Carbon nitride materials: impact of synthetic method on photocatalysis and immobilization for photocatalytic pollutant degradation

Abstract

Different synthesis routes for carbon nitride materials (CN) and the resulting products were compared to study the photocatalytic activity (pollutant degradation) in dependence on structure and properties. The CN materials were synthesized by thermal decomposition of dicyandiamide in air and under argon as well as in sealed ampoules with or without the use of a salt melt. The as-prepared materials were characterized by IR spectroscopy, nitrogen adsorption measurement, solid-state NMR spectroscopy, diffuse reflectance UV–Vis spectroscopy, elemental analysis and powder X-ray diffraction (PXRD). The surface polarity of the CN materials was estimated by adsorption of the dicyano-bis(1,10-phenanthroline)-iron(II) complex, which allows an evaluation of the degree of condensation. The CN materials were tested with regard to the photocatalytic degradation of rhodamine B (RhB). It is shown that the photocatalytic activity increases with higher surface polarity. Promising CN materials with high RhB degradation of 85% within 25 min and high surface polarity of 0.89 were selected for an immobilization approach to obtain coatings on a silicone substrate using a high-volume low-pressure (HVLP) spray coating technique. To study the photocatalytic activity of the catalyst coatings, the degradation rates of an aqueous RhB solution and solutions of organic pollutants such as triclosan and ethinyl estradiol were examined. Pollutants are decomposed with up to 63% of the initial concentration. Xenon lamps and different LEDs were used as light sources for comparison. Particularly high degradation efficiencies were obtained using LEDs, and the degradation rates are increased by adjusting the emission spectrum of the lamp to the pollutant and absorption edge of the catalyst, which results in a 40 times higher degradation efficiencies of LEDs compared to a Xe lamp. Graphical abstract

Photocatalytic water purification under visible light using carbon nitride materials and β-Bi2O3 immobilized on electrospun polyvinyl acetate fibers

“…5b). Thus, we conclude that in accordance with previous studies for CN and β-Bi 2 O 3 particles, RhB degradation results from a combination of indirect photocatalytic oxidation and photosensitive decomposition [29,36].…”

“…Embedding of CN particles directly into the fiber structure (CN-IF) during the spinning process was successful, but in comparison to fleeces with dip coated photocatalyst material (CN-ES) a low RhB degradation rate and photocatalytic activity per substrate area is observed (CN-ES2: 5.9 × 10 −11 mol cm −2 min −1 ; CN-IF: 2.2 × 10 −10 mol mg −1 min −1 ). Compared to CN-ES2 samples (2.0 × 10 −10 mol mg −1 min −1 ), a lower photocatalytic activity for substrates coated with β-Bi 2 O 3 (β-Bi 2 O 3 -ES 1.3 × 10 −10 mol mg −1 min −1 ) is determined, which is as expected since degradation experiments with catalyst dispersions using the same reactor setup showed higher degradation rates for CN materials compared to β-Bi 2 O 3 (SI 3) [29,51]. The use of the mixture of β-Bi 2 O 3 and CN (β-Bi 2 O 3 /CN-ES; 1.6 × 10 −10 mol mg −1 min −1 ) did not enhance the degradation rates as expected, thus better absorption capability of the mixture or formation of a heterojunction, as reported earlier for this combination of materials, is not observed [28].…”

“…The as prepared substrates were used for degradation experiments using RhB as model [52,53]. The excitation and transfer of the charge carriers contribute significantly to degradation of RhB, which was outlined in a previous study [29]. The corresponding timedependent RhB absorption diagram as a result of illumination is presented for CN-ES2 exemplarily (Fig.…”

“…They provide a suitable band gap for exposure with visible light, they are non-toxic and therefore particularly suitable for photocatalytic water purification of drinking water [16,25,27]. Due to the different band gap structure of β-Bi 2 O 3 (CB +0.33 V, VB +3.17 V vs. NHE, E g = 2.3 eV) and CN materials (CB 1.13 V, VB 1.60 V vs. NHE, E g = 2.8 eV) [25,28,29] the usage of both photocatalyst materials in a combined application might increase the visible light response and therefore was expected to be beneficial for photocatalytic pollutant degradation processes. Reactors for waste water treatment using photocatalysts are already in operation and they make use of immobilized photocatalyst materials to avoid a costly separation step of the catalyst material from the cleaned water feed [6,15,25,[30][31][32].…”

“…Based on previous studies [25], the highly soluble bismuth-containing molecular precursor [Bi 38 O 45 (OMc) 24 (EtOH) 13 ] [47] was used to synthesize β-Bi 2 O 3 particles, which are deposited on the fiber substrates by a dispersion based dip coating process. CN particles were obtained via a thermal decomposition of dicyandiamide according to a previous study [29] and deposited on the substrates. The degradation rates of RhB using either CN-or β-Bi 2 O 3 -coated fleeces, fleeces coated with a mixture composed of β-Bi 2 O 3 and CN, and polymer fleeces with partially embedded CN particles are compared.…”