The schematic representation for the synthesis of CdS/CTS is shown in Figure . Cadmium (Cd 2+ ) ions are stirred with CTS for 24h to ensure the coordination between Cd 2+ ions and the functional groups of CTS, i.e., -OH and -NH2 (Figure ). The S 2-ions have a molecular volume of about 25 Å 3 ; thus, they can penetrate easily into Cd-coordinated CTS to react with Cd 2+ forming CdS/CTS (Figure ). The formed material was then separated and dried via a freeze-drying method (Figure ). The mechanism of the material formation was further supported using XRD (Figure ), FT-IR (Figure ), and TGA (Figure ). The materials CdS/CTS QDs were characterized using XRD (Figure ), XPS (Figure ), TEM and HR-TEM images (Figure , Figure ), STEM and EDX analysis/mapping (Figure ), nitrogen adsorption-desorption isotherm (Figure The XRD pattern of CTS showed peaks for Bragg's diffraction angle (2θ) at 10.4° and 20.1° corresponding to the 020, and 110, respectively, representing their crystal lattice constant α of 8.470 and 4.075, respectively . The diffraction peaks indicate the high degree of crystallinity of chitosan. The CrI110 is 62.8% for pure CTS. The Cd-coordinated CTS shows an insignificant change in the diffraction pattern of CTS, except for low crystallinity (Figure ). After the coordination of Cd 2+ ions to the functional groups of CTS, the crystallinity is dropped significantly, making it difficult to determine CrI110 precisely. However, the crystallinity of CdS/CTS after the addition of S 2-is recovered. The CrI110 is 56.8% for Cd/CTS (Figure ). The XRD pattern of CdS/CTS shows peaks at Bragg's angles (2θ) = 26.5°, 30.6°, 43.9°, 51.9°, and 52.1°, which can be indexed as (111), ( ), ( ), ( ) and (311) planes, respectively for the hexagonal phase CdS (JCPDS 42-1411, Figure ). The major diffraction peaks correspond to the CdS phase that matched the simulated XRD pattern for CdS revealing the successful formation of a hexagonal phase of CdS on CTS via the in-situ biomimetic synthesis method (Figure ). According to Scherrer's equation, the average size of the crystallite CdS/CTS is approximately 3.7 nm (Eq. 1). The diffraction peaks' broadening is due to the small particle size of CdS QDs. The diffraction peaks of CTS are still present in the final product of CdS/CTS compared to CdS/CTS synthesized without freeze-drying (Figure ). No extra peaks except for CdS were observed in Figure . This observation indicates that drying CdS/CTS via freeze-drying maintains the crystallinity of CTS in the final product, i.e., CdS/CTS compared to the same materials without the freeze-drying process (Figure ) Before and after coordination with Cd 2+ ions, CTS was characterized using FT-IR spectra (Figure ). The vibration peaks in the wavenumber range of 3300-3450 cm -1 corresponding to the stretching vibrations for the functional groups of hydroxyl, amino, and amide groups showed shifts in the wavenumber's values (Table ). The change in the wavenumber indicates the strong interaction between the functional groups of CTS and Cd 2+ ions. The peaks at 1256 and 1056 cm -1 are assigned to C-H and C3-O stretching, respectively (Table ). The shift in N-H and O-H indicates that Cd 2+ interacted via those functional groups of CTS (Figure ).