Photocatalytic Hydrogen Generation Efficiencies in One-Dimensional

Oct 16, 2012 - Furthermore, Scheme 1 shows that the ECB of CdSe and CdS are both more negative than E0 such that MV+• radical cation formation is ex...
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Photocatalytic Hydrogen Generation Efficiencies in One-Dimensional CdSe Heterostructures Pornthip Tongying,† Vladimir V. Plashnitsa,† Nattasamon Petchsang,† Felix Vietmeyer,† Guillermo J. Ferraudi,‡ Galyna Krylova,*,† and Masaru Kuno*,† †

Department of Chemistry and Biochemistry, University of Notre Dame, 251 Nieuwland Science Hall, Notre Dame, Indiana 46556, United States ‡ Notre Dame Radiation Laboratory, University of Notre Dame, 338 Radiation Research Building, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: To better understand the role nanoscale heterojunctions play in the photocatalytic generation of hydrogen, we have designed several model one-dimensional (1D) heterostructures based on CdSe nanowires (NWs). Specifically, CdSe/CdS core/shell NWs and Au nanoparticle (NP)-decorated core and core/shell NWs have been produced using facile solution chemistries. These systems enable us to explore sources for efficient charge separation and enhanced carrier lifetimes important to photocatalytic processes. We find that visible light H2 generation efficiencies in the produced hybrid 1D structures increase in the order CdSe < CdSe/Au NP < CdSe/CdS/ Au NP < CdSe/CdS with a maximum H2 generation rate of 58.06 ± 3.59 μmol h−1 g−1 for CdSe/CdS core/shell NWs. This is 30 times larger than the activity of bare CdSe NWs. Using femtosecond transient differential absorption spectroscopy, we subsequently provide mechanistic insight into the role nanoscale heterojunctions play by directly monitoring charge flow and accumulation in these hybrid systems. In turn, we explain the observed trend in H2 generation rates with an important outcome being direct evidence for heterojunction-influenced charge transfer enhancements of relevant chemical reduction processes. SECTION: Spectroscopy, Photochemistry, and Excited States

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radial distances.10 Attesting to this, CdSe nanoribbons show enhanced photocatalytic activities toward H2 generation when compared to bulk CdSe.6 Furthermore, we expect that use of 1D NWs may enhance H2 production rates over those of comparable zero-dimensional (0D) materials because their volume normalized absorption cross sections are up to an order of magnitude larger.11,12 Nanostructures are also interesting because of the relative ease of making heterojunctions. This can dramatically improve the spatial separation of photogenerated charges and can enhance carrier lifetimes.13,14 Representative examples include both core/shell15,16 and dumbbell17,18 morphologies. In the case where the band alignment between two SCs follows a staggered type-II alignment, photogenerated electrons localize on one material while complementary holes localize on the other, promoting charge separation. Rather than utilize an SC heterojunction, a metal/SC interface can instead be pursued. In practice, this entails decorating low-dimensional SC nanostructures with noble metal nanoparticles (NPs) (e.g., Au,19−21 Pt,22−25 Pd23) as

ydrogen generation via photocatalytic water splitting has gained significant attention as a sustainable fuel generation process that utilizes two renewable resources: water and sunlight.1−3 Semiconductor (SC) systems used in these reactions must satisfy numerous requirements in order to be effective. In particular, they must possess suitable conduction and valence band potentials (ECB and EVB) that are reducing enough to produce hydrogen from water [ECB ≤ −0.41 V versus normal hydrogen electrode (NHE), pH = 7] while oxidizing enough to yield oxygen (EVB ≥ +0.82 V versus NHE, pH = 7). Furthermore, SC band gaps (Eg) must absorb most of the solar spectrum (i.e., Eg ∼ 2 eV)4 while simultaneously maintaining these appropriate ECB and EVB levels. Such stringent requirements have led researchers to explore the use of bulk heterostructures5 and, more recently, low dimensional materials6 as ways to overcome limitations of existing bulk systems. In this regard, nanostructures have attracted significant interest because of their size- and morphology-dependent optical and electrical properties. For the case of one-dimensional (1D) nanostructures, potential advantages of NWs or nanorods (NRs) include large surface areas, tunable band gaps,7 large absorption efficiencies,8 large biexciton lifetimes, suppressed Auger recombination kinetics,9 and enhanced charge separation efficiencies, stemming from a combination of sizable longitudinal carrier mobilities and short © 2012 American Chemical Society

Received: October 10, 2012 Accepted: October 16, 2012 Published: October 16, 2012 3234

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well as their alloys (e.g., PtFe,26 PtCo,19 PtNi19). Motivating this are the more positive metal NP Fermi levels relative to those of many SCs.27 This can then enhance charge separation efficiencies due to efficient SC/metal electron transfer.28 As added benefits, metal NPs are catalytically active,29−31 act as potential electron reservoirs28 and can lead to plasmonic enhancements of the nanostructure absorption.32 To better understand the role nanoscale heterojunctions play in the photocatalytic generation of hydrogen, we have designed several model 1D heterostructures based on CdSe nanowires (NWs).15,16 This includes CdSe/CdS core/shell NWs as well as Au NP-decorated core and core/shell NWs. Differences in the photocatalytic activities of these four systems (CdSe, CdSe/ CdS, CdSe/Au NP, and CdSe/CdS/Au NP) have been explained using femtosecond transient differential absorption (TDA) spectroscopy. The technique allows us to directly monitor the flow of charge and accumulation in the above systems and has enabled us to develop mechanistic insight into the roles that nanoscale heterojunctions play in relevant photocatalytic processes. An important outcome of this study is direct evidence for type-I heterojunction related charge transfer enhancements of relevant chemical reduction processes. CdSe NWs were synthesized using a procedure adapted from Puthussery et al.33 Details of the procedure can be found in the Supporting Information (SI). Resulting NWs have mean diameters of d = 14.0 ± 2.6 nm with lengths on the order of ∼6 μm (Figure 1a,b). A portion of these wires was then coated

Both CdSe and CdSe/CdS NWs were decorated with Au NPs using a photocatalytic deposition procedure based on irradiating the NWs in the presence of AuCl3 and octadecylamine (ODA).35 In the approach, photogenerated electrons created in the wires reduce surface adsorbed Au3+ ions while ODA acts as a hole scavenger. The photocatalytic deposition of Au is efficient and results in NWs uniformly decorated with NPs having an average diameter of 4.9 ± 1.1 nm (sample size = 100) on CdSe and 2.3 ± 0.2 nm (sample size = 100) on CdSe/ CdS. This is illustrated in Figure 2. NPs primarily deposit on

Figure 2. Low- and high-resolution TEM images of Au NP-decorated (a,b) CdSe and (c,d) CdSe/CdS NWs. Arrows in panel d point to individual Au NPs on the CdS shell.

the CdS shell in the case of core/shell NWs due to their dense coating. Additional transmission electron microscopy (TEM) images of Au NP-decorated core and core/shell wires (Figures S2−S3) as well as details of the procedure can be found in the SI. Prior to actual H2 generation measurements, the produced 1D systems were tested with methyl viologen (MV2+) to probe their photoinduced electron transfer efficiencies. MV2+ is an excellent system in this regard because it possesses a pH independent reduction potential of E0 (MV2+/MV+•) = −0.446 V,36 similar to that of H+/H2 at pH = 7. Furthermore, Scheme 1 shows that the ECB of CdSe and CdS are both more negative than E0 such that MV+• radical cation formation is expected in all systems. As an added benefit, the MV2+ + e− → MV+• reduction can be monitored in situ using the discernible rise of the MV+• absorption at λmax = 605 nm [corresponding molar extinction coefficient ε(λmax) = 1 × 104 M−1 cm−1].37 We find that all 1D hybrids produce MV+• in qualitative agreement with the energetics outlined in Scheme 1. Figure S4 illustrates results from this study showing a comparison of MV+• accumulated during 60 min of irradiation. Resulting MV2+ reduction efficiencies increase in the order: CdSe/Au NP < CdSe/CdS/Au NP < CdSe/CdS < CdSe. Interestingly, CdSe NWs show the largest efficiencies, while Au NP-decorated

Figure 1. Low- and high-resolution TEM images of (a,b) CdSe and (c,d) CdSe/CdS NWs.

with CdS to produce core/shell species. CdS shell thicknesses of approximately 5 nm were achieved as shown in Figure 1c,d as well as Figure S1 of the SI. The shell is porous and is composed of a dense network of CdS nanocrystals. This shell morphology is similar to that previously reported by Goebl et al.16 as well as by Petchsang et al. for ZnSe/CdSe NWs.34 Li et al. has similarly reported a bristlelike CdS NR shell on CdSe NW cores.15 3235

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Scheme 1. Potential Energy Level Diagram Showing the Conduction and Valence Flat Bands of Bulk CdSe and CdS38a

a

Also shown are the Au Fermi level39 and MV2+ reduction potentials.37

Figure 3. Comparison of CdS, CdSe, CdSe/CdS, CdSe/Au NP, and CdSe/CdS/Au NP NW photocatalytic efficiencies for H2 generation. Inset: photocatalytic H2 generation from CdSe/CdS core/shell NWs versus illumination time.

CdSe NWs show the least. This trend will be rationalized following detailed TDA experiments discussed below. All NW systems were now studied for their photocatalytic H2 generation efficiencies. Details of these experiments can be found in the SI. We find that CdSe NWs irradiated with visible light (λexc ≥ 320 nm, using a 0.01 M CuSO4 filter, 30 mW/cm2 at 500 nm) produce H2 at a rate of 1.98 ± 0.55 μmol h−1 g−1. CdSe NWs decorated with Au NPs exhibit a comparable H2 generation rate of 2.61 ± 0.75 μmol h−1 g−1. When CdSe/CdS core/shell NWs are tested, bubbles of H2 are continuously produced during irradiation. A video illustrating this can be found in the SI. The associated H2 generation rate (58.06 ± 3.59 μmol h−1 g−1) is 30 times larger than that of bare CdSe NWs. Corresponding CdSe/CdS/Au NP nanohybrids show a comparable H2 generation rate of 38.05 ± 6.26 μmol h−1 g−1. Note that observed H2 generation rates may be suppressed by the presence of residual organic surfactants on the NW surface despite extensive washing of samples. Core/shell NWs also appear relatively stable against photocorrosion even after 15 h of irradiation. This is demonstrated by TEM images of the wires taken after the experiment, revealing intact core/shell species (Figure S6). A summary of measured H2 generation rates for the four systems follows CdSe (1.98 ± 0.55 μmol h−1 g−1) < CdSe/Au NP (2.61 ± 0.75 μmol h−1 g−1) < CdSe/CdS/Au NP (38.05 ± 6.26 μmol h−1 g−1) < CdSe/CdS (58.06 ± 3.59 μmol h−1 g−1) NWs and is shown in Figure 3. For comparison purposes, we have also determined the H2 generation rate of CdS NWs to be ∼6.92 μmol h−1 g−1. By monitoring the H2 evolved from a suspension of CdSe/CdS NWs over 60 h [Figure 3 (inset)], an effective H2 generation rate of 0.14 μmol/h is found using 5.28 mg of catalyst (i.e., 26.52 ± 1.64 μmol h−1 g−1).40 We find that CdSe/CdS core/shell NWs exhibit the largest H2 generation efficiencies with CdSe NWs exhibiting the least. While the observed trend can readily be argued to result from the improvement of charge separation efficiencies and carrier lifetimes, as previously noted for core/shell nanocrystals,41 there is a simultaneous need to rationalize the observed trend seen in the earlier MV2+ study. Notably, that experiment showed CdSe NWs to possess the largest reduction efficiency of all systems studied. Thus, enhanced charge separation and

carrier lifetimes in core/shell NWs alone cannot fully explain the chemical reduction efficiencies of these 1D nanostructures. To explain what are apparently contradicting trends between the MV2+ reduction and H2 generation experiments, we conduct detailed ultrafast TDA measurements for direct monitoring of electron lifetimes and charge separation processes in all systems. TDA is an important spectroscopic tool for probing nanostructures9 since it allows one to track the movement and/or accumulation of photogenerated charges by monitoring the band edge bleaches of constituent materials. This is especially important in the hybrid 1D systems here given the multitude of heterojunctions present. It is also of particular use in II−VI materials due to sizable electron/hole effective mass differences in CdS and CdSe. This means that bleaches in the TDA experiment predominantly reflect state filling due to the presence of conduction band electrons.42 Figure 4a first compares the linear absorption of CdSe and CdSe/CdS core/shell NWs with their respective transient absorption spectra (λexc = 387 nm, 5.2 and 7.2 ps delays for CdSe/CdS and CdSe, respectively). In CdSe, a band edge peak occurs at ∼687 nm (∼1.80 eV) along with a second prominent feature at ∼570 nm (∼2.18 eV). This latter absorption is attributed to CdSe’s split-off hole.43 In CdSe/CdS, the band edge appears blueshifted. This is likely due to partial alloying of the NW surface with sulfur during the overcoating procedure (see SI). All TDA bleaches occur at the same frequencies as transitions in the absorption. Absorption and TDA spectra of CdS NWs were also taken as a control (λexc = 387 nm, 2.4 ps delay). Figure 4a shows that CdS NWs possess a band edge transition at ∼480 nm (∼2.58 eV). In core/shell wires, the CdS shell absorbs further to the blue at ∼448 nm (∼2.77 eV). This can be explained by confinement effects experienced by nanocrystals composing the shell. As with CdSe and CdSe/CdS, corresponding TDA bleaches occur at the same frequencies as features in the ensemble absorption spectrum. For Au NP-decorated core and core/shell NWs, no apparent differences in the CdS or CdSe linear absorption and TDA responses are seen. Absorption spectra comparing Au NP decorated species to their undecorated counterparts are shown in Figure S7 of the SI. Note that the expected plasmon 3236

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Figure 4. (a) Comparison of UV−visible absorption and corresponding TDA spectra of CdSe, CdS, and CdSe/CdS core/shell NWs. Numbers in parentheses indicate the delay at which the TDA spectra were acquired. (b) Comparison of CdSe band edge bleach kinetics in core/shell NWs when excited at 387 and 560 nm. (c) Comparison of CdS and CdSe band edge bleach kinetics in CdSe/CdS core/shell NWs when excited at 387 nm.

bleach at ∼680 nm (Figure 4c, left). The concurrent decrease of the CdS transient with an increase of the CdSe band edge bleach therefore demonstrates electron transfer from CdS into CdSe. This is in line with their expected type-I band alignment15,46 and is further corroborated by analogous TDA results from Goebl et al.16 Next, significant slowing of the CdSe bleach recovery occurs on both picosecond and nanosecond time scales. The extracted fast (slow) lifetime is 104 ps (2.45 ns) as illustrated in Figure 4c and more clearly in Figure 4b (λexc = 387 nm, top trace). The most notable result is a sizable increase of the long-lived electron fraction available to carry out subsequent reduction chemistries. Specifically, the charge fraction remaining in the wire over the course of a nanosecond increases from ∼8% in bare CdSe NWs to approximately 33% in the core/shell species. There are at least two reasons for this increase. The most obvious is the improved surface passivation of CdSe by CdS. Namely, the CdS shell suppresses underlying nonradiative recombination processes in the CdSe core related to the presence of surface defects. This can then lead to emission quantum yield (QY) enhancements as previously seen by Li et al.15 and verified here through single NW QY measurements [CdSe: QY=0.75 ± 0.48%, sample size=18; CdSe/CdS: QY=1.32 ± 0.70%, sample size=15]. An alternative explanation is the existence of a type-I interface between CdSe and CdS where the CdS shell can contribute photogenerated electrons to the core when excited due to the existence of a favorable band offset. These additional charges then add to those created in CdSe and can subsequently participate in relevant reduction chemistries. Additional TDA measurements were therefore conducted to investigate both possibilities, using λexc = 560 nm to excite only the core of CdSe/CdS NWs. The results are shown in Figure 4b (middle trace) where the graph clearly illustrates an increase

resonance of Au NPs, generally seen at ∼530 nm, is not observed. This is likely due to their small mean size, their relatively broad ensemble size distribution, and potential delocalization of metal electrons into the SC, which suppresses metal plasmon resonances, as previously noted.44 We now analyze TDA bleach recoveries for all four NW systems to develop a mechanistic picture that simultaneously rationalizes observed trends in both MV2+ reduction and H2 generation experiments. This entails analyzing the band edge decay kinetics of CdSe and CdS to directly monitor the flow and/or accumulation of photogenerated charges as well as to estimate their effective lifetimes. CdSe NWs are an important reference system in this regard since their photophysics and carrier dynamics have been extensively studied, making it a model 1D nanostructure.45 In Figure 4b, the CdSe band edge bleach dynamics (λexc= 387 nm, bottom trace) displays two components: a dominant picosecond recovery (relative fit amplitude ∼ 57%; τ ∼ 88 ps) and a longer nanosecond component (relative fit amplitude ∼ 43%; τ ∼ 686 ps). At 1 ns, 8% of the initial bleach magnitude remains. The fast decay contains important information about intrinsic recombination and charge separation processes in CdSe.42 The slower component, on the other hand, reflects the presence of long-lived electrons within the wire. These latter charges are likely responsible for both the MV2+ reduction and H2 evolution chemistries being studied given their lifetimes comparable to characteristic time scales of diffusion-limited processes. In core/shell NWs, significant differences are observed. First, by monitoring the CdS band edge bleach at ∼480 nm [Figure 4c (left)] we observe rapid carrier cooling, seen by growth of the bleach over the course of ∼1.6 ps. Its subsequent decay contains two fast components (τ ∼1 ps and τ ∼39 ps), accompanied by simultaneous growth of the CdSe band edge 3237

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recoveries. Both possess a fast (CdSe/CdS: τ ∼ 104 ps versus CdSe/CdS/Au NP: τ ∼ 126 ps) and a slow (CdSe/CdS: τ ∼ 2.45 ns versus CdSe/CdS/Au NP: τ ∼ 7.91 ns) decay component. Furthermore, the long-lived charge fraction in Au NP-decorated CdSe/CdS is near identical, comprising ∼45% of the total bleach (versus ∼33% for CdSe/CdS NWs). From this, we conclude that no significant interaction exists between the Au NPs and the CdSe core. As a corollary, the lack of a significant increase in the photocatalytic activity of Au NPdecorated core/shell species again allows us to conclude that CdSe is the primary reducing species responsible for the observed core/shell reduction chemistries. We now explain the different trends seen in the MV2+ and H2 generation experiments. While the observed order of H2 generation efficiencies (Figure 3) can be rationalized using the above TDA results (i.e., core/shell species possess the best combination of long carrier lifetimes and absolute numbers of long-lived carriers), the observed MV2+ reduction efficiencies provide some notable differences (Figure S4). Particularly surprising is the large discrepancy between the bare CdSe and CdSe/CdS core shell efficiencies since enhanced carrier lifetimes in the latter should translate into larger MV2+ reduction efficiencies. In practice, though, CdSe NWs perform significantly better. We explain this reversal by recalling from the TDA experiments that, in all cases, CdSe is the primary reducing species involved in the reduction processes being studied. Given this, the decrease in MV+• radical cation buildup in core/ shell NW studies likely illustrates mass transport issues caused by the presence of a nanocrystal shell. Namely, MV2+ molecules are presumed to be hindered from reaching the CdSe core by a dense CdS nanocrystal network. In the H2 generation experiments, this does not represent a significant hindrance given the smaller size of the proton (H2 covalent radius 0.37 Å vesus MV2+ molecular radius 3.6 Å50). The reversed trend between CdSe and CdSe/CdS NWs in the MV2+ reduction experiments therefore highlights the importance of shell porosity when dealing with sterically hindered species subjected to reduction by the NW core. We have designed several model 1D systems based on CdSe NWs to elucidate the influence nanostructure morphology as well as nanoscale heterojunctions have in the photocatalytic generation of hydrogen. These experiments reveal that CdSe/ CdS core/shell NWs possess the largest photocatalytic H2 generation efficiencies of the studied species with an experimental H2 production rate of 58.06 ± 3.59 μmol h−1 g−1. This is 30 times better than that of bare CdSe NWs. Detailed TDA measurements follow the flow and accumulation of photogenerated charges and, in turn, explain the origin of these enhancements. In all cases, CdSe is the active species responsible for relevant chemical reduction processes, despite the presence of a CdS shell in core/shell NWs, possessing favorable electron transfer energetics. This stems from ultrafast charge transfer between the shell and the core in CdS/CdSe NWs and supports the existence of a type-I interface between the two materials. Furthermore, while enhanced H2 generation efficiencies can generally be explained in terms of increased carrier lifetimes, a more quantitative accounting of H 2 generation efficiencies requires the participation of sizable numbers of photoexcited electrons from the CdS shell. This illustrates that carrier lifetimes alone do not determine the chemical reduction efficiencies of nanoheterostructures and aid detailed quantitative assessments of the role heterojunctions

in the long-lived carrier fraction relative to the case of bare CdSe NWs excited at 387 nm (∼16% versus ∼8%). This shows that introducing a CdS shell indeed increases the long-lived carrier lifetime. However, the remaining charge fraction is still smaller than that observed with core/shell wires (∼33%) when both the core and the shell are excited. From this, we conclude the following: (a) Adding CdS to the surface of CdSe NWs passivates their surface defects, effectively increasing electron lifetimes.41 (b) When excited, the CdS shell contributes electrons to the CdSe core. This indirectly supports the existence of a type-I interface. (c) The combination of the CdSe (surface) defect passivation as well as the introduction of additional carriers from the CdS shell, in turn, leads to large enhancements of the observed core/shell H2 generation rate relative to those of bare CdSe NWs. (d) Finally, CdSe is the dominant reducing species responsible for both MV 2+ reduction and H2 generation. This is an important conclusion since, energetically speaking, both CdSe and CdS can reduce MV2+ and protons (Scheme 1). However, the ultrafast electron transfer between CdS and CdSe (Figure 4c, left) likely prevents this. Thus, unlike core/shell nanocrystals,41 proton reduction primarily occurs at the surface of CdSe (core) NWs. We now consider the charge transient kinetics of Au NP decorated core and core/shell wires. This is because adding Au NPs can, in principle, improve charge separation efficiencies due to electron transfer/subsequent accumulation between the SC (CdSe or CdS in the case of core/shell wires) and the metal.21,28,47 For Au NP-decorated CdSe NWs, the CdSe band edge transient shows a fast and a slow decay component (Figure S8). The fast component consists of a ∼6.9 ps (relative fit amplitude ∼ 28%) and a 63 ps (relative fit amplitude ∼ 48%) contribution, both faster than the early component (τ ∼ 88 ps) of bare CdSe wires. While this rate enhancement can tentatively be ascribed to SC-to-metal electron transfer, what’s interesting and more notable is that the subsequent long-lived bleach has a comparable lifetime to that of bare CdSe NWs [τ ∼ 856 ps (Relative fit amplitude ∼ 24%) versus 686 ps]. As a consequence, it retains a similar ∼7% contribution to the overall decay on the ns time scale. This suggests that, even if electron injection into Au were to occur, the absolute number of charges transferred cannot be large, possibly due to Fermi level equilibration. Note that a decrease of the fast bleach recovery time scale can alternatively be explained in terms of energy transfer between the SC and the metal.21 In principle, electron injection into Au can be monitored by measuring a blueshift of its plasmon resonance.48 However, this is precluded in the current study by the absence of an apparent plasmon resonance in all Au-coated samples (Figure S7). Despite this, the similar hydrogen generation efficiencies of bare CdSe NWs compared to their Au NP-decorated counterparts allows us to conclude that CdSe is the primary photocatalytic species in either system. Any electron injection from CdSe into Au therefore does not significantly enhance H2 generation rates. The conclusion is made even more apparent in the earlier MV2+ reduction studies where CdSe NWs and Au NP decorated CdSe NWs were found to have the largest and smallest reduction efficiencies, respectively, the latter corroborating prior studies showing efficient MV+•-to-metal back electron transfer.49 Analyzing the transient absorption kinetics of core/shell and Au NP-decorated core/shell NWs leads to the same conclusion. Namely, CdSe/CdS NWs and their Au NP decorated counterparts show near identical CdSe band edge bleach 3238

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play in chemical reduction processes involving these and other photocatalytically active species.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the NW synthesis, processing, and characterization. Additional TEM images of CdSe/CdS NWs. Additional TEM images of Au NP decorated core and core/shell NWs. Details of the Au NP decoration process. Spectrum profile of Xe arc lamp. Bar graph showing MV2+ reduction efficiency. Details of the H 2 generation experiments. Movie illustrating H 2 generation from core/shell NWs. TEM images of CdSe/CdS NWs before and after H2 generation experiments. Linear absorption spectra of Au NP decorated core and core/shell species. Details of TDA measurements. TDA measurements on Au NP decorated core and core/shell wires. 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] (G.K.); [email protected] (M.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by the Center for Sustainable Energy at Notre Dame (cSEND). The authors thank the Center’s Materials Characterization Facility (MCF), the Notre Dame Integrated Imaging Facility (NDIIF), as well as the Notre Dame Radiation Laboratory/Department of Energy (DOE), Office of Basic Energy Sciences, for use of their facilities and equipment. P.T. thanks the Royal Thai Government Scholarships for partial financial support and Prof. Prashant V. Kamat for useful discussions.

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ABBREVIATIONS NWs, nanowires; NP, nanoparticle; SC, semiconductor; CB, conduction band; VB, valence band; NHE, normal hydrogen electrode; NRs, nanorods; TDA, transient differential absorption; ODA, octadecylamine; MV2+, methyl viologen; MV+., methyl viologen radical; QY, quantum yield

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REFERENCES

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