Solution Phase Approach to Tio2 Nanostructures

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 95120- 6099

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 mil- lion metric tons/yr), with catalysts, ceramics, coated fabrics and tex- tiles, floor coverings, printing ink, and roofing granules constituting other major uses on a consumption basis.[2] In terms of more high-

-tech domains, TiO2 is a material of interest in photonics,[3] membranes,[4] biological supports,[5-7] sensing,[8-9] electrochromics, 2

[10] and environmental applications[11] including 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 vi- tal 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 p- type 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 re- duce 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 opti- mize 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 arising 5 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 han- dling, 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 can be ob- 6 tained.[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 block-copolymers [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 pre- formed nanosized templates or templates that self-assemble during the film forming process such as in evaporation-induced self-assembly (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, decompo- sition/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, com- position of the calcination atmosphere, and confinement can be elu- cidated 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 self- assembly 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 ad- justed 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 support- ed 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 9 nanoparticles 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 sug- gested to be H2Ti3O7. Nanowires were calcined to 500ºC to trans- form 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] Polycrys- talline 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, reach- ing 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 co- workers 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(viny- lalcohol) (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 imprinting tools. 11

[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 re-

moved 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 magnifica- tion 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 cross-sectional 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 TiO2 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. Stack- ing 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 tem- 14 plated 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 block- copolymer 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

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 connect- ed 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 infilter- ation 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 de- scribed 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 TiO2 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 Hydro- thermal 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 single-crystal ZnO nanowires showed that the faster electron transport afforded by the single-crystalline nanowires led to improved charge-collection 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 sig- nificant body knowledge garnered from fundamental surface science research. The TiO2 surface, especially the rutile and anatase poly- morphs, 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 single- crystalline 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 free- standing 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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