Green synthesis of chemical converted graphene sheets derived from pulping black liquor

The present chemical converted graphene sheets are mostly dependent on the exfoliation of graphite with strong oxidants, which suffer from heavy burden of environmental pollution. Here, we report a simple, green and scalable method to synthesize graphene sheets by one-pot activation-synthesis method derived from black liquor, a biomass waste resource in pulping industry. The pulping black liquor consisting of alkali lignin as main components with phenyl skeletons and condensed aromatic segments was successively transferred from heterogeneous chemical states into micron-sized graphene sheets. The proposed conversion mechanism mainly consists of aromatic rebonding, refusion and deoxygenation reaction, in which alkaline species acting as situ-activation agent. The graphene sheets obtained from pulping black liquor (BL-G) show typical hexagonal honeycomb lattice in thickness of 1–3 atomic carbon layers with controllable lateral size ranging from 0.36 to 2.5 μm. After spin-coating single- or few-layer of BL-G sheets onto flexible substrate, it could produce transparent and conductive thin films (TCFs) with sheet resistance as low as 2.1 kΩ sq−1 and 85% transmittance for 550 nm light, suggesting its great potentials using as flexible electrode materials. This green synthesis route may pave a bright way in large-scale production of chemically converted graphene sheets.