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It sounds like were thinking of reproducing photosynthesis outside a plant, which is something many people have thought about. It's a tricky problem. One thing to keep in mind is that photosynthesis doesn't make energy from nothing. It requires an energy input in the form of photons, which the cell converts to chemical potential energy through a series of oxidation-reduction reactions. It is a multi-step process. So, replicating photosynthesis outside the cell definitely takes a form of energy (photons) and the combining of chemicals--that's what it happens inside the cell, too. Keep in mind that energy is always conserved.
Some kinds of plants are used to remediate environmental contamination, and you might be able to design an experiment that explores that avenue. Try a Google search for "bioremediation" and see if that sounds like an interesting topic. If so, brainstorm some questions you think would make an interesting science project. This Science Buddies page has excellent information about formulating a question:http://www.sciencebuddies.org/science-f ... tion.shtml
If you want to, you can then post your brainstormed questions here and we can help you design an experiment to answer one of them.