Publication: Chemically modified graphene as nano-adsorbent for environmental application
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Date
2023-01-01
Authors
Rabita Bt. Mohd Firdaus Achutan
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Abstract
Carbon dioxide (CO2) emissions and pollution by dyes have a significant environmental impact, and it is imperative that cost-effective and recyclable adsorbents are developed to reduce their effect on the ecosystem and human health. Research was conducted to develop porous graphene-based macrostructures for two different environmental applications including adsorption of CO2 gas and Congo red dye respectively. The first part of this research is focused on developing 2D graphene oxide (GO) and further modifications on GO were carried out with two different approaches: physical and chemical activation. The structural and the chemical properties of the prepared activated graphene were deeply characterized by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectrometry (XPS) and Brunauer–Emmett–Teller (BET) nitrogen adsorption. The temperature used for the activation process was found to be the key parameter leading to enhance CO2 adsorption capacity of the GO-based materials. The specific surface area was increased from 219.3 m2 g−1 for starting GO to 762.5 m2 g−1 and 1060.5 m2 g−1 after physical and chemical activation, respectively. The performance of CO2 adsorption was gradually enhanced with the activation temperature for both approaches. For the best performances, a factor of enhancement of 6.5 and 9 after physical and chemical activation, respectively was reached. The CO2 adsorption capacities for physically and chemically activated graphene were 27.2 mg g−1 and 38.9 mg g−1, respectively, at 25 °C and 1 bar. In addition, the impact of the used temperature for the activation treatment on the CO2 adsorption capacity has been investigated and discussed leading to a comprehensible mechanism. The second phase of this study was to construct 3D graphene based monoliths (GBMs) by self-assembling of GO in which GO was reduced by hydrothermal method using ascorbic acid, as the reductant agent. Different concentrations of GO (1-6 mg mL-1) in the starting stable aqueous dispersion obtained at pH 10 were investigated to optimize the self-assembly process. The structural properties including surface area, pore volume, pore size and surface chemistry were measured and discussed considering their performance for CO2 capture. The results showed that the optimized adsorbents (2 mg mL-1) exhibited the highest surface area and total pore volume of 331 m2 g-1 and 0.80 cm3 g-1 respectively. In the developed 3D GBMs, the best CO2 adsorption capacity (74.0 mg g-1) was measured at a GO concentration of 2 mg mL-1, nearly twice than that measured for activated 2D graphene. In the second part, a thin layer of alumina was deposited on the optimized 3D GBMs to prepare a 3D Al2O3 / GBM hybrids. In order to deposit alumina homogeneously on the inner walls of the porous structures, atomic layer deposition (ALD) technique was selected since its principle involves the use of gaseous precursors that facilitate their diffusion. Furthermore, alumina is shown to be deposited at the core of 3D GBMs using advanced and powerful characterisation techniques such as focused ion beam technology (FIB), transmission electron microscopy (TEM), scanning transmission electron microscope (STEM) and electron energy loss spectroscopy (EELS) mapping. This is an unprecedently detailed study of 3D Al2O3 / GBM hybrids, which has not previously been reported. The growth mechanism of alumina on the graphene surface was as well investigated. In contrast to the most approaches which require additives (surfactants or polymers) to enhance graphene reactivity for ALD deposition, reduced GO forming porous aerogel that offers sufficient seeding sites for alumina formation. The proposed growth mechanism comprises a nucleation stage that results in the formation of primarily islands and leading for the highest ALD duration to a continuous covering layer for high ALD cycles. The 3D Al2O3 / GBM hybrids were initially used for CO2 adsorption studies, however, the adsorption capacity gained was reduced from 74.0 mg g-1 to 25.95 mg g-1 compared to the 3D GBMs. Thus, in this section, the prepared 3D Al2O3 / GBM was used for another environment application which is dye adsorption study. The adsorption capacity of Congo red (CR) of the 3D Al2O3 / GBM is much higher than that of pristine / uncovered 3D GBMs. The dominant mechanism in the adsorption process of CR dye by the designed 3D Al2O3 / GBM is based on favourable electrostatic interactions between alumina surface and CR. This work is therefore proof-of-concept research in which the 3D Al2O3 / GBM hybrids was employed as a superior key component in the construction novel materials for adsorption system with promising adsorption performance toward dye pollutants