ZnO Nanorod/Multiwalled Carbon Nanotube Composites Sensitized with Cu-Based Metal–Organic Frameworks as Photoanodes for Solar-Driven Water Splitting

This research investigates the photoelectrochemical (PEC) water splitting capabilities of Cu-based metal–organic framework (MOF)-sensitized one-dimensional (1D) metal oxide nanostructures, specifically zinc oxide (ZnO) nanorod (NR)/multiwalled carbon nanotube (MWCNT) nanocomposites (ZC NCs). This study involves the synthesis of ZnO NRs as well as ZC NCs with varying content of MWCNTs (from 0.05 to 0.25 wt %) via the sol–gel technique, followed by a comprehensive analysis of their structural, optical, and surface properties. The incorporation of MWCNTs with ZnO revealed the enhanced optical and surface characteristics and high concentration of oxygen vacancies in ZC NCs as evidenced by electron spin resonance (ESR) analysis, contributing to improved PEC performance. Thereafter, binder-free thin films of the synthesized materials were deposited using the doctor-blade technique, and then, PEC water splitting performance was assessed through various electrochemical techniques. Herein, the PEC performance of ZC-0.1 NCs (with 0.1 wt % MWCNTs) was 4.5 times superior to that of bare ZnO NRs. Subsequently, the optimized ZC-0.1 NCs were further sensitized with CH3–Cu-BTC MOFs, forming ternary ZC-0.1/CH3–Cu-BTC NCs. For these ternary NCs, an impressive current density was observed (2.57 mA/cm2), nearly 9 times greater than that of pristine ZnO NRs (1.23 V) vs the reversible hydrogen electrode (RHE) and almost double that of ZC-0.1 NCs. Furthermore, the designed photoanode is stable over time, which was predicted and forecasted through the time series analysis (TSA) model. The higher PEC performance for ternary NCs was justified through surface topography, morphological, and contact angle studies to analyze the surface nature and its hydrophilicity. Hence, the present work demonstrates the systematic roadmap for a PEC water splitting study using valuable insights for the design and optimization of nanostructured metal oxides, photoactive organic moieties, and their composites.