Hybrid Photoanodes Based on Surface-Bound Host-Guest Molecular Assemblies

Energy Engineering Engineering Physical Chemistry Energy Chemistry Chemistry Catalysis Bioengineering Life Sciences
Hybrid photoanodes Host-guest strategy Water oxidation catalyst Photocatalysis Charge separation

Academic Background

In the field of solar energy conversion, molecular catalysts have attracted significant attention due to their high activity and structural tunability. However, most molecular catalysts operate under homogeneous conditions, which are not favorable for large-scale and recyclable utilization. Therefore, immobilizing molecular catalysts on solid supports has become a more practical research direction. On the other hand, narrow-band-gap inorganic semiconductors, as stable visible-light absorbers, have shown remarkable durability in photoelectrochemical (PEC) catalysis. Immobilizing molecular catalysts on light-absorbing semiconductors is considered a promising approach for solar energy conversion, such as water splitting and CO₂ reduction, as it combines the advantages of both molecular catalysts and semiconductor light absorbers.

However, existing strategies often suffer from poor charge transfer efficiency between the catalyst and the semiconductor, leading to unsatisfactory catalytic activity. Therefore, developing new strategies to construct highly efficient hybrid photoelectrodes has become a key focus of current research. This paper proposes a hybrid photoanode fabrication strategy based on host-guest interactions, where phosphonate-derivatized cyclodextrins (p-CDs) are anchored on the surface of a tungsten oxide (WO₃) film to form a versatile support, enabling the encapsulation of different molecular catalysts for water splitting or organic substrate oxidation.

Source of the Paper

This paper was co-authored by Jiaxuan Wang, Daokuan Li, Xiaona Li, Guoquan Liu, Yong Zhu, Licheng Sun, and Fei Li, affiliated with the State Key Laboratory of Fine Chemicals, Dalian University of Technology, the School of Environmental Science and Technology, the Department of Chemistry, Westlake University, and the KTH Royal Institute of Technology, Sweden. The paper was published in the journal Chem on April 10, 2025, with the DOI 10.1016/j.chempr.2024.11.003.

Research Process and Results

1. Preparation and Characterization of Hybrid Photoanodes

The study first selected a WO₃ film coated on fluorine-doped tin oxide (FTO) as the substrate. Phosphonate-derivatized cyclodextrin (p-CD) with seven ethylphosphonic acid functional groups was prepared using a modified literature procedure. Subsequently, p-CD was loaded onto the WO₃ film to form a p-CD-functionalized WO₃ film (WO₃|p-CD), serving as a platform for encapsulating different molecular catalysts. To validate the feasibility of this platform, an adamantyl cobaloxime complex (Co1) was chosen as a water oxidation catalyst and encapsulated into WO₃|p-CD, forming the WO₃|p-CD|Co1 photoanode.

Through UV-visible titration experiments, it was found that the stoichiometry between p-CD and Co1 in a stable inclusion complex was 1:1, with a binding constant of 2087.11 M⁻¹. Further characterization using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy confirmed the uniform distribution of p-CD and Co1 on the WO₃ film surface, with the crystal structure of WO₃ remaining unaffected by the molecular catalyst loading.

2. Electrochemical Behavior of Cobaloxime Catalysts

The study first explored the electrochemical behavior of Co1 in a homogeneous solution, revealing a catalytic current for water oxidation in a pH 2 aqueous solution. Continuous cyclic voltammetry scans confirmed the stability of Co1 during electrocatalysis. Subsequently, Co1 was immobilized on indium tin oxide (ITO) glass, and it was found that p-CD-mediated electron transfer between the catalyst and the substrate significantly enhanced catalytic activity.

3. Photoelectrochemical Water Oxidation Performance of Hybrid Photoanodes

In PEC water oxidation experiments, the WO₃|p-CD|Co1 photoanode exhibited significant photocurrent enhancement and a cathodic shift in the onset potential. At 1.23 V vs. RHE, the photocurrent density increased from 0.72 mA/cm² to 1.94 mA/cm², indicating that p-CD-mediated host-guest interactions significantly improved the photoanode performance. Moreover, the WO₃|p-CD|Co1 photoanode demonstrated good stability in a 6-hour chronoamperometry experiment, with only an 8% decay in photocurrent.

4. Application of Hybrid Photoanodes in Organic Substrate Oxidation

To validate the versatility of the WO₃|p-CD platform, a 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) derivative was selected as a catalyst for the oxidation of benzyl alcohol. Experimental results showed that the WO₃|p-CD|TEMPO photoanode exhibited significant photocurrent enhancement and accelerated reaction kinetics, with the conversion yield of benzyl alcohol increasing from 39% to 100% and the Faradaic efficiency improving from 76% to 96%.

Conclusions and Significance

This study proposes a hybrid photoanode fabrication strategy based on host-guest interactions, successfully immobilizing molecular catalysts on semiconductor photoanode surfaces and significantly improving the performance of photoelectrochemical water oxidation and organic substrate oxidation. Through p-CD-mediated catalyst encapsulation, the study achieved efficient photo-induced charge separation and long-term stability, providing new insights for solar energy conversion. Additionally, the versatility of the WO₃|p-CD platform allows its application in various photoelectrochemical reactions, demonstrating broad potential.

Research Highlights

  1. Innovative Application of Host-Guest Strategy: For the first time, host-guest interactions were applied to the construction of semiconductor/molecular catalyst hybrid photoanodes, significantly improving catalyst loading efficiency and charge transfer efficiency.
  2. High-Performance Water Oxidation Photoanode: The WO₃|p-CD|Co1 photoanode exhibited excellent photoelectrochemical water oxidation performance, with photocurrent density and stability significantly surpassing those of traditional covalently linked photoanodes.
  3. Versatile Platform: The WO₃|p-CD platform is not only suitable for water oxidation reactions but can also achieve efficient oxidation of organic substrates by selecting different molecular catalysts, demonstrating its potential in various photoelectrochemical reactions.

Other Valuable Information

This study also delved into the charge transfer mechanism of the photoanode using electrochemical impedance spectroscopy (EIS) and fluorescence spectroscopy, confirming the critical role of p-CD-mediated charge separation in enhancing photoanode performance. Additionally, the study explored the stability of molecular catalysts in photoelectrochemical reactions, providing important references for the future design of more efficient molecular catalysts.