Plasmonic Aerogels as a Three-Dimensional Nanoscale Platform for

Article pubs.acs.org/Langmuir

Plasmonic Aerogels as a Three-Dimensional Nanoscale Platform for Solar Fuel Photocatalysis Paul A. DeSario,*,† Jeremy J. Pietron,*,† Adam Dunkelberger,† Todd H. Brintlinger,‡ Olga Baturina,† Rhonda M. Stroud,‡ Jeffrey C. Owrutsky,† and Debra R. Rolison† †

Code 6100, Chemistry Division and ‡Code 6300, Material Science & Technology Division, U.S. Naval Research Laboratory, Washington, D.C. 20375, United States S Supporting Information *

ABSTRACT: We use plasmonic Au−TiO2 aerogels as a platform in which to marry synthetically thickened particle−particle junctions in TiO2 aerogel networks to Au∥TiO2 interfaces and then investigate their cooperative influence on photocatalytic hydrogen (H2) generation under both broadband (i.e., UV + visible light) and visible-only excitation. In doing so, we elucidate the dual functions that incorporated Au can play as a water reduction cocatalyst and as a plasmonic sensitizer. We also photodeposit non-plasmonic Pt cocatalyst nanoparticles into our composite aerogels in order to leverage the catalytic water-reducing abilities of Pt. This Au−TiO2/Pt arrangement in three dimensions effectively utilizes conduction−band electrons injected into the TiO2 aerogel network upon exciting the Au SPR at the Au∥TiO2 interface. The extensive nanostructured high surfacearea oxide network in the aerogel provides a matrix that spatially separates yet electrochemically connects plasmonic nanoparticle sensitizers and metal nanoparticle catalysts, further enhancing solar-fuels photochemistry. We compare the photocatalytic rates of H2 generation with and without Pt cocatalysts added to Au−TiO2 aerogels and demonstrate electrochemical linkage of the SPR-generated carriers at the Au∥TiO2 interfaces to downfield Pt nanoparticle cocatalysts. Finally, we investigate visible light−stimulated generation of conduction band electrons in Au−TiO2 and TiO2 aerogels using ultrafast visible pump/IR probe spectroscopy. Substantially more electrons are produced at Au−TiO2 aerogels due to the incorporated SPR-active Au nanoparticle, whereas the smaller population of electrons generated at Au-free TiO2 aerogels likely originate at shallow traps in the high surface-area mesoporous aerogel.

I. INTRODUCTION Solar-driven water splitting at nanoscale photocatalytic semiconductors offers a high-payoff pathway toward developing carbon-neutral fuels and is thus an extensively explored area of research.1−4 Our approach constructs surface plasmon resonance (SPR)-active catalytic aerogels and uses their inherent compositional and interfacial design flexibility to probe the critical mechanistic challenges affecting photocatalytic solar hydrogen (H2) generation. The primary variables limiting the performance of photocatalytic semiconductors include (1) inefficient absorption of incident light, especially at visible wavelengths; (2) limited lifetimes of photogenerated, reactive electron−hole pairs; and (3) poor catalytic efficiency for the reactions of interest.1−4 Titanium dioxide is one of the most heavily investigated semiconductors for solar fuel photocatalysis because it is inexpensive, chemically stable, and has conduction and valence band edges suitable for reductive and oxidative water splitting.1−7 A key barrier to applying TiO2 as an efficient solar-fuels photocatalyst is that its wide band gap (3.2 eV for anatase TiO2) requires ultraviolet (UV) light of λ < 400 nm for the excitation of reactive electron−hole pairs.1−7 One chemically robust means of sensitizing TiO2 to visible light is adsorbing SPR-active metal nanoparticles at the TiO2 surface to © 2017 American Chemical Society

generate reactive electron−hole pairs to drive visible-light photochemistry8−14 and solar-fuels chemistry.15−20 As a materials design platform for SPR photocatalysis, composite aerogels provide a high specific surface area (∼100− 1000 m2 g−1) that amplifies available reactive sites, whereas their mesoporous network facilitates the rapid flux of reactants to and from those reactive sites.21−29 We demonstrated previously that Au nanoparticles incorporated into the titania network (3D Au−TiO2)21,30 plasmonically sensitize the visiblelight photooxidation of water31 and methanol.32 The Au∥TiO2 interfacial reaction zone in 3D Au−TiO2 differs from that in typical Au/TiO2 catalysts in that each Au nanoparticle contacts more than one TiO2 nanoparticle, creating multiple Au∥TiO2 interfacial contacts per Au nanoparticle.21,30 This extended Au∥TiO2 interphase is advantageous for SPR-driven photocatalysis because the probability of successful hot carrier injection increases with the number of interfacial contacts33 and Special Issue: Fundamental Interfacial Science for Energy Applications Received: April 1, 2017 Revised: July 17, 2017 Published: July 19, 2017 9444

DOI: 10.1021/acs.langmuir.7b01117 Langmuir 2017, 33, 9444−9454

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Langmuir Table 1. Physical Characteristics of Standard [l] and Strengthened [h] TiO2 and Au−TiO2 Aerogels aerogel

BET surface area (m2 g−1)c

ave pore diameter (nm)

cumulative pore volume (cm3 g−1)

average Au diameter (nm)

SPR maximum (nm)

TiO2[l]a TiO2[h]a 8.5% Au−TiO2[l]b 4.0% Au−TiO2[l]b 2.2% Au−TiO2[l] 2.2% Au−TiO2[h]

139 128 133 144 127 149

24.0 10.8 19.3 24.7 16.6 11.0

1.0 0.46 0.77 1.1 0.73 0.53

4.7 ± 1.5 4.2d 4.5 ± 3.2 4.8 ± 2.7

545 547 565 565

a Structural data used with permission from ref 35. bStructural data used with permission from ref 31. cThe measurement technique features ca. 1− 2% precision, and the batch-to-batch variability for the various aerogel syntheses is ca. 10%. dThe average Au nanoparticle diameter in this sample was estimated using XRD data and the Scherer equation (ref 35); uncertainty was not calculated.

Figure 1. Incremental pore size distribution (Harkins and Jura analysis of N2 physisorption isotherm) of (A) 3D Au−TiO2[h] aerogels and (B) 3D Au−TiO2[l] aerogels with 2.2% Au weight loading.

II. RESULTS AND DISCUSSION II.A. Physical Characterization of Network-Modified Au−TiO2 Aerogels. We synthesized TiO2 and Au−TiO2 aerogels at the upper and lower gelation limits of the Ti precursor concentration (0.3 g mL−1 [h] and 0.2 g mL−1 [l], respectively, of titanium isopropoxide, Ti(iOPr)4) to modify the interconnect thickness between the networked nanoparticles (TiO2∥TiO2). We chose Au loadings of ∼2−8.5 wt % in order to maintain a consistent nanoparticle size and dispersion.21,31 The Au-free aerogels prepared at a high precursor ratio (denoted TiO2[h]) retain the high specific surface area (∼125− 150 m2 g−1) and cumulative pore volume (>0.5 cm3 g−1) characteristic of TiO2 aerogels prepared at the low precursor ratio, TiO2[l] (Table 1, Figure S1). Regardless of the precursor ratio and presence of Au, pore diameters are distributed in the 8−30 nm range (Figure 1). After calcination at 425 °C, all aerogels are anatase in crystal habit (Figure 2), with average crystallite diameters of ∼8.5−9.5 nm (Figures S2 and S3). The Au particle size in Au−TiO2 was analyzed using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) analysis rather than X-ray diffraction (XRD) because of the overlap of Au reflections with anatase reflections. The HAADFSTEM data reveal well-dispersed ∼5-nm-diameter Au with a small number of larger aggregates evident (Figure 3, Table 1), consistent with our previous findings where the ∼2−3 nm ligand-stabilized Au nanoparticles used in the synthesis of the composite aerogels undergo controlled coarsening during calcination.21,31 The size distribution of the incorporated Au nanoparticles in Au−TiO2[h] aerogels and the local structure around individual Au nanoparticles in the aerogel are essentially the same as those observed in Au−TiO2[l] aerogels (Figure 3, Figure S4, and Table 1).

is also advantageous for heterogeneous catalysis because the success of oxidative hole transfer depends on the dimensions of that interface.34 We also recently reported that TiO2 aerogels photocatalyze H2 generation, and the activity strongly depends on the concentration of the Ti precursor in the sol−gel synthesis, which affects the thickness of the covalently bonded TiO2∥TiO2 junctions. Eight-fold-higher surface-area-normalized H2 generation occurs with the thickest junctions courtesy of an increased number of reactive shallow trap sites.35 In the present study, we investigate the effect of combining this means of network modification with the inclusion of SPR-active Au on the photocatalytic activity of Au−TiO2 aerogels. We then introduce nonplasmonic ∼2 nm Pt to catalyze water reduction using SPR-generated electrons. Synthetic challenges can arise in placing and separating the plasmonic and cocatalyst nanoparticles within a catalytic reaction zone.36−41 The continuous nanostructured, highsurface-area network comprising the architectural backbone of the aerogel provides a superb matrix in which to spatially separate and electrochemically connect plasmonic sensitizers to metal nanoparticle cocatalysts. We compare herein the photocatalytic rates of H2 generation with and without Pt cocatalysts added to Au−TiO2 aerogels in order to demonstrate the viability of creating an aerogel-based composite that integrates all of the critical parts of a solar fuels photocatalyst: (1) plasmonic Au as a means of harvesting visible light to generate carriers; (2) networked nanoparticulate TiO2 serving as a 3D wire that separates the electron−hole pair; and (3) a Pt cocatalyst that provides the locus of water reduction. 9445


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Figure 2. X-ray diffractometry of 3D Au−TiO2 aerogels synthesized at the upper and lower gelation limits of Ti precursor concentration (0.3 g mL−1 [h] and 0.2 g mL−1 [l], respectively, of Ti(iOPr)4). Anatase (ICDD no. 01-071-1168), gold (ICDD no. 03-065-8601), and rutile TiO2 (ICDD no. 01-084-1284).

Figure 4. Diffuse reflectance UV−visible spectroscopy of 3D Au− TiO2[h], 3D Au−TiO2[l], and TiO2[l] aerogels.

basic electrolyte with a methanol hole scavenger. When holescavenging species such as alcohols are added to the reaction medium, the photocatalytic oxidation of water via photogenerated holes (eq 1) is circumvented in favor of the more facile photocatalytic oxidation of the hole scavenger, thus ensuring that water reduction (eq 2) becomes the ratedetermining step in the photocatalytic process.1 1 2 TiO2 (hVB+) + H 2O → 2H+ + O2 (1) 2 2 TiO2 (eCB−) + 2H 2O → 2OH− + H 2

(2)

Performing photocatalytic experiments with a hole scavenger also allows us to isolate what impact the structural modifications and Au inclusions have on the rates of the H2generating water-splitting half reaction. Consistent with our previous work,35 varying Ti precursor concentration over the range where gelation can occur (with the high end having an ∼40% higher Ti(iOPr)4 concentration than the low end) yields TiO2 aerogels with an ∼4-fold-higher mass-normalized H2 generation activity at the high-concentration end (Figure 5). The incorporation of Au nanoparticles into the synthetic protocol for the TiO2[l] aerogel improves the mass-normalized H2-generation activity by ∼10−15× over that of TiO2[h] aerogel and is 50−60× higher than for the TiO2[l] aerogel (Figure 5). The substantial increase in photocatalytic activity at Au− TiO2 is unsurprising: nanoparticulate Au supported on nanoscale TiO2 is well established as a cocatalyst for photocatalytic H2 production under UV illumination.45−48 We do not, however, observe a substantial variation in broad spectrum photocatalytic H2 generation activity upon increasing Au loading in the aerogels from 2.2 to 4.0 to 8.5 wt % (Figure 5). Substantial changes in photocatalytic H2 generation rates have been reported elsewhere, but the control of Au particle size was lost at higher weight loadings.46 In Au−TiO2 aerogels, the Au nanoparticle size is essentially invariant across these Au weight fractions.21,31 The incorporation of Au nanoparticles into the synthetic protocol for the TiO2[h] aerogel further enhances the photocatalytic H 2-generation activity: ∼2× the activity observed for standard Au−TiO2[l] aerogels and 75−100× the activity of TiO2[l] aerogels (Figure 5). This cooperative effect

Figure 3. Scanning transmission electron micrographs using highangle annular dark field (STEM-HAADF) of (A, C) a 3D 2.2 wt % Au−TiO2[h] aerogel and (B, D) a 3D 2.2 wt % Au−TiO2[l] aerogel at (A, B) low magnification, showing a distribution of Au nanoparticles (bright spots), and (C, D) high magnification, showing a single ∼5 nm Au nanoparticle incorporated into the aerogel network.

All of the aerogels exhibit the typical band gap feature for anatase TiO2 at 390 nm. Additionally, the spectra of Au−TiO2 aerogels exhibit an SPR signature, arising from the ca. 5 nm Au nanoparticle guests, that spans much of the visible spectrum with a wavelength maximum centered between 560−570 nm (Figure 4). Also apparent is a broad, flat extinction between the TiO2 band edge and the onset of the SPR feature at about 490 nm that corresponds to Au metal interband transitions. The similarity in position, intensity, and shape of the SPR spectra for Au−TiO2[l] and Au−TiO2[h] aerogels indicates that the local dielectric environment around the incorporated Au nanoparticles is not significantly impacted by the thickened TiO2 particle junctions that form at the higher precursor ratio.42−44 II.B. Cooperative Effects of Plasmonic Au Guests and a Network-Modified TiO2 Aerogel on Photocatalytic H2 Generation at Au−TiO2 Aerogels. The rate and quantum efficiency of H2 generation (based on moles of H2 generated per moles of incident photons) were determined for aerogels in 9446


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Figure 5. Mass-normalized rates of H2 generation under broad-band illumination at TiO2 aerogels, Au−TiO2[l] aerogels with 2.2−8.5 wt % incorporated Au nanoparticles, TiO2[l]/Pt aerogels, TiO2[h] aerogels, and Au−TiO2[h] aerogels with 2.2 wt % incorporated Au nanoparticles, where [l] and [h] designate respective low or high concentrations of the titanium precursor in sol−gel synthesis. (a) Mass-normalized H2-generation rates averaged over the entire 4 h experiment. (b) Mass-normalized H2 generation over time. Two batches of 2.2% Au−TiO2[h] were prepared to compare batch-to-batch synthetic consistency.

composite aerogels is merited but beyond the scope of the present work. II.C. Roles of Au in the Composite Aerogels under Broadband Illumination. Under broadband excitation (i.e., UV + visible light), Au can serve both as a water-reduction catalyst and a plasmonic sensitizer. In the cocatalyst role, carriers are photogenerated in the TiO2 and electrons migrate through the oxide network to the Au∥TiO2 interface to reduce water (Scheme 1). Nonplasmonic functionality offers two

of Au plus network-modified TiO2 increases the activity for H2 generation by ∼400−700 μmol g−1 h−1 vs the Au−TiO2[1] aerogel. Without the Au guest in the anatase host, the TiO2[h] aerogel increases H2 generation by ∼5× over that of the TiO2[l] aerogel, corresponding to an increase in massnormalized activity of only ∼75 μmol g−1 h−1. In our previous study of the effects of synthetically modifying the TiO2 networks on the photocatalytic activity of the resulting aerogels, we used dynamic photovoltage and photocurrent measurements and static photovoltage measurements to discern the effects of electron-trapping surface sites on the photoelectron lifetime, voltage, and mobility.35 In Au-free aerogels, the enhanced photocatalytic activity for H2 generation was attributed to a higher density of trapping sites in the network-modified aerogels. The cooperative effect observed for Au−TiO2 aerogels may originate from enhanced Au∥TiO2 interfacial contact at or near the thickened TiO2∥TiO2 interparticle necksregions already demonstrated to increase the number of reactive shallow trap sites in Au-free TiO2 aerogels.35 As of yet, we have no direct structural evidence for such a preferential sitingthe similarity in the SPR features for Au−TiO2[l] and Au−TiO2[h] aerogels (Figure 4) suggests that the Au∥TiO2 interfaces are structurally similar. Preliminary broadband and visible-light photovoltage measurements at films of Au−TiO2[l] and Au−TiO2[h] aerogels yield no reliable trends across multiple samples, although differences in photocatalytic H2 generation were present, with the highest rates obtained at Au−TiO2[h]. The nonlinear structure− property mapping among synthetic variations, photocatalytic trends, and fundamental photophysics highlights the challenge of characterizing multifunctional architectures, even more so at the more complicated Au∥TiO2 interface. Incorporating active sites can accelerate multiple forward and back reactions, complicating the straightforward interpretation of static photovoltage measurements. A more extensive dynamic photovoltage and photocurrent investigation of the substantial effect of synthetically modifying Au−TiO2 gels on the broad-spectrum H2 generation photoactivity of the resulting guest−host

Scheme 1

benefits: transferring electrons to Au hinders carrier recombination in the TiO2 and the Au surface is much more active for water reduction than is the TiO2 surface. In the SPR-driven pathway (Scheme 2), electron−hole pairs are generated near the Au∥TiO2 interface, with the hole performing oxidation chemistry at that interface while the hot electron is injected into the TiO2 conduction band19,32,49 and migrates through the oxide network until it performs reduction at an active site at the TiO2 surface. II.D. Roles of Au in the Composite Aerogels under Visible Illumination. Photocatalytic H2-generation experi9447


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Assessing the plasmonic contributions of Au in the composite aerogels is complicated by the fact that even at sub-band-gap excitation, the Au-free TiO2 aerogel shows some activity for H2 generation (Figure 6). We tested several TiO2 aerogels, with some showing no detectable H2 under our measurement condition and others having activity just above our detection limit (∼2 μmole gcat−1 h−1). The activity present with sub-band-gap excitation energy is consistent with the modest sub-band-gap photoactivity observed at other forms of defect-rich nanoscale TiO240 and can be explained by the excitation of electrons residing in visible light-accessible shallow trap sites, which form when TiO2 is expressed as lowdimensional, disordered, or porous nanostructures.50 We have also observed shallow trap-driven photocatalysis in very restricted wavelength ranges in our previous photoelectrochemical studies.31 In Au−TiO2 and TiO2 aerogels, shallow-trap sites that are useful for photocatalytic water splitting occur in the narrow region (∼420−500 nm) between the TiO2 bandgap and the wavelengths corresponding to the onset of the Au SPR. No photoelectrochemical water-splitting activity is observed at >∼500 nm in TiO2 aerogels, whereas in Au−TiO2 aerogels, photoelectrochemical water splitting dies off as the SPR diminishes at wavelengths approaching 800 nm.31 At Au− TiO2 aerogels under visible-light illumination, both SPR- and shallow trap-derived electrons (Schemes 1 and 2) are likely to lead to hydrogen generation as catalyzed at the Au∥TiO2 interface. To gain better insight into the origins of visible-light H2 generation activity at the photocatalytic aerogels, with and without sensitization by plasmonic Au nanoparticles, we performed ultrafast (sub-ps) visible pump−infrared (IR) probe time-resolved spectroscopy. The excitation of Au−TiO2 aerogels with 500 nm, 120 fs pulses produced free conduction−band51 electrons that were readily detected by the IR (5 μm) probe beam. The absorptions exhibit biexponential decays with lifetimes on the order of 2 and 20 ps (Figure 7, dark purple circles). Gold-free TiO2 aerogels also produce measurable conduction−band electrons, even when

Scheme 2

ments were performed under visible-light illumination by filtering out UV light with a long-pass filter (435 nm cutoff, Figure S5) in order to prevent direct band gap excitation of TiO2 and to isolate the plasmonic effects of Au on water reduction. Mass-normalized rates of photocatalytic H2 generation and quantum efficiencies at the composite aerogel photocatalysts (Figure 6) are 40−60× lower than under broadband illumination. Given the low H2 yields under visible illumination, it is likely that the predominant role of Au under broadband conditions is as an electron acceptor for reactive carriers generated by TiO2 bandgap excitation (Scheme 1). We previously demonstrated that the broad SPR of ∼5 nm Au nanoparticulate guests can sensitize photoelectrochemistry in TiO2 aerogel hosts, with the wavelength dependence of the photoactivity of the composite aerogel well matched to its SPR extinction.31 Plasmonic contributions, wherein carriers are injected from plasmonic Au into the TiO2 conduction band (Scheme 2), although still energetically possible, are apparently not as prominent under broadband illumination and account for a smaller fraction of the overall amount of H2 generated by the photoreduction of water.

Figure 6. Rates of photocatalytic H2 generation under visible illumination (λ > 435 nm) at TiO2 aerogels and composite Au−TiO2, TiO2/Pt, and Au−TiO2/Pt aerogels synthesized at upper [h] and lower [l] gelation limits of the Ti precursor concentration in 4:1 (vol/vol) 0.1 M NaOH/MeOH containing 0.02 M EDTA. The data for each aerogel represent (a) time-averaged values and (b) activity as a function of time. Duplicate batches of most composites were synthesized in order to capture batch-to-batch variation among the aerogel photocatalysts. 9448


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improve the utilization of SPR-derived electrons for H2 generation. We deposited Pt nanoparticles as reduction cocatalysts onto Au−TiO2 aerogels (Au−TiO2/Pt), adapting a strategy devised Scheme 3

Figure 7. Normalized transient-decay curves obtained for Au−TiO2[l] and gold-free TiO2[l] aerogels. All samples were pumped at 500 nm (2 μJ), with the probe maintained at 5 μm.

pumping at energies well to the red of the cutoff wavelength used in the photocatalytic experiments but at substantially lower numbers (Figure 7, red circles), and feature a singleexponential decay with a time constant of 7 ps. We surmise that this response arises because we excite shallowly trapped electrons with the 500 nm pulses.52 As another control experiment, we compared the effects of on- (585 nm) and offresonance (750 nm) pumping in visible pump−IR probe experiments at Au−TiO2 aerogels and observed similar discrimination in carrier generation rates within the plasmonic aerogels (strong carrier generation on-resonance and very weak carrier generation off-resonance, Figure S6). Taken together, the time-resolved spectroscopic data demonstrate the importance of both the plasmonic sensitizer and of exciting within the SPR wavelength distribution to generate the carriers (electron− hole pairs) that drive visible-light photochemistry at the photocatalytic Au−TiO2 aerogels. The direct observation of conduction−band electron generation in Au−TiO2 aerogels upon visible-light stimulation confirms the likely participation of SPR-generated electrons in photocatalytic water reduction. Gold-free TiO2 exhibits a smaller number of conduction−band electrons, corroborating the modest H2-generation activity observed at these highsurface-area defective materials, as also observed elsewhere in TiO2 nanowire photocatalysts.40 Given the generation of carriers under visible excitation with and without Au, it is likely that Schemes 1 and 2 are both operational in 3D Au− TiO2 aerogels under visible illumination. II.E. Wiring Pt Cocatalysts to the Plasmonic Au∥TiO2 Interface. Whether conduction−band electrons in TiO2 originate from direct band gap excitation, shallow traps, or hot carrier injection from plasmonic Au, their efficient utilization in the reduction of water to H2 requires that they ultimately transfer to a surface site that is highly active for the reaction. As we demonstrated in the broad spectrum photocatalysis experiments at Au−TiO2 aerogels, the direct excitation of TiO2 generates carriers and the incorporated Au primarily fulfills the role of cocatalyst. When illuminating with visible light, however, electrons are transferred from Au to the relatively less reactive TiO2 surface sites, thus potentially limiting the quantum efficiency of the reaction. The incorporation of another cocatalyst material that could accept an SPR-generated electron, if properly wired to the plasmonic sensitizer through the oxide network, could substantially

elsewhere to photodeposit Pt onto TiO2 nanomaterials.53 During Pt deposition under visible-light illumination, methanol is used as the sacrificial reagent in suspensions of Au−TiO2 aerogels in aqueous methanol/H2PtCl6. Photodeposition occurs via migration of plasmonically generated electrons and likely by visible light-generated shallow-trap electrons to surface sites where Pt4+ ions are preferentially adsorbed and electroreduced to Pt metal (Scheme 3). We applied a similar protocol to deposit Pt nanoparticles on Au-free TiO2 aerogels, but used broadband (UV−visible) illumination. Chemical−state analysis using X-ray photoelectron spectroscopy (XPS) confirms the presence of both metallic and ionic Pt in Au−TiO2[h]/Pt (Figure 8a) and TiO2[h]/Pt (Figure 8b) aerogels after photodeposition. The Pt 4f 7/2 peak centered at ∼71 eV is indicative of Pt(0), whereas the broad envelope from 72.5 to 78 eV contains contributions from the Pt(0) 4f5/2 doublet centered near a binding energy (BE) of 74.8 eV as well as a higher BE feature we attribute to adsorbed Pt(IV)Cl4. The total weight percentage of Pt on both composites, determined using the entire Pt 4f region for quantification, is ∼1% when weighted relative to the Ti 2p and O 1s regions (Figure S7). The total metallic Pt in the composite is determined using the fitted Pt 4f 7/2 peak at 71 eV, which reveals that ∼50% of the Pt is fully reduced, yielding a total weight fraction of ∼0.5% Pt(0) on both aerogel supports. It is likely that unreduced Pt ions arise from incomplete photocatalytic reduction of the chloropatinic precursor within the ∼8 h photodeposition time and an inability to remove adsorbed, ionic Pt even after extensive washing of the composite. A more detailed treatment of the XPS data, including the Au 4f spectral region, is given in the Supporting Information (Figure S7). The HAADF-STEM images of Au−TiO2/Pt aerogels (Figure S8) are visually indistinguishable from those of Au−TiO2 aerogels, showing well-distributed metallic nanoparticles on the oxide support. Although it is difficult to distinguish Pt and Au particles via HAADF-STEM as a result of their similar atomic numbers, the particle-size histogram that is collected appears to show two populations (Figure S8d). The mode 9449


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Figure 8. XPS for the Pt 4f region of (a) 2.2 wt % Au−TiO2/Pt aerogel and (b) TiO2/Pt aerogel.

centered at ∼4 nm is similar to the distributions for the Au− TiO2 aerogels, and the second mode centered at ∼2.4 nm is attributed to photodeposited Pt nanoparticles. The UV−visible spectrum of the Au−TiO2[h]/Pt aerogel presents nearly identical (in shape and magnitude) Au SPR features to that of its parent Au−TiO2[h] aerogel (Figure 9).

indirectly confirming the quality of electron wiring in the TiO2-bonded network. When Pt cocatalysts are incorporated into Au−TiO 2 aerogels, H2-generation activity under visible illumination improves (Figure 6), directly confirming that electrons generated by the excitation of the Au SPR are wired through the TiO2 oxide network to Pt nanoparticles. At 40−50 μmol gcat−1 h−1, the Au−TiO2/Pt aerogels achieve an ∼1.3−2× improvement in their H2-generation rate and quantum efficiency compared to the rates and efficiencies of Au− TiO2[l] and Au−TiO2[h] aerogels (20−35 μmole gcat−1 h−1) and at least an ∼50× improvement compared to the values for the TiO2 aerogel. Similar to the effects of aerogel-incorporated Au, the enhancements from Pt can be due to multiple pathways (Scheme 4), and similar comparisons of the relative improveScheme 4

Figure 9. Diffuse reflectance UV−visible spectroscopy of Au− TiO2[h]/Pt, parent Au−TiO2[h], and Au-free TiO2[h]/Pt aerogels.

The only difference is an intensity offset for the Au−TiO2[h]/ Pt aerogel, which likely originates from nonplasmonic scattering at the Pt nanoparticles. The UV−visible spectrum of the TiO2[h]/Pt aerogel is included for comparison and exhibits a broad scattering pattern typical of metallic nanoparticles at energies substantially removed from their SPR. In general, the contact between Au nanoparticles and even a small amount of Pt metal strongly diminishes the SPR response of the Au nanoparticle: Au nanoparticles with 3−5 nm diameters exhibit an ∼35% loss in Au SPR intensity after the incorporation of just 7 atom % (Pt/Au) Pt and >80% diminution at 15 atom % Pt.54 The Au−TiO2[h]/Pt composite aerogel contains >20 atom % Pt/Au, yet the intensity and shape of the Au SPR feature are entirely retained. It is therefore unlikely that even a small fraction of the Pt nanoparticles are in direct contact with the Au nanoparticles in the aerogel,

ments in activity under broadband and visible illumination help us to elucidate which mechanisms dominate under a given reaction condition. Under visible illumination, the consistent enhancement in the rate of photocatalytic H2 generation at Au−TiO2[h]/Pt aerogels is primarily due to the SPR-driven pathway described in Scheme 4: the SPR excitation originating at the Au∥TiO2 interface produces reactive electron−hole pairs, with the electrons wired through the nanoscale TiO2 aerogel network 9450


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photocatalysis as well as how the design of the plasmonic/ catalytic nanoscale interfaces and the mesoscale pathways between those interfaces affects the integration of all of the processes.

to the incorporated Pt nanoparticle cocatalyst. The gains in activity are due to the preferable water-reducing abilities of a Pt(0) surface vs a TiO2 surface. The increased H2 generation at TiO2/Pt aerogels relative to TiO2 aerogels under either visible light (13−20 vs 430 nm, Au−TiO2[h]) using a 500 W Xe arc lamp (Newport-Oriel) equipped with an AM 1.5 filter (Newport-Oriel) or using a cutoff filter (Newport-Oriel) for >8 h. The composite aerogels (TiO2[h]/Pt and Au−TiO2[h]/Pt) were collected by centrifugation, washed with water, and recentrifuged multiple (>5) times and dried at ∼100 °C overnight. IV.D. Structural and Physical Characterization. The crystalline phases of the calcined aerogels were characterized with X-ray diffraction (Rigaku SmartLab, 40 kV and 44 mA, 2° min−1 scan rate). Average crystallite diameters were determined by applying the Scherrer equation to the full width at half-maximum (fwhm) of the anatase (101) diffraction peak at 2θ = 25.2°. The Brunauer−Emmett− Teller (BET) surface area and Barrett−Joyner−Halenda (BJH) pore size distributions were calculated from N2 physisorption isotherms (Micromeritics ASAP2020). The isotherm data (Figure S1) were fitted with Micromeritics DFT Plus software using a density functional model theory assuming cylindrical geometry and Halsey curve thickness. The weight loadings of Au and/or Pt in the composite aerogels were confirmed in part by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, Al Kα X-rays), using a flood gun to prevent charging. Avantage software was used for peak fitting, peak position measurements, and quantification of the Au/Ti atomic ratio, which was converted to weight percentage. High-resolution spectra were recorded in the Au 4f and Pt 4f (where relevant) and Ti 2p and O 1s energy regions, and the integrated intensity of these peaks was used to quantify the relative atomic percentage of each element. All peak positions were referenced to the C 1s peak at 284.5 eV. Elemental analysis (Galbraith, Inc.) was also used to verify Au weight loading in Au−TiO2 aerogels. Diffuse reflectance UV−visible data (PerkinElmer Lambda 1050 spectrophotometer with an integrating sphere) were converted to absorption values using the Kubelka−Munk transformation. Optical band gaps were determined from Tauc plots by extrapolating the linear portion of the plot near the absorption edge to the energy axis. High-resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) analyses were conducted on Au−TiO2[l], Au− TiO2[h], and Au−TiO2[h]/Pt aerogels using a JEOL JEM2200FS TEM and an aberration-corrected Nion UltraSTEM200X, both operating at 200 kV. For these analyses, aerogel material was drop cast onto holey-carbon-coated copper grids from isopropyl alcohol following grinding via a mortar and pestle. The HRTEM images allow for structural determination through lattice-plane spacing as well as specific Au nanoparticle size. To develop statistics, HAADF-STEM provided a better contrast between gold particles and titania than did bright-field imaging and was used to quantify the average Au particle diameters. Gold particles were identified with an automated threshold routine (subsequently checked and edited manually for consistency), and the particle size distributions were then calculated for the resultant binary images. Average diameters (Figures S4 and S8) were determined by analyzing over 100 particles for both Au−TiO2[h] and Au−TiO2[h]/Pt. IV.E. Photocatalytic H2 Generation. Photochemical hydrogen generation rates of suspensions of TiO2, Au−TiO2, and Au−TiO2/Pt aerogels in mixtures of aqueous base and methanol, with ethylenediaminetetraacetic acid added as an additional hole scavenger,58 were measured in a slurry reactor. Aerogel photocatalyst powders were ground and then suspended at a concentration of 2 g of TiO2 L−1 in a 4:1 (v/v) 0.1 M NaOH/methanol solution containing 0.02 M ethylenediaminetetraacetic acid (EDTA, 99% Aldrich). A 50 mL Pyrex reactor was filled with 25 mL of the slurry and sealed; the slurry was stirred and the reactor headspace was purged for several hours with argon to remove oxygen before photocatalytic measurements were performed. The slurry was illuminated with broadband light from a 500 W Xe arc lamp (Newport-Oriel, Figure S5) equipped with an AM 1.5 filter (Newport-Oriel) or a visible light (λ > 435 nm) cutoff

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b01117. Nitrogen physisorption isotherms, bright-field transmission electron micrographs, scanning electron micrographs, particle-size histograms, spectral lamp intensity and a transmission spectrum, transient decay curves, and XPS spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail (P.D.): [email protected]. *E-mail (J.P.): [email protected]. ORCID

Paul A. DeSario: 0000-0003-2964-4849 Jeremy J. Pietron: 0000-0001-7761-2812 Debra R. Rolison: 0000-0003-0493-9931 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the Office of Naval Research for financial support. The authors thank Dr. Joseph F. Parker for his helpful discussions about XPS peak fitting.

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REFERENCES

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