Light-induced generation of radicals on semiconductor-free carbon photocatalysts

Abstract

This work provides an experimental evidence of the photoinduced generation of radical species upon UV irradiation of aqueous suspensions of carbon materials with varied textural, structural and chemical composition. The use of a powerful spectroscopic tool as spin trapping electron spin resonance (ESR) has allowed to detect and identify these radicals (among which are hydroxyl, superoxide and other organic radicals), which are the basis of the so-called Advanced Oxidation Processes. Our results demonstrate the ability of carbon materials-including activated carbons-to interact with UV light, and also to generate highly reactive species capable of promoting the photooxidation of an aromatic pollutant. Moreover, for some of the carbons the concentration of radicals was higher than that detected for titania powders. Although the photogeneration of radicals upon irradiation is a well-known process for inorganic semiconductors such as titanium oxide or zinc oxide, our results demonstrate a similar behavior on carbon materials in the absence of semiconductor additives. .

Carbon Materials as Additives to WO3 for an Enhanced Conversion of Simulated Solar Light

“…The ability of the hybrid photocatalysts to form radicals was confirmed by spin resonance spectroscopy using a nitrone spin trapping agent (Figure 7). Similar ESR patterns were obtained for all the samples, with the characteristic quartet peak profile with 1:2:2:1 intensity (g = 2.006, aN = abH = 14.8 G hyperfine splitting constants) of the DMPO-OH adduct attributed to hydroxyl and superoxide radicals (Finkelstein et al, 1980;Velasco et al, 2013a). Quantification of the relative abundance of ROS showed rather constant concentration levels in the photocatalysts, pointing out that the slightly higher basicity on the surface of WO3/carbons compared to bare WO3 does not affect their affinity to generate species with unpaired electrons (i.e., OH radicals).…”

“…In most cases, the improved performance is attributed to mass transfer effects (especially when using porous additives) and/or interfacial electronic effects in the case of highly conductive carbon materials such as graphenes and carbon nanotubes (Faria and Wang, 2009;Leary and Westwood, 2011;Ania et al, 2012;Velasco et al, 2014). More recently, we have demonstrated the photochemical activity of certain nanoporous carbons due to their ability to promote the photochemical splitting of water Velasco et al, 2014) and generate oxygen radical species capable of reacting with electron donors (i.e., pollutant in aqueous solution), hence boosting the photooxidation conversion Velasco et al, 2013a).…”

“…Thus, charge carriers are generated as a result of both WO3-photon and carbon-photon interactions; if splitting is favored (due to existence of reservoir, electron acceptors and hole scavengers), they can propagate and participate in charge transfer reactions (either involving direct hole oxidation due to the proximity of phenol molecules in the adsorbed state, and/or radical mediated mechanisms). All these processes may coexist simultaneously and may contribute to the observed superior activity of the hybrid WO3/carbon catalysts (Haro et al, 2012;Velasco et al, 2013a).…”

“…Quantification of the relative abundance of ROS showed rather constant concentration levels in the photocatalysts, pointing out that the slightly higher basicity on the surface of WO3/carbons compared to bare WO3 does not affect their affinity to generate species with unpaired electrons (i.e., OH radicals). In this regard, it should be noted that the carbon additives used in this work have the capacity themselves to photogenerate ROS upon illumination (Velasco et al, 2013a). Thus, charge carriers are generated as a result of both WO3-photon and carbon-photon interactions; if splitting is favored (due to existence of reservoir, electron acceptors and hole scavengers), they can propagate and participate in charge transfer reactions (either involving direct hole oxidation due to the proximity of phenol molecules in the adsorbed state, and/or radical mediated mechanisms).…”

Boosting visible light conversion in the confined pore space of nanoporous carbons

“…Our recent research has put forward the intrinsic photochemical activity of nanoporous carbons under adequate illumination conditions [1,2], demonstrating their ability to photogenerate radical oxygen species (ROS) in aqueous environments [3,4]. This has opened new perspectives in the field of applied photochemistry based on carbon materials covering from photooxidation reactions for environmental remediation and photoassisted regeneration of exhausted carbon adsorbents [5,6] to water splitting [7], enhanced adsorption/oxidation or photoluminescence [8][9][10][11].…”