# Promising Cr-Doped ZnO Nanorods for Photocatalytic Degradation Facing Pollution
## 2 Materials and Methods  
### Synthesis Route of Cr-Doped ZnO Nanorods  
Pure and Cr-doped ZnO nanorod samples were synthesized using the thermal decomposition technique. The typical synthesis route of Cr-doped ZnO followed the procedure given in our previous work [11]. Zinc acetate dihydrate \"Zn (CH\({}_{3}\)COO)\({}_{2}\). 2H\({}_{2}\)O\" (Winlab, purity: 99.999%) and chromium (III) acetate hydroxide \"(CH\({}_{3}\)CO\({}_{2}\))\({}_{7}\)Cr\({}_{3}\)(OH)\({}_{2}\)\" (Sigma-Aldrich, St. Louis, MO, USA) were used as precursors for synthetization of Cr-doped ZnO nanorods. The thermal decomposition route process can be summarized as follow. Prior to adding chromium (III) acetate hydroxide \"Cr III OAc hydroxide, (CH3 CO2)\({}_{7}\)Cr\({}_{3}\)(OH)\({}_{2}\)\", zinc acetate dihydrate \"Zn OAc dehydrate, Zn (CH3 COO)\({}_{2}\) 2H\({}_{2}\)O\" was grinded for 2 h via _a single vibrating mill machine with agate_ mortar and _a milling ball (3 cm in diameter). Then, the milled powder was subjected to another 2 h of grinding after adding the_ desired amount for each sample from chromium (III) acetate hydroxide. The obtained fine powder was introduced into a covered 50 mL alumina crucible and placed in an oven for 4 h at 350 \({}^{\circ}\)C for the decomposition process. The weight ratio of (CH\({}_{3}\)CO\({}_{2}\))\({}_{7}\)Cr\({}_{3}\)(OH)\({}_{2}\): Zn (CH\({}_{3}\)COO)\({}_{2}\) 2H\({}_{2}\)O varied from 0 to 5 wt.%. A summary of the prepared samples is given in Table 1.  
The characterization of the final powder confirmed the formation of wurtzite hexagonal phase without other ZnO phases, or any phases based on chromium as discussed elsewhere [11]. This refers to the stability of the host structure of ZnO crystal, although with the addition of Cr atoms due to substituting the Zn sites in the lattices by Cr, which is usually suggested for a low concentration of Cr [11]. The morphological and structural properties of the synthetized Cr-doped ZnO nanorod were shown via transmission electron microscopy \"TEM\" (type JEOL-JEM-1230). PL measurements were obtained through an He-Cd laser of 325 nm excitation wavelength. XRD was performed using monochromatized CuK\(\alpha\) radiation (\(\lambda\) = 1.5418 A). X-ray photoelectron spectroscopy (XPS) measurements were analyzed via ESCALSB 250 electron spectrometer under ultra-high vacuum (\(p\) < 10\({}^{-9}\) mbar). The data were collected at normal incidence, i.e., the sample surface was vertical to the emission angle. A mono-chromatic AlK\(\alpha\) source (1486.6 eV) functions using a power of 420 W (30 mA and 14 kV), where the C1s core (284.6 eV) was used as reference to calibrate the peak positions of other elements. The photocatalytic activity (PAC) of the synthesized Cr-doped ZnO nanorods was inspected under UV-vis irradiation via a UV-vis spectrophotometer (Unico UV-2100, Waltham, MA, USA).  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Weight Concentration of Cr III OAc** & **Photocatalytic Reaction:** \\  & **Hydrooxide and Zinc Acetate Dihydrate in** & **Methyl Orange (MO)** \\ \hline \multirow{2}{*}{\(ZnO\)-\(Cr_{0}\)} & \multirow{2}{*}{0} & 165 mL from 10 ppm aqueous of methyl orange \\  & & 3.3 mg from each sample \\ \hline \multirow{2}{*}{\(ZnO\)-\(Cr_{1}\)} & \multirow{2}{*}{1} & \multirow{2}{*}{Then stirred for 10 min followed by 5 min sonication until adsorption-desorption equilibrium.} \\  & & \\ \hline \multirow{2}{*}{\(ZnO\)-\(Cr_{3}\)} & \multirow{2}{*}{3} & \multirow{2}{*}{A 3 mL of each solution was filtered than examined each 10 min} \\  & & \\ \end{tabular}
\end{table}
Table 1: Cr-doped ZnO nanorod description: weight concentration of Cr III OAc hydroxide and zinc acetate dihydrate changes in percentage of the investigated sample ZnO-Cr\({}_{0}\), ZnO-Cr\({}_{1}\), ZnO-Cr\({}_{3}\) and ZnO-Cr\({}_{5}\) photocatalytic reaction sample treatment.