Contamination of your sample by the ball material?  More information needed to provide an answer - type of milling media, type of mill, material milled etc...

The longer the time spent milling, up to a point, the smaller the powder particles. You've mentioned in your question that you've gone from a micron sized to nanometre sized particle range. Particle size will affect the reflectivity of your sample. If you're measuring the UV-absorption in reflection mode than this reflectivity change may be due to particle size change. The wavelength or energy where you go from high to low absorption should still be pretty much the same unless you've significantly altered the structure (very possible) or contaminated the specimen.

Hi Mahesh

Alan Rawle and Rob Aughterson have given you two very good answers.

A further possibility is that by producing a nanopowder by ball milling, you can actually pack more of your material into the sample holder, thereby increasing the amount of material that is sampled during your UV-Vis absorption measurements (i.e. a micron sized powder will pack much less efficiently than a nanopowder).

Also, ball milling can generate an enormous amount of heat, which if not controlled can thermally transform your material into something quite different. You definitely want to do XRD on your material before and after ball milling to make sure there is not an unwanted phase change or undesirable chemical transformation.

Best

Geoff

Hello Mahesh, some time ago I had similar observations. Apart from the facts mentioned by the previous gentelmen in their answers, in my case above all, this effect was due to the presence of impurities coming from the high-energy grinding process. Note that, during high-energy milling process, for example in planetary ball mills, grinding media can be subjected to abrasion. The amount of these impurities depends on many parameters: the type of the mill, the material from which the grinding media are made (Zirconium oxide, Steel, Silicon nitride etc.) , the rotation speed of the mill, the grinding time, the mass ratio of the balls to the milling material etc. So what Alan also paid attention to in his answer.

Dear mahesh

the spectrum of UV-Vis commonly take in solution state.in this state the intensity of ball milled powder solution independ of particle size. also the intensity depend on amount of concentration of solution.do not mistake the intensity and wavelength (energy). if we observe the change in wavelength therefore we can conclude the impurity can be affected. varying the intensity just depend on concentration amount of solution.