# Tunable Coloring via Post-Thermal Annealing of Laser-Processed Metal Surface
## 3 Results and Discussion
### Mechanism of Color Tuning  
Figure 4a illustrates the annealing tuning process of the laser-generated surface color. First, the high energy of the laser pulse oxidizes the Ti surface. A thin film of oxide is formed and generates color with visible light illumination. This oxide film has different bands of thickness, due to the Gaussian distribution of the laser pulse intensity and laser irradiation overlapping. As a result, in micro-level, stripes of different colors can be seen. Together, they contribute to the macroscopic color--much like the principles behind the RGB color model. With thermal annealing, the thermal energy facilitates further oxidization of the Ti surface to create Ti\({}_{2}\)O\({}_{3}\), which allows for gradual tuning \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Area Ti\({}^{3+}\)/Ti\({}^{4+}\)** & **Calculated O 1s/Ti 2p** & **Atomic O 1s/Ti 2p** \\ \hline OL1\({}_{\rm Ref}\) & / & 2 & 2.19 \\ OL1\({}_{\rm 500^{\circ}C,1.5h}\) & 0.23 & 1.89 & 2.0 \\ OL2\({}_{\rm Ref}\) & / & 2 & 2.4 \\ OL2\({}_{\rm 500^{\circ}C,1.5h}\) & 0.12 & 1.94 & 2.21 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Ti cations and oxygen anion analysis. Figure 3: X-ray photoelectron spectroscope (XPS) spectra of original titanium substrate, (**a**) OL1, and (**b**) OL2 before and after 500 \({}^{\circ}\)C, 1.5 h annealing treatment. of the macroscopic color in micro-level. By varying the annealing temperature and length of time, the area and therefore intensity of the additional colors can be controlled to affect the macroscopic color. The color-tuning mechanism is further discussed below: The increase of the oxide layer thickness contributes to the color change. The EDX results show that the surface has been further oxidized after post-annealing. It is thus likely that the thickness of the oxide layer has increased. However, this increase in thickness is unlikely to be the key reason for the color change. Otherwise, the color of OL1 should have changed significantly (given the large changes in chemical composition). Another possible explanation is the Ti\({}_{2}\)O\({}_{3}\) generated by the annealing treatment, as shown by the XPS spectra. Researchers have reported that Ti\({}_{2}\)O\({}_{3}\) could be generated under an insufficient oxygen environment, which has an intrinsic color of yellow or blue depending on its lattice imperfections [23; 24]. Therefore, the appruttenance of Ti\({}_{2}\)O\({}_{3}\) to the Ti surface is thought to be the reason for the yellow or blue color. The model of the oxidation process during annealing is depicted in Figure 4b. As the initial laser oxidation process is intense and dynamic, with elevated-temperature exposure, the oxide film suffers from thermal and growing stress, and thus forms cracks and pores inside the film [25]. Therefore, the original oxide film is not airtight and contains internal defects. During the annealing process, titanium can continues to be oxidized through the inward diffusion of oxygen or with the permeation of oxygen through the oxide layer. Due to insufficient oxygen, Ti\({}_{2}\)O\({}_{3}\) is generated in these pores and cracks. At the same time, the original TiO\({}_{2}\) could also be reduced with the Ti\({}^{3+}\) or oxygen vacancy contained [26]. Since the TiO\({}_{2}\) layer is transparent, the yellow or blue Ti\({}_{2}\)O\({}_{3}\) is visible and able to tune the macroscopic color of the substrate surface. In addition, the increase in peak reflectance can also be explained by the generation of Ti\({}_{2}\)O\({}_{3}\). The absorbance of visible light spectra by Ti\({}_{2}\)O\({}_{3}\) is much lower than that of TiO\({}_{2}\)[27; 28]; thus, a TiO\({}_{2}\)/Ti\({}_{2}\)O\({}_{3}\) oxide layer would exhibit a higher reflectance than a TiO\({}_{2}\) layer. Although Ti\({}_{2}\)O\({}_{3}\) appears in both oxide layers after annealing, the colors are different. This may be attributed to the different laser parameters, which results in differences between the structures of the two oxide layers.