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Metal-free carbon-based nanomaterial coatings protect silicon photoanodes in solar water-splitting KunHo Yoon, Jae-Hyeok Lee, Joohoon Kang, Junmo Kang, Michael J. Moody, Mark C Hersam, and Lincoln J. Lauhon Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b02691 • Publication Date (Web): 10 Nov 2016 Downloaded from http://pubs.acs.org on November 11, 2016

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Metal-free carbon-based nanomaterial coatings protect silicon photoanodes in solar water-splitting KunHo Yoon§1, Jae-Hyeok Lee§1, Joohoon Kang1, Junmo Kang1, Michael J. Moody1, Mark C. Hersam*1,2,3,4, and Lincoln J. Lauhon*1 1

Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, United States 2

Department of Chemistry, Northwestern University, Evanston, IL 60208, United States

3

Department of Medicine, Northwestern University, Evanston, IL 60208, United States

4

Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, IL 60208, United States

ABSTRACT The decreasing cost of silicon-based photovoltaics has enabled significant increases in solar electricity generation worldwide. Silicon photoanodes could also play an important role in the cost-effective generation of solar fuels, but the most successful methods of photoelectrode passivation and performance enhancement rely on a combination of precious metals and sophisticated processing methods that offset the economic arguments for silicon. Here we show that metal free carbon-based nanomaterial coatings deposited from solution can protect silicon photoanodes carrying out the oxygen evolution reaction in a range of working environments. Purified semiconducting carbon nanotubes (CNTs) act as a hole extraction layer, and a graphene (Gr) capping layer both protects the CNT film and acts as a hole exchange layer with the electrolyte. The performance of semiconducting CNTs is found to be superior to that of metallic or unsorted CNTs in this context. Furthermore, the insertion of graphene oxide (GO) between the n-Si and CNTs reduces the overpotential relative to photoanodes with CNTs deposited on hydrogen passivated silicon. The composite photoanode structure of n-Si/GO/CNT/Gr shows

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promising performance for oxygen evolution and excellent potential for improvement by optimizing the catalytic properties and stability of the graphene protective layer.


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The oxidation of water to O2 is an important process for the complete photoelectrochemical production of solar fuels.1 Although silicon is a successful substrate for photovoltaics, it has been considered less suitable as a photoanode for water oxidation because of its narrow bandgap and its instability in oxidizing environments. These limitations have motivated the development of coatings that protect n-Si without compromising light absorption, afford a good photovoltage, and generate a sufficiently high rate of oxygen evolution. Coatings of transition metals (Ni,2, 3 Mn,4 Co,5 Ti,6 and Ir7) and their oxides can meet these requirements, but they are generally limited to working in an alkaline environment. Recently, an iridium catalyst was used to extend the working range of pH of Si photoanodes coated with TiO2 using atomic layer deposition (ALD).8 The performance of this type of photoanode is also maximized by diffusion doping of a p-type surface layer to form an inversion layer on the n-Si substrate. However, the costs associated with precious metal catalysts, vapor phase deposition processes, and substrate diffusion doping motivate alternative, potentially lower-cost approaches to highperformance coatings.9 Here we explore the use of solution-processed carbon-based nanomaterials10 to passivate and enhance the performance of n-Si photoanodes for water oxidation. We study semiconducting carbon nanotube (CNT) films on n-Si substrates in an aqueous environment over a wide range of pH values, and show that graphene and graphene oxide (GO) can enhance the stability and catalytic activity for the oxygen evolution reaction (OER). Carbon-based nanomaterials have previously shown promise as functional elements in photoanodes. For example, graphenecovered n-Si was shown to improve photoanode stability for ferrocene and bromine oxidation11, 12

and hydrogen reduction,13, 14 though these reactions are more energetically and kinetically

favorable than the OER.1 CNT-graphene complexes,15 graphene,16, 17 and graphene-oxide18 have


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been tested as oxygen reduction electrocatalysts, and CNTs have been used to promote charge separation with wide-gap semiconductor photocatalysts,19,

20

but their performance as a

passivation layer for high-performance narrow-gap semiconductor water-splitting photoanodes has not been reported. CNTs in particular have properties that could be advantageous for photoanode coatings. CNTs are chemically stable over a wide range of pH values, present a large surface area for reactions, and can be easily processed in solution.10 However, as synthesized, single-walled CNTs (SWCNTs) are generally a mixture of metallic and semiconducting phases due to the distribution in size and chirality.21 The broad absorption spectrum of unsorted CNTs has been exploited in solar cells, but performance is limited by the mixed electrical properties.22 When appropriately sorted by chirality and diameter, SWCNTs act as p-type semiconductors,23 and thus are considered here as an alternative to diffusion doping to form a p-n junction in contact with n-Si.22 To explore this concept, we used highly enriched semiconducting SWNTs (>99.8% purity) with a narrow size distribution (diameter = 1.2 ± 0.3 nm) to make thin films of approximately 20 nm thickness on n-Si substrates (Fig. S1 in Supporting Information). Briefly, a semiconducting SWCNT solution was filtered on an anodized aluminum oxide (AAO) membrane with 20 nm pore size, dried, and washed in ethanol to remove the remaining surfactant. The AAO membrane was dissolved so that the freestanding random network of SWCNTs could be transferred to an n-Si substrate and dried. The native oxide layer was etched with HF prior to the film transfer. Additional details of the processing are provided in the Supporting Information. n-Si/CNT film photoanodes were fabricated and tested using a conventional single workingelectrode configuration (Fig. 1a). A CH instruments CHI 760D potentiostat was used for all


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measurements, with the Si photoanode, Pt wire, and miniature Ag/AgCl (saturated) electrode serving as working, counter and reference electrodes, respectively. Under photoexcitation, holes generated in the silicon are injected into the CNT layer, and these holes can perform water oxidation when the bias is sufficiently positive. Indeed, current density vs. potential (J-E) curves show substantial photoactivity in acidic (1 M H2SO4), neutral (phosphate-buffered), and basic (1 M KOH) solutions (Fig. 1b), with some variation depending on the electrolyte. At pH 7, a current density of 0.1 mA cm-1 is achieved at 240 mV below the thermodynamic water oxidation potential. The photocurrent magnitude of the devices shown in this pH series ( 1000 s, samples with commercial graphene (Graphene Supermarket, >95% monolayer coverage) or graphene grown on 99.999% copper foil were not stable on the same timescale (Fig. S4). This could be due to a higher density of pinholes or incomplete coverage. However, defects may also play an important role in catalysis of the oxidation reaction. Annealed and solvent-rinsed films made from commercial graphene or graphene from 99.999% foil showed enhanced photocurrent densities of up to 19 mA/cm2 at 1.83 V vs. RHE (Fig S5). This suggests that future work to tailor the catalytic properties of the graphene as well as the band alignment of the CNTs could further improve water-splitting performance. Two control experiments show that a semiconducting CNT thin-film promotes the OER, and provide some insight into the role of the CNTs. First, we note that the current density of n-


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Si/2L graphene photoanodes under illumination at 0.9 V is over 100 times lower than that of the Si/CNT/2L graphene photoanodes (Fig. 3a). As expected, the bare graphene cannot maintain a sufficiently high photoexcited hole density to lower the quasi-Fermi level below the redox potential. Hence, we conclude that the CNTs act as efficient hole acceptors and transporters to the graphene, shifting the Fermi level26, 27 sufficiently to carry out the OER at a bias below the thermodynamic potential. Second, Si/CNT/graphene photoanodes fabricated with unsorted or metallic CNTs yield photocurrents of only 1.9 or 4.7 mA cm-2 at 1.0 V, respectively, compared to 7.9 mA cm-2 for p-type semiconducting CNTs, (Fig. 3b). While the metallic and p-type samples have similar current slopes, there is a large difference in overpotential (0.37 V and 0.23 V, respectively, at 1 mA cm-2, relative to the thermodynamic potential of 0.40 V). Because the catalytic properties of the graphene protective layers are the same, we conclude that the p-type CNTs produce a higher photovoltage. In fact, one might expect better performance from p-type CNTs if they help maintain the depletion layer in the n-Si absorber, which would reduce recombination and maintain the photovoltage. However, we also observe that the metallic CNTs perform better than the unsorted CNTs, which points to the role of good conductivity in the hole extraction layer. The unsorted sample has a similar onset voltage as the metallic sample, but the slope is smaller, which could be explained by the lower conductivity. The purified semiconducting CNTs were used in the experiments that follow because they show both a lower onset potential and a higher slope. When fabricating the photoanodes described above, the native oxide layer was removed by wet etching prior to transferring the CNT layer to make the Si surface hydrophobic, which improves the bonding to carbon nanotubes and the electrical properties of CNT thin-film devices.28 The etching process used (see Supporting Information) produces hydrogen terminated


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Si, which has a low recombination rate29 that should in principle improve the photoanode performance. However, several groups have fabricated photoanodes without removing the native oxide30 and have claimed that it blocks majority carrier electron transport into the depletion/inversion region, thereby reducing recombination and improving the open circuit voltage or photovoltage.22 Such an approach also requires that photogenerated hole transport is reasonably facile for a thin (< 3 nm) oxide.31 To further explore the potential applications of carbon-based nanomaterials in this context, we tested GO as an interlayer between the n-Si and CNT thin film to enhance CNT adhesion while inhibiting electron transfer and promoting hole transfer. In organic photovoltaics (OPVs), GO has been used as a hole-extracting interfacial layer (IFL) to address a known source of OPV failure.32 When GO is inserted between Si and CNTs, it is hypothesized that the hydrophobic CNTs form more a stable interface with the basal plane of GO than with SiO2.33 Thin monolayer films of the GO were deposited onto substrates using a LangmuirBlodgett (LB) technique as previously reported.34 In brief, a GO solution prepared via a modified Hummers method35 was spread dropwise onto the air/water interface in a Langmuir-Blodgett (LB) trough (Nima Technology, model 112D), and the GO LB film was compressed to a surface pressure of 13 mN/m. The GO LB films were then transferred to the substrates by vertically raising the sample at a speed of 2 mm/min. The LB method allows the layer-by-layer transfer of GO films, and thus provides tight control of thickness. As shown in Fig. 4a, the addition of two layers of GO (Si/2L-GO/CNT/Gr) improves photoanode performance compared to a Si/CNT/Gr photoanode without GO. Furthermore, the devices are effective (Fig. 4a) and stable (Fig. 4b) across a range of pH values from 0 to 14. Trends in the photocurrent (at 1.73 V versus RHE) with an increasing number of ~1.1 nm thick GO layers are suggestive of the roles that GO is


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playing (Figs. 4c & S4). With the addition of a single layer of GO, J-E sweeps show a significant increase in photocurrent compared to the sample without GO interlayers. The increase in photocurrent provides indirect evidence of reduced recombination, which is the expected consequence of an enhancement in hole transfer and/or improved electron blocking, both of which have previously been demonstrated in GO.34 We emphasize again that Si is the dominant absorber, and that performance is improved by maximizing the rate of hole transfer from silicon to the graphene/solution interface and by minimizing the transfer of electrons from the carbon-based layers to the Si (as reflected in the low dark current values). For one to three layers of GO, the photocurrent magnitudes are comparable. With four layers of GO, the photocurrent density decreases significantly, presumably due to the diminishing probability that photogenerated holes tunnel across the GO layers. In n-type Si and p-type CNT heterojunction photovoltaics,22 it has been proposed that a thin native SiO2 layer is necessary to maintain a reasonable photovoltage. In our photoanodes devices, GO appears to play a similar role in establishing the photovoltage, possibly via selective hole transport as shown previously for an organic photovoltaic cell.34 However, it is also possible that GO enhances depletion of the n-Si. Indeed, the potential to tailor the chemistry of the GO, CNT, and graphene layers to optimize photoelectrode performance suggests that an allnanocarbon coating approach could be readily extended beyond the oxygen evolution reaction demonstrated here.

AUTHOR INFORMATION Corresponding authors: *E-mail: [email protected], [email protected] §

KunHo Yoon and Jae-Hyeok Lee contributed equally.


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NOTES: Competing financial interests: The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This research was supported by the Materials Research Science and Engineering Center (MRSEC) of Northwestern University (NSF DMR-1121262). This work made use of the Northwestern University Atomic and Nanoscale Characterization Experimental Center (NUANCE), which has received support from the NSF MRSEC (DMR-1121262), State of Illinois, and Northwestern University.

ASSOCIATED CONTENT Supporting Information Available: CNT processing information, stability comparisons, and additional controls. This material is available free of charge via the Internet at pubs.acs.org.


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Figure Captions

Figure 1: Semiconducting carbon-nanotube photoanode performance. a. Schematic band alignment for Si/CNT photoanode. b. Cyclic voltammograms of CNT films on n-type Si in 1 M KOH (red), phosphate buffered (green), and 1 M H2SO4 (blue) electrolytes. Respective dark currents are shown with dotted lines. c. Cyclic voltammogram of 2 nm of Ni on CNT films on ntype Si in 1 M KOH. Figure 2: Improved performance with graphene overlayer. a. Cyclic voltammogram of Si/CNT/Graphene in 1 M KOH in the light (red) and dark (black). b. Chronoamperometry at 1.73 V versus RHE for an annealed Si/CNT/Graphene photoanode showing stable operation over 15 minutes. Figure 3: Control experiments using metallic and unsorted CNTs. a. Cyclic voltammogram of photoanode without SWCNT film (Si/Graphene) in 1 M KOH showing greatly reduced performance compared to Fig. 2a. b. Cyclic voltammograms of photoanodes with metallic or unsorted CNT films compared to photoanodes with semiconducting CNTs in 1 M KOH, showing lower current densities and higher onset potentials Figure 4: Effect of graphene oxide (GO) interlayer thickness on performance. a. Cyclic voltammograms for devices with 0 L (dotted) and 2 L GO (solid) in 1 M KOH (red), phosphate buffered (green), and 1 M H2SO4 (blue) electrolytes; 0 L refers to Si/CNT/Graphene samples. b. Chronoamperometry at 1.73 V versus RHE in 1 M KOH for 0 L, 1 L, and 4 L GO all carbonbased photoanodes. c. Photocurrent (at 1.73 V versus RHE in 1 M KOH) as a function of the number of interfacial GO layers, from cyclic voltammograms for photoanodes with different


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number of GO layers inserted prior to CNT film transfer (Fig. S6). Inset: photoanode schematic. Note that current density values differ slightly from those obtained from CV curves in (a).

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Figure 2


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Figure 4


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