CN/iodine-doped CN homojunction powder catalysts with excellent visible-light photocatalytic properties

Highlights

• A novel CN/CNI homojunction photocatalyst was constructed by a facile synthetic route.

• The catalyst shows excellent visible-light photocatalytic properties in RhB degradation, H2 evolution and CO2 reduction.

• The optimized CN/CNI-40% catalyst obviously shows a higher photocatalytic activity than CNI-R.

• The significantly enhanced photocatalytic properties were discussed in detail.

Abstract

Although carbon nitride (CN) catalyst has great attraction, the relatively low separation efficiency of photogenerated carriers leads to its unsatisfactory photocatalytic performance. Herein, CN/iodine-doped CN (CN/CNI) homojunction powder catalysts were fabricated via a facile water-bath combined with in-situ sintering method. The optimized CN/CNI-40% catalyst with a proper CN content shows prominent visible-light (λ > 420 nm) photocatalytic performance and stability. Its photocatalytic conversion ratio for Rhodamine B (RhB) is 5.1, 2.7, 3.0 time higher than that of CN, CNI and CNI-R (reference sample), respectively. Its photocatalytic H₂ evolution rate is 5.8, 2.1, 2.2 time higher than that of CN, CNI, CNI-R, respectively. And its photocatalytic CH₄ generation rate is 4.9, 3.5, 3.6 time higher than that of CN, CNI, CNI-R, respectively. This remarkably improved activity was mainly caused by the synergistic effect of CN and CNI, which can contribute to the excellent absorption performance in the 442–591 nm regions and the efficient separation of photoinduced electron-hole pairs provided by the well-matched band structures and homojunction interfaces between CN and CNI. The probable mechanism of remarkably improved photocatalytic property was also investigated. This study offers a novel and rational avenue to synthesize cheap and high-efficiency multifunctional photocatalysts applied in environmental and energy fields.

Introduction

With the global economy development, energy exhaustion and environmental pollution have been two main problems that human faced in the 21st century. Due to the advantages of mild reaction conditions and environment friendly, semiconductor photocatalysis technology has shown broad application prospects in the field of CO₂ photoreduction, pollutant degradation and organic photosynthesis [[1], [2], [3], [4], [5], [6]]. However, the synthesis of stable, inexpensive and highly efficient photocatalytic materials is still the key to restrict the practical application of photocatalytic technology. Among numerous photocatalytic materials, the polymeric graphite-like carbon nitride (CN) is regarded as a promising material, owing to its structural stability, easy preparation, suitable energy band [[7], [8], [9], [10]]. However, the intriguing CN visible-light photocatalyst possesses still some shortcomings of relatively small specific surface area, poor visible-light (λ > 420 nm) absorption property and relatively high recombination rate of photoinduced carriers, resulting in an unsatisfactory photocatalytic efficiency, which has greatly inhibited its practical application [[11], [12], [13], [14], [15]].

In the past ten years, using non-metal (boron, sulfur, phosphorus, etc.) doped CN was used as one of idea ways to improve the photocatalytic property of monomer CN [[16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]]. For example, the co-doping with carbon (C), phosphorus (P) of CN enhanced the photocatalytic H₂ production by enhancing the separation efficiency of electrons and holes [20]. Boron, phosphorus co-doped CN nanocomposite was used as an efficient visible-light-driven catalyst for both CO₂ reduction and pollutant control [21]. Iodine-doped CN (CNI) photocatalyts showed highly efficient activities for H₂ production and pollutant treatment, owing to the absorption performance enhancement of visible-light irradiation and the separation efficiency improvement of photoinduced carriers [[22], [23], [24], [25]], but there is still much room for improvement.

In addition, the synthesis of CN heterojunction materials by combining CN and proper semiconductor has also been used as a effective tool to improve the photocatalytic performance of CN [[27], [28], [29], [30], [31], [32], [33]]. For example, owing to the effective separation of photoinduced carriers, the CH₄ evolution rate of CN/Sn₂S₃-DETA heterostructure catalyst is obviously higher than that of CN [27]. Besides, the CN/Bi₂MoO₆ heterojunction facilitated the separation and transfer of photogenetrated carriers, showing a higher photocatalytic H₂ production performance than CN under visible-light irradiation [30]. However, there are some differences of lattice matching among different semiconductors for these CN heterojunction materials, which affects the transmission and separation of photoinduced carriers from the heterojunction interface to a certain extent.

Our research have shown that the conduction band (CB), valence band (VB) of CNI is −1.13 eV, +1.53 eV versus Normal Hydrogen Electrode (NHE), respectively, and there is a very good band structure matching between CN (CB, VB is −0.92 eV and + 1.78 eV, respectively) and CNI. Obviously, extremely matched band structure offers a vital chance to fabricate CN/CNI homojunction semiconductors.

Herein, making full use of the topology-induced band offset and the almost same characteristics of lattice matching, electronic affinity and work function between CN and CNI, a novel CN/CNI homojunction catalyst was fabricated by a facile synthetic route. Compared with CN or CNI, the photocatalytic performance of CN/CNI catalyst was greatly improved under visible-light irradiation. In particular, the CN/CNI-40% catalyst with an optimized CN content indicated an excellent photocatalytic property, and its degradation ratio for RhB, H₂ evolution rate, CO₂ reduction rate is respectively 3.0, 2.2, 3.6 time higher than that of CNI-R (reference sample) with the highest photocatalytic activity in the literature (Adv. Mater. 2014, 26, 805–809). This study provides a facile strategy to fabricate stable, cheap and highly active multifunctional materials.