Templated titania films with meso- and macroporosities

Materials Letters 61 (2007) 2191 – 2194 www.elsevier.com/locate/matlet

Templated titania films with meso- and macroporosities Lai Qi, Dunbar P. Birnie III ⁎ Materials Science and Engineering, Rutgers University, Piscataway, NJ 08854, USA Received 29 July 2006; accepted 22 August 2006 Available online 25 September 2006

Abstract Porous TiO2 films with both mesoporosity and macroporosity were fabricated by a templated sol–gel method for applications, such as dyesensitized solar cells (DSC), photocatalysis and catalysis. With the incorporation of differently sized pores, the resultant structures exhibit high surface areas and possess interpenetrating aligned pore channels, which are believed to be beneficial for applications where diffusion of reactants to interior surface can be rate limiting. Both liquid and solid TiO2 precursors can be applied for large area coating in this process. Almost crack-free films were produced by templated coating of pre-synthesized TiO2 nanoparticles. The measured specific surface area and porosity of synthesized films were in the range of 33–137 m2/g and 61–80%, respectively, depending on the size of the selected template. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; Microstructure; Solar energy materials; Porosity; Sol–gel

1. Introduction Dye-sensitized solar cells (DSC) that adopt porous TiO2 film as the photoelectrode demonstrate high solar conversion efficiencies (over 10%) at competitively low costs [1–4]. Under illumination, the photon excited electron–hole pairs disassociate at the interface between TiO2 and adsorbed sensitizers (dye). Free electrons are injected from sensitizers into the conduction band of TiO2; while the holes are conveyed away to the counter electrode by electron transfer from electrolyte to the oxidized sensitizers and subsequently electrolyte diffusion [2]. Decided by this operation mechanism, an ideal TiO2 electrode should possess a large surface area accessible to both dye molecules and redox species in the electrolyte. By interpenetrating a pore network into a TiO2 network, it provides pathways for both charges towards opposite electrodes after their separation. Optimal efficiency is acquired due to balanced charge transportation ability in both directions. Currently, the doctor–blade method [1,5] and screen printing [6] are the most frequently used techniques to prepare nanocrystalline TiO2 films for DSC. After depositing a layer of loosely packed nanoparticles and following sintering at 400– 500 °C, porous nanocrystalline films result due to random ⁎ Corresponding author. Tel.: +1 732 4455605; fax: +1 732 445 3258. E-mail address: [email protected] (D.P. Birnie). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.08.076

packing of particles. The average pore size usually falls in the same range of the particle size. Though this coating technique may offer a strikingly high internal surface area (e.g. a roughness factor of 460 μm− 1) [7], the influence of the randomly tortuous and narrowed pore channels on the diffusion rate of electrolyte is suspected to be negative. As pointed out by Grätzel [6,7], a desired TiO2 structure should possess a high degree of order with mesoporous channels aligned in parallel to each other and perpendicular to the electrode substrate. Templated growth combined with sol–gel coating technique provides approaches to fabricate fine nanostructure of desired features [8,9,15–22] Jiu and Adachi [9] reported templated growth of porous TiO2 films with mixed polymers of Pluronic F-127 and cetyltrimethylammonium bromide. Zukalová [8] reported a similar structure with Pluronic P-123. Both works achieved very high surface areas but with quite small pore diameters, 4–7 nm [9] and 6–8 nm [8], respectively. For the concerns of mass transportation in TiO2 [10] and embodiment of all-solid-state or quasi-solid-state DSC, decreased pore size may present difficulties for complete coating of sensitizers (due to the relatively large molecule size of the ruthenium based sensitizers, ∼ 1.5 nm [11]), the diffusion of redox species, and infiltration of organic hole conductor [12]. Somani [13] and Huisman [14] reported an inverse opal (IO) TiO2 structure for DSC, which was composed of macropores prepared by an assemble–infiltration process. Somani [13] reported improved

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Fig. 1. SEM micrographs of TiO2 films derived from a sol–gel precursor with differently sized templates. (a) 1 μm; (b) 600 nm; (c) 300 nm; (d) cross view of the 600 nm. The coating method was cast-coating after mixing the precursor solution and PS template solution by ultrasonication for 2 min.

performance of IO TiO2 cells with organic hole conductor over the traditional nanocrystalline cells. Inverse opal of TiO2 or other materials have been reported previously [15–22] by a template–infiltration two-step process. The template particles were assembled onto the substrate first; then a metal organic precursor solution was infiltrated by the capillary effect. Because organic metal compounds suffer large volume shrinkage during decomposing to oxides, cracking is inevitable and a complete filling of the voids between the template spheres is difficult. Repeated infiltration is necessary and may take extended time (e.g. days [20] or weeks [17]). In this paper, we report a simple mixing–casting method for templated TiO2 structures with improved film uniformity, less cracking and easier process from previously reported methods. The preparation is based on the self-assembly of polystyrene (PS) spheres in the presence of either TiO2 sol–gel precursor or presynthesized TiO2 nanocrystals. During drying, TiO2 precursors are forced into the voids between assembled PS spheres. The developed structures show aligned macropore channels interpenetrating into the mesopore network between TiO2 nanoparticles, which improves the charge transport or reactant diffusion.

of titanium tetra-isopropoxide (Aldrich, 98%) in distilled water, and then redissolving the precipitates in hydrochloric acid. The clear solution was found to be TiOCl2 [23]. The other Ti precursor (P25 precursor) is a suspension of pre-synthesized TiO2 nanoparticles, which is provided by Degussa (40–50 nm, 40 wt.% in water). The template is aqueous solutions of polystyrene mono-

2. Experimental Two Ti precursor solutions have been tested. One is a transparent liquid (sol–gel prescursor). It was prepared by hydrolysis

Fig. 2. X-ray diffraction pattern of the sol–gel derived porous TiO2. The lines are known anatase TiO2 (JCPDS# 21-1272).

L. Qi, D.P. Birnie III / Materials Letters 61 (2007) 2191–2194 Table 1 Specific surface area and porosity data of the synthesized TiQ2 films

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area was performed with a BET surface area analyzer (Micromeritics, Gemini).

Ave. pore size (nm)

Brunauer–Emmett–Teller (BET) Spec. surf. area (m2/g)

Roughness factor (μm− 1)

Porosity (%)

1000 600 300 160

33.2 60.1 119.1 137.2

53.7 69.7 148.1 197.1

61.5 71 70.4 65.8

sized nanoparticles (160 nm–1 μm), which is purchased from Duke Scientific Inc. (USA). Either the sol–gel or the P25 precursor solution was mixed with the PS suspension by mechanical stirring. While stirring, ethanol (50 vol.%), polyethylene glycol (5 wt.% of TiO2), a small amount of triton X-100 and hydrochloric acid were added sequentially to adjust the viscosity and pH, which are important for the coating uniformity and adhesion. The mixture solution was then coated onto F-doped tin oxide coated glass substrates by either doctor–blade coating or just cast-coating. After drying in air flow for 30 min, the coatings were calcined at 450 °C in air for 30 min to remove the organic contents. The microstructure was investigated by a scanning electron microscope (SEM, Zeiss, DSM 982). The X-ray diffraction was measured with an X-ray diffractometer (Siemens, Daco-Mp). The porosity of films was measured by a mercury porosimeter (Micromeritics, Autopore III). The measurement of the surface

3. Results and discussion Fig. 1 shows the SEM micrographs of the synthesized porous TiO2 from TiO2 sol–gel precursor. From the top views (Fig. 1a–c), it can be seen that the TiO2 forms a three-dimensionally honeycomb-like network with an interpenetrated network of closely packed spherical pores. All the pores are spatially aligned and interconnected with an average size slightly smaller than the size of PS templates. The pore size of synthesized TiO2 porous films can be controlled by adopting differently sized templates with average pore size varying from 1 μm to 160 nm. The cross-section view of the TiO2 film is shown in Fig. 1d. It can be observed that most of the pores in the top layers are selfassembled into a face-centered-cubic structure (A–B–C–A stacking), which is consistent with the top view observation. The pore arrangement in the lower part of the layer is suspected to have lower regularity than that of the upper part, possibly due to the substrate confinement. But all the pores are closely or nearly closely packed. The X-ray diffraction pattern of prepared porous TiO2 film is shown in Fig. 2. After annealing at 400 °C for 1 h, pure anatase phase was obtained without observation of other phases. The primary crystallite size is estimated to be around 45 nm by using the Scherrer equation. Varying the particle size of the template changes the specific surface area (the roughness factor), pore size and porosity of the resultant films. These factors are important for applications, such as DSC, fuel cells, electrochemical batteries, sensors and catalysts. Table 1 lists the specific surface area (SA) and porosity (Pr) data of the TiO2 films derived from

Fig. 3. SEM micrographs of porous P25 films with both macro- and mesoporosities templated by 1 μm PS at different magnifications (a) 1000×, (b) 20,000×, (c) 50,000×, (d) cross-view of a 15 μm thick film.

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surface more accessible to dyes and redox species. Considering the big size of organic hole conductors [12], these templated P25 films show higher potential than mesoporous nanocrystalline films towards efficient all-solid-state or quasi-solid-state junctions.

4. Conclusions We report a simple method for the preparation of TiO2 films with both meso- and macroporosity. The prepared films contain aligned and closely packed pore networks. Not only high porosity, large pore size and high surface area could be achieved at the same time, but the flexibility to customize those parameters also makes the synthesized materials promising for some applications. References Fig. 4. Pore size distribution of templated P25 films by mercury porosimetry (open triangle: non-templated P25; solid triangle: P25 templated with 1 μm PS). Inset: the isotherms of both films.

sol–gel precursors with different template sizes. The samples were powders scraped off from their substrates and measured by BET surface area analyzer and mercury porosimeter. With size decreasing from 1 μm to 160 nm, the specific surface area increases from 33.2 m2/g to 137.2 m2/ g. The roughness factor (Rf) is calculated by: Rf = 4.2 × (1 −Pr) ×SA, where 4.2 is the density of fully crystallized anatase with the unit of g/ cm3. The porosities of films with 300 nm and 600 nm pore size are higher than 70%. Another advantage for the large pore size of this honeycomb TiO2 is its potential to work with polymer electrolytes to form an all-solid-state junctions [2,24], because polymer electrolytes usually have high viscosity and large molecular size, and thus, are difficult to be filled into tiny pores. We reproduced the honeycomb TiO2 structure with pre-synthesized TiO2 nanocrystals (Degussa, P25, ∼50 nm). Fig. 3 shows SEM micrographs of porous TiO2 films of P25 nanoparticles templated by 1 μm PS nanospheres. With the P25 precursor, we observed very few cracking as shown in Fig. 3a, which might be due to the less shrinkage of the P25 particles during heating. At high magnification (Fig. 3b,c), individual P25 particles and the mesoporosity between them are distinguishable. The other space inside the film forms an interpenetrating and aligned macropore network. Mercury porosimeter was used to measure the pore size distribution, which is shown in Fig. 4. Without template, P25 formed nanocrystalline film with an average pore size of 32 nm and very narrow pore sized distribution. Templated with 1 μm PS, double peaks appear in the pore volume–pore size diagram, which are centered at 35 nm and 520 nm, respectively. The first peak indicates preserved mesoporosity; while, the second peak proves the co-existence of macroporosity from templates. The obvious size reduction of pores (530 nm) from their template (1 μm) is possibly due to the fact that mercury intrusion records actually the size of the pore opening that is significantly smaller than the pore diameter (Fig. 3c). In traditional nanocrystalline films prepared by the doctor–blade method [1–6], the pore spaces are typically tightly constricted and some fraction of closed pore volume is inevitable, which lowers the actual surface area accessible to dyes. In the templated TiO2 films of this work, the large pore size and aligned pore channels make the inner

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