Defect Control via Compositional Engineering of Zn-Cu-In-S Alloyed QDs for Photocatalytic H₂O₂ Generation and Micropollutant Degradation: Affecting Parameters, Kinetics, and Insightful Mechanism

Abstract

Photocatalytic H 2 O 2 production and recalcitrant pollutant degradation are regarded as promising clean technology toward achieving sustainable solar-to-chemical energy conversion. Herein, nonstoichiometric Zn-Cu-In-S (ZCIS) quaternary alloyed quantum dots (QDs) are rationally fabricated via a reflux method toward H 2 O 2 generation and ciprofloxacin degradation under visible light irradiation. The optimum catalyst (ZCIS-2) exhibits a notable H 2 O 2 production of 1685.2 μmole h −1 g −1 (solar-tochemical conversion efficiency (SCC), 0.19%), which is 5.3 times higher than that of CuInS 2 (CIS), and a ciprofloxacin (CIP) degradation efficiency of 96% in 2 h. The observed improvement in activity corresponds to optimized exciton separation/transfer, broad photon absorption, tunable band alignment, and effective adsorption/activation. In addition, oxygen reduction goes through both direct two-electron single-step reduction and single-electron two-step superoxide radical pathways, whereas CIP degradation proceeds via direct • O 2 − and indirect • OH radical pathways, as confirmed by scavenger experiments. An appropriate amount of defects improves the adsorption/activation of O 2 toward H 2 O 2 and active oxygen species generation that facilitates CIP degradation. The effect of operational parameters, such as pH, surrounding environment, presence of ions, sacrificial agent, etc., on both H 2 O 2 formation and CIP removal is vividly studied. Hence, the current study will provide an in-depth insight into O 2 photoreduction and micropollutant removal, which encourages further advancement of potent alloyed quantum dot-oriented photocatalytic systems.

Synthesis of Z-schemes 0D–3D heterojunction bi-functional photocatalyst with ZnInCuS alloyed QDs supported BiOI MF for H2O2 production and N2 fixation

“…28 The wurtzite phase of ZCIS verified from XRD analysis was well supported by the Raman bands at 294 cm −1 and 475 cm −1 (pink box), which may be ascribed to the A 1 and A 1 (LO) mode, respectively, in the spectra of both ZCIS and the ZCIS-BOI composite, indicating good formation of the binary hybrid. 13,29 It may be noted that the O v in the pristine BOI was preserved in the ZCIS-BOI hybrid, which plays a pivotal role in the photocatalytic mechanism supported by EPR analysis discussed in a later section. XPS (X-ray photoelectron spectroscopy) studies were performed for all the synthesized samples (represented in Fig.…”

“…The photo-reduction of O 2 to H 2 O 2 proceeds primarily through two routes, namely, (i) a direct 2e − reduction process achieved by the reduction of dissolved O 2 , and (ii) an indirect 1e − reduction process forming superoxide radicals, and isopropyl alcohol (i-PA) oxidation in presence of photo-induced holes generating protons. 13 The E CB values of all the synthesized photocatalysts except BOI, i.e., all the ZCIS-containing materials, are appropriate for O 2 reduction reaction via the generation of H 2 O 2 from dissolved oxygen through both the 2e − single step and 1e − two-step processes. The attributed reduction pathways are elaborated in the mechanism section.…”

“…Furthermore, the observed enhancement in H 2 O 2 production in the ZCIS-BOI systems is attributed to the presence of highly reducing electrons, surface defects and a reduction in the rate of unwanted exciton recombination by the formed heterojunction (in good agreement with the EIS, Bode phase, PL, TRPL, and photocurrent studies). 13,36 The Z-scheme heterojunction in ZCIS-BOI is mainly responsible for the accelerated carrier separation and migration, and the availability of high-reduction-potential electrons, which enhances the H 2 O 2 production rate. A comparative study of the photocatalytic H 2 O 2 production over various QDs is given in Fig.…”

“…Moreover, the superoxide radical production capability of ZCIS-BOI(1 : 1) was well verified by nitro blue tetrazolium (NBT) analysis, which is discussed in detail in the mechanism section. 13,39,40 Additionally, the group of An stated that existence of defect states reduces the Gibbs free energy for O 2 adsorption (the rate-determining step), ensuring the easy generation of the hydroperoxy intermediate (*OOH), which afterward transfers to H 2 O 2 . 41 Liu and co-workers also found that vacancies encourage O 2 adsorption and activation,…”

“…11,12 Furthermore, alloying CIS with Zn will be advantageous in bulk and surface defect-state regulation, tenable band gap energy, and variation of the conduction band (CB) potential of the QDs, which enhance the photocatalytic efficiency, as we have reported in our previous investigation. 13 Zhong and his research group examined the consequences of a core-shell and alloy composition of CIS, and they established that Zn-Cu-In-S alloyed QDs fabricated using a cation exchange technique illustrated better exciton separation and migration ability than the core/shell CIS/ZnS QDs. 14 However, to improve the photocatalytic efficiency further with superior charge-carrier separation/migration ability, ZCIS QD is coupled with other photocatalysts.…”