Adding the humic acid coating to the nanoparticles of zinc oxide (ZnO-NP) may improve the properties necessary for their colloidal stability. To show how humic acid coating affects the properties of ZnO-NP, three differently sol-gel synthesized ZnO-NP were synthesized: pristine zinc oxide nanoparticles without coating (p-ZnO-NP) and humic acid coated zinc oxide nanoparticles at two different initial concentrations of 20 mg/L (HA20-ZnO-NP) and 200 mg/L (HA200-ZnO-NP) of humic acids in the starting solution. All ZnO-NP were found to be nanocrystals of mineral zincite exhibiting wurtzite crystal symmetry. Transmission electron microscopy showed that capping by humic acids during synthesis decreased the size of HA20-ZnO-NP and HA200-ZnO-NP compared to p-ZnO-NP nanoparticles. Via experiments, HA20-ZnO-NP were found to dissolve less and have a similar or higher stability than both p-ZnO-NP and HA200-ZnO-NP.

Department of Soil Science and Geology Faculty of Agrobiology and Food Resources Slovak University of Agriculture in Nitra Tr A Hlinku 2 949 76 Nitra Slovakia

Institute of Electrical Engineering Slovak Academy of Sciences Dúbravská cesta 9 841 04 Bratislava Slovakia

Institute of Laboratory Research on Geomaterials Faculty of Natural Sciences Comenius University in Bratislava Ilkovičcova 6 842 15 Bratislava Slovakia

Institute of Nuclear and Physical Engineering Faculty of Electrical Engineering and Information Technology Slovak University of Technology in Bratislava Ilkovičcova 3 812 19 Bratislava Slovakia

Materials Research Centre Faculty of Chemistry Brno University of Technology Purkyčnova 464 118 612 00 Brno Czech Republic

Nanotechnology Centre VŠ B Technical University of Ostrava 17 listopadu 15 2172 708 33 Ostrava Poruba Czech Republic

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