Cost-effective synthesis of high-structural integrity graphene nanoplatelets (GNPs) remains a grand challenge. This study presents an innovative, environmentally friendly and efficient method for synthesizing high-quality GNPs by integrating wet thermal expansion with liquid-phase exfoliation. The thermal expansion of a wet graphite intercalation compound (GIC) in a semi-closed system improves the expansion effectiveness, minimizes the release of fine dust and improves the hydrophilicity of the resulting product. These improvements significantly enhance the effectiveness and efficiency of the subsequent liquid-phase exfoliation by shearing, which produces GNPs in pure water within 30 min with an overall productivity of 38.24 %. GNPs exhibit desirable properties, e.g. few layers (1–2 nm thick), large lateral dimension (a few microns), high sp2 hybridized carbon content (86 %), rich surface functional groups yet a low oxidation level (C/O ratio = 21.7 and electrical conductivity of ∼ 2055 S cm−1). The simplicity, adjustability and scalability of this method, combined with environmental friendliness, make it highly suitable for various applications and pave the way for further advancements in sustainable nanotechnology practices.