Sensitization of Nanocrystalline Metal Oxides with a Phosphonate

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Sensitization of Nanocrystalline Metal Oxides with a Phosphonate-Functionalized Perylene Diimide for Photoelectrochemical Water Oxidation with a CoO# Catalyst Joel Thomas Kirner, and Richard G. Finke ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05874 • Publication Date (Web): 20 Jul 2017 Downloaded from http://pubs.acs.org on July 26, 2017

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ACS Applied Materials & Interfaces

Sensitization of Nanocrystalline Metal Oxides with a Phosphonate-Functionalized Perylene Diimide for Photoelectrochemical Water Oxidation with a CoOx Catalyst Joel T. Kirner, Richard G. Finke*

Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States

KEYWORDS: dye-sensitized photoelectrochemical cell, perylene diimide, cobalt oxide, water oxidation catalysis, alumina overlayer, tin oxide

ABSTRACT

A

planar

organic

thin-film

composed

bis(phosphonomethyl)-3,4,9,10-perylenediimide,

of

a

perylene

PMPDI)

with

diimide

dye

(N,N´-

photoelectrochemically

deposited cobalt oxide (CoOx) catalyst was previously shown to photoelectrochemically oxidize water (DOI: 10.1021/am405598w). Herein, the same PMPDI dye is studied for the sensitization of

different

nanostructured

metal

oxide

(nano-MOₓ)

films

in

a

dye-sensitized

photoelectrochemical cell architecture. Dye adsorption kinetics and saturation decreases in the 1 ACS Paragon Plus Environment


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order TiO₂ > SnO₂ >> WO₃. Despite highest initial dye loading on TiO₂ films, photocurrent with hydroquinone (H₂Q) sacrificial reductant in pH 7 aqueous solution is much higher on SnO₂ films, likely due to a higher driving force for charge injection into the more positive conduction band energy of SnO₂. Dying conditions and SnO₂ film thickness were subsequently optimized to achieve light-harvesting efficiency >99% at the λmax of the dye, and absorbed photon-to-current efficiency of 13% with H₂Q, a 2-fold improvement over the previous thin-film architecture. A CoOx

water-oxidation

catalyst

was

photoelectrochemically

deposited,

allowing

for

photoelectrochemical water oxidation with a faradaic efficiency of 31 ± 7%, thus demonstrating the second example of a water-oxidizing, dye-sensitized photoelectrolysis cell composed entirely of earth-abundant materials. However, deposition of CoOx always results in lower photocurrent due to enhanced recombination between catalyst and photo-injected electrons in SnO₂, as confirmed by open-circuit photovoltage measurements. Possible future studies to enhance photoanode performance are discussed, including alternative catalyst deposition strategies or structural derivatization of the perylene dye.

INTRODUCTION

Global energy needs are growing due to an increasing population and a more energyintensive economy.1

In order to meet these energy demands while also minimizing CO2

emissions, the use of fossil fuels as energy feedstocks must be decreased relative to carbonneutral (and ideally, renewable) fuel sources.1 Of the available renewable energy feedstocks, energy from sunlight is by far the most abundant, yet greatly under-utilized.1 However, solar energy is diffuse, making efficient collection and conversion a challenge. Additionally, the intermittency and diurnal nature of sunlight means that the collected energy must be stored for 2 ACS Paragon Plus Environment

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future use, either in the form of heat, electrical energy, or chemical energy in the form of solar fuels. For this reason, photoelectrochemical water splitting, the light-driven conversion of abundant and renewable liquid water into hydrogen and oxygen gases, has been identified as one of the “Holy Grails of Chemistry”.2,3 A water-splitting photoelectrolysis cell (WS-PEC, note the list of abbreviations at the end of the text) requires a semiconductor for light absorption and photogenerated charge transport, and catalyst(s) for the efficient oxidation of water and reduction of protons. For practical application, a WS-PEC should be efficient, long-lived, and produce H₂ gas at a price competitive with steam reforming from natural gas.3 Hence, many researchers have employed earthabundant, organic materials as the light-collecting components of WS-PECs, as detailed in our recent review article.4 Such devices include both organic thin-film and dye-sensitized photoelectrolysis cell architectures,4 (OTF-PECs and DS-PECs, respectively) and have employed materials including aromatic heterocycle dyes such as phthalocyanines, porphyrins and subporphyrins, perylenes, all-organic Donor–Acceptor type dyes, as well as organic polymers and fullerenes.4 Previously, our group developed a water-oxidation photoanode based on an organic thinfilm composed of a novel perylene diimide (PDI) dye on planar ITO, with co-deposited CoOx water oxidation catalyst (WOCatalyst).5 The dye, N,N´-bis(phosphonomethyl)-3,4,9,10perylenediimide (PMPDI, Fig. 1), incorporates phosphonate groups in an attempt to help couple the organic dye to the inorganic CoOx catalyst. We previously found that the device efficiency was limited by low light-harvesting efficiency, LHE (only 12% of incident light was absorbed by the thin film at the dye’s λmax) and significant charge carrier recombination (only 6% charge transport efficiency).5 Given the planar architecture of the previous system, the LHE could not be

3 ACS Paragon Plus Environment


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improved by simply increasing the thickness of the PMPDI film,5 due to the intrinsically low exciton diffusion length of organic materials.6 We hypothesized that both the LHE and charge transport efficiency might be improved through the use of a nanostructured device architecture such a DS-PEC. There have been many examples of water-oxidizing DS-PEC systems in the literature, which have been summarized in several review articles.4,7–14

Figure 1. Structure of N,N´-bis(phosphonomethyl)-3,4,9,10-perylenediimide (PMPDI).

Perylene dyes have been studied as light-harvesting materials in many systems aimed towards light-driven water splitting, including homogeneous (dissolved or suspended) photocatalytic systems,15–23 and heterogeneous OTF-PEC4 and DS-PEC systems.4,24–29 Of the handful of perylene-based DS-PECs,24–29 successful water oxidation had remained elusive until two recently published systems.26,29 Specifically, a 2015 study by Ronconi et al.26 reported a water-oxidizing DS-PEC composed of a nanostructured WO₃ film sensitized with a cationic PDI dye and co-loaded with IrO₂ nanoparticle WOCatalyst. In pH 3 buffer and ~1 sun illumination, WO₃/PDI/IrO₂ photocurrent reached ~70 µA/cm² (approximately a 4-fold enhancement relative to WO₃/PDI without catalyst, though water oxidation was not confirmed by the direct detection of O₂ product).26 A second system by Kamire et al.29 is composed of nanostructured TiO₂ sensitized with a perylene monoimide (PMI) dye. The TiO₂/PMI film was subsequently treated by atomic layer deposition (ALD) to partially encapsulate the dye with an electronically insulating Al₂O₃ overlayer. Molecular WOCatalyst (either a dinuclear Ir2 catalyst or a mononuclear Ir catalyst


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with a phenyl spacer and silatrane anchor group) was subsequently loaded atop the Al₂O₃ layer.29 Kamire et al. found that the Al₂O₃ overlayer served to enhance the stability of the PMI dye on the surface, and also served to decrease the rate of recombination between photo-injected electrons in the TiO₂ conduction band and oxidized catalyst at the surface or oxidizable species in solution.29 Steady-state photocurrent for TiO₂/PMI/Al₂O₃/Ir anodes in aqueous electrolyte with 1-sun illumination reached as high as ~15 µA/cm², though faradaic efficiencies for O₂ production were low, ~20%.29 Herein we report an exploratory study on the dye-sensitization of several nanoparticle metal oxide (MOx) films with our PMPDI dye.5 While nano-TiO₂ is the most common substrate for DS-PEC studies,4,7–13 perylene derivatives often do not have sufficient excited-state energies to photo-inject an electron into the TiO₂ conduction band.25,28 Furthermore, transient absorption spectroscopy studies of perylene dyes on various MOx films have found different rates for electron injection and recombination depending on the identity of the MOₓ,26,28 including TiO₂, SnO₂, and WO₃. Therefore, we chose to broaden our initial studies to include these three MOₓ materials. We report on the preparation and characterization of TiO₂, SnO₂, and WO₃ nanostructured films. We then study the sensitization of these films with PMPDI dye and test photocurrent in the presence of sacrificial reductant. Overall, we find that: (i) the SnO₂ substrate shows the most promising results with our PMPDI dye; (ii) optimization of dying conditions and SnO₂ film thickness allowed for >99% light-harvesting efficiency at the λmax of PMPDI; (iii) the absorbed photon-to-current efficiency (APCE) measured for this DS-PEC architecture with sacrificial reductant is superior to our previous OTF-PEC architecture5; and (iv) photoelectrochemical deposition of CoOx as WOCatalyst results in decreased photocurrent,



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contrary to results for our previous architecture, and for reasons which will be described. This last finding illustrates the highly-recognized4,7–13,30 challenge of coupling catalysts with lightabsorbing units in such a way as to prevent recombination back reactions, so that “methods ensuring a photon-driven, unidirectional flow of charge through catalytic cycles need to be developed”.30 Hence, we describe several pathways currently under study to further improve the photoelectrochemical water-oxidation efficiency of the current and analogous devices.

EXPERIMENTAL SECTION Materials. The following starting materials and solvents were used as received: Perylene-3,4,9,10-tetracarboxylic

dianhydride

(PTCDA)

(97%,

Aldrich);

(aminomethyl)phosphonic acid (99%, Alfa Aesar); imidazole (99%, Acros Organics); formamide (certified ACS grade, Fisher); Co(NO₃)₂·6H₂O (Fluka, >99%); hydroquinone (Aldrich, >99%). Buffer solutions were prepared from water (Barnstead NANOpure ultrapure water system, 18 MΩ), KH₂PO₄ (Fisher, Certified ACS grade, 99.3%, 0.0005% Fe); KOH (Fisher, Certified ACS grade, 88.5%, 11.5% water, 0.00028% Fe, 0.0008% Ni). Synthesis and characterization details for the PMPDI dye are available in our previous publication.5 The fully-protonated form of PMPDI was used for all experiments, prepared as previously described.5 The identity and purity of the dye were confirmed by NMR, HPLC, and elemental analysis.5 Preparation

and

Characterization

of

MOₓ

Sintered

Nanoparticle

Films.

Experimental details for the preparation and characterization of nano-TiO₂, SnO₂, and WO₃ films are provided in Sections S2 and S3 of the Supporting Information. Briefly, nano-MOₓ pastes were either purchased or prepared in-house, doctor bladed onto fluorene-doped tin oxide 6 ACS Paragon Plus Environment

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coated glass (FTO) between Scotch tape spacers (1–4 layers, where stated, to vary the resulting film thickness), and the resulting paste films were calcined to form the sintered, mesoporous films. Where stated, alumina overlayers were also deposited on MOₓ films by a solution-based method, prior to dye loading (see Sec. S7c in the Supporting Information for experimental and characterization details). Dying of MOₓ Films. Preliminary dying experiments were done by placing MOₓ films (approximately 1.2×1.2 cm films on 1.2×2.5 cm FTO) in a covered petri dish filled with enough dye solution to cover the entire film surface with several millimeters of solution. Dye solutions usually consisted of 3 mg of PMPDI (fully protonated form) in 10 mL of formamide (sonicated to dissolve, saturated, 99% at the λmax of 490 nm. For comparison, the kinetics of dye adsorption on SnO₂ were much slower, taking an experimentally inconvenient ~3 weeks to reach a significantly lower saturation level. At saturation, the absorptance of PMPDI on 1-Scotch SnO₂ reached only 76% at the λmax of 490 nm.

Figure 3. Dye adsorption kinetics on various MOₓ films on FTO. (a) UV–vis spectra of dying TiO₂ (blue) and SnO₂ (orange) films over time (light to dark), normalized by average film thickness according to Table 1. Films were dyed from 0.5 mM PMPDI in formamide. Spectra correspond to absorption by PMPDI only, as spectra of bare MOₓ films have been subtracted out. The inset provides photographs of MOₓ films after 3 weeks of dying. Note that WO₃ had no appreciable dye absorbance. (b) Dye absorbance at 460 nm, normalized by average film thickness on TiO₂ (●) and SnO₂ (■) over time. This wavelength was chosen because it is relatively insensitive to dye aggregation.38,39 Dashed lines are provided to guide the eye. Similar behavior of significantly lower dye loading (per active surface area) on nanoSnO₂ than TiO₂ was previously reported by Kay and Grätzel,41 using the common Ru-based dye, N719. They rationalized this behavior based on the different isoelectric points (IEPs) of the MOₓ (that is, the pH at which the net surface charge of the oxide is zero). The IEP of TiO₂ (anatase) is generally 6–7, whereas that of SnO₂ (cassiterite) is 4–5.42–44 Hence, the SnO₂ surface is more acidic than TiO₂, and therefore the interaction between the SnO₂ surface and acidic dye anchor groups such as carboxylates or phosphonates is expected to be weaker than for TiO₂. An



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alternative/complementary hypothesis for the lower dye loading on SnO₂ is a lower internal surface area45 relative to TiO₂, as suggested by the higher degree of particle aggregation observed by SEM for SnO₂ films relative to TiO₂ films (Fig. S4). Consistently, WO₃ has the lowest IEP42 of WO₃, consistent with the decreasing isoelectric points (increasing acidity of the MOₓ surfaces) in that order. In fact, the acidic phosphonate anchor groups of PMPDI were not able to bind to the highly acidic WO₃ surface at all. While loading of PMPDI onto TiO₂ proved more facile than onto SnO₂, photocurrent measured in the presence of hydroquinone sacrificial reductant was much lower on TiO₂ than on SnO₂, consistent with a poor thermodynamic driving force for electron injection from photoexcited PMPDI into the more negative conduction band of TiO₂. Hence, SnO₂ was chosen for optimization studies. SnO₂ film thickness and dying conditions were optimized to achieve >99% light-harvesting efficiency at the λmax of PMPDI on ~6 µm-thick films. Photocurrent of ~1,100 µA/cm² is typical for optimized SnO₂/PMPDI films in pH 7 buffer in the presence of hydroquinone. Optimized SnO₂/PMPDI photoanodes in pH 7 buffer still exhibited photocurrent that decayed from ~40 to 20 µA/cm² over 5 min, even in the absence of hydroquinone sacrificial reductant and before depositing a water-oxidation catalyst. However, O₂ detection experiments by a Generator–Collector method revealed that such currents do not originate from water oxidation. An approximation of the dye concentration on SnO₂ indicates that a significant portion of photolysis current cannot correspond to degradative oxidation of PMPDI, which degraded by only ~12% during 10 min of photolysis. Therefore, oxidation of trace redox impurities such as Cl− remains a hypothesis behind such photocurrents. Loading CoOx onto SnO₂/PMPDI anodes by a photoelectrochemical deposition method allowed for water oxidation with a faradaic efficiency of 31 ± 7%, therefore demonstrating only


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the second example63 of photoelectrochemical water oxidation by a DS-PEC system composed entirely of earth-abundant elements. Catalyst deposition also appears to have kinetically enhanced the stability of PMPDI during photolysis, as indicated by UV–vis experiments before and after. However, anodes with CoOx always exhibited decreased photocurrent relative to those without CoOx. Controls and open-circuit photovoltage measurements indicated that this is due to enhanced recombination in the presence of CoOx, due to catalyst adsorbed directly to the SnO₂ surface intercepting conduction band electrons, thus emphasizing the recognized4,7–13,30 challenge of coupling catalysts with light-absorbing units in such a way as to prevent recombination back reactions. As such, “methods ensuring a photon-driven, unidirectional flow of charge through catalytic cycles need to be developed”,30 and are currently under further study. The present studies have yielded several readily testable hypotheses for the rational preparation of an improved device, which are currently under way and will be reported in due course.

ASSOCIATED CONTENT Supporting Information. Discussion on the selection of MOₓ materials; Experimental details for the preparation of nano-MOₓ films; Characterization details and data for nano-MOₓ films; Estimation of PMPDI energy states; Illumination setup for photoelectrochemical experiments; Photoelectrochemical experiments and controls for TiO₂ and SnO₂ anodes; SnO₂/PMPDI performance optimization experiments; Oxygen detection by Generator–Collector method; IPCE measurements for SnO₂/PMPDI. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION



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Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written primarily by the first author. All authors have given approval to the final version of the manuscript. Notes This manuscript was submitted by JTK to the Academic Faculty of Colorado State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. The authors declare no competing financial interests. ACKNOWLEDGMENTS This work was supported primarily by NSF Grant CHE-1057723 (to R.G.F.), and in early stages by DOE Grant DE-FG02-04ER15591 (to C. Michael Elliott). LIST OF ABBREVIATIONS ALD, atomic layer deposition; DS-PEC, dye-sensitized photoelectrolysis cell; DSSC, dyesensitized solar cell; ECB, conduction band edge energy; EF,n, quasi fermi energy of electrons; FTO, fluorine-doped tin oxide-coated glass; HOMO, highest occupied molecular orbital; H₂Q, hydroquinone; IEP, isoelectric point; IPCE, incident photon-to-current efficiency; KPi, potassium phosphate; :LHE, light-harvesting efficiency; LUMO, lowest unoccupied molecular orbital; MOₓ, metal oxide; OTF, organic thin-film; PDI, perylene diimide; PEC, photoelectrolysis cell; PMI, perylene monoimide; PMPDI, N,N´-bis(phosphonomethyl)-3,4,9,10perylenediimide; PTCDA, perylene-3,4,9,10-tetracarboxylic dianhydride; Voc, open-circuit


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voltage; WOCatalyst, water-oxidation catalyst; WS-PEC, water-splitting photoelectrolysis cell; XRD, X-ray diffraction.



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Sensitization of Nanocrystalline Metal Oxides with a Phosphonate

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