Antioxidant potential and increased photocatalytic efficiency of gallic acid-capped ZnO and NiO NPs for azo dye degradation - effect of heterojunction coupling and machine learning-assisted modeling

Sep 29, 2026

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Green synthesis produces gallic acid-capped ZnO nanoparticles (g-ZnO NPs), NiO nanoparticles (g-NiO NPs), and Z-scheme ZnO–NiO heterojunction nanocomposites (g-ZnO–NiO NCs). Structural and elemental characterization confirm nanoscale crystallinity and effective semiconductor coupling. Under solar light, the nanocomposites degrade crystal violet and Congo red with higher efficiencies (95% and 92%) than individual nanoparticles. Kinetics and radical-generation analysis attribute improvements to Z-scheme charge separation, optimized pH/catalyst/dye parameters, and reactive oxygen species. TOC indicates strong mineralization and cycling tests show stability, while DPPH/TPC/FRAP support improved antioxidant performance; machine learning models accurately predict degradation.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages16

Reading Time40 min

AccessFree to download

Introduction

Photocatalysis and reactive oxygen species (ROS)

Limitations of ZnO/NiO and heterojunction strategies

Z-scheme heterojunction rationale and research motivation

Nanoscale Advances PAPER Cite this: DOI: 10 .1039/d5na00827a Received 26th August 2025 Accepted 11th November 2025 DOI: 10.1039/d5na00827a[rsc.li/nanoscale-advances](rsc.li/nanoscale-advances) Antioxidant potential and increased photocatalytic eﬃciency of gallic acid-capped ZnO and NiO NPs for azo dye degradation: eﬀect of heterojunction coupling and machine learning-assisted modeling Aqeela Sikandar,a Abu Bakar Siddique, *a Azhar Abbas,ab Abdul Majid,c Bilal Sikandar,d Muhammad Ashraf Shaheen,e Umar Nishan fand Khaled Fahmi Fawyg This study presents the synthesis of gallic acid-capped zinc oxide nanoparticles (g-ZnO NPs), nickel oxide nanoparticles (g-NiO NPs), and their Z-scheme heterojunction nanocomposites (g-ZnO-NiO NCs) using a green approach with gallic acid as the reducing agent. Structural, morphological, and elemental analyses conﬁrmed nanoscale crystallinity and uniform distribution, with eﬀective ZnO–NiO coupling in the composite. XRD analysis revealed the minimum crystallite size of g-ZnO-NiO NCs (11 . 82 nm) in comparison to g-ZnO NPs (28 . 24 nm) and g-NiO NPs (17 . 93 nm) . Photocatalytic performance was assessed for the degradation of crystal violet (CV) and Congo red (CR) dyes under solar light. Kinetic studies showed that g-ZnO-NiO NCs exhibited the highest degradation eﬃciencies (95% for CV and 92% for CR), with rate constant values (2 .84 × 10 −2 min −1 for CV and 2 .56 × 10 −2 min −1 for CR) signiﬁcantly higher than those of individual g-ZnO and g-NiO NPs. The enhanced activity was attributed to eﬃcient charge separation through a Z-scheme mechanism, facilitating the generation of cO2 − and cOH radicals. Parameters such as pH, catalyst dose, dye concentration, and radical scavengers were optimized, conﬁrming the role of reactive oxygen species in degradation process. Total organic carbon (TOC) analysis indicated signiﬁcant mineralization (84% and 80% of CV and CR, respectively), and reusability tests showed high stability with a meager decrease of activity (∼6%) over ﬁve cycles. Machine learning models, including Decision Tree, Random Forest, and ANN, accurately predicted the photocatalytic degradation process. The antioxidant assay results depicted the higher eﬃciency of g-ZnO-NiO NCs than pristine NPs and gallic acid, assessed by DPPH, TPC, and FRAP assays. Conclusively, it was emphasized that the g-ZnO-NiO heterojunction is a promising, sustainable photocatalyst for organic pollutant removal under solar irradiation and has better antioxidant potential than g-ZnO NPs, g-NiO NPs, and gallic acid. 1. Introduction Direct mixing of untreated eﬄuents of textile and pharmaceutical industries with freshwater reservoirs is a major cause of water aInstitute of Chemistry, University of Sargodha, Sargodha 40100, Pakistan. E-mail: [abubakar.siddique@uos.edu.pk](abubakar.siddique@uos.edu.pk); [abubakar054@gmail.com](abubakar054@gmail.com) bDepartment of Chemistry, Government Ambala Muslim College, Sargodha 40100, Pakistan cDepartment of Botany, University of Sargodha, Sargodha 40100, Pakistan dDepartment of Physics and Applied Mathematics, Pakistan Institute of Engineering and Applied Sciences (PIEAS), Nilore, Islamabad 45650, Pakistan eDepartment of Allied Health Sciences, Superior University Lahore, Pakistan fDepartment of Chemistry, Kohat University of Science and Technology, Kohat, 26000 KP, Pakistan gDepartment of Chemistry, Faculty of Science, Research Center for Advanced Materials Science (RCAMS), King Khalid University, P.O. Box 960, Abha 61421, Saudi Arabia pollution. Synthetic azo dyes, including crystal violet (CV) and Congo red (CR), are widely used in these industries as coloring agents and poses serious environmental risks due to their toxicity, carcinogenic properties, and resistance to biodegradation.1–5 Among numerous remediation options, photocatalytic processes, like advanced oxidation processes (AOPs), are gaining signi􀀁cant attention. AOPs utilize the synergistic

FAQs

How were the ZnO and NiO nanoparticles and their Z-scheme nanocomposites prepared?

Gallic acid was used in a green approach as the reducing agent to synthesize gallic acid-capped ZnO NPs (g-ZnO NPs), NiO NPs (g-NiO NPs), and their Z-scheme ZnO–NiO heterojunction nanocomposites (g-ZnO–NiO NCs).

Which dyes were tested for photocatalytic degradation and under what light conditions?

Crystal violet (CV) and Congo red (CR) were degraded under solar light using the synthesized catalysts.

What explains the improved photocatalytic performance of the g-ZnO–NiO nanocomposites?

The enhanced activity is attributed to efficient charge separation via the Z-scheme mechanism, which supports generation of reactive species (including cO2− and cOH radicals) and sustained redox potential.

Sep 29, 2026

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