Urea substitutes noble metal catalysts for the photodegradation of organic pollutants

A new laser-based technique developed by the Institute of Materials Science (ICMAB-CSIC) uses urea, a common substance in the chemical industry and a low-cost alternative to noble metal co-catalysts, to enable a more efficient, one-step production of hybrid graphene-based organic-inorganic composite layers for environmental remediation; photodegradation of antibiotic contaminants from wastewater. The composition and chemical bonds of the urea-enriched thin layers were studied in detail using synchrotron light at the ALBA Synchrotron.

Human activity is increasing the amount of pollutants in water and air, as well as in all sorts of materials at home and work place. The existence of antibiotic contamination is undeniably one of the most threatening challenges to date, at a time when antibiotic-resistant bacteria has already been flagged as the next world-wide pandemic crisis.

Semiconductor photocatalysts have long been investigated for environmental remediation because they can degrade or mineralize a wide range of organic contaminants as well as pathogens. Research focuses on addressing some drawbacks that prevent their use on a large scale. On the one hand, many photocatalysts are activated only by UV radiation which represents solely a small fraction of the total available solar emission. On the other hand, the recombination of the photogenerated  electron-hole pairs that enable the decomposition of the pollutant is usually faster than the oxidation reactions that cause the degradation of organic molecules. As a consequence, noble metal co-catalysts acting as electron scavengers, such as gold or platinum, are needed in the process.

Urea, a common substance in the chemical industry as well as in the metabolism of mammals, has been identified as a useful material to tackle these problems. Urea is a precursor of graphitic carbon nitride (g-C3N4), a family of semiconductor compounds with an energy gap closer to that of visible light than typical semiconductors, such as TiO2, used in photodegradation of organic materials. Until now, g-C3N4-enriched layers were produced through a two-step energy-consuming technique, involving high calcination temperatures followed by a deposition process. Moreover, urea itself contributes to the separation of photogenerated charge carriers, preventing electron-hole recombination and, thus, increasing photodecomposition efficiency.

The new technique developed by scientists at the ICMAB-CSIC, together with the national institutes for lasers and materials physics in Romania and ALBA Synchrotron developed a laser-based method that permits the synthesis of g-C3Nfrom urea, formation of graphene-like platelets, and simultaneous deposition of hybrid materials in one stage.

Starting materials were inorganic titanium oxide nanoparticles, graphene oxide (GO) platelets, and organic urea. By submitting this mixture to UV laser radiation, in a technique called reactive matrix-assisted pulsed laser evaporation or reactive MAPLE, simultaneous chemical transformation and deposition in form of thin films take place.

The formed hybrid inorganic-organic layers encompassed high oxidation power from TiO2, enhanced electron mobility and pollutant adsorption from graphene, lower-energy and amplified photocatalytic activity from g-C3N4 and a longer oxidation stage thanks to separation and transfer of photo-induced charge carriers by urea. "High degradation efficiencies were achieved without the addition of noble metal co-catalysts, opening new pathway for light-induced synthesis of materials and new compounds starting from organic molecules" explains Enikö György, researcher at ICMAB-CSIC.

All these advantages together led to a high photodegradation efficiency for both organic dyes and antibiotics.

For the  detailed characterisation of the properties of this composite semiconductor synchrotron FTIR microspectroscopy at MIRAS infrared beamline of ALBA was used, among other techniques and facilities. Due to the low concentration of some elements, the brightness of this beam- up to 1,000 times more powerful than those used in conventional FTIR microspectroscopy - was critical for studying the chemical composition and the chemical bonds of the different constituents of the layers. The results showed that a significant part of the initial urea molecules was transferred to the substrate surface without chemical alterations, creating hybrid inorganic-organic thin layers.

The technique developed in this study shows a path for faster and more efficient ways to synthesize composite materials for environmental remediation. This one-step production of a highly photocatalytic material avoids the use of additional chemicals and high-temperature processes, which are important aspects when considering larger scale fabrication processes in industry.

Figure: (I) HRTEM image of TiO2-GO-urea thin film; (II) (a) self-degradation efficiency of MO under UV light irradiation and photocatalytic degradation of MO in the presence of (b) TiO2-GO (c) TiO2-GO-urea layers; (III) (a) Photolysis of chloramphenicol under UV light irradiation and (b) photocatalytic degradation of chloramphenicol in the presence of TiO2-GO-urea thin films; (IV) UV-visible spectra of chloramphenicol measured at regular time intervals under UV light irradiation in the presence of TiO2-GO-urea thin film.

Cover Figure: Researchers Ángel Pérez del Pino and Enikö György from the ICMAB-CSIC, together with Ibraheem Yousef, scientist responsible of MIRAS beamline at ALBA.

Reference: Laser-induced synthesis and photocatalytic properties of hybrid organic–inorganic composite layers. R. Ivan, C. Popescu, A. Perez del Pino, I. Yousef, C. Logofatu, E. György. Journal of Materials Science, 54 (2019) 3927-3941

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