A team of researchers recently submitted a paper to the journal Carbohydrate Polymers, which is available as a pre-proof, that demonstrated the feasibility of using photoluminescent transparent wood (PL) for light conversion using lignin-based quantum dots.
Study: Wood-cellulose photoluminescence material based on carbon quantum dot for light conversion. Image Credit: GiroScience/Shutterstock.com
Carbon Quantum Dots (CQDs) in Optoelectronics Applications
Nanosized CQDs, a relatively new type of luminescent materials, have gained considerable attention in optoelectronics applications owing to their biocompatibility and unique optical properties such as photostability and tunable emission. However, the fabrication of CQDs in the red emission wavelength has remained a significant challenge.
By adjusting the CQD synthesis process, the emission wavelengths of CQDs can be extended throughout the entire visible spectrum, including red emission, which can expand their applicability to optoelectronics applications such as white-light-emitting diodes (LEDs).
Although effective full-color emission CQDs can be synthesized by preventing aggregation-caused quenching (ACQ) of CQDs using silica gel or polymer matrix, the use of these materials does not support the concept of sustainable and green synthesis. Thus, simple fabrication techniques are required to synthesize robust CQDs with adjustable photoluminescence (PL) emissions.
Bio-Based Methods to Synthesize Full-Color Emission CQDs
Lignin, one of the abundant biomass materials in nature, can be a suitable material for the synthesis of full-color emission CQDs. Different active oxygen-containing functional groups and the complex aromatic structure of lignin provide favorable conditions for the surface functionalization and formation of conjugated graphite cores in CQDs. Additionally, the natural hierarchical porous structure present in the cellulose skeleton in delignified wood can be utilized as a solid-state matrix to impregnate quantum dots to effectively eliminate the ACD of CQDs.
Synthesis of Photoluminescent Transparent Wood (PTW)
Transparent wood (TW), a bio-based composite material, is synthesized by embedding a cellulose skeleton of delignified wood with an optically transparent polymer matrix, with both of them having a similar refractive index. TW possesses excellent mechanical properties, low thermal conductivity, and optical transmittance, which makes it suitable for optoelectronics-related applications.
In TW, the rigid cellulose framework provides suitable oxygen barriers and dense hydrogen bonding sites, which can help in generating a uniform fluorescence emission.
In this study, CQDs with stable and tunable fluorescence emission from blue to orange wavelengths were fabricated using lignin as the precursor and then embedding it into poplar-based TW/substrate to synthesize PTW. The synthesized CQDs and PTWs were later characterized using different characterization techniques.
Poplar Wood Delignification
The poplar wood was delignified using the chlorine oxidation method. Initially, the poplar wood chips were submerged in a mixture of glacial acetic acid (CH3COOH) and sodium chlorite (NaClO2) and heated in a water bath for four hours at 85o Celsius. Proper concentration of oxidizing chlorine was maintained until the poplar chip color was bleached to pure white.
Ultrapure water was used to eliminate the chemical residue from the chips, while acetone and ethanol were used to eliminate the residual moisture in order to increase the cellulose skeleton rigidity.
Synthesis of CQDs Doped with Nitrogen Elements
Single emission blue CQDs, green CQDs, and orange CQDs were synthesized by hydrothermal carbonization of lignin with various nitrogen-containing compounds. 300 milligrams of urea and 100 milligrams of kraft lignin were dissolved in 10 milliliters of deionized water, and the resultant mixture was heated for eight hours at 160o Celsius.
Subsequently, the reaction was cooled to room temperature, centrifuged at 10,000 rotations per minute for ten minutes using a high-speed centrifuge to remove the solid components, and then dialyzed in deionized water for 48 hours to eliminate small molecules. Eventually, the solution was freeze-dried to obtain blue CQDs. Green and orange CQDs were prepared through the same procedure using different heating temperatures, time, and solvents.
Finally, PTWs were fabricated by impregnating the CQDs into poplar-based TW using the vacuum impregnation method.
Characterization of CQDs and PTWs
Transmission electron microscope (TEM), DXR 532 Raman spectrometer, horizontal X-ray diffractometer (XRD), Ultima IV spectrometer, and ultraviolet (UV)-spectrophotometer were used to characterize the synthesized CQDs. Quanta 200 scanning electron microscope (SEM) combined with an energy dispersive spectrometer (EDS) and a Shimadzu AGS-X tester were employed to characterize the PTW samples.
All three types of CQDs were successfully synthesized and dispersed on the poplar wood cellulose network structure to fabricate PTW. The CQDs were quasi-spherical with average sizes between 1.8 and 3.0 nanometers, highly crystalline in nature, and contained a few nano-graphite layers. Specifically, orange CQDs displayed a typical honeycomb-like crystalline pattern.
The emission color of CQDs displayed a red-shift characteristic with increasing sizes of CQDs. The elemental composition of all three CQDs was almost similar. The surface oxidation of CQDs bolstered the redshift of PL wavelengths, and the chemical proportion and composition primarily impacted the PL performance of CQDs. The optical quantum yields of orange, green, and blue CQDs were 12.09%, 14.42%, and 41.41%, respectively.
The PTWs demonstrated excellent optical haze and transparency, and their mechanical properties were considerably enhanced compared to the original wood. The PTWs also displayed an exceptional multi-color luminous performance.
The Commission Internationale de l'Elcairage (CIE) coordinates of the three PTWs corresponding to standard orange, green, and blue emission areas were (0.628, 0.370), (0.272, 0.432), and (0.166, 0.101), respectively. By regulating the concentration of orange CQDs, blue CQDs, and green CQDs in the PTW, white PTWs were fabricated successfully with the CIE coordinates of (0.293, 0.341).
Taken together, the findings of this study demonstrated that PTWs can be effectively used in the field of fluorescence conversion and uniform light emission owing to their adjustable colorful luminescence performance and exceptional light scattering stability.
Wu, X., Wu, Y., Xu, R. et al. (2022) Wood-cellulose photoluminescence material based on carbon quantum dot for light conversion. Carbohydrate Polymers. https://www.sciencedirect.com/science/article/pii/S0144861722003344?via%3Dihub.