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.
Step-scheme
(S-scheme) heterojunctions comprising two semiconductors
having two sets of charge carriers at different sites with an outstanding
redox capability have emerged as a prospective tactic for H2O2 production and antibiotic remediation. Herein, 0D/2D
Fe2O3 QD/B-g-C3N4 (F-BN)
was successfully fabricated via in situ nucleation of Fe2O3 quantum dots (FQDs) over boron-doped g-C3N4 (BCN) sheets for H2O2 production
and photo-Fenton amoxicillin (AMX) degradation. Empirical results
demonstrate that the F-BN composite shows a superior catalytic activity
compared to the parent material and the optimized 3F-BN attains the
best activity for H2O2 generation (729 μmol
and solar-to-chemical conversion efficiency (SCC) of 0.12%) and photo-Fenton
AMX degradation (93%) with a “k” of
0.0891 min–1, which is 3.34 and 7.01 times higher
than those of the pristine materials. The outstanding activity could
be attributed to effective separation and utilization of excitons
through the S-scheme transfer pathway. Moreover, charge transfer through
the S-scheme transfer corridor along with continuous Fe3+/Fe2+ shuttling is responsible for the effective photo-Fenton
activity. Additionally, the influence of the variation of experimental
conditions is also studied in detail. The high photocurrent, lower
EIS semicircle, and low PL intensity indicate effective separation
efficiency of e–/h+ in the 3F-BN material.
Furthermore, the scavenging experiment and terephthalic acid (TA),
nitro blue tetrazolium chloride (NBT), and EPR measurements not only
evidence that the generated reactive oxygen species (•OH and •O2
–) participated
in photocatalytic activities but also validate the S-scheme charge-transfer
mechanism which is further confirmed from in-situ XPS analysis.
In recent years, S-scheme oriented photocatalytic systems have shown tremendous potential and possibilities toward efficient conversion of solar energy to clear and sustainable chemical fuel owing to superior exciton separation and strong redox ability. This minireview summarizes the background history of the S-scheme mechanism along with a brief discussion of the type of band alignment and the journey from Z-scheme type to Sscheme systems. The review also elaborates the detailed operational mechanism and beneficial features of the S-scheme photocatalyst, including construction tactics, necessary conditions, band bending, and built-in electric field along with charge transfer dynamics. More importantly, we have described thoroughly the frequently used characterization techniques that demonstrate the interfacial charge transfer mechanism through S-scheme systems (XPS, in situ XPS, KPFM, and TRPL analysis) referring to the relevant reported literature with a brief computational interpretation. Additionally, the review highlights recent studies on the H 2 O 2 evolution reaction with particular emphasis on S-scheme-oriented systems. The conclusion and future prospective section focuses on associated challenges and insightful newly reported information for the research community to fully understand the chemistry of S-scheme-based charge flow and design, promising systems for achieving benchmark efficiency in the clean energy generation sector.
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