Characterization of Zr-doped TiO2 nanocrystals prepared by a nonhydrolytic sol-gel method at high temperatures

Article abstract of DOI:10.1021/jp0626566

Highly crystalline and surface-modified Zr-doped TiO₂ nanorods were successfully prepared using a nonhydrolytic sol-gel method that involves the condensation of metal halides with alkoxides in anhydrous trioctylphosphine oxide (TOPO) at either 320 or 400°C. In addition, the interaction of the cross-condensation between the Ti and Zr species was studied by characterizing the morphologies, crystalline structures, chemical compositions, surface properties, and band gaps of the nanocrystals obtained at different reaction temperatures and Zr-to-Ti stoichiometric ratios. Increases in the concentration of Zr⁴⁺ and in the reaction temperature led to large nanorods and regular shapes, respectively. In addition, only the anatase form was observed in the Zr-doped TiO₂ nanorods. The Zr-to-Ti ratios obtained ranged from 0.01 to 2.05, all of which were far below the stoichiometric ratios used during the preparation of the samples (0.25-4). Moreover, the Zr⁴⁺ units accumulated mainly at the surface of the TiO₂ nanocrystals. The band gaps of the Zr-doped TiO₂ nanorods ranged from 2.8 to 3.8 eV, which are smaller than those of pure TiO₂ (3.7 eV) or ZrO₂ (5.2 eV). The Zr-doped anatase TiO₂ nanorods prepared at 400°C at an initial stoichiometric Zr-to-Ti ratio of 2:3 exhibited the highest photoactivities for the decomposition of rhodamine B because of the presence of trace amounts of Zr⁴⁺ (Zr/Ti = 0.03) in the TiO₂ and the regular shapes of these particles. DSC analysis indicated that the temperatures for forming nanocrystalline TiO₂ and ZrO₂ were 207 and 340°C, respectively. Moreover, the reactivities of condensation between the Ti species were reduced when Zr species were involved in the NHSG reactions. The results obtained in this study clearly demonstrate that the faster kinetics for the generation of TiO₂ controls the material properties as well as the photoactivities of the nonhydrolytic sol-gel-derived nanocrystals.

Full text of DOI:10.1021/jp0626566

20808
J. Phys. Chem. B 2006, 110, 20808-20814
Characterization of Zr-Doped TiO2 Nanocrystals Prepared by a Nonhydrolytic
Sol-Gel Method at High Temperatures
,†
‡
Sue-min Chang* and Ruey-an Doong
Institute of EnVironmental Engineering, National Chiao Tung UniVersity, 75, Po Ai Street,
Hsinchu, 30068, Taiwan, and Department of Biomedical Engineering and EnVironmental Sciences,
National Tsing Hua UniVersity, 101, Sec 2, Kuang Fu Road, Hsinchu, 30013, Taiwan
ReceiVed: May 1, 2006; In Final Form: August 11, 2006
2
Highly crystalline and surface-modified Zr-doped TiO nanorods were successfully prepared using a
nonhydrolytic sol-gel method that involves the condensation of metal halides with alkoxides in anhydrous
trioctylphosphine oxide (TOPO) at either 320 or 400 °C. In addition, the interaction of the cross-condensation
between the Ti and Zr species was studied by characterizing the morphologies, crystalline structures, chemical
compositions, surface properties, and band gaps of the nanocrystals obtained at different reaction temperatures
4
+
and Zr-to-Ti stoichiometric ratios. Increases in the concentration of Zr and in the reaction temperature led
to large nanorods and regular shapes, respectively. In addition, only the anatase form was observed in the
Zr-doped TiO
the stoichiometric ratios used during the preparation of the samples (0.25-4). Moreover, the Zr units
accumulated mainly at the surface of the TiO nanocrystals. The band gaps of the Zr-doped TiO nanorods
ranged from 2.8 to 3.8 eV, which are smaller than those of pure TiO (3.7 eV) or ZrO (5.2 eV). The Zr-
doped anatase TiO nanorods prepared at 400 °C at an initial stoichiometric Zr-to-Ti ratio of 2:3 exhibited
the highest photoactivities for the decomposition of rhodamine B because of the presence of trace amounts
2
nanorods. The Zr-to-Ti ratios obtained ranged from 0.01 to 2.05, all of which were far below
4
+
2
2
2
2
2
of Zr4 (Zr/Ti ) 0.03) in the TiO
+
and the regular shapes of these particles. DSC analysis indicated that the
and ZrO were 207 and 340 °C, respectively. Moreover, the
2
temperatures for forming nanocrystalline TiO
2
2
reactivities of condensation between the Ti species were reduced when Zr species were involved in the NHSG
reactions. The results obtained in this study clearly demonstrate that the faster kinetics for the generation of
TiO
nanocrystals.
2
controls the material properties as well as the photoactivities of the nonhydrolytic sol-gel-derived
Introduction
In addition, anatase TiO2 and tetragonal ZrO2 contain oxygen
vacancies and exhibit higher catalytic performances than their
Titanium dioxide (TiO2) is one of the most widely used
photocatalysts because of its suitable band gap (ca. 3.2 eV),
chemical stability, and nontoxicity. In addition, zirconium
dioxide (ZrO2), which has a wide band gap (ca. 5.0 eV) and
more-negative (-1.0 V vs NHE) and more-positive (4.0 V vs
NHE) reducing potentials in its conduction and valence bands,
respectively, relative to those of TiO2, is considered as a
promising alternative photocatalyst for the degradation of a
7
,10-12
other crystalline forms.
Therefore, the preparation and
characterization of nanosize, highly crystalline anatase and
tetragonal ZrxTi1-xO2 photocatalysts continues to attract a high
degree of attention.
1
The sol-gel method is a simple and feasible process for the
fabrication of nanocomposite oxides with tailored sizes. Con-
ventionally, the sol-gel method uses a hydrolytic route that
involves initial hydrolysis of the precursors and subsequent
continuous condensation between the hydrolyzed precursors to
form oxide gels. The hydrolytic sol-gel (HSG) route to metal
oxides proceeds very efficiently at room temperature. In addi-
tion, nanoparticulates can be prepared through a combination
of water-in-oil emulsion and sol-gel processing. Because the
as-prepared oxides are usually amorphous, they must be calcined
at high temperature for crystallization. Unfortunately, calcination
results in the coalescence of particles, which increases the size
and decreases the surface area of the photocatalysts.13
2-5
greater variety of pollutants.
It has been demonstrated that
incorporating Zr4 into TiO2 to form ZrxTi1-xO2 can introduce
+
lattice defects and lead to higher photoactivities than those of
the pure oxides. The introduced defects not only reduce the band
gaps, i.e., increase the wavelength of the activation light, but
also play a role as charge-trapping centers in inhibiting charge
recombination.6 Although the presence of a few defects
improves the photoactivities of ZrxTi1-xO2 systems, an excess
of defects promotes charge recombination and decreases cata-
lytic efficiency. The number of defects is dependent on the size,
7
-9
crystalline structure, and Zr-to-Ti ratio of the photocatalyst.
Nanosize photocatalysts have large surface areas and sufficient
numbers of surface defects to exhibit high catalytic efficiencies.
The nonhydrolytic sol-gel (NHSG) route is an alternative
pathway for the preparation of metal oxides. In contrast to the
HSG route, the NHSG route involves the reaction of metal
halides with oxygen donors, such as metal alkoxides, alcohols,
*
Corresponding author. E-mail: chang@mail.nctu.edu.tw. Tel.: +886-
14
or ethers, under nonaqueous conditions. Several studies have
3
-5712121 ext. 55506.
†
‡
investigated the nonhydrolytic sol-gel route to pure oxides and
National Chiao Tung University.
National Tsing Hua University.
15-17
binary oxides including TiO2, ZrTiO4, SiTiO4, and SiZrO4.
1
0.1021/jp0626566 CCC: $33.50 © 2006 American Chemical Society
Published on Web 09/07/2006

Characterization of Zr-Doped TiO2 Nanocrystals
J. Phys. Chem. B, Vol. 110, No. 42, 2006 20809
Because the NHSG process is relatively slower than the HSG
approach, it usually requires temperatures exceeding 80 °C.
Moreover, the lower reactivity makes the morphologies and
TABLE 1: Metal Halides, Oxygen Donors, and Reaction
Temperatures Used for the Synthesis of Zr
x 2
Ti1-xO
Nanoparticles
microstructures of NHSG-derived metal oxides more control-
metal halide
TOPO temp
lable. Trentler et al.18 prepared anatase TiO2 nanocrystals
sample
-320
(2.5 mmol)
oxygen donor(s)
(mmol) (°C)
through the injection of titanium alkoxides into titanium iso-
propoxide/heptadecane solutions in the presence of trioctylphos-
phine oxide (TOPO) at 300 °C. These TiO2 crystals had average
sizes of 3.8-9.2 nm, depending on the type of the metal halide
and the concentration of TOPO. Moreover, these TiO2 nano-
particles exhibited no surface hydroxyl groups because of the
TiO
2
TiCl
TiCl
4
4
Ti(OC
Ti(OC
3
3
H
H
7
)
)
4
(2.0 mmol) 13.5 320
(1.0 mmol), 13.5 320
2
Zr0.2Ti0.8O -320
7
4
Zr(OC
3
H
7
)
4
‚
3 2
(CH ) CHOH
(
1.0 mmol)
2
Zr0.4Ti0.6O -320
TiCl
4
Zr(OC
CH
(2.0 mmol)
3
7
H )
4
‚
13.5 320
(
3 2
) CHOH
coordination of the surface titanium atom to TOPO. Subse-
quently, Joo et al.19 reported that condensation between zir-
Zr0.2Ti0.8O -400
TiCl4
Ti(OC H ) (1.0 mmol), 13.5 400
2
3
7
4
Zr(OC
3
H
7 4
) ‚
conium halides and zirconium isopropoxide at 340 °C in pure
TOPO results in monodisperse, uniformly sized tetragonal ZrO2
nanoparticles (ca. 4 nm). In addition to pure metal oxides, binary
metal oxides can also be prepared through cross-condensation
3 2
(CH ) CHOH
(
1.0 mmol)
2
Zr0.4Ti0.6O -400
TiCl
4
Zr(OC
CH
(2.0 mmol)
3
7
H )
4
‚
13.5 400
(
3 2
) CHOH
2
0
between different metal halide and alkoxides. Tang et al.
Zr0.6Ti0.4
Zr0.8Ti0.2
O
O
2
-400
-400
ZrCl
ZrCl
4
4
Ti(OC
Ti(OC
3
H
H
7
)
)
4
(2.0 mmol) 21
(1.0 mmol), 21
400
400
adapted a process similar to that of Joo et al. to prepare
monodisperse HfxZr1-xO2 nanocrystals. Solid solutions with
various concentrations were obtained after nonhydrolytic cross-
condensations using different stoichiometric amounts of hafnium
halides and zirconium alkoxides had been performed. In
addition, the morphologies and crystalline phases within these
solid solutions were controllable by modifying the reaction
temperature and the combination of metal halide and alkoxide.
Unfortunately, ZrTiO4 solid solutions could not be formed using
this method. This phenomenon is different from that of ZrTiO4,
for which the desired Zr-to-Ti ratios can be controlled by the
initial metallic ratios in the solutions when the similar elimina-
2
3
7
4
Zr(OC
3
H
7
)
4
‚
3 2
(CH ) CHOH
(
1.0 mmol)
2
ZrO -400
ZrCl
4
Zr(OC
(CH
(2.0 mmol)
3
7
H )
4
‚
21
400
3 2
) CHOH
mixed solution was then heated to 320 °C and underwent the
condensation reaction at this temperature for 3 h with vigorous
stirring at 500 rpm. Afterward, the solution was cooled to 60
°C, and acetone was added to precipitate TiO2 nanocrystals.
The precipitate was harvested by centrifugation at 11000 rpm
and washed with acetone several times to remove excess TOPO.
For ZrO2 preparation, 0.6 g of ZrCl4 (2.5 mmol) and 0.8 g of
Zr(OC3H7)4-(CH3)2CHOH (2.0 mmol) were dissolved in 8.0 g
of TOPO (21 mmol). The ratio of metal chloride to alkoxide is
17
tion of alkyl halides is conducted at low temperatures (110 °C).
In any event, the interaction between the Zr/Ti halides and
alkoxides in the presence of TOPO at high temperatures has
yet to be addressed adequately. In addition, the detailed quanti-
fication of the Zr-to-Ti ratios, microstructures, electronic struc-
tures, surface properties, and photoactivities of the ZrxTi1-xO2
nanocrystals resulting from various combinations of Zr and Ti
species remain unclear.
1
9
similar to that used by Joo et al., which means that the metal
chloride was in excess. In contrast to TiO2, ZrO2 particles were
produced only when the temperature rose to 400 °C. For the
synthesis of Zr-doped TiO2 nanocrystals, various molar ratios
In this study, we synthesized and characterized Zr-doped TiO2
nanocrystals using an NHSG method conducted at 320 or 400
i
of M(O Pr)4 and M′Cl4 (M and M′ represent Ti and/or Zr) were
used. The samples are denoted as ZrxTi1-xO2-t, where x
represents the reactants in the ratio of Zr/(Zr + Ti) and t denotes
the reaction temperature. The nonhydrolytic reaction was carried
out at 320 or 400 °C when x was smaller than 0.5 and 400 °C
for x larger than 0.5. All recipes and reaction conditions are
summarized in Table 1. Unless otherwise mentioned, all of the
NHSG reactions were conducted in hot trioctylphosphine oxide
°
C in liquid TOPO. Titanium chloride (TiCl4) and zirconium
chloride (ZrCl4) were used as the precursors for TiO2 and ZrO2,
respectively, and titanium isopropoxide and zirconium iso-
propoxide were used as the oxygen donors. To understand
explicitly the chemistry of the cross-condensation leading to
the Zr-doped TiO2 nanocrystals, we examined the material
propertiessincluding particle sizes, shapes, crystalline phases,
surface compositions, surface properties, and optical propertiess
of the products prepared at various stoichiometric Zr-to-Ti ratios
and reaction temperatures. Moreover, we investigated the
photocatalytic activities of the Zr-doped TiO2 samples toward
the degradation of rhodamine B (RhB) and elucidated how they
relate to the material properties.
(
TOPO, Strem Chemicals, 99%) liquid and under N2 atmo-
sphere.
Characterization. The thermal behaviors of the NHSG reac-
tions were investigated by differential scanning calorimetry
DSC, Setaram Labsys DSC 131) under a N2 flow of 20 mL/
(
min and at a heating rate of 10 °C/min from 25 to 550 °C. The
samples for DSC were prepared by dissolving the reactants in
TOPO at 150 °C and then cooling the mixture to room tem-
Experimental Section
i
perature under N2 atmosphere. The TiCl4/Ti(O Pr)4/TOPO mix-
i
NHSG-Derived Nanocrystals. Titanium dioxide (TiO2)
nanocrystals were prepared by condensation between titanium
chloride (TiCl4, Fluka, 99%) and Ti(OC3H7)4 (Aldrich, 99.9%),
whereas zirconium dioxide (ZrO2) nanocrystals were fabricated
by zirconium chloride (ZrCl4, Strem Chemicals, 99.5%) and
zirconium isopropoxide propanol complex [Zr(OC3H7)4-(CH3)2-
CHOH, Aldrich, 99.9%]. Titanium chloride (0.5 g, 2.5 mmol)
and Ti(OC3H7)4 (0.6 g, 2.0 mmol) were first dissolved in TOPO
ture was a viscous yellow liquid, and the ZrCl4/Zr(O Pr)4/TOPO
mixture was an opaque white solid. The particle sizes and shapes
of the NHSG-derived nanocrystals were characterized by high-
resolution transmission electron microscopy (HRTEM, JEOL
JEM-4000EX) at an accelerating voltage of 400 kV. The crys-
talline properties of the nanocrystals were identified by X-ray
diffractometry (XRD, Rigaku) using Cu KR radiation (λ )
1.5405 Å) and operating at an accelerating voltage of 30 kV
and an emission current of 20 mA. The X-ray diffraction patterns
(5.2 g, 13.5 mmol) at 150 °C under N2 atmosphere. This well-

20810 J. Phys. Chem. B, Vol. 110, No. 42, 2006
Chang and Doong
were acquired over the 2θ range from 20° to 90° at a sampling
width of 0.02° and a scanning speed of 4°/min. The elemental
compositions (Zr/Ti) of the binary metal oxides were analyzed
by inductively coupled plasma-mass spectrometry (ICP-MS,
Perkin-Elmer, SCIEX ELAN 5000). The surface chemical
compositions of the nanocrystals were examined by X-ray
photoelectron spectroscopy (XPS, Physical Electronics, ESCA
PHI 1600) using an Al KR X-ray source (1486.6 eV). The
photoelectrons were collected into the analyzer with a 23.5 eV
passing energy. The collection step was 1.0 eV for the wide-
range scan and 0.1 eV for high-resolution analysis in selected
energy intervals. All analytical processes were carried out under
ultrahigh vacuum conditions with the pressure maintained below
-
9
1
.4 × 10 Torr. The shifts of the photoelectron peaks in the
XPS spectra resulting from charging effects were referenced to
the O(1s) line taken as 530.2 eV. The surface elemental ratios
were estimated from the integrated peak areas of the elements
and normalized to their sensitivity factors. The functional groups
of the nanoparticles were identified by Fourier transform infrared
Figure 1. DSC curves of the NHSG reactions leading to TiO
line), ZrO (dashed line), and Zr0.25Ti0.75 (dashed-dotted line) in the
presence of TOPO.
2
(solid
2
O
2
TABLE 2: Zr-to-Ti Molar Ratios and Actual Compositions
of the Binary Oxide Nanoparticles
-
1
spectrometry (FTIR, Horiba) scanning from 400 to 4000 cm
with a resolution of 4 cm-1 for 100 scans. Samples for FTIR
measurements were well-mixed with KBr and then pressed as
pellets. The optical properties and band gaps of the nanoparticles
were determined by UV-vis spectrophotometry (Hitachi 3010)
scanning from 800 to 190 nm using the absorption of hexane
as the reference. Samples for the UV-vis measurements were
well-dispersed in hexane in a quartz cuvette with an optical
length of 10 mm.
Zr-to-Ti molar ratioa
Zr
x
Ti1-xO
2
nanocrystals
ICP-MS
XPS
actual composition
Zr0.2Ti0.8
Zr0.4Ti0.6
Zr0.2Ti0.8
Zr0.4Ti0.6
Zr0.6Ti0.4
Zr0.8Ti0.2
2
O -320
2
O -320
2
O -400
2
O -400
2
O -400
2
O -400
0.03
0.05
0.01
0.03
0.34
2.05
b
0.02
b
0.05
0.33
2.29
Zr0.03Ti0.97
Zr0.05Ti0.95
Zr0.01Ti0.99
Zr0.03Ti0.97
Zr0.25Ti0.75
Zr0.67Ti0.33
O
2
O
2
O
2
O
2
O
2
O
2
-320
-320
-400
-400
-400
-400
Photocatalytic Activities. Rhodamine B (RhB) was selected
as the target compound to examine the photoactivity of the
NHSG-derived ZrxTi1-xO2 materials. The nanocrystalline cata-
lysts were suspended in RhB solutions (0.05 mM) to give a
final concentration of 1 g/L in the dark. Prior to illumination,
the solutions were purged with O2 in the dark for 30 min to
ensure equilibrium among the catalysts, RhB, and oxygen.
Thereafter, the solutions were kept under constant O2 equlibrium
condition during photocatalysis. Photocatalysis was carried out
in a fused-silica tube irradiated by UV light at a wavelength of
a
Molar ratios were determined by ICP-MS and XPS. b Detection
of Zr was not possible.
the surface layer of the nanocrystals. When the value of x was
greater than 0.5, the actual Zr-to-Ti ratios in the nanocrystals
increased with increasing content of Zr in the reaction mixture.
No such trend occurred, however, when the value of x was
below 0.5. The similar Zr-to-Ti ratios (0.01-0.05) in the Zr0.2-
Ti0.8O2 and Zr0.4Ti0.6O2 samples prepared at 320 and 400 °C
indicate that increasing temperature had little effect on the
degree of incorporation of Zr into the TiO2 lattice.
3
05 nm. The decolored solutions were analyzed by UV-vis
spectrophotometer scanning from 700 to 400 nm.
Regardless of the initial Zr-to-Ti ratio used for the synthesis
of the binary oxides, the final Zr-to-Ti ratios in the products
were always much smaller than the theoretical stoichiometry.
This phenomenon is attributed to the different kinetics of
condensation between Ti and Zr species. Actually, the formation
of TiO2 through the condensation of TiCl4 with Ti(OC3H7)4
occurs at 320 °C, but a higher temperature (400 °C) is required
for the preparation of ZrO2. Because the actual compositions
of ZrxTi1-xO2 are different from the stoichiometric values, we
use the actual ratios based on ICP-MS results for further
discussion.
Heat Flows during NHSG Reactions. To understand the
different dynamic properties of the NHSG reactions that form
TiO2, ZrO2, and ZrxTi1-xO2, we used DSC to examine the heat
flows of these reactions. Figure 1 displays the heat flows of the
NHSG reactions forming TiO2, ZrO2, and Zr0.25Ti0.75O2-400 in
the presence of TOPO. The NHSG reaction leading to TiO2
exhibited two endothermic peaks in the temperature range from
180 to 270 °C. The endothermic peak centered at 207 °C is
ascribed to the condensation between tTisCl and tTis
(OC3H7), and the peak ranging between 218 and 270 °C is
attributed to the halide/titanyl exchange. This exchange results
in the breaking and reforming of TisO bonds, which is consid-
ered to be necessary for the crystallization of TiO2.18 The DSC
trace for the NHSG reaction leading to ZrO2 exhibited two
Results and Discussion
Chemical Compositions. To examine the chemical states of
the Ti and Zr species after the nonhydrolytic sol-gel reaction,
the NHSG-derived samples were analyzed using XPS (see
Supporting Information, Figure S1). The binding energy of
Zr(3d5/2) centered at 182.3 eV in the ZrO2 spectrum, and that
of Ti(2p3/2) was at 459.0 eV in the TiO2 spectrum. Similarly to
pure TiO2 and ZrO2, Zr(3d5/2) and Ti(2p3/2) in ZrxTi1-xO2
nanocrystals exhibited binding energies in the ranges 182.3-
1
82.5 and 458.7-459.0 eV, respectively. These values are in
good agreement with the reported values for TiO2, ZrO2, and
2
1
ZrTiO4, which indicates the formation of TisO and ZrsO
bonds in the samples via the nonhydrolytic reactions. Table 2
lists the actual Zr-to-Ti ratios of the ZrxTi1-xO2 nanocrystals as
analyzed using both ICP-MS and XPS. The Zr-to-Ti ratios of
the binary oxides determined by ICP-MS ranged from 0.01 to
2
.05, whereas a range of 0.02-2.29 was obtained through XPS
analysis. It is noted that ICP-MS is an instrumental method that
can determine the total concentrations of elements in ZrxTi1-xO2
nanocrystals suspended in solution, whereas XPS is usually
employed for the analysis of chemical species on the surfaces
of nanocrystals. The similar Zr-to-Ti ratios obtained in these
two methods reveal that most of the Zr4+ ions accumulated in

Characterization of Zr-Doped TiO2 Nanocrystals
J. Phys. Chem. B, Vol. 110, No. 42, 2006 20811
significant endothermic peaks centered at 51 and 340 °C, which
i
are attributed to the liquefaction of the ZrCl4/Zr(O Pr)4/TOPO
complex and the condensation of tZrsCl with tZrs(OC3H7),
respectively. The DSC results clearly indicate that the temper-
atures for the formation of TiO2 nanocrystals (207 °C) are lower
than those required for the generation of NHSG-derived ZrO2
(340 °C), which means the the condensation reactivity of Ti
species is higher than that of Zr species.
To understand the interaction between the Ti and Zr species,
the DSC curve of Zr0.25Ti0.75O2 was further examined. Similarly
to ZrO2, Zr0.25Ti0.75O2 exhibited endothermic melting of the
TOPO complex at 65 °C. However, the endothermic melting
process was not obvious in the DSC curve of TiO2. This is
i
mainly attributed to the fact that TiCl4/Ti(O Pr)4/TOPO mixture
i
is a viscous liquid, whereas the ZrCl4/Zr(O Pr)4/TOPO and
i
ZrCl4/Ti(O Pr)4/TOPO mixtures are solids at room temperature.
The Zr0.25Ti0.75O2 sample exhibited an endothermic peak of
condensation centered at 300 °C. Because the elemental analysis
showed that TiO2 is the dominant component in Zr0.25Ti0.75O2,
this condensation mainly occurs between tTisCl and tTis
i
i
(
O Pr), which result from ligand exchange between tTis(O -
Pr) and tZrsCl at low temperatures. Ligand exchange between
metal chlorides and metal alkoxides has been demonstrated to
produce metal chloroalkoxides, and thus homogeneous as well
17
as heterogeneous condensations can occur. It is noted that the
temperature for condensation between the Ti species for Zr0.25-
Ti0.75O2 (300 °C) was higher than that for TiO2 (207 °C), which
suggests that the Ti species in the Zr0.25Ti0.75O2 precursor
solution is different from that in the TiO2 precursor solution.
Arnal et al. reported that metal chloroalkoxides are present in
oligomeric structures because of the formation of alkoxo bridges.
In this study, titanium zirconium chloroalkoxide oligomers could
be produced in the Zr0.25Ti0.75O2 precursor solution after ligand
exchange. Compared to the pure titanium chloroalkoxide
isomers, the Ti species have a lower reactivity to condense with
each other to form TisOsTi bonds when Zr species coexist
with Ti species.
Figure 2. HRTEM images of NHSG-derived nanocrystals: (a) Zr0.03
-
Ti0.97
Ti0.75
O
O
2
-320, (b) Zr0.01Ti0.99
-400.
2 2
O -400, (c) Zr0.03Ti0.97O -400, (d) Zr0.25
-
2
TEM and XRD. Figure 2 shows HRTEM images of Zr0.03-
Ti0.97O2-320, Zr0.01Ti0.99O2-400, Zr0.03Ti0.97O2-400, and Zr0.25-
Ti0.75O2-400 nanocrystals. The HRTEM images indicate that
the NHSG-derived TiO2 and ZrO2 nanocrystals were almost
spherical, except for a few that have irregular shapes as a result
of aggregation (see Supporting Information, Figure S2). The
sizes of the TiO2 and ZrO2 particles, as observed from low-
magnification TEM images, were ca. 4.5 and 3.0 nm, respec-
tively. Highly crystalline structures and single crystallinity were
observed for both the TiO2 and ZrO2 particles in high-
magnification TEM images. Because TOPO plays the role of a
capping agent that isolates the NHSG-derived nanocrystals, these
relatively small ZrO2 nanocrystals arose from the lower metal-
to-TOPO ratio that we used in these NHSG reactions.
The Zr0.03Ti0.97O2 nanocrystals prepared at 320 °C exhibited
irregular shapes and a wide size distribution. A similar phenom-
enon was also observed for Zr0.05Ti0.95O2-320 (see Supporting
Information, Figure S2). The sizes and shapes became more
uniform when the reaction temperature rose to 400 °C. The
Zr0.03Ti0.97O2-400 and Zr0.25Ti0.75O2-400 nanoparticles became
faceted nanorods at 400 °C (Figure 2c,d). The lengths and widths
of the Zr0.03Ti0.97O2-400 nanorods were of ca. 15.0 and 8.0 nm,
respectively, whereas the corresponding dimensions of the Zr0.25-
Ti0.75O2-400 nanorods were ca. 30.0 and 14.0 nm, respectively.
The high-magnification HRTEM images showed that the
nanorods existed in a single crystalline domain but contained a
considerable number of defects on their edges. In addition, the
Zr0.25Ti0.75O2-400 nanorods were covered with a thin layer of
amorphous material. A similar amorphous structure was domi-
nant in the Zr0.67Ti0.33O2-400 sample.
1
6
The endothermic peak for Zr0.25Ti0.75O2 appearing at 334 °C
in the DCS curve can be assigned to the formation of ZrsOs
Ti bonds through cross-condensation. This cross-condensation
is responsible for the incorporation of Zr into the TiO2 lattice.
However, the low Zr-to-Ti ratios reveal that the formation of
ZrxTi1-xO2 nanocrystals is kinetically controlled because the
temperature required for the formation of TiO2 is lower than
that for the formation of binary oxides. The relatively rapid
formation of TiO2 nanocrystals results in the inefficient
condensation of Zr species during a dynamic equilibrium. The
Zr components were lost either because the reaction temperature
(320 °C) was too low to form ZrO2 or because the ZrO2 particles
that formed were too small to be recovered through centrifuga-
tion at 11000 rpm at 400 °C. In contrast to the ineffective incor-
17
poration of Zr into TiO2 in this study, Vioux et al. reported
that amorphous zirconium titanate gels with Zr-to-Ti ratios equal
to those of the initial solutions were obtained using a similar
cross-condensation process at 110 °C. In addition, the temper-
i
ature of condensation between TisCl and TisO Pr was reported
1
6
to be 100 °C, which is lower than the temperature for the
condensation of pure TiO2 (207 °C) obtained in this study. This
discrepancy is due to the fact that TOPO modifies the thermo-
dynamic properties and kinetics of the nonhydrolytic sol-gel
reactions by forming Lewis adducts with the precursors.
Figure 3 displays the XRD patterns of TiO2, ZrO2, and
ZrxTi1-xO2 samples with various values of x. The TiO2 and ZrO2
particles obtained crystallized in the anatase and tetragonal
forms, respectively, without further calcination. In addition, the
Zr0.03Ti0.97O2-320, Zr0.05Ti0.95O2-320, Zr0.01Ti0.99O2-400, Zr0.03-
Ti0.97O2-400, and Zr0.25Ti0.75O2-400 samples exhibited only
Microstructures. To identify the effect of the preparation
conditions on the microstructures of Zr-doped TiO2, the NHSG-
derived ZrxTi1-xO2 samples were further analyzed using HR-

20812 J. Phys. Chem. B, Vol. 110, No. 42, 2006
Chang and Doong
reaction temperature makes the crystallite sizes larger and more
uniform. The Zr0.25Ti0.75O2-400 sample exhibited the largest
crystallite size of 12.6 ( 2.5 nm because it was prepared at a
high Zr-to-Ti ratio and reaction temperature.
The TEM observations and XRD results reveal that the initial
Zr-to-Ti ratio and reaction temperature affect the sizes and
shapes of the NHSG-derived ZrxTi1-xO2 nanocrystals. The initial
Zr-to-Ti ratio affects the morphology of the ZrxTi1-xO2 samples
by affecting the condensation rate. It has been suggested that
the rate of alkyl halide elimination governs the nucleation and
1
8
growth of nanocrystals. In this study, because the presence of
Zr species retarded the formation of TiO2, the sizes of the
nanocrystals increased as the Zr-to-Ti ratio increased. In addition
to the crystallite sizes, the different reactivities between Ti and
Zr species also play the crucial role in the shapes and size
distributions of the NHSG-derived nanocrystals. At 320 °C, the
growth of TiO2 nanocrystals is inhibited at the end where the
Ti species are hybridized with less-reactive Zr species, thus
resulting in the irregular shape of Zr-doped TiO2. However, this
low reactivity can be kinetically promoted by increasing the
2 2 x 2
Figure 3. XRD patterns of TiO , ZrO , and Zr Ti1-xO nanocrystals.
anatase structures, and no ZrO2 or ZrTiO4 diffraction peaks were
identified, even though substantial amounts of Zr reactants had
been added to participate in the nonhydrolytic reactions. This
result is mainly due to the fact that the Zr4 ions serve only as
dopants in the TiO2 matrix. Moreover, the diffraction peak of
the (101)a profile of Zr0.25Ti0.75O2-400 shifted slightly to a lower
value of 2θ when compared to that of the pure TiO2 nanocrys-
+
18
reaction temperature. Trentler et al. suggested that the forma-
tion and breaking of TisO bonds at elevated temperatures helps
to erase the defects and, hence, result in more-crystalline
structures. In this study, increasing temperature encourages the
rearrangement of Ti and O atoms, leading to the elimination of
4+
tals, revealing that the Zr ions, with a larger ionic radius (0.79
Å) than Ti4 (0.68 Å), were incorporated into the TiO2 lattice
to form dilute ZrxTi1-xO2 solid solutions. The XRD results show
the poor crystallinity of the Zr0.67Ti0.33O2-400 sample, which is
in agreement with the observation of the amorphous structure
in its HRTEM image. The Zr-to-Ti ratio of 2.05 in the Zr0.67-
Ti0.33O2-400 sample clearly indicates that substantial amounts
of TiO2 are present in the ZrO2 matrix. These results suggest
that higher temperatures are required for crystallization of the
binary oxide solutions than for crystallization of pure TiO2 and
+
4+
the defects caused by Zr and the formation of regularly shaped
nanocrystals. This phenomenon was not observed when a similar
reaction protocol was used for the preparation of HfxZr1-xO2
2
0
nanoparticles. This discrepancy might be due to the fact that
Zr and Hf species have similar reactivities, whereas the Ti
species have a higher reactivity than the Zr species in the
nonhydrolytic sol-gel reactions.
17
Surface Properties. In contrast to the hydrophilic surfaces
of the metal oxides obtained using the conventional HSG
method, the surfaces of the metal oxides obtained in this study
were hydrophobic. The nanocrystals were readily soluble in
hexane, but they were difficult to suspend in water. The XPS
results indicate the existence of substantial amounts of carbon
and small amounts of phosphorus on the surfaces of the
nanocrystals, suggesting that some TOPO residues existed on
the surface of the metal oxides (see Supporting Information,
Figure S3). The C-to-P molar ratio ranged between 18 and 20,
which is lower than that expected for TOPO (C/P ) 24). In
ZrO2 in the nonhydrolytic sol-gel reactions. Vioux et al. found
that an NHSG-derived ZrTiO4 sample crystallized at 700 °C.
The high temperatures required for crystallized ZrxTi1-xO2 solid
solutions at high Zr concentrations probably arise because the
anatase TiO2 and tetragonal ZrO2 structures are not compatible
with each other. The anatase Zr0.25Ti0.75O2-400 nanocrystals
contained amorphous structures at their edges. It has been
previously demonstrated that the solubility of Zr4 units in sol-
+
2
2
gel-derived anatase TiO2 is below the Zr-to-Ti ratio of 0.075.
4
+
The excess Zr ions (i.e., 0.34 > 0.075) in the Zr0.25Ti0.75O2-
00 sample would primarily accumulate at the surface of the
4
*
*
addition, the O -to-P ratios (where O denotes oxygen atoms
other than MsO atoms) were in the range 2-3, which is larger
than the intrinsic value (O/P ) 1) expected for TOPO (see
Supporting Information, Table 1S). These results imply that the
TOPO molecules were decomposed during the high-temperature
reactions. To investigate the interactions between TOPO and
the nanocrystals, we used FTIR spectroscopy to further char-
acterize the functional groups present on the surfaces of the
nanocrystals. Figure 4 displays the FTIR spectra of TOPO, the
as-prepared nanocrystals, and the nanocrystals obtained after
photocatalysis. The spectrum of TOPO exhibited a significant
anatase TiO2 nanocrystals because of slow condensation involv-
ing the Zr species and segregation of the Zr4+ ions to maintain
phase stability. Therefore, the amorphous material at the edges
of the Zr0.25Ti0.75O2-400 nanorods presumably comprises cross-
linked Zr and Ti binary oxides that are unable to crystallize in
this nonhydrolytic sol-gel reaction.
The average crystallite sizes of anatase TiO2 and tetragonal
ZrO2, calculated using Scherrer’s equation from the broadening
of diffraction peaks of the (101)a and (101)t profiles, were 4.6
and 3.0 nm, respectively. These values are close to the particle
sizes observed in the TEM images. Because of the single
crystallinity of the NHSG-derived nanocrystals, the size distri-
butions and variations were examined from the TEM images.
The TiO2 and ZrO2 nanoparticle sizes have uncertainties of (1.0
and (0.9 nm, respectively. The crystallite sizes of the Zr0.03-
Ti0.97O2-320 and Zr0.05Ti0.95O2-320 samples were 5.2 ( 1.4 and
-
1
absorption at 1146 cm that was due to PdO stretching. The
corresponding most intense absorptions for the TiO2 and ZrO2
-
1
nanocrystals appeared at 1065 cm . The shift from 1146 to
-
1
1065 cm indicates that the PdO groups were chelated to the
4
+
4+
surface Zr or Ti centers. Figure 5 presents a cartoon illus-
tration of the surface structure of an NHSG-derived nanocrystal
based on the XPS and FTIR spectroscopic data. The surfaces
of the nanocrystals are covered by TOPO units, which are chem-
ically bonded to the surface through chelation of the PdO
oxygen atoms or through defunctionalization of the PdO groups
6
.7 ( 2.5 nm, respectively; i.e., an increased Zr-to-Ti ratio
increased the crystallite sizes. In addition, both the crystallite
sizes and the size distributions of the Zr0.01Ti0.99O2-400 (5.6 (
1
.0 nm) and Zr0.03Ti0.97O2-400 (7.7 ( 1.9 nm) samples were
larger than those obtained at 320 °C, indicating that increasing

Characterization of Zr-Doped TiO2 Nanocrystals
J. Phys. Chem. B, Vol. 110, No. 42, 2006 20813
2
Figure 4. FTIR spectra of pure TOPO and TOPO-capped ZrO and
TiO nanocrystals, the latter recorded before and after photocatalysis.
2
Figure 5. Surface chemical structures of NHSG-derived nanocrystals.
Figure 6. UV-vis spectra of NHSG-derived nanocrystals.
to form new PsOsM bonds. After photocatalysis, the intensity
of the signal for TOPO decreased, implying that a certain
amount of TOPO decomposed during photocatalysis. The P-to-
Ti and C-to-P elemental ratios of the NHSG-derived TiO2 before
and after photocatalysis were further analyzed by XPS. The P-to-
Ti ratio remained at a similar value (2.01), whereas the C-to-P
ratio decreased from 19.1 to 11.1 after photocatalysis. These
XPS and IR results reveal that photocatalysis causes the
decomposition of the surface TOPO through the removal of
organic parts and the transformation of PdO functional groups
to other phosphorus species.
Optical Properties. The band gaps and quantum effects on
the optical properties of the nanocrystals were determined by
UV-vis spectrophotometry. Figure 6 presents the UV-vis
spectra of the TiO2, ZO2, and ZrxTi1-xO2 nanocrystals suspended
in hexane. The TiO2 nanocrystals showed a steep increase in
their absorptions at wavelengths below 333 nm, and the ZrO2
nanocrystals exhibited significant absorptions at wavelengths
below 234 nm. The band gaps of the pure TiO2 and ZrO2
nanocrystals, derived from the onsets of these significant
absorptions, were 3.7 and 5.3 eV, respectively, which are larger
Figure 7. Photodegradation of RhB in the presence of NHSG-derived
nanocrystals.
with TiO2 in a comparative amount (Zr/Ti ) 2.1) and exhibited
1
,23
4+
than those of bulk TiO2 (3.2 eV) and ZrO2 (5.0 eV).
This
a band gap of 3.8 eV. Large amounts of Ti in the ZrO2 matrix
phenomenon indicates that quantum effects occurred and that
the nanocrystals obtained were quantum-sized. The anatase Zr-
doped TiO2 crystals exhibited absorptions that extended to
longer wavelengths. The band gaps of the Zr-doped TiO2
crystals ranged between 2.8 and 3.6 eV, i.e., they were smaller
than those of pure TiO2 or ZrO2 nanocrystals. Presumably, these
red shifts and reduced band gaps resulted either from defects
caused by trace amounts of Zr4 in the TiO2 lattice or from
increased crystal domains. We note that the Zr0.25Ti0.75O2-400
sample exhibited marked absorptions up to 600 nm. This
extrinsic band gap absorption of Zr0.25Ti0.75O2-400 is attributed
to the accumulation of substantial amounts of Zr4 at the surface
of the TiO2 anatase crystals. A similar effect occurred for
amorphous Zr0.67Ti0.33O2-400, which contained mainly ZrO2
not only inhibited the crystallization of ZrO2 but also greatly
reduced the band gap of ZrO2 from 5.3 to 3.8 eV.
Photoactivities of the NHSG-Derived Nanocrystals. We
examined the photoactivities of the NHSG-derived nanocrystals
by considering the decoloration of RhB monitored at 554 nm.
Figure 7 shows the change in RhB concentration as a function
of irradiation time. In the absence of a photocatalyst, RhB was
stable when illuminated with UV light at 305 nm. The rapid
degradation of RhB in the presence of the prepared nanocrystals
indicates that each of the TOPO-capped photocatalysts exhibited
photoactivity. Although the band transitions of ZrO2 nanocrys-
tals occur below 234 nm, electron transitions between the defect
levels and bands result in the ZrO2 nanocrystals having photo-
catalytic functions at 305 nm. The photodecomposition of RhB
+
+

20814 J. Phys. Chem. B, Vol. 110, No. 42, 2006
Chang and Doong
followed pseudo-first-order kinetics. The Zr0.03Ti0.97O2-400
regular shapes of the nanorods. The photoreactivities of the Zr-
doped TiO2 materials are associated with the amounts of dopants
and the shapes of the nanorods. The photocatalysts having the
optimal Zr-to-Ti ratio of 0.03 and regular shapes exhibit the
highest photoactivities. The TOPO moieties, which we used as
capping agents, were chemically bound to the surface of the
nanocrystals, either through direct donation of their PdO groups
to the surface metal centers or through reformation. The
influence of surface TOPO on the mechanisms and dynamics
of photocatalysis requires further studies to better understand
the photocatalytic chemistry of such surface-modified photo-
catalysts.
-
1
sample (k ) 0.40 min ) exhibited the highest rate of decom-
position of RhB, followed by Zr0.25Ti0.75O2-400 (k ) 0.28
-
1
-1
min ), Zr0.01Ti0.99O2-400 (k ) 0.13 min ), Zr0.05Ti0.95O2-320
-
1
-1
(
k ) 0.10 min ), Zr0.03Ti0.97O2-320 (k ) 0.07 min ), TiO2-
20 (k ) 0.04 min ), Zr067Ti0.33O2-400 (k ) 0.01 min ), and
then ZrO2 (k ) 3.2 × 10 min ).
-1
-1
3
-
3
-1
The ZrxTi1-xO2 nanocrystals exhibited higher degradation
efficiencies than did the pure TiO2 and ZrO2 samples. Moreover,
the stoichiometry of the reactants and the reaction temperature
during the synthesis of the nanoparticles affected the photo-
activities of these NHSG-derived nanocrystals. The degradation
efficiencies increased with increasing Zr-to-Ti ratio. For the
samples prepared at 400 °C, we observed the highest photo-
activity when the Zr-to-Ti ratio in the ZrxTi1-xO2 nanocrystals
was 0.03. In contrast, the ZrxTi1-xO2 sample prepared with the
same Zr-to-Ti ratio (0.03) but at 320 °C exhibited a lower rate
of degradation of RhB. It has been demonstrated that the
photoactivities of photocatalysts are mainly related to the
amounts of defects. A few defects in a TiO2 matrix, resulting
from the presence of dopants, can play the role of trapping
centers to inhibit charge recombination and improve the
Acknowledgment. The authors thank the MOE ATU
Program and the National Science Council, Taiwan, R.O.C.,
for financial support under Grants NSC95-2221-E-009-110 and
NSC94-2113-M-007-018.
Supporting Information Available: HRTEM images of
NHSG-derived TiO2, ZrO2, Zr0.05Ti0.95O2-320, and Zr0.67Ti0.33O2-
4
00 nanocrystals; Ti(2p) and Zr(3d) XPS spectra of ZrxTi1-xO2
nanocrystals; O(1s), P(2p), and C(1s) XPS spectra of NHSG-
*
photoactivity.24 On the contrary, an excessive number of defects
derived TiO nanocrystals; C-to-O -to-P molar ratios estimated
2
from the integrated peak areas of the XPS spectra normalized
by their atomic sensitivity factors; and photodecomposition of
can induce charge recombination and decrease the degree of
effective charge. We attribute the highest photoactivity of the
4+
RhB in the presence of Degussa P-25 TiO . This material is
Zr0.03Ti0.97O2-400 samples to their moderate content of Zr ions
in the TiO2 lattice (Zr-to-Ti ratio of 0.03) and the poor
photoactivity of the photocatalysts prepared at low temperature
to the great number of surface defects that resulted from the
irregular shapes of these particles.
2
available free of charge via the Internet at http://pubs.acs.org.
References and Notes
(1) Kabra, K.; Chaudhary, R.; Sawhney, R. L. Ind. Eng. Chem. Res.
We also compared the photoactivity of the NHSG-derived
photocatalysts with that of Degussa P25 TiO2 in terms of the
decoloration of RhB (see Supporting Information, Figure S4).
Similarly, the photodecomposition of RhB by Degussa P25 TiO2
followed pseudo-first-order kinetics, and the pseudo-first-order
rate constant (k) was 0.58 min . Although Degussa P25 showed
a higher apparent photoactivity than the NHSG-derived nano-
crystals, the intrinsic photoactivities of the NHSG-derived TiO2
2004, 43, 7683.
(2) Emeline, A. V.; Panasuk, A. V.; Sheremetyeva, N.; Serpone, N. J.
Phys. Chem. B 2005, 109, 2785.
(
3) Karunakaran, C.; Senthilvelan, S. J. Mol. Catal. A: Chem. 2005,
233, 1.
(4) Navio, J. A.; Colon, G.; Macias, M.; SanchezSoto, P. J.; Augugliaro,
V.; Palmisano, L. J. Mol. Catal. A: Chem. 1996, 109, 239.
-1
(5) Sayama, K.; Arakawa, H. J. Photochem. Photobiol. A 1996, 94,
67.
(6) Schattka, J. H.; Shchukin, D. G.; Jia, J. G.; Antonietti, M.; Caruso,
-
2
-1
nanocrystals (k ) 0.02 g‚m ‚min ) were 1.7 times higher than
R. A. Chem. Mater. 2002, 14, 5103.
-
2
-2
-1
(7) Chang, S. M.; Doong, R. A. Chem. Mater. 2005, 17, 4837.
that of Degussa P25 (k ) 1.2 × 10 g‚m ‚min ) when
(
8) Zhang, Z. B.; Wang, C. C.; Zakaria, R.; Ying, J. Y. J. Phys. Chem.
B 1998, 102, 10871.
9) Dondi, M.; Matteucci, F.; Cruciani, G. J. Solid State Chem. 2006,
179, 233.
normalized to the surface area, because the TOPO-capped TiO2
2
(
2 m /g) has a much smaller specific surface area than Degussa
(
2
P25 (50 m /g). The high intrinsic photoactivities of the NHSG-
derived nanocrystals are presumably due to the fact that the
surface TOPO enhances the adsorption of RhB onto the
photocatalysts.
(
10) Shibata, T.; Irie, H.; Ohmori, M.; Nakajima, A.; Watanabe, T.;
Hashimoto, K. Phys. Chem. Chem. Phys. 2004, 6, 1359.
(
11) Zhang, Q. H.; Gao, L.; Guo, J. K. Appl. Catal. B: EnViron. 2000,
6, 207.
12) Vera, C. R.; Pieck, C. L.; Shimizu, K.; Parera, J. M. Appl. Catal.
A: Gen. 2002, 230, 137.
2
(
Conclusions
(
(
(
13) Udompom, A.; Ananta, S. Mater. Lett. 2004, 58, 1154.
14) Vioux, A. Chem. Mater. 1997, 9, 2292.
15) Andrianainarivelo, M.; Corriu, R.; Leclercq, D.; Mutin, P. H.;
We have successfully prepared Zr-doped anatase TiO2
nanorods through the cross-condensation of Zr/TiCl4 with Zr/
Vioux, A. J. Mater. Chem. 1996, 6, 1665.
16) Arnal, P.; Corriu, R. J. P.; Leclercq, D.; Mutin, P. H.; Vioux, A.
Chem. Mater. 1997, 9, 694.
17) Andrianainarivelo, M.; Corriu, R. J. P.; Leclercq, D.; Mutin, P.
H.; Vioux, A. J. Mater. Chem. 1997, 7, 279.
18) Trentler, T. J.; Denler, T. E.; Bertone, J. F.; Agrawal, A.; Colvin,
i
Ti(O Pr)4 in anhydrous TOPO at either 320 or 400 °C. The
(
kinetics of the alkyl halide elimination involving the Ti and Zr
species plays a crucial role in determining the morphologies,
chemical compositions, and crystalline phases of the Zr-doped
TiO2 nanorods. Because the condensation between the Ti species
is faster than that between the Zr species, the kinetically
controlled reactions cause anatase TiO2 to be the dominant
structure in the nanorods, with the Zr4 ions appearing only as
dopants accumulating at the surfaces of the nanocrystals for
various initial Zr-to-Ti ratios used in preparation. The existence
of less-reactive Zr species in the reaction mixtures retards the
nucleation of TiO2. Therefore, increasing the concentration of
Zr species in the precursor solutions increases the sizes of the
nanocrystals, and increasing reaction temperatures promote
(
(
V. L. J. Am. Chem. Soc. 1999, 121, 1613.
(19) Joo, J.; Yu, T.; Kim, Y. W.; Park, H. M.; Wu, F. X.; Zhang, J. Z.;
Hyeon, T. J. Am. Chem. Soc. 2003, 125, 6553.
(
20) Tang, J.; Fabbri, J.; Robinson, R. D.; Zhu, Y. M.; Herman, I. P.;
Steigerwald, M. L.; Brus, L. E. Chem. Mater. 2004, 16, 1336.
21) Sham, E. L.; Aranda, M. A. G.; Farfan-Torres, E. M.; Gottifredi,
J. C.; Martinez-Lara, M.; Bruque, S. J. Solid State Chem. 1998, 139, 225.
+
(
(22) Yang, J.; Ferreira, J. M. F. Mater. Res. Bull. 1998, 33, 389.
(23) Emeline, A.; Kataeva, G. V.; Litke, A. S.; Rudakova, A. V.;
Ryabchuk, V. K.; Serpone, N. Langmuir 1998, 14, 5011.
24) Choi, W. Y.; Termin, A.; Hoffmann, M. R. J. Phys. Chem. 1994,
98, 13669.
(