Photocatalytic improvement of Mn-adsorbed g-C3N4

https://doi.org/10.1016/j.apcatb.2017.01.034Get rights and content

Highlights

  • The photocatalytic efficiency of Mn-adsorbed g-C3N4 is three times higher that of pristine g-C3N4.

  • The Mn atoms adsorbed on g-C3N4 stably form ionic bond with N atoms, as confirmed by DFT calculation and experiment.

  • The photocatalytic efficiency enhancement of Mn-adsorbed g-C3N4 is attributed to the decrease of band gap, the half-filled Mn 3d states in the forbidden gap, and the up-shifting of band edges.

Abstract

This study employed experimental results and theoretical calculations to investigate Mn-adsorbed g-C3N4 as a potential photocatalyst with high efficiency. Mn was chosen as the incorporating element, because among the 3d transition metals it exhibits the highest binding energy and most suitable band edge positions. The photocatalytic efficiency of Mn-adsorbed g-C3N4 is 3 times higher that of pristine g-C3N4. Although small variations in the phase and surface morphology were observed, which were confirmed to not be the determining factors to improve efficiency. The factors that affect the high photocatalytic efficiency are therefore the electronic structure, optical absorption, and band edge variations after Mn-adsorption. The Mn atoms stably are bonded with N atoms, due to the strong absorption energy and ionic bond. Moreover, reduction of the g-C3N4 band gap after Mn-adsorption results in a red shift of the absorption band edge. The half-filled Mn 3d state introduces impurity states into the forbidden band gap, which will increase the life time of charge carriers. In addition, the up-shifting of band edges of Mn-adsorbed g-C3N4 leads to inhibition of the electron-hole recombination. As a consequence, the photocatalytic efficiency of Mn-adsorbed g-C3N4 is enhanced due to the combination of the aforementioned effects.

Introduction

Recently, intense research activities have sought suitable materials as photocatalysts for splitting water or environmental pollutants by using solar energy. The optimal material should not only have strong light absorption ability, suitable band gap, and redox potential, but also be stable, non-toxic, abundant, and easily processable into a desired shape. Unfortunately, most semiconductors do not meet all of these criteria. TiO2 has been the dominant material used, but its band gap is too wide (3.26 eV) to efficiently absorb visible light [1]. In recent years, growing interest has focused on the non-TiO2-based catalysts. However, the present achievements are still far from the ideal materials [2], [3]. Most of the available catalysts are active only under UV irradiation, while others that are capable of absorbing visible light are not stable during the reaction process [4]. Recently, graphitic carbon nitride (g-C3N4) materials have been explored for various applications, especially in visible light photocatalysis, owing to their large specific surface area, short diffusion length, high chemical stability, and appealing electronic structure, with a band gap of 2.70 eV that is suited to absorbing blue light [5], [6]. In addition, the high nitrogen content in g-C3N4 plays an important role in boosting photoelectric conversion efficiency, as the nitrogen atoms affect the spin density and charge distribution of carbon atoms, inducing activated regions on the surface [7].

On the other hand, the photocatalytic efficiency of g-C3N4 under visible light has been found to be low, absorbing only 0.1% of the irradiated visible light [8]. Further study has ascribed the low activity to many factors, for example the high excitation energy determined by the band gap energy, low charge mobility originating from the nature of the polymer, low specific surface area (2–10 m2/g), and insufficient sunlight absorption [8]. Recently, some effective methods such as nanostructure modification [9] and the introduction transition metals (TMs) or nanoparticles into g-C3N4 [10], [11] were reported to improve the photocatalytic activity of g-C3N4 under visible light. Also, chemical doping with nonmetallic elements or deposition with metal atoms is an alternative strategy to modify the electronic structures as well as their surface properties to improve their performance [12]. Among all doping elements exploited so far, the 3d TMs have widely been employed, since the d-orbitals of TMs hybridize with the pπ-orbitals of the g-C3N4 framework, and meanwhile decrease the band gap of TMs-incorporated g-C3N4. Importantly, most of these materials possess magnetic properties, which is important to the photocatalyst recycling process [13].

To select properly doping/adsorbing TM atoms, some key factors such as the stability, carrier density, and synthesis should be considered. First, the stability is important for photocatalsyts. The binding energy after incorporation/adsorbed can be applied to evaluate the stability. The small or negative binding energy implies that these TM atoms have a tendency to segregate and ultimately form metal clusters in the catalyst. Among the TMs-adsorbed g-C3N4 systems previously investigated, the binding energy of Mn is the strongest (i.e., Mn is 4.48 eV, Cr is 3.65, Zn is −1.40, etc.). Second, the carrier density of the g-C3N4 system after TMs-incorporation/adsorbed is one of the key factors for the photocatalyst, as higher carrier density materials make the photogenerated e/h+ pair easier to migrate onto the surface. Among all the TMs investigated, the electron carrier density of the Mn-adsorbed system is the highest (5.0 × 1013 cm−2) [13]. Finally, the Mn-adsorbed systems have been easily and successfully synthesized by mixing MnCl2 with substrate materials [14], [15]. To the best of our knowledge, there is no report on the photocatalytic properties of Mn-adsorbed g-C3N4. Therefore, systematic investigation of the electronic structure and optical properties of Mn-adsorbed g-C3N4 is urgent and important for photocatalytic applications.

This report employed experimental and theoretical methods to examine the photocatalytic activity of Mn-adsorbed g-C3N4. Our results indicate that Mn is the most promising adsorbed element among all of the 3d TMs. XRD, XANES, XPS, Raman and SEM, TEM measurements were performed to determine the morphology and phase change. The photocatalytic efficiency was also studied, and was found to be significantly improved. The results show that the chemical compositional variation is the contributing factor to the optical absorption range extension and band edge shift, which results in the improvement in photocatalytic efficiency.

Section snippets

Sample preparation

The g-C3N4 powders were synthesized directly by heating melamine in a semi-closed system according to the literature [16]. First, 10 g of melamine was heated in a muffle furnace from room temperature to 520 °C with a rate of 4 °C/min. After calcination at 520 °C for 2 h, the as-prepared g-C3N4 was naturally cooled to room temperature, and ground to powder. The Mn-adsorbed g-C3N4 powders were synthesized by the following process. 1 g of g-C3N4 and different amounts of MnCl2 were mixed with 20 mL

The transition metals-adsorbed g-C3N4

To improve the photocatalytic efficiency, investigations of the un-fulfilled 3d TMs-adsorbed g-C3N4 were performed to choose the optimum element. As is well known, these TMs on the top of g-C3N4 are quite mobile in nature and form clusters due to strong d-d interaction [23]. Therefore, we first initiate investigation of the binding energy of TM-adsorbed g-C3N4. The average binding energy is calculated by using the following formula [13],Eb=ECN+ETMETM/CN,where ECN and ETM are the energy of bare

Conclusions

In this paper, a combined experimental and theoretical approach was used to identify the chemical origin of photocatalytic efficiency improvement after Mn-adsorption on g-C3N4. Due to its highest binding energy and most suitable band edge positions, Mn was chosen among the TMs as the adsorption element. Photocatalyst efficiency investigations show the noteworthy improvement of efficiency by 3 times after Mn-adsorption on g-C3N4. The phase, surface area, and surface morphology of the samples

Acknowledgments

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (No. 2014R1A1A2058415, No. 2015R1D1A1A01058991, and No. 2016R1A6A1A03012877), and partially supported by the China Scholarship Council (No. 201408260037).

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