Self Assembly of Honeycomb Tio 2 Coatings by Tea-leaf Extracts as the Dispersoids

Photo catalytic TiO 2 coatings with well defined honeycomb structure were successfully self-assembled on glass sub-strates by a simple " dip and burn sol-gel method ". The crux of this new method is that the starting material for the sol-gel coating (the water solution of ammonium titanium citrate) was " doped " with " tea-leaf extracts ". Tea-leaf extracts were completely dissolved in the water solution of ammonium titanium citrate and nucleate uniformly during the drying procedure of the coating precursor formation. This uniformly dispersed nucleation of the " tea-leaf extracts (tan-nin, catechin, etc.) " were extracted during calcinations and leaving honeycomb-cell like nano-structure in the TiO 2 coatings. Self-assembled cells were uniformly distributed in the TiO 2 coatings and the diameter of the cells was in the range of 200-1000 nm. Fine honeycomb structures were clearly observed with microscopic observations.


Introduction
Growth process for the porous photo-catalytic Titanium-dioxide (TiO 2 ) thin films has attracted significant attention since it is essential for the dye-sensitized solar cells.The porosity of the coatings enlarges the effective surface area of the coatings and enhances the photocatalytic efficiency of the TiO 2 thin films.Coating process capable of providing large-area Sol-gel processes with additives in the coating solutions have been reported to be successful for obtaining porous TiO 2 thin films with enhanced photo-catalytic efficiencies [1][2][3][4].These additives play the principal role in the microstructure of the self-assembled porous TiO 2 coatings.Survey studies for better additives have been intensely performed since the additives vaporize during the annealing process and the materials properties of the additives dominate the microstructure of the porous films.The ideal morphology of the porous TiO 2 coatings is nanosized honeycomb structure.In this study for the formation of the well-defined honeycomb structure in the coatings, a new additive for the honeycomb TiO 2 films "tea leaf extracts" is successfully combined with the water soluble ammonium titanium citrate.

Experimental
In this study, a simple "dip and burn sol-gel method" was applied as the coating process for TiO 2 thin films.The water solution of ammonium titanium citrate (NH 4 ) 4 [Ti 2 (C 6 H 4 O 7 ) 2 ]•4H 2 O was adopted as the starting material for sol-gel coating.The Ti concentration in the solution was 2 wt% (i.e. 2% of the weight of the solution correspond to Ti).The reference samples (Sample Ref.) were fabricated as follows: Quartz glass (0.5 mmt) plates were dipped into this 2 wt% water solution of ammonium titanium citrate and dried in the ambient for 1 -2 hours to form coating precursors on the glass plates.Afterwards, these samples were sintered at 450℃ for 5 hours in the air for crystallization of TiO 2 .
In order to obtain TiO 2 coatings with the porous (honeycomb) structure, "red-tea leafs" (0.6 g) were added in the above water solution of ammonium titanium citrate (10 cc) and extracted for 24 -72 hours at room temperature.As a result, the yellowish color of the water solution of ammonium titanium citrate turned into red-brownish color which is familiar to us as the color of the strong red tea.The coating, drying and sintering procedure was identical to those for the sample Ref. Six grams of Sugar was also applied as additives into the in the above water solution of ammonium titanium citrate (10 cm 3 ) and stirred until complete dissolution.The details of the preparation conditions for three kinds of samples (Ref, Tea, and Sug) are summarized in Table 1.The crystalline structure of the films was investigated by x-ray diffraction method with Cu K line (RIGAKU RINT 1500).Digital Optical microscope (KEYENCE VHX) with the magnification up to 5000× was applied for the observation of the surface morphology of the TiO 2 films.The thickness of the films was investigated by the optical thickness measurement system (FILMET-RICS F-20).The photo-catalytic activity of the films was investigated by the photo-catalytic decomposition of isopropanol (CH 3 CH(OH)CH 3 ) under ultraviolet irradiation.The TiO 2 sample for evaluation was placed in an airtight quartz vessel.The initial atmosphere of this vessel was controlled carbon dioxide (CO 2 ) free by gas substitution.Isopropanol (CH 3 CH(OH)CH 3 ) with the concentration of 60 -70 ppm was introduced into this CO 2 free quartz vessel with the TiO 2 sample inside and succeeded by UV irradiation by black-light (wavelength: 300 -400 nm, intensity: 0.27 mW/cm 2 , time: 30 hours).The change of the composition of the gas inside the vessel during UV irradiation was periodically monitored by gas sampling and checking by gas chromatography.

Results and Discussion
The crystalline structure of the three kinds of samples, Ref, Tea and Sug showed broad halo pattern with faint diffractions assigned to TiO 2 rutile or anatase structures.The thickness of the films (Ref, Tea, Sug) were 100 nm, 80 nm, 100 nm, respectively.Figures 1(a Well defined honeycomb structure seems to be the fruit of the additive (red tea leaf extracts) which is completely soluble to the coating solution (water solution of ammonium titanium citrate) and shows uniform nucleation in the coating precursor followed by vaporization during calcinations.The coating solutions used in this  study (water solution of ammonium titanium citrate added with red tea extracts or sugar) are very stable and showed no precipitation or separation.The additives such as acryl beads, which are not soluble to the coating solution is not for the honeycomb structure but for the films with random pores.For the purpose of obtaining TiO 2 films with uniform honeycomb structure, the completely soluble additives (red tea extracts, sugar, etc.) seem to have the advantage1.
The photocatalytic activities of the samples Ref, Tea and Sug were evaluated by photo-catalytic decomposition of isopropanol (CH 3 CH(OH)CH 3 ) under ultraviolet irradiation and the results are shown in Figure 2. The initial concentration of isopropanol (60 -70 ppm) gradually decreased according to the ultraviolet irradiation in all samples (Ref, Tea, and Sug) (Figure 2(a)).Accordingly, the concentration of acetone (CH 3 COCH 3 ) and CO 2 gradually increased with the ultraviolet irradiation (Figures 2(b) and (c)).
Figure 3 shows the change of the concentration of the isopropanol, acetone, and CO 2 with ultraviolet irradiation up to 30 hours on the sample Sug.First period up to 8 hours, the concentration of isopropanol consistently decreased and reached around zero value, while the concentration of acetone consistently increased and showed a maximum value (~60 ppm) at 8 hours.The second period (8~30 hours) the concentration of acetone gradually decreased and reached zero value around 24 hours, while the concentration of CO 2 showed consistent increase throughout (8 -24 hours) and saturate.These results are consistent with the fact that photo catalytic decomposition of isopropanol to CO 2 takes place via acetone [5][6][7].
Effective surface area is the one of the major factor for dominating photo catalytic activity of the photo catalytic coatings.The larger the surface area is, the more active the photo catalytic activity becomes.The sample Tea showed well-defined honeycomb structure with the cell diameter of 200 -1000 nm while the samples Ref and Sug showed flat surface with no particular structures (Figure 1).However, the photo catalytic activities of the samples of Tea and Ref were very close and no clear advantage in the photo catalytic activity was found in sample Tea.This paradox can be explained by the dimensions of the honeycomb cell and the film thickness.
Generally, honeycomb structured films possess larger effective surface area compared with that of the films with flat surfaces since the surface area inside the honeycomb cells acts as extra surface area.However, we have to take the relation of the film thickness and the cell diameter of the honeycomb structure into account.Figure 4 shows the schematic drawing of the sample "Tea", which possesses well defined honeycomb structure with the cell diameter between 200 -1000 nm and the thickness of 80 nm as described above.The point is that the cell diameter is larger than the film thickness as schematically illustrated in Figure 4.
There do exist the gain of the effective surface area with the honeycomb structure, however this gain is rather small because of the shallowness of the cells.Moreover, at the bottom of the shallow and wide honeycomb cells, not the surface of the photo catalytic film but the surface of the raw glass is exposed.Accordingly, the decrease in the surface coverage takes place in the honeycomb film compared with that of the films with flat surfaces which is a direct decrease in the effective surface area.The gain in the effective surface area originate from the inside area of the honeycomb cells is counterbalanced by the loss in the surface area caused by the surface coverage of the film itself forced by the shallow and wide dimensions of the cells.Hence the photo catalytic activities of the samples of Tea (honeycomb) and Ref (flat) were almost identical according to the photo decomposition of isopropanol (Figure 2).
Another paradox observed in Figure 2 is that the sample Sug grown by sugar as additive showed higher photo catalytic activity than those of the samples Ref and Tea.The reproducibility of this superiority of the samples Sug (checked by several samples grown under same conditions shown in Table 1) was quite good and cannot be accidental.The samples Sug showed flat surface similar to that of the samples Ref by optical microscopy.One possibility is that the samples Sug possess nano pores which are invisible by optical microscopy; however scanning electron microscopy could not provide evidences for the nano pores so far.Another interesting possibility is that the surface of the sample Sug was chemically improved by sugar additives during film growth processes, resulting in surfaces with higher photo catalytic activity.
) and (b) show the optical micrographs of the samples Tea (a) and Sug (b) respectively.Well-defined honeycomb structures with the cell diameter of 200 -1000 nm were clearly observed in Figure 1(a).However, sample Sug (Figure 1(b)) showed flat surface with no particular structures identical to that of the sample Ref (not shown).The above described decrease in the film thickness in the sample Tea by 20% compared with those of the samples Ref and Sug is related to the successful porous structure formation in the sample Tea.

Figure 1 .
Figure 1.The optical micrographs of the samples (a) Tea and (b) Sug, respectively.Well-defined honeycomb structures with the cell diameter of 200 -1000 nm were clearly observed.

Figure 2
Figure 2 The photocatalytic activities of the samples Ref, Tea, and Sug evaluated by photo-catalytic decomposition of isopropanol (CH 3 CH(OH)CH 3 ) under ultraviolet irradiation.

Figure 3 .Figure 4 .
Figure 3.The change of the concentration of the isopropanol, acetone and CO 2 with ultraviolet irradiation up to 30 hours on the sample Sug.