## 3 Results and Discussion
Figure 6 presents the TEM images of the nontreated Au/TiO\({}_{2}\) nanocomposite. One can easily see that the AuNPs are no longer homogenous and uniform in shape, but their size is distributed in the range from about 10 nm to around 55 nm. This is in line with the broader absorption band in their UV-vis absorption spectra (Figure 7). It is likely that these AuNPs are formed in the solution before being attached to TiO\({}_{2}\) surface because they show similar size distribution as those generated from the pure HAuCl\({}_{4}\). All generated AuNPs are seen immobilized on the TiO\({}_{2}\) surface with clear and sharp boundaries indicating a rather weak connection between them. This also supports a small red shift of about 15 nm compared to the situation with the pristine AuNPs. The influence of TiO\({}_{2}\) on the reduction mechanism of Auric ions may be twofold: catalyst effect under UV radiation from plasma which enhances the particle formation, resulting in bigger particles and growth termination by capturing the growing particles onto the titania surface. Unlike the non-treated Au/TiO\({}_{2}\), there are two types of AuNPs existing in the treated Au/TiO\({}_{2}\), one with a similar size distribution as in the non-treated Au/TiO\({}_{2}\) composites and the other with very small sizes ranging from 4 nm to 9 nm (Figure 8). Both of these types exhibit noticeably close contacts to the TiO\({}_{2}\) surface with seemingly undetectable boundaries. The small particles even seem to be submerged into the TiO\({}_{2}\) matrix (see Figure 8). The UV-vis spectra (Figure 7) of this sample show a good agreement with the TEM results. The spectra of the treated Au/TiO\({}_{2}\) show two absorption peaks which can be attributed to the two classes of AuNP particle size (there is no AuNP aggregation). Figure 5: TEM image (**a**) and particle size distribution histogram (**b**) of gold nanoparticles (AuNPs) produced by the plasma–liquid interaction method with a precursor concentration of 0.06 mM. Figure 6: TEM image (**a**) of non-treated Au/TiO\({}_{2}\) nanocomposites produced by the plasma–liquid interaction method with a precursor concentration of 0.06 mM, the inset shows the particle size distribution histogram of gold nanoparticles. Magnification of TEM image (**b**) elucidating the sharp boundary between TiO\({}_{2}\) surface and gold nanoparticle. The absorption peaks are strongly red shifted from about 523 nm to 538 nm (for smaller AuNPs) and from 534 nm to around 680 nm (for larger AuNPs). Using the refractive index difference of 1.279 (rutile TiO\({}_{2}\) has refractive index of 2.609) applied for the situation presented in Figure 5 in [21], one can easily estimate the red shift for particles with the original plasmonic resonance of 534 nm in water (\(n\) = 1.33) to be about 130 nm which is in good agreement with the red shift of the larger sized AuNPs in the treated Au/TiO\({}_{2}\). This result again confirms a strong contact between the treated TiO\({}_{2}\) and the AuNPs in treated Au/TiO\({}_{2}\) nanocomposites. Figure 8: TEM images (**a**) of the treated Au/TiO\({}_{2}\) nanocomposite produced by the plasma–liquid interaction method with the precursor concentration of 0.06 mM. Magnification of TEM images elucidating (**b**) the submerging of small gold nanoparticles into TiO\({}_{2}\) surface and (**c**) the close contact between gold nanoparticle and TiO\({}_{2}\) surface. The particle size distribution histogram (**d**) of small gold nanoparticles of the same sample. Figure 7: Extinction UV-vis spectra of (**a**) non-treated rutile TiO\({}_{2}\) (red curve), treated rutile TiO\({}_{2}\) with 3% NH\({}_{3}\), 3% H\({}_{2}\)O\({}_{2}\) under a 5 min plasma treatment (black curve) and (**b**) bare Au-nanoparticles generated from 0.06 mM HAuCl\({}_{4}\) (red curve), non-treated Au/TiO\({}_{2}\) nanocomposite (blue curve), and treated Au/TiO\({}_{2}\) nanocomposite (black curve).