Extract-Derived Organic Capping Layers in Plant-Mediated Metal-Oxide Photocatalysts: Surface Evidence, Thermal Persistence, and Photocatalytic Consequences—A Systematic Review

Abstract

Introduction: Plant-mediated synthesis of metal-oxide photocatalysts is widely described as a sustainable route, yet the chemical identity, persistence, and functional role of extract-derived organic layers after nanoparticle formation remain unresolved. This systematic review evaluates whether phytochemical capping layers can be distinguished from concurrent effects of particle size, morphology, crystallinity, and calcination-induced defects.

Methods: Following PRISMA 2020, Scopus, Web of Science Core Collection, PubMed, and Google Scholar were searched for original English-language studies published between January 2015 and June 2026. Studies were eligible when they reported plant-extract-mediated metal-oxide photocatalysts, photocatalytic testing, and at least one extract- or surface-sensitive measurement, including FTIR, XPS, TGA/DTG, zeta potential, chromatographic fingerprinting, or washing/calcination controls.

Results: Preliminary evidence mapping indicates that FTIR is the most common basis for assigning phytochemical capping, whereas direct confirmation of residual surface organics by XPS, thermal analysis, extract fingerprinting, or controlled removal experiments is uncommon. Across different oxides, extract identity is associated with variations in particle growth, surface charge, optical response, defect chemistry, and photocatalytic performance. However, causal claims regarding organic layers are often confounded by nonstandardized synthesis conditions and photocatalytic test protocols. Evidence for reactive oxygen species pathways, dark adsorption, mineralization, and cyclic stability is comparatively limited.

Conclusions: Extract-derived organic layers should be assessed as potentially functional organic–inorganic interfaces rather than treated as generic green additives. An evidence-ranking framework and a minimum reporting checklist are proposed to connect extract composition, surface characterization, thermal persistence, and photocatalytic performance.