Análisis de la interacción cuando finaliza la secuencia didáctica

T he next step o f the investigation was to look at the photocatalytic properties o f the film s that w ere produced. Photocatalytic activity w as evaluated b y looking at the rate o f rem oval o f stearic acid, CH3(CH2) i6C0 2H, from the surface o f the coating w hen irradiated by U V light.

5.4.1 Sam ple preparation

The sam ples w ere first cleaned prior to testing by w iping w ith petroleum spirit 60-80 to rem ove any grease from the surface o f the glass. The coatings w ere then activated by irradiating under uv light (2 x 8W 254 nm fluorescent tubes) for 4 hours. 7.5 pi o f a solution o f 2.5 g dm'^ stearic acid in propanol w as then spin coated onto the surface o f the glass substrate. This produced an even layer o f stearic acid that could be reproducibly produced allow ing com parisons to be m ade.

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5.4.2 Testing o f samples

IR spectroscopy was used to exam ine the coated specimens. The thickness o f stearic acid on the surface o f the glass could then be quantified by looking at the absorbance area o f the C-H stretches produced by the stearic acid that occur between 2700 cm ' to 3000 cm '. This area could be quantified to a rate o f removal o f stearic acid molecules, typically, there are 3.17xlO'^ molecules o f stearic acid per cm^ per unit o f absorbance^''’^'.

To simulate accelerated sunlight the same 254 nm fluorescent tube lamp was used. By irradiating the samples for 30 minute periods and then re-examining them by IR spectroscopy it is possible to see the reduction in C-H peak area and thus the reduction in amount o f stearic acid present on the surface o f the glass. Figure 5.8 shows an example o f the reduction in C-H stretches with irradiation

0) 85 83 73 4- 3 000 2 9 5 0 2 9 0 0 2 850 W a v e n u m b e r / cm-1 — - N o Irradiation — 3 0 mins — 6 0 mins — 9 0 mins — 12 0 mins 2800 F igure 5.8 sh o w in g the reduction in C-H peak area under U V irradiation.

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N oim alised graphs o f peak area versus time could then be used to com pare the samples activity.

5 .4 .3 P h o to c a ta ly tic stu d ie s o f thick film s

It was found that the thick films were very capable o f removing stearic acid from the surface o f the coatings, however, the most interesting result was the dependence upon the thickness o f the coating. Films that were grown under exactly the same flow and temperature conditions, but for different lengths o f time had greatly differing rates o f removal o f stearic acid. This is shown by figure 5.9 a graph that shows the rate o f removal o f stearic acid from the surface o f glass by using a normalised peak area plot versus time. All o f the coatings shown in figure 5.9 were grown from methanol and TiCfr, at a temperature o f 550 °C for different growth times.

1 0.9 0.8 0.7 3 0 Seco nd s 6 0 secon ds 9 0 secon ds 1 2 0 s e con ds — plain g lass - - V 0.6 Q . 0.5 0.4 0.3 0.2 0.1 0 10 20 30 40 50 60 0 t i m e / m i n

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This variation in activity w as explained because longer grow th tim es give thicker film s, and that thicker films have a greater surface roughness. G reater surface roughness m eans that the films have a larger area in contact w ith the stearic acid, therefore a greater am ount o f electron hole transfers can take place, and the stearic acid is rem oved m ore quickly. This show ed that in order to com pare fairly the photocatalytic ability o f films, com parison m ust be m ade betw een film s o f sim ilar thickness.

5.4.4 C om paring film thickness

The m ethod used to ascertain w hether two film s w ere o f the sam e thickness w as to look at the uv / vis absorption spectra o f the films. If the film s had very sim ilar uv / vis spectra then it w as assum ed that the film s w ere o f com parable thickness and therefore surface roughness. This is in show n in figure 5.10 w here it can be seen that film s that are grow n for the sam e tim e under sim ilar conditions have a very sim ilar profile com pared to either thinner or thicker films.

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I

Figure 5 .1 0 sh o w in g the reproducibility o f uv / v is spectra for film s o f différent thickn ess

Therefore to compare films first uv / vis spectra had to be taken to ascertain the point at which the film had a thickness that gave the correct curve.

On average after 60 minutes o f irradiation most thick films had reduced the amount o f stearic acid on the surface by between 60% and 80%. However, since these films are not suitable for window applications they were not investigated further.

5.5 Photocatalytic studies o f thin film s

Thin films were grown using deposition times from 5 to 15 seconds, at temperatures from 300 to 600 °C. Films deposited using these conditions had thicknesses o f less than 200 nm. These films were transparent films and still photocatalytically active.

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It was found that films could be produced that would reduce stearic acid levels to less than 30 % in 20 minutes, as can be seen in figure 5.11.

0 .9