*3.3. Photocatalytic Properties*

Figure 6a,b show the results of the photocatalytic measurements. From the adsorption isotherms thus obtained the MB saturation coverage was determined, and found to be 8.17 ± 0.6 × 10<sup>í</sup><sup>3</sup> ȝmol·cm<sup>í</sup>2 for the <101> sample and 8.29 ± 0.8 × 10í<sup>3</sup> ȝmol·cm<sup>í</sup><sup>2</sup> for the <001> sample, *i.e*., an increase of 1.5% for the latter film. This is much smaller than the 10% increase in porosity inferred from analysis of the optical data described in Section 2.3, suggesting that the difference in porosity is related to pores inside the film structure which are inaccessible for the MB adsorption. Control experiments in a reactor containing uncoated glass substrates show that MB adsorption on reactor surfaces, other than TiO2 film, is about 12% of the initial concentration.

**Figure 5.** Tauc plots of ξȽܧ *vs*. photon energy, *E*, and least square linear fit of data for films with (**a**) no orientation and (**b**) preferential <001> orientation. In both cases the optical bandgap was estimated to be 3.3 eV.

After MB equilibration the UV lamp was switched on and the photodegradation of MB was measured by the increasing colometric signal. The photodegradation of MB was modeled as a pseudo-first order reaction with the apparent rate *kƍ*, viz.:

$$\frac{\text{d}C}{\text{d}t} = -k't\tag{10}$$

or

$$-\ln\frac{C}{C\_0} = k't\tag{11}$$

where *C*0 is the initial concentration of MB (after 40 min equilibration); *C* is the concentration at time *t*. Figure 6b shows a plot of Equation (11) for a <101> and a <001> film. The apparent rate constants averaged over a set of four samples from each batch were determined to be *kƍ*<101> = 0.99 ± 0.09 × 10<sup>í</sup><sup>3</sup> min<sup>í</sup><sup>1</sup> and *kƍ*<001> = 1.29 ± 0.17 × 10<sup>í</sup>3 miní<sup>1</sup> , corresponding to an increase of *kƍ* by approximately 30% for the preferentially <001> oriented films compared to randomly oriented films.

The effect of UV intensity on the photocatalytic rate for the two sets of films was investigated. Repeated experiments were performed where the distance between the sample and the UV light source was systematically varied. It was changed from 8.5 cm, which is the closest distance, limited by the height of the reaction cell, up to 13.5 cm, 18.5 cm and 23.5 cm, yielding UV intensities at the sample position of 0.382 mW·cm<sup>í</sup><sup>2</sup> , 0.158 mW·cm<sup>í</sup><sup>2</sup> , 0.085 mW·cm<sup>í</sup><sup>2</sup> and 0.053 mW·cm<sup>í</sup><sup>2</sup> , respectively. The dependence of the rate constant of the UV light intensity was fitted using Equation (12):

$$k = k''I^a \tag{12}$$

where *k*Ǝ is the intensity independent rate constant, *k* is the apparent rate constant, *I* is the UV light intensity, and Į is the reaction order by light intensity.

**Figure 6.** (**a**) Adsorption of methylene blue dye as a function of time, and (**b**) semilogarithmic plot of the normalized MB concentration, *C*, as a function of UV irradiation time over <001> and <101> TiO2 films. The gray line denotes the amount of dye adsorbed by reaction cell walls and uncoated glass slide in blank experiments.

Figure 7 shows the effect of UV intensity on the photodegradation rate with different degree of preferential <001> orientation. In each case the experiments were conducted using four different samples. It was found that the increased orientation changes the way the catalyst is affected by the intensity of UV light. The less-oriented sample <101> showed an almost UV intensity independent rate constant for MB photodegradation with a rate constant *kƎ*<101> = 1.19 × 10<sup>í</sup><sup>3</sup> min<sup>í</sup><sup>1</sup> and reaction order of Į = 0.18. In contrast the <001> sample yielded a rate constant of *kƎ*<001> = 1.95 × 10<sup>í</sup><sup>3</sup> min<sup>í</sup><sup>1</sup> and a reaction order of Į = 0.42. Thus the UV independent rate constant *k*Ǝ is 64% larger for the <001> oriented film. Again, this cannot be accounted for by a larger exposed surface area (higher porosity of surface roughness) as shown by the microscopy data, the estimates of film porosity, and surface coverage of MB. Instead it must be attributed to an intrinsic higher reactivity for the preferentially <001> oriented films. Mills and coworkers have reported that for thick, porous catalysts the rate constant, the intensity dependence can be divided into three regions [39]. The first region, at very low intensities, where the rate increases linearly with light intensity; a second region, at medium light intensities, where a square root dependence is observed, and a third region, at high intensities, where photon flux no longer limits the photo-degradation rate, and rate becomes independent of further increase of the light intensity. We can conclude from our measurements that for the randomly oriented grains dominated by {101} surfaces, the photo-degradation rate is not limited by UV intensity (with an almost constant rate as a function of UV intensity, Į = 0.18). Given that our films are thin and non-porous with undeveloped surface (based on the AFM measurements) we assign this to a small number of reactive sites and/or exposure of reactive sites with low reactivity. In contrast, for the preferentially <001> oriented films, we find Į = 0.42, which suggests that these films have a larger number of reactive sites and/or expose a larger fraction of more reactive sites (Table 2).

**Figure 7.** Methylene blue photo-degradation rate as a function of UV intensity for (**a**) a randomly oriented (<101>) TiO2 film, and (**b**) a preferentially <001> orientated TiO2 film. The dashed line is a guide to the eye.

Furthermore, the UV intensity independent rate constant points to a total increase of activity by 61%. This is a dramatic increase, keeping in mind that the XRD analysis points to only 25% of the crystallites, oriented in the <001> direction (which does not directly translate to 25% increase of surface {001} coverage). For comparison, Yang *et al*. [40] demonstrated a novel synthesis of colloidal catalyst with 70% exposed highly-reactive {100} facets. Compared to Degussa P25 it showed 3-fold increase in the photo-oxidation rate of MB, which is similar to our results.

**Table 2.** Kinetic parameters for the photocatalytic degradation of MB for the samples with <001> preferential orientation, and with dominant <101> orientation (randomly oriented grains).

