Uniform Gold-Nanoparticle-Decorated {001}-Faceted Anatase TiO2

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Uniform Gold Nanoparticles Decorated {001}-Faceted Anatase TiO2 Nanosheets for Enhanced Solar Light Photocatalytic Reactions Huimin Shi, Shi Zhang, Xupeng Zhu, Yu Liu, Tao Wang, Tian Jiang, Guanhua Zhang, and Huigao Duan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12470 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 9, 2017

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ACS Applied Materials & Interfaces

Uniform Gold Nanoparticles Decorated {001}-Faceted Anatase TiO2 Nanosheets for Enhanced Solar Light Photocatalytic Reactions Huimin Shi,† Shi Zhang,‡ Xupeng Zhu,† Yu Liu,# Tao Wang,§ Tian Jiang,# Guanhua Zhang,‡ Huigao Duan*,‡

†School

of Physics and Electronics, State Key Laboratory of Advanced Design and

Manufacturing for Vehicle Body, Hunan University, Hunan 410082, P. R. China §

College of Physics and Electronic Engineering, Northwest Normal University,

Lanzhou 730070, P. R. China #

College of Opto-Electronic Science and Engineering, National University of Defense

Technology, Changsha, 410073, P. R. China ‡State

Key Laboratory of Advanced Design and Manufacturing for Vehicle Body,

National Engineering Research Center for High Efficiency Grinding, College of Mechanical and Vehicle Engineering, Hunan University, Hunan 410082, P. R. China E-mail: [email protected]

KEYWORDS: {001}-faceted anatase TiO2 nanosheets, solar light photocatalytic reactions, plasmon resonance, Au nanoparticles, tandem-type electrons separation

ACS Paragon Plus Environment


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ABSTRACT The {001}-faceted anatase TiO2 micro/nanocrystals have been widely investigated for enhancing the photocatalysis and photoelectrochemical performance of TiO2 nanostructures but their practical applications still require improved energy conversion efficiency under solar light and enhanced cycling stability. In this work, we demonstrate the controlled growth of ultrathin {001}-faceted anatase TiO2 nanosheets on flexible carbon cloth for enhancing the cycling stability, and the solar light photocatalytic performance of the synthesized TiO2 nanosheets can be significantly

improved

by

decorating

with

vapor-phase

deposited

uniformly-distributed plasmonic gold nanoparticles. The fabricated Au-TiO2 hybrid system shows an eight-fold solar light photocatalysis enhancement factor in photo-degrading Rhodamine B, a high photocurrent density of 300 µA cm-2 under the illumination of AM 1.5G, and 100% recyclability under consecutive long-term cycling measurement. Combined with electromagnetic simulations and systematic control experiments, it is believed that the tandem type separation and transition of plasmon-induced hot electrons from Au nanoparticles to {001} facet of anatase TiO2, and then to neighboring {101} facet is responsible to the enhanced solar light photochemical performance of the hybrid system. The Au-TiO2 nanosheet system well addresses the problems of limited solar-light response of anatase TiO2 and fast recombination of photo-generated electron-hole pairs, representing a promising high-performance recyclable solar light responded system for practical photocatalytic reactions.

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1. INTRODUCTION Anatase titanium dioxide (TiO2) has been proved the most reactive phase for photocatalysis in environmental purification and phtotoelectrochemical (PEC) energy conversion systems.1-6 The inherent fast recombination of photo-generated electron-hole pairs (e--h+) and limited light response range (< 387 nm) are two main obstacles that limit its photocatalytic performance.7-9 A great number of efforts have been made to mitigate these photocatalysis inefficiencies, in which controllably synthesizing and tailoring high-reactivity facets dominated anatase micro/nanocrystal particles by crystal engineering is supposed to be an effective strategy since the surface stability and reactivity of anatase are closely correlated to its surface chemistry properties.10-16 The equilibrium shape of an anatase crystal is a slightly truncated bipyramid enclosed by more than 94% {101} and fewer {001} facets according to surface energies and the Wulff construction calculated in vacuum.13 The {100} is the most stable facet for anatase TiO2 among oxygenated surfaces, and the {101} facet is the most stable facet under clean and hydrogenated conditions.17 Although both theoretical and experimental studies indicated that the {001} facet is especially reactive in the equilibrium owing to its higher electron mobility rate and reactivity, the synthesis of {001} facets dominated anatase TiO2 is thermaldynamic unfavorable because the average surface energy of typical anatase TiO2 facets follows 0.90 J m-2 for {001} > 0.53 J m-2 for {100} > 0.44 J m-2 for {101}.12, 13 Recently, inspired by the pioneering work by Yang et al. who successfully synthesized uniform anatase TiO2 single crystals with 47% of {001} facets,14 various {001}-faceted anatase TiO2 micro/nanocrystals with different morphologies, structures and ratio of {001} facets were exploited using different capping agents for photocatalytic and PEC reaction applications.11,

18-21

2


In addition, different


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{001}-faceted anatase micro/nanocrystal composites with improved photocatalytic and PEC activity were also achieved by doping foreign atoms, introducing surface defects and hybridizing with other materials etc.,22-27 owing to the inhibited recombination of e--h+ and broadened light response in composite systems. Among them, Au nanoparticles (NPs)/{001}-faceted anatase TiO2 micro/nanocrystals system is particularly interesting due to its improved separation of e--h+ and enhanced visible light absorption enabled by localized surface plasmon resonance (LSPR) of Au NPs.28, 29

Diverse Au/{001}-faceted anatase TiO2 nanocrystals with enhanced photocatalytic

performance have been demonstrated for visible light photocatalysis, PEC water splitting and solar-to-hydrogen conversion.30, 31 Though much progress has been made on the synthesis of {001}-faceted TiO2 structures, however, the existing work focused mainly on substrate-free powder photocatalysts composed by micro/nanocrystal particles, which is unsuitable for aqueous-based photocatalytic and PEC reactions. Specifically, in photocatalytic water purification process, the aggregation of photocatalysts will not only degrade the photocatalytic activity and stability of whole systems, but also may cause second pollution by the incomplete removal of photocatalysts.20, 24, 26, 32 Meanwhile, in PEC water splitting process, casting powder photocatalysts on conductive substrate as work electrode is inevitable in order to achieve efficient collection and fast transfer of photo-generated electrons.30 In this case, the randomly assembled micro/nanocrystal particles may hinder the separation and transition of electrons because of the energy barriers formed on the interfaces between micro/nanocrystals. Besides, the PEC water splitting is an interfacial reaction process, the cast film electrode may reduce the effective active surface between photocatalysts and solution, lowering the utilization efficiency of light. In addition, current existing Au/TiO2 hybrid structures usually 3


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involve aqueous wet-chemical synthesis process of Au nanoparticles, in which the LSPR induced excitation and transfer of hot electrons may be weakened due to the coating layer on the surface of Au NPs resulted from the surfactant/reductant (sodium sulfocyanate, sodium borohydride, oleylamine and polyvinylpyrrolidone etc.).7, 33, 34 Even in surfactant/reductant-free photoreduction process, the uniform distribution and size control of Au NPs on target photocatalysts are extremely challenging in aqueous solution due to the aggregation of micro/nanocrystals.33, 35, 36 Therefore, sophisticated design and synthesis of a photocatalytic system to address the aforementioned issues is highly desirable in practical environmental purification and PEC water splitting reactions. In this work, we demonstrate an Au-NPs-decorated {001}-faceted anatase TiO2 nanosheets (Au-TiO2 NS) hybrid structure on flexible carbon cloth (CC) for solar light photocatalysis. The obtained structure is extremely favorable to aqueous photocatalytic and PEC reactions due to the broadened visible light absorption from the strong LSPR, the efficient separation of e--h+ achieved by the tandem type transition for electrons, the high absorbance ability of {001}-faceted anatase and the fast electrons collection by the binder-free conductive substrate. The Au-TiO2 NS shows an eight-fold enhancement on the photo-degrading of Rhodamin B (RhB) dye under the solar light, a high photocurrent density of 300 µA cm-2 under the illumination of AM 1.5G and 100% recyclability under consecutive long-term cycling measurement. The electromagnetic simulations in company with systematic control experiments revealed that the broadened visible light absorption and the suppressed recombination of e--h+ enabled by the tandem type separation and transition path for LSPR-induced hot electrons from Au NPs to {001} facet of TiO2, and then to neighboring {101} facet are responsible to the enhanced solar light photocatalytic 4



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performance of the hybrid system. With the high photocatalytic performance and stability, our engineered flexible Au-TiO2 NS system holds the promise in applications such as solar light environmental purification and PEC water splitting.

2. EXPERIMENTAL SECTION Typically, the chemicals include hydrochloric acid (HCl, 37%, Tianjin Chemicals), acetone (CH3COCH3, Tianjin Chemicals), ethanol (C2H5OH, Tianjin Chemicals), chloroauric acid (HAuCl4. 4H2O, purity > 99.999%, Sinopharm Chemical Reagent Co., Ltd), titanium butoxide (C16H36O4Ti, ≥ 99%, Aladdin Industrial Co., Shanghai, China), hydrofluoric acid (HF, ≥ 99%, Aladdin Industrial Co., Shanghai, China), benzoquinone (BQ, ≥ 99%, Aladdin Industrial Co., Shanghai, China), isopropyl alcohol (IPA, ≥ 99%, Aladdin Industrial Co., Shanghai, China) and sodium oxalate (Na2C2O4, ≥ 99%, Aladdin Industrial Co., Shanghai, China), high-purity nitrogen (N2, 99.999%)and carbon cloth (WOS1002, Rocktek Co. Ltd, China). Noting that all the chemicals were of analytic grade and used as received without further purification. The deionized water (18.25 MΩ) was used during whole experiments. The synthesis of Au-TiO2 NS included two stages. The TiO2 NS was synthesized by hydrothermal process, and then the Au NPs were deposited by our previously developed vapor phase deposition method.37, 38 In details, 1.2 mL of C16H36O4Ti was slowly dropwise added into 30 mL mixture solution of HCl and H2O with volume ratio of 2:1 and stirred at 600 rad min-1 for 3 h until the yellowish transparent solution formed. Then 0.8 mL of HF was added into the above solution and stirred for another 15 min (Cautions: the HF is extremely dangerous and the full protection and attention are needed during experiments). After the precursor was transferred into a 50 mL polytetrafluoroethylene (Teflon)-lined stainless-steel autoclave, the cleaned 5


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CC (3*3 cm2) with 3 nm seed layer (deposited by ALD) was immersed into the solution and sent for hydrothermal growth at 180℃ for 16 h in a drying oven. As the autoclave cooled down to room temperature naturally, the sample was dried at 90℃ for 12 h in a drying oven after rinsed with deionized water and ethanol for several times to obtain TiO2 NS. And then the dried TiO2 NS was sent into a vacuum tube furnace for the deposition of Au NPs. In brief, a tiny quartz boat with 50 µL 100 mM HAuCl4 solution was placed at high temperature zone, the TiO2 NS was placed right above the quartz boat by a quartz holder with the distance of 3 cm. The deposition of Au NPs was carried out on the pressure of 800 mTorr with the N2 flow rate of 2 sccm. The temperature increased from room temperature to 500℃ with the heating rate of 35℃ min-1, and held on at 500℃ for another 2 h. After the system cooled down to room temperature naturally, the final Au-TiO2 NS was synthesized. The morphology and structure of sample were characterized by field emission scanning electron microscopy (FE-SEM, Sigma HD, accelerating voltage of 10 kV) with an energy dispersive spectrometer (EDS). The crystal phase structure was examined

by

a

powder

X-ray

diffraction

(XRD,

Siemens

D-5000,

Cu Kα λ=0.15418 nm). The microstructure was characterized by a high resolution transmission electron microscope (HR-TEM, Tecnai GZ F20) with an energy dispersive X-ray detector (EDX). Raman spectrum was investigated by a confocal Raman microscopy system (WITec alpha 300R, with laser wavelength of 532 nm). The X-ray photoelectron spectroscopy (XPS) was performed by a Thermo Fisher Scientific K-Alpha 1063 system. UV-Vis absorption spectrum was carried out on a UV-Vis spectrophotometer (TU-1901) with an integrating sphere. The total organic carbon (TOC) was tested on TOC-V CPH, Shimadzu Corporation, Japan. 6



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3. RESULT AND DISCUSSION

Figure 1. (a)-(c) Schematic illustration of the synthesis process flow for Au-TiO2 NS. (d)-(f) Digital photographs demonstrating the corresponding color evolution of the samples at each synthesis stage.

The synthesis process of the Au-TiO2 NS is schematically illustrated in Figure 1. The {001}-faceted anatase TiO2 NS were directly grown on CC by hydrothermal process. Typically, a piece of CC coated with a TiO2 seeding layer (Figure 1a) was immersed into the precursor, and then the {001}-faceted anatase TiO2 NS (Figure 1b) were grown after the hydrothermal process at 180℃ for 16 h. The Au NPs were deposited on as-prepared {001}-faceted anatase TiO2 NS via our previously developed vapor deposition process using HAuCl4 as the precursor (Figure 1c).37,

38

The color

evolution of the samples at each stage is demonstrated by the corresponding digital photograph in Figure 1d-f. The light blue TiO2 NS on CC (Figure 1d) turned into aubergine Au-TiO2 NS (Figure 1f) after Au NPs deposition. The distinct color 7


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evolution was resulted from the strong LSPR absorption and scattering of Au NPs in the visible light region. The conformal surfactant-free vapor deposition process enabled uniform distribution of Au NPs on the surface of whole TiO2 NS

Figure 2. (a)-(c) SEM images of TiO2 NS. (d) and (e) SEM images of Au-TiO2 NS. (f) The statistic size distribution of Au NPs from (e).

As displayed by SEM images in Figure 2a-c, the carbon fibers were totally covered by the thin truncated octahedral TiO2 NS with the thickness of around 50 nm. The intercrossed TiO2 NS vertically anchored on the carbon fibers. Such a binder-free tight bonding between TiO2 NS and conductive CC not only provides fast transfer path for electrons, but also guarantees the lossless recycle of photocatalysts during the photocataltyic process, improving the long-term stability of the whole system.37, 39 In addition, the three-dimensional structure assembled by two-dimension ultrathin nanosheets possesses not only higher surface specific areas comparing with other planar structures, but also more active reaction sites resulted from the exposed {001} high energy reactive facet. Particularly, previous study demonstrated that the surface heterojunction could form between holes accumulated oxidative {001} facets and 8



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electrons accumulated reductive {101} facets owing to the more negative flat-band potential of anatase {001} facets, which is favorable for the separation of e--h+.40 Therefore, the as-synthesized {001} and {101} facets composed TiO2 NS are particularly benefiting to photocatalytic applications. Figure 2d-e demonstrates that the Au NPs were uniformly distributed on the exposed surfaces of TiO2 NS. The main size distribution of Au NPs, obtained from the statistic result in Figure 2e, was around 10 nm (Figure 2f). The uniform distribution of Au NPs on all exposed surfaces of TiO2 NS derived from the crucial advantage of our vapor deposition process: the homogeneous evaporation and deposition of vaporized Au precursor on the spatial location confined TiO2 NS.

Figure 3. (a) and (b) TEM, (c) High resolution TEM images and (d) SAED pattern of Au-TiO2 NS. The inset in (b) is the EDX curve of the sample. 9


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Figure 3 displays TEM images of Au-TiO2 NS. Figure 3a-b reveals that the thickness of nanosheets was about 50 nm and the Au NPs were uniformly distributed on the surface of exposed nanosheets. The EDX inset in Figure 3b reveals the existence of Au, Ti, and O in Au-TiO2 NS. High resolution TEM image in Figure 3c displays the obvious monocrystal lattice pattern of anatase phase TiO2 material, in which the conspicuous crystal plane with the interspacing of 0.35 nm corresponding to the (101) crystal plane of anatase TiO2, was observed. The (111) crystal plane of Au nanoparticle with the interspacing of 0.23 nm was also obviously displayed in the same sample. Further selected area electron diffraction (SAED) pattern of the sample in Figure 3d demonstrates the orderly arranged diffraction spots of monocrystal anatase TiO2 material, indicating the monocrystalline nature of as-synthesized TiO2 NS. Besides, a blurry diffraction ring corresponding to (220) diffraction plane of Au NPs was also displayed in the SAED pattern (identified with white dash arrow), further proving the existence of Au NPs.

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(b)

TiO2 NS

(101)

Intensity (a.u.)

(204)

(116) (220)

(200)

(105) (211)

(004)

Intensity (a.u.)

TiO2 NS Eg

Au-TiO2 NS

(215)

(a)

Au-TiO2 NS

B1g

A1g+B

1g

Eg

D

TiO2-JCPDS: 21-1272

20

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O 1s

(c)

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C 1s

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Ti 2p3/2

Au 4d

Au 4f

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1000

G

Raman shift (cm )

(d)

Ti 3p

0

500

Full spectrum

Ti 2p

Intensity (a.u.)

50

2θ Degree

Ti 2p1/2

450

455

460

465

470

Binding energy (eV)

Binding energy (eV)

(f)

(e) Au 4f

O 1s

Intensity (a.u.)

Au 4f7/2

Au 4f5/2

Intensity (a.u.)

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78

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90

92

525

94

528

531

534

537

540

Binding energy (eV)

Binding energy (eV)

Figure 4. (a) and (b) The XRD patterns and Raman spectra of the samples, respectively. (c)-(f) The full XPS spectrum, the fine spectrum of Ti 2p region, Au 4f region and O 1s region of Au-TiO2 NS, respectively.

The crystalline structure of sample was further characterized by powder XRD and Raman spectra. The sharp diffraction peaks displayed in XRD patterns of TiO2 NS (solid black) and Au-TiO2 NS (solid red) in Figure 4a fit well with the standard diffraction pattern of anatase TiO2 (JCPDS: 21-1272). There were no observable diffraction peaks related to Au NPs observed in the XRD pattern of Au-TiO2 NS due 11


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to the relatively low integral content of Au in the whole sample. The Raman bands centered at 143 cm-1 (Eg), 393 cm-1 (B1g), 512 cm-1 (A1g+B1g) and 637 cm-1 (Eg) in Figure 4b were character optical modes of anatase TiO2.41, 42 Both samples displayed the conspicuously identical Raman bands, indicating unchanged chemical bonds of TiO2 after the deposition of Au NPs. The weak Raman bands located at 1350 cm-1 and 1582 cm-1 are D band and G band from CC substrate, respectively. The detailed surface chemical composites and oxidation states of Au-TiO2 NS were further confirmed by XPS. The Ti 2p, Ti 3p, Au 4d, Au 4f, O 1s and C 1s peaks are displayed in the full XPS curve of Au-TiO2 NS in Figure 4c, in which the C 1s peak is mainly resulted from the CC substrate. It should be pointed out that no peaks of F element were observed, which was in agreement with the previous study that the absorbed F element from the HF can be eliminated at high temperature.43 In our synthesis process, the absorbed F element was removed spontaneously during the deposition of Au NPs, simultaneously eliminating the negative effect of absorbed F on the photocatalytic reaction.11 The Ti 2p spectrum in Figure 4d displays typical bonding of Ti-O at the binding energy of 459.2 eV (Ti 2p3/2) and 464.8 eV (Ti 2p1/2), indicating the state of Ti4+ in sample. The Au 4f spectrum in Figure 4e reveals the peaks at the binding energy of 83.9 eV (Au 4f7/2) and 87.6 eV (Au 4f5/2) from the metallic state Au. Figure 4f displays an asymmetric XPS pattern of O 1s, which can be fitted by two peaks localized at the binding energy of 530.35 eV and 532.15 eV, corresponding to the lattice oxygen O2- from Ti-O and surface absorbed low amount of organic remnant contaminations or ambient moistures, respectively.

12



(a)1.0

(b) Blank, kapp=3.43*10

4

Ct/C0

-lnCt/C0

0.6 0.4 Blank TiO2 NS

-30 -20

-10

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2nd 3rd 4th

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-2

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TiO2 NS, kapp=1.27*10

Dark

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300 250 200 150 100 50 0 0.0

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4.0

Figure 5. (a)-(c) Photo-degradation of RhB by Au-TiO2 NS under solar light, the corresponding reaction rate constant and the consecutive circulation test, respectively. (d) Photoresponse of TiO2 NS and Au-TiO2 NS under chopped solar light illumination (AM 1.5G) at 0.5 V vs Ag/AgCl. (e) The long-term PEC stability test of Au-TiO2 NS.

To evaluate the photocatalytic activity of the sample, the photo-degradation of RhB aqueous solution was first performed under the illumination of simulated solar light. The photocatalytic activity test in Figure 5a reveals that the RhB aqueous was totally degraded by Au-TiO2 NS after 40 min illumination, while only about 40% of RhB was degraded under the same conditions by TiO2 NS. The corresponding 13


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reaction rate constant of Au-TiO2 NS was more than 8 times higher than that of bare TiO2 NS (Figure 5b). Note the self-degradation of RhB under the illumination was rather low (solid black in Figure 5a and 5b) and was negligible during the photocatalysis evaluation. In addition, the by-product of RhB dye was analyzed through measuring the content of TOC of solution during photo-degradation. The corresponding results were displayed in Figure S1 (Supporting Information, SI), which indicated that the content of TOC in target solution decreased to about 30% after degraded by Au-TiO2 NS under the illumination of AM 1.5G light. The decrease of TOC after photocatalytic process confirmed that the RhB solution was indeed decomposed instead of the de-coloration effect of photocatalyst, indicating the high mineralization ability of the sample. The cycling test in Figure 5c indicates that the photocatalytic activity of Au-TiO2 NS almost underwent no loss after consecutive 10 circulations, demonstrating the excellent cycling stability of the sample. The remarkable cycling stability of Au-TiO2 NS was inherited from the intrinsic outstanding chemical stability of TiO2 and Au NPs. In addition, neither evident desquamation of Au NPs from TiO2 NS nor TiO2 NS from CC were observed in the SEM images in Figure S2 after 10 times cycling test, confirming the strong binding between Au NPs, TiO2 NS and CC. Note that the sample was 100% recyclable owing to the tight bonding between Au-TiO2 NS photocatalysts and the substrate, which well addresses the problems related to the loss of photocatalysts and concomitant second pollution generally encountered by the micro/nanocrystal photocatalysts (caused by the incomplete removal of powder during the practical applications). The high photocatalytic degrading ability and long term stability of Au-TiO2 NS originated, on the one hand, from the strong LSPR enhancement of Au NPs in visible light. On the other hand, the flexible three dimensional CC substrate with high specific areas 14



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provided perfect support for photocatalysts, and its micro-pores acted as convenient channels for pollutants transport. Moreover, we also carried out a series of scavenger experiments to investigate the critical reactive species in current hybrid system. The corresponding results were .

displayed in Figure S3, which indicated that both superoxide radical ( O2-) and .

hydroxyl radical ( OH) were active species in photo-degrading RhB solution when the .

.

BQ, IPA and Na2C2O4 were used as the scavenger for O2-, OH and trapped holes.44, 45 It is reasonable that the trapped hole impact the slightest effect on the photocatalytic activity in photo-degrading process due to the good monocrystalline and less defects of our hydrothermal synthesized TiO2 NS (confirmed by previous SEM, TEM and .

.

XRD results). Considering that the O2- and OH were mainly resulted from the reduction of O2 by photo-generated electron and the oxidation of H2O by photo-generated hole, respectively. It is reliable to conclude that the photo-generated electron-hole pairs were effectively separated and participated in photo-degrading process in current hybrid system. To further verify the PEC performance of the synthesized Au-TiO2 NS, the PEC measurements were carried out on three-electrode electrochemical system in 1 M KOH solution under the simulated solar light. The as-synthesized Au-TiO2 NS with an area of 1×1 cm2 was directly used as work electrode, and a decent photocurrent density of 300 µA cm-2 at 0.5 V vs Ag/AgCl was achieved under the illumination of a simulated AM 1.5G light source with a power density of 100 mW cm-2, which is more than four times larger than that of the TiO2 NS (about 70 µA cm-2, Figure 5d), indicating the significantly enhanced solar light PEC performance of Au-TiO2 NS. Taking into the consideration of the structural difference between bare TiO2 NS and 15


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Au-TiO2 NS hybrid system, it is trustworthy to know that the increase photocurrent density of Au-TiO2 NS originated from the transfer of LSPR induced hot electrons from Au nanoparticles to TiO2 NS and then collected by conductive CC collector.46 It is worth noting that the CC in Au-TiO2 NS here not only acted as a flexible substrate to provide large specific surface area and micro-channels for solution diffusion, but also a binder-free collector for the efficient photo-generated electrons transfer due to its high conductivity, which is unremarkable in photo-degrading process. The long-term PEC stability test displayed in Figure 5e shows that the photocurrent density of Au-TiO2 NS kept at about 300 µA cm-2 at 0.5 V vs Ag/AgCl after uninterrupted 4 h illumination, confirming its outstanding long-term stability in PEC application. The gradual increase of photocurrent density during long-term stability test is supposed to be resulted from the slight temperature increase of KOH solution during the continuously illumination. It has been proved that the Au nanoprticles played different roles in Au/TiO2 system in enhancing the photocatalytic reaction under the illumination of different light, which served as sinks for electrons to improve the separation of photo-generated electrons excited from valence band of TiO2 under the illumination of UV light and a plasmon source under visible light.37 In order to understand how Au NPs enhanced the solar light photocatalytic reactions of Au-TiO2 NS, we carried out a systematic study on the optical response of Au NPs under different light illumination and analyzed the possible transfer path of LSPR induced hot electrons. We first compared the photocatalysis enhancement of Au-TiO2 NS under the illumination of UV-light (λ< 400 nm, UVREF400) and visible light (λ> 420 nm, UVCUT420), and the corresponding results were displayed in Figure S4 and Figure S5, respectively. The reaction rate constants depicted in Figure S4b and Figure S5b demonstrated that, 16



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compared with that TiO2 NS, about 1.4 and 21 times improvement were achieved for Au-TiO2 NS under the illumination of UV-light and visible light, respectively. The UV light photocatalysis enhancement was rather low, therefore, the remarkable solar light photocatalytic activity of Au-TiO2 NS mainly originated from the strong enhancement of Au NPs in visible light. One of the dominant positive effects of Au NPs in enhancing photocatalysis is their strong LSPR absorption in the visible light range.47-54 In this process, as illuminated by visible light, the surface plasmon polariton (SPP) was first excited on the surface of Au nanoparticles by visible light, then the SPP decays quickly through Landau damping to excite hot electrons, the excited hot electrons generally lose their energy in three ways, including emitting photons through recombining with holes, transferring their kinetic energy to thermal by non-elastic interactions between electron-electron and electron-phonon and participating in photocatalytic reactions by injecting into conduction band of neighboring semiconductors.16,

49, 53

To investigate the specific enhancement

mechanism in current system, the UV-Vis absorption spectra of the samples were first carried out in Figure 6a. Both samples revealed a conspicuous absorption edge at the wavelength of around 390 nm, which agreed well with the band edge absorption of anatase TiO2 (~387 nm), corresponding to the band gap energy of 3.2 eV. As expected, a broad absorption peak in the range of 400 nm to 750 nm, corresponding to the LSPR of Au NPs, was observed in the Au-TiO2 NS. In which, the small peak located at the wavelength of 520 nm was inter-band transition and LSPR absorption of Au NPs, while the peak at 580 nm is supposed to be resulted from the intercoupling of Au NPs due to their small gaps.55, 56 The broad LSPR absorption of visible light is especially advantageous to the photocatalytic reaction of Au-TiO2 NS due to the broadened light response range. The broad LSPR absorption of Au-TiO2 NS 17


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originated from not only the relatively broad size distributions of Au NPs (4-20 nm), but also the intercoupling of Au NPs due to their narrow gaps, as displayed in Figure 2e.51, 57 As a comparison, no absorption peak was observed in TiO2 NS. The gradual intensity increase of spectra in visible light came from the absorption of the CC substrate (displayed in Figure S6).

Figure 6. (a) The experimental measured UV-Vis light absorption spectra of TiO2 NS and AuTiO2 NS. (b) The simulated absorption cross section of individual TiO2 NS and Au-TiO2 NS. (c) The simulated electric field distributions of individual Au-TiO2 NS with different polarizations of incident light. (d) The schematic illustration of possible LSPR enhancement mechanism in Au-TiO2 NS.

Further electromagnetic simulations were conducted by finite-different time-domain (FDTD) method on an individual nanosheet to understand the LSPR enhancement of Au-TiO2 NS. The schematic model used for FDTD simulation was displayed in Figure S7. The simulated absorption cross sections in Figure 6b (the 18



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polarization of the incident light follows oz direction) agreed well with the UV-Vis absorption in Figure 6a. The peak centered at the wavelength of 522 nm was the inter-band transition and intrinsic LSPR peak of Au NPs, and the small absorption peak at 554 nm derived from the intercoupling of Au NPs. The slight blueshift of coupling peak compared with that of experimental result was mainly resulted from the deviation between the model used in simulations and the practical experimental sample. No absorption was observed in the simulated spectrum for TiO2 NS in visible light region, which was in line with the light response property of TiO2. Besides, the simulated electromagnetic field distributions in Figure 6c demonstrated that lots of strong electric field localized regions (hot spots) were formed at the interfaces of Au/TiO2 and the interfaces of neighboring Au/Au when the polarization of incident light parallels to oy axis and oz axis, respectively.51 The strong localized electric fields enabled the direct excitation of hot electrons in Au NPs and the generation of electron-hole pairs in TiO2 by direct resonance energy transfer from the LSPR dipole to TiO2, improving the photocatalytic reactions in Au-TiO2 NS.50 However, no obvious field localized region were observed for an individual TiO2 NS in Figure S7, confirming the essential effect of Au NPs in the LSPR absorption and field localization. The fast separation and transition of e--h+ were the most determinate factors in photocatalysis and PEC reactions owing to their interfacial reaction features. Regarding to current Au-TiO2 NS system, since the work function of gold is larger than that of anatase TiO2, the position of Fermi level of Au nanoparticle located between the conduction band and the valence band of TiO2 NS. When the Au naoparticle and TiO2 nanosheet contacted, the Schottky barriers formed at the Au/TiO2 interfaces.58 As schematically displayed in Figure 6d, when the Au-TiO2 NS 19


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was illuminated by solar light, the LSPR induced hot electrons in Au NPs first transfer to {001}-faceted TiO2 over the Schottky barriers formed at the Au/TiO2 interfaces,58 and then the electrons in {001} facet quickly transfer to {101} facet through the surface heterojunction between them.37, 40 Therefore, the electrons and holes were effectively separated by such a tandem type separation path, leading to hole-rich Au NPs while electron-rich {101} facets. Subsequently, the separated holes .

and electrons participated in photo-oxidation reaction, H2O + h+ OH and .

photo-reduction reaction, O2 + e- O2-, respectively.37 The separated photocatalytic reactions performed at the different surfaces of Au NPs and {101} facets of TiO2 NS greatly boosted the photocatalytic efficiency due to the suppressed recombination of e--h+. The previous tests on active species by using different scavengers in Figure S3 have proved both the photo-generated electrons and hole were taking part in the photocatalytic process, confirming the effective separation of e--h+. To verify this hypothesis, photoluminescence (PL) was carried out to investigate the recombination of e--h+ of samples. The PL spectra in Figure S9 shows that the PL intensity of Au-TiO2 NS decreased tremendously compared with that of TiO2 NS, indicating the reduced recombination of e--h+ in Au-TiO2 NS. In other words, the average lifetime of e--h+ in Au-TiO2 NS was prolonged. It is worth noting that the electrons excited from valence band to conduction band of TiO2 NS by UV-light also have prolonged lifetime due to the effective separation caused by the surface heterojunctions between {001} and {101} facets.40, 59 In addition, the strong absorption ability of high energy {001} facets and enhanced interactions between light, photocatalysts and the pollutants enabled by the light confinement of the hierarchical structures also contributed to the solar light photocatalytic reactions of the system through their complex synergetic interactions. 20



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4. CONCLUSIONS In conclusion, we designed a uniform Au nanoparticles decorated {001}-faceted anatase TiO2 NS structure on flexible carbon cloth for enhanced solar light photocatalytic reactions. The as-synthesized binder-free hierarchical structure well addressed the loss of photocatalysts and concomitant second pollution that the micro/nanocystal particles generally encountered in aqueous photocatalytic process. The Au-TiO2 NS achieved an eight-fold solar light enhancement on the photo-degradation of RhB and a photocurrent density of 300 µA cm-2 under the illumination of AM 1.5G. Further cycling test on the degradation of RhB and long-term PEC stability measurement confirmed the excellent stability and 100% recyclability of the sample. Combined with the electromagnetic simulations, the broadened visible light absorption and the tandem type separation and transition process of plasmon-induced hot electrons from Au nanoparticles to {001} facet of anatase TiO2, and then to the neighboring {101} facet are responsible to the enhanced photocatalytic performance of the hybrid system. The designed Au-TiO2 NS hybrid system demonstrated a high solar light photocatalytic activity and decent long-term stability, which provides an alternative photocatalysts for photocatalytic and PEC applications.

ASSOCIATED CONTENTS Supporting Information The supporting information includes the experimental details related to photocatalytic and photoelectrochemical test, Figure S1-S7.

AUTHOUR INFORMATION 21


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Corresponding Author *

E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We gratefully acknowledge the financial support from National Natural Science Foundation of China (Grant nos. 11574078), Foundation for the authors of National Excellent Doctoral Dissertation of China (201318), the Natural Science Foundation of Hunan Province (2015JJ1008, 2015RS4024), and the Fundamental Research Funds for the Central Universities (531107040992).

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Uniform Gold-Nanoparticle-Decorated {001}-Faceted Anatase TiO2

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