Elsevier

Applied Surface Science

Volume 427, Part A, 1 January 2018, Pages 375-387
Applied Surface Science

Full Length Article
Synthesis and characterization of novel Sm2O3/S-doped g-C3N4 nanocomposites with enhanced photocatalytic activities under visible light irradiation

https://doi.org/10.1016/j.apsusc.2017.08.051Get rights and content

Highlights

  • Sulfur-doped graphitic carbon nitride (CNS) materials were synthesized in situ.

  • The Sm2O3/CNS photocatalysts containing different Sm2O3 contents were prepared.

  • The Sm2O3(8.9)/CNS sample exhibited the highest methylene blue photodegradation.

  • The optimum Sm2O3(8.9)/CNS provided ∼93% MB photodegradation after 150 min.

  • Trapping tests proved that radical dotO2̄ radical was the major oxidative species in the reaction.

Abstract

Novel Sm2O3/S-doped g-C3N4 (CNS) composites were synthesized with in situ method by simultaneous combining S doping in carbon nitride structure to produce CNS as well as hybridization of CNS with the Sm2O3 semiconductor. The obtained composite photocatalysts with different Sm2O3 contents were characterized by XRD, FT-IR, XPS, TEM, BET, DRS and PL techniques and their photocatalytic activities were investigated for the degradation of methylene blue (MB) as a model pollutant in aqueous solution under visible-light irradiation. The XRD structure phase and TEM morphology results showed that stacking degree of π-conjugated system in the CNS structure was disrupted in the precense of Sm2O3 particles. The optimal Sm2O3 loading value was determined to be 8.9 wt% and its corresponding MB photodegradation rate was about 93% after 150 min light irradiation, which was indeed greater compared with those of the individual CNS and Sm2O3 samples. This enhanced photocatalytic performance was originated from characteristics of the hybrid formed between the Sm2O3 and CNS so that it improved the effective charge transfer through interfacial interactions between both components. In addition, the CNS synthesized by S doping exhibited a significant enhancement in the photocatalytic activity relative to that of the pure g-C3N4; this was mostly caused by the increase in its visible light harvesting ability and charge mobility. The possible mechanism for the photocatalytic degradation of MB was suggested and discussed in detail based on the findings acquired from radical/hole trapping experiments.

Introduction

Commercial dyes with different structures are extensively used in numerous fields such as textile, cosmetic and paper industries and their unintended discharge into the environment leads to serious environmental pollution [1]. On the other hand, removing of dyes is ineffective by conventional wastewater treatment technologies and therefore new efficient methods need to be immediately addressed [2]. In the past decades, the degradation of organic contaminants in water using the semiconductor photocatalysts such as TiO2 has attracted extensive attention because of its chemical stability, nontoxicity and inexpensiveness [3], [4], [5]. However, a major drawback for such metal oxide is that it can absorb only 3–5% of sunlight in the ultraviolet region which greatly limits its applications in wider range of the solar spectrum [5], [6]. The well-organized photocatalysts matched with the solar spectrum should possess well architectures, proficient separation ability of electron-hole pairs and have suitable band gaps to highly harvest the visible light [7], [8], [9], [10].

Graphitic carbon nitride (g-C3N4) as a fascinating polymeric semiconductor is of worldwide interest because it has appropriate absorption threshold in the visible light region (450–460 nm) with a band gap of 2.70 eV and high thermal and chemical stability resulting from its tri-s-triazine tectonic units; these properties make g-C3N4 a promising photocatalyst in photodegradation of organic pollution or water splitting under visible light irradiation [11], [12], [13]. However, practical applications are still hindered by several shortcomings of bulk g-C3N4 such as its high recombination rate of charge carriers, low surface area (<10 m2/g) and the restricted visible-light harvesting capacity up to 460 nm [14]. To solve these problems, several attempts have been made in order to rationally design the photocatalytic systems based on g-C3N4 to enhance their photocatalytic activities. Due to polymeric nature of g-C3N4, the physicochemical and textural properties as well as surface chemistry can be engineered by means of surface modification for specific applications [15]. To date, various strategies have been applied including non-metal (e.g., B [16], [17], S [18] and O [19]) and metal doping (e.g., Fe [20], [21] and Cu [22]), molecular doping (copolymerization) [23], exfoliation [24], nanoarchitecture design by hard/soft templating strategies [11], [25], supramolecular preorganization [26] and preparation of g-C3N4/metal oxide heterojunctions (e.g., g-C3N4/TiO2 [27], g-C3N4/NiO [28] and Fe2O3/g-C3N4 [29]).

Doping, especially anion doping, is an efficient strategy to broaden the light absorbance range to visible region and consequently increasing the photocatalytic activity of bulk g-C3N4 under the visible light. In principle, due to incorporation of dopant in the g-C3N4 framework, modified material shows a slightly narrower band gap with a weak absorption tail in the visible light region, resulting from midgap states generated within the band gap [30], [31]. For example, it was found that ex situ sulfur-doped g-C3N4, synthesized by annealing the g-C3N powder at 450 °C under gaseous H2S atmosphere, exhibited 8.0 times higher photocatalytic activity for the H2 evolution rate than that of the bulk g-C3N4 under visible light [32]. Nevertheless, it is believed that using H2S is not desirable from an environmental point of view because of its toxicity. In this content, in situ sulfur-doped mesoporous g-C3N4 (mpgCNS) was also employed using thiourea as an inexpensive starting material, which displayed 30 times higher photocatalytic H2-production activity than native g-C3N4 [18]. Wang et al. found that sulfur-doped g-C3N4 endowed carbon nitride materials with enhanced physicochemical properties, and in consequence, higher photocatalytic CO2 reduction than that of un-doped g-C3N4 under UV–vis irradiation [33]. Although numerous efforts have so far been reported for application of sulfur doped carbon nitride materials in enhancing the photocatalytic efficiencies of water splitting and CO2 reduction reactions, there are few investigations on the photocatalytic degradation of aqueous organic pollutants under visible light irradiation [34].

Another promising strategy for engineering the physicochemical properties of g-C3N4 is combining this compound with other semiconductors having the well-matched band gaps. This approach not only extends the light absorption region, but also improves the interfacial charge transfer. Besides, due to the potential difference between two sides, band bending is formed at the interface of both components. This phenomenon in turn leads to increasing lifetime of photogenerated electron-holes pairs [15], [35], [36], [37], [38], [39], [40]. For instance, Katsumata et al. synthesized g-C3N4/WO3 composites using a two-step mixing-calcination route which exhibited higher activity for the photodegradation of CH3CHO compared to the pure g-C3N4 under visible light irradiation [41]. Similarly, Zang et al. reported that g-C3N4/SnO2 nanohybride prepared with the same method revealed much higher MO photodegradation than those of its components including g-C3N4 and SnO2 [42]. However, the degradation efficiency was obviously decreased after the second run in the photocatalyst reusing experiment.

Rare earth oxides are the most prominent materials with 4f configurations in which f-orbitals can interact with functional groups such as organic compounds [43], [44], [45]. These oxides have shown improved photocatalytic and luminescent properties [46]. For example, Stengl et al. reported that doping TiO2 with Eu3+ increased the adsorption capacity of organic pollutant at the semiconductor TiO2 surface in aqueous solution and therefore improved the photoactivity of titania [47]. In another study, Sm3+-doped nanocrystalline TiO2 presented significantly superior photocatalytic activity for the MB degradation under the UV light irradiation [48].

On the basis of the above concepts, simultaneous combining two strategies in one material may further improve the photocatalytic activity of g-C3N4. In this regard, sulfur-doped g-C3N4/BiVO4 composite showed an oxygen evolution rate of 750 μmol h−1 g−1 that was >2-fold greater than that of pristine BiVO4 under the visible light irradiation (λ > 420 nm) [49].

Herein, we present (for the first time) synthesis of novel Sm2O3/S-doped g-C3N4 (SCN) composites by combination of S doping in carbon nitride (to prepare CNS) and hybridization of Sm2O3 semiconductor with CNS. The composite photocatalysts with different amounts of Sm2O3 were simply synthesized by in situ method via thermal condensation of thiourea and samarium(III) nitrate as starting materials. The effects of Sm2O3 content and operating factors were also assessed on the photocatalytic performance. The synthesized Sm2O3/CNS samples exhibited outstanding photocatalytic activities toward methylene blue (MB) photodegradation under the visible light irradiation. Besides, to investigate the origin of such a high photocatalytic activity, experimental studies were conducted and the mechanism behind this behavior was investigated in detail based on the results achieved from radical/hole capturing experiments.

Section snippets

Experimental

Some experimental sections including Materials, Characterization techniques, Photoelectrochemical measurements and Photocatalytic test are provided in supporting information file.

TGA, XRD, FT-IR and XPS analyses

The TGA diagrams are used to investigate thermal stability as well as the Sm2O3 content in Sm2O3/CNS composites by heating up the samples from room temperature to 900 °C at a heating rate of 10 °C min−1. It is observed in Fig. 1 that when the temperature increases to 450 °C, the CNS phase in the Sm2O3/CNS composites becomes unstable. The decomposition of CNS is completed with further increasing the temperature from 450 to 620 °C. Compared with the pure CNS, the Sm2O3/CNS composites start to burn at

Conclusions

This work focused on the modification of carbon nitride materials via two simultanious strategies including S doping as well as creation of hybrid structure. The Sm2O3/sulfur-doped g-C3N4 (CNS) composites were in situ synthesized for the first time by the one-pot thermal condensation of mixed thiourea and samarium(III) nitrate hexahydrate as starting materials. The obtained composite photocatalysts with different Sm2O3 contents were characterized by XRD, FT-IR, XPS, TEM, BET, DRS and PL

Acknowledgment

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

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