**3. Results and Discussions**

*3.1. Synthesis of CuI Nanostructured Materials*

CuI powdered materials show a fine powder consistency and light creamy color. A high yield of the synthesis reaction was observed (>90%). White homogeneous colloidal suspensions of CuI were also fabricated (Figure 2). The colloids show stability for about two weeks. There are no significant differences in the appearance of the colloidal suspensions, concerning the polymer employed. The absence of greenish-blue tonalities confirms the reduction of Cu <sup>2</sup>+to Cu <sup>1</sup><sup>+</sup> species.

**Figure 2.** Synthesis of CuI nano-structured materials by a controlled precipitation method followed of reduction. Biopolymer (Arabic gum and chitosan were employed to increase colloidal stability). The materials (powders and colloids) exhibit a white color, characteristic of Cu(I) materials.

#### *3.2. Characterization of CuI Nanostructured Materials*

**SEM-EDX.** Figure 3 shows the SEM characterization of CuI nanostructured materials. At low magnifications, the materials exhibit a shell-like morphology (Figure 3A,C). Figure 3C clearly shows the 3D shell-like architectures with controlled internal microstructures. Figure 3B,D show the materials at higher magnifications. The materials are composed of smaller particles that are orderly aggregated in 3D shell structures. SEM-EDX analysis (Figure 3E,F) reveals the composition and purity of the materials. The weight percentages of the elements are in agreement with a CuI formulation (33.4% Cu and 66.6% I). Figure S1 (Supplementary information) shows a representative micrograph of CuI with the corresponding EDX elemental mapping for copper and iodine. Only Cu and I are present and well dispersed in the CuI lattice, demonstrating the successful preparation of the material.

**Figure 3.** SEM-EDX analysis of CuI powdered nano-structured materials. (**A**) low magnification SEM micrograph of CuI powdered material, the shell like morphology of the materials is easily seen. (**B**) high magnification SEM micrograph of CuI powdered material, illustrating the ordered nanostructures that compose the 3D shell microstructures. (**C**) magnification of dotted zone in A. (**D**) magnification of dotted zone in B. (**E**) SEM micrograph for EDX analysis. (**F**) EDX analysis of CuI powdered materials.

**AFM.** Figure 4 portrays the AFM analyses of AG and CuI@AG thin films. Figure 4A–C show the morphology of AG films. The polymeric films (AG) are very thin (≈4 nm) and exhibit a uniform structure and composition. In comparison, CuI@AG films are thicker (≈30 nm) due to the insertion of CuI NMs in the polymeric matrix (Figure 4D–F). AFM analysis confirms the shell morphology of the CuI materials observed with SEM analysis. Tapping phase analysis (Figure 4C) of AG demonstrates the presence of pure polymer, while the CuI@AG phase analysis (Figure 4F) denotes phase changes due to inorganic (CuI) and organic (polymer) composition [21].

**Figure 4.** AFM analysis of Arabic gum (AG) and CuI@AG films. (**A**–**C**) Arabic gum thin films; height, amplitude and phase analysis, respectively. (**D**–**F**) CuI@AG thin films; height, amplitude and phase analysis, respectively.

AFM analyses of chitosan and CuI@Ch films are presented in Figure 5. Twodimensional topography analysis of chitosan is shown in Figure 5A, illustrating its smooth surface and homogeneity. Figure 5D–L display the height, amplitude, and phase images of CuI@Ch composites at increasing magnifications. The AFM images at low magnifications reveal the presence of uniformly distributed CuI nanostructured materials with shell-like morphology. At higher magnification, a detailed inspection of the composite nanostructure denotes the presence of small (≈50 nm) ordered particles with a hierarchical 3D shell-like morphology.

In tapping mode, phase analysis is capable of imaging the nanostructure of blend or multiphase materials with high resolution of submicron length scales. We can observe evident differences in the phase images of chitosan films (Figure 5C) in comparison to CuI@Ch films. For Ch films, only one phase is observed, corresponding to the biopolymer, whereas for the composite, inorganic (CuI) and organic (polymer) phases are represented.

**Figure 5.** AFM analysis of Chitosan (Ch) and CuI@Ch films. (**A**–**C**) Chitosan thin films; Height, amplitude and phase analysis respectively. (**D**–**L**) CuI@Ch composite films; height, amplitude and phase analysis at different magnifications.
