High Stability of Immobilized Polyoxometalates on TiO₂ Nanoparticles and Nanoporous Films for Robust, Light-Induced Water Oxidation

Abstract

Solutions to two general limitations in the immobilization of molecular water oxidation catalysts (WOCs) on nanoparticle or photoelectrode surfaces have been investigated: (a) a straightforward electrostatic method to bind charged WOCs more effectively to these surfaces and (b) a method to increase the concentration of the semiconductor- and/or electrode-immobilized WOCs so they can be spectroscopically characterized in this form. Polyoxometalate (POM) WOCs, known to be fast, selective, and oxidatively stable under homogeneous conditions, and their high negative charges are a good test case to assess the viability of electrostatic surface immobilization. The POM WOCs, [RuIV 4O5(OH)­(H2O)4(γ-PW10O36)2]9– (Ru 4 P 2 ) and [{RuIV 4(OH)2(H2O)4}­(γ-SiW10O34)2]10‑ (Ru 4 Si 2 ), have been immobilized by silanization on TiO2 nanoparticles and nanoporous electrodes and have been found to retain catalytic water oxidation activity. In photoelectrochemical systems, increased current density of WOC-TiO2/FTO electrodes is consistent with water oxidation occurring on the derivatized, modified semiconductor surface. The simple use of semiconductor nanoparticles (versus conventional larger particles or surfaces) provides sufficient concentrations of immobilized POM WOCs to enable their spectroscopic and electron microscopic characterization after photoelectrocatalytic use. Multiple techniques have been used to observe the effectiveness of silanization for POM WOC immobilization on nanoparticle surfaces as well as TiO2/FTO electrodes before and after photoelectrocatalysis.

Metal Organic Framework Derived Materials: Progress and Prospects for the Energy Conversion and Storage

“…On the other hand, understanding the mechanism of water splitting, detecting active species and correlating the activity with structure have been followed by different groups. [107a,b] Synthesis of new materials involved use of metal salts, metal organic frameworks, and polyoxometalates as the precursor. MOF derived nano and microstructure have been recently explored for the water splitting .…”

Tailored Assembly of Molecular Water Oxidation Catalysts on Photoelectrodes for Artificial Photosynthesis

“…The fabrication of dye‐sensitized PEC cells by stable co‐immobilization of molecular photosensitizers and electrocatalysts with APTES chemistry using TiO 2 and indium tin oxide was reported (Figure g) . In addition to organometallic electrocatalysts, Hill's group reported that negatively charged tetranuclear polyoxometalate (POM) OER catalysts can be electrostatically adsorbed on TiO 2 or Fe 2 O 3 photoanodes after modification with APTES. However, their works have limitations regarding the low stability of immobilized POM catalysts without protective layer by atomic layer deposition to prevent the detachment of POM electrocatalysts .…”

“…Rather than modification of molecular electrocatalysts, photoelectrode surfaces can also be covalently modified with functional groups to avoid altering the activity of molecular electrocatalysts, to immobilize molecular electrocatalysts difficult to covalently modify, or to form more stable chemical bonding , . In particular, alkoxysilanes with a specific end‐functional group have been widely used for surface modification of various photoelectrodes and electrodes, such as TiO 2 ,, , ZnO,, SnO 2 ,, and Si, due to their well‐known and simple chemistry. When using 3‐aminoproyltriethoxysilane (APTES) in particular, amine groups can be readily displayed on photoelectrode surfaces by its hydrolytic condensation and used to immobilize molecular electrocatalysts covalently and electrostatically.…”

Multi-Tasking POM Systems

“…Several studies have focused on direct binding of the POM to photoactive metal oxides, such as TiO 2 and hematite. Recently, our group published a report in which we treated TiO 2 with 3-aminopropyltrimethoxysilane (APS) to generate a cationic surface that strongly bound the highly negatively charged POM WOC, Ru 4 Si 2 (Lauinger et al, 2015 ). This was effective for surface immobilization of the POM and light driven water oxidation.…”