## 3 Results
### Characterization of the Coatings
As the substrate bias voltage increases from \(-50\) V to \(-75\) V in the mixture of the anatase and rutile phase, the rutile phase increases but the anatase phase remains dominant (Table 2). The observed results are in agreement with those reported in ref. [32,33]. It follows that the creation of a film structure and its phase composition depend more on the energy differences between the bombarding positive ions and the depositing atoms than they do on the thermal mobility of the depositing atoms on the surface. The phase transition of anatase to the more thermodynamically stable rutile TiO\({}_{2}\) is caused by the positively charged particles increased kinetic energy as the substrate's bias voltage increases. It has been demonstrated that anatase forms at lower maximum power values and rutile forms at higher power values when using high-energy deposition techniques such as high-power impulse magnetron sputtering [23]. In our case, the high bias applied to the substrate may cause a more vigorous process that alters the crystal structure and results in smaller crystallite sizes (Table 2). The presence of a broad, low-intensity peak associated with the rutile phase could indicate poor crystalline phase quality in the thin film. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample Bias Voltage** & **Ti Content, (wt.\%) \(\pm\) SD** & **Cu Content, (wt.\%) \(\pm\) SD** \\ \hline \(-25\) V & 68.95 \(\pm\) 0.15 & 2.62 \(\pm\) 0.02 \\ \(-50\) V & 68.48 \(\pm\) 0.41 & 2.34 \(\pm\) 0.16 \\ \(-75\) V & 67.98 \(\pm\) 0.76 & 2.13 \(\pm\) 0.38 \\ \(-100\) V & 68.14 \(\pm\) 0.33 & 2.11 \(\pm\) 0.21 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Copper and titanium content (in wt.% \(\pm\) standard deviation (SD)) in the examined coatings. Figure 6: XRD diffraction patterns of the substrate and Cu-doped-TiO\({}_{2}\) coated samples at different bias values. No obvious peaks corresponding to secondary phases like CuO, Cu\({}_{2}\)O, Cu are observed. Similar observations of no peaks of copper oxide visible in the XRD patterns, with copper concentrations ranging from 0 to 65.97 at%, were reported by [PERSON] et al. [15]. However, they applied no bias during the reactive magnetron sputtering process. In our previous study, we found that when increasing the negative bias from \(-\)50 up to \(-\)100 V during the glow discharge sputtering of Ti-Cu target material in an oxygen atmosphere, the Cu\({}^{2+}\) state decreased at the expense of an increase in Cu\({}^{1+}\) state in the oxide phase [34]. In the present study, we observed that the Cu-doped films did not exhibit any discernible shifting of (101) anatase Bragg reflections, as would be predicted during substitutional or interstitial doping, due to the ionic radius differences between Ti(IV) (0.60 A), Cu(I) (0.77 A), and Cu(II) (0.73 A). [PERSON] and his co-authors [35] also reported no shifting in the position of the (101) anatase peak up to 38 at% Cu in the oxide. According to their DFT simulations, substitutional Cu doping of anatase was predicted to result in negligible lattice distortion, because the Cu-O distances remained at 2.00 A under all charge states, staying in line with the normal Ti-O bond length of 1.98 A in anatase. This clarifies the lack of TiO\({}_{2}\) lattice deformation seen in our investigation. The largest peak for the most coated samples is the (101) diffraction peak of anatase and (110) maxima of rutile, which are used to determine the crystallite sizes. As shown in Table 2, when the bias voltage was adjusted from \(-\)25 to \(-\)100 V, the crystallite size dropped from 9.72 nm to 5.60 nm, indicating the grain refinement brought about by applying a negative substrate bias. The substructural features (dislocation density and microstrain) show a general trend toward increasing microstrain and dislocation density as the bias increases. By varying the bias voltage from \(-\)25 V to \(-\)100 V, the dislocation density and microstrain were raised from 10.58 up to 31.93 \(\times\) 10\({}^{15}\) lines/m\({}^{2}\) and 0.016 to 0.026, respectively.