Abstract
This article reports on the formation of pyrolytic carbon/TiO2 nanocomposite (p-C/TiO2) by pyrolysis of a mixture of the P25 TiO2 and kraft lignin at 600 °C. The result was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, UV-visible spectroscopy, electron paramagnetic resonance spectrometry (EPR), thermogravimetry (TGA) and SEM microscopy. Its photocatalytic activity was ascertained using three classes of chemical probes, namely (i) degradation of methylene blue (MB) and rhodamine-B (RhB) dyes in UV light-irradiated aqueous suspensions, (ii) depletion of phenol and (iii) degradation of antibiotics. The p-C/TiO2 nanocomposite is a strong phisisorbent of both MB and RhB nearly twofold with respect to neat TiO2. Although it is nearly twofold more photoactive toward the degradation of MB (0.091 min−1 versus 0.047 min−1), it is not with regard to RhB degradation (0.064 min−1 versus 0.060 min−1). For the degradation of phenol in aqueous media (pH 3), pristine TiO2 was far more effective than p-C/TiO2 for oxygenated suspensions (17.6 × 10−3 mM min−1 versus 4.3 × 10−3 mM min−1). Under an argon atmosphere, the kinetics were otherwise identical. The activity of the material was tested also for a real application in the degradation of a fluoroquinolone antibiotic such as enrofloxacin (ENR) in tap water. It is evident that the photoactivity of a semiconductor photocatalyst is not a constant, but it does depend on the nature of the substrate used and on the experimental conditions. It is also argued that the use of dyes to assess photocatalytic activities when suspensions are subjected to visible light irradiation is to be discouraged as the dyes act as electron transfer photosensitizers and or can undergo photodegradation from their excited states.
Highlights
Titanium dioxide (TiO2 ) is the most valuable semiconductor material used widely in many fields that include photochromics, pigments, solar energy conversion devices, sensors and, within the present context, as a photocatalyst able to decay organic contaminants in aquatic and air environments [1,2,3,4,5].It has been examined and used extensively because of its high photoactivity, long-term stability, low toxicity, ecofriendly characteristics and relatively low cost
Taking into account that above a pyrolysis temperature of 600 ◦ C the fraction of the anatase phase in P25 TiO2 is converted into rutile, all pyrolysis experiments were carried out at or below this temperature so as not to influence the photocatalytic activity of this metal oxide, as rutile TiO2 is well-known to be less photoactive than anatase TiO2
0.047 ± 0.005 using UV-visible light irradiation, it is important to note that such dyes as methylene blue and rhodamine-B absorb significantly light causing them to act as photosensitizers via electron injection for their respective excited states onto the conduction band of the photocatalysts, which causes them to undergo self-inflicted oxidative degradation through an entirely different mechanistic pathway from the pathway that sees the semiconductor photocatalysts being activated by UV irradiation [23]
Summary
Titanium dioxide (TiO2 ) is the most valuable semiconductor material used widely in many fields that include photochromics, pigments, solar energy conversion devices, sensors and, within the present context, as a photocatalyst able to decay organic contaminants in aquatic and air environments [1,2,3,4,5]. Du and coworkers [30] succeeded in incorporating graphene into macro-mesoporous titania frameworks through in situ reduction of graphene oxide, which resulted in improved mass transport via the film, reduced length of the mesopore channel and increased accessibility of the surface area of the thin film, while effectively suppressing charge recombination in TiO2 that led to significant enhancement of the photocatalytic activity in degrading the MB dye. Computational results showed that the nanocomposites are potentially more efficient than pure TiO2 because of their lower experimental bandgaps (2.94 to 2.75 eV versus 3.08 eV for TiO2 ) and to charge separation at the interfaces that reduces recombination of electron–hole pairs For their part, Zhang and coworkers [34] reported a unique route for the synthesis of graphene/TiO2 continuous fibers by using force spinning combined by water vapor annealing method. Pyrolytic carbon increases the adsorption of chemicals before irradiation, rendering the material useful for a preconcentration of pollutants from waters and cleaning of the catalyst after irradiation
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