In presence of sodium hydroxide, reducing and stabilizing agents (eg. modified starch, cellulose or chitosan), the salts of copper, iron or aluminum are converted to the corresponding copper, iron or aluminum oxide nanoparticles, while in case of silver it is converted to silver nanoparticles.
Reduction of the metal ions to its corresponding metal i.e., zero oxidation state, depends on the reduction potential,of the metal. The value of reduction potential you ll get from the electrochemical series. If the value is more positive there is more chance of formation of the NPs rather than oxide. For example Au+/ Au = +1.692 ; Ag+/Ag = +0.7996, Cu2+/Cu = +0.34 and so on. Hence, it is easy to reduce gold and silver than the copper.
Reduction of the metal ions to its corresponding metal i.e., zero oxidation state, depends on the reduction potential,of the metal. The value of reduction potential you ll get from the electrochemical series. If the value is more positive there is more chance of formation of the NPs rather than oxide. For example Au+/ Au = +1.692 ; Ag+/Ag = +0.7996, Cu2+/Cu = +0.34 and so on. Hence, it is easy to reduce gold and silver than the copper.
It's the same reason why iron rusts and gold doesn't! It's basic chemistry involving the reduction potential of the metals and the fact that we live in an atmosphere with oxygen. I'm sorry but before trying to make nanoparticles you should learn the chemistry involved.
It can be understood like this. The salts of metals that are easy to be oxidized in nature will be converted to metal oxide nanoparticles. The salt of the metal that are the more stable in nature, such as gold, the more easy to form metal nanoparticles.
Silver is noble metal and so are its salts mostly. As discussed in above answers it has much to do with its reduction potential. In my view if you maintain the pH of the medium carefully you can get a metal or an oxide. Similar is the condition with Gold but Platinum could be a bit tricky.
Thank you Amaresh Sahoo, Pedro Quaresma, Guohua Zhou and Anal Jha for your answers. In a typical experiment ( same temperature, same time, same carbohydrate ( reducing and stabilizing agenrt), Ag was converted to AgNPs while Cu was not. By adding NaOH equivalent to copper salt, copper was converted to CuNPs.However, in other experiment, while using different carbohydrate, same copper salt, NaOH equivalent to copper salt, different temperature, CuO NPs was produced instead of CuNPs!!!! Meaning that in addition to reduction potential of the metal, the reaction conditions have much to do.
All answers above are absolutely right. Electrochemical (reduction) potentials of silver (gold and some other metal ions) are quite high in wide pH range to be easily converted into metallic form. So silver ions and, especially gold ions, form metal particles of different size. Another reason is relatively low stability of as-prepared oxides of aforementioned metals (especially gold). Moreover keep in mind that (1) in non-deaerated conditions silver nanoparticles in many cases have silver oxide shell and (2) crystal latice of silver oxide is similar with metallic silver (similar XRD patterns).
Thank you Alexander Tolstov. It is really good and valuable discussion.
According to the electrochemical series the corresponding value of reduction potential in each case should be taken into consideration. On top of that, the reduction potential plays the main rule in the proses of converting the metallic nanoparticles.
As discussed above about the reduction potential, formation of nanoparticle is also determined by the reduction ability of reductive reagents. High reduction ability is needed to reduce copper salt to CuNP. So, different carbohydrates result in different nanoparticles.
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