The heating was done under inert atmosphere, which enabled the reduction of the number of oxygen vacancies thereby eventually generating more free carriers. Additionally, the tin-complexes on the surfaces of ITO crystals are also expected to break at this temperature leading to the generation of more free tin ions, which could diffuse into the crystalline structure of ITO and contribute more electrons to the surface plasmon. While I could Proof/explain the presence of additional free electrons based on the shifting optical properties in the IR data, I still have little/no clue on why the solution color change from tranparent to dark green since ITO has LSPR in the NIR region.