Plasmonic Photoanodes for Solar Water Splitting with Visible Light

Aug 23, 2012 - We report a plasmonic water splitting cell in which 95% of the effective charge carriers derive from surface plasmon decay to hot elect...
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Plasmonic Photoanodes for Solar Water Splitting with Visible Light Joun Lee,† Syed Mubeen,† Xiulei Ji, Galen D. Stucky, and Martin Moskovits* Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States S Supporting Information *

ABSTRACT: We report a plasmonic water splitting cell in which 95% of the effective charge carriers derive from surface plasmon decay to hot electrons, as evidenced by fuel production efficiencies up to 20-fold higher at visible, as compared to UV, wavelengths. The cell functions by illuminating a dense array of aligned gold nanorods capped with TiO2, forming a Schottky metal/semiconductor interface which collects and conducts the hot electrons to an unilluminated platinum counter-electrode where hydrogen gas evolves. The resultant positive charges in the Au nanorods function as holes and are extracted by an oxidation catalyst which electrocatalytically oxidizes water to oxygen gas. KEYWORDS: Surface plasmons, gold nanorods, alumina, solar water splitting, H2 production

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semiconductor system that can harvest photons over the entire solar spectrum and beyond.35,36 Several recent studies have reported significant photocatalytic reaction rates in systems in which plasmonic metal nanoparticles were resident either on the surface or inside a semiconductor matrix, with the increased photocatalysis attributed either to direct injection of energetic charge carriers to the semiconductor, or to radiative energy transfer through near-field electromagnetic coupling and resonant photon scattering.37−49 A central question in plasmonic photosensitization is the extent to which one can harvest light by plasmonic antennae then convert that energy to useful entities such as electron hole pairs. In the context of the current study, the seminal question is to what extent can plasmonic light conversion be used either directly to carry out a useful photochemical process or to promote increased photocatalytic activity at the surface of a proximate wide-bandgap semiconductor, thereby essentially photosensitizing it by extending the photoactivity of the device into the visible region of the spectrum. To date, most such studies show rather small photocatalytic rates for water splitting in the visible as compared to the ultraviolet regions.44−49 Somewhat better plasmonic photosensitization was recently reported for a TiO2 film impregnated with nonpercolating gold nanoparticles, in which the device’s photoconductance with visible illumination was as large as ∼50% of its conductance in UV.34 In the current study we show that by engineering a device which incorporates an array of aligned gold nanorods, functioning as a plasmonic nanostructure with a high surface area, in contact with TiO2, one could improve the visible solarto-fuel conversion efficiency so that it greatly exceeds its UV efficiency.

n abiding paradigm in the development of renewable and sustainable carbon-free energy source is the utilization of solar energy to produce storable clean fuels such as hydrogen by, for example, using sunlight to drive a thermodynamic uphill chemical reaction such as the electrolysis of water to produce H2 and O2.1−12 Among the challenges of designing and fabricating such systems is the development of a device that produces H2 efficiently under panchromatic illumination for sustained periods of time. One problem is that the oxidative half reaction, a 4e− process producing O2, induces photocorrosion of broad band semiconductor absorbers such as Si, Cu2S, CdSe, CdS, and so forth.13−18 Hence, until recently, the choice of photoanodes was limited to a few stable wide band gap oxide semiconductors such as TiO2, WO3, and CeO2, which respond only in the ultraviolet, thereby seriously limiting the efficiency.19−23 Fruitful strategies have been developed for overcoming this narrow-band photoactivity, for example, by doping or photosensitizing wide band gap oxide semiconductors to improve their activity at sub-bandgap photon energies.24−27 A recent challenge has been to determine the extent to which useful photovoltaics or photosynthesis devices could be created based on charge carriers derived from the decay of localized surface plasmons in an appropriate nanostructured metal or metal/semiconductor junction. Metal nanostructures of, for example, gold, silver, and copper, which possess strong absorptions in the visible spectrum due to localized surface plasmon (LSP) resonances, have been shown to act as photosensitizers transferring some of the absorbed plasmonic energy to an adjacent semiconductor either through resonant energy transfer or directly through hot-electron injection, thereby improving the apparent photoactivity of the semiconductor in the visible region of the spectrum, so far very inefficiently.28−34 One attraction of plasmonic devices is the strong morphology and geometry dependence of LSPs, in principle making it possible to design a composite plasmonic/ © 2012 American Chemical Society

Received: July 27, 2012 Revised: August 16, 2012 Published: August 23, 2012 5014

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Figure 1. Schematic of the fabrication of gold/TiO2 nanostructures on ITO-coated glass. Left panel: ITO-coated glass slides were coated with a 20 nm TiO2 layer and a 1 μm layer of Al prepared by electron-beam evaporation, and the aluminum film was anodized to form porous aluminum oxide (PAO). Middle panel: Gold was electrochemically deposited in the PAO and the PAO template removed by wet etching. Right panel: TiO2 was evaporated on the gold nanorods using electron-beam evaporation, and an oxygen evolution catalyst (OEC) electrodeposited on the gold. All scale bars correspond to 150 nm. Extreme right: Schematic of the working cell in which only the plasmonic anode is exposed to light. The cathode is a platinum mesh that is not illuminated.

The important novelty in the present study is a plasmonic photoanode fabricated using a series of top down and bottom up techniques, which shows unprecedented oxygen evolution ability when illuminated with visible light. Briefly, highly ordered and vertically oriented gold nanorod (Au NR) arrays with rod diameters ∼90 nm were electrodeposited in a porous anodic aluminum oxide template that was prepared on a transparent indium tin oxide (ITO)/TiO2 conducting substrate (see Experimental Methods section for details). An approximately 20 nm thick TiO2 was e-beam deposited on the upper portion of the Au nanorods and, in the process, also as small TiO2 nanoparticles deposited sporadically along the sides of the Au nanorods. The TiO2 forms metal-to-semiconductor Schottky junctions.50−52 On some of the samples, cobalt/ borate, a well-known oxygen evolution catalyst (OEC)12 that promotes efficient water oxidation, was electrochemically deposited on the TiO2-decorated Au nanorods (which we show below deposits exclusively on the bare Au portions of the nanowires). In a separate compartment, Pt mesh (which was not illuminated) was used as the cathode at which H2 evolution occurs. The anode and cathode compartments were separated by an ion-permeable nafion membrane. The resulting device will be referred to as the plasmonic electrochemical cell (PEC). A schematic of the fabrication steps along with corresponding scanning electron micrographs (SEM) and a schematic of the working cell are shown in Figure 1. Various control samples were also fabricated in which certain components, for example, the photocatalyst or the TiO2 on the gold nanorods, were omitted to determine their roles in the overall device. High-resolution transmission electron micrographs of a typical Au nanorod following TiO2 e-beam deposition and Co-OEC electrodeposition are shown in Figure 2. The TiO2 is seen to reside primarily near the tops of the Au nanorods and as small discontinuous nodules along the side of the nanorods (Figure 2a,b). Even though Co-OEC electrodeposition was carried out (in the dark) after the TiO2 was in place, the catalyst deposited preferentially on the bare patches of the Au nanorods (Figure 2c). This is not unexpected since the electrochemical fields at the gold would exceed those at places already covered by TiO2. Nevertheless, the physical separation

Figure 2. TEM images of a composite plasmonic photoanode (CoOEC/AuNR/TiO2) structure (a) Under low magnification, two distinct areas are observed with the e-beam deposited TiO2 at the top of the nanorod (green arrow) and electrochemically deposited CoOEC residing directly on the gold surface (red arrow). Higher magnification images show that (b) no Co-OEC was deposited on the TiO2 and (c) electrochemical deposition of Co-OEC occurred exclusively on the bare AuNR surfaces.

of the site at which oxygen separation takes place from the TiO2, which we will argue later becomes an electron reservoir, might prove useful in creating an autonomous plasmonic water splitting device which carries out both oxygen and hydrogen evolution at two (nearby) locations along the same nanowire. The photoelectrochemical properties of the PECs described above, loaded with 1 molar potassium borate electrolyte (pH 9.6), were characterized using a three-electrode setup (Aunanorods/semiconductor working electrode, Pt mesh counter electrode, and saturated calomel reference electrode). Figure 3a shows the water oxidation currents as a function of potential under chopped illumination, which allows the dark and light 5015

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Figure 3. (a) Water oxidation currents as a function of potential under chopped AM 1.5 white light illumination. (b) Photocurrent vs time plots for Au NR/TiO2 with Co-OEC present on the Au nanorods. UV and Vis correspond, respectively, to light passed through the UV bandpass filter and the visible high-pass filter described in the text. Note the high photocurrent obtained with visible light illumination. The visible light response shows a fast current rapid component and a slow component that continues to increase over the ∼100 s measurement interval. (c) As in b but for a device in which the Co-OEC was omitted. The measured photocurrents were lower both with UV and visible illumination; however the photocurrent obtained with visible illumination continued to be greater than that measured with UV. Also the slow current component was no longer observed.

Figure 4. (a) Energy band diagram of composite plasmonic photoanode unit. The electron−hole pairs created in AuNR upon excitation in visible light are separated as electron hole pairs with energetic electrons injected into TiO2. The energetic holes are efficiently extracted by Co-OEC and used for water oxidation. (b) The photocurrent action spectra (blue) of Co-OEC/AuNR/TiO2 shows a large responsivity in the visible spectra which coincides with the UV−vis absorbance (red) of the composite structure. (c) The quantity of evolved hydrogen (blue trace) measured (gaschromatographically) as a function of time. (Black curve) The photocurrent simultaneously recorded at 1 V vs RHE with visible light illumination. (Red trace) The photocurrent calculated from the evolved H2. (d) Faradaic efficiency of the process, calculated by comparing the red and black traces in c, showing that there is an initial process with efficiency ∼40% (the fast process) and a slower process that grows in over ∼25 min to an eventual efficiency ∼80%.

currents to be monitored simultaneously. Only small anodic currents were measured during the “dark” cycles for all potentials E ≤ 0.5 V. During “light on” cycles, anodic currents were observed for E ≥ 0.15 V with current densities reaching 0.1 mA/cm2 at 0.5 V for the best device (all potentials are referenced to the reversible hydrogen electrode (RHE)). Photocurrents were measured with the photoanode illuminated

either with UV or visible light, which were provided by passing the AM 1.5 solar light source alternately through a UV−vis band-pass filter (320−510 nm) or high-band-pass filter that passed wavelengths longer than 410 nm. Figure 3b shows the photocurrent versus time plot obtained for a device that incorporates all of the constituent elements: Au nanorod array, TiO2 deposited near the top of the 5016

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array (Figure 4b) is strong evidence that the initiating photoprocess is surface plasmon excitation of the gold nanorods. The UV−visible extinction spectrum of the Au NRs capped with TiO2 trends upward toward the UV, reflecting the UV absorption of the TiO2. In the visible it shows two pronounced localized surface plasmon resonance (LSPR) with maxima at 508 and 610 nm (Figure 4b). The resonances correspond to the transverse and longitudinal modes of plasmonic excitations in the assembly.53−55 The wavelength of longitudinal modes resonance is strongly dependent on the aspect ratio and the interspacing distance of the nanorods.53 In the nanorod arrays the interspacing distance is sufficiently close for strong coupling between the nanorods to occur, blue-shifting the higher energy absorption with respect to the isolated longitudinal mode.53,55 The photocurrent action spectra (Figure 4b) track the two surface plasmon extinction bands faithfully, indicating incontrovertibly that visible-light response of the device arises from surface plasmon excitations in the AuNRs. Moreover, although the extinction spectrum has its largest values in the UV, the UV response of the photocurrent action spectrum is rather modest in the UV indicating that direct bandgap excitation of the TiO2 contributes negligibly to the observed photoelectrochemical processes (Figure S4). Summarizing the proposed water splitting mechanism, a significant fraction of hot electrons generated following the decay of the plasmon breech the Schottky barrier with the TiO2 and immediately flow to the (unilluminated) Pt mesh electrode where hydrogen ions are reduced to H2. This is the source of the “fast” component. The positive charges left behind are collected in the Co-OEC catalyst, enabling oxygen oxidation to O2 and replenishing the lost electrons in the Au nanorods. Some electrons become trapped in the deposited TiO2 leading to a reduced Schottky barrier, which results in an increased photocurrent over time (the “slow” component) until after ∼25 min the TiO2 becomes saturated and the systems achieves steady state (Figure 4c). The time rate of H2 production confirms this picture. Referring to Figure 4c one sees that the H2 production rate in the cathode chamber, when expressed as the equivalent electrochemical current, tracks the shape of the measured photocurrent rather faithfully, but not with 100% efficiency. On calculating and plotting the (Faradaic) efficiency as a function of time, by dividing the photocurrent calculated from the rate of H2 production by the experimentally determined photocurrent (Figure 4d), one sees that there is indeed a slow and fast component. The fast component contributes ∼40% efficiency in terms of H atom per electron passed. This efficiency grows steadily over ∼25 min, eventually reaching a more or less steady ∼80% efficiency. The efficiency of the plasmonic photoanode is the highest so far reported for such plasmonic systems and certainly the first to show efficiencies with visible light significantly in excess of what is observed with UV illumination,44−49 with our best devices showing 20-fold quantum yields for visible light illumination as compared to UV. Additional efficiency gains are likely possible by, among other improvements, electrodepositing more efficient OECs such as IrO2, RuO2, and so forth. Experimental Methods. PAO Synthesis. ITO-coated glass substrates (sheet resistance: 20 ± 2 Ω/sq, purchased from Thin Film Devices Inc.) were ultrasonically cleaned in acetone, IPA, and DI-water, then dried with argon. A TiO2 film (20 nm

nanorods, and the Co-OEC in direct contact with the Au nanorods (this device will be shorthanded Co-OEC/Au NR/ TiO2). Observed photocurrents were small under UV illumination and plateaued at ∼0.02 mA/cm2 (at 1 V versus RHE). By contrast, photocurrents were significantly larger under visible light illumination, exceeding 0.1 mA/cm2, and as high as 0.35 mA/cm2 for the best devices. Under visible light, the photocurrent consists of two temporal components: a fast component that rises with a time constant ∼2.9 s (see Figure S1 of the Supporting Information) and raises the photocurrent to ∼0.07 mA/cm2. Additionally, there is a slow component that increases with a time constant ∼1100 s (see Figure S1) and adds another 0.05 mA/cm2 over the first 100 s (Figure 3b), the overall photocurrent reaching a plateau at a value of ∼0.35 mA/ cm2. TiO2-based devices fabricated without Au nanorods produced insignificant photocurrents when illuminated with visible light (Figure S2). Devices fabricated without the CoOEC still produced larger photocurrents with visible light than with UV illumination but with lower overall values of the photocurrent, and a much reduced slow component (Figure 3c). Gas chromatographic (GC) measurements carried out after 1 h illumination with visible light showed unequivocally that H2 and O2 evolved, each in its respective chamber (Figure S3) with only oxygen evolving in the chamber containing the Co-OEC/ Au NR/TiO2 electrode. No gas evolution was detectable in either chamber after 1 h illumination with UV. In the absence of the Co-OEC catalyst much reduced gas evolution was observed with either visible or UV illumination. The plasmonically driven solar water splitting process can be understood qualitatively with the aid of the schematic band diagram of the photoanode structure shown in Figure 4a. A Schottky junction is established at the interface between the Au NRs and TiO2 resulting in charge transfer from the TiO2 to the Au NRs and a potential barrier of ∼1.1 eV. Illuminating the AuNRs with visible light excited surface plasmons that rapidly decay, producing many hot electrons (often referred to as electron−hole pairs, even though both these species coexist in the metal’s conduction band). The hot electrons transiently occupy normally empty states in the gold’s conduction band above the Fermi energy. A significant fraction of these excited electrons are transferred to the TiO2. This process leaves energetic positive charges (sometimes referred to figuratively as holes) on the gold. Some of the hot electrons may also reduce dissolved O2 and participate in trap-mediated recombination with holes. Electrons transferred from Co-OEC to the AuNRs fill the photogenerated holes in the gold, and the Co-OEC is resupplied with electrons as it catalyzes water oxidation. The much lower photocurrents for metal-semiconductor anodes without OEC (Figure 3c) emphasize the crucial role of a charge carrier mediator at the interface between the Au NR and the solution. The above observations imply a mechanism in which hot electrons resulting from the decay of surface plasmons transfer from the Au NRs to the contiguous semiconductor as a first step, followed by the repopulation of the positive charges left behind on the AuNRs by electrons transferred to the AuNRs in the oxygen evolution reaction through the beneficial intermediacy of the CoOECthe positive charges on the Au essentially functioning as holes. The strong similarity between the photoelectrochemical action spectrum and the visible portion of the extinction spectrum of the device which is dominated by the surface plasmon bands of the Au nanorod 5017

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electron scanning microscope, and transmission electron microscopy (TEM) images were obtained using FEI Tecnai G2 Sphere microscope. The presence of the electrodeposited catalyst on AuNR-TiO2 sample was verified by energydispersive X-ray analysis (EDX) spectra obtained from both multitude and single photoanode unit (Figure S6, Supporting Information). The extinction spectra for gold nanorods and gold nanorods with TiO2 and Co-OEC (Figure S7) were measured on a UV−vis−NIR spectrometer (Shimadzu UV3600).

thickness) was evaporated onto the ITO-coated glasses by electron-beam evaporation at a rate of 0.3 Å/s (pressure: 9 × 10−5 with O2 bleeding). A small stripe of ITO area was left unexposed during TiO2 evaporation for future electrical contacts. The samples were annealed in a tube furnace at 500 °C in air for 1 h. Before aluminum evaporation, the samples were treated in O2 plasma for 1 min (300 mTorr, 100W). A 1 μm film of pure aluminum was e-beam evaporated on the substrates at an initial rate of 1 Å/s for the first 5 nm, then increased to 2 Å/s, and finally increased to 5 Å/s after 50 nm. The aluminum was anodized by making electrical contact to it using copper tape and masking off all exposed areas other than those where anodization was desired, with electroplating tape (3M, type 470). The sample was placed in 0.3 M oxalic acid facing a flat graphite cathode electrode. Anodization was carried out at 60 V with the electrolyte maintained at 2 °C. The anodized samples were then placed in 5 wt % phosphoric acid for 65 min to remove the oxide barrier layer and increase the pore diameter. Electrodeposition of Nanorods. Electrical contact was made to the ITO with copper tape, and exposed areas, other than those on which electrodeposition was desired, were masked with electroplating tape. Electrodeposition of gold was performed in a glass cell containing 20 mL of Orotemp Au (Technic Inc.), 20 mL of DI-water, and 2 wt % of gelatin at 40 °C. A platinum wire and saturated calomel electrode (SCE) were used as counter and reference electrode, respectively. A potential of −1.1 V was initially applied for 30 s and then decreased to −0.7 V for the next 1 h. TiO2 Deposition. The sample containing electrochemically grown gold nanorods was placed in 0.1 M NaOH, while maintaining the masked tapes, for 30 min to remove the PAO template. The sample was carefully rinsed in DI-water, and then the cover tapes were removed. The sample was then placed in ethanol and hexane and dried in air. TiO2 was evaporated using e-beam evaporation. TiO2 was unidirectionally deposited by ebeam evaporation on the Au nanorod array resulting in gold nanorods partially coated with TiO2. OEC Deposition. Co/borate was electrochemically deposited on the AuNR/TiO2 composite by applying 1 V vs SCE for 5 min in 0.25 mM cobalt nitrate, 1 M potassium borate in which the pH was adjusted to 9.6. Photoelectrochemical Characterization. All photoelectrochemical measurements were performed in a two-compartment cell. Electrical contact was made to ITO using copper tape. The measurement was performed using three-electrode configuration with a Pt-wire counter electrode and SCE reference electrode. One molar potassium borate electrolyte (pH 9.6) was used as electrolyte solution. Samples were illuminated through glass/ITO/TiO2 side. The photocurrents as a function of light-intensity were measured using simulated AM 1.5 solar irradiation. Light intensity was measured using a calibrated Si photodiode (Hamamatsu, S2387-66R, 5.8 × 5.8 mm2). The photocurrents as a function of source wavelength were measured using the light from 300 W xenon lamp passed through a monochromator and focused with a microscope objective (33×) to yield a beam size of 2.5 mm2. The intensity of the incident light from the monochromator was measured by a calibrated Si photodiode. Current density veresus voltage (J− V) data was obtained using a potentiostat (EG&G Princeton Applied Research, Potentiostat/Galvanostat 273A). Material Characterization. Scanning electron microscopy (SEM) images were taken using FEI XL30 Sirion FEG digital

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ASSOCIATED CONTENT

* Supporting Information S

GC results and PEC performance of various devices. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Address: University of California, Department of Chemistry and Biochemistry, Santa Barbara, California 93106, USA. Author Contributions †

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge research support from the Institute for Energy Efficiency, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Award Number: DESC0001009. The authors made extensive use of the MRL Central Facilities at UCSB; the MRL central facilities are supported by the MRSEC program of the NSF National Science Foundation under award no. DMR-1121053, a member of the NSF-funded materials research facilities network (http:// www.mrfn.org). We also made extensive use of the MRL Central Facilities at UCSB for the HRTEM/STEM microscopy supported by the National Science Foundation under award no. DMR-0080034 and DMR-0216466. The contribution of X.J. was supported by a postdoctoral fellowship from the Natural Sciences and Engineering Research Council of Canada and by National Science Foundation grant no. DMR 0805148. We also thank Mr. Nirala Singh and Prof. Eric W. McFarland for GC measurements.

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