## 3 Results and Discussion
### Photocatalytic Activity  
To compare the effect of PANI composition in the PANI/TiO\({}_{2}\) composites, different weight ratios of PANI (2%, 4%, 6%, 8%, 10%) were added when the catalyst was prepared. Plots of C/C\({}_{0}\) versus visible-light-irradiation time were constructed for the different catalysts (Figure 6a).  
Figure 4: X-ray diffraction (XRD) pattern of PANI/TiO\({}_{2}\) composite.  
Figure 5: Diffuse reflectance spectra (DRS) spectra of TiO\({}_{2}\) and 4% PANI/TiO\({}_{2}\) composite (insert shows digital image of TiO\({}_{2}\) (**left**) and 4% PANI/TiO\({}_{2}\) composite (**right**)).  
As the weight ratio of PANI increased from 0% to 4%, the degradation efficiency of MB increased from 38.65 \(\pm\) 2.68% to 48.10 \(\pm\) 4.27%, but with 10% PANI composition, it decreased to 30.33 \(\pm\) 2.33%  
Figure 6: (**a**) Photocatalysis of methylene blue (MB) under visible-light irradiation with different photocatalysts, (**b**) dark adsorption of MB with 4% PANI/TiO\({}_{2}\), and (**c**) photolysis of MB (MB = 5 uM, catalyst dose: 0.8 g/L).  
in 240 min. This phenomenon shows that the proper amount of PANI is needed to optimize the degradation efficiency of a contaminant owing to obstruction of the molecular structure of PANI [24]. As a result, 4% PANI/TiO\({}_{2}\) composite displayed enhanced photocatalytic activity compared with commercial TiO\({}_{2}\) under visible-light irradiation. This is due to the synergetic effect of the well-matched energy levels of TiO\({}_{2}\) and PANI, PANI/TiO\({}_{2}\) composite could interfere the recombination of holes and electrons, which can improve the production of the reactive oxygen species that degrades MB. In addition, since PANI is a photosensitizer with a narrow bandgap, PANI/TiO\({}_{2}\) composite could enhance the response to visible light by inserting the energy level of PANI into the energy level of TiO\({}_{2}\), which was consistent with the DRS result. These electron transfer between the PANI and TiO\({}_{2}\) semiconductors and the enhanced photocatalytic activity of the composites has already been reported experimentally in other literature [12; 25]. Adsorption of MB by the 4% PANI/TiO\({}_{2}\) did not occur (Figure 6b), and the direct photolysis of MB by visible-light irradiation was relatively low (\(4.69\pm 1.67\%\), Figure 6c), indicating that MB degraded predominantly by photocatalytic activity.  
Different amounts of 4% PANI/TiO\({}_{2}\) (0, 0.32, 0.8, 1.12, 1.6 g/L) were added to the MB solution to optimize the catalyst dose. Results of the removal ratio of MB by different photocatalyst doses (Figure 7) showed that MB degradation increased from \(3.88\pm 1.51\%\) to \(67.37\pm 7.82\%\) as the dose of the photocatalyst increased to 1.12 g/L but adversely decreased to \(58.40\pm 0.38\%\) at the dose of 1.6 g/L. This result could be explained in terms of photocatalyst agglomeration and visible-light penetration into the suspension [26]. As the photocatalyst concentration in the solution is increased, the photocatalyst tends to agglomerate, which will reduce the amount of photocatalyst surface exposed to the visible-light. Furthermore, the agglomeration and sedimentation of photocatalyst particles also decrease the penetration depth of visible light from the reactor wall, causing light scattering, which lowers the light intensity entering the suspension.