Carbon Nanotube-Based Membrane for Light-Driven, Simultaneous

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Carbon Nanotube Based Membrane for LightDriven, Simultaneous Proton and Electron Transport Gregory A Pilgrim, Amanda R Amori, Zhentao Hou, Fen Qiu, Sanela Lampa-Pastirk, and Todd D. Krauss ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00578 • Publication Date (Web): 07 Dec 2016 Downloaded from http://pubs.acs.org on December 10, 2016

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Carbon Nanotube Based Membrane for LightDriven, Simultaneous Proton and Electron Transport Gregory A. Pilgrim†, Amanda R. Amori†, Zhentao Hou†, Fen Qiu†, Sanela Lampa-Pastirkǂ, and Todd D. Krauss†‡* †University of Rochester Department of Chemistry and ‡Institute of Optics ǂNazareth College Department of Chemistry and Biochemistry

Corresponding Author *Todd D. Krauss, [email protected]

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Abstract: Here we discuss the photon driven transport of protons and electrons over hundreds of microns through a membrane based on vertically aligned single walled carbon nanotubes (SWNTs). Electrons are photogenerated in colloidal CdSe quantum dots that have been noncovalently attached to the carbon nanotube membrane and can be delivered at potentials capable of reducing earth-abundant molecular catalysts that perform proton reduction. Proton transport is driven by the electron photocurrent and is shown to be faster through the SWNT based membrane than through the commercial polymer Nafion. The potential utility of SWNT membranes for solar water splitting applications is demonstrated through their excellent proton and electron transport properties as well as their ability to interact with other components of water splitting systems, such as small molecule electron acceptors.

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Photochemical water splitting systems use sunlight to split water into its constituents, oxygen and hydrogen ions; the latter of which will be reduced to molecular hydrogen (H2), serving as a source of renewable energy.1 As such, water splitting systems are conceived of as having two


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regions, one devoted to photoinduced oxidation of water, and the other to photochemical reduction of protons to H2.1 In one concept of a potential water splitting system, the two regions would be separated by a membrane capable of transporting protons with electrons traveling in an external circuit.2 While it is possible to develop a water splitting system that does not require a membrane, strong pH gradients forming around the electrodes and the inherent safety issue of the coevolution of H2 and O2 limit their potential applicability.3 Ideally, a membrane in a water splitting system would use a single material to transport both electrons and protons. A single material has advantages with respect to simplicity and cost, and removes limitations on the transport of one charged species by devoting separate areas of the system to each. To date, most membranes for water splitting systems use two materials4-7: one, typically based on sulfonated polymer systems,5 such as Nafion,6,7 for proton transport, and a second, such as carbon nanotubes6 or silicon microwires7 for electron transport. Examples exist of membranes that can transport protons and electrons under chemical environments common for water splitting4-8 but none have been demonstrated to do so simultaneously while interacting with other critical components of heterogeneous water splitting photocatalytic systems, such as photosensitizers and hydrogen reduction catalysts. SWNTs have several material properties that make them potentially attractive as components for artificial photosynthesis membranes. Carbon nanotubes (NTs) are electrically conductive9 and, following from their geometry, have a hollow bore through which ions can move in a directed manner in an enclosed solvent.10 Furthermore, the geometry11 of a SWNT can be used to tune its intrinsic electronic properties such as bandgap and oxidation/reduction potentials.12 Crucially for water splitting applications, some semiconducting NTs have reduction potentials negative enough to reduce hydrogen evolution catalysts.13,14 A number of covalent15,16 and non-


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covalent17,18 techniques have been used to electrically connect NTs to other light harvesting molecules,18 and to hydrogen evolving19 and hydrogenase20 catalysts, suggesting NTs as desirable modular components in a variety of photochemical systems. NTs grown in vertically aligned arrays21 are particularly attractive as a basis for membranes,6,8 as alignment of NTs also aligns their long axis, allowing for directed transfer of electrons and ions. The general scheme used to measure light-driven proton and electron transport through SWNT membranes is shown in Figure 1. Vertically aligned SWNTs (Figure 2) serve as the basis for the membranes, and were fabricated using a commercial chemical vapor deposition apparatus. To make the membranes impermeable, and give them mechanical rigidity, an epoxy was deposited between the NTs.8 Subsequently, the NT bores were opened by physical and chemical methods, allowing for cross membrane transport of charged species (See Supplemental Information). CdSe quantum dots (QDs) in hexanes14,22 were drop cast onto one side of the SWNT membrane for use as a photosensitizer. We believe that the aliphatic capping ligands of the QDs interact noncovalently with the NTs of the membrane, leading to a coating that is stable even under constant illumination. Photoexcited electrons in the QDs easily reduce NTs because NTs are favorable electron acceptors with respect to the conduction band energy of CdSe QDs (Figure S6). To demonstrate simultaneous crossing of NT membranes by protons and electrons we employed two indicators, bromophenol blue (BpB)23 and methyl viologen (MV)24 on opposite sides of the NT membrane, as well as ascorbic acid (AA) as a proton source and sacrificial electron donor.14 BpB is a pH indicator, and its absorption spectrum exhibits distinct features as the proportions of protonated (BpBH) to nonprotonated (BpB-) BpB change with proton concentration.25 MV changes from colorless to blue upon addition of electrons (MV2+ to MV+) and has been used as a stand-in for proton reduction catalysts.14 Following irradiation, electrons


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travel along the sp2 hybridized sidewalls of the NTs and reduce MV2+ on the other side of the membrane, in a manner similar to previous reports of photocurrent generated from QDs cast onto multi-walled nanotube (MWNT) membranes.8 In addition to being a source of protons, AA serves as a sacrificial electron donor, filling holes left in QDs by transfer of photoexcited electrons to NTs.14 Protons move though NT bores, likely via a Grotthuss type mechanism,26 drawn across the membrane by charge balance requirements. A custom glassware apparatus (Figure S2) was assembled such that charged species were required to pass through a NT membrane, taking advantage of the two separate transport pathways provided for protons and electrons.

Figure 1. Illustration showing simultaneous crossing of a carbon nanotube membrane by protons and electrons. Protons, from an aqueous solution of excess ascorbic acid and 4 µM bromophenol blue (BpB), cross through a water-filled nanotube bore. Movement of protons is reflected in the spectral changes observed in BpB, a pH indicator, which changes from its protonated (BpBH) to


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non-protonated (BpB-) form. Photoexcited electrons, generated by continuous irradiation of quantum dots with a 488 nm laser, enter the nanotube conduction band, cross the membrane, and reduce MV from MV2+ to MV+ on the other side. Holes left behind in quantum dots are filled by electrons from the sacrificial donor, ascorbic acid. Electron current is determined from spectroscopic measurements and is directly proportional to the amount of MV+ present.

Figure 2. Images of the carbon nanotube membrane. A) Scanning electron micrograph of a cross section of a vertically aligned array of single walled carbon nanotubes. Nanotubes are densely packed and associate strongly with one another. B) Epoxy completely fills what spaces remain between nanotubes in the array forming the membrane. C) Membranes are macroscale objects,


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approximately 2 cm x 2 cm in area, 350-400 µm thick, and self-supporting. Membrane is shown with a nickel to provide a relative scale of size.

Figure 3 shows absorption spectra resulting from the transport of protons and electrons through the SWNT membrane upon photoexcitation of the QDs. An intensity increase in the peak at 595 nm corresponds to an increase in [BpB-],25 indicating deprotonation of BpBH due to movement of protons through the membrane from the right to left chamber (Figure S2). A decrease in the peak at 449 nm is associated both with a decrease in [BpBH]25 and with laser induced photobleaching (See Figures S3 and S4). Photobleaching also indirectly causes a red shift in the peak at 595 nm (Figure S4). The inset shows that the oxidation state of the MV changes over the same 150 minutes from MV2+, which is colorless, to MV+ with a characteristic peak at 605 nm,24 indicating reduction of MV2+ by photogenerated electrons moving across the membrane. Importantly, concurrent changes in the spectra of BpB and MV indicate simultaneous membrane crossing by protons and electrons. The net proton current, determined as the number of protons required to cause the spectral change observed in BpB in Figure 3, which is corrected for a small amount of photobleaching, was 1.44 x 10-7 A. The calculated electron current was 3.97 x 10-8 A based on the intensity of the peak at 605 nm associated with [MV+], which agrees fairly well with the magnitude of the proton current. Due to charge balancing requirements the values for the electron and proton current should theoretically be equal. However, we hypothesize that the value observed for the electron current is lower than what was expected due to several possible factors. First, only electrons that were in fairly narrow diameter semiconducting SWNTs were able to reduce MV2+. Metallic SWNTs in the membrane have reduction potentials around that of highly oriented


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pyrolytic graphite (HOPG) at +550 mV,27 which is energetically downhill from the semiconducting SWNTs. Electrons that through charge transfer ended up in metallic SWNTs have reduction potentials far too positive to reduce MV2+. Thus, these electrons would traverse the membrane but not be detected spectroscopically. Also, reduced MV+ is very sensitize to oxidation. Exposure of MV+ to even trace amounts of oxygen, which was unavoidable during the extraction of the solution containing MV from the glassware apparatus, converts MV+ to MV2+, thus lowering the observed [MV+], and therefore the calculated number of electrons that crossed the membrane. While it may be possible to perform the measurements air-free, we found that rigorously excluding oxygen from all aspects of the measurement to the level needed to ensure higher accuracy in the calculated electron current was not possible.

Figure 3. Absorbance spectra of bromophenol blue (main) and methyl viologen (inset) taken from zero (red) to 150 minutes (blue). BpB data was taken in 15 minute increments. An important parameter relevant for potential water splitting applications is the reduction potential of an electron as delivered from the NTs. Each NT geometry, defined more rigorously as chirality14 and named in terms of two integers n and m, has a specific reduction potential. The chiralities of NTs present in the membranes were determined by optical spectroscopic methods28 (see Figure S6) and have been reported to have reduction potentials in aqueous solution as follows: (6,5) at -800 mV, (7,5) at -646 mV and (7,6) at -626 mV vs. SHE respectively. Exact


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values for NT reduction potentials depend on environment and are not well established29,30 since reduction potentials can change somewhat depending on NT environment.12,29,31 Since electrons flow from materials with more negative reduction potentials to those with more positive potentials, CdSe QDs having reduction potentials more negative than -1 V vs. SHE transfer electrons into NTs in the membranes. The reduction potential of MV2+ is -446 mV vs. SHE24 and is more positive compared to the NTs observed in the membrane. Therefore, overall electron flow in this system occurs from the QD conduction band to the SWNTs to the MV2+. Reduction of MV2+ implies the electrons that traverse the membrane, and which are not transferred into metallic SWNTs, are delivered to the far side with a potential more negative than -446 mV vs. SHE.26 We investigated whether some electrons were being delivered at an even more negative potential by substituting MV2+ with a diquat molecule,32 DQ4, which also serves as a redox indicator but with a reduction potential of -700 mV vs. SHE. No reduction of DQ4 was observed, although based on their more negative reduction potential isolated (6,5) NTs could be expected to reduce DQ4. However, if found in close proximity, electrons in (6,5) NTs will likely charge transfer into other NTs within the membrane that have more positive reduction potentials, such as the (7,5) or (7,6) NTs. Therefore, due to downhill electron transfer between nanotubes within the membrane, the effective reduction potential of the membrane is likely defined primarily by the largest diameter, or smallest bandgap, nanotubes within the membrane. Several control experiments were performed to ensure that the simultaneous electron and proton crossing was light-driven. Without irradiation the system did not produce a spectral change in either indicator (Figure S5), and irradiating an NT membrane that had not been coated with QDs produced only effects caused by photobleaching of BpBH (Figure S4). Membranes were also evaluated for leakage by filling one chamber of the glassware apparatus (Figure S2)


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and tipping the apparatus such that the filled chamber was directly over the unfilled chamber, separated by the membrane. Over a period of ten minutes no liquid was observed to enter the empty chamber, indicating that solution does not flow through the membrane, either through leaks or through the NT bores themselves, even when motivated by gravity. For this reason, the transfer of protons across the membrane was not caused by movement of solution. In addition to demonstration and evaluation of light-driven simultaneous crossing and reduction of chemical targets, we also quantified the proton transport properties and electron conductivity of membranes separately as described for other membranes in the literature.4-8 We quantified the rate of proton transport driven by an applied potential of 0.8V (see Supplemental Information) and found that the proton current is an improvement over Nafion or MWNT membranes under identical conditions (Figure 4). A comparison of the independently measured transport values for the SWNT based membrane with others in the literature4-8 is presented in Table S1. The SWNT membrane transports protons at a rate approximately 1.2 times that of Nafion, the previous best material for proton transport in mixed proton and electron transporting membranes.7,33 Electron conductivity was quantified using a Kelvin bridge apparatus (Supplemental Information) and it was found to be 558 ± 18 mS cm-1. This value is in good agreement with the conductivity of similarly fabricated MWNT membranes8 and reflects that the interstitial spaces between individual NTs in an array are filled with electrically insulating epoxy.


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Figure 4. Comparison of rates of independently measured proton crossing in membranes based on single walled (triangles) and multi walled (circles) carbon nanotubes as well as Nafion (squares).

Slope is directly proportional to rate of proton transport. The SWNT based

membranes described here transport protons more quickly than Nafion under the same conditions.

The demonstration of a single material, embedded in a membrane, that can simultaneously transport protons and electrons significant distances, accompanied by the delivery of photogenerated electrons at reduction potentials approximately 0.5 V more negative than SHE, may have significant implications for applications that require coupled electron and proton transport, such as artificial photosynthesis. When evaluating membranes for water splitting it is common to have independent and separate evaluation of proton and electron crossing.4-8 Membrane evaluations with accompanying proton reduction typically use platinum catalysts33 due to their exceptionally low reduction potential (0 versus SHE in strong acid).33,34 By contrast, SWNT membranes have the interesting quality of light-driven crossing of both photogenerated electrons and protons at the same time, accompanied by the reduction of a specific molecular target at a potential compatible with earth abundant metal-based hydrogen reduction catalysts.13


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While the SWNT membrane possesses several favorable aspects regarding its transport properties, the proton and electron currents demonstrated here are too low to produce appreciable amounts of hydrogen, likely because of poor photocurrent injection into NTs from QDs.8 The electron current could be improved by covalent coupling of photoreceptors to NTs and by putting a larger number of QDs per NT.16 Drop casting QD films on top of the NT membrane, while simple, seems to not place many QDs in the irradiation area that are well coupled to the NTs.

In conclusion, we have demonstrated simultaneous transport of protons and electrons macroscopic distances through a membrane in an aqueous environment using one material to transport both species. By fabricating that membrane from long SWNTs, we have further demonstrated reduction of molecules hundreds of microns away from the photoexcited electron and at a potential suitable for light-driven proton reduction. Lastly, we have demonstrated that the SWNT membrane performs competitively in independent tests of proton and electron crossing rates and thus in this respect compares extremely well with other membranes in the literature.

Supporting Information Discussion of nanotube growth, as well as spectra for control experiments, nanotube chirality identification, and a table of independent transport values for the membrane described in this work as well as for others in the literature can be found in the Supporting Information file. The following files are available free of charge. Supporting Information (PDF)


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Acknowledgements The authors gratefully acknowledge Dr. Kara Bren, and Dr. Lisa DeLouise for fruitful conversations, Stephanie Daifuku and Michael Mark for their assistance with Raman measurements, and Richard Eisenberg’s group for their gift of DQ4 molecules. Portions of this work were performed at the Institute for Electronics and Nanotechnology at Georgia Tech and at the URnano Facility at the University of Rochester. This work is supported by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy, Grant No. DE-FG02-09ER16121.

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