I interested to know how can prepare the metal oxide by green synthesis....

In my opinion, green synthesis refers to synthesis with natural substances. For example, the flavonoid rutin, quercetin can reduce the ions of gold, silver, platinum elements to metal nanoparticles. Otherwise, only low-active metal ions can be reduced. From ions of low-active metals can be obtained oxides. For the synthesis using plant extracts, bacteria, broths. We obtained gold nanoparticles in plants. When the minerals of gold, platinum, and silver are abundant in the soil, they can penetrate into the plants and use for their recovery the precursors of gold that are in the soil. On this principle, we have created an electrochemical cell.

Article Preparation and bactericidal properties of silver nanopartic...

Conference Paper Mirgorod Yu. A., Maslobrod S. N. Using Plants Saturated by N...

These articles will be helpful for your study. Please check:-https://www.researchgate.net/publication/317277385_Green_synthesis_of_ZnO_nanoparticles_by_using_Calotropis_procera_leaves_for_the_photodegradation_of_methyl_orange

https://www.researchgate.net/publication/319558032_Biosynthesis_of_zinc_oxide_nanoparticles_using_leaf_extract_of_Calotropis_gigantea_characterization_and_its_evaluation_on_tree_seedling_growth_in_nursery_stage

Thank @Yuri and Devi

@Devi I have gone through the synthesis procedure, but is it necessary to use NaOH in order to get ZnO NPs? Again, is it possible to get Zn NPs not ZnO NPs through green means? What about drying at low temperature such as less than 80oC and calcination at 500oC. Which one is appropriate.

Dear colleague@zaccheus

From my little experience in nanomaterials fabrication through green route

1- NaOH considered as accelerator in the reaction

2- For the appropriate calcination temperature either you measure the melting point or go through literatures to find the temperature that fit with your metal/ metal oxide

3- Be aware to wash your precipitated NPs with dH2O and ethanol or methanol then proceed drying in suitable conditions (oxygen free to get metallic or in air to get metal oxide)

Thank you for your input sir @Yasser