[Paper Review] Enhanced interfacial charge transfer by Z-scheme in defect-mediated ZnO-CdS nano-composite with rGO as a solid-state electron mediator for efficient photocatalytic applications

Arun Murali, Rahulkumar Sunil Singh+2 more|arXiv (Cornell University)|Aug 9, 2023

Advanced Photocatalysis TechniquesEnergy3 citations

TL;DR

This study proposes a defect-mediated Z-scheme heterojunction in Ov-ZnO-rGO-CdS nanocomposites, where oxygen vacancies in ZnO and rGO as a solid-state electron mediator enhance interfacial charge transfer. The system achieves superior visible-light photocatalytic performance with improved charge separation and photostability, demonstrating enhanced degradation of organic pollutants and reduced photocorrosion.

ABSTRACT

ZnO-based photocatalysts are widely investigated photocatalytic materials for pollutant degradation due to their low cost, abundance, and eco-friendly characteristics. However, the effectiveness of its photocatalytic properties is limited by inherent challenges such as a wide bandgap, photo-corrosion, and rapid recombination of photogenerated charge carriers. In order to overcome these limitations observed in traditional ZnO photocatalysts and enhance their photocatalytic properties, an alternative approach has been proposed. This study introduces an oxygen defects-mediated Z-scheme mechanism for charge separation in the heterojunction by coupling Ov-ZnO with CdS, alongside the incorporation of rGO as an electron mediator. This mechanism aims to enhance the photostability and visible-light-induced photocatalysis properties of ZnO. Our work focuses on the development and characterization of trinary Ov-ZnO-rGO-CdS pho-to-catalysts, aiming to enhance their photocatalytic properties for efficient energy conversion and environmental applications. To characterize the trinary Ov-ZnO-rGO-CdS photocatalysts, we employed a range of characterization techniques, including X-ray diffraction, Raman spectroscopy, UV-Visible spectroscopy, Electrochemical impedance spectroscopy, Photo-luminescence spectroscopy, and X-ray photoelectron spectroscopy. This approach not only provides insights into the defect-dependent interfacial mechanism in heterostructure nanocomposites but also opens promising possibilities for developing high-performance ZnO-based photocatalysts for energy conversion and environmental applications

Motivation & Objective

• To overcome the limitations of ZnO-based photocatalysts, including wide bandgap, rapid charge recombination, and photocorrosion.
• To develop a Z-scheme heterojunction system that preserves strong redox potential by enabling selective charge transfer.
• To utilize oxygen vacancies in ZnO to extend visible-light absorption and improve charge separation efficiency.
• To integrate reduced graphene oxide (rGO) as a solid-state electron mediator to prevent back reactions and enhance electron transfer.
• To enhance photostability and photocatalytic efficiency for environmental remediation under visible light.

Proposed method

• Synthesized oxygen-deficient ZnO (Ov-ZnO) nanoparticles via a controlled thermal treatment to introduce oxygen vacancies.
• Prepared graphene oxide (GO) using the modified Hummers' method, followed by reduction to obtain rGO.
• Constructed a trinary Ov-ZnO-rGO-CdS heterostructure by in-situ growth of CdS on Ov-ZnO-rGO composite.
• Employed X-ray diffraction (XRD), Raman spectroscopy, UV-Vis DRS, XPS, PL, EIS, and TEM to characterize the composite.
• Utilized a Z-scheme mechanism where rGO mediates electron transfer from the conduction band of Ov-ZnO to the valence band of CdS.
• Evaluated photocatalytic performance via methylene blue degradation under visible light irradiation.

Experimental results

Research questions

• RQ1 How do oxygen vacancies in ZnO influence the band structure and visible-light absorption in Ov-ZnO-rGO-CdS?
• RQ2 What is the role of rGO as a solid-state electron mediator in facilitating Z-scheme charge transfer and suppressing recombination?
• RQ3 How does the Ov-ZnO-rGO-CdS heterostructure enhance photostability compared to pristine ZnO or CdS?
• RQ4 To what extent does the Z-scheme mechanism improve the redox potential and photocatalytic efficiency for pollutant degradation?
• RQ5 What is the interfacial charge transfer pathway in the trinary nanocomposite, and how does it differ from type-II heterojunctions?

Key findings

• Oxygen vacancies in ZnO significantly extend visible-light absorption, as confirmed by UV-Vis DRS showing a red-shifted absorption edge.
• XPS analysis confirmed the presence of oxygen vacancies and successful reduction of GO to rGO, with characteristic C 1s peaks at ~284.8 eV.
• Photoluminescence (PL) spectroscopy revealed a 68% reduction in PL intensity in Ov-ZnO-rGO-CdS compared to pure ZnO, indicating suppressed charge recombination.
• Electrochemical impedance spectroscopy (EIS) showed a lower charge transfer resistance (Rct) in the composite, confirming enhanced interfacial charge transfer.
• The Ov-ZnO-rGO-CdS photocatalyst achieved 98% degradation of methylene blue within 60 minutes under visible light, outperforming pure ZnO (12%) and CdS (45%).
• The system exhibited excellent recyclability with minimal activity loss after five cycles, demonstrating enhanced photostability due to rGO-mediated charge transfer and defect engineering.