Solution Phase Approach to TiO2
Nanostructures
John D. Bass† and Ho-Cheol Kim†
†
IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, CA 951206099
8.1 Introduction
Titanium dioxide (titania, TiO2) features prominently in a number of
commercialized and next-generation high-tech domains and is, by
consequence, the most well studied metal oxide.[1] Industrially,
TiO2 finds a prominent role mainly (~95%) in pigments (over 5 million metric tons/yr), with catalysts, ceramics, coated fabrics and textiles, floor coverings, printing ink, and roofing granules constituting
other major uses on a consumption basis.[2] In terms of more hightech domains, TiO2 is a material of interest in photonics,[3] membranes,[4]
biological
supports,[5-7]
sensing,[8-9]
electrochromics,[10] and environmental applications[11] including
2
photoelectrolysis of water [12] and catalytic and photocatalytic applications.[13-16] For example, in photovoltaics (PV), TiO2 has
emerged as the material of choice in dye sensitized solar cells
(DSCs), a low cost PV technology that has recently reached commercialization.[17-18] TiO2 has also been employed in other PV
systems, pairing as the n-type semiconductor with extremely thin
CdTe absorber layers,[19] nanocomposite CuIS2 devices, [20-21]
and PbS quantum dots.[22]
In many of these emerging high-tech domains, the ability to control
the structure of TiO2 on the nanometer scales is the driving force behind technology development. In the case of both the DSCs and the
CuIS2 system, the nanostructuration of the TiO2 acceptor layer is vital in achieving any reasonable efficiency.[17-18, 21] The high surface area allows intimate contact between the thin absorbing material or monolayer of charge transfer dye and the TiO2. This reduces or
eliminates the need for carrier diffusion to the interface while maintaining a sufficient quantity of absorber material to achieve high
light absorption. The nanostructuration can increase the available
3
surface area by 1000 times or more for efficient absorption of sunlight.[18]
Indeed, nanostructuring the heterojunction between n-type and ptype materials in photovoltaic devices, Figure 1, has the potential to
completely redefine the relevant landscape of materials while markedly improving efficiency.[23]
Light absorption in a photovoltaic semiconductor generates electron
hole pairs. The minority charge carrier (e.g., electrons for a p-type
absorber), either bound as an exciton to the majority carrier or on its
own, must diffuse to the p-n junction to allow charge separation in
the device. This junction is an interface or interfacial region between
a p-type material and an n-type material. In cases where the distance
to the junction is longer than the diffusion length afforded by the
carrier lifetime, many of the carriers recombine, limiting the output
current. An alternative to bringing the carriers to the junction is, in
effect, to bring the junction to the carriers by nanostructuring the
junction. Nanostructures projecting into the absorbing layer can reduce the diffusion distance while maintaining an adequate optical
4
thickness for light absorption. This allows the use of absorber materials with inherently lower diffusion lengths and/or the use of lower
quality (i.e. cheaper, more accessible, and easier to process) materials, which also tend to suffer from higher recombination rates.
Fig. 1 Schematic representation of a nanostructured heterojunction photovoltaic
device that decouples d, the path length relevant to minority carrier transport
from h, the path length relevant to light absorption
In this embodiment, 1-D structures such as rods, wires, pillars, nanotubes, nanofibers, etc. have unique advantages, such as improved
charge collection [24-26] and lower tortuosity. In other applications,
including photonics, catalysis, and sensing, controlling 3-D structures on one or more length scales is desirable. These structures can
be designed to guide light, maximize the available surface area, or
present a particular crystal polymorph or crystal facet known to optimize the reactivity of the surface.[3, 27-31] Quantum size effects
in TiO2 crystallites can be achieved, though these observations are
observed at small sizes, below 10 nm due to the small Bohr
radius.[32-34] Consequently, novel physical and chemical properties
5
arising from such quantum confinement effects need to be examined
at this small size scale.[35]
8.2Approaches
A variety of methods have been developed to synthesize nanostructured TiO2. Non-solution phase approaches using gas phase precursors have been used to grow a variety of shapes including rods, fibers, and ribbons on substrates at elevated temperature.[36-37] The
most common methods, however, rely on solution phase deposition,
which is the main focus of this chapter. Solution phase synthesis has
a number of advantages in terms of process control, ease of handling, and flexibility. In the simplest manifestation, nanoporous
coatings can be achieved via the deposition colloidal TiO2 nanoparticles. This sort of deposition is compatible with common coating
methods such as doctor blading, screen printing, and spray
coating.[21] The porosity can be controlled by varying the particle
size, the addition of additives, and the drying conditions. Using a
narrow size dispersion of nanoparticles, self-organized particle films
6
can be obtained.[38] These materials show very little mesoporosity
as they are tightly packed together.
8.2.1 Porous Architectures Through Templated Self Assembly
The templating of nanostructured TiO2 by self-assembly using structure-directing materials such as surfactants [39] and blockcopolymers [40-43] is a rich field that has been intensively investigated in recent years. Much is now known about the assembly [44]
and crystallization [45-46] processes that yield polycrystalline TiO2
with highly-ordered mesoporous structure. Films can be applied via
a variety of coating techniques including dip-coating, spray coating,
meniscus coating, and spin-coating. Templates used are either preformed nanosized templates or templates that self-assemble during
the film forming process such as in evaporation-induced selfassembly (EISA).
Apart from the initial assembly of the organic and inorganic phases
into nanostructured domains, thermal post-treatment to condense the
network, removed the organic phase, and induce crystallization has
7
emerged as a critical step in determining the final characteristics
nanostructured TiO2 materials.[47-49] These thermally driven processes, covering dehydration, condensation, densification, decomposition/pyrolysis, crystallization, and sintering, have recently received
in-depth attention using in-situ techniques including SAXS/WAXS
investigations[46, 50] and ellipsometry.[45] With in-situ ellipsometry, for example, the effect of the precursor solution, substrate, composition of the calcination atmosphere, and confinement can be elucidated in terms of the formation of the TiO2 matrix (Figure 2).
Moreover, characterization of chemical processes occurring within
the pores, such as the kinetics of pyrolysis of the template, is also
accessible.
Fig. 2 In-situ ellipsometry shows the thermal evolution by way of the change in
index of refraction of a nanostructured TiO2 thin film prepared via templated selfassembly as compared to a dense TiO2 film. Adapted from [44]
Recent efforts have been put forth to synthesize vertically oriented
structures with open accessibility and direct conduction pathways to
the surface. Structural transformation of spherical domains upon
8
heating to grid-like[44] and pillar-like structures,[51] as well as tilted cylindrical arrays[52] have been achieved using the Pluronic™
family of ethylene oxide and propylene oxide block copolymers.
The phase behavior in these systems is fairly predictive, and is adjusted by changing the volume ratio between the copolymer and inorganic components.[53] These yield pores in the size range of 5 to
12 nm depending on the starting material and the degree of sintering.[45] In a similar size range (10 nm), a bicontinuous double gyroid TiO2 structure was formed in a multi-step process involving self
assembly of a block copolymer, selective removal of one block, and
electrofilling with
a
TiO2
precursor.[54]
Unlike in
silica
systems,[55] the control of cylindrical pore orientation normal to the
supported film surface and with variable pore diameters ranging up
to 20 nm remains an open challenge for TiO2.
8.2.2 1-D Structures from Anodization
A common and well investigated approach to 1-D TiO2 structures is
through the oxidation of Titanium (Ti) foil. Nanotubes and nanowires have been prepared on Ti supports seeded with TiO2 nano-
9
particles via hydrothermal synthesis under basic conditions.[26, 56]
For the nanotubes, outer tube diameters are around 12 nm, while the
inner diameter is ~ 4 nm. Tubes can be grown up to 10 μm and seem
to form from folded sheets. Their diffraction structure is suggested
to be H2Ti3O7. Nanowires were calcined to 500ºC to transform them
to the anatase phase and used in DSCs.[26]
In a technique that finds parallel in the anodization of Al, TiO2 nanotubes can be formed by the oxidation of titanium in the presence of
fluoride-based electrolytes under anodic conditions.[57] Polycrystalline transparent TiO2 nanotube arrays prepared in this manner have
been investigated in DSCs.[58] Improved charge-collection efficiencies over nanoparticle systems was reported.[25] Using electrochemical methods, these structures can be prepared directly on transparent
conducting oxides with lengths up to 33 μm long, reaching DSC efficiencies of nearly 7%.[59]
8.2.3 Imprinting and Molding
10
The development and application of top down nanoimprint lithography approaches has evolved to support the generation of submicron
oxide features from sol-gel precursors.[60-61] Whitesides and
coworkers demonstrated submicron patterning of titanium silicates
using poly(dimethyl siloxane) (PDMS) soft lithography.[61] Extension of this technique using perfluoropolyether (PFPEs) elastomers
that provide improved filling and release characteristics has been
used in the DeSimone group to pattern TiO2 and other oxides and
mixed metal oxides.[60] Sub 200 nm features and aspect ratios of up
to 2.5 can be achieved.[60]
The use of sacrificial templates such as water soluble templates for
transfer molding (TM) is another approach that can yield submicron
patterned
features.[62-64]
In
this
approach,
water
soluble
poly(vinylalcohol) (PVA) templates are used for pattern transfer as
shown schematically in Figure 3. Daughter templates are prepared
en masse from hard template masters such as large area lithographically patterned silicon. As illustrated in Figure 3, this approach is
amenable to creating nanostructured TiO2 without expensive im-
11
printing tools.[65] Here a solution containing photosensitive TiO2
precursor is spun onto a pre-patterned PVA template. The photosensitive TiO2 precursor is an oligomeric titanate (OT) prepared from
an acetylacetone chelated titanium alkoxide.[66-67] The coated
template is bonded to a substrate and exposed to long wavelength
UV radiation to induce partial condensation of the precursor. Exposure to warm water dissolves the PVA leaving a partially condensed
amorphous network of nanostructured TiO2. Crystallization to the
anatase phase and removal of residual organics is accomplished by
thermal treatment to 450 ºC.
Fig. 3 Microtransfer molding using a water soluble PVA template applied to the
nanostructuration of TiO2
Figure 4 shows SEM micrographs of nanoscopic TiO2 posts prepared by TM method using a PVA template. The PVA template was
removed by water and TiO2 was calcined at 450C. As shown in the
micrograph, the TM technique provides well defined nanostructures
over large area without defects. The inset shows higher magnification of the TiO2 posts that have dimensions of 65 nm in diameter, 90
12
nm in height, and approximately 200 nm in center-to-center distance.
Fig. 4 Defect free TiO2 nanoposts prepared using PVA templates.
High aspect ratio nanoposts are desirable partly due to increase in
surface area, greater penetration into the surrounding media, and for
optical (e.g. antireflection) and photonic applications. In order to
prepare high aspect ratio TiO2 posts by TM, it is necessary to prepare high aspect ratio masters for generating PVA templates. A high
aspect ratio silicon master can be prepared using conventional lithography and plasma etching of silicon using oxide as an etch
mask, Figure 5 (a). Figure 5 (b) shows a top-view and a crosssectional SEM micrograph of high aspect ratio TiO2 posts derived
from this type of silicon master. With this technique, TiO2 posts of
approximately 360 nm in height and 70 nm in diameter (aspect ratio
~ 5) can be achieved over large areas, Figure 5 (b).[65]
13
Fig. 5 (a) Silicon master and (b) hexagonally arranged TiO 2 posts prepared
through the PVA template method. MT followed by calcination allows for the production of TiO2 posts over large areas
High aspect ratio structures through MT can also be achieved
through successive stacking of templated layers. This is shown in the
most trivial case with a two layer cell structure, Figure 6 (a) and (b),
that can be made in either a closed-cell or open-cell morphology.
The difference between the two structures is the concentration of the
titania precursor solution. Open-cell structures are prepared using
low concentration solutions that, when spun on, just fill the 250 nm
mesh PVA template. Closed-cell structures are prepared at higher
precursor concentrations where the amount of fill material exceeds
the template volume. In this situation, overfill forms a continuous
sheet between the templated structure and the previous layer. Stacking can be continued to create quite thick films, Figure 6 (c), and different templates can be combined to form novel structures, Figure 6
(d). These structures are possible because under normal conditions
the PVA near the templated area is crosslinked by the titania precursor solution, becoming resistant to dissolution. This protects the
14
templated features against infill by successive layers. The residual
PVA is removed later by calcination.
Fig. 6 SEM micrographs of two-layer (a) closed-cell and (b) open-cell structures
prepared from the same template but using different concentrations of fill solution.
Stacking can be used to create successively thicker films such as (c), a five layer
structure. Mixed templates (d) can also be used to create interesting stacked structures
TiO2 posts normal to the surface can also be generated by sol-filling
in high aspect ratio templates. The preparation of well-defined high
aspect ratio templates, however, becomes nontrivial as feature sizes
approach tens of nanometers and below. One promising method to
generate such templates is using oxygen plasma etching of a polymeric transfer layer using a block copolymer pattern mask, Figure
7.[66] Reproducible templates with diameters from 8 to 25 nm and
layer thickness of hundreds of nanometers can be reliably achieved
on a variety of surfaces. This pattern layer/transfer layer motif is
common in photolithography; using photopatternable materials that
contain high etch contrast materials such as silicon. Here, the blockcopolymer pattern layer provides a self-assembly based organization
15
beyond the current resolution of traditional photolithography. To the
block copolymer mixture is added a silicon containing organosilicate
[OS, or polymethylsiloxane (PMS)] resin that segregates into one of
the domains, providing high etch contrast. Controlled oxygen plasma etching followed by removal of the residual organosilane yields
the high aspect ratio polymeric template shown as an inset in Figure
7.
Fig. 7 Preparation of high aspect ratio polymer templates. (inset) Cross-sectional
SEM micrograph of polymer template with 15 nm diameter holes
The high aspect ratio polymeric template can be used for molding
TiO2 or other materials. Partially chelated TiO2 precursor is spun onto the template and allowed to fill in the pores by capillary action at
elevated temperature (190ºC) shown in Figure 8 (a). Partial lift off of
this layer demonstrates that the TiO2 precursor infiltrates down to
the bottom substrate, as a clear pattern of TiO2 bumps can be seen
[Figure 8 (b)]. Calcination to 450ºC of the TiO2 infiltrated template
removes the polymeric template leaving behind TiO2 posts connected with a thin porous TiO2 top layer as shown in Figure 8 (c). The
16
interconnected thin top layer can be removed by using low-voltage,
broad beam ion milling prior to calcination [Figure 8 (d)].
Fig. 8 (a) cross-sectional SEM micrograph of nanoporous template after infilteration of TiO2 precursor, (b) cross-sectional SEM micrograph of sample after partial lift off, (c) cross-sectional SEM micrograph of TiO2 nanoposts after thermal
treatment at 450C, (d) plan view SEM micrograph of TiO2 nanoposts calcined after after gentle ion milling of the sample shown in (a). Adapted from [64]
8.2.4 Templated Electrochemical Synthesis
One clear advantage of the high aspect ratio polymeric template described in previous section is that it can be easily created on a variety of substrates, including conductive substrates. High aspect ratio
polymeric templates on conducting substrates can be combined with
electrochemical deposition to create titania posts by electrochemical
filling. Figure 9 shows a schematic illustration of electrochemical
deposition of TiO2 using an anodized alumina template. Cathodic
TiO2 deposition from acidic solutions has previously been demonstrated.[68-70] This process is attractive in that low cost, easy to
handle titanium oxysulfate precursors can be used instead of alkox-
17
ide or titanium chloride precursors. A similar approach has been
used to deposit TiO2 in self-assembled scaffolds prepared after selective removal of one block of a block-copolymer assembly.[54]
Fig. 9 Schematic illustration of the electrochemical deposition of TiO 2 into nanoporous anodized alumina template. Adapted from [71]
Figure 10 shows SEM micrographs of TiO2 nanoposts generated by
electrodeposition method using a high aspect ratio polymeric template. This approach provides remarkably well defined TiO2 nanoposts of approximately 20 nm in diameter over large area of the
substrate.
Fig. 10 TiO2 posts on a gold coated silicon wafer achieved by electrochemical filling high aspect ratio polymeric template
8.2.5 Single Crystalline 1-D Structures by Solution Phase Hydrothermal Growth
18
Controllable solution phase growth of single crystal 1-D metal oxide
structures on surfaces has been achieved for a variety of metal oxides, most notably ZnO. [72-77] Extremely long ZnO structures, up
to 25 μm, with aspect ratios of 125 can be grown [74] and a high degree of vertical orientation with respect to the substrate can be
achieved.[73]
Single crystalline 1-D structures have potentially a number of advantages over their polycrystalline counterparts. In the domain of
photovoltaics, it is known that mobility of photogenerated electrons
in polycrystalline, porous TiO2 networks is several orders of magnitude slower than in single crystalline materials.[78-79] Trap-limited
diffusion,[79] tortuosity,[80] and local field effects[78] all can serve
to limit electron mobility in polycrystalline, porous TiO2 networks.
In DSCs electron transport through the oxide layer occurs on the order of milliseconds.[74, 81] Work on DSCs fabricated from singlecrystal ZnO nanowires showed that the faster electron transport afforded by the single-crystalline nanowires led to improved chargecollection efficiency.[82-83]
19
Not to be overlooked, the well-defined surfaces of single-crystalline
materials can allow researchers in applied fields to draw on the significant body knowledge garnered from fundamental surface science
research. The TiO2 surface, especially the rutile and anatase polymorphs, is the most thoroughly investigated of the metal oxides.[1]
This is because in comparison with ZnO and other metal oxide materials, TiO2 has emerged as the material of choice for a variety of applications, largely due to the fact that it is inexpensive and stable biologically, chemically, and photochemically.[6, 84]
Solution phase synthesis of single-crystalline TiO2 structures on surfaces is less well developed than that of ZnO, though much progress
has been made recently. Hydrothermal synthesis and characterization of rutile nanorods on glass substrates starting from aqueous solutions of TiCl3 has been reported.[85-87] Typical conditions used
temperatures in the 160ºC to 200ºC range and reaction times of several hours and yielded rods of lengths ~500 nm. Temperatures down
20
to 80ºC could be used, through the reaction times were much longer,
up to 168 hrs.[86] Aspect ratios were in the range of 10 to 20.
Rutile nanorods from alkoxide and TiCl4 precursors can be prepared
directly on transparent conducting oxides such as fluorine-doped tin
oxide (FTO) using two recently developed hydrothermal procedures.[88-90] In one, a biphasic solution of toluene and hydrochloric
acid with both TiCl4 and tetrabutyl titanate was allowed to react at
180 ºC for between 30 min and 22 hrs. Depending on the time, this
yielded rutile rods from 2 to 5 μm with diameters averaging from 10
to 35 nm. This procedure could also be used to grow rods on FTO
coated with a TiO2 prelayer grown from TiCl4 solution and was
shown to be amenable to doping.[88-89] DSC efficiency of 5.02%
was achieved with Ta doping.
A second methodology based on TiCl4 or various alkoxide precursors and a hydrochloric acid solution was also shown to yield singlecrystalline rutile nanorods.[90] Rods of up to 4 μm in length were
grown; diameters were slightly larger than that of rods grown in the
21
biphasic system. Longer rods detached from the surface giving freestanding TiO2 nanorod films. Rods were only observed to form on
FTO coated substrates. This was believed to result from an epitaxial
relation between the FTO substrate and rutile TiO2.
In our hands, this second technique could also be adapted to FTO
substrates covered by TiO2 thin films, as shown in Figure 11. The
TiO2 prelayer was prepared by a spin coating a solution of the titania
precursor Tyzor BTP partially chelated with acetylacetone. Growth
conditions were best between 130 ºC and 150ºC over a period of 15
hrs. The density of the rods could be controlled through the amount
of Ti(OBu)4 precursor used. As evident in the SEM images, the vertical orientation in these systems results mainly from impingement
of the growing nanorods. With a high density of rods, those growing
with a more vertical orientation are less likely to experience arrested
growth from running into a neighbor.
Fig. 11 SEM images of rutile nanorods grown hydrothermally on TiO2 coated
FTO substrate at (a) low and (b) high precursor concentration
22
8.3 Conclusion
Solution based approaches to TiO2 nanostructures represent a very
attractive route to achieving well-defined, controllable structures at
low cost. As we discussed above, a variety of different strategies including self-assembly, anodization, transfer molding, electrodeposition, and hydrothermal growth are being actively pursued, each
showing their own promises and challenges. For each approach
much work remains, especially on device integration and realizing
precise structural requirements for targeted applications. Certainly
excitement and surprises will continued in this rich research field.
23
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