Objective Cellulose is a complex macromolecular polysaccharide formed by glucose units connected by β-1,4-glycosidic bonds. It is a natural polymer compound primarily found in plant cell walls, gaining attention for its unique biocompatibility, renewability, and environmental friendliness. The molecular chains of cellulose can intertwine like ropes, forming various structural morphologies. Cellulose has several allomorphic crystalline forms, which lead to diverse physical properties. Generally, at the molecular level, cellulose has various forms of external defects, including disorder in molecular chain arrangement and the presence of voids, cracks, and impurities in its microstructure. On the one hand, the defects would weaken the mechanical properties of celluloses and severely affect the chemical stability and reactivity. On the other hand, the defects in the cellulose such as impurities change the electrical properties. As one example, the doping of oxygen (O) in the cellulose would transform the polymer from an insulator to a semiconductor. However, the basic mechanisms at the electronic scale of the cellulose are still unclear.
Method To deepen the understanding of the changes in electrical characteristics of cellulose after doping, in this work, we investigated the regulation of electrical properties by O doping through the first-principles calculations based on the density functional theory. Calculations were carried out using the Vienna Ab-initio simulation pack-age (VASP). The energy cutoff for the plane wave basis set was set to 450 eV, and the interactions between ionic cores and valence electrons were described using the projector augmented wave (PAW) method. For the exchange-correlation functional, the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional was employed. During structural relaxation, the force convergence criterion was set to less than 0.01 eV/Å, and the total energy convergence criterion was set to less than 1.0 × 10-5 eV.
Results The results indicate that the perfect cellulose has a band gap of 4.938 eV, showing almost no conductivity and exhibiting insulating properties. The conduction band minimum is located to the left of the high-symmetry point D, while the valence band maximum is situated to the right of point D, indicating an indirect band gap. The energy bands are more curved between 5 eV and 8 eV, while they are relatively flat between 0 eV and -8 eV, showing a higher degree of electron localization. When carbon (C) atoms in cellulose were replaced by oxygen (O) atoms, significant changes occurred in its electrical properties. Secondly, when the O2 atom replaced C1 or C3, the band gap of the system decreased significantly, exhibiting semiconductor characteristics. The interaction was changed accordingly between the replaced atom and surrounding atoms, resulting in enhanced electronic localization. The density of states was relatively flat between 0 eV and -10 eV, with a high degree of electronic localization primarily contributed by the p orbitals of O1 and O2. Thirdly, when the O2 atom replaced C2 or C4, the valence band of the system shifted toward the conduction band, even surpassing the Fermi level, displaying half-metallic characteristics. Compared to undoped cellulose, the density of states near the Fermi level changed significantly, primarily contributed by the O2 atom, but with a smaller effect on the conduction band. When the O2 atom replaced C5, the system still behaved as an insulator, the band gap of the system increased and the energy bands remain relatively curved between 5 eV and 6 eV, while being flat between 0 eV and -9 eV, with a high degree of electronic localization.
Conclusion In summary, the replacement of C atom in cellulose with O atom significantly alters its electronic properties due to the higher electronegativity of O, which tends to form stable covalent bonds with surrounding C atoms, thereby limiting electron transitions. However, different substitution sites exhibit distinct properties. Therefore, when preparing and functionally modifying cellulose, the substitution sites of O atom should be given special consideration. By applying doping effects to cellulose, its electrical properties can be significantly altered, offering new insights for the application of cellulose in the engineering field of electronic devices.
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