Synthesis Of Lignin-based Graphene Nanomaterials And Their Applications

As the low-valued utilization of lignin,and also the high cost conversion of graphene materials,the lignin with phenyl skeletons and condensed aromatic segments was successively transferred from heterogeneous chemical states into 0D and 2D homogenous graphene materials including Graphene quantum dots?GQDs?and micron-sized graphene sheets.These high quality GQDs with good biocompatibility have potential to be excellent nanoprobes for multicolour bioimaging.The micron-sized graphene sheets with good transparency and conductivity transparent have great potentials using as flexible and conductive thin films?TCFs?.Furthermore,the as-synthesized GQDs and BL-G sheet could be assembled into ultrafine electrode?GQDs/BL-G?for boosting the specific capacitance of lignin-based supercapacitor.The lignin derived graphene materials open a bright window for the high valued utilization of lignin,and allows lignin for the development of high performance and sustainable applications.The main component consists three parts listed as following:The high quality single-crystalline GQDs from biomass waste,lignin was prepared via a two-step method in a large scale.The synthetic processes mainly consisted of the oxidized cleavage step and aromatic refusion step of alkali lignin?AL?molecules.The as-synthesized single-crystalline GQDs with a typically hexagonal honeycomb graphene network,present 1–3 atomic layers thickness.Due to the well construction of perfect graphene lattice,the GQDs could exhibit good luminescence with of 21%quantum yield?QY?,much higher than most reports.The GQDs presenting good long-term photostability,water solubility and biocompatibility,could be used as excellent nanoprobes for multicolour bioimaging,which could well solved problems of high expense,heavy toxicity and poor photoluminescence property.Based on the previous works related with the notice of the conversion from amorphous lignin to zero dimensional ordered graphene structure?GQDs?,a simple,green and scalable one-pot activation-synthesis method was developed to synthesize mirometer-sized graphene sheets from black liquor mainly consisting of alkali lignin?AL?.The AL with phenyl skeletons and condensed aromatic segments was successively transferred from heterogeneous states to well ordered micron-sized graphene sheets.The conversion mechanism could be concluded as three steps as 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 typically hexagonal honeycomb lattice with thickness of 1-3 atomic carbon layers.Due to the structure oriented property,the controllable adjust of lateral size ranging from 0.36 to 2.5?m could be of interests for graphene synthesis to meet demands of diverse fields requiring specific structure.After spin-coating few-layered BL-G sheets onto flexible substrate,it could prepare transparent and conductive thin films?TCFs?with low sheet resistance of 2.1k?sq^-1 and 85%transmittance for 550 nm light,suggesting its great potentials used as flexible electrode materials.This green synthetic route may pave a bright way in versatile,green,large-scale production of chemically converted graphene sheets and well make up the weakness including heavy burden of environmental pollution for current chemically converted graphene.After the well controllable synthesis of GQDs and graphene sheet,the assembling of GQDs and BL-G junction could be well constructed to modify the electrochemical property of energy storage devices.Green,sustainable and high-performance supercapacitor was highly demanded especially from low-cost or even biomass waste lignin.The majority of lignin-based supercapacitor always presents low capacitance and inferior electrochemical performance due to the easy stacks and uncontrollable morphology.Based on the controllable synthesis of GQDs and BL-G sheet,herein,we designed a hybrid sandwich-like structure of GQDs and BL-G sheet?GQDs/BL-G?from lignin via a one-step,low cost and facile method.Both of GQDs and BL-G exhibited highly crystalline structure with obvious graphene lattice.This all-lignin based novel GQDs/BL-G hybrid hetero-junction was firstly designed to greatly boost the specific capacitance of 404.6 F g^-1,over two times higher than unmodified AL electrode of 168 F g^-1,and with a 7.5 times charge transportation over AL-based supercapacitor.This porous hybrid structure endow highly dispersed BL-G sheet in which the GQDs play an additive role for stopping the stacks and agglomerates between two BL-G layers with enhanced specific surface area.This work paves a novel path towards lignin for further green,sustainable and high-valued energy storage devices.