[yswong90]

In the synthesis of Silver nanoparticles, one of the step is to ice bath the sodium borohydride (2mM) and silver nitrate (1mM) while mixing both of them together slowly to produce yellow colloidal silver.

My question is, why is the ice-bath needed? Does it mean that the reduction process between silver nitrate and sodium borohydride require a cold condition?

However, in the reduction process of a more highly concentrated Silver Nitrate (1M) and Sodium Borohydride (2M), there was no mention of ice water bath in the entire process. Why is that so? In fact, the reaction was aggressive and the solution was quite hot.

As for the other reduction process like the one in silver nitrate and sodium citrate solution, a hot condition (involving boiling of silver nitrate solution) is needed. Why is that so?

Would really appreciate it if someone can explain these three questions for me. Thanks and have a nice day ahead!

[Corribus]

Citrate is a much milder reducing agent than borohydride.  I've never made AgNPs with borohydride, but I suspect it is cooled to slow the reaction and give better control over final particle size/shape.  Citrate serves a dual function of both reducing agent and capping agent, and citrate-reduced particles tend to have nice spherical shapes but also be fairly large.

[yswong90]

Corribus, thanks for your reply

This is what I found from the internet :

"In most reactions with sodium borohydride, the aldehyde or ketone is dissolved in the reaction solvent and
a solution of sodium borohydride is added, with external cooling if necessary, at a rate slow enough to keep
the reaction temperature below 25°C. Higher temperatures may decompose the hydride, and adding the
carbonyl compound to the alkaline sodium borohydride solution may cause side reactions of base-sensitive
substrates. The amount of solvent is not crucial, but enough should be used to completely dissolve the
reactants"

Although there is no carbonyl compound involved in my case, could the high temperature decomposition of the hydride which might cause side reactions of base-sensitive substrates be the answer to my question?

As usual, all opinions are welcomed! thanks!

[AWK]

http://www.ktu.edu/lt/mokslas/zurnalai/medz/medz0-87/03%20Electronic...(pp.287-291).pdf
http://www.rad.nd.edu/recent_pubs/Site/Subtask%203_files/3_2004_Pillai_JPCB.pdf

[yswong90]

Thanks for the links..

Unfortunately, non of the links (and most of the links / papers i found in the internet) mentioned about the REASON why ice bath cooling is needed for sodium borohydride. 

[AWK]

Excerpt from the first link
"The purity of water and reagents, cleanliness of the
glassware are critical parameters. Solution temperature,
concentrations of the metal salt and reducing agent, reaction
time influences particle size"

[yswong90]

Thanks AWK 

[yswong90]

Dear all, I am recently reading about the reduction of silver nitrate by sodium borohydride to form silver nanoparticles.

However, upon searching for the procedures in journals on the internet, I discovered that sodium borohydride needs to be cooled down by ice bath. This is not only true in the synthesis of silver nanoparticles but also other organic compound (for example, reduction of vanillin by sodium borohydride).

My question is : Why is the ice bath on sodium borohydride needed? Is it be due to the fact that reaction of sodium borohydride is more stable at low temperature? If yes, can anyone please explain in more details to me?

Thanks in advance.. and have a great day ahead