Fast charge separation and photocurrent enhancement on black TiO2 ...

Accepted Manuscript Fast charge separation and photocurrent enhancement on black TiO2 nanotubes cosensitized with Au nanoparticles and PbS quantum dots Kang Du, Guohua Liu, Xuyuan Chen, Kaiying Wang PII:

S0013-4686(18)31019-3

DOI:

10.1016/j.electacta.2018.05.014

Reference:

EA 31800

To appear in:

Electrochimica Acta

Received Date: 26 April 2018 Accepted Date: 1 May 2018

Please cite this article as: K. Du, G. Liu, X. Chen, K. Wang, Fast charge separation and photocurrent enhancement on black TiO2 nanotubes co-sensitized with Au nanoparticles and PbS quantum dots, Electrochimica Acta (2018), doi: 10.1016/j.electacta.2018.05.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT Fast Charge Separation and Photocurrent Enhancement on Black TiO2 Nanotubes co-sensitized with Au Nanoparticles and PbS Quantum Dots Kang Du, Guohua Liu, Xuyuan Chen, and Kaiying Wang*

Raveien 215, Horten 3184, Norway *

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Department of Microsystems, University College of Southeast Norway, Campus Vestfold,

Corresponding Author: [email protected]

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Tel: +47 3100 9317

Abstract: Reduced TiO2 nanomaterials (so-called black TiO2) attract intensive research

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interests due to their excellent capability for harvesting solar energy. To obtain a high conversion rate of solar energy to electrical energy, we loaded Au nanoparticles (NPs) and PbS quantum dots (QDs) on as-prepared TiO2 nanotubes (TNT) and black TiO2 nanotubes by magnetron sputter technique and subsequently dip coating for enhancing

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their photoelectric behaviors. It was discovered that the loadings of Au NPs and PbS QDs result in over 45% enhancement of photocurrent for the black TNT under visible light, and only ~ 9.12% enhancement on the as-prepared TNT (without reduction process). The

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loadings of Au NPs and PbS QDs might act as separation centers rather than only light absorbers to accelerate electron-hole separation and transport. The black TiO2 nanotube

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composites could be considered as an excellent photoelectrode material applied in the field of solar energy conversion due to their high visible light harvesting and photoelectric response.

Keywords: black TiO2 nanotubes; Au Nanoparticles; PbS quantum dots; fast charge separation.

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1. Introduction The increasing energy demand and polluting environment push researchers and

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engineers to develop high-efficiency renewable energy technologies [1]. The ideal strategy is to convert sunlight into electricity or directly convert it into chemical fuels such as hydrogen, methane, etc. [2]. It is well-known that nanomaterials can be employed

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as an efficient mediator to improve the efficiency of conversion [3]. Fujishima and Honda firstly reported the phenomenon of photocatalytic water splitting by TiO2 as photoanode

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material [4]. Since then, TiO2 materials have gained intensive interests and used as one of the most significant mediators in several fields due to their high photoactivity, low-cost, non-toxicity, abundance as well as excellent chemical stability [5]. Unfortunately, TiO2 materials have a relatively large band gap (anatase of 3.2 eV and rutile of 3.0 eV) [6], and thus retard its applications due to relatively low absorption ratio of solar energy (only 4-5%

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UV light in solar spectrum) [7]. To sufficiently utilize visible light (~ 46%) of solar energy, various efforts and methods have been used to synthesize solar-driven TiO2 based

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nanomaterials in the past decades [8]. Generally, there are two strategies for the purpose: one is bandgap narrowing by doping non-metallic elements (H, C, N, F, P, S, etc.) [9];

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alternative one is sensitizing with metallic elements (Au, Ag, Pt, Cu, etc.) [10-13], or narrow bandgap semiconductors (PbS, Cu2O, CdS, CdSe) [14, 15]. Chen et al. put forward the black TiO2 material prepared by a high-pressure

technique in the H2 atmosphere [16]. Compared with conventional TiO2 materials, the black TiO2 nanomaterial exhibits advantages: high absorption in visible spectrum [17] and the narrowed bandgap of 1.2 eV [18]. Inspired by these advantages, several approaches have been initiated for synthesizing black TiO2 nanomaterials, such as 2

ACCEPTED MANUSCRIPT hydrogen-argon treatment at high temperature [19], H2S gas treatment [20], aluminum thermal reduction [21], magnesiothermic reduction [22], NaBH4 reduction [23], flame reduction method [24] and electrochemical reduction [25]. Through the processes of the

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above methods, TiO2 nanomaterials display various colors, such as gray [26], brown [27], dark blue [28] and black [29], which induce absorption enhancement in visible regime [30-32]. Although the mechanism of absorption enhancement for “black” TiO2 nanomaterials is still not well explained, several researchers have proved that the

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crystalline structure (disordered lattice/layer [33]), chemical species (Ti2O3 shell [34],

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Ti3+ sites, Ti-OH, and Ti-H groups), and/or energy level (band-edge shifting) are formed on the very top surface (several nanometers) of black TiO2 nanomaterials [35-39]. One common understanding is that the formed surface defects/species or disordered layer on black TiO2 nanomaterials could facilitate the enhancement of charge carrier density by several orders of magnitude as compared to that of pristine TiO2 nanomaterials [40, 41].

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However, the photoelectrochemical performance of black TiO2 nanomaterials is not significantly improved as their dramatic absorption enhancement in visible spectrum [30, 42, 43]. The possible reason is that the large majority of photo-generated carriers on

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black TiO2 nanomaterials are recombined and cannot involve into redox reactions, this

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ultimately results in the decrease of photoelectrochemical activities [39, 44]. The most of the photo-induced carriers may generate on the surface defected/disordered layer rather than the bulk pristine TiO2, because the “black” disordered layer exhibits strong absorption in visible regime. However, the majority of photo-induced carriers are prone to the bulk recombination of pristine TiO2 due to the seamless barrier at Ti3+/TiO2 interface. Only electrons or holes migrated on the surface of TiO2 could involve into the subsequent redox reactions. To suppress the bulk recombination, one strategy is to 3

ACCEPTED MANUSCRIPT decorate TiO2 surface with noble metallic nanoparticles [45, 46], act as electron traps, to accelerate electron-hole separation and further improve photoelectrochemical activities. The other approach is that coupling quantum dots (QDs) sensitizers. The adoption of the

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QDs could offer effective separation centers to transfer the carriers from one semiconductor to another one and thus reducing the electron-hole recombination [47-49]. In this work, we have fabricated TiO2 nanotubes by electrochemical anodization and then electrochemically reduce them as black TiO2 nanotubes. To reduce the

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recombination of photo-generated carriers, Au nanoparticles (NPs) and PbS quantum dots

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(QDs) were loaded on as-prepared TiO2 and black TiO2 nanotubes respectively. Then, morphological, structural, and optical properties of the as-prepared and black TiO2 nanotubes composites were studied by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and UV-Visible absorption spectroscopy.

Furthermore, photocurrent performances of the TNT

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photoelectrodes were evaluated by photoelectrochemical (PEC) measurements. The enhanced photocurrent verifies that the carrier separation is increased on the black TiO2

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nanotubes after introducing Au NPs and PbS QDs. 2. Experiments

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2.1 Chemicals and materials

All chemicals and materials were used as received without further purification.

Titanium foils (99.8% purity) were obtained from Baoji Titanium Industry Co., Ltd. and then cut into rectangular pieces with a dimension of 20 mm × 11 mm × 0.3 mm. Ethylene glycol (C2H6O2) and ammonium fluoride (NH4F, VWR International) were used for preparing anodization electrolyte. Lead sulfide (PbS) core-type QDs and Nafion (5 wt. % in a mixture of lower aliphatic alcohols and water, contains 45% water) were purchased 4

ACCEPTED MANUSCRIPT from Sigma-Aldrich for preparing QDs solution. Sodium sulfate decahydrate (Na2SO4·10H2O) was purchased from Sigma-Aldrich for electrochemical reduction and measurement. Acetone, isopropanol, and absolute alcohol were used for cleaning Ti foils

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and TiO2 nanotubes. 2.2 Fabrication of TiO2 nanotubes

Before electrochemical anodization, titanium foils were rinsed with acetone, isopropanol and deionized (DI) water in an ultrasonic bath (FinnSonic M12) for 10

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minutes, and then dried by blowing nitrogen gas. After cleaning, both sides of Ti foil

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were covered with Scotch tape, and partial tape on the front side was peeled off for specifying the location of electrochemical anodization (~1 cm2) and electrical contact. The TNTs were fabricated by electrochemical anodization of Ti foil in a fluoridecontaining electrolyte, which consisted of 0.5 wt.% ammonium fluoride (NH4F), 97 vol% ethylene glycol (EG) and 3 vol% water [50]. The anodization was performed in a two-

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electrode cell, where Ti foil (0.3 mm) was used as working electrode and thick Ti sheet (1 mm) as the counter electrode. The anodization potential was set at the constant of 60 V (VWR power supply) and anodization time was 5 h at room temperature (~ 20˚C). After

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anodization, the electrodes were rinsed with absolute alcohol in an ultrasonic bath

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(BRANSOIC 3510E-MTH) for 15 minutes to remove “grass-like” debris. Then, the electrodes were annealed at 500˚C (LENTON WHT6) for 3 h in air to form anatase TNT for electrochemical reduction and loading experiments. 2.3 Preparation of H-TNT-Au-PbS nanocomposites Black TiO2 nanotubes were obtained by electrochemical reduction in 0.5 M Na2SO4 electrolyte under the applied potential of 5 V for 15 seconds (denoted as H-TNT) [51]. After electrochemical reduction, the electrodes were rinsed with DI water and dried with 5

ACCEPTED MANUSCRIPT flowing nitrogen. Au NPs were loaded on H-TNT by magnetron sputtering deposition (Polaron SC500 Sputter Coater) under the deposition current of 15 mA for 70 seconds (denoted as H-TNT-Au). OA-capped PbS QDs were loaded on the H-TNT-Au by a

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dipping process. In a typical synthesis, 30 µL (PbS core-type quantum dots, 10 mg/mL in toluene) and 20 µL Nafion (5 wt.%) were dissolved in 950 µL of ethanol to prepare PbS QDs solution and then dispersed by ultrasonic for 10 min at ambient temperature (~20˚C). Then, 40 µL (in 8 cycles, ~12 µg PbS QDs) dipping solution was dropped on H-TNT-Au,

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and then the electrode was dried in an air atmosphere (denoted as H-TNT-Au-PbS). The

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fabrication procedures of H-TNT-Au-PbS nanocomposites are shown in Fig. 1. Finally, the TNTs electrodes were packaged by photoresist (SU8-2150, Microchem) to avoid the contact of bare Ti foil with electrolyte during electrochemical measurements. 2.4 Characterization

The surface morphology was studied by scanning electron microscopy (SEM, Hitachi

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SU8230) with accelerating voltage of 5 kV and field emission transmission electron microscopy (FE-TEM, FEI Tecnai G2-F20) with the accelerating voltage of 200 kV. The X-ray diffraction (XRD) patterns were recorded by using Cu Kα radiation (ARLTM

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EQUINOX 1000 X-ray Diffractometer, λ=1.540598 Å) with accelerating voltage 40 kV

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from 10º to 80º. Elemental constituents were evaluated by energy dispersive X-ray spectroscopy (EDX, assembled on Hitachi SU3500) with accelerating voltage of 15 kV. The optical properties were recorded from wavelength 220 nm to 850 nm by using UVVIS spectrophotometer (SHIMADZU, UV-2600 with ISR-2600 Integrating Sphere Attachment) with the fine BaSO4 plate as a reference. 2.5 Electrochemical measurements

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ACCEPTED MANUSCRIPT All electrochemical measurements were conducted by using Zahner Elektrik IM6 in a standard three-electrode configuration, where Pt sheet was used as a counter electrode, and Ag/AgCl electrode (filled with saturated KCl solution) was applied as the reference

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electrode. Besides, Na2SO4 solution (0.5 M) was used as the electrolyte. Linear sweep voltammograms (LSV) were measured in a potential range of 0 to 1.5 V vs. Ag/AgCl with a scan rate of 10 mV/s, and chronoamperometry were performed at bias potential of 1.0 V (vs. Ag/AgCl) with the illumination “on” and “off” for 30 seconds intervals.

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Electrochemical impedance spectroscopy (EIS) were obtained at 0 V DC bias potential

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with an oscillation amplitude of 10 mV in the frequency range from 100 mHz to 1 MHz. The stability measurements of the electrodes were performed under UV and visible light illumination at the bias potential of +1 V. A Xenon lamp (the illumination current of ~ 5A) and a UV-LED (HAMAMATSU L10561) were used as light sources, and the illumination intensity of light sources was ~100 mWcm-2 for visible light and ~170

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mWcm-2 for UV light. The light intensities were calibrated by a solar power meter

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(Sanpometer SM206) for visible light and UV power meter (OAI 306) for UV light.

3. Results and discussion

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3.1 Morphologies

Fig. 2 shows top-view SEM images of TNT, TNT-Au, TNT-Au-PbS, H-TNT, H-

TNT-Au, and H-TNT-Au-PbS. It can be seen in Fig. 2(a) that the TNTs present a porous periodical structure with an average diameter of ~130 nm and a wall thickness of ~ 15 nm. After loading with nanoparticles Au, the inner diameter decreases to ~ 115 nm and its wall thickness is about 20 nm, see the inset of Fig. 2(b). Further loading with quantum dots PbS, some clusters (~30 nm) are observed in the inset of Fig. 2(c) and they randomly 7

ACCEPTED MANUSCRIPT distribute on the TNT walls. There are no obvious morphological differences between TNT and H-TNT, as shown in Fig. 2(d), illustrating that the electrochemical reduction has less effect on the TNT dimensional configuration. Fig. 2(e) and Fig. 2(f) show the

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SEM images of H-TNT loaded with Au NPs and PbS QDs, respectively. These morphologies are similar to those of TNT-Au and TNT-Au-PbS.

HR-TEM and selected area electron diffraction (SAED) were used to study further the morphology and crystal structure of reduced TiO2 nanotubes loaded with Au NPs and

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PbS QDs. In Fig. 3 (a), it is shown that Au NPs and PbS are well dispersed on the surface

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of TiO2 nanotubes. The size of sputtered Au NPs is range from 1 nm to 6 nm and the size of PbS QDs is about to 1.5~3 nm, as shown in Fig. 3 (b). Also, the distances between adjacent lattice fringes are 0.24 nm and 0.34 nm, which are close to the d spacing of Au (111) planes (d=2.3654 Å) and PbS (111) planes (d=3.4246 Å). Fig. 3 (c) shows a set of circular spots on SAED patterns which is in accordance with XRD data shown in Fig. 4

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(a), informing that H-TNT-Au-PbS nanocomposites are the anatase phase. Fig. 3 (d) indicates that the disordered layer is formed on the surface of TiO2 nanotubes after electrochemical reduction treatment. A fringe spacing of 0.35 nm points out that the

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crystalline structure of TiO2 is anatase phase in (101) planes (d=3.5169 Å). The thickness

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of disorder layer is around ~1 nm, which is close to several published data [16, 34]. Fig. 3 (f)~(j) show the EDX mapping images of Ti, O, Au, PbS, and S element, suggesting Au NPs and PbS are uniformly loaded on TiO2 nanotubes by the distribution of colors. 3.2 Phase and element analysis XRD was employed to analyze structure and orientation of the as-prepared electrodes, as shown in Fig. 4(a). TNT and H-TNT show the diffraction peaks at 2θ of 25.3˚, 37.8˚, 48.0˚, 53.9˚, 55.0˚, 62.7˚, 68.8˚, 70.3˚ and 75.1˚ which are indexed as crystal planes (101), 8

ACCEPTED MANUSCRIPT (004), (200), (105), (211), (204), (116), (220), and (215) [52]. These peaks are ascribed to anatase phase patterns, indicating that electrochemical reduction does not obviously change the crystal structure and crystalline orientation. Besides, the peak at 2θ of 40.1˚ is

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the diffraction pattern from the Ti substrate of crystal planes (101). The obvious peaks at 2θ of 38.0˚, 44.2˚, 64.3˚, and 77.2˚ on TNT-Au and H-TNT-Au are the Au NPs diffraction patterns with the crystal planes of (111), (200), (220), and (311). An overlapped peak is noticed as the anatase phase at 37.8˚ is approaching the Au diffraction

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pattern of 38.0˚. No new peaks are observed on the diffraction patterns of TNT-Au-PbS

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and H-TNT-Au-PbS because of the relatively low amount of PbS QDs are loaded on these electrodes, the diffraction patterns might be submerged in the background noise. Fig. 4(b) shows EDX spectra of TNT, TNT-Au, TNT-Au-PbS, H-TNT, H-TNT-Au, and H-TNT-Au-PbS. The spectra of TNT and H-TNT show similar signals at 0.277 keV, 0.452 keV, 0.525 keV, 4.508 keV and 4.932 keV, which are the characteristic peaks for

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C-Kα, Ti-Lα, O-Kα, Ti-Kα, and Ti-Kβ. An additional signal at 2.120 keV (Au-M) for TNTAu and H-TNT-Au means that metallic element Au has been deposited on TNT and HTNT by magnetron sputtering technique [53]. Furthermore, another three additional

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peaks are observed at 0.677 keV, 2.307 keV and 2.342 keV on TNT-Au-PbS and H-TNT-

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Au-PbS. These peaks are the characteristic behavior of F-Kα, S-Kα, Pb-M [54], and show the existence of PbS QDs on TNT-Au and H-TNT-Au. Table 1 presents mean atom concentration and normalized concentration of as-

prepared electrodes by using EDX measurements. To improve measurement accuracy, ten positions on each sample were selected for EDX analysis. For TNT and H-TNT, the atomic ratio of Ti and O are close to 1:2. Essentially speaking, the difference between TNT and H-TNT is hard to investigate by EDX technique since the used e-beam with the 9

ACCEPTED MANUSCRIPT accelerating voltage of 15 kV can penetrate a depth of 1~2 µm underneath the top surface of the electrode. However, the thickness of the disordered layer formed on the surface of TiO2 nanotubes only several nanometers, as shown in Fig. 3 (d). The minor peaks of

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elements C and F are detected due to the residual of Ethylene glycol (EG) and ammonium fluoride (NH4F). Atom concentration of Au on TNT-Au and H-TNT-Au are at a similar level (~ 0.14%). The elements S and Pb atom concentration on TNT-Au-PbS and H-TNT-Au-PbS are close to 0.12%, and the ratio of the two elements is near 1:1. We

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also notice that the atom concentration of C and F on TNT-Au-PbS and H-TNT-Au-PbS

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are increased to several percentages caused by using Nafion (chemical formula: C7HF13O5S·C2F4) as a binder for preparing PbS QDs solution [55]. Furthermore, the mean normalized concentrations of each sample show similar tendencies with the mean atom concentrations. 3.3 UV-vis spectroscopy analysis

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Fig. 5 (a) shows UV-vis absorption spectra of TNT, TNT-Au, TNT-Au-PbS, H-TNT, H-TNT-Au, H-TNT-Au-PbS. The TNT spectrum shows strong absorption below wavelength ~ 400 nm, and weak absorption at the visible light above ~ 400 nm. After

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loading with Au NPs and PbS QDs, the absorption intensity of the TNT-Au and TNT-

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Au-PbS are slightly increased at the wavelength above 400 nm. On the contrary, the absorption intensity of H-TNT increases dramatically in the visible light region. The absorption spectrum of H-TNT-Au shows further enhancement from wavelength 325 nm to 615 nm as compared with that of H-TNT. This phenomenon can be understood that Au NPs have absorption peak from 450 nm to 580 nm, which is superimposed on the absorption spectra of H-TNT. However, the absorption intensity of H-TNT-Au is decreased at the range from 615 nm to 850 nm due to the light reflection of Au 10

ACCEPTED MANUSCRIPT nanoparticles. The absorption pattern of H-TNT-Au-PbS is similar to that of H-TNT-Au, but its intensity decreases from wavelength 480 nm to 850 nm. Although PbS QDs could absorb visible light, the Nafion might act as a scattering layer and leads to the decrease of

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absorption on H-TNT-Au-PbS [56]. Fig. 5 (a) shows that the absorption edge wavelengths of the TNTs are located 422 nm, 431nm, 465 nm, 443 nm, 471 nm, and 482 nm, respectively. Fig. 5(b) shows the bandgap calculated by Kubelka Munk equation for the TNT electrodes [57]. The bandgap energy(Eg) of TNT is about 2.97 eV, which is

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close to the well-known bandgap of anatase (3.2 eV). The band gap of TNT-Au, TNT-

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Au-PbS, H-TNT, H-TNT-Au, and H-TNT-Au-PbS are 2.90 eV, 2.69 eV, 2.83 eV, 2.64 eV and 2.59 eV, suggesting that bandgaps of the electrodes are shifted to the low number after loading with Au NPs and PbS QDs.

Fig. 6(a) shows photocurrent characteristics of TNT, TNT-Au, TNT-Au-PbS, HTNT, H-TNT-Au, H-TNT-Au-PbS under UV illumination (~ 170 mWcm-2). The

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photocurrent density of TNT is increased from 0 V to 0.6 V and reached maximum value 9.5 mAcm-2, and then slightly decreased from 0.6 V to 1.5 V. After loading with Au NPs and decorated with PbS QDs, the photocurrent densities of TNT-Au and TNT-Au-PbS

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are increased to 9.96 mAcm-2. The photocurrent densities of H-TNT, H-TNT-Au, H-

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TNT-Au-PbS increase as a function of bias potential, and maximum current densities of 22-24 mAcm-2 are recorded at the bias potential 1.5 V. Fig. 6(b) shows the photocurrent characteristics of as-prepared electrodes under visible illumination (~ 100 mWcm-2). The similar phenomena are also observed on TNT, TNT-Au, and TNT-Au-PbS under visible light. The current densities are saturated at 586 µAcm-2, 641 µAcm-2, and 691 µAcm-2 at the potential of 1.5 V. The photocurrent enhancements of TNT-Au and TNT-Au-PbS are 9.39% and 17.92% as compared with that of TNT. While, the photocurrent densities of 11

ACCEPTED MANUSCRIPT H-TNT, H-TNT-Au, and H-TNT-Au-PbS are dramatically enhanced under visible illumination, and maximum current densities reach 1.29 mAcm-2, 1.51 mAcm-2, and 1.79 mAcm-2 at the potential of 1.5 V. The corresponding enhancement ratios are 17.05% and

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38.76% as compared with that of H-TNT. However, the dark current densities of H-TNT, H-TNT-Au, and H-TNT-Au-PbS (75-91 µAcm-2 @ 1.5 V) are higher than that of TNT electrodes (0.4-3 µAcm-2 @ 1.5 V).

Fig. 6(c) and Fig. 6(d) show transient photocurrent behaviors of the as-prepared TNT,

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TNT-Au, TNT-Au-PbS, H-TNT, H-TNT-Au, and H-TNT-Au-PbS with an applied

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potential of +1 V vs. Ag/AgCl under UV and visible light. All electrodes show obvious current pulses during the illumination “on” and “off” at 30 seconds intervals. The photocurrent densities on fifth pulse for TNT, TNT-Au, and TNT-Au-PbS are 8.07 mAcm-2, 9.01 mAcm-2, and 10.09 mAcm-2 under UV illumination, and 0.592 mAcm-2, 0.616 mAcm-2, and 0.646 mAcm-2 under visible illumination. The photocurrent density of

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H-TNT, H-TNT-Au, and H-TNT-Au-PbS are 17.04 mAcm-2, 18.16 mAcm-2, and 18.62 mAcm-2 under UV light, and 0.789 mAcm-2, 0.999 mAcm-2, and 1.149 mAcm-2 under visible light. Obviously, the photocurrent densities of TNT-Au and TNT-Au-PbS are

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enhanced by 4.05% and 4.87% as compared to that of as-prepared TNT. However, the

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photocurrent of H-TNT-Au and H-TNT-Au-PbS are enhanced by 26.62% and 15.92% of H-TNT. Interestingly, we found that photocurrent density of H-TNT is 33.28% higher than that of TNT. The photocurrent enhancements of H-TNT-Au and H-TNT-Au-PbS are 62.18% and 77.86% higher than that of TNT-Au and TNT-Au-PbS. Fig. 7 (a) presents Nyquist plots for TNT, TNT-Au, TNT-Au-PbS, H-TNT, H-TNTAu and H-TNT-Au-PbS at 0 V DC bias potential with 10 mV oscillation amplitude in the frequency range from 100 mHz to 1 MHz. Obviously, the real and imaginary impedance 12

ACCEPTED MANUSCRIPT of H-TNT, H-TNT-Au, and H-TNT-Au-PbS are both lower than that of TNT, TNT-Au, TNT-Au-PbS. Figure 7 (b) shows the impedance curves at the high-frequency region, and the intercept on real axis is equivalent series resistance (ESR). It is indicated that the ESR

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of H-TNT, H-TNT-Au, and H-TNT-Au-PbS are smaller than that of TNT electrodes. Also, semi-circle curve at high-frequency region represents the charge transfer resistance at the electrolyte/electrode interface, and its diameter is proportional to the charge transfer resistance. Three semi-circle curves are observed on H-TNT, H-TNT-Au, and H-

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TNT-Au-PbS, suggesting H-TNT-Au-PbS has the lowest charge transfer resistance. The

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inset image of Fig. 7 (b) shows the equivalent circuit for the Nyquist plots which are simulated by EIS spectrum analyzer. In this equivalent circuit, Rs is the equivalent series resistance (ESR) and which is also known as the resistance of electrolyte (0.5 M Na2SO4 solution). It is the resistance of the electrolyte combined with the internal resistance of the electrodes. RCT is the charge transfer resistance to characterize the rate of redox

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reactions at the interface of electrode/electrolyte. CPE1 is the constant phase element to depict the separation of ionic and charges at electrode/ electrolyte interface, and its behavior likes a double-layer capacitor. Wo is a Warburg open element which is related to

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the diffusion process of ions along a porous electrode [58, 59]. Table 3 lists the fitting

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parameters of the equivalent circuit for the Nyquist plots of as-prepared electrodes. P1 and n1 are the preexponential factor and exponent for CPE1. Wor1 and Woc1 are the parameters of the Wo. As shown in Table 3, the increased ESR (3.99 ohms → 4.24 ohm and 3.53 ohm → 3.84 ohms) may relate to the internal resistance of electrodes as the loadings of Au NPs and PbS QDs. The RCT of electrodes is significantly reduced from 642.8 ohms to 2.15 ohm after electrochemical reduction treatment and further decreased from 2.15 ohm to 0.72 ohms after loaded with Au NPs and PbS QDs. Low charge 13

ACCEPTED MANUSCRIPT transfer resistance represents the rapid ion transfer and fast redox reactions within the electrode [60]. The reduced RCT may primarily attribute to the formation of Ti3+ sites and/or hydroxyl radicals (Ti3+-OH) on the surface of TiO2 nanotubes and good

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conductivity of Au NPs and PbS QDs. Fig. 8 shows the as-prepared electrode stability measurement under UV and visible light illumination. Before light illumination, as-prepared electrodes were measured for 2000 seconds (~33 mins) in dark environment at the bias potential of +1 V. Under UV

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illumination, the reduced electrodes (H-TNT, H-TNT-Au, and H-TNT-Au-PbS) give

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higher photocurrent densities than that of the TNT electrodes (TNT, TNT-Au, and TNTAu-PbS). The initial photocurrent densities of the electrodes are 8.29 mAcm-2, 8.11 mAcm-2, 8.40 mAcm-2, 16.0 mAcm-2, 16.9 mAcm-2, and 17.0 mAcm-2. After UV illumination for more than 4 hours, the photocurrent densities for the electrodes are changed by -10.1%, +9%, +8%, +2%, +12%, and +12% of their initial values. Under

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visible light illumination, the electrodes show small photocurrent densities which are 579 µAcm-2, 610 µAcm-2, 641 µAcm-2, 760 µAcm-2, 979 µAcm-2, and 998 µAcm-2. Similarly, the final current densities of electrodes are changed by -0.2%, -2.6%, -0.2%, +1%, -

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14.8%, and -7% compared with their initial values. In summary, the pristine electrodes

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show better stability than the reduced electrodes under UV and visible light illumination. This phenomenon is understood by: (1) under UV illumination, the electrodes generates more oxygen bubbles than under visible illumination, the nanocomposites may fall off from the electrodes by these bubbles; (2) the formed Ti3+sites by electrochemical reduction may slowly oxidize to Ti4+ (Ti4+→ Ti3+) under the bias potential of +1 V for several hours.

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ACCEPTED MANUSCRIPT Discussion Fig. 9 (a) illustrates the electron-hole pairs generation and recombination in (I) TNT, (II) H-TNT, and (III) H-TNT-Au-PbS. For TNTs, electron-hole pairs are generated after

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absorbed the photon energy. The large amounts of electrons and holes are recombined on the surface and bulk of TNTs. A small number of charges could be transported and involved the subsequent photoelectrochemical reactions. H-TNT can generate more electron-hole pairs than that of TNT due to the contribution of the black Ti3+ shell layer

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which exhibits strong absorption in visible regime. However, the majority of electrons

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and holes may be migrated from Ti3+ shell layer to bulk TiO2 for bulk recombination due to the seamless low barrier at Ti3+/TiO2 interface. For H-TNT-Au-PbS, more electrons and holes may migrate to the loadings of Au NPs and PbS QDs rather than to bulk TiO2, thus suppressing the bulk recombination. Simultaneously, Au NPs and PbS QDs could also act as light absorbers to contribute extra electron-hole pairs. Fig. 9(b) shows the

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schematic setup of electrochemical measurements for evaluating the TNT-based photoelectrodes in 0.5 M Na2SO4 and Fig. 9(c) shows bandgap positions of TiO2 and PbS quantum dots, and work function of Ti and Au [61, 62]. Under UV or visible light

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illumination, electrons are excited from valence band (VB) into the conduction band

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(CB), and holes are remained on valence band (VB). The generated electron-hole pairs are separated by applied bias. The holes move to TiO2/electrolyte interface and water molecules are oxidized to generate oxygen gas and hydrogen cations. Meanwhile, the electrons move toward to Ti substrate and hydrogen cations are reduced to form hydrogen on Pt counter electrode. However, the separated electrons and holes might be recombined during carrier transport along tubular structures. The current density of TNTs reaches its maximum until the dynamic equilibrium between generation and 15

ACCEPTED MANUSCRIPT recombination of carriers, which shows as a plateau in Fig. 6(a) and (b). For H-TNT electrodes, the Ti3+ shell is formed on the tubular structure after electrochemical reduction in 0.5 M Na2SO4. The formed Ti3+ shell on H-TNTs brings three distinct

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advantages: (i) the color of H-TNT is black or blue-black, which enhances the absorption at visible regime, shown in Fig. 5(a); (ii) the narrow bandgap of H-TNT is liable to electron excitation, shown in Fig. 5(b); (iii) the decreased charge transfer resistance of HTNT is beneficial to the rapid ion transfer and fast redox reactions within the electrode,

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shown in Table 3. Therefore, the current densities of H-TNT electrodes are continuously

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increased as a function of bias potential since the separation efficiency are surpass the carrier’s recombination efficiency. After loading with Au NPs and PbS QDs, heterogeneous junctions are formed at the interface TiO2/PbS, TiO2/Au, and TiO2/Au/PbS, and related edge positions are shown in Fig. 9(e), (f), and (g). A suitable nanostructured formation of these heterogeneous junctions between TiO2 and metallic

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elements or semiconductors might increase the generation and separation of the charge carriers [45, 63-64]. In Fig. 9(e), the band diagram of H-TNT is combined with a narrower bandgap semiconductor PbS QDs to form a composited band diagram. Under

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the positive bias potential, the photo-generated electrons are cascaded through the CB of

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PbS QDs and TNT, and down to the work function level of Ti. Meanwhile, the generated holes are separated rapidly and react with water. After loading with Au NPs, the electrons generated on Au NPs usually transfer to H-TNT by two ways: hot electron injection (HEI) and plasmonic resonance energy transfer (PRET) [65, 66], shown in Fig. 9(f). Due to Au NPs directly interact with electrolyte, thus, Au NPs could play as main hole-accumulation centers for further reactions under positive potential. In addition, the holes generated on H-TNT are easily transported to Au NPs and further achieve charge separation. Fig. 9(g) 16

ACCEPTED MANUSCRIPT shows the combined band diagram of H-TNT, Au NPs, and PbS QDs. Under visible illumination, the induced electrons on H-TNT, Au NPs, and PbS QDs are cascade down to Ti foil, meanwhile, holes are pushed to the VB of PbS QDs under positive bias

NPs and PbS QDs as separation centers and light absorbers.

Conclusions

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potential. In total, the photocurrent density of H-TNT-Au-PbS is enhanced by using Au

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In summary, TiO2 nanotubes (TNT) have been fabricated by electrochemical

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anodization and then subsequently reduced them as black TiO2 nanotubes by an electrochemical method in 0.5 M Na2SO4 solution. The black TNTs show a strong light absorption from the wavelength 400 nm to 850 nm. To retard photo-generated carrier recombination, Au nanoparticles (NPs) and PbS quantum dots (QDs) were loaded on asprepared and black TNT by magnetron sputtering technique and dip coating approach.

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The EDX analysis shows that Au NPs and PbS QDs are randomly distributed on the asprepared and black TNTs. Compared with photocurrent density of as-prepared TNTs (0.592 mAcm-2), the transient photocurrent density of H-TNT (0.789 mAcm-2) is

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increased by 33.28% at the applied potential of +1 V under visible illumination.

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Meanwhile, the photocurrent densities of H-TNT-Au (0.999 mAcm-2) and H-TNT-AuPbS (1.149 mAcm-2) are enhanced by 62.18% and 77.86% as compared with the asprepared TNT electrodes. However, the photocurrent densities of as-prepared TNT electrodes are only increased about 4.05% and 4.87% after loaded with the same amount of Au NPs and PbS QDs. Therefore, the loaded Au NPs and PbS QDs on black TNT could consider as separation centers rather than light absorbers for accelerating carrier

17

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Acknowledgments The authors are grateful to Zekija Ramic, Ragnar D. Johansen, Thomas Marthinsen, and Muhammad Tayyib for helping experimental preparation. The author Kang Du acknowledges financial support from KD program at University College of Southeast

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Norway, Norwegian Research Council-FRINATEK programme (231416/F20), EEA-

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Poland (237761), National Natural Science Foundation of China (Grant No. 61574117 and 61274120) and partial funding for this work was obtained from the Norwegian Ph.D. Network on Nanotechnology for Microsystems, which is sponsored by the Research

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Council of Norway, Division for Science, under contract No. 221860/F40.

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Table Captions Table 1. The mean atom concentration and normlized concentration of as-prepared samples by EDX data.

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Table 2. The comparison of photocurrent densities of TNT and H-TNT photoelectrodes on fifth pulse under visible light.

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Table 3. EIS fitting parameters of the equivalent circuit for the Nyquist plots.

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TNT-Au

1.72±0.23 64.33±1.04 0.67±0.10 33.29±1.04 -

1.68±0.20 68.44±0.92 0.90±0.15 28.83±1.16 0.14±0.02 -

C O F Ti Au S Pb

0.78±0.10 38.76±1.11 0.48±0.08 59.99±1.15 -

0.82±0.07 43.03±0.96 0.68±0.08 54.22±0.84 1.25±0.41 -

TNT-Au-PbS H-TNT Atom. Concentration [at. %] 4.56±0.54 1.73±0.24 64.46±1.31 66.33±1.24 4.19±0.54 0.72±0.11 26.39±1.48 31.23±1.26 0.15±0.03 0.12±0.03 0.12±0.03 Norm. Concentration [wt. %] 2.24±0.41 0.80±0.06 41.77±1.90 40.79±1.38 3.30±0.74 0.51±0.08 50.30±1.79 57.90±1.41 0.36±0.35 0.85±0.48 1.18±0.34 -

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Table 1

H-TNT-Au

H-TNT-Au-PbS

1.75±0.24 68.25±0.86 0.92±0.15 28.94±1.05 0.14±0.02 -

4.63±0.52 64.39±1.29 4.28±0.54 26.29±1.43 0.15±0.03 0.13±0.03 0.13±0.03

0.83±0.11 42.77±1.13 0.67±0.12 54.54±1.17 1.19±0.38 -

2.279±0.439 42.106±1.948 3.389±0.775 49.855±1.574 0.338±0.319 0.89±0.51 1.15±0.37

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TNT

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Samples Elements C O F Ti Au S Pb

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Enhancement →

0.592 mA/cm2

0.616 mA/cm2

4.05%

→

↓ /

33.28% →

0.646 mA/cm2

↓

/ 62.18%

0.789 mA/cm2

0.999 mA/cm2

26.62%

77.86%

→ 15.92%

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Table 2

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Au NPs + PbS QDs

4.87%

↓ Enhancement

H-TNT

Enhancement

1.149 mA/cm2

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TNT

Au NPs

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As-prepared

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Rs (ohm) 3.99 4.06 4.24 3.53 3.68 3.84

RCT (ohm) 642.8 242.2 192.6 2.15 1.13 0.72

P1 8.93e-5 11.6e-5 9.61e-5 2.75e-4 4.48e-4 1.84e-4

Wor1 808.5 699.2 530.7 51.84 49.94 42.76

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Table 3

n1 0.792 0.723 0.765 0.496 0.654 0.799

Woc1 0.02 0.01 0.02 0.14 0.11 0.09

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Sample TNT TNT-Au TNT-Au-PbS H-TNT H-TNT-Au H-TNT-Au-PbS

ACCEPTED MANUSCRIPT Figure Captions Figure 1. Schematic diagram of H-TNT-Au-PbS nanocomposites fabrication procedures: (a) Ti foil, (b) anodization to form TNT, (c) annealing to form anatase phase, (d)

technique, and (f) loading PbS QDs by dip coating approach.

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electrochemical reduction to obtain black TNT, (e) loading Au NPs by magnetron sputter

Figure 2. SEM images of (a) TNT, (b) TNT-Au, (c) TNT-Au-PbS, (d) H-TNT, (e) HTNT-Au, and (f) H-TNT-Au-PbS.

(a) TEM and (b) HRTEM of H-TNT-Au-PbS, (c) SAED patterns, (d)

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Figure 3.

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disordered surface on TiO2 nanotube after electrochemical reduction, and EDX mapping images of (e) selected region on H-TNT-Au-PbS, (f) titanium, (g) oxygen, (h) gold, (i) lead, (j) sulfide.

Figure 4. (a) XRD patterns of TNT, TNT-Au, TNT-Au-PbS, H-TNT, H-TNT-Au, and HTNT-Au-PbS, (b) EDX element spectra of as-prepared electrodes.

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Figure 5. (a) UV-VIS absorption spectra of TNT, TNT-Au, TNT-Au-PbS, H-TNT, HTNT-Au and H-TNT-Au-PbS, (b) the band gap energy determination of as-prepared electrodes.

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Figure 6. The photocurrent characteristics of TNT, TNT-Au, TNT-Au-PbS, H-TNT, H-

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TNT-Au and H-TNT-Au-PbS in (a) dark and under UV illumination with the intensity of ~ 170 mWcm-2, (b) dark and under visible illumination with the intensity of ~ 100 mWcm-2, (c) the transient photocurrent responses of as-prepared electrodes under UV illumination at the applied potential of +1 V, and (d) under visible illumination at the applied potential of +1 V.

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ACCEPTED MANUSCRIPT Figure 7. (a) Nyquist plots for as-prepared TNT, TNT-Au, TNT-Au-PbS, H-TNT, HTNT-Au, and H-TNT-Au-PbS, and the inset image shows at high frequency, (b) Nyquist plots of electrodes at the high-frequency region.

(b) visible light illumination at the bias potential of +1 V.

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Figure 8. The as-prepared electrode stability measurement under (a) UV illumination and

Figure 9. (a) the schematic setup of electrochemical measurements for H-TNT-Au-PbS

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photoelectrode in 0.5 M Na2SO4 solution, (b) the bandgap energy of TiO2 and PbS quantum dots, and work function of Ti, Au and Pt, band diagram of (c) H-TNT, (d) H-

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TNT with PbS QDs, (e) H-TNT with Au nanoparticles, and (f) H-TNT with Au and PbS

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QDs in the positive bias potential under visible light illumination.

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TiO2 (204)

TiO2 (105) TiO2 (211)

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Au (311)

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TiO2 (116) TiO2 (220)

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Absorbance (a. u.)

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443 nm 431 nm 422 nm

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2.90 eV 2.69 eV 2.83 eV 2.64 eV 2.59 eV

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Highlights: TiO2 nanotube arrays are synthesized by anodization.

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Black TiO2 nanotube arrays are obtained by electrochemical reduction treatment.

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Au nanoparticles (NPs) and PbS quantum dots (QDs) on as-prepared TiO2 nanotubes

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(TNT) and black TiO2 nanotubes by magnetron sputter technique and subsequently dip

The loadings of Au NPs and PbS QDs might act as separation centers rather than only

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light absorbers to accelerate electron-hole separation and transport.

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coating process.

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