Abstract
Undoped metal-free graphene oxide (GO) materials prepared by either a modified Hummers’ (GO-H) or a Brodie’s (GO-B) method were tested as photocatalysts in aqueous solution for the oxidative conversion of phenol. In the dark, the adsorptive capacity of GO-B towards phenol (~35%) was higher than that of GO-H (~15%). Upon near-UV/Vis irradiation, GO-H was able to remove 21% of phenol after 180 min, mostly through adsorption. On the other hand, by using less energetic visible irradiation, GO-B removed as much as 95% in just 90 min. By thorough characterization of the prepared materials (SEM, HRTEM, TGA, TPD, Raman, XRD, XPS and photoluminescence) the observed performances could be explained in terms of their different surface chemistries. The GO-B presents the lower concentration of oxygen functional groups (in particular carbonyl groups as revealed by XPS) and it has a considerably higher photocatalytic activity compared to GO-H. Photoluminescence (PL) of liquid dispersions and XRD analysis of powders showed lower PL intensity and smaller interlayer distance for GO-B relative to GO-H, respectively: this suggests lower electron-hole recombination and enhanced electron transfer in GO-B, in support of its boosted photocatalytic activity. Reusability tests showed no efficiency loss after a second usage cycle and over three runs under visible irradiation, which was in line with the similarity of the XPS spectra of the fresh and used GO-B materials. Moreover, scavenging studies revealed that holes and hydroxyl radicals were the main reactive species in play during the photocatalytic process. The obtained results, establish for the first time, that GO prepared by Brodie’s method is an active and stable undoped metal-free photocatalyst for phenol degradation in aqueous solutions, opening new paths for the application of more sustainable and metal-free materials for water treatment solutions.
Highlights
Temperature programmed desorption (TPD) analysis of the graphene oxide (GO)-H sample revealed peaks at 176 and 193 °C in both CO2 (Fig. 2a) and CO profiles (Fig. 2b), which are attributed to epoxy and hydroxyl groups (190– 200 °C), respectively [47]
The additional peak observed in the CO2 profile at 254 °C corresponds to carboxylic acids, whereas the evolution of groups as CO at higher temperatures corresponds to phenols and carbonyl/quinones
Regarding GO-B, there is a sharp peak at 266 °C in both CO and CO2 profiles, which cannot be attributed to carboxylic acids since these functionalities evolve as CO2 only
Summary
Graphene and its derivatives have been employed to produce photocatalysts with applications in different fields, such as environmental remediation and energy conversion [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]. Undoped metal-free GO materials are still uncommon in photocatalysis for the degradation of organic pollutants [30,31,32,33,34,35] These GO materials are either commercial or prepared the by Hummers’ method, while the particular application of Brodie’s-based GO materials for the photocatalytic degradation of organic contaminants has not been reported in the literature, as far as we know. To the best of our knowledge, we are reporting for the first time the comparison of undoped metal-free GO materials prepared by Hummers’ and Brodie’s methodologies for the photocatalytic degradation of phenol as a well-recognized probe molecule, considering the surface chemistry, interlayer distance and photoluminescence in the liquid dispersions
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