Photoactivity of Au/TiO 2 under different irradiation

18

3.2.1 Effect of Au content

The degradation of MO over Au/TiO₂ samples under visible illumination was not very pronounced and considerably less than that under UV illumination. This can be attributed to the absence of a suitable hole-trapper on the Au NPs, such as Cl^– and Br^– anions^29-31. As a consequence, the visible-light-responsive activity was not

investigated in this study. However, the UV-induced photocatalytic activity of Au/TiO2 was enhanced significantly by use of the GLED. Figure 6 shows the degradation rate constants using various Au content photocatalysts under UV and UV/visible irradiations. Under UV illumination, the MO degradation rates for all Au-containing TiO₂ samples showed no improvement over that for pristine TiO₂. In general, the noble metal gold is typically used to produce the highest Schotty barrier among metals to facilitate electron capture^38,39. Thus, the presence of Au on TiO₂ favors the migration of photo-produced electrons to gold, improving the electron-hole separation. This phenomenon is evidenced in the photoluminescence (PL) intensity, as shown in Figure 7. It can be seen that pristine TiO2 and Au/TiO2 particles exhibit similar curve shapes in their PL spectra, demonstrating that Au particles do not give rise to a new PL phenomena. However, the PL intensities of Au/TiO2 catalysts are considerably less than that for TiO₂. The quenching effect increases with the gold content. If we assume that the PL emission results mainly from the recombination of excited electrons and holes, the Au/TiO₂ sample with lower PL intensity should exhibit better activity than pristine TiO2 under UV irradiation. However, the aggregation effect, leading to an increase in the secondary particle size of the photocatalyst, is also important in determining the photocatalytic rate^35,40. The turbidity values of 0%, 0.1%, 0.3%, and 0.5% Au/11-TiO₂ were 250, 181, 175, and 157 NTU, respectively, indicating that aggregation of TiO2 particles occurred during

the impregnation process of Au/TiO₂. The lowest activity for 0.5% Au/11-TiO₂ is probably because of excess Au particles covering some active sites on the TiO2

surface.

Figure 6 also shows how the photocatalytic activity of all Au/TiO2 samples under UV/visible illumination is better than that for samples under only UV

illumination. The irradiation of GLED did not have a significant effect on the activity of pristine TiO₂. This indicates a synergistic effect between UV and green light, which enhances the degradation rate of dye over Au/TiO2 samples. This synergistic effect is improved by increasing the Au content (i.e., SPR peak intensity). The degradation rate of the dye on the 0.5% Au/11-TiO2 with an additional illumination of GLED was 1.25 times faster than that in the absence of visible light. However, the

visible-light-responsive photocatalytic activity of these Au/TiO2 samples is not obvious, so the enhancement of photocatalytic activity is not a result of the visible-light-excited electrons at the gold NPs being injected into TiO2. The electromagnetic field of the incident light couples with the oscillations of the conduction electrons in Au NPs, resulting in strong enhancement of the local

electromagnetic fields near the surface of Au NPs. Thus, it is suggested that when the wavelength of the incident light matches the SPR band of Au/TiO2, it acts as an additional electron magnetic field, thus increasing the generation rate of photoexcited holes and electrons at TiO2. This is depicted in Figure 8. Thus, the photocatalytic behavior of Au/TiO₂ is remarkably boosted as it is assisted by the enhanced electric near-field amplitudes of SPR^32,33. The plasmonic photocatalysis under UV/visible illuminations for Au/TiO₂ is observed in this work. This composite photocatalyst can be applied practically in an indoor environment as the ordinary fluorescent tubes used indoors emit both UV and visible light⁴¹.

3.2.2 Effect of TiO₂ size

20

The photocatalytic activity for Au/TiO₂ samples with different TiO₂ size under UV and UV/visible illumination is shown in Figure 9. The activities of these

photocatalysts under UV irradiation were of the order: 0.5%Au/21-TiO₂ >

0.5%Au/11-TiO2 > 0.5%Au/31-TiO2. The highest UV-induced photoactivity of 0.5%

Au/21-TiO₂ is a consequence of its proper crystallization. The significant decrease in specific surface area results in a low photoactivity of 0.5% Au/31-TiO2 under UV irradiation⁴².

The plasmonic photocatalysis under UV/visible illuminations decreased with increasing TiO₂ crystal size, as shown in Figure 9. The previously mentioned

synergistic effect was not observed in the use of 0.5%Au/31-TiO2. The intensities of the SPR absorptions of these photocatalysts were not very different, but the red-shift of the SPR peak was enhanced as the TiO2 crystal size increased due to the change in refractive index. The illumination of GLED peaked at 524 nm, hence its irradiation will not be absorbed by 0.5%Au/31-TiO₂ (λSPR = 570 nm) to result in the electric near-field amplitudes of SPR. Figure 8 also illustrates the effect of TiO₂ size on the SPR of Au/TiO2. The result indicates that the plasmon resonance of Au/TiO2 and Ag/TiO₂ should be adjusted properly to be consistent with the light source for the enhancement of photoactivity²³. Tian et al. also report that the largest incident photon to current conversion efficiency (IPCE) of Au/TiO₂ film is generated when the wavelength of incident light matches the SPR absorption region³². Our results of photocatalytic activity are consistent with the previous report³². Thus, both the proper optical metal content and semiconductor size for the plasmonic photocatalysis depend on the irradiation source and reaction conditions.

4. CONCLUSION

In the study, Au/TiO2 is proposed as a new type of photocatalyst that uses the

enhanced electric field amplitude on the surface of Au particles in the spectral vicinity of its plasmon resonance. The UV-responsive photoactivity of Au/TiO2 was

significantly enhanced by under illumination of GLED, which emitted

SPR-corresponding irradiation. The plasmonic photocatalysis under UV/visible illumination for Au/TiO₂ increases with the increase in Au content and SPR

adsorption. Furthermore, as the wavelength of the SPR is shifted toward the emission region of GLED with a proper refractive index of TiO₂, the synergistic effect is enhanced. Results indicate that the absorption intensity and peak position of SPR are both important for the plasmonic photocatalytic activity of Au/TiO₂.

Acknowledgements

The authors wish to acknowledge our two anonymous reviewers for their excellent comments. The authors would like to thank the National Science Council of Taiwan, for financial support of this research under contracts NSC-98-2218-E-011-005.

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