I am in the process of collecting material for my story. The underlined word is a/an-

Material is used as a noun here. It is the substance or content being collected (উপাদান), and a thing you can collect is a noun.

Material (noun): the matter from which a thing is or can be made (বস্তু; জড় উপাদান).
Example: Goats can eat more or less any plant material.

Material (adjective): physical rather than mental or spiritual (বস্তুগত); also significant or important (গুরুত্বপূর্ণ).
Examples: The material world; The insects did not do any material damage to the crop.