ArticleIn situ construction of protonated g-C3N4/Ti3C2 MXene Schottky heterojunctions for efficient photocatalytic hydrogen production
Graphical Abstract
In addition to the protonation of graphitic carbon nitride, conductive 2D MXene was introduced as a co-catalyst to further accelerate electron-hole separation and interfacial charge transport for improved hydrogen production.
Introduction
Over the past few decades, the increasingly severe energy crisis and the need for environmental remediation have become important challenges facing human society [1, 2, 3, 4, 5]. Clean hydrogen has emerged as an ideal new energy source to replace traditional fossil fuels. The photochemical splitting of water using sustainable solar energy is a new technology to produce hydrogen. In this method, semiconductor photocatalysts are excited after absorbing photons of specific energies and generate reductive electrons and oxidative holes that induce redox reactions [6, 7, 8, 9, 10]. Therefore, an effective semiconductor must have a suitable band gap (2–2.5 eV) and a matched band position for the activation of water splitting under light illumination [11, 12]. Moreover, it is also essential that the semiconductor be earth-abundant and not generate secondary pollution.
Recently, various semiconductor-based photocatalysts have been employed as hydrogen-producing catalysts for water splitting; in particular, graphitic carbon nitride (g-C3N4) has long been regarded as one of the most promising candidates for photocatalytic hydrogen production owing to its tunable electronic structure and good chemical stability [13, 14, 15, 16]. However, g-C3N4 has inherent drawbacks, such as low specific surface area, poor charge transport capacity, and a high electron-hole pair recombination rate, which leads to a low efficiency of hydrogen production [17, 18, 19, 20, 21]. Therefore, follow-up treatment of pristine g-C3N4 is essential to enhance its catalytic activity. For example, minor element doping, especially doping with non-metal elements, can result in defect energy levels, which can improve its light absorption properties and promote effective charge separation. Morphology regulation can also significantly increase its specific surface area and reactive sites [22, 23, 24, 25]. Surface functionalization is another approach for the modification of g-C3N4. Protonation (i.e. acid pretreatment) is a functionalizing approach that can convert the negative charges on the surface of g-C3N4 into positive charges and adjust its electronic bandgap and ionic conductivity, thus improving its hydrogen generation performance [26, 27, 28, 29, 30, 31]. In addition to the optimization of g-C3N4 alone, the fabrication of composite systems is also a common method to enhance photocatalytic capacity [32, 33, 34, 35, 36, 37]. The introduction of co-catalysts such as noble metals, transition metal phosphides, graphene, and carbon materials has also been explored for the acceleration of charge separation and interfacial charge transport [38, 39, 40]. Transition metal carbides (MXenes) obtained from the etching and exfoliation of MAX materials (Ti3AlC2) are a new family of 2D materials that has attracted attention for their excellent structural stability, tunable surface terminations, and metal conductivity. Thus, 2D MXenes can act as excellent electron transport media for enhancing the photocatalytic performance of catalysts [41, 42, 43, 44, 45].
Inspired by the aforementioned achievements, we designed and synthesized PCN/2D Ti3C2 MXene heterojunctions and investigated their use as photocatalysts for efficient visible-light-responsive hydrogen production. The as-prepared PCN/MXene composite photocatalysts exhibit a significantly enhanced hydrogen generation rate. In particular, when 20 mg of the co-catalyst MXene is added, the hydrogen production efficiency of the optimal sample PCN-20 is 2181 μmol·g−1, which is 5.5-fold and 2.7-fold greater than that of pure CN and PCN. The morphology, light absorption, and photoelectrochemical properties of the composites were studied. The protonated g-C3N4, which has an optimized lamellar structure, is integrated with 2D MXene nanosheets to form a close interfacial contact, and the optimal sample PCN-20 shows significantly improved photoelectrochemial properties that facilitate charge carrier separation and interfacial migration due to the synergistic effect of the protonation of g-C3N4 and its hybridization with MXene. The findings in this work provide insights for optimization and performance improvement of semiconductor photocatalysts.
Section snippets
Synthesis of g-C3N4
Pristine g-C3N4 (CN) was calcined by a previously reported thermal polymerization method in a tube furnace [46, 47, 48, 49]. Specifically, 5 g of dicyandiamide (DCDA) was placed in a semi-closed ceramic crucible with a cover, heated to 550 °C at a moderate rate of 5 °C·min−1, and then maintained at this temperature for 4 h under a N2 atmosphere. Subsequently, the sample was cooled naturally to room temperature, and the obtained yellow product was ground into a fine powder and collected.
Synthesis of MXene nanosheets
MXene
Results and discussion
The PCN/MXene composites were synthesized by electrostatic-driven interactions, as shown in Fig. 1. First, the cambium-like MXene obtained by etching MAX with HF was intercalated by DMSO molecules, which facilitated the disaggregation of the nanosheets with negative surface charges by ultrasonic treatment. CN synthesized by traditional thermal polymerization has been demonstrated to have a negatively charged surface, which would not be conducive to electrostatic self-assembly with MXene.
Conclusions
In summary, we have described the protonation of nano-structured g-C3N4 by treatment with hydrochloric acid. The obtained protonated carbonitride (PCN) demonstrates not only altered surface morphology but also increased absorption in the visible light range via adjustment of the band gap width. From the photocurrent and PL measurements, it can be seen that the modified PCN is able to improve the efficiency of electron separation and also minimize the electron-hole recombination, providing more
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This work was supported by the the National Natural Science Foundation of China (21975129), Six Talent Peaks Project in Jiangsu Province (2015-XCL-026), the Natural Science Foundation of Jiangsu Province (BK20171299), the Start-up Fund from Nanjing Forestry University, and the State Key Laboratory of Photocatalysis on Energy and Environment (SKLPEE-KF201705), Fuzhou University.
Published 5 January 2021