Expanding the applications of the ilmenite mineral to the preparation of nanostructures: TiO2 nanorods and their photocatalytic properties in the degradation of oxalic acid

Article abstract of DOI:10.1002/chem.201202451

The mineral ilmenite is one of the most abundant ores in the Earth′s crust and it is the main source for the industrial production of bulk titanium oxide. At the same time, methods to convert ilmenite into nanostructures of TiO₂ (which are required for new advanced applications, such as solar cells, batteries, and photocatalysts) have not been explored to any significant extent. Herein, we describe a simple and effective method for the preparation of rutile TiO₂ nanorods from ball-milled ilmenite. These nanorods have small dimensions (width: 5-20nm, length: 50-100nm, thickness: 2-5nm) and possess large specific surface areas (up to 97m² g^-1). Dissolution/hydrolysis/precipitation is proposed as a growth mechanism. The nanorods were found to have attractive photocatalytic properties in the degradation of oxalic acid. Their photocatalytic activity is close to that of the benchmark Degussa P25 material and better than that of a commercial high-surface-area rutile powder. Hot rods: Ilmenite (FeTiO₃) is a major source of bulk TiO₂ but ways to convert it into nanostructures of titanium oxide are scarce. Single-crystal TiO₂ nanorods were synthesized from natural ilmenite by simple ball milling and acid treatment (see figure). Copyright

Full text of DOI:10.1002/chem.201202451

FULL PAPER
DOI: 10.1002/chem.201202451
Expanding the Applications of the Ilmenite Mineral to the Preparation of
Nanostructures: TiO Nanorods and their Photocatalytic Properties in the
2
Degradation of Oxalic Acid
[
a]
[a]
[b]
[b]
[c]
[c]
Tao Tao, Ying Chen, Dan Zhou, Hongzhou Zhang, Sanly Liu, Rose Amal,
[
d]
[a]
Neeraj Sharma, and Alexey M. Glushenkov*
2
ꢀ1
Abstract: The mineral ilmenite is one
of the most abundant ores in the
Earth’s crust and it is the main source
for the industrial production of bulk ti-
tanium oxide. At the same time, meth-
ods to convert ilmenite into nanostruc-
a simple and effective method for the
preparation of rutile TiO2 nanorods
from ball-milled ilmenite. These nano-
rods have small dimensions (width: 5–
20 nm, length: 50–100 nm, thickness:
2–5 nm) and possess large specific sur-
face areas (up to 97 m g ). Dissolu-
tion/hydrolysis/precipitation is pro-
posed as a growth mechanism. The
nanorods were found to have attractive
photocatalytic properties in the degra-
dation of oxalic acid. Their photocata-
lytic activity is close to that of the
benchmark Degussa P25 material and
better than that of a commercial high-
surface-area rutile powder.
tures of TiO (which are required for
2
new advanced applications, such as
solar cells, batteries, and photocata-
lysts) have not been explored to any
significant extent. Herein, we describe
Keywords: ball milling · il
nanostructures photocatalysis
photochemistry · rutile · titan um
ACTHUNGTNERNUNGm en CAHTUNGTRNENUNGi te ·
·
·
A
H
U
G
R
N
U
G
A
T
N
T
E
N
N
ACTHUNGTRENNUNiG ACHTUNGTRENNNUG
Introduction
battery within a single integrated power pack can be pro-
[5]
duced. Last, but not least, nanostructured TiO materials
2
Nanostructured titanium oxide (TiO ) has emerged as a key
remain the most-popular choice for modern photocatalysts
for the destruction of harmful organic chemicals in water
and air.
The large-scale synthesis of TiO2 nanostructures from
cheap and abundant precursors is crucial for the transfer of
dye-sensitized solar cells, new batteries with incorporated ti-
tania anodes, and efficient photocatalysts from the laborato-
ry to “real life” applications. The natural mineral ilmenite
2
material in the fields of energy and environmental science.
Dye-sensitized solar cells, next-generation photovoltaic devi-
ces that mimic natural photosynthesis, use nanostructures of
[6–8]
TiO as the material of choice for the semiconducting com-
2
[1,2]
ponent of their photoelectrodes.
Titanium oxides are low-
and zero-strain materials for anodes of lithium-ion batteries
and are receiving much current interest for the storage of
[3,4]
electrochemical energy.
Owing to the dual functionality
(FeTiO ) is the primary source for the production of bulk
3
[9]
of TiO as both a photoelectrode material for solar cells and
TiO2. Ilmenite mineral is relatively cheap (80–107 USD/
2
[9]
as an anode material for lithium-ion batteries, state-of-the-
art devices that combine a dye-sensitized solar cell and a
metric tonne in 2004–2008, with a recent peak of 250–350
[10]
USD/metric tonne in 2012 ) and its deposits are located on
all five major continents (North and South America, Eur-
AHCTUNGTRENNUNGa sia, Africa, and Australia). According to recent estimates,
the world’s total reserves of ilmenite exceed 680 million tons
[
a] Dr. T. Tao, Prof. Y. Chen, Dr. A. M. Glushenkov
Institute for Frontier Materials
Deakin University, Waurn Ponds, VIC 3216 (Australia)
Fax : (+61)3-52271103
and are sufficient for the preparation of 350–400 million
[9]
tons of TiO2. Current industrial routes for the preparation
[11,12]
of TiO use either the sulfate or chloride process,
whilst,
2
E-mail: alexey.glushenkov@deakin.edu.au
ironically, methods for the preparation of nanostructures of
[
b] D. Zhou, Dr. H. Zhang
TiO (e.g., nanorods, nanosheets, or mesoporous materials)
2
School of Physics and Centre for Research on
Adaptive Nanostructures and Nanodevices (CRANN)
Trinity College Dublin, Dublin 2 (Republic of Ireland)
from ilmenite are virtually non-existent. Instead, as we have
[13]
discussed previously,
sophisticated chemistry and exotic
[
c] Dr. S. Liu, Prof. R. Amal
School of Chemical Engineering
ARC Centre of Excellence for Functional Nanomaterials
The University of New South Wales
Sydney, NSW 2052 (Australia)
precursors are often used to prepare nanostructured TiO2.
Considering the existing abundant reserves, cheap price, and
easy availability of ilmenite, there is a compelling case for
developing production methods of TiO nanostructures di-
2
rectly from this material.
Only in the last few years has it finally been demonstrated
that ilmenite can be used directly in the preparation of
[
d] Dr. N. Sharma
Australian Nuclear Science and Technology Organisation
Locked Bag 2001, Kirrawee DC NSW 2232 (Australia)
Chem. Eur. J. 2013, 19, 1091 – 1096
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1091

[15]
nanostructures of TiO and iron titanate (FeTiO ). Li et al.
where
and it has been concluded that the formation of
2
3
reported the formation of mesoporous TiO2 from ball-
FeTiO nanoflowers (diameter: 1–2 mm) occurs during the
3
[14]
milled ilmenite in aqueous solutions of H SO .
Aggre-
course of the hydrothermal treatment of ball-milled ilmenite
in an aqueous solution of NaOH through a dissolution/pre-
cipitation mechanism. The surface area of the flower-like
2
4
gates of mesoporous rutile TiO nanoparticles with a con-
2
trollable bimodal pore-size distribution have been obtained
from ilmenite through a combination of carbothermal reduc-
2
ꢀ1
nanostructures (26 m g ) is significantly higher than that of
[13]
2
ꢀ1
tion, ball milling, and acid leaching.
Finally, flower-like
the ball-milled ilmenite (2.7 m g ).
FeTiO nanostructures have been produced by a mild hydro-
Another type of nanostructure, nanorods, form after sub-
sequent treatment in an aqueous solution of HCl. The mor-
phology of the nanorods is shown in Figure 1d (inset shows
a high-magnification SEM image). The typical length of the
rods is in the range 50–100 nm, their width is between 5 and
20 nm, and their thickness is in the range 2–5 nm. We later
established that preliminary leaching in NaOH was not re-
quired for the formation of the nanorods; they could also be
formed by direct acid leaching of the ball-milled ilmenite.
However, it is worthwhile noting that a longer time (6 h in-
stead of 4 h) is needed for the nanorods to form is this case.
XRD patterns of the milled ilmenite and the samples
after the wet-chemistry treatments are presented in
Figure 2. The diffraction peaks of the milled ilmenite
3
thermal treatment of ball-milled ilmenite in a 2m aqueous
[15]
solution of NaOH.
Herein, the formation of nanorods, which are another
technologically important morphology of TiO , from ilmen-
2
ite through a simple two-step procedure of ball milling and
acid treatment is demonstrated. The phase composition,
morphology, textural parameters (surface area and pore-size
distribution), and formation mechanism of these nanorods
are discussed. The obtained TiO nanorods were found to
2
exhibit improved photocatalytic activity in the mineraliza-
tion of oxalic acid, which is a typical contaminant in indus-
trial waste water.
Results and Discussion
A SEM image of the starting ilmenite material is shown in
Figure 1a. This material consists of large particles with a
typical size of about 100 mm. Ball milling leads to a substan-
tial change in the morphology of ilmenite, as shown in Fig-
ure 1b, and results in a significant decrease in their particle
size. These sub-micron particles of the ball-milled ilmenite
typically aggregate into larger agglomerates. A SEM image
of the material after leaching in NaOH (for the removal of
SiO impurities from the ball-milled ilmenite) is shown in
2
Figure 1c. As a result of the leaching, ilmenite changes its
morphology from irregular aggregates of sub-micron parti-
cles into a flower-like nanoarchitecture. This morphological
transformation has been previously studied in detail else-
Figure 2. XRD patterns of the materials that were obtained from ilmen-
ite: a) after ball milling; b) after leaching of the milled ilmenite in
NaOH; c–e) after leaching of the NaOH-treated sample in HCl for 2, 8,
and 24 h, respectively.
powder are broadened owing to small grain sizes (Fig-
ure 2a). No new phases were observed in the XRD pattern
after leaching of the milled ilmenite in a 2m aqueous solu-
tion of NaOH at 1208C for 2 h (Figure 2b). The XRD pat-
terns of the samples that were subsequently treated in a 4m
aqueous solution of HCl for 2, 8, and 24 h (Figure 2c–e)
matched the standard pattern of tetragonal rutile TiO2
(
JCPDS No. 01-076-1939). It follows from these results that
the FeTiO phase transforms completely into the rutile TiO2
3
phase after 2 h of leaching in dilute HCl. Thus, we conclude
that rutile TiO nanorods can be prepared from ilmenite by
2
ball milling, treatment in an alkaline solution (optional),
and leaching in dilute HCl.
The nanorods were also evaluated by TEM; low-magnifi-
cation images are shown in Figure 3a,c. Aggregates of short
nanorods are visible and their electron-diffraction pattern
Figure 1. SEM images of the materials at various stages of processing:
a) original ilmenite powder; b) milled ilmenite powder; c) flower-like
structures of FeTiO
3
after leaching of the milled ilmenite in NaOH for
(
Figure 3b) matches the expected pattern of a rutile TiO2
2
h; d) nanorods that were obtained after leaching of the flower-like
phase. These observations are in an excellent agreement
3
FeTiO structures in HCl for 8 h. Inset: higher-magnification image of the
nanorods.
with the SEM and XRD data. The TiO nanorods appear to
2
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Application of Ilmenite to the Preparation of Nanostructures
FULL PAPER
2
Figure 3. TEM characterization of the TiO nanorods that were produced
by leaching of the milled and NaOH-treated ilmenite in HCl for 8 h:
a,c) Low-magnification images; b) SAED pattern taken from (a);
d) high-resolution image.
be single crystals and the single-crystalline nature of a TiO2
nanorod is demonstrated in Figure 3d. During the TEM
analysis, we observed that the nanorods were sensitive to
beam damage and easily broke into tiny nanocrystallites
under the electron beam.
XPS spectra of the TiO nanorods were recorded and the
2
data are presented in Figure 4. The Ti 2p (Figure 4a) and
O 1s (Figure 4b) signals are in a very good agreement with
[16]
the reference XPS spectra of rutile TiO2. A week Fe 2p
peak (Figure 4c) was also observed, but the intensity of the
peak was comparable to the noise level, thus suggesting that
the presence of Fe was quite low. These results further con-
Figure 4. XPS spectra of rutile TiO nanorods that were obtained by acid
leaching of the NaOH-treated ilmenite: a) Ti 2p; b) O 1s; c) Fe 2p.
2
firm that the product is rutile TiO with a low level of Fe im-
2
purities.
that was leached for 2 h in hydrochloric acid showed two
pore-size maxima at around 20 and 40 nm, whereas a single
maximum at 23 nm was observed in the pore-size distribu-
tion of the sample that was leached for 4 h. The contribution
of the macropores was no longer significant in samples that
were leached for 8 and 24 h. Moreover, the pore distribu-
tions of these two samples were very similar and were domi-
nated by mesopores that were 4–30 nm in size. Thus, leach-
ing intervals of longer than 8 h did not lead to significant
textural changes in the formed titanium-oxide nanorods and
their growth was essentially complete. A narrow distribution
of mesopores that was centered at around 10–15 nm can be
correlated with space between the nanorods in the aggre-
gates (cf. the TEM data, Figure 3a).
Changes in the surface area of the ilmenite material
during the course of its acid leaching are plotted in Fig-
ure 5a. The initial surface area of nanostructured flower-like
2
ꢀ1
FeTiO is 26 m g . A large surface area is beneficial for the
3
dissolution of ilmenite because it leads to a faster dissolu-
tion rate. The BET surface area increases significantly
during the first eight hours of leaching and reaches a maxi-
2
ꢀ1
mum value of 97 m g . Almost-constant values of approxi-
2
ꢀ1
mately 90 m g were recorded after 12 and 24 h from the
start of the leaching. These results indicate that the transfor-
mation of ilmenite flower-like nanostructures into TiO2
nanorods predominantly takes place in the first 8 h of leach-
ing and only minor changes occur upon longer acidic treat-
ment.
A dissolution/hydrolysis/precipitation mechanism is pro-
This assumption correlates well with the pore-size distri-
butions, shown in Figure 5b. The pores in the samples that
were leached for 2 and 4 h covered a wide range of pore
types, including both meso- and macropores. The sample
posed to explain the formation of the TiO nanorods during
2
[17]
the acid leaching of ilmenite.
When the milled ilmenite
comes into contact with the HCl solution, both iron and tita-
nium migrate into the solution to form FeCl and TiOCl .
2
2
Chem. Eur. J. 2013, 19, 1091 – 1096
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1093

A. M. Glushenkov et al.
[23–25]
the preparation of synthetic rutile.
They prepared high-
quality synthetic rutile TiO with a purity of 92.3 wt.% from
2
Panzhihua ilmenite after milling for a short time (only
1
5 min) in air and subsequently leaching it in a 20% aque-
ous solution of HCl.
The increase in surface area and the changes in pore-size
distribution (Figure 5) relate to the extent of hydrolysis and
precipitation of the dissolved titanium in the aqueous solu-
tion of HCl. A short time of acid leaching (less than 8 h)
cannot lead to the complete hydrolysis and precipitation of
the dissolved titanium; the hydrolysis product is dispersed in
the HCl solution. When the leaching time exceeds 8 h, hy-
drolysis and precipitation of the dissolved titanium in the
form of nanorods are essentially completed.
Figure 6a shows the optical absorption properties of rutile
TiO nanorods, a commercial rutile powder with a high sur-
2
Figure 5. Surface areas and pore-size distributions: a) Evolution of the
surface areas of the materials after acid leaching of the NaOH-treated il-
menite; b) pore-size distributions of the samples after acid leaching for
various periods of time: 2 (i), 4 (ii), 8 (iii), and 24 h (iv).
Subsequently, TiOCl may be hydrolyzed and precipitate in
2
the form of tiny TiO crystals that continue their growth in a
2
1D fashion. The main chemical reactions during the leaching
of ilmenite by HCl can be described according to Equa-
[18,19]
tions (1) and (2).
FeTiO þ4 HCl ! TiOCl þFeCl þ2 H O
ð1Þ
ð2Þ
3
2
2
2
TiOCl þH O ! TiO þ2 HCl
2
2
2#
Although, in our view, this proposed mechanism is the
most likely and the presence of TiOCl as the predominant
2
titanium-containing species during the intermediate stage
[20,21]
correlates with previous opinions,
it is premature to
draw final conclusions at this stage. Other species may be in-
volved and further investigation is required to clarify the
formation mechanism.
The ball milling of ilmenite or its nanostructuring (for ex-
ample, its transformation into flower-like nanoarchitectures)
appear to be crucial for the efficient dissolution of FeTiO3
in acid solution, whilst no noticeable dissolution of bulk il-
menite happens in such solutions. This result also correlates
well with observations in the literature. Chen et al. have ob-
Figure 6. Photocatalytic properties: a) UV/Vis diffuse reflectance spectra
for P25, rutile TiO
b) photocatalytic tests for the mineralization of oxalic acid with P25,
rutile TiO nanorods, and a commercial rutile powder. The total carbon
2 2
nanorods, and commercial rutile TiO powders;
served that, in contrast to the unmilled FeTiO , ilmenite
3
that was milled for 200 h under vacuum could be completely
2
[22]
dissolved. Li at al. investigated the dissolution of mechan-
ically activated Panzhihua ilmenite in HCl and H SO for
0
loading in each experiment was 1000 mg; c) plot of ln ACTHUNGTRNENUG( C/C ) as a function
of irradiation time for the same materials.
2
4
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Application of Ilmenite to the Preparation of Nanostructures
FULL PAPER
2
ꢀ1
face area (Sigma Aldrich; surface area>130 m g ), as well
as a reference photocatalytic material, P25 (Degussa). The
samples were characterized by UV/Vis diffuse reflectance
spectroscopy. The commercial rutile powder and P25 do not
exhibit much absorption beyond 350 nm, whereas the TiO2
nanorods absorb light up to wavelengths of 400–450 nm.
This extension of light absorption to longer wavelengths
may arise from the contribution of impurities (such as iron
that is incorporated during the synthesis) or defects. Indeed,
defects or interstitial impurities often give rise to local states
P25, a mixture of anatase and rutile nanoparticles that is
commonly used as the benchmark material in photocatalytic
studies.
Experimental Section
Preparation of the nanorods: Ilmenite powder (10 g) and four hardened
steel balls (diameter: 25.4 mm) were loaded into the stainless-steel con-
[
33]
tainer of a magneto-ball mill (described in detail elsewhere ). Ball mill-
ing was conducted for 150 h at RT under an argon atmosphere (100 kPa).
The magnet was located on the bottom of the mill at a 458 angle relative
to the vertical direction and the rotation speed was 160 rpm. Ilmenite
[26]
below the conduction edge.
The photocatalytic activities of the rutile TiO nanorods,
2
powder (FeTiO
used as the starting material. Its chemical composition can be expressed
as TiO (dry basis) 49.6%, Fe (total) 35.1%, FeO 32.8%, Fe 13.7%,
Al 0.47%, Cr 0.25%, SiO 0.45%.
3
, 99% purity; Consolidated Rutile Ltd, Australia) was
the commercial rutile powder, and P25 were evaluated by
monitoring the decomposition of oxalic acid in aqueous sol-
ution under UV irradiation. The results are shown in Fig-
ure 6b and the kinetics of the mineralization of oxalic acid
by rutile TiO nanorods is comparable with that demonstrat-
ed by P25. The activity of TiO nanorods is considerably
better than that of the commercial high-surface-area rutile
powder. An improved photocatalytic behavior of single-crys-
talline nanorods has been demonstrated previously in a
range of publications and has been attributed to a larger
aspect ratio and the good crystallinity of nanorods.
kinetics of oxalic-acid degradation is also presented as a plot
of ln(C/C ) versus time (Figure 6c). The rate of photocata-
2
2 3
O
2
O
3
2
O
3
2
In all of the wet-chemistry experiments, the powders, stirring rods, and
their corresponding aqueous solutions were placed in Schçtt bottles.
Next, the bottles were immersed in paraffin-oil baths on the top of a hot
plate. Magnetic stirring at 800 rpm was used to stir the solutions. First,
the milled ilmenite (3 g) was treated with a 2m aqueous solution of
NaOH (300 mL) at 1208C for 2 h. The filtered samples were washed and
2
2
2
dried at 908C for 4 h. The rutile TiO nanorods were prepared by leach-
ing a NaOH-treated sample (1 g) in 4m HCl (100 mL) at 908C for 4 h.
The suspension was filtered after leaching and the leached samples were
washed and dried at 908C for 4 h.
[27–31]
The
Structural characterization: The samples were characterized by X-ray dif-
fraction (XRD, PANalytical X-ray diffractometer, Cu Ka radiation, l=
ACHTUNGTRENNUNG
0
lytic degradation of oxalic acid over P25 can be approximat-
ed by a first-order reaction (almost-linear profile), whereas
the degradation rates for the other two catalysts do not fit
the first-order reaction well. It is an open question whether
the iron impurity affects the photocatalytic properties of the
nanorods. It has been reported previously (for example, see
reference [32]) that a few atom% of Fe impurity may en-
1
.5418 ꢁ), SEM (Carl Zeiss UltraPlus instrument), and TEM (FEI Titan
80–300, operating voltage: 300 kV). The diffuse reflectance spectra of the
dry powders were measured on a Cary 300 UV/Vis spectrophotometer
(
4
Varian) that was equipped with an integrating sphere. BaSO was used
as a reference sample. Surface areas, pore volumes, and pore-size distri-
butions of the samples were determined on a Micromeritics Tristar 3000
adsorption instrument. Surface characterization by X-ray photoelectron
spectroscopy (XPS) was carried out on an ESCALab220i-XL (Thermo
Scientific, UK) by using
1
a monochromated X-ray source (energy
486.6 eV) that was induced by Al Ka radiation (10 kV, 15 mA, spot size
hance the photocatalytic activity of TiO . On the other
2
hand, we have a very low content of iron impurity (Fig-
ure 4c) and it is not clear at this stage if this affects the pho-
tocatalytic properties of the nanorods.
diameter: 1 mm). The pass energy was 20 eV with a scan step of 0.1 eV.
Photocatalytic measurements: The photocatalytic activity of the samples
was measured by using a small batch photocatalytic reaction system,
which consisted of a photoreactor with a near-UV illumination source
and a conductivity monitor for measuring carbon dioxide that was gener-
ated as the organic substance was oxidized. Oxalic acid was chosen as the
model organic compound. The amount of mineralized oxalic acid was
equivalent to 1000 mg of carbon. The catalyst loading was 1 gL and the
volume of the reaction mixture was 100 mL. A 20 W black-light blue-flu-
orescent lamp (NEC) was used as the light source. This lamp had a wave-
length emission peak at approximately 370 nm and emitted radiation be-
tween 320–410 nm. The photocatalytic reaction was initiated by turning
Conclusion
ꢀ
1
A new method for the synthesis of single-crystal TiO nano-
2
rods from natural ilmenite has been demonstrated. This
method includes ball milling and two sequential wet-chemis-
try treatments in 1) aqueous NaOH (optional) and 2) aque-
ous HCl. Dissolution/hydrolysis/precipitation is proposed as
the growth mechanism. Initially, titanium and iron migrate
into the solution to form TiOCl and FeCl . During the hy-
on the UV lamp. A detailed description of the process has been given by
Matthews. Degussa P25 and a commercial rutile TiO powder (particle
2
size<100 nm, surface area>130 m g ; Sigma Aldrich) were also tested
for comparison.
[34]
2
ꢀ1
2
2
drolysis of TiOCl , tiny TiO crystals precipitate and contin-
2
2
ue to grow in a 1D fashion. Electron microscopy demon-
strated that the nanorods had good crystallinity and their di-
mensions were 50–100 nm (length) by 5–20 nm (width) by
Acknowledgements
Financial support from the Australian Research Council is acknowledged.
The work that was conducted at Trinity College Dublin was supported by
the Irish Government’s Programme for Research in Third Level Institu-
tions, Cycle 4, National Development Plan 2007–2013, within the frame-
work of the INSPIRE programme and Science Foundation Ireland under
grant no. 07/SK/I1220a.
2
–5 nm (thickness). The measured BET specific area is in
2
ꢀ1
the range 90–97 m g . In addition, the TiO nanorods have
2
excellent photocatalytic properties in the photodegradation
of oxalic acid. They are more active than commercial rutile
2
ꢀ1
powder with a high surface area (>130 m g ; Sigma–Al-
drich). Moreover, the activity is similar to that of Degussa
Chem. Eur. J. 2013, 19, 1091 – 1096
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1095

A. M. Glushenkov et al.
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Received: July 10, 2012
Revised: October 12, 2012
Published online: November 23, 2012
1096
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