**3. Conclusion**

*Colloids - Types, Preparation and Applications*

**56**

**Figure 16.**

**Figure 15.**

*observation [6].*

**Figure 14.**

*readily seen [6].*

*conditions as in (a) [6].*

*Degradation of MB. (a) Absorption spectra of pure MB sample illuminated by a white light lamp taken at fixed time intervals. (b) Absorption spectra of sample containing small concentration of NCs under the same* 

*High resolution and double-corrected electron microscopy analysis in HAADF-STEM mode for NCs* 

*Fluorescence spectra of nanocolloid obtained by FLASiS (600* μ*J pulse energy) of Ag solid target in water for different centrifugation times. Excitation wavelength is* λ = exc 220nm *. A dominant band at 284 nm is* 

The MB absorbance decreases progressively faster than for pure MB. Dye degradation is easily identified by color change in the solution, from deep blue to faint light blue after exposure to white light (insets in the Figures). Degradation

efficiency amounts to 80%, while for pure MB is only 55%.

FLASiS was used as a "green" method for synthesizing Ag, Ni and Fe metal nanocolloids in liquid media.

Comparison of Ag nanocolloids synthesized by FLASiS and chemical route in aqueous solutions of TSC and st has been discussed. Micro-Raman spectroscopy indicated that stabilizer molecules are adsorbed on the NPs surface and inhibit agglomeration, even up to one year in TSC. FLASiS synthesized NPs seem to stabilize faster that those generated by chemical route, as evidenced by the evolution of their optical extinction spectra.

Magnetic NPs like Ni and Fe colloids were generated in water and *n*-heptane. Spherical NPs shape is almost fully dominant, with a bimodal log-normal size distribution centered at roughly 4 nm and 9 nm radii. Different structures, like hollow type NPs and NiO species were observed. Nanocolloids in *n*-heptane exhibit a greater magnetic response than in water, both showing superparamagnetic behavior.

Fe NPs lack the characteristic optical plasmonic resonance exhibited by noble metals. Raman spectra of the Fe nanocolloids in water and ethanol show Raman peaks of magnetite, hematite or mixtures of them. For the case of water, maghemite signals were also detected.

Few atoms (2–20) Ag NCs can also be synthesized by FLASiS followed by several step centrifugation processes. Fluorescence spectra yield band structures in the range 250 nm to 625 nm, in agreement with the jellium model which predicts HOMO-LUMO type transitions. HRTEM analysis show clusters in different aggregation stages, with coexistence of 1 nm radius NPs together with few atoms NCs. Photocatalytic activity of Ag NCs was assessed against degradation of freshly prepared pure methylene blue. It was found that this efficiency rises 25% in the mixture of Ag NCs and MB.

### **Acknowledgements**

We want to thank Dr. M.B. Fernandez Van Raap and Dr. D. Coral from Instituto de Física de La Plata (IFLP - CONICET) for the VSM measures. Besides, we acknowledge Dr. D. Muraca from Instituto de Física "Gleb Wataghin" (IFGW), Universidade Estadual de Campinas, Brasil and Dr. A. Caneiro from Y-TEC S.A., Argentina, for TEM analysis. We gratefully acknowledge their commitment and dedication.

These works were granted by PIPs 0394, 0280 and 0720 of CONICET, PME 2006-00018, PICTs 2012-1817 and 2016-3205 (ANPCyT), grants 11/I151 and 11/I197 (Facultad de Ingeniería), and 11/X651 and 11/X680 (Facultad de Ciencias Exactas) of Universidad Nacional de La Plata (UNLP), Argentina. We thank C2NANO-Brazilian Nanotechnology National Laboratory (LNNano) at Centro Nacional de Pesquisa em Energia e Materiais (CNPEM)/MCT (#14825, #14827, #16976, #18425, #19927 and #22345) and Research proposal TEM-16976 for the use of TEM. VSM was carried out at IFLP. Synthesis of nanocolloids by FLASiS, and OES, MRS, fluorescence and photocatalysis studies were performed at CIOp (CONICET - CIC - UNLP), La Plata. We acknowledge Y-TEC S.A. for the use of TEM FEI TALOS F200X.

D.C.S. and V.B.A. are researchers of CIC. L.B.S., J.M.J.S. and D.M.A. are researchers of CONICET, Argentina.

*Colloids - Types, Preparation and Applications*
