Synthesis of mesoporous titania film

The mesoporous titania films are obtained from diblock copolymer template assisted sol-gel synthesis. There are two titania precursors for the sol-sol-gel chemistry. One is TTIP, which is applied to fabricate titania films at high temperature. The other is EGMT, a titania precursor for achieving working photoanodes at low temperatures.

Sol-gel synthesis with TTIP

The formation of the mesopores in titania films relies on the self-assembly of the template polymer PS-b-PEO in a so-called good-poor solvent pair. However, the sol-gel-synthesized titania films from the precursor TTIP normally suffer from a low crystallinity with a low electron conductivity even after long-term calcination at 450 ^◦C [19]. To overcome the shortcomings pre-synthesized crystalline titania nanoparticles are introduced into the framework of meosoporous titania films. The nanoparticles are synthesized by the group of Prof. Dina Fattakhova-Rohlfing from Ludwig-Maximilians-Universit¨at M¨unchen. The detailed synthsis procedure is described in reference [107, 108]. The enhancement of the conductivity of nanoparticle incorporated titania films is shown in the literature [109].

There are two routes for producing mesoporous titania films in this work.

For the first route, the pre-weighed polymer PS-b-PEO is dissolved in 1,4-dioxane (C4H8O2), which is achieved by being stirred with a magnetic bar for 30 min (800 rpm).

1,4-dioxane is a good solvent, i.e., non-selective solvent for both PS and PEO blocks.

Separately, TTIP is added to ethanol under 800 rpm stirring. 10 min later, a mixture of acetic acid, DI water and ethanol is added in drop-wise with a pipette. 30 min later, the preprepared polymer solution is added. Here, ethanol is a selectively poor solvent for the PS block. As a result of the introduction of ethanol, polymer micelles with PS cores and PEO coronas are formed, and the titania species are bound to PEO chains through coordination bonds [110]. Finally, the solution is stirred at 800 rpm for 20 h. For the nanoparticle suspension, the pre-weighed nanoparticles are added into the polymer solution and then stirred 20 h at 800 rpm until the nanoparticle are dispersed homogeneously in the polymer solution. After 20 h, the prepared sol-gel solution is mixed with the nanopartilce suspension under vigorous stirring. Afterwards, the whole solution is ultrasonicated for 5 min to mix titania-sol and nanopartiles homogeneously in the solution before being deposited onto substrates as described in section 4.2.3. After film deposition, a high temperature sintering (section 4.2.2) is required to obtain the final mesoporous titania films.

For the second route, a binary azeotrope of toluene and 1-butonal is used as good-poor solvent pair, where toluene is the good solvent and 1-butonal is the good-poor solvent for the polymer template PS-b-PEO. The azeotrope with an approximate composition of 72.2 wt% toluene and 27.8 wt% 1-butonal has a constant boiling point of 105.5^◦C, which allows simultaneous evaporation of the solvent pair during film deposition, i.e., the fraction of good and poor solvent remains constant. Therefore the driving force of polymer self-assembly is unchanged during solvent evaporation. Particularly, this is crucial for spray deposition, which is a slow process of solvent removal as compared to spin coating. The

self-assmbly of the polymer template may change during slow solvent evaporation if the good-poor solvent pair is not an azeotrope. The polymer template PS-b-PEO is dissolved in an azeotrope mixture of toluene and 1-butonal by stirring at 800 rpm for 30 min.

Afterwards, TTIP is added precisely via a pipette. After another 30 min stirring, HCl (8 M) is added drop-wise under vigorous stirring. The resulting solution is aged for 20 h at room temperature. The procedure for nanoparticle incorporation is unchanged, except using toluene instead of 1,4-dioxane for the nanoparticle suspension. Mesoporous titania films are synthesized by different deposition methods and subsequent high-temperature process. A schematic overview of both routes is displayed in figure 4.7.

Figure 4.7: Schematic illustration of fabricating mesoporous titania films with incorporated nanoparticles. It involves several steps: polymer template assisted sol-gel synthesis using the titania precursor TTIP, film deposition and high-temperature calcination.

Sol-gel synthesis with EGMT

Mesoporous titania films obtained from sol-gel synthesis of EGMT can be directly used as photoanodes for solar cells without high-temperature processing. The EGMT powder is weighed in a pre-cleaned glass vial before adding the required amount of HCl (12 M), and then the mixture is put into an ultrasonic bath for 10 min until the solution is clear.

Separately, the polymer template PS-b-PEO is dissolved in dimethylformamid (DMF) by stirring (800 rpm) for at least 30 min. Afterwards, the polymer solution is carefully transferred to the glass vial with the EGMT solution by a pipette. The final mixture is

stirred at 90^◦C for 30 min. A clear solution with the color of pale yellow is obtained, which is then stirred for another 24 h after cooling down to room temperature The weight ratios in the final solution are 20:1:1:2 of DMF:PS-b-PEO:EGMT:HCl. A schematic overview of the hydrolytic sol–gel route at low temperature is shown in figure 4.8a. After 24 h aging the solution is deposited onto substrates and UV irradiation is subsequently used to remove the PS-b-PEO template. Furthermore, the obtained sol-gel solution can be mixed with nanoparticle suspension under vigorous stirring, as illustrated in figure 4.8b.

Figure 4.8: Schematic overview of the sol-gel chemistry using EGMT as titania precursor.

Annealing and calcination

After film deposition, annealing of the resulting TiO2/PS-b-PEO composite films is per-formed at a temperature above the Tg of PS. On one hand, the annealing step improves the self-assembly of the polymer template. On the other hand, it completely removes residual solvent. The composite film is more stable under ambient conditions without the existence of solvents, which is beneficial for further film characterization.

The annealed composite films made from TTIP require high-temperature calcination to burn away the diblock copolymer template and to transform titania from its amorphous phase to the crystalline anatase phase. After calcination crystalline mesoporous titania

films are achieved, which are desirable for photovoltaic applications [10,111–114]. In this work, the calcination is performed in a tube furnace (GERO or Hereaus instruments) under ambient conditions. A calcination protocol of heating ramp, calcination tempera-ture, calcination time can be set according to different requirements. If not specifically metioned, a standard calcination scheme in the present thesis consits of a heating ramp of 5 ^◦C/min and a calcination temperature of 500 ^◦C for 2 h.