Role of Metal Ion-Linked Multilayer Thickness and Substrate Porosity in Surface Loading, Diffusion, and Solar Energy Conversion

Abstract

Metal ion-linked multilayers offer an easily prepared and modular architecture for controlling energy and electron transfer events on nanoparticle, metal oxide films. However, unlike with planar electrodes, the mesoporous nature of the films inherently limits both the thickness of the multilayer and subsequent diffusion through the pores. Here, we systematically investigated the role of TiO 2 nanoparticle film porosity and metal ion-linked multilayer thickness in surface loading, through-pore diffusion, and overall device performance. The TiO 2 porosity was controlled by varying TiO 2 sintering times. Molecular multilayer thickness was controlled through assembling Zn II -linked bridging molecules (B = p-terphenyl diphosphonic acid) between the metal oxide and the Ru(bpy) 2 ((4,4′-PO 3 H 2 ) 2 bpy)]Cl 2 dye (RuP), thus producing TiO 2 -(B n )-RuP films. Using attenuated total reflectance infrared absorption and UV−vis spectroscopy, we observed that at least two molecular layers (i.e., TiO 2 -B 2 or TiO 2 -B 1 -RuP) could be formed on all films but subsequent loading was dependent on the porosity of the TiO 2 . Rough estimates indicate that in a film with 34 nm average pore diameter, the maximum multilayer film thickness is on the order of 4.6−6 nm, which decreases with decreasing pore size. These films were then incorporated as the photoanodes in dye-sensitized solar cells with cobalt(II/III)tris(4,4′-di-tert-butyl-2,2′-bipyridine) as a redox mediator. In agreement with the surface-loading studies, electrochemical impedance spectroscopy measurements indicate that mediator diffusion is significantly hindered in films with thicker multilayers and less porous TiO 2 . Collectively, these results show that care must be taken to balance multilayer thickness, substrate porosity, and size of the mediator in designing and maximizing the performance of new multilayer energy and electron management architectures.

Metal Ion-Linked Molecular Multilayers on Inorganic Substrates: Structure and Applications

“…Extending beyond trilayers becomes challenging due to the fundamental limits as to how many layers can load within the pores of the mesoporous film. 84 Nonetheless, there are several assemblies containing four or more molecular layers with some examples shown in Figure 14.…”

“…Crucial to the formation of metal ion-linked multilayers is geometrically opposed metal ion binding groups for subsequent ion loading. In terms of linking ions, similar to the planar multilayers ( vida supra ), Zr 4+ and Zn 2+ are the most common choice, but Cd 2+ , La 3+ , Sn 4+ , Cu 2+ , Fe 2+ , and Mn 2+ have also been used to generate multilayers. − The binding of the metal ion has been probed using XPS, ICP-MS, and ATR-IR measurements, ,,, but the specific coordination environment around the metal linking ion and its impact on the structure of the multilayer is not yet well understood. Regardless, it is known that the choice of metal ion can impact subsequent layer addition, excited state quenching, energy/electron transfer dynamics, and also performance in solar energy conversion devices.…”

Harnessing near-infrared lightviaS0to T1sensitizer excitation in a molecular photon upconversion solar cell

“…1. A was chosen as the annihilator molecule because the parent 9,10bis(phenylethynyl)anthracene, 10,36 is known to facilitate TTA and also has sufficient potential for electron injection into the conduction band of TiO 2 (À0.5 V vs. NHE) 37 from the singlet ( 1 A*/A + = À0.64 V) but not triplet ( 3 A*/A + = À0.15 V) excited state. 10 Phosphonate binding groups were chosen for A because they exhibit greater surface stability than COOH, 38 and will prevent competitive desorption during subsequent loading steps.…”

Layer-by-Layer Naphthalenediimide/Zn Phosphonate Hybrid Films Grown from Aqueous Solutions by a Simple Deposition Technique

“…25,26 Metal phosphonate films using Zn 2+ as the binding metal cation, on the other hand, have scarcely been reported, in spite that the bulk zinc phosphonates are well-known. 15,27 Recently, organic zinc phosphonate films, generated by layer-by-layer techniques, were reported by Hanson et al 28,29 However, mesoporous TiO 2 nanoparticle films were employed as substrates, limiting the film growth to a few layers. It is also worth of mention the work of Shen et al, who prepared titanium phosphate and lanthanide phosphate layer-by-layer thin films.…”

“…Different photoactive and electroactive molecules have been incorporated into zirconium phosphonate thin films. − The NDI derivatives are particularly easy to functionalize with ligand groups at opposite ends, yielding compounds of the NDI-L 2 type. Our group has constructed stable and organized zirconium phosphonate films incorporating a water-soluble bis-phosphonic acid derivative, N , N ′-bis­(2-phosphonoethyl)-1,4,5,8-naphthalenediimide (PNDI, L = CH 2 CH 2 PO 3 H 2 ) (Figure A). , Metal phosphonate films using Zn 2+ as the binding metal cation, on the other hand, have scarcely been reported, in spite that the bulk zinc phosphonates are well-known. , Recently, organic zinc phosphonate films, generated by layer-by-layer techniques, were reported by Hanson et al , However, mesoporous TiO 2 nanoparticle films were employed as substrates, limiting the film growth to a few layers. It is also worth of mention the work of Shen et al, who prepared titanium phosphate and lanthanide phosphate layer-by-layer thin films. − …”