UV-Visible Optical Absorbance of Graphene Oxide Synthesized from Zinc-Carbon Battery Waste via a Custom-Made Ultrasound Generator based on Liquid Sonication Exfoliation Method - Rhyko Irawan Wisnuwijaya; Colloids and Soft Material Group, Department of Physics Education, Universitas Negeri Yogyakarta, Karangmalang Complex, Yogyakarta 55281 (Directorate of Research and Community Engagement, Universitas Indonesia, 2017)