Designing oxygen vacancy mediated bismuth molybdate (Bi₂MoO₆)/N-rich carbon nitride (C₃N₅) S-scheme heterojunctions for boosted photocatalytic removal of tetracycline antibiotic and Cr(VI): Intermediate toxicity and mechanism insight

Highlights

• Oxygen vacancies (OVs) mediated Bi₂MoO₆/C₃N₅ S-scheme heterojunctions was fabricated.

• Bi₂MoO₆/C₃N₅ obtains exceptional photocatalysis performance for removal of TC and Cr(VI).

• Analyze the degradation intermediate eco-toxicity and pathway of tetracycline combined LCMS with QSAR calculation.

• S-scheme mechanism together with OVs contributes to improved charge separation and stronger redox ability.

Abstract

Polymeric N-rich carbon nitride of C₃N₅ is being utilized as a new visible-light-driven catalyst due to its narrower bandgap (∼2.0 eV). Building step-scheme (S-scheme) heterojunction by coupling with other semiconductors especially those own oxygen vacancies (OVs) can further upgrade the photocatalytic performance of C₃N₅-based photocatalysts. Herein, a novel S-scheme heterojunction of OVs mediated Bi₂MoO₆/C₃N₅ was fabricated by in-situ growing Bi₂MoO₆ nanoparticles with OVs on C₃N₅ nanosheets. Benefiting from the efficient separation and transfer of high energetic charge carriers by S-scheme charge migration, enriched structural defects, as well as the close contact by the in-situ growth, the heterojunction exhibited superior visible-light photocatalytic performance toward the removal of tetracycline (TC) and Cr(VI) than C₃N₅, Bi₂MoO₆, and their mechanical mixture under visible light. The TC degradation routes and the bio-toxicity evolution of TC were explored. Moreover, the photocatalytic mechanism for TC decomposition and Cr(VI) reduction over Bi₂MoO₆/C₃N₅ with OVs were elucidated. This work presents a newfangled vision for designing promising C₃N₅-based S-scheme heterojunction photocatalysts for pollution control.

Introduction

Nowadays, environmental pollution is becoming increasingly serious. Pharmaceuticals and heavy metals are two major kinds of toxic contaminants widespread in the water environment, greatly threatening the ecosystem and even the sustainable development of society. Thus, developing eco-friendly and effective technologies for environmental remediation is a significant issue [1]. Photocatalysis as a green and sustainable technology utilizing ubiquitous solar energy holds tremendous potential in environmental restoration [2], [3], [4], [5], [6]. The significant prerequisite for the realization of the industrial application of photocatalysis technique mainly depends on the exploration of outstanding photocatalysts [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21].

Polymeric carbon nitride family (e.g., C₃N₄, C₂N₃, C₃N₅, and C₆N₇) has been a rising “star” in photocatalysts by virtue of the excellent thermal/chemical stability, fascinating photoelectronic property, non-toxicity, and rich source [6], [22], [23], [24], [25], [26], [27], [28], [29], [30]. Among them, C₃N₅ with a higher N content, as a newly synthesized graphite carbon nitride photocatalyst, has displayed promising potential in visible-light-driven (VLD) photocatalytic water splitting, mitigation of nitric oxide and refractory organic pollutants due to its better sunlight absorption (Eg ≈ 2.0 eV) and more negative conduction band (CB) potential in comparison with popular g-C₃N₄ [24], [31], [32], [33]. For instance, mesoporous C₃N₅ was demonstrated to be more active for photocatalytic H₂ production than C₃N₄ in 2017 [34]. Besides, C₃N₅ also exhibited superior photocatalytic PMS activation behavior to C₃N₄ [35]. Although pristine C₃N₅ has great potential in photocatalytic applications, the severe electron-hole reunion and unsatisfactory redox capability impair the catalytic performance owing to the intrinsic drawback of the band energy structure. It is imperative to design novel C₃N₅-based photocatalytic system with efficient charge-carrier separation and superior redox capacity simultaneously.

Very currently, building S-scheme heterojunctions consisting of an oxidative semiconductor with more positive valence band (VB) potential and a reductive semiconductor with more negative conduction band (CB) potential has been an attractive approach to obtain high-performance photocatalysts via synchronously ensuring the efficient charge separation and preserving the highest redox capacity [36], [37], [38], [39], [40], [41], [42], [43], [44]. Further, hetero-structure coupled with structural defect can synergistically optimize the sunlight utilization and foster carrier separation, thereby leading to an enhancement in photocatalytic efficacy [39], [45], [46]. Therefore, integrating C₃N₅ with an oxidant semiconductor to develop S-scheme heterojunction with structure defects is a promising strategy for optimizing the photocatalytic performance.

Bi₂MoO₆ (Eg ≈ 2.5–2.8 eV) has been widely researched and recognized as an active VLD photocatalyst for wastewater treatment due to its advantages of unique layered architecture, good optical and chemical property [47], [48], [49], [50]. More significantly, the intrinsic band structure of Bi₂MoO₆ and C₃N₅ signifies the feasibility of building the S-scheme heterojunction between Bi₂MoO₆ and C₃N₅.

Herein, we designed and fabricated a novel S-scheme heterojunction of oxygen vacancies (OVs) modified Bi₂MoO₆/C₃N₅ (BMO/CN) by depositing Bi₂MoO₆ nanoparticles (NPs) with OVs onto C₃N₅ nanosheets (NSs) for mitigation of pharmaceuticals and Cr(VI) in wastewater. This work aims to illustrates these points: (i) The catalytic behavior of the BMO/CN heterojunction in photocatalytic antibiotic tetracycline hydrochloride (TC) degradation and Cr(VI) reduction; (ii) the band energy structures, the dominant reactive substances, carrier migration mechanism, the degradation pathways of antibiotic and detoxication performance in the BMO/CN photocatalytic system; (iii) the activity enhancement mechanism.