## 2 Materials and Methods  
### Mechanochemical Synthesis and Characterization of CuBi\({}_{2}\)O\({}_{4}\) NPs  
Copper (II) acetate dihydrate Cu(CH\({}_{3}\)COO)\({}_{2}\cdot\)2H\({}_{2}\)O (99.9%, Aldrich, Saint Louis, MO, USA), bismuth (III) acetate Bi(CH\({}_{3}\)COO)\({}_{3}\) (99.9%, Aldrich, Saint Louis, MO, USA), sodium hydroxide NaOH (98%, Aldrich, Saint Louis, MO, USA) and acetone CO(CH\({}_{3}\))\({}_{2}\) (99.5%, Aldrich, Saint Louis, MO, USA) were purchased and used as received, without further purification. Ultra-pure water (18 M\(\Omega\) cm\({}^{-1}\)) was obtained from a Barnstead E-pure deionization system (Thermo Scientific, Waltham, MA, USA). The nanoparticles of CuBi\({}_{2}\)O\({}_{4}\) were obtained following the procedure reported by [PERSON] et al. [13], as follows: 5 \(\times\) 10\({}^{-4}\) mol (0.09 g) of Cu(CH\({}_{3}\)COO)\({}_{2}\cdot\)2H\({}_{2}\)O and 1 \(\times\) 10\({}^{-3}\) mol (0.38 g) of Bi(CH\({}_{3}\)COO)\({}_{3}\) were ground in an agate mortar for approximately 10 min, and then 4 \(\times\) 10\({}^{-3}\) mol (0.16 g) of previously ground NaOH was added. The mixture was ground until a dark brown powder was obtained. This product was washed four times with water and twice with acetone. In each case, it was separated by centrifugation at 3000 rpm for 10 min and finally air dried. This procedure does not necessarily carry out any heat post-treatment and is environmentally friendly because it excludes the use of a large quantity of solvents. The chemical reaction is as follows:  
\[\text{Cu(CH${}_{3}$COO)}_{2}\bullet 2\,\text{H${}_{2}$O}+2\,\text{Bi(CH${}_{3}$COO)}_{3 }+8\,\text{NaOH}\;\rightarrow\;\text{CuBi${}_{2}$O${}_{4}$ NPs}+8\,\text{NaCH${}_{3}$COO}+6\, \text{H${}_{2}$O}\]  
The yield of the reaction was 85%, and it was carried out in triplicate to guarantee the reproducibility of the synthesis method.  
A Raman spectrum of 100 to 900 cm\({}^{-1}\) was acquired by a dispersive Raman spectrometer, Nicolet Almega XR (Thermo Scientific Nicolet, Waltham, MA, USA), and detected by a CCD camera, at 25 s and a resolution of ~4 cm\({}^{-1}\). The excitation beam was a Nd:YVO\({}_{4}\) 532 nm laser, and the incident power on the sample was ~3 mW. FTIR analysis was performed on KBr pellets with powdered CuBi\({}_{2}\)O\({}_{4}\) NPs, in a Thermo Nicolet Nexus spectrometer (Thermo Scientific, Waltham, MA, USA). X-ray diffraction patterns were acquired at room temperature with Cu K\(\alpha\) radiation (\(\lambda\) = 1.5406 A\({}^{\circ}\)) in a D5000 Siemens diffractometer (Bruker, Billerica, MA, USA); diffraction intensity was measured between 2.5\({}^{\circ}\) and 70\({}^{\circ}\), with a 20 step of 0.02\({}^{\circ}\), for 0.8 s per point. The average crystal size (D) of the NPs was estimated from their diffractograms, using the Debye-Scherer formula, D = k\(\lambda\)/ \(\beta\)cos\(\theta\), where \(\kappa\) is the shape factor equal to 0.9, \(\lambda\) is the CuK\(\alpha\) radiation, \(\beta\) is the full width at half maximum intensity of selected peaks (FWHM) and \(\theta\) is the Bragg angle. High-resolution transmission electron microphotographs (HR-TEM) were obtained with a JEOL ARM200F analytical microscope (JEOL, Aksishima, Tokyo, Japan) operating at 200 kV, by deposition of a drop of the powdered CuBi\({}_{2}\)O\({}_{4}\) NPs dispersed in ethanol onto 300-mesh Cu grids coated with a carbon layer. UV-visible electronic absorption spectra of the powdered samples were obtained by the diffuse reflectance technique, with an Ocean Optics USB2000 miniature fiber-optic spectrometer (Ocean Optics, Dunedin, FL, USA).