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.

The photosensitised degradation of 4-chlorophenol (4-CP) by iron meso-tetrakis(2,6-dichloro-3-sulfophenyl)porphyrin (FeTDCPPS) has been studied in aerated aqueous solution, and is shown to lead to formation of p-benzoquinone (BQ) and p-hydroquinone (HQ) as main photoproducts. In deaerated solution no p-benzoquinone was formed. The photolysis products were identified by high performance liquid chromatography (HPLC) and UV-visible spectroscopy. The photodegradation in aerated solution was also carried out in the presence of sodium azide (NaN₃) as a singlet oxygen [¹O₂(¹Δ_g)] quencher, and showed a significant decrease in the rate of photolysis, suggesting under these conditions, that Type II sensitisation is one of the dominant mechanisms of 4-CP degradation. Support for this comes from laser flash photolysis and time-resolved singlet oxygen phosphorescence measurements. However, these also show direct reaction between the excited porphyrin and 4-CP, indicating that there are two mechanisms involved in the chlorophenol photodegradation.