Photoactivity Experiments and Morphology Effects

As described in the methods section, the photocatalytic activity of the A-TiO2

nanocrystals was characterized by exposing acetaldehyde-dosed samples to UV light followed by TPD. TPD spectra for the 18 nm bipyramids following a 2 h UV exposure are presented in Figure 3.2c. These data show that the UV exposure did not cause a decrease in the desorption signal for acetaldehyde, indicating that photo-desorption of acetaldehyde does not occur to any appreciable extent as has been reported in some instances for other TiO2 surfaces.35,101-103 Exposure to UV light did, however, induce photochemical reactions

as evidenced by small changes in the product distribution and more importantly the

Figure 3.7. Fractional yield of carbon-containing products during CH3CHO TPD for 14 nm

appearance of ketene as a significant product at 670 K. Analogous data for the 18 nm platelets are displayed in Figure 3.2d and also show ketene as a desorption product following UV exposure for 2 h. The TPD spectra in Figures 3.2c and d show that the amount of ketene produced from the platelet sample is significantly larger than that from the bipyramidal sample. These TPD results are similar to those obtained in another study of ours, published in Chapter 4 of this dissertation, with B-TiO2 nanorods which were also

found to produce ketene from acetaldehyde following UV light exposure. In that study, ketene production was found to follow the reaction sequence shown in Equation 3.1 which proceeds via photo-oxidation of adsorbed acetaldehyde to produce stable acetate species which thermally decompose above 600 K to produce ketene:

𝐶𝐻 𝐶𝐻𝑂( )+𝑂( )→ 𝐶𝐻 𝐶𝑂𝑂( )+ 𝐻( ) ⎯ 𝐶𝐻 𝐶𝑂 + 𝐻 𝑂 (3.1)

Thus, the ketene product formed by UV exposure indicates that these samples are active for photo-oxidation of acetaldehyde to surface acetate. As mentioned above, ketene production was also observed for the nanocrystals predosed with oxygen, suggesting that a separate thermal pathway for the partial oxidation of acetaldehyde to acetate occurs in the presence of adsorbed oxygen.

It should be noted that other studies have reported photo-induced methyl radical ejection from adsorbed acetaldehyde on TiO2 resulting in the formation of a surface

formate species. Like acetates, formates are relatively stable on TiO2 and decompose

between 550 and 600 K to produce CO and CO2.35,104 The absence of these formate

decomposition products in the present study indicates that this additional photo-oxidation pathway does not readily occur on the bipyramidal and platelet A-TiO2 nanocrystals.

To further investigate the differences in activity for the photo-oxidation of adsorbed acetaldehyde to acetate between the bipyramidal and platelet nanocrystal morphologies, the amount of ketene produced during a TPD run was measured as a function of the UV light exposure interval. These data for the 18 nm bipyramidal and platelet samples are displayed in Figure 3.8. Note that the data have been normalized to account for difference in the surface area between the two samples. The trends in the data are similar with the amount of ketene produced increasing with increasing UV exposure, further confirming the photochemical nature of the overall reaction pathway. The data also show that for a given UV exposure interval the platelet nanocrystals produce significantly more ketene than the bipyramidal nanocrystals, thus, indicating that they have higher photocatalytic activity. As will be discussed below, this difference in activity is likely related to the proportion of exposed (101) and (001) planes in the two samples and suggests that the (001) surface which is preferentially exposed in the platelets has higher activity for the photo-oxidation of acetaldehyde to acetate compared to the (101) plane. This shape- dependency is also evident when one compares the ketene yields as a function of UV exposure for the 10 nm bipyramids and the 14 nm platelets (see data in Figures 3.9 and 3.10), although the respective nanocrystal sizes are slightly less comparable here.

Various studies of the photocatalytic activity of oxide nanocrystals,51,59,69,94 _{including our}

investigation of B-TiO2 nanorods in chapter 4, have demonstrated that crystallite size can

have a large impact on the photocatalytic activity of a material. In the present study, we also investigated the impact of crystallite size on photoactivity for the different A-TiO2

as a function of UV exposure for the 10 nm, 18 nm, and 25 nm bipyramidal nanocrystal films. Similarly, Figure 3.10 presents ketene yields from acetaldehyde TPD as a function of UV exposure for the 14 nm and 18 nm platelet nanocrystal films. These data have again been normalized to correct for the differences in total surface area between the samples. These two figures illustrate that crystallite size plays a large role in influencing photocatalytic activity and show that in both cases photo-activity increases with increasing particle size.

Figure 3.8. Ketene yields from TPD of 5 L CH3CHO-dosed 18 nm A-TiO2 bipyramids and

Figure 3.9. Ketene yields from TPD of 5 L CH3CHO-dosed A-TiO2 bipyramids as a function

of UV illumination interval and nanocrystal size.

Figure 3.10. Ketene yields from TPD of 5 L CH3CHO-dosed A-TiO2 platelets as a function of

Since predosing with O2 was found to increase the activity of the A-TiO2

nanocrystals for the thermal oxidation of adsorbed acetaldehyde to acetate, the effect of adsorbed oxygen on the photocatalytic activity of the nanocrystals was also investigated. In these experiments the nanocrystal films were sequentially dosed with 20 L O2 and then

5 L acetaldehyde and then exposed to UV light for a predetermined time interval, followed by TPD. As previously mentioned, the samples were kept at 165 K throughout dosing and UV exposure. Figures 3.11 and 3.12 display the yield of ketene produced during each TPD run as a function of UV exposure with and without O2 predosing for the 10 nm bipyramids

and 14 nm platelets, respectively. As was previously discussed, for both samples, the ketene yield increased under dark conditions upon O2 predosing; however, the presence of

adsorbed oxygen appears to have little to no impact on the rate of photo-generated adsorbed acetate, as the increase in ketene yield for the UV exposed O2-predosed samples is

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Figure 3.11. Ketene yields from TPD of 5 L CH3CHO-dosed 10 nm A-TiO2 bipyramids, with

and without pre-adsorbed oxygen, as a function of UV illumination interval.