WO2016028738A1 - Stabilization of chromophores or catalysts with polymer overlayers - Google Patents

Abstract

Chromophores are stabilized on the surface of nanoparticle metal oxide electrodes, even in aqueous environments, with an overlayer comprising poly(methyl methacrylate), poly(tetrafluoroethylene), or a combination thereof. Molecular chromophores, catalysts, and combinations thereof, can be so stabilized and yet retain their useful reactivity. Facile application of an overlayer occurs by dip-coating into a polymer monomer or oligomer solution, for example.

Description

STABILIZATION OF CHROMOPHORES OR CATALYSTS WITH POLYMER OVERLAYERS

Cross-Reference to Related Applications

[0001] This application claims benefit of priority under PCT Chapter I, Article 8, and 35 U.S.C. § 1 19(e) of U.S. Provisional Patent Application No. 62/038,634, entitled "STABILIZATION OF CHROMOPHORES OR CATALYSTS WITH

POLYMER OVERLAYERS," filed on August 18, 2014, which is incorporated herein by reference.

Statement Regarding Federally Sponsored Research or Development

[0002] This invention was made with government support under Grant No. DE-SC000101 1 awarded by the Department of Energy. The U.S. Government has certain rights in the invention.

Field of Invention

[0003] This invention relates to electrochemistry, photochemistry, and catalysis. Some embodiments relate to the stabilization of molecular chromophores, catalysts, and combinations thereof on metal oxide surfaces, such as nanoparticle metal oxide electrodes in aqueous environments, using a polymer overlayer.

Background of the Invention

[0004] Energy storage with solar energy used to create chemical fuels by artificial photosynthesis is an important research area but one with significant challenges. Hurdles exist to high efficiency energy conversion and storage arising from design, scale-up, and long-term stability of devices and components. Use of photoelectrochemical cells for solar energy conversion and storage is a promising approach. This includes dye-sensitized photoelectrosynthesis cells (DSPECs) which integrate high band gap oxide semiconductors with the molecular-level light absorption-catalytic properties of molecular assemblies or clusters. The use of molecular assemblies offers the advantage of exploiting chemical synthesis to make rapid and systematic changes in the active components. In a DSPEC photoanode, light absorption by a chromophore (dye)-catalyst assembly initiates electron transfer catalysis by excited state injection with subsequent electron transfer activation of the catalyst for water oxidation.

[0005] Long-term stability of assemblies on metal oxide surfaces under the working conditions of a DSPEC, most likely over a broad pH range in water, is an essential element of a working device. The long-term performance of DSPECs depends on both the stability of the active forms of surface-bound assemblies and the stability of the assembly on oxide surfaces.

Summary of the Invention

[0006] We report here a straightforward approach to overlayer stabilization based on poly(methyl methacrylate) (PMMA) overlayers. It provides a simple alternative that results in surface stabilization without significantly modifying the chemical or physical properties of pre-bound molecules. PMMA films have been used as a medium for immobilizing chromophores, including Ru(bpy)3²⁺ (bpy is 2,2'- bipyridine), in an optically transparent, semi-rigid matrix convenient for photophysical studies and industrial uses.

[0007] Accordingly, some embodiments relate to methods of stabilizing a molecule on a metal oxide surface, comprising: forming an overlayer of at least one polymer on the metal oxide surface comprising the molecule.

[0008] Other embodiments provide electrodes comprising:

a conductive substrate;

a metal oxide surface in electrical communication with the conductive substrate; at least one molecule in electrical communication and anchored to the metal oxide surface; and

an overlayer of at least one polymer on the metal oxide surface comprising the at least one molecule.

[0009] Still other embodiments relate to methods of harvesting light to perform useful chemistry, comprising:

providing an electrode such as any described herein;

engaging the electrode in an electrochemical cell; illuminating the electrode with light under conditions suitable to cause one or more chemical reactions, wherein one or more products of the one or more chemical reactions are useful;

thereby harvesting light to perform useful chemistry.

[0010] While the disclosure provides certain specific embodiments, the invention is not limited to those embodiments. A person of ordinary skill will appreciate from the description herein that modifications can be made to the described embodiments and therefore that the specification is broader in scope than the described embodiments. All examples are therefore non-limiting.

Brief Description of the Drawings

[0011] Figure 1 . Schematic depictions of an added PMMA coating on a metal- oxide surface (Ti0₂ or nano\JO) and the results of contact angle measurements of a mesoporous Ti0₂ film before and after soaking in a PMMA/DCM coating solution. Also shown are the chemical structures of the Ru(ll) dyes (RuP²⁺ or RuC²⁺) and PMMA (n ~ 3500).

[0012] Figure 2. FIB-SEM images of PMMA-coated Ti0₂-RuP²⁺ films, Ti0₂- RuP²⁺(PMMA), prepared by soaking in 2.0 wt.% PMMA containing stock solutions in DCM.

[0013] Figure 3. FIB-SEM images of PMMA-coated Ti0₂-RuP²⁺ films, Ti0₂- RuP²⁺(PMMA), prepared by soaking in 3.0 wt.% PMMA containing stock solutions in DCM.

[0014] Figure 4. TEM image of a 2.0 wt.% PMMA-coated Ti0₂ nanoparticle with the PMMA thickness indicated.

[0015] Figure 5. Variation in PMMA overlayer thickness with solution concentration on Ti0₂-RuP²⁺.

[0016] Figure 6. Absorption spectra of Ti0₂-RuP²⁺ films before and after soaking in 0.5 wt.% to 3.0 wt.% PMMA solutions. Inset: relative A_t/A₀ absorbances of PMMA-coated Ti0₂-RuP²⁺ films at 458 nm in pH 12 phosphate buffer solution containing 0.1 M HP0₄ ²7P0₄ ^3", 0.5 M KN0₃ in the dark.

[0017] Figure 7. Photo-stability of a PMMA-coated Ti0₂-RuP²⁺ film in aqueous 0.1 M HCI0₄ (pH 1 ). Inset: desorption rate constant (k_des) as a function of PMMA overlayer thickness. Plot shows K_des (x 10^"V¹) versus PMMA thickness in nm.

[0018] Figure 8. Normalized transient absorbance kinetics probed at 400 nm following 3.2 mJ, 425 nm, 5 nsec excitation of Ti0₂-RuP²⁺(PMMA) electrodes (Γ/Γ₀ = 1 .0) from -3.0 wt.% PMMA, immersed in 0.1 M HCI0₄ at 22 ± 2 °C.

[0019] Figure 9. Schematic depictions of PMMA coating protecting RuP²⁺ or RuC²⁺ functionalized mesoporous nanoparticle metal oxide, with TEM image showing PMMA coating on Ti0₂.

[0020] Figure 10. Schematic depictions of procedure for PMMA coating on

RuP²⁺ or RuC²⁺ functionalized mesoporous nanoparticle metal oxide.

[0021] Figure 1 1 . CVs of 0.0 - 4.0 wt.% PMMA-coated FTO|ranolTO-RuP²⁺ in 0.1 M HCI0 aqueous (pH 1 ) condition. (Scan rate: 30 mV/s).

[0022] Figure 12. CVs of 0.0 - 3.0 wt.% PMMA-coated FTO|Ti0₂-RuP²⁺ in 0.1

M HCI0₄ solution. (Scan rate: 20 mV/s).

[0023] Figure 13. Contact angle of PMMA-coated (0.0, 0.5, 1 .0, 2.0, and 3.0 wt.%) Ti0₂-RuP²⁺ film.

[0024] Figure 14. PMMA stock solution concentration versus contact angle of PMMA-coated Ti0₂-RuP²⁺ film.

[0025] Figure 15. SEM image of the FTO|Ti0₂-RuP²⁺ film without the PMMA coating (i.e. the control sample) with a gold-palladium sputter coating (-10 nm) and a down cut milling technique to expose the inside of the film.

[0026] Figure 16. SEM image of the FTO|Ti0₂-RuP²⁺ film without the PMMA coating (i.e. the control sample) with a gold-palladium sputter coating (-10 nm) and a down cut milling technique to expose the inside of the film.

[0027] Figure 17. SEM image of the 0.5 wt.% PMMA-coated FTO|Ti0₂-RuP²⁺ film with a sputter coating with a gold-palladium sputter coating (-10 nm) and a down cut milling technique to expose the inside of the film.

[0028] Figure 18. SEM image of the 0.5 wt.% PMMA-coated FTO|Ti0₂-RuP²⁺ film with a sputter coating with a gold-palladium sputter coating (-10 nm) and a down cut milling technique to expose the inside of the film.

[0029] Figure 19. SEM image of the 0.5 wt.% PMMA-coated FTO|Ti0₂-RuP²⁺ film with a sputter coating with a gold-palladium sputter coating (-10 nm) and a down cut milling technique to expose the inside of the film. [0030] Figure 20. SEM image of the 1 .0 wt.% PMMA-coated FTO|Ti0₂-RuP²⁺ film with a sputter coating with a gold-palladium sputter coating (-10 nm) and a down cut milling technique to expose the inside of the film.

[0031] Figure 21 . SEM image of the 1 .0 wt.% PMMA-coated FTO|Ti0₂-RuP²⁺ film with a sputter coating with a gold-palladium sputter coating (-10 nm) and a down cut milling technique to expose the inside of the film.

[0032] Figure 22. SEM image of the 1 .0 wt.% PMMA-coated FTO|Ti0₂-RuP²⁺ film with a sputter coating with a gold-palladium sputter coating (-10 nm) and a down cut milling technique to expose the inside of the film.

[0033] Figure 23. SEM image of the 2.0 wt.% PMMA-coated FTO|Ti0₂-RuP²⁺ film with a sputter coating with a gold-palladium sputter coating (-10 nm) and a down cut milling technique to expose the inside of the film.

[0034] Figure 24. SEM image of the 2.0 wt.% PMMA-coated FTO|Ti0₂-RuP²⁺ film with a sputter coating with a gold-palladium sputter coating (-10 nm) and a down cut milling technique to expose the inside of the film.

[0035] Figure 25. SEM image of the 2.0 wt.% PMMA-coated FTO|Ti0₂-RuP²⁺ film with a sputter coating with a gold-palladium sputter coating (-10 nm) and a down cut milling technique to expose the inside of the film.

[0036] Figure 26. SEM image of the 3.0 wt.% PMMA-coated FTO|Ti0₂-RuP²⁺ film with a sputter coating with a gold-palladium sputter coating (-10 nm) and a down cut milling technique to expose the inside of the film.

[0037] Figure 27. SEM image of the 3.0 wt.% PMMA-coated FTO|Ti0₂-RuP²⁺ film with a sputter coating with a gold-palladium sputter coating (-10 nm) and a down cut milling technique to expose the inside of the film.

[0038] Figure 28. TEM images of the Ti0₂-RuP²⁺ nanoparticles without PMMA coating (i.e. the control sample).

[0039] Figure 29. TEM images of the 0.5 wt.% PMMA-coated Ti0₂-RuP²⁺ nanoparticles.

[0040] Figure 30. TEM images of the 1 .0 wt.% PMMA-coated Ti0₂-RuP²⁺ nanoparticles.

[0041] Figure 31 . TEM images of the 2.0 wt.% PMMA-coated Ti0₂-RuP²⁺ nanoparticles. [0042] Figure 32. TEM images of the 3.0 wt.% PMMA-coated Ti0₂-RuP²⁺ nanoparticles.

[0043] Figure 33. Absorption spectra of PMMA-coated on mesoporous

FTO|Ti0₂-RuP²⁺ nanoparticle films prepared by soaking in 0.0 ~ 3.0 wt.% PMMA stock solution for 10 seconds.

[0044] Figure 34. Absorption spectra of PMMA-coated on mesoporous FTO|nanolTO-RuP²⁺ nanoparticle films prepared by soaking in 0.0 ~ 3.0 wt.% PMMA stock solution for 10 seconds.

[0045] Figure 35. Absorbance changes of 0.0 wt.% PMMA-coated FTO|Ti0₂- RuP²⁺ film (i.e. the control sample) in pure water under dark. Arrow shows trend in absorbance over time. Inset: Absorbance changes at 458 nm versus time.

[0046] Figure 36. Absorbance changes of 0.5 wt.% PMMA-coated FTO|Ti0₂- RuP²⁺ film in pure water under dark. Inset: Absorbance changes at 458 nm versus time.

[0047] Figure 37. Absorbance changes of 1.0 wt.% PMMA-coated FTO|Ti0₂- RuP²⁺ film in pure water under dark. Inset: Absorbance changes at 458 nm versus time.

[0048] Figure 38. Absorbance changes of 2.0 wt.% PMMA-coated FTO|Ti0₂- RuP²⁺ film in pure water under dark. Inset: Absorbance changes at 458 nm versus time.

[0049] Figure 39. Absorbance changes of 0.0 wt.% PMMA-coated FTO|Ti0₂- RuP²⁺ film (i.e. the control sample) in pH=12 phosphate buffer solution containing 0.1 M HP0₄ ²7P0₄ ^3", 0.5 M KN0₃ under dark.

[0050] Figure 40. Absorbance changes of 0.5 wt.% PMMA-coated FTO|Ti0₂- RuP²⁺ film in pH=12 phosphate buffer solution containing 0.1 M HP0₄ ²7P0₄ ^3", 0.5 M KNO₃ under dark. Arrow shows trend in absorbance over time. Inset: Absorbance changes at 458 nm versus time.

[0051] Figure 41. Absorbance changes of 1.0 wt.% PMMA-coated FTO|Ti0₂- RuP²⁺ film in pH=12 phosphate buffer solution containing 0.1 M HP0₄ ²7P0₄ ^3", 0.5 M KNO₃ under dark. Arrow shows trend in absorbance over time. Inset: Absorbance changes at 458 nm versus time.

[0052] Figure 42. Absorbance changes of 2.0 wt.% PMMA-coated FTO|Ti0₂- RuP²⁺ film in pH=12 phosphate buffer solution containing 0.1 M HP0₄ ²7P0₄ ^3", 0.5 M KNO₃ under dark. Arrow shows trend in absorbance over time. Inset: Absorbance changes at 458 nm versus time.

[0053] Figure 43. Absorbance changes of 3.0 wt.% PMMA-coated FTO|Ti0₂- RuP²⁺ film in pH=12 phosphate buffer solution containing 0.1 M HP0₄ ²7P0₄ ^3", 0.5 M KNO₃ under dark. Arrow shows trend in absorbance over time. Inset: Absorbance changes at 458 nm versus time.

[0054] Figure 44. Photostability of 0.5 wt.% PMMA-coated FTO|Ti0₂-RuP²⁺ mesoporous film in 0.1 M HCI0₄ aqueous (pH 1 ) condition under constant 455 nm LED irradiation (475 mW/cm²) from 0 to 16 h recorded every 15 min. Arrow shows trend in absorbance over time. Inset: Absorbance at 480 nm versus time and desorption rate constant (k_des).

[0055] Figure 45. Photostability of 1 .0 wt.% PMMA-coated FTO|Ti0₂-RuP²⁺ mesoporous film in 0.1 M HCI0₄ aqueous (pH 1 ) condition under constant 455 nm LED irradiation (475 mW/cm²) from 0 to 16 h recorded every 15 min. Arrow shows trend in absorbance over time. Inset: Absorbance at 480 nm versus time and desorption rate constant (k_des).

[0056] Figure 46. Photostability of 2.0 wt.% PMMA-coated FTO|Ti0₂-RuP²⁺ mesoporous film in 0.1 M HCI0₄ aqueous (pH 1 ) condition under constant 455 nm LED irradiation (475 mW/cm²) from 0 to 16 h recorded every 15 min. Arrow shows trend in absorbance over time. Inset: Absorbance at 480 nm versus time and desorption rate constant (k_des).

[0057] Figure 47. Photostability of 3.0 wt.% PMMA-coated FTO|Ti0₂-RuP²⁺ mesoporous film in 0.1 M HCI0₄ aqueous (pH 1 ) condition under constant 455 nm LED irradiation (475 mW/cm²) from 0 to 16 h recorded every 15 min. Arrow shows trend in absorbance over time. Inset: Absorbance at 480 nm versus time and desorption rate constant (k_des).

[0058] Figure 48. Photostability of 0.5 wt.% PMMA-coated FTO|Ti0₂-RuP²⁺ mesoporous film in pure water under constant 455 nm LED irradiation (475 mW/cm²) from 0 to 16 h recorded every 15 min. Arrow shows trend in absorbance over time. Inset: Absorbance at 480 nm versus time and desorption rate constant (k_des).

[0059] Figure 49. Photostability of 1 .0 wt.% PMMA-coated FTO|Ti0₂-RuP²⁺ mesoporous film in pure water under constant 455 nm LED irradiation (475 mW/cm²) from 0 to 16 h recorded every 15 min. Arrow shows trend in absorbance over time. Inset: Absorbance at 480 nm versus time and desorption rate constant (k_des).

[0060] Figure 50. Photostability of 2.0 wt.% PMMA-coated FTO|Ti0₂-RuP²⁺ mesoporous film in pure water under constant 455 nm LED irradiation (475 mW/cm²) from 0 to 16 h recorded every 15 min. Arrow shows trend in absorbance over time. Inset: Absorbance at 480 nm versus time and desorption rate constant (k_des).

[0061] Figure 51. Photostability of 3.0 wt.% PMMA-coated FTO|Ti0₂-RuP²⁺ mesoporous film in pure water under constant 455 nm LED irradiation (475 mW/cm²) from 0 to 16 h recorded every 15 min. Arrow shows trend in absorbance over time. Inset: Absorbance at 480 nm versus time and desorption rate constant (k_des).

[0062] Figure 52. Photostability of PMMA-coated FTO|Ti0₂-RuP²⁺ film in pure water: RuP²⁺ loss from the surfaces was monitored by absorbance changes at 480 nm which were also corrected for light scattering from Ti0₂ under constant 455 nm LED irradiation (475 mW/cm²) from 0 to 16 h recorded every 15 min.

[0063] Figure 53. Photostability of 1 .0 wt.% PMMA-coated FTO|Ti0₂-RuP²⁺ mesoporous film in in pH 12 phosphate buffer solution containing 0.1 M HP0₄ ^2" /P0₄ ^3", 0.5 M KN0₃ under constant 455 nm LED irradiation (475 mW/cm²) from 0 to 16 h recorded every 15 min. Arrow shows trend in absorbance over time. Inset: Absorbance at 480 nm versus time.

[0064] Figure 54. Photostability of 2.0 wt.% PMMA-coated FTO|Ti0₂-RuP²⁺ mesoporous film in in pH 12 phosphate buffer solution containing 0.1 M HP0₄ ^2" /P0₄ ^3", 0.5 M KNO₃ under constant 455 nm LED irradiation (475 mW/cm²) from 0 to 16 h recorded every 15 min. Arrow shows trend in absorbance over time. Inset: Absorbance at 480 nm versus time.

[0065] Figure 55. Photostability of 3.0 wt.% PMMA-coated FTO|Ti0₂-RuP²⁺ mesoporous film in in pH 12 phosphate buffer solution containing 0.1 M HP0₄ ^2" /P0₄ ^3", 0.5 M KNO₃ under constant 455 nm LED irradiation (475 mW/cm²) from 0 to 16 h recorded every 15 min. Arrow shows trend in absorbance over time. Inset: Absorbance at 480 nm versus time.

[0066] Figure 56. Photostability of PMMA-coated FTO|Ti0₂-RuP²⁺ film in pH 12 phosphate buffer solution containing 0.1 M HP0₄ ²7P0₄ ^3", 0.5 M KN0₃: RuP²⁺ loss from the surfaces was monitored by absorbance changes at 480 nm which were also corrected for light scattering from Ti0₂ under constant 455 nm LED irradiation (475 mW/cm²) from 0 to 1 6 h recorded every 1 5 min.

[0067] Figure 57. Absorbance-time changes of a 3.0 wt.% PMMA-coated FTO|ranolTO-RuP²⁺ electrode in pH 1 2 solution (0.1 M HP0₄ ²7P0₄ ^3" phosphate buffer with 0.5 M KNO₃) for 24 h under dark. Arrow shows trend in absorbance over time. Inset: Absorbance changes at 458 nm versus time.

[0068] Figure 58. CVs of before and after 24 h soaking a 3.0 wt.% PMMA- coated FTO|A7anolTO-RuP²⁺ electrode in pH 1 2 solution measured in 0.1 M HCI0 aqueous solution.

[0069] Figure 59. Photostability of 3.0 wt.% PMMA-coated FTO|nanolTO- RuP²⁺ mesoporous film in pH 1 solution containing 0.1 M HCI0₄ under constant 455 nm LED irradiation (475 mW/cm²) from 0 to 1 6 h recorded every 1 5 min. Inset:

Absorbance at 480 nm versus time and desorption rate constant (k_des).

[0070] Figure 60. Photostability of 3.0 wt.% PMMA-coated FTO|nanolTO- RuP²⁺ mesoporous film in pH 1 2 phosphate buffer solution containing 0.1 M HP0₄ ^2" /P0₄ ^3", 0.5 M KNO3 under constant 455 nm LED irradiation (475 mW/cm²) from 0 to 1 6 h recorded every 1 5 min. Arrow shows trend in absorbance over time. Inset: Absorbance at 480 nm versus time and desorption rate constant (kdes).

[0071] Figure 61 . Absorbance changes of 3.0 wt.% PMMA-coated FTO|Ti0₂- RuC²⁺ in pH 1 2 phosphate buffer solution containing 0.1 M HP0₄ ²7P0₄ ^3", 0.5 M KNO₃ in dark condition. Arrow shows trend in absorbance over time. Inset:

Absorbance changes at 458 nm versus time.

[0072] Figure 62. Normalized transient absorbance kinetics at 22 ± 2 °C probed at 400 nm following 3.2 mJ, 425 nm, 5 nsec excitation of FTO|Ti0₂-RuP²⁺ electrodes (Γ/Γ₀ = 1 .0), previously soaked in 3.0 wt.% PMMA solution, immersed in 0.1 M HCI0₄, pH 7.5 (0.1 M phosphate buffer, 0.5 M NaCI0₄) and pH 1 2 (0.1 M phosphate buffer, 0.5 M NaCI0 ) solutions.

[0073] Figure 63. Normalized luminescence spectra collected at 22 ± 0.1 °C following 450 nm excitation of FTO|Ti0₂-RuP²⁺ electrodes (Γ/Γ₀ = 1 .0), previously soaked in 3.0 wt.% PMMA solution, immersed in 0.1 M HCI0₄, pH 7.5 (0.1 M phosphate buffer, 0.5 M NaCI0₄) and pH 1 2 (0.1 M phosphate buffer, 0.5 M NaCI0₄) solutions. [0074] Figure 64. Luminescence spectra collected at 22 ± 0.1 °C following 450 nm excitation of FTO|Ti0₂-RuP²⁺ electrodes (Γ/Γ₀ = 1 .0), previously soaked in 3.0 wt.% PMMA solution, immersed in 0.1 M HCI0₄, pH 7.5 (0.1 M phosphate buffer, 0.5 M NaCI0₄) and pH 12 (0.1 M phosphate buffer, 0.5 M NaCI0₄) solutions.

[0075] Figure 65. Normalized luminescence spectra collected at 22 ± 0.1 °C following 450 nm excitation of FTO|Ti0₂-RuP²⁺ electrodes (Γ/Γ₀ = 1 .0), previously soaked in 0, 0.5, 1 .0, 2.0 and 3.0 wt.% PMMA solutions, immersed in 0.1 M HCI0₄ solution.

[0076] Figure 66. Luminescence spectra collected at 22 ± 0.1 °C following 450 nm excitation of FTO|Ti0₂-RuP²⁺ electrodes (Γ/Γ₀ = 1 .0), previously soaked in 0, 0.5, 1 .0, 2.0 and 3.0 wt.% PMMA solutions, immersed in 0.1 M HCI0₄ solution.

Detailed Description

[0077] As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various forms. The figures are not necessarily to scale, and some features may be exaggerated to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention.

[0078] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this disclosure belongs. In the event that there is a plurality of definitions for a term herein, those in this section prevail unless stated otherwise.

[0079] Where ever the phrase "for example," "such as," "including" and the like are used herein, the phrase "and without limitation" is understood to follow unless explicitly stated otherwise. Similarly "an example," "exemplary" and the like are understood to be non-limiting.

[0080] The term "substantially" allows for deviations from the descriptor that don't negatively impact the intended purpose. Descriptive terms are understood to be modified by the term "substantially" even if the word "substantially" is not explicitly recited. [0081] The term "about" when used in connection with a numerical value refers to the actual given value, and to the approximation to such given value that would reasonably be inferred by one of ordinary skill in the art, including

approximations due to the experimental and or measurement conditions for such given value.

[0082] The terms "comprising" and "including" and "having" and "involving" (and similarly "comprises", "includes," "has," and "involves") and the like are used interchangeably and have the same meaning. Specifically, each of the terms is defined consistent with the common United States patent law definition of

"comprising" and is therefore interpreted to be an open term meaning "at least the following," and is also interpreted not to exclude additional features, limitations, aspects, etc. Thus, for example, "a device having components a, b, and c" means that the device includes at least components a, b and c. Similarly, the phrase: "a method involving steps a, b, and c" means that the method includes at least steps a, b, and c.

[0083] Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

[0084] Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.

[0085] It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

[0086] In some embodiments of the present invention, any suitable molecule can be stabilized on a metal oxide surface. A suitable molecule can be, but is not limited to a chromophore, a catalyst, a chromophore-catalyst assembly, or a combination of two or more thereof. Chromophores that may be mentioned include from ruthenium coordination complexes, osmium coordination complexes, copper coordination complexes, porphyrins, phythalocyanines, and organic dyes, and combinations thereof. Suitable transition metal catalysts include, but are not limited to iron catalysts, ruthenium catalysts, osmium catalysts, and combinations thereof. Generally speaking, a chromophore absorbs one or more photons of light to reach an excited state, whereupon electron transfer may occur. A catalyst, in contrast, facilitates the chemical reaction of another molecule. A catalyst may absorb one or more photons, and may engage in electron transfer. Broadly, suitable molecules may be chosen from those that include one or more alike or different transition metal atoms.

[0087] Exemplary catalysts include, but are not limited to, complexes having the structure of formula (I):

L₁ M 1_2 (I)

wherein M is chosen from Ru, Ir, Fe, Co, Ni, and Os, and L-i , L₂, and L₃ may be any combination of ligands as long as the combination meets the bonding requirements for M. In some embodiments, U may be any applicable bidentate ligand that is known to one skilled in the art, L₂ may be any applicable tridentate ligand that is known to one skilled in the art and L₃ may be any applicable monodentate ligand that is known to one skilled in the art. In one embodiment, L₃ is H₂0.

[0088] As used herein, a ligand is either an atom, ion, or molecule that binds to a central metal to produce a coordination complex. The bonding between the metal and ligand generally involves formal donation of one or more of the ligand's electrons. The monodentate ligand is a ligand with one lone pair of electrons that is capable of binding to an atom (e.g. a metal atom). Exemplary monodentate ligands include, but are not limited to, H₂0 (aqua), NH₃ (ammine), CH₃NH₂ (methylamine), CO (carbonyl), NO (nitrosyl), F^" (fluoro), CN^" (cyano), CI^" (chloro), Br^" (bromo), I^" (iodo), N0₂ ^" (nitro), and OH^" (hydroxyl). In some embodiments, the monodentate ligand is H₂0. The bidentate ligand is a ligand with two lone pairs of electron that are capable of binding to an atom (e.g. a metal atom). Exemplary bidentate ligands include, but are not limited to, bipyridine, phenanthroline, 2-phenylpyridine

bipyrimidine, bipyrazyl, glycinate, acetylacetonate, and ethylenediamine. Exemplary tridentate ligands include, but are not limited to, terpyridine, DMAP, and 2,6-bis(1 - methylbenzimidazol-2-yl)pyridine (Mebimpy). In some cases, L₂ can be a

tetradentate ligand such as, for example, porphyrin, and U and L₃ are alike or different monodentate ligands such as those described for L₃ above. The tridentate ligand and tetradentate ligand are ligands with respectively three or four lone pairs of electron that are capable of binding to an atom (e.g. a metal atom). [0089] Among suitable catalysts, transition metals such as, for example, Ru, Ir, Fe, Co, Ni, Os, Mn, and Mg and combinations thereof can be mentioned. Further catalysts can be chosen from [Ru(tpy)(bpy)(OH₂)]²⁺, [Ru(tpy)(bpm)(OH₂)]²⁺,

[Ru(tpy)(bpz)(OH₂)]²⁺, [Ru(tpy)(Mebim-pz)(OH₂)]²⁺, [Ru(tpy)(Mebim-py)(OH₂)]²⁺, [Ru(DMAP)(bpy)(OH₂)]²⁺, [Ru(Mebimpy)(bpy)(OH₂)]²⁺, [Ru(Mebimpy)(Mebim- pz)(OH₂)]²⁺, [Ru(Mebimpy)(Mebimpy)(OH₂)]²⁺, {Ru(Mebimpy)[4,4'- ((HO)₂OPCH₂)₂bpy](OH₂)}²⁺ and Os(tpy)(bpy)(OH₂)²⁺, and combinations thereof. In addition to those ligands defined elsewhere herein and in the literature, bpm is 2,2'- bipyrimidine; bpz is 2,2'-bipyrazine; Mebim-pz is 3-methyl-1 -pyrazylbenzimidazol-2- ylidene; Mebim-py is 3-methyl-1 -pyridylbenzimidazol-2-ylidene; Mebimpy is 2,6- bis(1 -methylbenzimidazol-2-yl)pyridine; and DMAP is 2,6- bis((dimethylamino)methyl)pyridine. Chromophore-catalyst assemblies such as, for example, [(P0₃H₂)₂bpy)₂Rua(4-Mebpy-4'-bimpy)Ru_b(tpy)(OH₂)]⁴⁺ ((P0₃H₂)₂bpy is 4,4'-bisphosphonato-2,2'-bipyridine; 4-Mebpy-4'-bimpy is 4-(methylbipyridin-4'-yl)-/V- benzimid-A/-pyridine; tpy is 2,2':6',2"-terpyridine)), may be mentioned. For more discussion, see U.S. Patent Application Publication No. 2013/00201 13 A1 , published on January 24, 2013, which is incorporated herein by reference.

[0090] The molecules can be anchored to the metal oxide surface in any suitable manner. Covalent bonding, ionic bonding, and combinations thereof can be mentioned. In some cases, phosphonate moieties, carboxylate moieties, and combinations thereof can anchor a molecule to a metal oxide surface. Certain cases provide a molecule comprising a Ru(ll) polypyridyl derivatized phosphonate complex, a Ru(ll) polypyridyl derivatized carboxylate complex, or a combination thereof. For example, the molecule can comprise [Ru(bpy)₂((4,4'-(OH)₂PO)₂bpy)]²⁺,

[Ru(bpy)₂(4,4'-(COOH)₂bpy)]²⁺, or a combination thereof.

[0091] Any suitable metal oxide surface can appear in certain embodiments of the present invention. Conductive, semi-conductive, and insulating metal oxides may be mentioned. In some cases, the metal oxide surface comprises titanium dioxide, tin-doped indium oxide, or a combination thereof. Also, the metal oxide surface may comprise tin-doped indium oxide (ITO), fluorine-doped tin oxide (FTO), antimony tin oxide (ATO), gallium zinc oxide (GZO), indium zinc oxide (IZO), copper aluminum oxide, fluorine-doped zinc oxide, and aluminum zinc oxide, and

combinations thereof. In still further cases, the metal oxide is chosen from Sn0₂,Ti0₂, Nb₂0₅, SrTi0₃, Zn₂Sn0₄, Zr0₂, NiO, Ta-doped Ti0₂, Nb-doped Ti0₂, and combinations thereof. The metal oxide surface also can be of any suitable form. Monolithic, single crystal, nanocrystalline, porous or nonporous, micron-size particles, and nanoparticles having an average size of less than about 1 μιη can be mentioned. In some cases, the metal oxide surface comprises nanoparticles of the metal oxide.

[0092] Any suitable polymer can be used to provide the stabilizing overlayer. Poly(methyl methacrylate) is illustrated extensively herein, but embodiments of the present invention are not limited thereto. Poly(methyl methacrylate),

poly(tetrafluoroethylene), and combinations thereof can be used.

[0093] Any suitable substrate can be used in certain embodiments of the present invention. For example, a conductive substrate can be used. In another example, a transparent conductive substrate can be used. Fluorine-doped tin oxide- coated glass, or tin-doped indium oxide-coated glass, for example, can provide a suitable transparent conductive substrate in some cases.

[0094] As used herein, "electrical communication" exists between two points if electron transfer can happen under suitable circumstances. In some cases, electrical communication indicates metallic electrical conduction. In other cases, electrical communication indicates electron transfer such as from an excited chromophore to a metal oxide surface.

[0095] Electrodes and other embodiments of the present invention can be engaged to harvest light to perform useful chemistry. Any suitable light source can be used. In some cases, sunlight is used; in other cases, an artificial light source is used. Useful chemistry includes, but is not limited to catalysis, photo catalysis, electro catalysis, the reduction of carbon dioxide, splitting water, and the like. In some cases, an electrode exhibits enhanced long-term performance relative to a substantially-similar electrode that does not have the overlayer of at least one polymer on the metal oxide surface comprising the at least one molecule. Long-term performance can be measured in any suitable manner. For example, Figures 52 and 56 (among others) illustrate experiments demonstrating the loss of RuP²⁺ over time from electrodes treated with varying amounts of PMMA, illuminated while immersed in water or pH 12 phosphate buffer solution, respectively. In those figures, the 3.0 wt. % sample shows enhanced long-term performance relative to the 1 .0 wt. % sample.

[0096] Electrodes, photo electrochemical cells such as DSPECs, and the components thereof can be made in any suitable manner. Molecules, metal oxide surfaces, polymer layers, and other components can be synthesized as taught in the literature and/or as illustrated herein. In some cases, forming an overlayer comprises contacting the metal oxide surface comprising the molecule with a composition comprising polymer monomers oligomers, or a combination thereof under conditions sufficient to form the overlayer. For example, monomers or oligomers of PMMA and/or poly(tetrafluoroethylene) can be dispersed in any suitable solvent, such as, for example, dichloromethane or Fluorinert FC-20 (CAS:51 142-49- 5). Then the metal oxide surface comprising the molecule is contacted with the monomer/oligomer composition, such as by dip coating, brushing, spraying, spin coating, or combinations thereof. Optionally, after contact, the overlayer can be dried even as polymerization continues by any suitable methods, such as, for example, blowing a dry inert gas such as N₂ across the surface for a time, drying under vacuum, and combinations thereof. In some cases, oligomers of

poly(tetrafluoroethylene) are dispersed in Fluorinert FC-20 in an amount of oligomer of up to 2 wt %, in certain instances up to 1 wt %, and an electrode comprising nanoparticles of metal oxide such as Ti0₂ and a catalyst such as RuP²⁺ is dipped into the oligomer composition, withdrawn, dried under flowing N₂ for five minutes, and then dried further under high vacuum. Additional examples of forming an overlayer appear herein. Polymerization of the monomers and/or oligomers can be effected by any suitable method, such as, for example, catalyzed polymerization, electropolymerization, photopolymerization, and combinations thereof. In some cases, an electrochemical or photoelectrochemical cell comprises an electrode such as a described herein, and a counter electrode, wherein the electrode and the counter electrode are in ionic contact with an electrolyte, and in electrical communication with each other via an external circuit. In other cases, an

electrochemical or photoelectrochemical cell further comprises a reference electrode. Any suitable counter electrode can be chosen, such as, for example, platinum, gold, copper, nickel, glassy carbon, and the like. Any suitable reference electrode can be chosen, such as for example, standard hydrogen electrode (SHE), normal hydrogen electrode (NHE), silver chloride electrode, saturated calomel electrode (SCE), and saturated sodium calomel electrode (SSCE).

[0097] Here we report the use of thin film overlayers of PMMA to stabilize a Ru(l l) polypyridyl complex bound to mesoporous, nanoparticle metal-oxide films over the pH range 1 -12 in water. Stabilization is accomplished by adding PMMA to metal oxide films pre-derivatized with the surface-bound dye by a dip-coating technique. The thickness of PMMA coatings was varied from 0.8 ~ 2.1 nm, as measured by transmission electron microscopy (TEM), controlled by varying the concentration of polymer in precursor stock solutions. With PMMA overlayers, up to ~100-fold enhancements in photostability have been achieved compared to the surface-bound complex without significant perturbation of optical, electrochemical, or excited state properties.

[0098] The PMMA overlayer procedure is illustrated in Figures 1 , 9, and 10. PMMA coatings were added as an overlayer after surface attachment of

[Ru(bpy)₂((4,4^"-(OH)₂PO)₂bpy)]²⁺ (RuP²⁺)- or [Ru(bpy)₂((4,4^"-(COOH)₂bpy)]²⁺

(RuC²⁺) as chloride salts to mesoporous metal-oxide films, either Ti0₂ or conducting Sn(IV)-doped ln₂03, on fluorine-doped tin oxide (FTO) electrode substrates giving either FTO|Ti0₂-RuP²⁺ or FTO|/7a/7olTO-RuP²⁺, or the corresponding RuC²⁺

electrodes. The dipping procedure was performed by brief (seconds) soaking of the derivatized electrodes in dichloromethane (DCM) solutions containing pre-dissolved PMMA oligomer. In order to establish the maximum PMMA wt.% in the dipping solutions that still allowed electrochemical activity, cyclic voltammetry was performed on FTO|/7a/7olTO-RuP²⁺(PMMA) films following dip-coating in solutions 0.5 wt.% to 4.0 wt.% PMMA in DCM. As shown in Figure 1 1 in 0.1 M HCI0 aqueous solution with up to ~ 3.0 wt.% PMMA, FTO|nanolTO-RuP²⁺(PMMA) films retain reversible electrochemistry for the RuP^3+/2+ couple with E_1/2(Ru^m/") = 1 .36 V vs NHE compared to 1 .35 V for the surface-bound complex and with a minimal attenuation of current. With 4.0 wt.% PMMA in the coating solution, FTO|/7a/7olTO-RuP²⁺ films no longer gave an observable electrochemical response for the Ru(lll/ll) couple presumably due to an inhibition to charge compensation by counter ions. Subsequent studies were conducted with < 3.0 wt.% PMMA to avoid electron transfer inhibition. [0099] A similar conclusion was reached from results obtained in a parallel set of experiments on Ti0₂ rather than on nano\JO. As shown in Figure 12, RuP²⁺ on ΤΊΟ2 can be detected electrochemically but with electron transfer inhibited. E1/2 for the surface redox couple falls within the semiconductor band gap and surface oxidation occurs by cross-surface, electron transport to the underlying FTO substrate by site-to-site electron transfer hopping.

[00100] Contact angle measurements were conducted on PMMA-coated surfaces by using a water droplet method to probe the hydrophobicity of the PMMA coating. As shown in Figures 1 , 13 and 14, hydrophobicity of PMMA-coated surfaces is markedly enhanced compared to uncoated surfaces. A linear increase in contact angle was observed with increasing PMMA content in the dip-coating solution with 12.5° (0.0 wt.%, control) « 52.6° (0.5 wt.%) < 57.1 ° (1 .0 wt.%) < 70.7° (2.0 wt.%) < 71 .6° (3.0 wt.%).

[00101] Soaking time at fixed concentrations was also investigated as a way to obtain controlled thicknesses. While a minimum soaking time of a few seconds was required, further soaking periods did not lead to significantly thicker coatings. As described above, thickness and morphology of PMMA coatings is dependent on the concentration in the loading solutions.

[00102] To provide evidence for conformal PMMA coating throughout oxide films, focused ion beam scanning electron microscopy (FIB-SEM) was used to image PMMA-coated Ti0₂-RuP²⁺ films. A down cut milling technique was used to expose the inside of the film. As seen in Figures 2-3 and in Figures 15, 17, and 20, images of 0.5 wt.% to 2.0 wt.% PMMA-coated Ti0₂-RuP²⁺ films show that the Ti0₂ films retain their porosities compared to PMMA-free samples. Further detail can be seen in Figures 16, 18, and 19. A molten PMMA layer was observed in cross-sectional images of 1 .0 wt.% (Figures 20-22) and 2.0 wt.% (Figures 2 and 23-25) PMMA because the organic PMMA overlayer melted under ion beam excitation during the milling process. For 3.0 wt.% PMMA, a decrease in porosity was observed along with formation of a thick (-0.4 μιη) PMMA layer on top of the mesoporous Ti0₂- RuP²⁺ film, Figures 3, 26, and 27.

[00103] These results suggest that overlayers produced from higher concentrations of PMMA (> 3.0 wt.%) inhibit diffusion of electrolyte solution into the films by blocking the pores of the mesoporous oxide by formation of a thick, non- conformal, blocking layer of PMMA on the outside of the film. The blocked pores inhibit counter ion diffusion and impede electrochemical oxidation or reduction.

[00104] The PMMA conformal coating thickness on individual Ti0₂

nanoparticles was confirmed by transmission electron microscopy (TEM) (Figures 28-32). Figure 4 shows a TEM image of a 2.0 wt.%, 1 .70 nm PMMA-coated film on 20-30 nm diameter Ti0₂-RuP²⁺ nanoparticles. From the TEM images and contact angle data, the PMMA coating appears to form with relative uniformity on individual Ti0₂-RuP²⁺ nanoparticles with film thicknesses increasing linearly with the wt.% of PMMA in the dipping solution, Figure 5. These results suggest that the adsorbed PMMA overlayers are conformal.

[00105] The electrochemical response of RuP²⁺ is maintained up to -2.1 nm (3.0 wt.%), a thickness at which the protective overlayer is comparable in thickness to the extended diameters of the surface-bound complexes and associated perchlorate counter ions. As shown in Figures 6, 33, and 34, PMMA-coated Ti0₂- RuP²⁺ and surface-bound nanolTO-RuP²⁺ films exhibit the characteristic broad metal-to-ligand charge transfer (MLCT) absorptions from 400 to 550 nm found for other Ru(ll) polypyridyl complexes. There are additional contributions to the spectra from absorption/scatter on Ti0₂, <400 nm, and nano\JO, <350 nm. The spectrum of nano ITO-RuP²⁺ decreases negligibly upon addition of PMMA overlayers showing that it is retained on the surface in the dip coating procedure. In contrast to atomic layer deposited Ti0₂ or AI2O3 overlayers, only slight spectral shifts are observed in the MLCT absorptions with the shifts due to a PMMA medium effect.

[00106] Stabilities of PMMA-coated Ti0₂-RuP²⁺ and nanolTO-RuP²* films in aqueous solutions as a function of pH were investigated both in the dark and under irradiation. As shown in Figures 35-43, the desorption of RuP²⁺ from Ti0₂-RuP²⁺ films, immersed in pure water (pH ~6) and in a pH 12 phosphate buffer solution (0.1 M HP0₄ ²7P0₄ ^3", 0.5 M KN0₃) in the dark were monitored spectrophotometrically over a period of 16 h at 15 min intervals. Ti0₂-RuP²⁺ is unstable toward surface hydrolysis and desorption of the complex above pH 6 in water (Figures 35 and 39). Desorption was dramatically reduced with added PMMA both in water and in the pH 12 phosphate solution. Stabilization by PMMA was most obvious in the phosphate buffer as shown in the inset of Figure 6. From these results, more than 90% of the initial RuP²⁺ remained after a 16 hr soaking period in 3.0 wt.% PMMA. Similar stabilizations were observed for nanolTO-RuP²⁺ (Figures 57-58) and Ti02-RuC²⁺ films (Figure 61 ) under the same conditions in the dark.

[00107] Photo-stabilities as function of film thickness and pH on both Ti0₂ and nano\JO were evaluated by using a previously published protocol with constant irradiation at 455 nm (fwhm -30 nm, 475 mW/cm²). RuP²⁺ loss from the films was monitored by absorbance changes at 480 nm from 0 to 16 h at intervals of 15 min with corrections made for light scattering. Absorbance changes are shown in Figures 44-56 for aqueous 0.1 M HCI0₄, pure water, and the pH 12 buffer solution. For neutral and acidic conditions, desorption kinetics were biphasic with a single average rate constant (k_des) calculated as the inverse of the weighted average lifetime (k_des = (τ)^"1). At pH 12 in 0.1 M HP0₄ ²7P0₄ ^3" with 0.5 M KN0₃, the kinetics of surface loss were complex but clear evidence for PMMA stabilization was obtained. For films coated from 0.5 wt.% PMMA, the surface coverage of RuP²⁺ on Ti0₂ decreased < -30% during the 16 hr light irradiation period in 0.1 M HCI0₄, while -60% was lost for the control sample. An exponential decrease in k_des with overlayer thickness was observed with k_des decreased by 20 fold (0.23 10^~5 s^"1) at 3.0 wt.% PMMA, Figure 7.

[00108] In a relative sense, photo-stability was enhanced as the pH was increased. In pure water, k_des decreased from k_des > 30 * 10^~5 s^~1 without stabilization to 0.31 10^~5 s^"1 with 3.0 wt.% added PMMA. PMMA overlayers on nano ITO-RuP²⁺ displayed similarly enhanced photostabilities (Figures 59-60) with similar results obtained in acidic and basic solutions.

[00109] Electronic connection between the oxide surfaces and RuP²⁺ through the surface-bound phosphonate groups appears to be maintained through the extended photolysis cycles. As shown by the emission spectra for FTO|Ti0₂-RuP²⁺ coated from 3.0 wt.% PMMA immersed in acid, at pH 7.5, and at pH 12 in Figures 65 and 66, there is no change in the weak emission from the complex even after extended photolysis periods. If the complexes were desorbed from the surfaces and trapped in the PMMA overlayer, an enhancement in emission would have been expected. Normalized luminescence spectra (Figure 63) and emission intensity (Figure 64) are shown for the sample coated from 3.0 wt. % PMMA.

[001 10] The impact of PMMA overlayers on electron injection and

recombination kinetics for FTO|Ti0₂-RuP²⁺ was investigated by nanosecond transient absorption measurements. Figure 8 illustrates back electron transfer recombination kinetics probed at 400 nm, an isosbestic point for the excited state, following 425 nm excitation. The excitation-injection sequence is illustrated in reactions 1 -3. Although there are minor differences with and without the PMMA overlayer, the kinetic behavior is similar and essentially independent of the extent of PMMA loading. Quantifying the extent of electron injection was problematic due to the change in the index of refraction from PMMA infiltration which augments the transient absorption response. Nevertheless, from the data in Figure 8 and the absence of emission, injection is efficient.

Ti0₂-RuP²⁺ Ti0₂-RuP²⁺* (1 )

Ti0₂-RuP²⁺* → Ti0₂(e^")-RuP³⁺ (2)

Ti0₂(e^")-RuP³⁺→ Ti0₂-RuP²⁺ (3)

[001 11] Back electron transfer kinetics for FTO|Ti0₂-RuP²⁺(PMMA, 3 wt.%) were monitored at 400 nm as a function of pH and solution composition (Figure 62). Analysis of the data shows that back electron transfer kinetics are independent of pH and relatively independent of the thickness of the PMMA overlayer. The absence of a pH dependence is a striking observation in light of the known pH dependence of recombination kinetics for RuP²⁺ and other surface-bound Ru chromophores on nanocrystalline Ti0₂. In earlier studies on FTO|Ti0₂-RuP²⁺, back electron transfer was shown to occur by fast and slow components with the slow component (τ >10 μβ) representing 15% of the total absorbance change at pH 1 , increasing to 30% by pH 5. Our results point to control of the effective surface pH by the PMMA overlayer with surface pH frozen at the pH used in the surface loading solution.

[001 12] In summary, we describe here a facile protocol for stabilizing Ru(ll) polypyridyl-derivatized phosphonate and carboxylate complexes on oxide surfaces in water over an extended pH range. It is based on a dip-coating procedure that results in conformal PMMA overlayers of controlled thicknesses on nanoparticle, mesoporous films of Ti0₂ or nano\JO. Electron transfer reactivity is retained on the stabilized surfaces with Ei_/2 and peak currents for the Ru(lll/l l) couples relatively unaffected by overlayer thicknesses of up to 2.1 nm with potential shifts in Ei_/2 for surface-bound Ru(lll/ll) couples of less than 0.02 V. The PMMA overlayer procedure results in stabilization of surface binding with thermal and photochemical stabilities of up to 100-fold achieved under basic conditions.

[001 13] Excitation of the surface-bound, stabilized, Ru(l l) polypyridyl dye RuP²⁺ on Ti0₂ is followed by efficient MLCT excited state injection, Ti0₂-RuP²⁺*(PMMA)→ Ti0₂(e^")-RuP³⁺(PMMA). Back electron transfer, Ti0₂(e^")-RuP³⁺(PMMA)→ Ti0₂- RuP²⁺(PMMA), is pH independent pointing to control of the effective surface pH by the PMMA overlayer dictated by the pH at the surface in the surface loading solution.

[001 14] The results described here point to PMMA dip-coating as an appealingly simple and surprisingly effective strategy for preparing surface-stabilized chromophore or chromophore-catalyst assembly structures on mesoporous nanostructured metal-oxides in both acidic and basic aqueous solutions for long-term stability in DSPEC and electrocatalytic applications.

Experimental

[001 15] Materials. [Ru(bpy)₂((4,4'-(OH)₂PO)₂bpy)]²⁺ (RuP²⁺) and

[Ru(bpy)₂(4,4'-(COOH)₂bpy)]²⁺ (RuC²⁺), where (bpy) = (2,2'-bipyridine), (4,4'- (OH)₂PO)₂bpy) = 2,2'-bipyridine-4,4'-diyldiphosphonic acid, and (4,4'-(COOH)₂bpy) = (2,2'-bipyridine-4,4-diylcarboxylic acid), were synthesized as their chloride salts by a literature procedure. Sn-doped ln₂0₃ (ITO) nanoparticles (20 wt% dispersion in ethanol) were purchased from Evonik Industries (TC8DE). Distilled water was further purified by using a Milli-Q ultrapure water purification system. Poly(methyl methacrylate) (PMMA, MW~350,000, Sigma-Aldrich chemical company),

spectrophotometric grade dichloromethane (DCM), and all other reagents were purchased and used without further purification.

[001 16] Metal-Oxide Film Preparation. Mesoporous titanium dioxide nanoparticle films (Ti0_2, -20 nm particle diameter, ~5 μιη film thickness) and reduced Sn(IV)-doped ln₂0₃ nanoparticle films (nano\JO, -10 nm particle diameter, ~3 μιη film thickness) were prepared, according a literature procedure, onto an area of 1 1 mm x 25 mm on top of fluoride-doped tin oxide (FTO)-coated glass electrode (Hartford Glass; sheet resistance 15 Ω cm^"2). Metal oxide-coated electrodes were derivatized by soaking in 150 μΜ RuP²⁺ or RuC²⁺ methanol solutions overnight followed by neat methanol soaking for an additional 12 h to remove any loosely bound RuP²⁺ or RuC²⁺ (complete surface coverage of Γ_Τιο₂ = ~ 8 ^χ 10^"8 mol cm^"2 and _nanoiTo = ~ 3 x 10^"8 mol cm^"2). In accord with previous reports, soaking electrodes in sensitizer solutions with concentrations≥ 1 50 μΜ results in complete monolayer coverage throughout the metal-oxide films.

[001 17] PMMA Coating. A PMMA coating was formed on Ti0₂ or nano\TO films functionalized with RuP²⁺ or RuC²⁺ by simply dipping these electrodes in dichloromethane (DCM) with various (0.5 wt.%, 1 .0 wt.%, 2.0 wt.%, 3.0 wt.%, and 4.0 wt.%) concentrations of PMMA oligomers. Once the electrode had soaked in the DCM/PMMA solution for a few seconds (< 10 sec), the film was air dried. This sequence is depicted in Figure 1 0.

[001 18] Contact Angle Measurement. Contact angles were measured on a KSV Instruments Cam 200 Optical Contact Mater lens for imaging purposes with Cam 200 Optical Contact Angle. A PM MA-coated FTO|Ti0₂-RuP²⁺ substrate was maintained 20 °C before a 5 μΙ_ drop of pure water was placed on the surface. The drop was allowed to equilibrate for 15 seconds before measuring the contact angle.

[001 19] Scanning Electron Microscopy (SEM) and Transmission Electron Micrograph (TEM). Focused ion beam scanning electron microscopy (FIB-SEM) analysis was performed on an FEI Helios 600 Nanolab Dual Beam System equipped with an Oxford instrument, INCA PentaFET-x3 detector by applying an accelerating voltage of 5 kV. PM MA-coated FTO|Ti0₂-RuP²⁺ samples were mounted on an aluminum sample holder using double-sided sticky carbon tape followed by sputter coating with gold-palladium (-10 nm). A down cut milling technique was applied to expose the inside of the film. TEM analysis was performed on a JEOL 201 OF FasTEM by applying an accelerating voltage of 200 kV.

[00120] Photophysical and Electrochemical Measurements. Absorption spectra were obtained by placing the dry, derivatized films perpendicular to the detection beam path of the spectrophotometer using a Agilent Cary 60 UV-vis spectrophotometer. The expression, Γ = A(A)/£(A_MLCT,₄58nm)/1000, was used to calculate surface coverage. Electrochemical measurements (Cyclic Voltammetry, CV) were conducted by using a CH Instruments 660D potentiostat with a Pt-mesh or Pt-wire counter electrode, and a Ag/AgCI (3 M NaCI; 0.197 V vs. NHE) reference electrode. Ei/₂ values were obtained from the anodic potentials at peak current values (E_{p c} and E_{p a}) in CVs. Photostability measurements were performed under air condition by a previously reported procedure. The light from a Royal Blue (455 nm, FWHM -30 nm, 475 mW/cm²) Mounted High Power LED (Thorlabs, Inc., M455L2) powered by a T-Cube LED Driver (Thorlabs, Inc., LEDD1 B) was focused to a 2.5 mm diameter spot size by a focusing beam probe (Newport Corp. 77646) outfitted with a second lens (Newport, Corp 41230). The light output was directed onto the derivatized thin film placed at 45° in a standard 10 mm path length cuvette containing 3 mL of the solution. The illumination spot was adjusted to coincide both with the thin film and the perpendicular beam path of a Varian Cary 50 UV/Vis spectrophotometer. The absorption spectrum (350 - 700 nm) of the film was taken every 15 minutes over 16 hours of illumination. The incident light intensity was measured with a thermopile detector (Newport Corp 1918-C meter and 818P-020-12 detector). The solution temperature, 22 ± 2°C, was consistent throughout the duration of the experiment. The absorption-time traces at 480 nm for both pH 1 (0.1 M HCI0₄) and pH -6.2 (pure H₂0) could be satisfactorily fit with the biexponential function (Eq. 1 ). Similar data collected for pH 12 (0.1 M HP0₄ ²7P0₄ ^3", 0.5 M KN0₃) was not biphasic with Eq. 1 inadequate to fit this data. For comparison, the results of each multi-exponential analysis was represented by a single rate constant by calculating the weighted average lifetime (<τ>) using Eq. 2.

y = A-,e-^(1/Tl)x + Α₂β ^(1/τ2)χ + yo (Eq. 1 )

1/kdes = <τ> at 480nm; < τ > =∑Ατ² / ΣΑ,τ (Eq. 2)

[00121] Transient Absorption. These measurements were conducted by using an experimental apparatus previously described. Pulsed laser excitation was at 425 nm, 3.2 mJ, 0.5 cm beam diameter. Kinetic transients were the result of 50 averages. A 380 nm long pass filter was placed between the probe source and the sample to prevent direct excitation of Ti0₂ but also of PMMA.

[00122] Photoluminescence. Steady-state emission spectra were collected by using a commercial instrument as previously described.

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Embodiments

[00123] Embodiment 1 . A method of stabilizing a molecule on a metal oxide surface, comprising:

forming an overlayer of at least one polymer on the metal oxide surface comprising the molecule.

[00124] Embodiment 2. The method of embodiment 1 , wherein the metal oxide surface comprises nanoparticles of the metal oxide.

[00125] Embodiment 3. The method of any one of embodiments 1 -2, wherein the metal oxide surface comprises titanium dioxide, tin-doped indium oxide, or a combination thereof.

[00126] Embodiment 4. The method of any one of embodiments 1 -3, wherein the metal oxide surface comprises tin-doped indium oxide (ITO), fluorine-doped tin oxide (FTO), antimony tin oxide (ATO), gallium zinc oxide (GZO), indium zinc oxide (IZO), copper aluminum oxide, fluorine-doped zinc oxide, and aluminum zinc oxide, or a combination of two or more thereof.

[00127] Embodiment 5. The method of any one of embodiments 1 -4, wherein the metal oxide surface comprises Sn0₂,Ti02, Nb₂05, SrTi03, Zn₂Sn0₄, Zr0₂, NiO, Ta-doped Ti0₂, Nb-doped Ti0₂, or a combination of two or more thereof.

[00128] Embodiment 6. The method of any one of embodiments 1 -5, wherein the molecule is a chromophore, a catalyst, a chromophore-catalyst assembly, or a combination of two or more thereof.

[00129] Embodiment 7. The method of any one of embodiments 1 -6, wherein the molecule comprises a chromophore chosen from ruthenium coordination complexes, osmium coordination complexes, copper coordination complexes, porphyrins, phythalocyanines, and organic dyes, and combinations thereof.

[00130] Embodiment 8. The method of any one of embodiments 1 -7, wherein the molecule comprises a catalyst chosen from iron catalysts, ruthenium catalysts, osmium catalysts, and combinations thereof.

[00131] Embodiment 9. The method of any one of embodiments 1 -8, wherein the molecule comprises a catalyst chosen from complexes having the structure of formula (I):

L₁ M 1_2 (I)

wherein M is chosen from Ru, Ir, Fe, Co, Ni, and Os,

Li is a bidentate ligand,

L₂ is a tridentate ligand; and

L₃ is a monodentate ligand.

[00132] Embodiment 10. The method of embodiment 9, wherein l_i is chosen from bipyridine, phenanthroline, 2-phenylpyridine bipyrimidine, bipyrazyl, glycinate, acetylacetonate, and ethylenediamine.

[00133] Embodiment 1 1 . The method of any one of embodiments 9-10, wherein L₂ is chosen from terpyridine, DMAP, and 2,6-bis(1 -methylbenzimidazol-2- yl)pyridine (Mebimpy).

[00134] Embodiment 12. The method of any one of embodiments 9-1 1 , wherein L₃ is H₂0 (aqua), NH₃ (ammine), CH₃NH₂ (methylamine), CO (carbonyl), NO (nitrosyl), F^" (fluoro), CN^" (cyano), CI^" (chloro), Br^" (bromo), I^" (iodo), N0₂ ^" (nitro), and OH^" (hydroxyl).

[00135] Embodiment 13. The method of any one of embodiments 1 -8, wherein the molecule comprises a catalyst chosen from complexes having the structure of formula (I): L₃

L₁ M L₂ (I)

wherein M is chosen from Ru, Ir, Fe, Co, Ni, and Os,

Li and L3 are alike or different monodentate ligands, and

L₂ is a tetradentate ligand.

[00136] Embodiment 14. The method of embodiment 13, wherein

L-i and L₃ are independently chosen from H₂0 (aqua), NH₃ (ammine), CH₃NH₂

(methylamine), CO (carbonyl), NO (nitrosyl), F^" (fluoro), CN^" (cyano), CI^" (chloro), Br^"

(bromo), I^" (iodo), N0₂ ^" (nitro), OH^" (hydroxyl), and combinations thereof;

and L₂ is porphyrin.

[00137] Embodiment 15. The method of any one of embodiments 14, wherein the molecule comprises at least one transition metal atom.

[00138] Embodiment 16. The method of any one of embodiments 1 -15, wherein the molecule comprises a Ru(ll) polypyridyl derivatized phosphonate complex, a Ru(ll) polypyridyl derivatized carboxylate complex, or a combination thereof.

[00139] Embodiment 17. The method of any one of embodiments 1 -16, wherein the molecule comprises [Ru(bpy)₂((4,4'-(OH)₂PO)₂bpy)]²⁺, [Ru(bpy)₂(4,4 - (COOH)₂bpy)]²⁺, or a combination thereof.

[00140] Embodiment 18. The method of any one of embodiments 1 -17, wherein the polymer comprises poly(methyl methacrylate), poly(tetrafluoroethylene), and combinations thereof.

[00141] Embodiment 19. The method of any one of embodiments 1 -18, wherein the forming an overlayer comprises contacting the metal oxide surface comprising the molecule with a composition comprising polymer monomers, oligomers, or a combination thereof under conditions sufficient to form the overlayer. [00142] Embodiment 20. An electrode, comprising:

a conductive substrate;

a metal oxide surface in electrical communication with the conductive substrate; at least one molecule in electrical communication and anchored to the metal oxide surface; and

an overlayer of at least one polymer on the metal oxide surface comprising the at least one molecule.

[00143] Embodiment 21 . The electrode of embodiment 20, wherein the conductive substrate is transparent.

[00144] Embodiment 22. The electrode of any one of embodiments 20-21 , wherein the conductive substrate comprises fluorine-doped tin oxide, tin-doped indium oxide, or a combination thereof.

[00145] Embodiment 23. The electrode of any one of embodiments 20-22, wherein the metal oxide surface comprises titanium dioxide, tin-doped indium oxide, or a combination thereof.

[00146] Embodiment 24. The electrode of any one of embodiments 20-23, wherein the metal oxide surface comprises tin-doped indium oxide (ITO), fluorine- doped tin oxide (FTO), antimony tin oxide (ATO), gallium zinc oxide (GZO), indium zinc oxide (IZO), copper aluminum oxide, fluorine-doped zinc oxide, and aluminum zinc oxide, or a combination of two or more thereof.

[00147] Embodiment 25. The electrode of any one of embodiments 20-24, wherein the metal oxide surface comprises Sn0₂,Ti0₂, Nb₂0₅, SrTi0₃, Zn₂Sn0 , Zr0₂, NiO, Ta-doped Ti0₂, Nb-doped Ti0₂, or a combination of two or more thereof.

[00148] Embodiment 26. The electrode of any one of embodiments 20-25, wherein the at least one molecule comprises a chromophore, a catalyst, a chromophore-catalyst assembly, or a combination of two or more thereof. [00149] Embodiment 27. The electrode of any one of embodiments 20-26, wherein the at least one molecule comprises a chromophore chosen from ruthenium coordination complexes, osmium coordination complexes, copper coordination complexes, porphyrins, phythalocyanines, and organic dyes, and combinations thereof.

[00150] Embodiment 28. The electrode of any one of embodiments 20-27, wherein the at least one molecule comprises a catalyst chosen from iron catalysts, ruthenium catalysts, osmium catalysts, and combinations thereof.

[00151] Embodiment 29. The electrode of any one of embodiments 20-28, wherein the molecule comprises a catalyst chosen from complexes having the structure of formula (I):

L₁ M 1_2 (I)

wherein M is chosen from Ru, Ir, Fe, Co, Ni, and Os,

Li is a bidentate ligand,

L₂ is a tridentate ligand; and

L₃ is a monodentate ligand.

[00152] Embodiment 30. The electrode of embodiment 20-29, wherein U is chosen from bipyridine, phenanthroline, 2-phenylpyridine bipyrimidine, bipyrazyl, glycinate, acetylacetonate, and ethylenediamine.

[00153] Embodiment 31 . The electrode of any one of embodiments 29-30, wherein L₂ is chosen from terpyridine, DMAP, and 2,6-bis(1 -methylbenzimidazol-2- yl)pyridine (Mebimpy).

[00154] Embodiment 32. The electrode of any one of embodiments 29-31 , wherein L₃ is H₂0 (aqua), NH₃ (ammine), CH₃NH₂ (methylamine), CO (carbonyl), NO (nitrosyl), F^" (fluoro), CN^" (cyano), CI^" (chloro), Br^" (bromo), I^" (iodo), N0₂ ^" (nitro), and OH^" (hydroxyl). [00155] Embodiment 33. The electrode of any one of embodiments 20-28, wherein the molecule comprises a catalyst chosen from complexes having the structure of formula (I):

L₁ M 1_2 (I)

wherein M is chosen from Ru, Ir, Fe, Co, Ni, and Os,

Li and L₃ are alike or different monodentate ligands, and

L₂ is a tetradentate ligand.

[00156] Embodiment 34. The electrode of embodiment 33, wherein

L-i and L₃ are independently chosen from H₂0 (aqua), NH₃ (ammine), CH₃NH₂

(methylamine), CO (carbonyl), NO (nitrosyl), F^" (fluoro), CN^" (cyano), CI^" (chloro), Br^'

(bromo), I^" (iodo), N0₂ ^" (nitro), OH^" (hydroxyl), and combinations thereof;

and L₂ is porphyrin.

[00157] Embodiment 35. The electrode of any one of embodiments 20-34, wherein the at least one molecule comprises at least one transition metal atom.

[00158] Embodiment 36. The electrode of any one of embodiments 20-35, wherein the at least one molecule comprises a Ru(ll) polypyridyl derivatized phosphonate complex, a Ru(ll) polypyridyl derivatized carboxylate complex, or a combination thereof.

[00159] Embodiment 37. The electrode of any one of embodiments 20-36, wherein the at least one molecule comprises [Ru(bpy)₂((4,4'-(OH)₂PO)₂bpy)]²⁺, [Ru(bpy)₂(4,4'-(COOH)₂bpy)]²⁺, or a combination thereof.

[00160] Embodiment 38. The electrode of any one of embodiments 20-37, wherein the at least one polymer comprises poly(methyl methacrylate),

poly(tetrafluoroethylene), and combinations thereof.

[00161] Embodiment 39. A method of harvesting light to perform useful chemistry, comprising: providing an electrode of any one of embodiments 20-38;

engaging the electrode in an electrochemical cell;

illuminating the electrode with light under conditions suitable to cause one or more chemical reactions, wherein one or more products of the one or more chemical reactions are useful;

thereby harvesting light to perform useful chemistry.

[00162] Embodiment 40. The method of embodiment 39, wherein the electrode exhibits enhanced long-term performance relative to a substantially-similar electrode that does not have the overlayer of at least one polymer on the metal oxide surface comprising the at least one molecule.

[00163] As previously stated, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various forms. It will be appreciated that many modifications and other variations stand within the intended scope of this invention as claimed below. Furthermore, the foregoing description of various embodiments does not necessarily imply exclusion. For example, "some" embodiments may include all or part of "other" and "further" embodiments within the scope of this invention. In addition, "a" does not mean "one and only one;" "a" can mean "one and more than one."

Claims

We Claim:

1 . A method of stabilizing a molecule on a metal oxide surface, comprising:

forming an overlayer of at least one polymer on the metal oxide surface comprising the molecule.

2. The method of claim 1 , wherein the metal oxide surface comprises nanoparticles of the metal oxide.

3. The method of any one of claims 1 -2, wherein the metal oxide surface comprises titanium dioxide, tin-doped indium oxide, or a combination thereof.

4. The method of any one of claims 1 -3, wherein the molecule is a chromophore, a catalyst, a chromophore-catalyst assembly, or a combination of two or more thereof.

5. The method of any one of claims 1 -4, wherein the molecule comprises a chromophore chosen from ruthenium coordination complexes, osmium coordination complexes, copper coordination complexes, porphyrins, phythalocyanines, and organic dyes, and combinations thereof.

6. The method of any one of claims 1 -5, wherein the molecule comprises a catalyst chosen from iron catalysts, ruthenium catalysts, osmium catalysts, and combinations thereof.

7. The method of any one of claims 1 -6, wherein the molecule comprises at least one transition metal atom.

8. The method of any one of claims 1 -7, wherein the molecule comprises a Ru(ll) polypyridyl derivatized phosphonate complex, a Ru(ll) polypyridyl derivatized carboxylate complex, or a combination thereof.

9. The method of any one of claims 1 -8, wherein the molecule comprises

[Ru(bpy)₂((4,4'-(OH)₂PO)₂bpy)]²⁺, [Ru(bpy)₂(4,4'-(COOH)₂bpy)]²⁺, or a combination thereof.

10. The method of any one of claims 1 -9, wherein the polymer comprises poly(methyl methacrylate), poly(tetrafluoroethylene), or a combination thereof.

1 1 . The method of any one of claims 1 -10, wherein the forming an overlayer comprises contacting the metal oxide surface comprising the molecule with a composition comprising polymer monomers, oligomers, or a combination thereof under conditions sufficient to form the overlayer.

12. An electrode, comprising:

a conductive substrate;

a metal oxide surface in electrical communication with the conductive substrate; at least one molecule in electrical communication and anchored to the metal oxide surface; and

an overlayer of at least one polymer on the metal oxide surface comprising the at least one molecule.

13. The electrode of claim 12, wherein the conductive substrate is transparent.

14. The electrode of any one of claims 12-13, wherein the conductive substrate comprises fluorine-doped tin oxide.

15. The electrode of any one of claims 12-14, wherein the metal oxide surface comprises titanium dioxide, tin-doped indium oxide, or a combination thereof.

16. The electrode of any one of claims 12-15, wherein the at least one molecule comprises a chromophore, a catalyst, a chromophore-catalyst assembly, or a combination of two or more thereof.

17. The electrode of any one of claims 12-16, wherein the at least one molecule comprises a chromophore chosen from ruthenium coordination complexes, osmium coordination complexes, copper coordination complexes, porphyrins,

phythalocyanines, and organic dyes, and combinations thereof.

18. The electrode of any one of claims 12-17, wherein the at least one molecule comprises a catalyst chosen from iron catalysts, ruthenium catalysts, osmium catalysts, and combinations thereof.

19. The electrode of any one of claims 12-18, wherein the at least one molecule comprises at least one transition metal atom.

20. The electrode of any one of claims 12-19, wherein the at least one molecule comprises a Ru(ll) polypyridyl derivatized phosphonate complex, a Ru(ll) polypyridyl derivatized carboxylate complex, or a combination thereof.

21 . The electrode of any one of claims 12-20, wherein the at least one molecule comprises [Ru(bpy)₂((4,4'-(OH)₂PO)2bpy)]²⁺, [Ru(bpy)₂(4,4'-(COOH)₂bpy)]²⁺, or a combination thereof.

22. The electrode of any one of claims 12-21 , wherein the at least one polymer comprises poly(methyl methacrylate), poly(tetrafluoroethylene), or a combination thereof.

23. A method of harvesting light to perform useful chemistry, comprising:

providing an electrode of any one of claims 12-22;

engaging the electrode in an electrochemical cell;

illuminating the electrode with light under conditions suitable to cause one or more chemical reactions, wherein one or more products of the one or more chemical reactions are useful;

thereby harvesting light to perform useful chemistry.

24. The method of claim 23, wherein the electrode exhibits enhanced long-term performance relative to a substantially-similar electrode that does not have the overlayer of at least one polymer on the metal oxide surface comprising the at least one molecule.