nanostructure arrays for efficient plasmonic photocatalysis

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Template-guided programmable Janus heteronanostructure arrays for efficient plasmonic photocatalysis liaoyong wen, Rui Xu, Can Cui, Wenxiang Tang, Yan Mi, Xingxu Lu, Zhiqiang Zeng, Steven L. Suib, Pu-Xian Gao, and Yong Lei Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b01675 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 9, 2018

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Template-guided programmable Janus heteronanostructure arrays for efficient plasmonic photocatalysis Liaoyong Wen, †∥Rui Xu, ‡∥ Can Cui, † Wenxiang Tang, † Yan Mi, § Xingxu Lu, † Zhiqiang Zeng,‡ Steven L. Suib, † Pu-Xian Gao,†* Yong Lei‡* †

Department of Materials Science and Engineering & Institute of Materials Science, University

of Connecticut, Storrs, CT 06269-3136, USA ‡

Institute of Physics & IMN Macro Nanos (ZIK), Ilmenau University of Technology,

Unterpoerlitzer Straße 38, 98693, Ilmenau, Germany §

Guangxi Key Laboratory of Chemistry and Engineering of Forest Products, Guangxi University

for Nationalities, 530006, Nanning, PR China *Corresponding author. E-Mail: [email protected], [email protected]

ABSTRACT: Janus hetero-nanostructures (HNs), as an important class of anisotropic nanomaterials, could facilitate synergistic coupling of diverse functions inherited by their comprised nanocomponents. Nowadays, synthesizing deterministically targeted Janus HNs remains a challenge. Here, a general yet scalable technique is utilized to fabricate array of programmable Janus HNs based on anodic aluminum oxide binary-pore templates. By designing and employing an over-etching process to partially expose four-edges of one set of nanocomponents in a binary-pore template, selective deposition and interfacing of the other set

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of nanocomponents is successfully achieved along the exposed four-edges to form a densely packed array of Janus HNs on a large scale. In combination with an upgraded two-step anodization, the synthesis provides high degrees of freedom for both nanocomponents of the Janus HNs, including morphologies, compositions, dimensions, and interfacial junctions. Arrays of TiO2–Au and TiO2/Pt NPs–Au Janus HNs are designed, fabricated, and demonstrated about 2.2 times photocurrent density and 4.6 times H2 evolution rate of that obtained from their TiO2 counterparts. The enhancement was mainly determined as a result of localized surface plasmon resonance induced direct hot electron injection and strong plasmon resonance energy transfer near the interfaces of TiO2 nanotubes and Au nanorods. This study may represent a promising step forward to pursue customized Janus HNs, leading to novel physicochemical effects and device applications. KEYWORDS: anodic aluminum oxide (AAO), binary-pore template, Janus hetero-nanostructure, localized surface plasmon resonance, plasmonic photocatalysis Hetero-nanostructures (HNs) that integrate two or more nanocomponents by solid-state interfaces are emerging as an important family of advanced nanoarchitectures, due to the synergistic effects induced by direct electronic and magnetic communications between the constituent nanocomponents.1-6 Of particular interest are Janus HNs comprising a metal and a semiconductor component, which have been applied to different areas including optoelectronics, catalysis, solar energy conversion, sensing, environmental remediation, and biomedicine.1, 3, 7-10 The main merits of such Janus HNs for solar energy conversion are that they could not only increase charge injection and optical path length, but also enhance light absorption from the visible to the IR range, as compared to their single semiconductor counterparts or their physical mixture counterparts.11 So far, there are three important strategies for constructing targeted 2 ACS Paragon Plus Environment

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plasmonic metal−semiconductor HNs: 1) Tuning localized surface plasmon resonance (LSPR) properties of plasmonic metals through adjusting size, shape, and arrangement of metals;11-14 2) Adjusting the optical microcavities of semiconductors with the forms of photonic crystal,15 whispering gallery mode,16 and Fabry–Perot resonator;17 3) Optimizing coupling effects between the plasmonic metals and semiconductors by choosing isotropic configurations (e.g., core@shell) that are formed by completely coating metals with semiconductors or anisotropic configurations (e.g., Janus and dumbbell-like) that formed by partially integrating both of them at selected locations.9, 18 Many studies showed that the anisotropic HNs enhanced photoconversion as compared to isotropic configurations due to the strong LSPR that locates at the junction area between plasmonic metals and semiconductors.9-10, 19-21 Concurrently, the exposed domains of both the metal and the semiconductor can facilitate charge carrier transfer from the HNs to their reaction partners such as oxidization agents and reduction agents in liquid media. In order to assemble deterministically targeted Janus HNs on a large scale, various synthetic techniques have been intensively explored including bottom-up self-assembly and top-down lithography.5, 6, 19-22 The self-assembly strategies are mainly focused on wet chemical processes, in which careful regulation of thermodynamic parameters and growth kinetics under the assistance of selected solvents, ligands, surfactants or catalyst additives are usually required case by case for different HNs.5 On the other hand, lithography guided by templates (e.g., polystyrene and anodic aluminum oxide) mainly relies on electrodeposition and physical/chemical vapor deposition.21-25 However, the resultant HN interfaces are always restricted to longitudinal direction with few options of sizes and shapes for different nanocomponents.21, 22 Therefore, the development of an approach that enables precise control over each nanocomponent and their interface is still imperative to fully exploit the synergistic effects of Janus HNs.


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Herein, we present an approach to fabricate arrays of Janus HNs with the assistance of binarypore templates.26 We chose TiO2−Au Janus HNs as a model system, because they are typical plasmonic−semiconductor HNs that are stable, readily synthesized and among the most studied HNs.18 We employed a sequential process of over-etching and electrodeposition on a TiO2−filled binary-pore template to grow TiO2−Au Janus HNs. We showed that each nanocomponent of the TiO2−Au Janus HNs could be independently controlled at the different steps ranging from pore widening, over-etching, to material deposition. By incorporating an upgraded two-step anodization process, this general strategy allows synthesis of even more complex Janus HNs that are otherwise difficult to obtain. By conjunction with theoretical finite-difference time-domain (FDTD) modelling, we determined that the enhanced photocatalytic performance of the TiO2−Au Janus HNs was ascribed to the direct hot electron injection and strong Plasmon resonance energy transfer (PRET) effect induced formation of electron-hole pairs and suppression of carrier recombination on adjacent TiO2 surface. A simply schematic flow diagram of the fabricating process of Janus HNs, and the corresponding scanning electron microscope (SEM) images, are shown in Figure 1a. The detailed process can be found in the experimental section and Figure S1. The crucial step is over-etching B-pores of the binary-pore template, which uniformly exposes the four edges of the square TiO2 NTs that were deposited in the anodized A-pores via atomic layer deposition (ALD) (Figure 1a and b). Subsequently, an electrodeposition process was used to grow Au NRs in the over-etched Bpores, where intimate contacts between the Au NRs and TiO2 NTs were formed along the exposed four edges of the TiO2 NTs (Figure 1a and c). The interfacial area can be adjusted by changing the length of Au NRs. When a short Au deposition time of 30 min was used, only partial length (∼ 1.4 µm) of the TiO2 NT edges (∼ 2.0 µm) was covered by the Au NRs (Figure


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S3a). With the deposition time extended to 45 min, the isolated Au NRs in the over-etched Bpores overgrew from the template and converged to form a continuous Au nanopore film around the tips of the TiO2 NTs (Figure S3b). Thus, such overgrowth should be avoided to assure that both the TiO2 NTs and Au NRs have exposed faces simultaneously. After removing the remaining template, an array of TiO2−Au Janus HNs with four exposed faces for both components were presented (Figure 1a and d). Considering the fact that all the nanocomponents are densely packed with each other, the integrity of the TiO2−Au HNs is retained with superior robustness over many other reported HNs.21, 22 The size and shape of the Au NRs and TiO2 NTs can be readily adjusted as well as their interfaces. For example, quasi-square shaped Au NRs with a size of about 220 nm were obtained (Figure 1e) through over-etching of template in NaOH solution for one hour. As we changed the over-etching time in NaOH solution for 40 min and then another one hour in H3PO4 solution, four identical pits were observed at the middle of each side of the square Au NR (Figure 1f). Concurrently the sizes of the Au NRs were enlarged to about 300 nm with 70 min over-etching in NaOH and then another one hour in H3PO4 solution (Figure 1g). On the other hand, the size of TiO2 NTs could also be enlarged using different pore-widening times for the A-pores (Figures S3c and S3d). Accordingly, the interfaces of the Au NRs and TiO2 NTs were also changed due to the different perimeter exposure of the TiO2 NT edges. Moreover, the photograph and lowmagnified SEM in Figure 1h demonstrated a uniform square-centimeter area of TiO2−Au Janus HNs, which could be scaled up to wafer size with other advanced imprinting techniques, such as roll to roll nanoimprinting.27, 28 Therefore, large-scale arrays of TiO2−Au Janus HNs with customized nanocomponents and interfaces could be realized by adjusting the different parameters ranging from over-etching, pore widening, to material deposition.


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Figure 1. Arrays of Janus HNs. (a) Schematic fabrication flow diagram of Janus HNs. (b) A tilted SEM image of an A-pore TiO2 filled template after 1 h over-etching. (c) A tilted and an enlarged (inset) SEM images of the A-pore TiO2 filled template after electrodeposition of Au NRs in over-etched B-pores. (d) A tilted and an enlarged (inset) SEM images of TiO2−Au Janus HNs after removal of template. Diverse TiO2−Au Janus HNs after the different treatments for Apore TiO2 filled templates: (e) in NaOH solution for 1 h; (f) in NaOH solution for 40 min and then in H3PO4 for another 1h; (g) in NaOH solution for 70 min and then in H3PO4 for another 1h.


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(h) Photograph (left) and SEM image of a typical large-scale array of freestanding TiO2−Au Janus HNs. Scale bars: 200 nm. The composition of as-prepared TiO2−Au Janus HNs was confirmed by energy-dispersive X-ray (EDX) mapping and line-scanning. The EDX maps of an array of Janus HNs confirmed the perfect matching of the spatial distributions of Ti (cyan) and Au (orange) with the arrangement of the A- and B-pores in the binary-pore template (Figure 2a). While the O (green) and Al (dark cyan) elemental distributions coincide with the frame of the remaining template. The array of TiO2−Au Janus HNs was also confirmed by an EDX line-scan along a row of six HNs after dissolving the template (Figure 2b). The periodic elemental distributions of Ti (cyan curve) and O (green curve) perfectly match each other to the square wires, while the elemental distribution of Au (yellow curve) fits with the quasi-square wires with pits on each side. Moreover, noble metal and transition metal oxide nanoparticles (NPs) could be incorporated into the HNs as functional co-catalysts.29 For example, Pt NPs were deposited inside the A-pores via 2 ALD cycles before the deposition of TiO2 NTs. Transmission electron microscopy (TEM) elemental mapping of the as-prepared TiO2/Pt NTs shows that the Ti and Pt elemental intensities are much stronger along the perimeter edges of the nanopores, confirming the existence of the Pt and TiO2 inside the nanopores (Figure S4a). After scratching the TiO2/Pt NTs from the substrate, it was clearly indicated that the Pt NPs with an average size of about 2 nm were anchored on the outside surface of TiO2 NT (Figure 2c). X-ray photoelectron spectroscopy (XPS) was also used to examine the metal Pt NPs in TiO2/Pt NPs−Au Janus HNs (Figure S4b). The high-resolution Ti 2p3/2 and Ti 2p1/2, as well as O 1s core-level are consistent with the typical values of the TiO2. The observed intense doublet of Au (83.8 and 87.4 eV) and Pt (71.2 and 74.5 eV) is due to the obvious metallic Au0 and Pt0, respectively (Figure 2d).


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Figure 2. Characterization of Janus HNs. (a) SEM image and the corresponding EDX elemental maps of TiO2−Au Janus HNs within template (Ti, cyan; Au, orange; O, green; Al, dark cyan). (b) EDX elemental line-scan of TiO2−Au Janus HNs, where the profiles of Ti (cyan), O (green), and Au (orange) elements along the green line are plotted and imbedded in SEM image (red arrow indicated). (c) TEM image of scratched TiO2/Pt NTs. (d) XPS spectra of TiO2/Pt NPs−Au Janus HNs, including Ti 2p, O 1s, Au 4f, and Pt 4f, respectively.

Considering that the over-etching process will expose not only the desired four edges of the nanocomponent in A-pores, but also the undesired tip of the nanocomponent, it might be difficult to construct desired HNs when some other semiconductors and metals are integrated in A-pores. For example, when SnO2 NTs were filled in A-pores, instead of realizing pre-defined Au NRs in 8 ACS Paragon Plus Environment

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B-pores, random Au NPs were observed on the tips of SnO2 NTs with a 10 min deposition. When the deposition time was extended to 30 min, a continuous Au film was formed on the tips as well as the side walls of the SnO2 NTs (Figure S5). Owning to a better conductivity of the SnO2, as well as a greatest electrostatic field and a depletion of Au ion formed in the electrolyte near the tip, the Au NPs will preferentially initialize and grow on the tips of the SnO2 NTs.30 To insulate these tips from the electrodeposition process, an additional cap is needed. Thus, a twostep anodization was carried out to grow a template with dual-diameter A-pores (Figure 3a), which resulted in a variable wall thickness from ~ 73 nm to ~ 152 nm along the A-pore axes (Figure S6a). Concurrently the B-pores could also be selectively etched on the templates with dual-diameter A-pores (Figure S6b). Most importantly, the four edges of the large-diameter SnO2 NTs at the bottom-segment of A-pores were exposed after the over-etching, while the smalldiameter SnO2 NTs at the top-segment of A-pores were still wrapped by the remaining barrier layer (Figures S6c-d). Due to the isolation from the remaining barrier layer at the top-segment, the Au electrodeposition was restricted in B-pores to form the desired SnO2−Au interface. Largescale array of SnO2−Au HNs is observed after removal of the remaining template, where their four edges of ~ 1.5 µm length at the bottom-segment are intimately connected while their topsegment of ~ 300 nm length is physically separated from each other (Figures 3b and S7). Furthermore, we could also expand the options of the Janus HNs by taking other advantages of the two-step anodized templates. For example, the length and diameter ratios of the top- and bottom-segments in A-pores could be designed by using different anodizing and pore widening durations, and the length and diameter ratios of the two segments in B-pores could be defined with different electrodeposition and etching times (Figure 3c).


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Figure 3. Janus HNs based on two-step anodized templates. (a) Schematic flow diagram of the fabrication process. Illustrations and the corresponding SEM images of SnO2−Au Janus HNs with different diameter and length ratios from top- to bottom-segment (top: bottom), including length ratios of SnO2 NTs (b) 1:5 and (c) 1:1, diameter ratios of SnO2 NTs (b) 1:2 and (c) 1:2, length ratios of Au NRs (b) 1:5 and (c) 1:2, and diameter ratio of Au NRs (b) 1:1 and (c) 1:1.6, respectively. Scale bars: 200 nm. The most exciting feature of the TiO2−Au Janus HNs is their ability to improve the photoconversion efficiency. Firstly, the TiO2 NT length was rationally chosen as 2 µm, since 2 µm is the maximum penetration depth of the incident light in TiO2, and any further increase of NT length would increase its electrical resistance while without the benefit of enhanced light 10 ACS Paragon Plus Environment

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capture.31-33 Meanwhile, to minimize the negative effects of incorporating Au NRs, the Au NR length in TiO2−Au Janus HNs was also carefully considered with the assistance of FDTD simulation. With an increase of Au NR length up to 1.0 µm, the simulated light absorption of the TiO2 NTs in TiO2−Au Janus HNs showed no obvious decrease. While the light absorption began to dramatically decrease with a further increase of the Au NRs length to 2 µm (Figure S8a). On the other hand, only a small variation was observed regarding the visible light absorption of the Au NRs with different lengths in TiO2−Au Janus HNs (Figure S8b). The simulated results are in accordance with the measured diffuse reflectance UV-Vis spectra (Figure S9), where no obvious distinction was found in the UV light region for the TiO2 NTs (2 µm) and the TiO2 (2 µm) −Au (800 nm) Janus HNs. Therefore, the TiO2 (2 µm)−Au (800 nm) Janus HNs were selected to demonstrate the plasmonic photocatalysis. Figure 4a shows chopped photocurrents of the TiO2−Au Janus HNs and TiO2 NTs under AM 1.5G illumination (100 mWcm−2) with and without a 420 nm long-pass filter. The photocurrent of the TiO2−Au Janus HNs reached about 1.7 mA cm-2 at 1.23 V (vs. RHE) under AM 1.5G illumination, which was about 2.2 times that of the TiO2 NTs. The onset potential of the photocurrent remained at around 0.14 V (vs. RHE) for both samples (Figure S10), indicating that the Au NRs had negligible influence on the surface catalysis process at the semiconductorelectrolyte interface.34-36 Meanwhile, the TiO2−Au Janus HNs showed an obvious visible light response with a density of 0.2 mA cm-2 at 1.23 V (vs. RHE), while no visible light response was found for the TiO2 NTs. To further elucidate the role of Au NRs, the incident photon-to-electron efficiency (IPCE) spectra of the TiO2−Au Janus HNs and TiO2 NTs were collected (Figure 4b). The IPCE spectra revealed that the observed photoconversion was dominated by the photoactivity of TiO2 in the UV region, where the TiO2−Au Janus HNs showed a much higher 11 ACS Paragon Plus Environment

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and broader spectrum comparing to that of the TiO2 NTs. Moreover, an obvious IPCE spectrum in the visible region was also observed with a peak of 1.21% at around 540 nm (inset of Figure 4b). LSPR have been identified for photocatalytic performance through the three primary mechanisms: 1) the scattering of resonant which contributes to strong photon absorption by the semiconductor, 2) the existence of intense oscillating electric fields around the metal, called Plasmon Resonance Energy Transfer (PRET), and 3) the production of hot electrons in the metal.11, 13 The LSPR effect is affected readily by shape, size, and content of metals, as well as characteristics of the semiconductor support. Thus, the possible mechanism for the photoactivity enhancement of TiO2−Au Janus HNs can be understood as follow. Firstly, as the TiO2 NTs were selected optically thick with limited reflection loss prior to the addition of Au NRs and therefore scattering is not expected to significantly increase the proportion of light that is absorbed by the semiconductor. Thus, the scattering effect cannot be assigned to the main responsibility of the photoactivity enhancement in the UV light region. Secondly, considering the unique shape and high aspect ratio of the Au NRs, the restoring force to the displaced electron gas and electromagnetic coupling among neighboring Au NRs are strengthened, which could generate strong multipole LSPR modes in the UV light region.37 Therefore, large spectrum overlaps between the Au NRs and TiO2 NTs in the UV light region became possible, which was supported by the measured absorption spectrum of the Au NRs and TiO2 NTs (Figure S9). Meanwhile, the non-centrosymmetric arrangement of the TiO2−Au Janus HNs generated dielectric mismatch that the near-field distribution on the TiO2 NTs was much larger than that of the Au NRs.10 The FDTD simulated electric-field distribution (|E/E0|) at 370 nm revealed that an intensive electric-field was centralized at the interface of the TiO2−Au Janus 12 ACS Paragon Plus Environment

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HNs and spread over to a large portion of the adjacent TiO2-liquid interface above the top of Au NRs (Figure 4d), suggesting that PRET induced formation of electron-hole pairs should be greatest in these regions.38 In addition, as the near-field enhancement is proportional to the dipole or multipole moment, highly density array of TiO2−Au Janus HNs with strong hot-spots could allow for large PRET.39 Therefore, the large PRET could result in an enhancement of probability of photo-oxidation relative to the probability of carrier recombination on the adjacent TiO2 surface (essentially at the TiO2-liquid interface). Besides, the photocurrent transients (PT) curves are commonly observed during water photoelectroysis on oxide semiconductor photoanodes, which were used to reveal the overall behavior of charge recombination and lifetime of the charge carriers.36, 40-43 To quantitatively determine the charge recombination behavior, a normalized parameter D is introduced: D = ( − )⁄( − )

(1)

where It, Ist and Iin are the time-dependent, steady-state and initial photocurrent, respectively, as shown in the inset of Figure 4c.44 The PT decay time (τ) is defined as the time when lnD = -1 in the normalized plots of lnD – t, which reflects the general behavior of charge recombination and lifetime of the charge carrier (Figure 4c and S11). τ was estimated to be 5.2 s for the TiO2−Au Janus HNs under the illumination of 100 mWcm-2, which was about three times that of the TiO2 NTs (1.7 s). On the one hand, since the onset potential and the flat-band potential (apparent Fermi level) did not change with the incorporation of Au NRs (Figure S10 and S12), the increase of PT decay time for the TiO2−Au Janus HNs should not due to the Fermi-level equilibration at the interface.45, 46 On the other hand, differing from the conventional phenomenon observed for the TiO2 NTs, the PT decay time for the TiO2-Au HNs increased with an increase of light


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intensity (Figure 4c). Owning to that the strong near-field intensity gradient is also proportional to the light intensity, the PRET effect could be the major reason for the lower carrier recombination.36, 41, 47 In addition, the ambient photoluminescence (PL) spectra measurement indicated that the PL intensities of the TiO2-Au Janus HNs were much lower than that of the TiO2 NTs, also demonstrating that the Au NRs effectively suppressed the carrier recombination on TiO2−Au Janus HNs (Figure S13). Therefore, it could be concluded that the large PRET effect induced formation of electron-hole pairs and suppression of carrier recombination play an effective role for the IPCE improvement of the TiO2−Au Janus HNs in UV light region.

Thirdly, since no detectable photocurrent was observed for bare Au NRs (Figure S14), the Schottky contact formed between the TiO2 NTs and Au NRs played a crucial role for the extraction of hot electrons and the subsequent IPCE improvement in the visible light region.48 This behavior can be seen in a simple FDTD model at 540 nm (Figure 4e). The LSPR created strong electric-field (|E/E0|) distribution mainly focused along the interface of TiO2 NTs and Au NRs below the top of Au NRs, differing from that at 370 nm (Figure 4d). When the electric-field distribution was extracted versus the wavelength, it indicated that the intensity of the electric field peaks at around 540 nm and becomes weaker at the shorter and longer wavelengths (Figure S15), which closely correlates with the measured absorption spectrum of the TiO2−Au Janus HNs (Figure S9). Thus, the potential carrier transfer paths under AM 1.5G illumination was plotted with a detailed discussion in Figure S16. Based on the above results, it could be concluded that both the PRET effect and hot electron injection contribute to the photoactivity improvement of the TiO2-Au Janus HNs under white light illumination.


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Figure 4. Photoactivities of Janus HNs. (a) Chopped photocurrents of the TiO2−Au Janus HNs and TiO2 NTs under AM 1.5G and visible light illumination. (b) IPCE spectra of the TiO2−Au Janus HNs and TiO2 NTs in the range of 300−800 nm without applying external bias. (c) Anodic photocurrent transients of TiO2−Au Janus HNs under AM 1.5G illumination with different irradiation intensities. Inset: the scheme for the calculation of the transient dynamics. FDTD simulated electric field (|E/E0|) distributions for the TiO2−Au Janus HNs at (d) 370 nm and (e) 540 nm. Left: top-down view; right: cross-sectional view. It is also important to directly examine the evolution of H2 gas. By using methanol as the hole scavenger, the TiO2−Au Janus HNs presented about three times H2 evolution rate (12.8 ± 1.5 µmolh-1cm-2) as compared to that of the TiO2 NTs (4.8 ± 0.6 µmolh-1cm-2) under AM 1.5G illumination (Figure 5a and S17a). When Pt NPs (~ 2 nm) were incorporated as a co-catalyst to form TiO2/Pt NPs−Au Janus HNs, the H2 evolution rate was further increased to 17.5 ± 2.6 µmolh-1cm-2, which was about 161% that obtained from the TiO2−Au Janus HNs. Moreover, the TiO2/Pt NPs−Au and TiO2−Au Janus HNs showed an obvious H2 evolution under visible light 15 ACS Paragon Plus Environment

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illumination, while negligible H2 was detected from the TiO2 NTs (Figure 5a and S17b). Since the H2 evolution rate of the TiO2/Pt NPs−Au Janus HNs (4.3 ± 0.4 µmolh-1cm-2) is higher than that of TiO2−Au Janus HNs (3.1 ± 0.3 µmolh-1cm-2), indicating that the Pt NPs improved the collection of hot electrons. This point was also demonstrated by visible light photoreduction of methylene blue (MB), a model acceptor molecule. After 50 min of visible light irradiation, the TiO2/Pt NPs−Au and TiO2−Au Janus HNs exhibited ∼96% and 80% reduction of the MB dye (Figure 5b and S16c-d), while the TiO2 NTs showed insignificant activity (less than 3%, part of the activity may come from photobleaching).49, 50 Thus, the overall mechanism of how LSPR affects the photocatalytic activity of the TiO2/Pt NPs−Au under white light irradiation was plotted in Figure 5c. Electrons in Au NRs are excited by the LSPR with visible light absorption and subsequently transferred to the conduction band of the TiO2 NTs. Meanwhile, the Au NRs can enhance the electron-hole pairs generation and suppress the carrier recombination on the adjacent TiO2 surface via PRET effect. While small Pt NPs on TiO2 NTs act as co-catalysts to trap not only the photogenerated electrons from the conduction band of TiO2 NTs but also the hot electrons from the Au NRs to generate H2.51, 52

Figure 5. (a) H2 evolution rates of the three samples (TiO2 /Pt NPs−Au Janus HNs, TiO2 −Au Janus HNs, and TiO2 NTs) under AM 1.5G and visible light illumination and in the presence of


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methanol and water. (b) Normalized concentration of MB vs irradiation time of the three samples under visible light illumination and in the presence of methanol and water. Inset: photos of the concentration of MB after different periods of photodegradation with the TiO2−Au Janus HNs. (c) Schematic for the electron paths of TiO2/Pt NPs−Au Janus HNs to show that both hot electron injection and PRET effect work in the system. In summary, we have developed a generic and efficient strategy to fabricate large-scale arrays of customized Janus HNs based on one- and two-step anodized templates. Each nanocomponent of the Janus HNs was precisely programmed with pre-determined size, shape, composition, dimension, and interfaces at different processing steps ranging from the anodization, porewidening, over-etching, to material deposition. Rationally selected TiO2−Au and TiO2/Pt NPs−Au Janus HNs were used for plasmonic photocatalysis and exhibited about 2.2 times photocurrent density and 4.6 times H2 evolution rate that obtained from the TiO2 NTs, which was ascribed to the LSPR induced direct hot electron injection and strong PRET effect. A further improvement on Janus HNs based plasmonic photocatalysis could be expected by tuning each nanocomponent and their interface,53 as well as incorporating proper co-catalysts for each nanocomponent.29 Multi-component arrays of customized HNs with an excellent integrity could be realized in conjunction with other template based nanostructuring strategies such as on-wire54 and coaxial lithography.55 The enabled heterogeneous nanoarchitectures demonstrated here could allow us to study the structure–property relationship from the microscopic single HN to macroscopic arrays of HNs on a large scale. ASSOCIATED CONTENT Supporting Information


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Additional details on the experimental methods. Detailed schematic process of fabricating TiO2−Au Janus HNs; SEM image of barrier layer and overgrowth TiO2 on top surface of template; SEM images of TiO2−Au Janus HNs; TEM image and XPS spectrum of TiO2/Pt NPs−Au Janus HNs; SEM images of SnO2−Au nanostructures; SEM images of two-step anodized templates with/without SnO2 filled in; SEM image of large-scale SnO2−Au Janus HNs; FDTD simulated absorption spectra of TiO2−Au Janus HNs; Diffuse reflectance absorption spectra of TiO2−Au Janus HNs, Au NRs, and TiO2 NTs; Photocurrents, Mott-Schottky plots, and ambient PL spectra of TiO2−Au Janus HNs and TiO2 NTs; FDTD simulated electric field distributions of TiO2−Au Janus HNs; Potential charge carrier transfer paths of TiO2−Au Janus HNs; H2 generation and MB photodegradation rates of TiO2/Pt NPs−Au Janus HNs and TiO2−Au Janus HNs. This Supporting Information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Author Contributions ∥

These authors contributed equally to this work.

NOTES The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors are grateful for the financial support from the U.S. National Science Foundation (Award # CBET-1344792) and the U.S. Department of Energy (Award # DE-EE0006854),


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European Research Council (HiNaPc, 737616), and German Research Foundation (DFG: LE 2249/4-1 and LE 2249/5-1). L.Y. Wen is partially supported by Uconn 2018 inaugural Postdoctoral Seed Grant. X.X. Lu is partially supported by a ThermoFisher Scientific Graduate Fellowship. Table of Contents

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nanostructure arrays for efficient plasmonic photocatalysis

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