Visible Light Photo-oxidation in Au Nanoparticle Sensitized SrTiO3:Nb

Jul 23, 2013 - ... which is significantly higher than the estimate (1.0 eV) based on the difference between the electron affinity/Fermi position of Sr...
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Visible Light Photo-oxidation in Au Nanoparticle Sensitized SrTiO3:Nb Photoanode Tian Ming,†,‡ Jin Suntivich,*,⊥,# Kevin J. May,†,‡ Kelsey A. Stoerzinger,†,§ Dong Ha Kim,†,‡,¶ and Yang Shao-Horn*,†,‡,§

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Electrochemical Energy Laboratory ‡Department of Mechanical Engineering §Department of Materials Science and Engineering Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ⊥ Harvard University Center for the Environment, Harvard University, Cambridge, Massachusetts 02139, United States ¶ Department of Chemistry and Nano Science, Division of Molecular and Life Sciences, College of Natural Sciences, Ewha Womans University, Seoul 120-750, Korea S Supporting Information *

ABSTRACT: Au nanoparticle decorated SrTiO3:Nb (Au/SrTiO3:Nb) has been demonstrated as a photoanode for photo-oxidation under visible light. The visible light photo-oxidation is a result of the generation of the hot electrons in the Au nanoparticles, which are subsequently injected into the SrTiO3:Nb conduction band. This leaves behind holes for electrochemical oxidation, which can oxidize a variety of chemical species with redox potentials ranging from 0 to 1.23 V vs RHE. The visible light response is most enhanced at the surface plasmon resonance energy of the Au nanoparticles, which we believe reflect the high optical density near the plasmon resonance for the sensitization. This work demonstrates a general route to impart visible light utilization to a wide band gap semiconductor for various energy conversion and environmental applications, including solar fuel generation and organic pollutant decomposition.

1. INTRODUCTION Titanates or titanium-containing oxides are one of the few photoanode materials with remarkable photostability, nontoxicity, and cost effectiveness.1,2 Titanates have been used for a variety of solar energy harvesting and environmental applications such as photoelectrochemical synthesis of carbon-neutral solar fuels,3 and removal of organic pollutants at low cost.4 However, titanates suffer from poor visible light absorption. Only the ultraviolet region (4%) of the solar irradiation spectrum can be absorbed by titanates because of its high intrinsic band gap regardless of the structure (3.2 eV for anatase and 3.0 eV for rutile TiO2)5 or composition (3.2 eV for SrTiO3 and BaTiO3).6 Significant efforts have been devoted to extend the absorption range of titanates, including bulk7,8 and surface sensitization.2 The bulk sensitization concept is based on the creation of a gap state. This is generally accomplished by doping, which thus far has seen little success due to the shortened photoexcited carrier lifetime as a result of increased recombination.9−11 Surface sensitization, on the other hand, is based on the idea of placing a separate absorber of lower energy photons, for example, a dye molecule2 or a quantum dot,12,13 on the surface of titanates. The electrons in these surface absorbers can be excited to the titanate conduction band upon illumination, leaving a hole behind for subsequent electrochemical oxidation.2 However, these surface sensitizations are severely limited by the oxidative corrosion14 and/or desorption of the sensitizers, though recent progress has shown that this © 2013 American Chemical Society

issue can be circumvented via the use of a proper ligand chemistry that facilitates faster hole consumption.15 In this study, we explore an alternative method. Instead of using dyes and/or quantum dots that require stabilization against oxidation, we use Au nanoparticles (AuNPs), which have an innate immunity against oxidative corrosion. Our general synthesis route to sensitizing titanate surfaces to visible light for photo-oxidation is based on a simple physical adsorption by the surface decoration of AuNPs, further simplifying the system studied. In addition, AuNPs exhibit surface plasmon resonance in the visible range, offering a path toward a highly efficient absorption of photons from 1.5 to 2.5 eV. We demonstrate the enhanced visible light photo-oxidation of water, ferrocyanide, hydrogen, and methanol, the results of which we will use to propose the origin of the observed photocurrent in the visible range. The use of metallic materials such as Au to sensitize semiconductors such as titanate relies on the principle of hot electron injection.16 Researchers have reported this concept via a variety of metal-semiconductor devices17,18 including AuNPs on TiO2, which have been demonstrated to photocatalytically decompose organic molecules19−21 and photo-oxidize water upon visible illumination.18,22−25 These devices operate by Received: April 26, 2013 Revised: July 9, 2013 Published: July 23, 2013 15532

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forming a Schottky junction,22−25 which harnesses photons by providing a barrier to prevent recombination and facilitate charge separation. In the study of TiO2 surfaces, there is a large uncertainty with the donor concentration and consequently the Fermi level, both of which depend on the intrinsic defect concentration.26,27 In this work, we elect to study the sensitizing effect on a well-defined junction using single crystalline Nb-doped SrTiO3 (SrTiO 3:Nb) with (100) termination, which have a known donor concentration and Fermi level. This gives us the ability to compare the experimental observation with the theoretical band structure at the Schottky junction, consequently enabling us to postulate the physical origin of the photo-oxidative effect. The use of AuNPs as sensitizers to SrTiO3:Nb is driven in part by the presence of the localized surface plasmon resonance (LSPR),28,29 which offers a large optical density for a single layer of Au nanoparticles and consequently increases the light absorption within the LSPR. In the limit at which the physical dimensions of the metal and the semiconductor are larger than the space charge layer, the Schottky barrier can be estimated as the difference between the work function of the metal and the electron affinity of the semiconductor. In the case of Au (20 nm thick) and SrTiO3:Nb at 0.01% Nb concentration, the Schottky barrier has been estimated to be about 1.1 eV, in agreement with the work function and the electron affinity.30 This Au/SrTiO3:Nb interface can yield a photocurrent at 0.3 V vs RHE, suggesting that a band bending at a potential of 0.3 V vs RHE is required for recombination prevention and that at a 15534

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states on SrTiO3:Nb0.01wt% surface, the shift could be a result of an additional dipole and/or surface state formed in Au/ SrTiO3:Nb0.01wt%. To assess this possibility, future works with Kelvin probe force microscopy in combination with X-ray absorption/photoemission would be essential. A more trivial possibility is that the shift arises from an artifact of the Mott− Schottky analysis, which is a convolution of all Au/electrolyte, Au/SrTiO3:Nb0.01wt%, and SrTiO3:Nb0.01wt%/electrolyte interfaces. To deconvolute those factors, systematic impedance analysis using more comprehensive models is indispensable; this is however beyond the scope of this paper. Although the information from the Mott−Schottky analysis does not yield us the exact energy diagram for the SrTiO3:Nb/ Au/electrolyte system, we can postulate the level alignment based on certain basic assumptions. We first focus on the SrTiO3:Nb/electrolyte system, which has the flatband potential at −0.53 V vs RHE. At 1.23 V vs RHE, this would translate to a Schottky barrier of 1.76 eV, which is significantly higher than the estimate (1.0 eV) based on the difference between the electron affinity/Fermi position of SrTiO3:Nb (4.1 eV) and the electrochemical potential of O2/OH− (5.1 eV). This discrepancy is likely a result of the reduced near-surface dopant concentration of SrTiO3:Nb, which has been reported previously. 53 When this finding is compared to the SrTiO3:Nb/Au/electrolyte system, the Schottky barrier of the SrTiO3:Nb/Au/electrolyte system is of lesser magnitude (1.66 eV) at the 1.23 V vs RHE. We emphasize that this is a convolution of three interfaces, which are SrTiO3:Nb0.01wt%/ electrolyte, Au/electrolyte, and Au/SrTiO3:Nb, all of which contribute to the current in the SrTiO3:Nb/Au/electrolyte system. Assuming that the SrTiO3:Nb0.01wt%/electrolyte interface is not significantly affected by AuNPs, the current path with a lower Schottky barrier can be attributed to a series of Au/electrolyte and Au/SrTiO3:Nb junctions. This forms the basis of our first assumption, which is that current contribution from the SrTiO3:Nb/electrolyte interface is minimal. Because the Au work function (∼5.1 eV)55 is similar to the O2/OH− redox at 0.1 M KOH (∼4.9 eV), we can further assume that the potential drop across Au and the electrolyte redox is minimal at 1.23 V vs RHE. This assumption can be extended to other potentials, provided that the SrTiO3:Nb/Au barrier is the limiting step in the electron transfer in the SrTiO3:Nb/Au/ electrolyte junction. This second assumption is supported by the observation that the SrTiO3:Nb/Au/electrolyte junction rectification is similar for both O2/OH− and Fe(CN)63−/ Fe(CN)64− redox (discussed in the next section). The Mott− Schottky response in the SrTiO3:Nb/Au/electrolyte junction is thus assumed to the SrTiO3:Nb/Au junction. Using this assumption, we extract the estimated donor concentration in the SrTiO3:Nb/Au junction to be 1.0 × 1018 cm−3, which is consistent with the theoretical value (3.11 × 1018 cm−3). Using the fact that the Fermi level of SrTiO3:Nb can be assigned to be close to the conduction band minimum in the SrTiO3:Nb/Au/ electrolyte junction configuration,56,57 and the 3.2 eV band gap of SrTiO3,6 our estimate of the energy diagram at 1.23 V vs RHE is shown in Figure 3b. Our proposed energy diagram is consistent with the expectation of the energetics of semiconductor/metal and semiconductor/redox Schottky junctions between SrTiO3:Nb/Au58 and SrTiO3:Nb/electrolyte,50 as shown electrochemically. The observed barrier height explains the behavior of the photoelectrochemistry of the Au/SrTiO3:Nb0.01wt% samples. In the dark, near 1.23 V vs RHE, there is virtually neither

potential above 0.3 V vs RHE, further band bending does not help with the hot electron collection (discussed below). We further study the wavelength dependence of the photoinduced OER by the Au/SrTiO3:Nb0.01wt% electrode by measuring the IPCE.42 As shown in Figure 2c, the IPCE of bare SrTiO3:Nb0.01wt% approaches zero for a light energy smaller than the SrTiO3:Nb0.01wt% band gap energy (approximately 400 nm). In contrast, the Au/SrTiO3:Nb0.01wt% electrode shows an increased IPCE in the visible regime, with a peak value at 570 nm. The IPCE exhibits a similar shape to the LSPR response as a result of the AuNP extinction (Figure 1d and Supporting Information, Figure S2), with the IPCE spectrum being blueshifted with respect to the extinction. We attribute this observation to the contribution of the plasmon excitation to the photo-oxidation, with the peak shift reflecting the higher injection probability with increasing photon energy.31,43 This result indicates that the AuNP decoration sensitizes SrTiO3:Nb0.01wt% for visible-light water oxidation via the plasmon resonance. It is noted that our proposed AuNP functionality is thus fundamentally different from the use of AuNP for photon concentration,44 where the effect is purely photonic in nature. Instead, we take advantage of the energetics of the Au/SrTiO3:Nb structure, where Au is also a hot electrongenerating absorber, instead of only an optical concentrator. We however cannot rule out that the AuNP could also serve in addition as a scattering center, especially to amplify the absorption of the substrate near 550 nm. The fact that AuNP has a higher absorption cross section than scattering below 100 nm45 however supports our hot electron hypothesis, although future works in exploring different substrates would be essential to reach this conclusion. To understand the origin of the photo-oxidation behavior, we investigate the energy alignment of SrTiO3:Nb, Au, and the electrolyte using Mott−Schottky analysis.46−49 Figure 3a shows

Figure 3. Mott−Schottky analysis and band bending of AuNPdecorated SrTiO3:Nb0.01wt%. (a) A Mott−Schottky analysis of SrTiO3:Nb0.01wt% with and without AuNP decoration. (b) Energy diagram of Au/SrTiO3:Nb0.01wt% in contact with OH−/O2 in electrolyte.

the Mott−Schottky plot for SrTiO3:Nb0.01wt% with and without AuNP decoration. The flat-band potential of bare SrTiO3:Nb0.01wt% was determined to be −0.53 V vs RHE, in agreement with previous findings.50−52 Assuming the dielectric constant of 300ε0,11,53 we estimate the donor concentration to be 1.2 × 1018 cm−3 for SrTiO3:Nb0.01wt%, also in agreement with the theoretical value of 3.11 × 1018 cm−3. With AuNPs, the flatband potential of Au/SrTiO3:Nb0.01wt% was shifted to −0.43 V vs RHE. The origin of this 0.1 V shift could be explained by multiple reasons and thus cannot be distinguished currently. One possibility is that the Au modifies the surface electronic structure of SrTiO3:Nb.53,54 As Au decoration can both induce charge transfer from SrTiO3:Nb0.01wt% and generate surface 15535

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For comparison, we include the photo-oxidation current due to the oxygen evolution. From here, we can see that the photooxidation current is higher for the Fe(CN)63−/Fe(CN)64− redox due to the faster Fe(CN)63−/Fe(CN)64− kinetics. The photocurrent gain (30%) over the sluggish oxygen evolution photokinetics demonstrates the need for a longer lifetime for hole injection into the O2/OH− redox, the result of which causes higher recombination and thus lower photocurrent. It is interesting to note that the open circuit potentials are roughly the same in either condition, despite the difference in the photocurrent. This again could reflect the Fermi level pinning by the surface state at that potential. Future experiments would be essential to verify this hypothesis. To further explore the generality of the visible-light sensitizing ability of the AuNP decoration of a SrTiO3:Nb0.01wt% electrode, we performed photo-oxidation experiments on two other chemical species with redox potentials near 0 V vs RHE. Our specific interests are H2 and methanol, which are used as a model system for the photo-oxidation of organic pollutants. This choice is driven by the near ∼0 V versus RHE redox of these two molecules, which have redox potentials of 0 and ∼0.03 V versus RHE, respectively. In these experiments, we use H2-saturated 0.1 M KOH as the electrolyte for H2 oxidation and Ar-saturated 0.1 M KOH for methanol oxidation, respectively. For both molecules, we observe dark reduction current at potentials at 0.6 V vs RHE, similar to the O2/OH− and the Fe(CN)63−/Fe(CN)64− cases. Given that the redox potentials at 0 and 0.03 V versus RHE, the observed reduction current cannot be H2 or methanol reduction. We therefore assign the observed current to a parasitic reduction. This parasitic reduction can counteract the photo-oxidation current, effectively reducing the efficiency of the photocurrent. Upon illumination with a 10 mW 532 nm laser source, we found that the photocurrent of H2 oxidation was nearly constant over a wide voltage range from 0.3 to 0.9 V, similar in both response and value to the Fe(CN)63−/Fe(CN)64− result (Figure 5a,b). In addition, the Au/SrTiO3:Nb0.01wt% electrodes for the photo-oxidation of H2 are active for a wide wavelength range from 400 to 700 nm (Figure 5c) with a visible IPCE peak at around the LSPR frequency, similar to that for the O2/OH− and Fe(CN)63−/Fe(CN)64− redox cases. For the visible light photo-oxidation of methanol, the photocurrent was found to be smaller than that of the H2 oxidation, as shown in Figure 5d,e. This difference could be due to the slower kinetics of the multielectron, multistep reaction of methanol oxidation. Nonetheless, it can be seen from the IPCE results (Figure 5c,f) that both H2 and methanol, like in the O2/OH− and Fe(CN)63−/Fe(CN)64− cases, benefit from the addition of AuNPs in the visible response, with maximum IPCE at approximately the LSPR frequency. AuNP sensitization thus nonselectively enhances the visible response to all the photooxidation of all the species studied in this paper and can thus serve as a general strategy to increase the visible light utilization, provided that the working potential maintains a barrier against reduction counter-current. The energy diagrams for H2 and methanol are however not straightforward to resolve. These two chemicals have much lower redox potentials than those of both Fe(CN)63−/ Fe(CN)64− and O2/OH−. Hence, for these two redox pairs, the energy diagram for Au/SrTiO3:Nb0.01wt% differs from that of Figure 3b. To propose the energy diagram, we rely on the same assumption from the O2/OH− case, which is that the Schottky barrier arises primarily from the Au/SrTiO3:Nb junction. The

oxidation due to the trapping of hole nor reduction due to the huge electron barrier (∼1.6 eV). During the negative-going dark CV scan to 0.6 V vs RHE, the barrier for electron injection decreases to below 0.9 V, where the thermionic emission of electron can cross the barrier (so-called forward-biased current). This results in a small but measurable reduction current below 0.5 V vs RHE. The requirement of the band bending acting as a barrier to thermionic emission sets the working range of our Au/SrTiO3:Nb system. Specifically, photo-oxidation can be effective only when the barrier is >0.9 V. This limits the working potential of our system to above 0.5 V vs RHE. Upon visible light illumination, AuNPs absorption leads to hot electron generation, in which some of the eletrons have sufficient energy to cross the Schottky barrier and be injected into the SrTiO3:Nb conduction band.43 This leaves behind holes, which can readily be injected into the electrolyte to oxidize species present near the surface.23 The open circuit voltage is thus determined by the potential at which the electron injection into the SrTiO3:Nb0.01wt% conduction band matches the thermionic emission of electrons across the barrier into the electrolyte. Although the Au/SrTiO3:Nb0.01wt% is sensitized in the visible regime and demonstrates enhanced catalytic activity with Au decoration, the system still owes a significant efficiency loss to the sluggish kinetics of the OER. To demonstrate this point, we investigated the photoresponse of the Au/SrTiO3:Nb electrode in a fast one-electron outer-shell ferrocyanide/ferricyanide redox couple, which has similar redox potential (1.2 V vs RHE) as the O2/OH− (1.23 V vs RHE). The fast redox minimizes the need to consider reaction kinetics and should thus help verify the nature of the O2/OH− band bending. In the dark, the Au/ SrTiO3:Nb0.01wt% electrode shows little current between 0.3 V and 0.96 vs RHE, with reduction current below 0.6 V vs RHE (Figure 4a, gray line), similar to the case in O2-saturated 0.1 M

Figure 4. Photo-oxidation of Fe(CN)64− on AuNP-decorated SrTiO3:Nb0.01wt%. (a) CV in dark and under 532 nm 10 mW laser illumination. (b) Photocurrent obtained by subtracting the dark curve from the illuminated curve shown in panel a, shown as reference is the same analysis for the O2 redox (Figure 2b). The Fe(CN)63−/ Fe(CN)64− redox was carried out in 5 mM Fe(CN)63−/Fe(CN)64−, 0.1 M Ar-purged KOH electrolyte.

KOH without the Fe(CN)63−/Fe(CN)64− redox couple. The similarity between the ORR and the Fe(CN)63−/Fe(CN)64− reduction measured in dark suggests that the band bending plays a much more significant role in controlling the reduction current than the catalytic activity in the case of the ORR. Figure 4a shows the CV behavior when the electrode is under a 10 mW 532 nm light illumination (green curve). The polarization shows a similar response to the CV in dark (gray line), except for an upward shift from the visible-light-induced oxidation current. We quantify this photo-oxidation current by subtracting the dark curve from the light curve (Figure 4b). 15536

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Figure 5. Photo-oxidation of H2 (a−c) and methanol (d−f) on the AuNP decorated SrTiO3:Nb0.01wt%. (a,d) CV in dark and under 532 nm 10 mW laser illumination. (b,e) Photocurrent obtained by subtracting the dark CV from the CV under laser illumination. (c,f) Incident photon to current efficiency at 0.86 V vs RHE at 10 nm grating resolution.

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resulting energy diagram (see Supporting Information, Figure S5) predicts a similar band bending as the O2/OH− case, with E = 0.43 V versus RHE being the Schottky barrier height. Using this proposed energy diagram, we explain the CV response as follows. Above 0.5 V versus RHE, there is no oxidation current in the dark due to the band bending preventing electron from being injected into the electrolyte. Upon illumination, hot electrons are generated, which can be injected and collected as photo-oxidation current, and in the process, oxidize H2 and methanol. Below 0.5 V versus RHE, competing parasitic reduction current that is not due to H2 or methanol sets in. This neutralizes the photo-oxidation current, causing the light current to decrease. To eliminate the parasitic reduction current, we subtract the dark from the light response (Figure 5b,e). The result demonstrates a uniform photocurrent persisting as low as 0.15 V vs RHE similar to that in the O2/ OH− and Fe(CN)63−/Fe(CN)64− cases (Figure 4b). Future studies on understanding the nature of the parasitic reduction current would be essential to boost the photoelectrochemical performance of the Au/SrTiO3:Nb junction.

ASSOCIATED CONTENT

S Supporting Information *

Details of particle size distribution, cyclic voltammograms, and IPCE information of bare SrTiO3:Nb0.01wt%, Fe(CN)63−/ Fe(CN)64− redox determination, energy diagram scheme for H2/methanol, prebackground-corrected cyclic voltammogram, and Mott−Schottky analysis This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(J.S.) E-mail: [email protected]. Tel: +1 (607) 255-9617. (Y.S.-H.) Tel: +1 (617) 253-2259. E-mail: [email protected]. Present Address #

J.S.: Department of Materials Science and Engineering, Cornell University, Ithaca, New York, 14853, United States.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Patrick Brown and Joel Jean are acknowledged for their help with the Au film preparation. Zhenxing Feng is acknowledged for his help in AFM measurements. J.S. acknowledges the Ziff Environmental Fellowship from Harvard University Center for the Environment. This work carried out at MIT was supported in part by the MRSEC Program of the National Science Foundation under Award No. DMR-0819762, and we thank the King Fahd University of Petroleum and Minerals in Dharam, Saudi Arabia, for funding the research reported in this paper through the Center for Clean Water and Clean Energy at MIT and KFUPM. K.A.S. was supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1122374.

4. CONCLUSIONS (001)-Oriented SrTiO3:Nb0.01wt% substrates with a band gap of 3.2 eV were sensitized to visible light for photo-oxidation applications by AuNP decoration. The photo-oxidation activity is ascribed to the formation of a Schottky junction between SrTiO3:Nb0.01wt% and Au, where the injection of hot electrons from Au to SrTiO3:Nb0.01wt% conduction band gives rise to the photocurrent. The device can carry out photo-oxidation of water, ferricyanide, hydrogen, and methanol, which demonstrate the generality of this strategy. The visible light response was found to occur within a wide range of visible light from 400 to 700 nm, with peak response near 580 nm, corresponding to the plasmonic excitation of the AuNPs. This work demonstrates a general route to sensitize a large band gap semiconductor to solar light photo-oxidation of a wide range of chemicals.

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