Article
pubs.acs.org/JPCC

Fundamental Strategy for Creating VLS Grown TiO2 Single Crystalline
Nanowires
Fuwei Zhuge,† Takeshi Yanagida,*,† Kazuki Nagashima,† Hideto Yoshida,† Masaki Kanai,† Bo Xu,†
Annop Klamchuen,† Gang Meng,† Yong He,† Sakon Rahong,† Xiaomin Li,‡ Masaru Suzuki,§ Shoichi Kai,§
Seiji Takeda,† and Tomoji Kawai*,††

The Institute of Scientiﬁc and Industrial Research, Osaka University, 8-1 Mihogaoka Ibaraki, Osaka, 567-0047, Japan
State Key Laboratory of High Performance Ceramics and Superﬁne Microstructures, Shanghai Institute of Ceramics, Chinese
Academy of Sciences, Shanghai 200050, P. R. China§
Department of Applied Quantum Physics and Nuclear Engineering, Faculty of Engineering, Kyushu University, 744 Motooka,
Nishi-ku, Fukuoka 819-0395, Japan‡

ABSTRACT: A single crystalline TiO2 nanowire grown by a sizeand position controllable vapor−liquid−solid (VLS) method is apromising candidate to control and design the physical and chemical
properties for various TiO2-based applications. However, creating
TiO2 nanowires by VLS has been a challenging issue due to adiﬃculty on controlling and understanding the complex materialtransport events across three phases. Here we propose a fundamental
strategy to create a TiO2 single crystalline nanowire by the VLS
mechanism. We show that a VLS growth of TiO2 nanowires canemerge intrinsically only within a quite narrow range of material ﬂux,
which is a sharp contrast to typical VLS oxides including MgO, SnO2,
In2O3, and ZnO, whose nanowires are easily grown by VLS withmuch wider ranges of material ﬂux. We reveal that a condensation of
Ti atoms at a vapor−solid interface, which is detrimental for VLS, is
responsible to limit a window of material ﬂux for TiO2 nanowires. In addition, we found that our rutile-TiO2 nanowires
preferentially grow along ⟨001⟩ direction, which interestingly diﬀers from a typical ⟨110⟩ oriented growth of TiO2 nanowires
formed by the vapor-phase method. The present approach based on a control of material ﬂux provides a foundation to tailor VLS
grown TiO2 nanowires based on a scientiﬁc strategy rather than a rule of thumb.

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INTRODUCTION
Titanium dioxide (TiO2) has shown a great promise for various
applications, including e.g. photocatalyst, dye-sensitized solar
cells, and water splitting systems.1−4 Since the functionalities
strongly rely on an electron transfer event at a surface of TiO2,
various TiO2 nanostructures, including nanoparticles, nanoporous structures, and nanowires, have been proposed toenhance a device performance via increasing a surface area.5
Among such TiO2 nanostructures, a single crystalline 1D
nanowire is especially an attracting candidate because thisnanostructure essentially allows us to avoid a detrimental
degradation of carrier mobility via reducing grain boundaries, atwhich a carrier tends to scatter.6,7 There are various methods tofabricate TiO2 nanowires. Among them, a hydrothermal
method has most successfully fabricated 1D TiO2 nanostructures.8−13 Although hydrothermal methods are very attractive
due to the applicability for various substrates, they often suﬀer
from suppressing detrimental crystal defects due to the lowtemperature process.14 In addition, controlling intentionally thesize ranges of diameter/length or the spatial locations seems to
be unfeasible for hydrothermal synthesis. A size and position© 2012 American Chemical Society

controllable VLS method for fabricating a nanowire is apromising to overcome above issues. Although a few reports
have demonstrated the occurrence of a VLS growth for TiO2
nanowires,13,15,16 knowledge to create single crystalline TiO2
nanowires by VLS has been substantially limited due to the lackof understanding the complex material transport events of VLS
across three phases. Here we demonstrate a fundamental
strategy to realize a TiO2 single crystalline nanowire by VLS
mechanism. We found that a VLS growth of TiO2 nanowires
can emerge intrinsically only within a quite narrow range ofmaterial ﬂux. Interestingly, our rutile-TiO2 nanowires preferentially grow along ⟨001⟩ direction, which diﬀers from a typical⟨110⟩ oriented growth in TiO2 nanowires formed by the vaporphase method.
Received: September 8, 2012
Revised: October 27, 2012
Published: October 29, 2012
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The Journal of Physical Chemistry C

ArticleFigure 1. (a) SEM image of succsesufully fabricated TiO2 nanowires grown on a sapphire (110) substrate. The image of 60° tilted angle is shown.
(b) TEM image of fabricated TiO2 nanowire with the Au catalyst at the tip. (c) HRTEM and SAED data of the selected area depicted in the dashedline within (b). A growth direction [001] is indicated. (d) EDS data at the selected position A and B in (b). Cu peaks are due to the use of Cu meshfor TEM analysis. This indicates the occurrence of VLS growth by showing the composition of Au catalysts.

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quantitatively the material ﬂux of metal species, ﬁlm depositionsat room temperature were carried out at various laser ablation
conditions in an Ar atmosphere. This is because in the lowtemperature conditions the re-evaporation events at thesubstrate surface are negligible. After measuring the thicknessof obtained thin ﬁlm by a surface proﬁler (Alpha-Step IQ), thematerial ﬂux of Ti was then estimated by using the density ofmetal element.

EXPERIMENTAL SECTION
Nanowire Fabrication. We have fabricated TiO2 nanowires by utilizing a pulse laser deposition technique with Au
metal catalysts. The details of fabrication systems can be seen inelsewhere.17−25 TiO2 nanowires were fabricated onto a sapphire(110) and TiO2 (001) or (110) single crystalline substrates(Crystal Base Co. Ltd.). Before the nanowire growth, an Au
thin layer (∼1 nm) was deposited onto a substrate by dc
sputtering. To supply a vapor source of Ti atom, a Ti metaltarget (∼99.9% purity) was utilized, which was ablated by ArF
excimer laser (λ = 193 nm). The ablation area and frequency
were 0.02 cm2 and 10 Hz, respectively, while the laser energy
was changed from 40 to 80 mJ to vary the material ﬂux of Ti.
The material ﬂux of oxygen was varied by changing an oxygen
partial pressure ranged from 10−4 to 1 Pa in the mixed gas of O2
and Ar under the constant total pressure of 10 Pa. The use of
constant total pressure is important to keep identical transportkinetics of Ti species to the substrate surface from the target.
The typical growth time for TiO2 nanowire was 90 min. During
depositions, a substrate was heated up to 850 °C by resistive Si
heater. After nanowire growth, the substrate temperature wascooled down to room temperature within 30 min. For
comparisons, we have fabricated SnO2 nanowires on Al2O3(110) single crystal substrate and MgO nanowires on MgO(100) single crystal substrate.
Characterization Methods. The surface morphology offabricated TiO2 nanowire was evaluated by a ﬁeld-emission
scanning electron microscopy (JEOL, JSM-7001F) at theacceleration voltage of 30 kV. All images were undertaken froma 60° tilted angle if no speciﬁc notiﬁcation was mentioned. The
crystalline structure and composition of TiO2 nanowire wereanalyzed by a transmission electron microscopy (JEOL, JEMARM200F) with integrated energy dispersive spectroscopy(EDS) at the acceleration voltage of 200 kV. Cu mesh covered
with a carbon ﬁlm was utilized to support nanowire samples for
TEM measurements. Selected area electron diﬀraction (SAED)
is performed to analyze the crystal structure. To examine■

RESULTS AND DISCUSSION
Figure 1 shows the typical microscopy images and structural
data of successfully fabricated TiO2 nanowires on a sapphire(110) substrate. The fabrication conditions are the oxygen
partial pressure of 10−2 Pa and the laser energy of 50 mJ. TEM,
SAED, and EDS measurements reveal that the fabricated TiO2
nanowire is a single crystalline nanostructure with the presenceof Au catalyst at the tip, highlighting the occurrence of VLS
growth for the present TiO2 nanowires. SEM image shows thepreferential growth of nanowires along 2-fold symmetricorientation. The cross angle between the nanowire growth
direction and a sapphire (110) surface was found to be around57°. This angle almost corresponds to an angle between ⟨001⟩
direction and {101} facets57.2° in rutile TiO2. This trend
can be interpreted in terms of the small lattice mismatch(0.55%) between rutile TiO2 (101) and sapphire (110), as
illustrated in Figure 2. This result implies the ⟨001⟩ oriented
growth direction of obtained TiO2 nanowires. The ⟨001⟩
growth direction of nanowires can be directly conﬁrmed byanalyzing HRTEM image and SAED data, as shown in Figure
1c. The averaged lattice space in HRTEM image along the
growth direction is 0.29 nm, which is almost consistent with thed-value (0.296 nm) of rutile TiO2 (001) plane. In addition, the⟨001⟩ oriented growth of our TiO2 nanowires is further
validated by performing an epitaxial growth of TiO2 nanowires
on a TiO2 single crystal substrate, as shown in Figure 3. In thecase of TiO2 (001) substrate (Figure 3a), all TiO2 nanowires
were grown only along the perpendicular direction to the24368

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The Journal of Physical Chemistry C

Articlenanowires has been mainly observed for hydrothermal
synthesis,10−12 where the side wall of nanowires is obviously
surrounded by liquid. One possible scenario, to explain thediscrepancy between our ⟨001⟩ growth direction andcommonly observed ⟨110⟩ growth direction of TiO2 nanowires
formed by the vapor-phase method, is the nanowire diameter
dependence of a minimum free energy of growth systembecause our TiO2 nanowires show relatively small diameters
less than 10 nm, which are smaller than previously reported size
rangestypically above 50−100 nm. The diameter dependence
on growth orientation has been experimentally and theoretically investigated in Si nanowire growth.30,31 In VLS grown Si
nanowires, ⟨111⟩ orientation is the most observable growth
direction; however, as the nanowire diameter decreases, ⟨112⟩and ⟨110⟩ oriented growth direction emerge especially below
20 nm diameter.30 This trend of altering the growth direction
has been interpreted in terms of a minimum free energy in all
interfaces, including LS interface, VS interface at the side wall,and LV interface of the metal catalyst.23 If the similar eﬀect
exists even for a VLS growth of TiO2 nanowire, the growth
direction might be altered from LS interface dominated ⟨110⟩
growth direction to VS interface dominated ⟨001⟩ growth
direction. To validate this scenario, we have examined the metal
catalyst diameter dependence on the growth direction of TiO2
nanowires, as shown in Figure 4. We have varied the thickness

Figure 2. Schematic illustration of the orientation relationship of
epitaxial grown TiO2 nanowires on sapphire (110) substrate.
Nanowires grow along rutile ⟨001⟩ direction with an epitaxial planeof (101) with (110) of Al2O3 substrate, which exhibits the 2-foldsymmetry of nanowire growth.

Figure 3. (a) Top view of TiO2 nanowires grown on rutile TiO2 (001)substrate. The inset shows the nanowire structure from a 60° tilted
observation. (b) Top view of TiO2 nanowires grown on rutile TiO2(110) substrate. (c) Schematic illustration of growth directions ofTiO2 nanowires grown on rutile TiO2 (001). (d) Schematic illustrationof growth directions of TiO2 nanowires grown on (110) substrates.

substrate surface. On the other hand, in the case of TiO2 (110)substrate (Figure 3b), nanowires grew along the direction
parallel to the substrate surface. Thus, all these experimental
results consistently demonstrate that the present TiO 2
nanowires grow along ⟨001⟩ direction with mainly the {110}facets of nanowire surface. Interestingly, this ⟨001⟩ oriented
growth direction of TiO2 nanowires diﬀers from a typical ⟨110⟩
oriented growth direction of TiO2 nanowires formed by thevapor-phase method.13,15,16
Since a surface energy of {110} plane is the lowest in TiO2
crystal planes, e.g., {110} (0.47 J/m2) < {100} (0.60 J/m2) <{101} (0.95 J/m2) < {001} (1.21 J/m2),26−29 the previously
reported ⟨110⟩ growth direction seems to be consistent with ascenario based on a minimum free energy at the growthinterface (i.e., liquid−solid (LS) interface for VLS growth).
Clearly, this existing scenario is not applicable to our ⟨001⟩
oriented TiO2 nanowires. Since the side wall planes of our TiO2
nanowires are mainly {110}, as shown in TEM, SAED, andepitaxial data, a scenario based on a minimum free energy at theside wall surface of nanowires (i.e., vapor−solid (VS) interface)
seems to be able to explain our present results with {110}surfaces. This ⟨001⟩ oriented growth direction of TiO2

Figure 4. (a, b) Eﬀect of catalyst diameter size on the growth
direction. SEM images of TiO2 nanowires grown on rutile TiO2 (110)substrate. The data of Au deposition time 3 and 7 s are shown for thecomparison. Insets show the magniﬁed top views. (c, d) SEM imagesof Au catalysts on TiO2 (110) substrate when varying Au catalyst sizes.
(e) Schematic illustration of the catalyst diameter dependence on thegrowth direction of VLS grown TiO2 nanowire.

of Au catalysts to change the catalyst diameter, as shown in
Figure 4c,d. In Figure 4b, when increasing the diameter of Au
catalysts, the TiO2 nanowires grown in TiO2 (110) substrate
were grown along the direction perpendicular to the substratesurface, which clearly diﬀers from the results for smaller sized
Au catalysts in Figure 4a. This result demonstrates that
increasing the catalyst diameter alters the growth direction of24369

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ArticleFigure 5. (a−d) SEM images of TiO2 nanowire grown under various laser energies of (a) 40, (b) 50, (c) 60, and (d) 70 mJ, with a ﬁxed oxygen
partial pressure of 1 × 10−2 Pa. (e−h) SEM images of SnO2 nanowire grown under the laser energies of (e) 40, (f) 50, (g) 60 and (h) 70 mJ, with aﬁxed oxygen partial pressure of 1 × 10−1 Pa. (i−l) SEM images of MgO nanowire grown under the laser energies of (i) 40, (j) 50, (k) 60, and (l) 70
mJ, with a ﬁxed oxygen partial pressure of 1 × 10−2 Pa.

TiO2 nanowires from ⟨001⟩ to ⟨110⟩ direction. As illustrated in
Figure 4e, for relatively small diameter sizes of catalysts, having{110} VS interface with {001} LS interface (i.e., growth
direction for VLS growth) is the most preferred to minimize a
total free energy, whereas for larger size ranges ⟨110⟩ growth
direction can most eﬃciently minimize a total free energy due
to large LS interface areas. Thus, our present results as to ⟨001⟩
growth direction can be interpreted in terms of a growth mode
dominated by a minimum free energy at a nanowire surface.
Next we show that above successful VLS growths of TiO2
nanowires require a precise control of material ﬂux comparedwith conventional VLS oxides. The term “VLS oxides” isdeﬁned as oxide materials whose nanowires are easily grown by
VLS mechanism. Figure 5a−d shows the eﬀect of metal species
ﬂux (by varying the laser energy) on the morphology variationof TiO2 nanostructures deposited onto a sapphire (110)substrate. The fabrication temperature is 850 °C, and theoxygen partial pressure is 10−2 Pa. For comparison, results of
SnO2 nanowires on Al2O3 (110) substrate under 10−1 Pa ofoxygen partial pressure and MgO nanowires grown on a MgO(001) substrate under 10−2 Pa of oxygen partial pressure arealso shown in Figure 5e−h and Figure 5i−l, respectively. TiO2
nanostructures exhibit a nanowire-like morphology above thelaser energy of 50 mJ (Ti ﬂux around 5.0 × 1016/cm2 s) and theoxygen partial pressure of 10−2 Pa. The occurrence of VLS
growth was already discussed in Figure 1b as a TEM image withthe presence of catalyst at the tip. Below the oxygen partialpressure of 10−2 Pa, only ﬁlm-like morphology emerged andany nanowires were not observable. The diameter of nanowires
tends to be larger as increasing the material ﬂux of Ti, as can beseen in Figure 5c,d. The nanowire morphology tends to be acone shape from pillar shape. This tapering is mainly caused by
the occurrence of vapor−solid (VS) growth,32−37 which is

detrimental for a VLS growth. In contrast to these TiO2
nanostructures, MgO nanostructures showed only the nanowire
morphology for all material ﬂux conditions employed. The
growth window of SnO2 nanowires is wider than that of TiO2
nanowires but narrower than that of MgO nanowires. Thus, a
VLS grown TiO2 nanowire can only emerge within a quite
narrow range of material ﬂux compared with conventional VLS
oxide−SnO2 and MgO nanowires. To specify more quantitatively such narrow window of material ﬂux for VLS grown TiO2
nanowires, Figure 6 shows the nanowire density mapping whenvarying Ti ﬂux and oxygen partial pressure. The mapping dataclearly demonstrate the narrow window for a VLS growth of
TiO2 nanowires, and therefore the precise material ﬂux control
has been required.
Here we question what essentially determines the narrowwindow of material ﬂux for a VLS growth of TiO2 nanowires
and also why there is a huge diﬀerence among TiO2, SnO2, andMgO on the material ﬂux dependence of VLS growth. Within
the framework of VLS principles, an occurrence ofcondensation at a vapor−solid interface (i.e., VS growth) is
highly detrimental to promote only a VLS growth.38 This isbecause an occurrence of VS growth prevents an area selective
crystal growth underneath catalysts. Our previous theoretical
study for VLS has identiﬁed that a material ﬂux plays a majorrole on emerging only VLS growth mode rather than VS
growth due to a diﬀerence between a LS interface and a VS
interface on a critical nucleation size.38 Therefore, weinvestigate a VS growth rate by measuring a ﬁlm thickness inthe absence of catalysts to extract the nature of VLS grown
TiO2 nanowires. Figure 7a,b shows measured VS growth rates
when varying a material ﬂux including a Ti ﬂux and an oxygen
partial pressure. In addition, a comparison between TiO2 andMgO is also shown to manifest the material dependence.
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ArticleFigure 7. (a) Comparison between TiO2 and MgO on the ﬁlm growthrate (VS growth rate) when varying a ﬂux of metal species at constanttemperature of 850 °C and constant oxygen pressure of 10−2 Pa. The
VLS growth rate of TiO2 nanowires is also shown. (b) Comparison
between TiO2 and MgO on the ﬁlm growth rate (VS growth rate)
when varying an oxygen partial pressure at constant temperature of
850 °C and constant metal ﬂux of (1.8−2.1) × 1017 cm−2 s−1. In both(a) and (b), the gray area corresponds to the optimal nanowire growth
window, where the TiO2 nanowire density is over 15 nanowires/μm2.
On the contrary, MgO nanowires are formable all over theexperimental conditions.

vapor pressures of their metal elements are also relatively high.
On the other hand, the occurrence of VS growth for TiO2
system strongly limits a window of material ﬂux for VLS
growth. This is because increasing a material ﬂux promotes a
VS growth due to an enhanced supersaturation at a VS
interface, which results in suppressing a VLS growth via
preventing a material collection into catalysts. In an ideal VLS
growth, since a VS growth is negligible, increasing a material
ﬂux can only contribute a supersaturation at a LS interface,allowing an area selective crystal growth underneath catalysts.
On the other hand, in the coexistence of VS and VLS growths,
optimizing strictly a window of material ﬂux is essential torealize a VLS growth rate higher than a VS growth rate. Thus,
controlling a material ﬂux is intrinsically required to realize a
VLS growth of TiO2 system in the presence of VS growth.
These comparisons between TiO2 system and conventional
VLS oxides highlight that the presence of VS growth in TiO2
system due to the intrinsic low equilibrium vapor pressure of Ti
can reasonably explain why there is a narrow window ofmaterial ﬂux for a VLS growth of TiO2 nanowires. These
implications also propose that increasing an operation temperature might be a promising way to suppress a VS growth of
TiO2 and enhance a VLS growth. Another possibility might be
the use of metal−organic source with a higher equilibrium
vapor pressure for Ti supply. Thus, our results present a
strategy based on the fundamental principles rather than a ruleof thumb to design and control a VLS growth of single
crystalline TiO2 nanowires.

Figure 6. Nanowire density mapping data of (a) TiO2, (b) SnO2, and(c) MgO nanowires when varying the supply of metal species andoxygen partial pressure. The gray points indicates the conditions which
we performed experiments. The comparison clearly highlights that a
VLS growth of TiO2 nanowires requires a much narrow range ofmaterial ﬂux compared with typical VLS oxide−SnO2 and MgO.

The clearest feature in Figure 7 is the absence of a VS growthin MgO for all experimental conditions whereas a VS growth of
TiO2 clearly exists. In other words, a condensation of MgO at
VS interface can be avoidable, while suppressing condensation
for TiO2 at VS interface seems to be rather diﬃcult withincurrent experimental ranges. This remarkable discrepancy is
due to a large diﬀerence between Ti and Mg on theirequilibrium vapor pressures. For example, for current
experimental conditions, 850 °C, a equilibrium vapor pressureof Ti is 3.1 × 10−10 Pa, which is much lower than theequilibrium vapor pressure of Mg, 9.4 × 103 Pa. In fact, theabsence of condensation process at the VS interface for Mg
source allows us to fabricate VLS grown MgO nanowires for
relatively wide range of material ﬂux in terms of the VLS
principle.38 This must be also same for other conventional VLS
oxides including ZnO, SnO2, and In2O3 because the equilibrium■

CONCLUSIONS
We have demonstrated a critical role of material ﬂux on a TiO2
single crystalline nanowire by the VLS mechanism. We showthat a VLS growth of TiO2 nanowires can emerge only within aquite narrow range of material ﬂux, which is a sharp contrast to
typical VLS oxides including MgO, SnO2, In2O3, and ZnO,
where their available material ﬂux ranges are much wider. We
reveal that a condensation of Ti atoms at a vapor−solidinterface, which is detrimental for VLS, is essential to explain anarrow window of material ﬂux for TiO2 nanowires. In
addition, we found that our rutile-TiO2 nanowires preferentially
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The Journal of Physical Chemistry C

Articlegrow along ⟨001⟩ direction, which interestingly diﬀers from atypical ⟨110⟩ oriented growth in previous reports. The ⟨001⟩
growth direction can be interpreted in terms of a growth mode
dominated by a minimum free energy at a nanowire surface
when a TiO2 nanowire diameter is small.

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AUTHOR INFORMATION

Corresponding Author*E-mail: yanagi32@sanken.osaka-u.ac.jp (T.Y.); kawai@sanken.
osaka-u.ac.jp (T.K.).
Notes

The authors declare no competing ﬁnancial interest.

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ACKNOWLEDGMENTS
This study was in part supported by a Grant-in-Aid for
Scientiﬁc Research on Innovative Areas [20111004] from the
Ministry of Education, Culture Sports, Science and Technology(MEXT) of Japan. A part of this work was supported by “LowCarbon Research Network (Handai satellite)” of Ministry of
Education, Culture, Sports, Science and Technology (MEXT),
Japan. F.Z., B.X., G.M., and Y.H. were supported by NEXT.
T.K. and S.R. were supported by the FIRST program.

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