Elsevier

Applied Catalysis B: Environmental

Volume 230, 15 August 2018, Pages 260-268
Applied Catalysis B: Environmental

One-step synthesis of nanostructured g-C3N4/TiO2 composite for highly enhanced visible-light photocatalytic H2 evolution

https://doi.org/10.1016/j.apcatb.2018.02.056Get rights and content

Highlights

  • Nanosizing of g-C3N4 and its compounding with nano-TiO2 were realized in one step.

  • Nanostructured g-C3N4/TiO2 is 10.8 times better than bulk g-C3N4 in visible-light H2 evolution.

  • The synergistic effects of nanostructure and heterojunction were revealed.

  • This work provides a guidance for the one-step synthesis of g-C3N4-based nanocomposites.

Abstract

Improving the photocatalytic property of g-C3N4 by combined strategies has attracted increasing attention recently. In this work, we realized the structure nanosizing of g-C3N4 and its synchronous compounding with TiO2 nanoparticles in one step, using a facile melamine-involved vapor deposition method coupled with a simple and easy setup. Nanostructured g-C3N4/TiO2 heterojunction was well-established and the resultant nanocomposite demonstrated an excellent visible-light photocatalytic H2 evolution performance 10.8 times higher than that of bulk g-C3N4. The structure nanosizing coupled with the heterojunction construction contributed together to the improvement of photoinduced electron-hole separation and final photocatalytic efficiency. The proposed simple method and setup have the potential to be used for preparing other g-C3N4-based nanocomposites with advanced photocatalytic properties.

Graphical abstract

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The nanostructured g-C3N4/TiO2 composites are synthesized in one step through a simple reaction device, in which a core-shell nanostructure are prepared, and show enhanced photocatalytic property for H2 evolution under visible-light.

Introduction

Environmental pollution and energy crisis are two major problems to be solved at present [[1], [2], [3]]. Hydrogen energy, a kind of clean energy, has attracted close attention from governments and scientists all over the world [[4], [5], [6]]. Photocatalytic water splitting is considered as an ideal technique for hydrogen production [7]. Among all the catalysts, nanostructured TiO2 has been extensively studied and commercialized already, e.g. P25 [8]. However, TiO2 only absorbs little solar photons (∼5%) and hardly responds to visible light due to its large band gap of 3.2 eV [9,10]. In recent years, graphitic carbon nitride (g-C3N4) has been a topic of wide concern owing to its unique electronic structure and properties [11,12]. Its narrow band gap of 2.7 eV and strong visible-light absorbing ability of 460 nm make it perform well in photocatalysis [13,14]. Nevertheless, the high photogenerated electron-hole recombination rate still limits the photocatalytic efficiency.

Nano-structuring [[15], [16], [17]], element-doping [18,19], composite-constructing [[20], [21], [22]], etc. are useful strategies to improve the activity of catalysts in photocatalytic H2 evolution. Nano-structuring primarily increases specific surface area of photocatalysts, and element-doping and composite-constructing mainly alters the electronic structure. In general, different strategy contributes in different way to improve a certain aspect of catalyst’s properties [23]. Thus, the combination of multifold strategies could bring additive effect on the improvement of photocatalytic activity and becomes a new tendency in the design of advanced photocatalysts [24,25].

Over the years, constructing the composite of g-C3N4 and nano-TiO2 has been proved useful to obtain enhanced performance in photocatalysis, benefiting from their appropriate band levels [26,27]. For example, Zhong et al. successfully designed a g-C3N4/TiO2 heterostructure by calcining the mixture of melamine and TiO2 nanobelt, where 2 times higher performance of H2 evolution are obtained [28]. Wang et al. synthesized g-C3N4/TiO2 by calcining their precursors, where 6 times higher performance of H2 evolution are obtained [29]. However, the g-C3N4 phase of these composites is still bulk g-C3N4 in the micron size range. It is supposed that, if nanosized g-C3N4 is compounded with nano-TiO2, the resultant nanostructured g-C3N4/TiO2 composite will have a further increased efficiency in photocatalytic H2 evolution. Several researchers have investigated the synthesis of nanostructured g-C3N4/TiO2. For instance, Chen et al. controlled the growth of g-C3N4 on mesoporous TiO2 spheres, resulting in a better fused g-C3N4/TiO2 heterostructure [30]. Su et al. loaded g-C3N4 nanodots on TiO2 nanotube arrays for better efficiency of pollutant degradation under solar light [31]. Nevertheless, the reported processes for synthesizing nanostructured g-C3N4/TiO2 are generally complex, requiring two or more steps to realize the nano-structuring of g-C3N4 and its recombination with nano-TiO2. Moreover, the photocatalytic H2 evolution of the nanostructured g-C3N4/TiO2 was not investigated yet and the potential influence has not been revealed. Therefore, it is still of great interest and urgency to realize the facile synthesis of nanostructured g-C3N4/TiO2 composites for enhanced photocatalytic H2 evolution.

Herein, we proposed and demonstrated a facile one-step method to achieve the nano-structuring of g-C3N4 and its compounding with commercialized TiO2 nanoparticles (P25). The method was actualized via vapor deposition using an easy and simple setup. The as-obtained nanostructured g-C3N4/TiO2 realized 10.8 times higher efficiency than bulk g-C3N4 in visible-light H2 evolution. The detailed process was introduced, the comprehensive characterization of catalysts was conducted and the mechanism of photocatalytic activity improvement was revealed.

Section snippets

Nanostructured g-C3N4/TiO2 composite

The one-step vapor deposition of nanostructured g-C3N4 onto TiO2 nanoparticles was carried out using an easy and simple setup as illustrated in Scheme 1. It comprises of a covered crucible (300 mL) with a cylinder (4 cm in height) inside. In a typical experiment, 0.5 g TiO2 nanoparticles (commercialized P25) were placed on the top of the cylinder and 24 g melamine was loaded on the bottom of the crucible. The covered crucible was put in a muffle furnace, heated at 520 °C for 4 h using a heating

Formation of nanostructured g-C3N4/TiO2 composite

The XRD pattern of as-synthesized CN/TiO2-24 is shown in Fig. 1a in comparison with those of control samples: TiO2 and bulk-CN. Obviously, CN/TiO2-24 is a composite of TiO2 and g-C3N4. For detailed comparison, the g-C3N4 phase (nano-CN) of CN/TiO2-24 was abstracted by removing the TiO2 phase from CN/TiO2-24 and its XRD pattern is exhibited in Fig. 1a. It can be seen that, the peak locations belonging to the g-C3N4 phase of CN/TiO2-24 (nano-CN) are slightly different from those of bulk-CN.

Conclusion

Nanostructured g-C3N4/TiO2 composites were synthesized by controlling the growth of nanoscale g-C3N4 on TiO2 nanoparticles via a facile one-step melamine-involved vapor deposition method using an easy and simple setup. The nanosizing of g-C3N4 and its good interfacial connection with TiO2 promote both visible light absorption and photogenerated electron transfer and separation. The g-C3N4 content can be adjusted and the optimal g-C3N4/TiO2 nanocomposite presents a favorable visible-light

Acknowledgements

The authors gratefully acknowledge the support from the National Natural Science Foundation of China (41502030, 51502272), the Zhejiang Provincial Natural Science Foundation of China (LQY18D020001), the Fundamental Research Funds for the Central Universities, the Open Foundation of Engineering Research Center of Nano-Geomaterials of Ministry of Education (NGM2017KF008) and Hubei Environmental Protection Bureau (2013HB10). The helpful comments of two anonymous reviewers are also highly

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