This link can give you an idea about optical properties of AuNPs.

http://www.sciencedirect.com/science/article/pii/S2090123210000056.

These specific physio-chemical properties of AuNPs may affect their synthesis in cold temperatures. Another concern is about the rate of synthesis. For more clear search you can look for temperature based synthesis procedures.

You must take a look to the redox process. Broadly speaking, to synthesize Ag NPs, you need just 1 electron (From Ag(I) to Ag(0)). In the case of Au NPs you need 3 electrons (From Au(III) to Au(0)). Therefore, the REDOX reaction kinetics of Ag NP´s synthesis is faster.

Due to different redox potential of Ag and Au. 

[AuCl4]- + 3 e− ↔ Au(s) + 4 Cl-             E0 = +0.93

Ag+ +  e−  ↔ Ag(s)                             E0 = +0.7996

It depends on the reduction agent used; with some of them AuNPs can also be obtained at room temperature. But generally, Ag(I) is easier to be reduced to metal then Au(III), due to the different position of Au and Ag in the electrochemical series (Volta) of redox potentials. 

Guys, thanks for all the answers.

Sukirtha, I've read that report already, and I've read about temperature based synthesis, thus being the reason that I asked this question. I was wondering why there wasn't anything written on the subject.

Shi-yi, Rafael, Rahul and Ossi, thanks for the clarifiying, I guess that should really be the answer.

Thanks again guys,

Brian.

To tell you upfront, I don't know the answer to your question but here are some thoughts: First, I am not sure your observation is valid. For both Au and Ag nanoparticles there are preparative methods that involve high and low temperatures. Silver nanoparticles are more sensitive to oxidation, and that may be the reason why you want to avoid higher temperatures if you can. Gold nanoparticles, on the other hand, are more stable and can often be annealed to become more uniform by boiling the suspension for some time. I believe this is the main reason for what you have observed. While the quality of samples of gold nanoparticles usually improves at higher temperature, silver particles tend to deteriorate under the same conditions. 

It actually depends upon the reducing agent or the product you are trying to generate. For example NaBH4 reduction of AuNPs can be done in room temperature whereas for citrate reduction you need more than 60 degree. Again to synthesis AuNPs rod often scintists have used cold temperature.