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Facile one-pot synthesis of cerium oxide/sulfur-doped graphitic carbon nitride (g-C3N4) as efficient nanophotocatalysts under visible light irradiation

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Abstract

Porous CeO2/sulfur-doped g-C3N4 (CeO2/CNS) composites were synthesized by one-pot thermal condensation of thiourea and cerium nitrate as starting materials. The obtained CeO2(x)/CNS composites (x = 8.4, 9.5 and 10.4 wt%) with different CeO2 contents were characterized by the XRD, FT-IR, XPS, TEM, BET, DRS and PL analyses. The TEM images displayed a nonporous and platelet-like morphology for pure CNS but a nanoporous structure with numerous uniform pore sizes of ∼40 nm for the CeO2(9.5)/CNS composite. The XRD phase structures and TEM morphologies confirmed that structural evolution trend and stacking degree of CNS were disrupted in precense of the CeO2 nanoparticles. The optimized photocatalyst, i.e. CeO2(9.5)/CNS nanocomposite, exhibited the highest visible light photocatalytic activity (91.4% after 150 min) with a reaction rate constant of 0.0152 min−1 toward methylene blue (MB) degradation which was greater compared with the individual CNS (0.0044 min−1) and CeO2 (0.0031 min−1) photocatalysts. This enhanced photocatalytic performance was originated from heterojunctions formed between CeO2 and CNS that improved the effective charge transfer through interfacial interactions between both components. The heterojunction prepared displayed excellent stability for the photocatalytic activity under the optimized conditions including catalyst dosage 0.08 g, initial dye concentration 7 mg/L and irradiation time 150 min which was obtained using response surface methodology (RSM). The trapping experiments using isopropanol, benzoquinone and ethylenediaminetetraacetic as the radical dotOH, radical dotO2 and h+ scavengers, respectively, verified that the radical dotOH and radical dotO2 as major species directly attacked onto the MB molecules while h+ showed a negligible role. Finally, it could be stated that simultaneous doping of both sulfur and CeO2 within the g-C3N4 structure using a simple one-pot synthetic process produced very active photocatalysts illustrating their potential for practical applications in industrial water treatment purposes.

Introduction

Environmental pollution is certainly dangerous for human health and ecological system in the current age which is created because of unintended release of chemical and textile wastewaters into the surrounding environment [1], [2], [3], [4], [5], [6], [7], [8]. In the past decades, the degradation of various organic pollutants (as an eco-friendly technology) by semiconductor photocatalysts has attracted considerable interest worldwide [9], [10]. One of the reasons behind this fact is that they have giant potential for absorbing solar energy in order to remove the contaminants from industrial wastewaters [11], [12], [13]. To date, photocatalysts based on TiO2 have widely been used for the degradation of organic compounds, but they can absorb only 3–5% of sunlight in the UV region which is a very small range of the solar spectrum [14], [15], [16]. Therefore, the production of visible-light-driven photocatalysts has become an important topic in recent years in order to expand sunlight utilization.

Nowadays, graphitic carbon nitride (g-C3N4) materials have received substantial interest in the academic communities as a novel generation of photocatalysts because of their facile synthesis procedures and appropriate electronic band structures which are related to their narrow band gaps of about 2.7 eV [17]. In addition, g-C3N4 possesses high physicochemical stability due to its π-conjugated framework which connected the two-dimensional layered structure of tri-s-triazine building blocks [18]. These unique properties make g-C3N4 a promising metal-free semiconductor photocatalyst in several applications such as solar energy conversion and wastewater treatment [19], [20]. However, the photocatalytic activity of pristine g-C3N4 is restricted owning to its low surface area (<10 m2/g), limited visible light harvesting capacity, and especially rapid recombination of photo-generated electron–hole pairs [21]. To overcome these problems, numerous strategies have been made to enhance the efficiency of pristine g-C3N4. Due to its polymeric nature, the surface chemistry can be distorted by means of surface engineering without obviously changing the theoretical composition [18]. Different modification methods including elemental doping at an atomic level [22], [23], [24], [25], [26], molecular doping (copolymerization) [27], nanoarchitecture design by hard/soft templating strategies [28], [29], exfoliation [30], supramolecular preorganization [31] and synthesis of g-C3N4/metal oxides heterojunctions have been performed in order to enhance the photocatalytic activity of pristine g-C3N4.

Among mentioned strategies, anion doping plays a predominant role to modulate the physicochemical properties of g-C3N4, in which dopants are uniformly introduced as impurities into the host backbone and lead to the formation of localized states in bandgap [32], [33], [34]. This phenomenon with creating tail absorption in the visible light region also increases the photogenereted charge transfer mobility [35]. For instance, Hong et al. reported that in situ sulfur-doped mesonanoporous g-C3N4 (mpgCNS) prepared by hard templating approach exhibited 30 times greater photocatalytic H2-production activity than native g-C3N4 [36]. Very recently, sulfur-doped g-C3N4 (TCN) prepared by simply heating mixed thiourea and melamine demonstrated 1.38 times superior photocatalytic CO2 reduction than un-doped g-C3N4 [37]. It was found that TCN showed light absorption up to 475 nm with a tail peak extending to near IR region, corresponding to sulfur doping. Even though sulfur doping improved the photocatalytic performance of pristine g-C3N4 especially for water splitting [38], [39], only a few works have been conducted on photocatalytic degradation of organic pollutants in wastewater. In this context, sulfur-doped g-C3N4 nanoporous rods were synthesized by heating melamine-tri-thiocyanuric acid supramolecule as a starting material and its photocatalytic activity was investigated for degradation of Rhodamine B under visible light irradiation [40].

Another promising strategy widely used to overcome the above intrinsic weaknesses is to couple carbon nitride materials with other semiconductors to enhance photocatalytic activity of bare g-C3N4. In theory, due to combining the g-C3N4 with another semiconductor that has well-matching VB and CB edge potentials, heterojunctions are formed between two components, which improve efficient separation of the photogenerated electrons and holes in the interfacial contact [18], [41], [42]. To date, several types of g-C3N4 based heterostructures have been studied such as g-C3N4/SnO2 [43], g-C3N4/NiO [44], g-C3N4/SmVO4 [45] and g-C3N4/Fe2O3 [46].

Among the semiconductors, CeO2 as one of the rare earth oxides is of great attention as a result of its unique properties such as its high oxygen storage ability, nontoxicity and the valence change between Ce4+ and Ce3+ states which can be helpful in retarding the recombination of photogenerated electron-hole pairs [47], [48], [49]. Cerium oxide has extensively been investigated in preparing catalysts, hydrogen evolution reactions, solar cells and photodegradation of organic pollutants [50], [51]. It has a wide band gap with a valence band (VB) potential around 2.5 eV and a conduction band (CB) potential close to −0.4 eV (vs. NHE) which are lower than those of g-C3N4 photocatalyst [52]. Therefore, it is speculated that g-C3N4 could be matched with CeO2 and heterojunctions can be formed between them and consequently effectively enhance the photocatalytic performance. Huang et al. synthesized CeO2/g-C3N4 composites via a two-step mixing-calcination method which revealed the highest photocatalytic activity in pollutant treatment among those of CeO2 and g-C3N4 [52]. However, the degradation efficiency was obviously decreased after the third run in the photocatalyst reusing experiment.

On the basis of the above concepts, it is possible that combining two strategies including the elemental nonmetal doping and hybridization of another semiconductor results in the enhanced photocatalytic activity of g-C3N4 for degradation of organic pollutant. In this context, Wang et al. most recently coupled sulfur-doped g-C3N4 with BiVO4 by impregnation precipitation method and the prepared composite indicated an oxygen evolution rate of 750 μmol h−1 g−1 that was higher than that of pure BiVO4 [53]. Herein, one-pot fabrication of CeO2/S-doped g-C3N4 photocatalysts with different CeO2 content was accomplished for the first time via a thermal condensation approach using thiourea and Ce(IV) nitrate as starting materials. The hybride samples were characterized by several techniques and then their photocatalytic activities were examined for the photodegradation of methylene blue (MB) as a model pollutant in aqueous solution under visible light irradiation. It was found that the synergistic effects of CeO2 and sulfur doping improved the visible light absorption by the CNS so that photodegradation activity of composite was obviously enhanced compared with the bare g-C3N4 and CeO2. In addition, the effects of CeO2 content and operating factors on the photocatalyst activity were investigated. A possible mechanism was also proposed by elucidating the active species via capturing experiments.

Section snippets

Materials

Cerium(III) nitrate hexahydrate and dicyandiamide were purchased from Sigma-Aldrich. Methylene blue (MB) as a model pollutant in textile industry, thiourea (CH4N2S), isopropanol (IPA), benzoquinone (BQ) and ethylenediaminetetraacetic (EDTA) were supplied from Merck Company (Germany). All chemicals were of analytical grade purity and utilized without further purification. Distilled water was used for the preparation of all solutions.

Synthesis of CeO2/S-doped g-C3N4 photocatalysts

In a typical synthesis, a desired amount of cerium(III) nitrate

Phase structures

Fig. 1a shows the XRD patterns of the CNS, CeO2 and CeO2/CNS photocatalysts. For the pure CNS, two distinct diffraction peaks are seen at 27.4 and 13.1° attributed to the typical (0 0 2) interlayer stacking of conjugated aromatic layers and the in-plane structural packing motif such as the hole-to-hole distance of the nitride pores, respectively [17]. The diffraction intensities of the mentioned peaks for CeO2/CNS composites at 13.1 and 27.4° are obviously decreased with increasing CeO2 content

Conclusions

Sulfur-doped g-C3N4 (CNS) was successfully coupled with CeO2 nanoparticles (CeO2/CNS) to achieve CeO2(x)/CNS nanocomposites (x = 8.4, 9.5 and 10.4 wt%) characterized by XRD, FT-IR, XPS, TEM, BET, DRS and PL techniques. Specific surface area and pore volume of CeO2(9.5)/CNS composite were 34.9 m2/g and 0.12 cm3/g, which were approximately 3.9 and 4.9 times greater than those of the pure CNS (8.9 m2/g and 0.026 cm3/g), respectively. The HR-TEM image of CeO2(9.5)/CNS composite specified that lattice

Acknowledgments

The financial support of this work by the Research Office of Amirkabir University of Technology (Tehran Polytechnic) is gratefully acknowledged.

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