One-pot synthesis of isotype heterojunction g-C₃N₄-MU photocatalyst for effective tetracycline hydrochloride antibiotic and reactive orange 16 dye removal

Highlights

• Facile synthesis of g-C₃N₄ from mixed precursors via thermal polycondensation.

• Efficient visible light photodegradation of RO-16 and TC-HCl by g-C₃N₄-M6U10.

• The major reactive species in photodegradation under study is O₂^–.

• The photocatalyst can be reused for at least 3 cycles.

• The photocatalyst remained stable and can be regenerated.

Abstract

The one-pot synthesis of g-C₃N₄-MU isotype heterojunction has been produced by the thermal polycondensation method by mixing different ratios of precursors between melamine and urea. The isotype heterojunction g-C₃N₄-MU samples were characterized by X-ray diffraction spectroscopy, scanning electron microscope and energy-dispersive X-ray-spectroscopy, UV–Visible diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy. The band-gap energy of these photocatalysts reveals that they can work well under visible light. The photocatalytic performance of the samples was investigated over the photodegradation of reactive orange-16 (RO-16) dye and tetracycline hydrochloride (TC-HCl) under visible light irradiation. The isotype heterojunction of g-C₃N₄-M6U10 showed the highest degradation of 95 and 85.6% for RO-16 and TC-HCl, respectively under irradiation time of 100 and 120 min. The major reactive species was identified as O₂^–. Moreover, the reusability of the photocatalyst was investigated up to 3 cycles with good efficiency. The present synthesized isotype heterojunction g-C₃N₄-MU could be applied as a facile pathway for synthesis and as an effective pathway to resolve various environmental problems.

Introduction

In recent decades, water pollution has emerged as one of the major threats in the world. The organic compounds such us reactive orange 16 dye (RO-16) and tetracycline hydrochloride (TC-HCl) have been widely used to fulfill human needs. However, both of them are poisonous and hazardous organic pollutants which if unconsciously discharged can pollute the environment, endangering the ecosystem and human health [1], [2], [3]. Consequently, it has become an imminent problem that must be addressed for the removal of organic pollutants from water [4], [5], [6].

The semiconductor photocatalysis has been considered as an efficient and attractive method for water pollutants separation, particularly organic pollutants degradation [7], [8]. A graphitic carbon nitride (g-C₃N₄) has received great attention due to its matching band gap energy [9], [10], high thermal and chemical stabilities [11], good electrical and optical properties [12], suitable electronic band structure and elemental abundance [13]. As such g-C₃N₄ is a very interesting material for photocatalytic application especially degradation of the pollutant [14], [15]. A facile synthesis of g-C₃N₄ has been reported by thermal polycondensation of precursors using melamine, dicyanamide, thiourea, and urea [16], [17], [18], [19].

Nevertheless, poor specific surface area and photo-absorption efficiency together with fast charge recombination of g-C₃N₄ become an obstacle to fully promote its photocatalytic activity [20], [21]. To take better advantages of g-C₃N₄, there are various methods to modify the photocatalytic ability of g-C₃N₄ including metal/non-metal doping [22], metal deposition [23], constructed heterojunction [13], and copolymerization [24].

The band structure among semiconductors could be well-matched formation which can promote charge separation between the interface of two semiconductors [25]. The constructed heterojunction between suitable semiconductors is based on g-C₃N₄ with other materials, for example, TiO₂-g-C₃N₄ [20], BiOI-g-C₃N₄ [26], BiVO₄- g-C₃N₄ [27], BiFeO₃-g-C₃N₄ [28], CeO₂- g-C₃N₄ [29], etc.

Furthermore, combining two semiconductors of g-C₃N₄ into the isotype heterojunction of g-C₃N₄ has been reported as an alternative pathway to resolve the limitation of g-C₃N₄. Isotype heterojunction of g-C₃N₄ prepared from thiourea and urea was able to promote the efficiency of electron-hole separation, thus improved the photocatalytic ability [30]. Likewise, isotype heterojunction of g-C₃N₄ prepared from cyanimide and urea could enhance photocatalytic performance due to the widening of band gap, more competitive CBM or VBM potentials, higher BET surface area, and thinner sheet morphology [31]. It is reported that isotype heterojunction of g-C₃N₄ prepared by dicyanamide-urea showed high photocatalytic performance [32]. Melamine and urea derived g-C₃N₄ were successfully prepared and showed improved photocatalytic activity [33]. These previous works have proven that combining two components of g-C₃N₄ precursors into isotype heterojunction of g-C₃N₄ can overcome the drawbacks and enhance the photocatalytic efficiency.

In this work, the isotype heterojunction of g-C₃N₄ -MU (MU = melamine and urea) was synthesized through thermal polycondensation employing melamine and urea as precursors. Urea, besides acts as a precursor, it could also be considered as a modifier or promoter to repair the deficiency of g-C₃N₄ as photocatalyst. Higher content of urea seems to affect the morphology into favoring more sheet-like which benefits the photocatalysis. The photocatalytic activity of g-C₃N₄-MU was investigated using RO-16 dye and TC-HCl under low energy (55 W Xe-lamp) visible light. The plausible photo-degradation mechanism was also proposed.