Hi, your metal nanoparticles are very reactives with the environment, while they be smaller will react mainly with the oxygen, it is necesary protect them in inert atmosphere. On the other hand, seriously I find a terrible mistake in your basic concepts. In metals you can`t speak of band gap, because the metals are gapless or their bands are overlaped. The absorption peak seen in UV-Vis is due to surface plasmon resonance.

Hi, as Esteban pointed out, there is no band gap in metal nano particles, and the peak you observe in Uv-Vis is because of SPR (Surface Plasmon Resonance) effect. Or is it  "metal oxide" nano particles you are dealing with? In general, are you referring to the absorption onset? because it becomes increasingly complicated to correlate the change in absorption onset and relate it to band gap when you are carrying out analysis in liquid environment!!. 

Still, if its metal nanoparticles you are dealing with, there is no such thing called band gap in them and the peaks you are dealing with is Surface Plasmon Resonance peaks and they tend to change if your sample aggregates after a specific amount of time. It happens when you haven`t capped/stabilized the nano-particles appropriately. You should probably support yourself with additional experiments (DLS, zeta analysis)

Warm wishes,

Ram.

In the case of a metal, when the size is reduced to nano-scale the quasi-continuous density of states split into discrete electronic levels. This generates a bandgap which increases with the decrease in particle size.  Such effects could be observed in both semiconductor and metal nanoparticles, where with a decrease in the particle size the band structure evolves. The phenomenon is  purely an electronic effect.

On the other hand, once the particle tends to agglomerate their energy levels are no more discrete and a gradual decrease in band gap could be observed. Since the particle was tested after 10 days, it is quite possible to get agglomerated with an increase in particle size which corroborates with the above explanation.

Hope this answers your question. 

Reference:

1. G. Schön, G., Simon, U., Colloid Polym. Sci. 1995, 273 , 101. 

2. Weller, H., Eychmüller, A. Semiconductor Nanoclusters; Elsevier Science; Amsterdam, 1996. 

Hi tanzeeha,

U haven't mentioned your particle size. If you are dealing with very small particle sizes, you could experience a lot of difference in properties as Rohit suggested. You should check your stability first then go on do other experiments.

sorry Eteban i didnt explain my work properly im studing different condition when metal transform in to semiconductor and average particle size is 4 to 5 nanometer

Tanzeeha,

There are various metal nanoparticles ,Ag,Au,Fe,Ni,..... etc the behavior of each of them with the time to the environment will be different,even rate of oxidation,agglomeration,aggregation will also be different ,colour of your sample depends size,concentration,degree of oxidation even coating on nanoparticles if any or others factors, YOU can keep your sample without change for several weeks if proper parameters such as ratio of nanoparticles,coating agent,surfactants,stabilizer maintained properly as reported in various articles ,my suggestion is that you should ask to your supervisor about your observations.he/she will give you better suggestions 

Regard 

Muhammad imran khattak