If I remember well, in classic photographic film AgBr is reduced into Ag0 wil the Br0 is captured by the gelatin emulsion. The gelatine catalyzes the reduction of Ag+ into Ag0.

PVP selectivily bind to Ag(100) plane which promotes the growth of silver nanocubes. On the other hand, citrate ions bind to Ag(111) plane which allows the growth of silver triangular and discal nanoplate. I suggest you the next paper about silver nanoplates

Article Photoinduced shape transformation from nanospheres to silver...

a water soluble stabilizer like SDS and a

strong reducing agent NaBH4 is commonly used. Silver nitrate is reduced

by sodium borohydride in the presence of stabilizer (SDS), resulting

in silver nanoparticles, according to the following Eq:

Ag++BH4−+3H2O→Ago+B(OH)3+3.5 H2

Also, in the formation of silver nanoparticles by the chemical reduction

method, the order of reactant addition, dropping silver nitrate solution

into NaBH4 solution with stabilizer, is important to obtain stable

silver nanoparticles. The reverse order of reactant addition causes

the immediate precipitation of silver nanoparticles. Moreover, when using Stirrer, by increasing the stirring speed it you may be able to have smaller nanoparticles.... hope this works.