The significance of the synthesis method for graphite-like carbon: effects of acidifying agents on properties and carbon monoxide sensitivity

Abstract

This study aimed to synthesize graphite-like activated carbon with different morphologies using the solvothermal method and to evaluate the effects of different acidifying agents on the structure, morphology, porosity, CO gas sensitivity, and optical properties. Graphite-like activated carbon was synthesized using the solvothermal method in an autoclave at 160 °C with four acidic agents: hydrochloric acid (HCl), phosphoric acid (H₃PO₄), sulfuric acid (H₂SO₄) and nitric acid (HNO₃). The activated carbon was produced using carbon black powder obtained from the carbonization process and the combustion of almond skin waste at 600 °C. The properties of the synthesized activated carbon were analyzed by XRD, FESEM, BET, UV–Vis spectrophotometry, and FTIR spectroscopy. The XRD analysis revealed that the AC-HNO₃ sample had the most graphite-like structure, with a d002 value of 3.67 Å and an Lc value of 10.02 Å. The 002 peak appeared near 2θ = 23°, and the 10L peak was observed as a broad peak near 2θ = 43°. The FESEM images showed that the carbon black sample had no pores, while the AC-HNO₃ sample had the highest porosity and the most uniform pore size, with pore sizes around 950 nm. The BET analysis also confirmed the largest specific surface area of the AC-HNO₃ sample with a surface area (\({S}_{BET}\)) = 340 m²/g, which correlates with the superior light absorption capabilities compared to 67 m²/g for carbon black. The absorption spectrum demonstrated enhanced light absorption for all synthesized activated carbons compared to carbon black, with the AC-HNO₃ sample exhibiting the highest optical absorption coefficient (α) in the order of 10⁶. Optical band gap calculations showed a reduction in the energy gap (E_g) from 3.83 eV for carbon black to 3.28 eV for the AC-HNO₃ sample through activation with different acidic solvents using the solvothermal method. The FTIR spectroscopy detected several functional groups in all samples, including OH, CH₂, C=C, and CH aromatic, across all samples. According to the carbon monoxide (CO) gas sensing characterization, the AC-HNO₃ sample exhibited the highest sensitivity at S = 9.3%, while carbon black showed the lowest sensitivity at 2.1%. These results indicated that the AC-HNO₃ sample, with its high porosity, large specific surface area, and superior gas sensitivity, could have been effectively utilized in industrial applications such as water treatment, air filtration, and gas sensing.