I wrote some thoughts on nootropics over here.
I’m personally not a believer in taking supplements and nootropics (except when recommended or prescribed by a health professional). I think that the supplement industry tends to be full of scams, because supplements are cheap to produce and easy to sell at a high markup. They appeal to people’s desire to want results without doing the actual work required. There may be detrimental side effects even with things like taking beta carotene in pill form.
If you want better attention: get exercise , take naps, don’t multitask , etc.