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Surface Plasmon Resonance Enhanced Photoelectrochemical Water Splitting from Au Nanoparticle-Decorated 3D TiO Nanorod Architectures 2

Hongxia Li, Zhaodong Li, Yanhao Yu, Yangjin Ma, Weiguang Yang, Fei Wang, Xin Yin, and Xudong Wang J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 12 May 2017 Downloaded from http://pubs.acs.org on May 13, 2017

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The Journal of Physical Chemistry

Surface Plasmon Resonance Enhanced Photoelectrochemical Water Splitting from Au Nanoparticle-Decorated 3D TiO2 Nanorod Architectures Hongxia Li,†,‡,§ Zhaodong Li,‡,§ Yanhao Yu, ‡ Yangjin Ma,ǁ Weiguang Yang,‡ Fei Wang,‡ Xin Yin, ‡ Xudong Wang*,‡ †

College of Materials and Environmental Engineering, Hangzhou Dianzi University,

Hangzhou 310018, People’s Republic of China ‡

Department of Materials Science and Engineering, University of Wisconsin-Madison,

Madison, WI, 53706, USA ǁ

Department of Electrical & Computer Engineering, University of Delaware, Newark

DE 19716, USA

ABSTRACT: Surface plasmonic resonance (SPR) is a new paradigm in photoelectrochemical (PEC) research, which realizes the persistent supply of green energy in a sustainable manner. However, typical approaches for Au nanoparticle (NP) decoration, such as colloidal chemical method, nano lithography, and in-situ photo/thermal reductions, involve multiple complex steps and often introduce unwanted surface/interface chemicals that jeopardize the SPR effect and charge transport. Here we report a largely enhanced PEC performance by decorating Au NPs onto the surface-reaction-limited pulsed chemical vapor deposition grown 3D titanium dioxide (TiO2) nanorod architectures via one-step sputtering process. The Au NPs size and amount could be well manipulated. Compared to the pristine TiO2, 1 ACS Paragon Plus Environment


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Au-TiO2 electrode achieves the highest photocurrent density enhancement of 42% and 267% under simulated sunlight and visible light illumination, respectively. After employing

amorphous aluminum

oxide

(Al2O3) films over the

Au-TiO2

photoelectrode, the PEC performance is further elevated by 87.8%. Moreover, 3D finite-difference time domain simulation is applied to investigate the spatial distribution of electric-field intensity around Au NP in different cases. The PEC enhancement is confirmed to follow the localized electromagnetic enhancement and hot electron injection mechanism on SPR excitation. This facile and effective Au decoration approach makes it possible to design plasmonic metal/semiconductor structures for other general photoanode improvement.

1. Introduction Owing to the appropriate electronic energetic levels, excellent surface chemical activity, superior stability, and moderate cost, titanium dioxide (TiO2) has attracted numerous research interests in photoelectrochemical (PEC) water splitting application.1-17 However, one critical limitation of TiO2-based solar energy harvesting is its large bandgap (~3.0-3.2 eV), which excludes it from absorbing the majority of solar energy within the visible light range.18-20 Coupling TiO2 with other visible light photoactive components, such as dyes, small bandgap semiconductors, quantum dots and metal nanoparticles (NPs), has been broadly investigated as a common strategy to design TiO2-based PEC electrodes with visible-light activity.19, 21-23

Among these

coupling strategies, decorating noble metals such as Au NPs shows particular advantages for efficient solar water splitting via the surface plasmon resonance (SPR) 2 ACS Paragon Plus Environment

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effect. It offers excellent photostability and tunable interactions with light in the visible and near-infrared range.

SPR is defined as photo-induced collective resonance of free electrons on a metal surface, where the plasmonic oscillation frequency and intensity are highly sensitive to the size and shape of the metal as well as the dielectric constant of the surrounding environment. Typical approaches to Au NP decoration include colloidal chemical method, nano lithography, and in-situ photo/thermal reductions. Nevertheless, these approaches involve multiple complex steps and often introduce undesired surface/interface chemicals that jeopardize the SPR effect and charge transport. Research also showed that tuning the dielectric environment could effectively influence the SPR effect. For example, with an aluminum oxide (Al2O3) overcoating by atomic layer deposition (ALD), a Au-TiO2 system exhibited red-shifted and intensified plasmon resonance, which improved the overall PEC performance.24 In addition, the geometry of photoelectrode is another critical aspect that can drastically impact the generation and transfer of hot carriers during PEC water splitting. Three-dimensional (3D) branched TiO2 nanorod (NR) architecture has been recently developed by surface-reaction-limited pulsed chemical vapor deposition (SPCVD) technique. It demonstrated a drastically enhanced PEC performance owing to the significantly enlarged surface area density and well-preserved charge transport property. In this paper, we introduced Au NP decoration and amorphous Al2O3 thin film coating onto the SPCVD-grown 3D TiO2 NR architectures by one-step sputtering and ALD processes, respectively. The size and quantity of Au NPs were well 3 ACS Paragon Plus Environment


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controlled by the sputtering time. Higher PEC efficiencies, particularly in the visible-light region, were observed from these 3D Au-TiO2 NR architectures compared to the pristine samples. Further elevated PEC efficiency was obtained when a 5 nm ALD Al2O3 film was uniformly coated. Finite-difference time domain (FDTD) simulations were implemented to understand the SPR spectrum and plasmonic field configurations. This work validates a unique combination of geometric, plasmonic and dielectric enhancements that are accumulative in one complex heterogeneous system.

2. Experimental Section Synthesis of 3D Branched TiO2 NW Architecture: Highly orientated ZnO NW arrays were synthesized on fluorine-doped tin oxide (FTO) glass via a hydrothermal method following the procedures described in our previous publications.25-27 TiO2 NWs were then grown by SPCVD on ZnO NWs arrays in a home-made ALD system using TiCl4 and H2O as precursors. In a typical TiO2 NRs growth process by SPCVD, the as-synthesized ZnO NW arrays were placed at the center of the ALD chamber. A constant flow of 40 sccm N2 was applied into the chamber as the carrier gas, which provided a background pressure of ~4.1 Torr. The chamber temperature was kept at 600 °C during the entire growth process. TiCl4 and H2O vapor precursors were separately pulsed into the chamber for 0.5 s by purging N2 for 60 s. Thus, one growth cycle included 0.5 s of TiCl4 pulsing + 60 s of N2 purging + 0.5 s of H2O pulsing + 60 s of N2 purging. Subsequently, the furnace was cooled down naturally under N2 flow. The 400-cycle deposition yielded the 3D TiO2 NR branch architecture on the FTO 4 ACS Paragon Plus Environment

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substrate.

Decoration of Au NP on 3D TiO2 NR Architecture: Au NPs were deposited onto the TiO2 NR surfaces by DC sputtering (Denton Vacuum Desk II sputter coater). The sputtering was conducted at a base pressure of 50 mTorr under Ar gas. The working current was set at 45 mA and the output power of the sputtering was ~ 5.4 W. The sputtering time of 1s, 3s, 5s, and 10s were selected to achieve different Au NP loading. The as-sputtered Au NP-decorated 3D TiO2 NR architecture is denoted as Au-X-TiO2 (X = sputtering time). As a comparison, Au NPs were also prepared from the colloidal solution method and then decorated onto TiO2 NR surfaces. Specifically, 20 mL of 2.0 mM HAuCl4 in a 50 mL beaker was stirred and heated to a rolling boil. Thereafter, 4 mL of a 2% solution of trisodium citrate dehydrates (Na3C6H5O7·2H2O) was quickly added to the boiling solution. Stirring and heating were stopped when the solution turned to deep red, where Au NPs with sizes of ~8-10 nm were received.28 Subsequently, the as-received Au NP solution was spin coated on the 3D TiO2 NR architectures for 30 s at a spinning rate of 5000 rpm, and the samples were then dried at 80°C for 15 minutes. The spin coating and drying process was repeated for three times to achieve optimal Au NP coating.

Amorphous Al2O3 layer overcoating: A thin layer of amorphous Al2O3 was coated on the Au NP-decorated 3D TiO2 NR architectures through a typical ALD process. The growth conditions were 500 ms Trimethylaluminum (TMA) pulsing + 60 s N2 purging + 500 ms H2O pulsing + 60 s N2 purging at 120 °C. ALD cycles were selected

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to be 20, 50, and 100, which yielded the Al2O3 film thickness of ~2 nm, ~5 nm, and ~10 nm, respectively.

Characterizations: The nanostructure morphology was imaged by LEO 1530 GEMINI SEM (Zeiss, Germany). Tecnai TF-30 TEM (FEI, OR, USA) was implemented to study the Au NP distribution and crystal structures. EDS equipped with SEM analysis and XPS (Thermo Fisher Scientific Inc., Waltham, MA, USA) measurements were conducted to obtain elemental information. In the XPS measurement, the survey binding energy range was set from 0 to 1200 eV. The Au 4f spectra were individually scanned to characterize the chemical structure of Au. Diffuse reflectance

UV-visible

spectra were

performed

on a UV-visible

spectrophotometer (UV-3600, Shimadzu, Japan) in the wavelength range of 320-800 nm and using BaSO4 as the reference. PEC Measurement: The PEC photoanodes were prepared by covering as-prepared samples with epoxy, leaving an average exposed active area of ~1.42 and 21 mm2. PEC characterizations were performed in a 1 mol L−1 NaOH (pH = 14) aqueous solution using a three electrode electrochemical cell configuration. A SCE was used as the reference electrode, and a Pt wire was used as the counter electrode. All electrodes were connected to a potentiostat system (Metrohm Inc., Riverview, FL) for J−V measurements. Light illumination was provided by a 150 W Xe arc lamp (6255, Newport Corporation, Irvine, CA). An AM 1.5G filter and a UV cutoff filter were utilized with the lamp for PEC characterization, where the intensity at the PEC anode


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position was adjusted to be 100 mW cm−2 and 86 mW cm−2, respectively. Amperometric I–t photoresponse was evaluated under chopped light irradiation (light on or off cycles: 50 s) at an applied potential of 0 V vs. SCE.

FDTD simulation: 3-D FDTD method was performed in a commercial software package (Lumerical FDTD Solutions) to simulate the plasmonic enhancement. The automatic non-uniform conformal mesh was applied to the entire simulation domain with mesh level of 30 points per wavelength resolution. An even finer mesh of 0.4 nm is then applied to cover the Au NP. The simulation time was set long enough to be numerically stable. The field source was a transverse electric plane wave with the E-field oscillating in the x-direction and propagates from the electrolyte into the designed nanostructures. The amplitude of the light source was set to 1 V/m.

3. Results and Discussion Figure 1a schematically illustrates the fabrication procedure of a Au NP-decorated 3D branched TiO2 NR architecture. The pure TiO2 NRs were assembled following our reported procedure. 25 Briefly, ZnO nanowires (NWs) were used as the template. Through a combined process of Kirkendall effect and SPCVD, ZnO NWs were entirely consumed and converted to a pure TiO2 3D architecture. Au NPs were loaded onto the 3D TiO2 by either sputtering or colloidal spin coating. The top view scanning electron microscopy (SEM) images (Figure 1b) illustrate the treelike morphology of the 3D TiO2 NR architectures with sputtered Au NPs. TiO2 NRs densely covered the NW backbone with a fairly uniform dimension of ~250 nm in 7 ACS Paragon Plus Environment


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length and ~30 nm in diameter. Higher magnification image in Figure 1c clearly reveals that 1 second sputtering introduced homogeneously separated Au NPs on the entire TiO2 surfaces, which is a desired feature to ensure minimal surface plasmonic field intensity drop due to NP aggregation.29

Transmission electron microscopy (TEM) was implemented to further characterize the morphology and crystal structure of the Au NPs, TiO2 NRs and their interfaces. As shown in Figure 1d, the density of TiO2 NRs was found to be as high as ~70 NRs per µm. They were branched from the backbone and protruded outward radially. Such TiO2 NRs were high-quality single crystals with the anatase phase. The NR phase and morphology evolution has been thoroughly investigated in our previous papers.26,30,31 The Au NP coverage was fairly uniform along the entire length of the backbone. This revealed the successful diffusion of Au atoms deep into the confined spaces of the 3D structures in the sputtering process. Notably, the coverage of Au NPs on individual TiO2 NRs was not uniform (inset of Figure1d), as expected from the shadowing effect against the Au atom flux. Most Au NPs exhibited a spherical shape. From high-resolution transmission electron microscopy (HRTEM) image, the lattice spacing measured from the Au NP was 0.20 and 0.24 nm, which matched perfectly to the (200) and (111) planes of FCC Au (JCPDS Card No. 4-784), respectively (Figure 1e). HRTEM also revealed sharp interfaces between Au NPs and TiO2 NRs (Figure S1). The high-quality Au lattice and Au-TiO2 interface are two essential factors for effective hot electron injection from Au to TiO2 upon SPR excitation.32


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The elemental information and surface chemical structure were further characterized by X-ray photoelectron spectroscopy (XPS). Strong Ti, O and Au peaks were identified from the full spectrum of the Au-TiO2 samples (Figure 1f). No Zn signal was detected in the full spectrum, confirming the complete removal of ZnO template, which was consistent with previous report. 25 The doublet peaks of Au 4f 7/2 and Au 4f 5/2 (83.7 and 87.4 eV) as well as the 3.7 eV difference were identified from the Au 4f region near ~85 eV (Figure 1g). These characteristics matched well to the metallic Au 4f state, confirming the formation of pure metal Au NPs on the TiO2 surface.33-36 The bonding situation between Au and TiO2 can be further illustrated by deriving the Wagner plot from the XPS spectra. The modified Auger parameters of Au in all Au-TiO2 samples are located at the same position as the referencing metallic Au

37

in the Wagner plot (Figure S2), further confirming that all the Au NPs are

metallic and no chemical bonding was evolved between Au and TiO2 during the sputtering process.



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Figure 1. a. Schematic fabrication process for making Au NP decorated-3D branched TiO2 NR architectures. b. Top-viewed SEM image of Au NP-3D TiO2 NR architecture arrays. c. Higher resolution SEM image showing homogeneously distributed Au NPs on the entire TiO2 surface. d. TEM image of an individual branched TiO2 NR array. Inset shows a single TiO2 NR decorated with Au NPs. e. HRTEM image of a Au NP resided on the TiO2 surface. f. Full range XPS spectrum acquired from the Au NP-TiO2 NR architecture. g. Au 4f scan extracted from the XPS spectrum, where the spin-energy separation of 3.7 eV between Au 4f 7/2 and Au 4f 5/2 peaks verified the pure Au metal phase. 10 ACS Paragon Plus Environment

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Four different sputtering time periods, 1, 3, 5, and 10 seconds, were selected to deposit Au NPs on the TiO2 NR surfaces, which were denoted as Au-X-TiO2 (X=1, 3, 5, and 10 seconds, respectively). Figure 2a shows the TEM images of the four different Au-X-TiO2 samples. In general, longer deposition time yielded less but larger Au NPs, where the average NP sizes were 4±2, 8±3, and 15±6 nm for Au-1s-TiO2, Au-3s-TiO2, and Au-5s-TiO2, respectively. The increasing size can be attributed to the diffusion and agglomeration of Au NPs under longer sputtering time.38 For the 10s sputtering sample (Figure 2a and S3), most Au NPs merged into large and continuous pieces on the front side facing the Au flux, whereas a large quantity of small Au NPs appeared on the shadowing side of the TiO2 NRs. Due to the large particle size and shape variation, the average size was not quantified from this 10s sample. The average Au weight ratios were measured by energy-dispersive X-ray spectroscopy (EDS) (Figure S4) and summarized in Table S1. The weight percentage of Au gradually increased from ~3.06 % to ~11.75 % when the sputtering time increased from 1 to 10s.

Theoretical and experimental studies suggested that Au NPs could introduce localized SPR driven by the electromagnetic field of incident light, enabling photoactivity in the visible/infrared light range.39 Controlling size and shape of Au NPs are able to tune the resonance frequency and intensity in the specific wavelength region. Therefore, the Au NPs deposited on TiO2 NR surfaces were expected to realize a promising high-performance PEC photoanode across a wide spectrum range. To investigate the PEC properties associated with different Au NP decorations, the 11 ACS Paragon Plus Environment


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four Au-X-TiO2 samples and the pristine 3D TiO2 NRs sample were used as PEC photoanodes and their photocurrent density (Jph) characteristics versus bias potential were measured under the same conditions (see experiment section for details). Figure 2b and 2c illustrates typical linear-sweep voltammograms of all the photoelectrodes obtained under simulated sunlight (AM 1.5G) and visible light illuminations. The current density under dark remained at a very low level from −1.0 to 1.0 V (vs. saturated calomel electrode (SCE)), indicating high quality crystal surfaces of the electrodes. Prompt Jph increase appeared after applying the AM 1.5G illumination. The pristine 3D TiO2 yielded a stable Jph of 1.75 mA cm-2 at 0 V vs. SCE for more than 17 hours (Figure S5). The Jph of Au-X-TiO2 electrodes were significantly higher than that of the bare TiO2 electrode. Enlarging the active area from 1.42 mm2 to 21 mm2 didn’t introduce observable PEC performance change (Figure S6). This phenomenon witnessed the positive role of the sputtered Au NPs in PEC process. The enhancement varied as the Au NP size changed. The Au-3s-TiO2 electrode exhibited the highest Jph of 2.69 mA cm-2 at 0 V, which was 42% larger than the Jph of pristine TiO2 electrode. Shorter sputtering time (1s) yielded lower Au NP coverage and smaller particle size, and thus less enhancement was obtained (32% enhancement compared to the pristine sample). However, further increase the sputtering time to 5s and 10s introduced excess Au coverage, which blocked large TiO2 surfaces and reduced PEC reaction sites. As a result, the Jph enhancement dropped to 37% and 26% for Au-5s-TiO2 and Au-10s-TiO2, respectively. The maximum Jph enhancement as a function of Au sputtering time is plotted in Figure 2d, where a parabolic relationship 12 ACS Paragon Plus Environment

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can be identified.

Because the SPR effect is effective in the visible light range, to further evaluate the SPR enhancement from Au NP decoration, the PEC performance of Au-X-TiO2 electrodes was investigated under visible light (≥400 nm) illumination. The Jph of all samples were much lower than those under AM 1.5G illumination due to the dominating absorption of near UV light by TiO2. Nevertheless, the Au-X-TiO2 electrodes exhibited significant visible-light PEC activity, where the maximum Jph was 267% higher than the pristine TiO2 electrode (i.e., 37.5 vs. 10.2 µA cm-2) (Figure 2c). The enhancement ratio and sputtering time relationship is plotted in Figure 2d (blue squares) in comparison to the AM1.5G results. It can be noticed that Jph decay was also less sensitive to the higher Au coverage (sputtering time >3s) under visible light illumination. When extending the cutoff wavelength to 430 nm (Figure S7), the photocurrent of the pristine TiO2 and TiO2-3s Au were both decreased compared to the visible light case (Figure 2c) due to the loss of the light absorption in 400-430 nm. Though the Jph of the pristine TiO2 was reduced to a very low level (2 µA cm-2 at 0.5 V vs. SCE), the TiO2-3s Au still yielded a Jph of 0.27 mA cm-2 at 0.5 V vs. SCE, two orders of magnitude higher than the pristine TiO2 case. These results confirmed that the photocurrent enhancement from Au NPs was mainly caused by the SPR effect in the visible light region, rather than the enhanced charge separation of TiO2 due to the interface electric field between TiO2 and Au. The visible-light PEC enhancement from Au NP decoration and its dependence on sputtering time were further analyzed by calculating the charge transfer kinetics from Au to TiO2 (Supporting information S8). 13 ACS Paragon Plus Environment


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Significant amount of electrons (at the level of 1017 cm2) passed through the active interface of the An/TiO2 photoanode under AM1.5G illumination. This number droped to 1015 cm2 when visible light illumination was applied. In addition, chronoamperometric I−t curves were collected under chopped light illumination at 0 V (vs. SCE) to examine the instant photoresponse of all photoelectrodes. Unlike previous work,32,

40

no emblematical anodic current spikes were observed at the

light-on points for any Au NP-decorated TiO2 NR electrodes under both AM1.5G (Figure 2e) and visible light illuminations (Figure 2f). This result, together with the high charge-to-hydrogen conversion efficiencies and charge transfer rates of Au-X-TiO2 photoanodes (Supporting information Part S8) signifies that the sputtering method could provide excellent charge transport kinetics at the Au/TiO2 interface for efficient and instant water oxidation reactions.41

To further illustrate the advantageous of Au NP sputtering, Au NPs-decorated TiO2 NRs made from colloidal solution was characterized for PEC performance comparison. As shown in Figure S9 the Jph values of Au-3s-TiO2 electrode under AM 1.5G and visible illuminations were 12.7 % and 34.3 % larger than those from colloidal Au NPs photoanodes, respectively, although the size and quantity of Au NPs coating (3.97±0.26 wt%) for both electrodes were very close. This comparison suggests that the sputtering method could introduce Au NP decoration with significantly higher SPR effect.


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Figure 2. a. TEM images of Au NP-decorated TiO2 NRs collected at different sputtering times. b. Jph-V curves of four Au-X-TiO2 photoanodes obtained under dark and AM 1.5G illuminations. c. Jph-V curves of four Au-X-TiO2 photoanodes obtained under dark and visible light illuminations. d. The peak photocurrent density (at 0 V) as a function of Au sputtering time under AM 1.5G (black squares) and visible light (blue squares) illuminations. e. and f. Chronoamperometric I-t curves of four



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Au-X-TiO2 photoanodes measured at 0 V vs. SCE under chopped AM1.5G e. and visible light f. illumination. It is known that SPR-mediated hot electron transfer is the main mechanism for the PEC activity enhancement.42,43 Figure 3a illustrates the hot electron transfer mechanism between Au NP and TiO2 under illumination, where hot electrons can transiently occupy the surface plasmon states above the Fermi level and subsequently inject into the conduction band of TiO2 for water reduction and leaving energetic positive charges functioning as holes in Au for water oxidation.44,

45

In order to

analyze the SPR-mediated hot electron injection in our Au NP-decorated TiO2 NR system, 3-D FDTD simulation was conducted to calculate the spatial distribution of electric-field intensity at the interface of Au and TiO2 as a function of incident light wavelength. Inset of Figure 3b depicts the model used in the FDTD calculation, where a spherical Au NP was placed on a flat TiO2 surface. The average electric field intensity (x-axis) in the simulation domain versus wavelength was collected by assuming the incident light traveling along the y-axis. As shown in Figure 3b, a strong and broad electric field peak in the visible region (400-700 nm) are observed from Au NP-decorated TiO2 samples. The peak intensity increased monotonically as the size of Au NP increased. Meanwhile, a noticeable electric-field strengthen at the wavelength below 350 nm was observed for Au decoration, which overlapped with the absorption edge of TiO2. This phenomenon indicates that the localized electromagnetic enhancement mechanism can also apply to the TiO2 by selectively increasing the rate of interband transitions in TiO2 due to the interaction between the localized electric 16 ACS Paragon Plus Environment

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field and the neighboring TiO2.46 Figure 3c illustrates the spatial distribution of electric-field intensity at the Au/TiO2 interface as a monochromatic incident light travels along the specific direction (y-axis). Under both ultraviolet (320 nm) and visible light (550 nm) illuminations, the electric-field intensity around the Au NPs was strengthened as the size of Au NP increased. The Au-5s-TiO2 sample exhibited a maximum local field enhancement factor of 2.2 and 8 under 320 nm and 550 nm illumination, respectively, consistent with the fact that larger NPs retained more effective plasmonic resonance.47-49 Stronger SPR electric field and deeper penetration depth were obtained under the wavelength of 550 nm. The wavelength-dependent FDTD study proves that the significant PEC enhancement in the visible-light region can be attributed to the hot electron injection mechanism upon SPR excitation.

To further support the visible light interaction, diffused reflectance UV-visible absorption spectra of all samples were measured within the wavelength range from 320 to 800 nm (Figure S10a). Significantly enhanced absorption was observed in the visible light regime (400-700 nm) on Au-X-TiO2 samples supporting the calculated SPR effect from Au NPs decoration. These enhancements made the as-synthesized Au-X-TiO2 samples appearing darker under increased sputtering time (Figure S10b), consistent with the more effective SPR for higher visible-light absorption.



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Figure 3. a. Schematic electronic energy level diagram for the Au NP-TiO2 system showing SPR-mediated electron transfer from Au NP to TiO2 excited by visible light and direct electron-hole excitation in TiO2 by UV light. b. Calculated electric-field intensity as a function of incident light wavelength for pristine 3D TiO2 NR photoanode and Au-X-TiO2 photoanodes with sputtering times of 1, 3, and 5 seconds. Inset shows the model of spherical Au NP on flat TiO2 surface used in FDTD calculation. c. Spatial distribution of electric-field intensity at the Au/TiO2 interface. The monochromatic incident light is assumed along the y axis.


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The plasmon field could be distorted when the dielectric medium surrounding the metal NP varied anisotropically.50 The SPR peak of the NP could be broadened and shifted to long wavelength range by increasing the dielectric constant of the surrounding medium. To observe this effect in PEC performance, an ultrathin dielectric Al2O3 layer was deposited on the Au-3s-TiO2 electrode by ALD. Three Al2O3 layer thicknesses (2 nm, 5 nm, and 10 nm) were selected and their Jph-V curves were measured under visible light (Figure 4a). The 5 nm Al2O3 coating exhibited the highest enhancement with a photocurrent density of 0.07 mAcm-2 at 0 V (vs. SCE), which is 87.8% higher than that of un-coated Au-3s-TiO2 electrode. Moreover, no additional redox peak was observed in Figure 4a other than the OER peak suggesting the Al2O3 coating was chemically stable in the reaction environment. This observation is consistent with number of literature reports on the extreme stability of ALD Al2O3 coating.24,50,51 A slight cathodic shift of the onset potential was observed from 5nm-Al2O3-Au-3s-TiO2 sample. This enhancement could be attributed to the suppressed photo-excited electron-hole recombination and improved the OER kinetics at the interface due to amorphous Al2O3 coating.50,52,53 Absorption measurements showed that 5 nm Al2O3 coating could further improve the absorbance in the visible light region (Figure 4b). FDTD simulation confirmed the enhanced spatial distribution of electric field after adding 5 nm Al2O3 layer to Au NPs (inset of Figure 4b). As the dielectric constant of the surrounding medium increased by Al2O3 coating, the frequency and intensity of SPR peaks shifted to longer wavelength, which was consistent with previous observations.54-56 The 19 ACS Paragon Plus Environment


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absorption enhancements of Au NP decoration and Al2O3 dielectric coating were calculated and compared in Figure 4c, where Au NP yielded a maximum 150% enhancement and Al2O3 coating further raised the enhancement to 260% in the visible light regime (470 nm). To quantitatively illustrate the enhancement from surrounding dielectrics, the electric field profiles across the Al2O3-Au-TiO2 interfaces were extracted from the simulation results under 570 nm illumination. As shown in Figure 4d, the electric field at the Au/TiO2 interface was intensified by 800 %, which penetrated ~2.1 nm into TiO2. In the presence of Al2O3, the electric field enhancement occurred at both Au/TiO2 and Al2O3/Au interfaces. Besides, the penetration depth was extended to ~6.15 nm into TiO2. Both experimental and simulation results suggested that appropriate thickness of dielectric coating can bring positive influence to light absorption, and thus improve the PEC performance.


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Figure 4. a. J-V curves of Au-3s-TiO2 photoanodes without and with ALD Al2O3 coating obtained under dark and visible light illumination. Inset shows a TEM image of a Au-3s-TiO2 NR with ALD Al2O3 coating revealing a uniform and complete coverage of all Au NPs. b. Diffuse reflectance UV-visible absorption spectra of the bare TiO2, Au-3s-TiO2 and 5nm-Al2O3-Au-3s-TiO2. Inset shows the FDTD simulated spatial distribution of electric field around a 3s Au NP without (left) and with (right) a 5nm Al2O3 coating. The 570 nm monochromatic incident light is perpendicular to the TiO2

surface.

c.

The

absorption

enhancement

of

Au-3s-TiO2

and

5nm-Al2O3-Au-3s-TiO2 photo-electrodes in reference with bare 3D TiO2 NRs. d. Calculated electric field profiles across the TiO2-Au (pink) and TiO2-Au-Al2O3 (purple) interfaces. The profiles were plotted horizontally across the middle of the Au sphere. The size of the Au sphere was ~8 nm.

4. Conclusion In summary, 3D branched TiO2 NR architectures synthesized by SPCVD were decorated with Au NPs via a simple and effective sputtering process. This one-step Au NPs decoration approach offered a dense coverage of Au NPs on TiO2 NR surfaces, and excellent Au/TiO2 interface quality. Owing to the intrinsic SPR effect from Au NPs, the 3D Au-TiO2 NR architectures exhibited significantly higher PEC performance compared to the pristine 3D TiO2 NR photoanodes. The optimal sputtering time was found to be 3s, which produced ~8 nm Au NPs. Corresponding 21 ACS Paragon Plus Environment


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Au-3s-TiO2 electrode exhibited a 42% higher photocurrent density under AM 1.5G illumination. The improvement even reached 267% under visible light illumination. FDTD simulation suggested that the enhanced photoactivity of Au decoration is mainly caused by the electrical field amplification effect and hot electron generation upon SPR excitation in the visible light region. Ultrathin dielectric Al2O3 coating was further introduced by ALD. 5 nm was found to be the optimal coating thickness that led to more intensified SPR electric field, larger radiation areas, and deeper electric field penetration in the visible light region. Corresponding PEC performance was 87.8% higher than that of the uncoated Au-3s-TiO2 electrode. This work demonstrated a facile and very effective approach to further enhance the PEC performance novel 3D nanostructure-based photoanodes. The knowledge obtained from this work can be readily applied to other general photoanode improvements.

ASSOCIATED CONTENT

Supporting Information.

The following files are available free of charge.

The wight percentage of Au corresponding to different sputtering times, high resolution TEM image of a single TiO2 NR decorated with Au NPs, Wagner plot of Au in all Au NP-3D branched TiO2 NR architectures, SEM images and EDS spectra of Au NP-3D branched TiO2 NR architectures with different sputtering time, Stability 22 ACS Paragon Plus Environment

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evaluation of pristine 3D TiO2 NR photoelectrode, Jph-V curves of TiO2 and TiO2-3s Au samples, Analysis of electron transfer efficiency, rate and number, Jph-V curves for Au NP-decorated 3D TiO2 NR architectures made from 3s sputtering and Au colloidal solution method, diffuse reflectance UV-visible absorption spectra of the Au NP-3D branched TiO2 NR samples with different sputtering times (PDF).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Author Contributions §These

authors contributed equally.

ACKNOWLEDGMENT This work is primarily supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award # DE-SC0008711. H. Li thanks the China Scholarship Council for financial support.

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