) shows the SAED pattern of loca-
tion area denoted by 09sadp in Fig. 9(a), revealing the (101), (103),
(200) and (215) reflections from anatase TiO2. Moreover, Fig. 9(d)
also shows the (111), (210), (211), (220), (221) and (301) reflec-
tions from the rutile TiO2. Fig. 9(d) also provides an evidence for a
phase transition from anatase to rutile TiO2 occurred when the
TiO2 precursor powder was calcined at 773 K for 2 h. From the
indexed result in Fig. 9(d), it is also apparent that Fig. 9(b) and (c)
show crystallites of anatase and rutile TiO2, respectively. Fig. 9(e)
and (f) shows HRTEM images of location area denoted by 27hrtem
and 24hrtem, respectively, in Fig. 9(a). They reveal that the calcined
product exhibited good crystallinity, clear lattice fringes and was
mesoporous. Fig. 9(e) shows that the d-spacing of the anatase TiO2
(100) is 3.670 Å, and rutile TiO2 (110) and (111) are 3.236 and
2.195 Å, respectively. On the other hand, Fig. 9(f) also shows that
the d-spacings of the anatase TiO2 (101), (004) and (200) reflec-
tions are 3.488, 2.353 and 1.871 Å, respectively, and the rutile TiO2
(110) reflection is 3.227 Å.
(2) Before calcining the precursor powder contained type I
NH4Cl and anatase TiO2 as the primary and secondary phases,
respectively. When calcined at 573 K for 2 h, phases of ana-
tase TiO2 and type II NH4Cl coexisted in the calcined product.
However, the calcined product only contained a single phase
of anatase TiO2 when the precursor powder was calcined at
673 K for 2 h. Moreover, anatase and rutile coexist as the pri-
mary and secondary phases in the calcined products.
(3) The crystallite size of anatase increased from 3.3 to 14.3 nm
when the precursor powder calcination temperature was
increased from 473 to 773 K. In addition, the crystallite size
of rutile TiO2 was 26.6 nm when the precursor powder was
calcined at 773 K for 2 h.
(4) When the TiO2 precursor powder with residual NH4Cl con-
tent was calcined at 673 K for 2 h, the BET specific surface
area was affected. The maximum value of the BET specific
surface area and pore volume were 172.8 m2/g and
0.392 cm3/g, respectively. When the powder was at various
temperatures for 2 h, the average pore size increased from
3.80 to 14.0 nm as the calcination temperature rose from
473 to 773 K.
(5) Mesoporous anatase TiO2 nanopowders were successfully
synthesized when the TiO2 precursor powder with residual
NH4Cl content and without an added surfactant agent was
calcined at 673 K for 2 h.
Acknowledgements
The authors sincerely thank the National Science Council of
Taiwan for its financial support under NSC 100-2221-E-037-001.
The authors also gratefully acknowledge the help of Mr. S.Y. Yao
for TEM.
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