*3.6. Photocatalytic Properties of Films*

The sol-gel derived thin films all demonstrated photocatalytic activity for the degradation of resazurin intelligent ink. The addition of surfactant was found to increase the photocatalytic activity of the thin films as shown in Table 2 and Figure 7. The samples prepared without surfactant, samples A1–A3 exhibited half-lives in the range of 9.5–16.50 min. In contrast, the samples produced with surfactant had half-lives ranging from 3 to 5 min. The fastest half-lives observed were for samples C2 and E2, which both had a half-life of 3 min for resazurin degradation. This indicates that the addition of Tween® 40 or Tween® 20 surfactant in the lowest concentration (0.003 mol·dmí<sup>3</sup> ) has a beneficial effect on the thin film microstructure and morphology, such that the functional properties are improved for photocatalytic activity. The photocatalytic activity is attributed to the surfactant role as the particles grow within the sol-gel. The surfactant acts as a spacer between growing titania particles that enables great control over their size and shape, preventing agglomeration. The particles produced as a result are angular and smaller in size (average size for C2 29 nm, compared to 130 nm for A2). This modified morphology results in an increased surface area to volume ratio upon which the organic dye can be adsorbed and photocatalytically degraded.

It has been widely acknowledged that photocatalytic activity can be influenced and enhanced by a number of factors, and within this study it has been found that a combination of factors, particularly morphology and surface roughness have caused significant changes to the photocatalytic properties of the thin films produced. For example, all samples produced with surfactant exhibit a much higher average surface roughness, which can be seen morphologically in the SEM images (Figures 2 and 3) where particles are more angular and a variety of sizes, and also in the 3D AFM images in Figure 4. Samples prepared without surfactant showed a surface roughness ranging from 9 to17 nm, attributable to the poor definition of the larger, more spherical particles ranging in size 40–130 nm. In contrast, samples prepared with Tween® surfactants showed average surface roughness values ranging from 135 to 366 nm, with much more angular and variable sized particles ranging from 25 to 55 nm as their average size. The increased surface roughness enables an increased surface area to volume ratio, and as described enables greater absorption of the dye on the surface, and an increased area for the photocatalytic reaction to take place.

**Figure 7.** Normalised decrease in absorption of resazurin peak at 630 nm with UV irradiation (365 nm) over time for samples: Blank; A = no surfactant; B = Tween® 40, 0.006 mol·dm<sup>í</sup><sup>3</sup> ; C = Tween® 40, 0.003 mol·dmí<sup>3</sup> ; E = Tween® 20, 0.003 mol·dmí<sup>3</sup> . Sample D has been removed for graph clarity.
