Polydopamine-Inspired Design and Synthesis of Visible-Light-Driven

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Polydopamine-inspired Design and Synthesis of Visiblelight-driven Ag NPs@C@elongated TiO2 NTs Core-shell Nanocomposites for Sustainable Hydrogen Generation Xinnan Zhang, Mingzheng Ge, Jianing Dong, Jianying Huang, Ji-Huan He, and Yuekun Lai ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04088 • Publication Date (Web): 18 Nov 2018 Downloaded from http://pubs.acs.org on November 23, 2018

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ACS Sustainable Chemistry & Engineering

Polydopamine-inspired Design and Synthesis of Visible-light-driven Ag NPs@C@elongated TiO2 NTs Core-shell Nanocomposites for Sustainable Hydrogen Generation Xinnan Zhang,[a] Mingzheng Ge,[a] Jianing Dong,[a] Jianying Huang,[b] Jihuan He[a] and Yuekun Lai*[a],[b] [a]

National Engineering Laboratory for Modern Silk, College of Textile and Clothing Engineering,

Soochow University, Suzhou 215123, P. R. China [b]

College of Chemical Engineering, Fuzhou University, Fuzhou 350116, P. R. China

*Corresponding author: Tel: +86 591 22865220 Mailing address: [email protected] (Yuekun Lai)

ACS Paragon Plus Environment

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Abstract: The primary challenge of photocatalytic hydrogen generation is to exploit a catalyst with good durability and low cost. Here, we designed a facile and efficient process of loading silver nanoparticles (Ag NPs) in situ on the core-shell nanocomposite of the elongated titanium dioxide nanotubes with carbon layer (C@TiO2 NTs) by polydopamine (PDA) without addition of any binder. The combination of C@TiO2 NTs with Ag NPs has excellent performance toward photocatalytic hydrogen production and degradation of rhodamine B (RhB) under visible light irradiation. The characterizations (SEM, TAM, XRD, XPS, etc.) showed that the carbon layer on nanotubes could conformably cover the whole of TiO2 NTs to form core-shell structure, which not only prevented Ag NPs from aggregating, but also acted as the electronic transport channel. Meanwhile, Ag NPs were uniformly distributed on the surface of C@TiO2 NTs. In conclusion, under the synergistic effect of Ag nanoparticles and outer-carbon layer, the utilization efficiency of visible light has been enhanced and the recombination of electrons/holes has been suppressed for Ag@C@TiO2 NTs nanocomposites. Therefore, the degradation efficiency of RhB and hydrogen generation rate is 2.5 times and 4.8 times as higher as pure TiO2 NTs, respectively. This work may provide fruitful experience for developing novel strategy into the design and fabrication of stable and highly active visible-light catalysts with noble metal modification toward sustainable hydrogen production in the energy filed. Keywords: Ag nanoparticle, elongated TiO2 nanotube, carbon shell, core-shell structure, visiblelight induced photo-catalyst Introduction With the rapid development of the economy, the environment pollution and energy crisis have become more and more serious. Hydrogen energy is a kind of clean, efficient and renewable ideal energy, which can be generated from abundant solar energy.1,2 In this concern, many investigations


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have been widely explored to solve environmental pollution problems and energy shortage crisis.3,4 As one kind of low consumption and environmentally friendly materials, semiconductor is frequently utilized in photocatalytic pollutant degradation and water splitting.5,6 TiO2, one of the most promising semiconductor nanomaterials,7 has attracted extensive interests in several areas such as photocatalysis, supercapacitors, lithium-ion batteries and photovoltaics,8,9 owing to the advantages of low-cost,10 non-toxicity,11 high reactivity12 and photochemical stability.13 When compared to other forms, 1D TiO2 nanomaterial (nanowires, nanorods, nanotubes and nanofibers) possess high aspect ratio and relatively low recombination ratio of electron/hole pairs to transfer charge rapidly.14 Especially, TiO2 nanotube with hollow structure and ultra-long size intrinsically owns large surface areas to provide more active sites. However, the wide band-gap (anatase: 3.2 eV, rutile: 3.0 eV)15 makes TiO2 absorb UV light, which only occupies around 3%-5% of the solar spectrum.16,17 Besides, the recombination of photoelectrons-holes also hinders TiO2 wide applications.18,19 To solve these problems, many studies have been performed to modify the pristine TiO 2 by doping of metal/non-metal,20 modifying with noble metal nanoparticles21,22 or forming heterojunction by coupling other semiconductors with narrow bandgap.23,24 The decoration of noble metal nanoparticles (Au, Pt, Ag etc.) on TiO2 is favourable to promote the transfer of photogenerated carriers and the effective separation of photo generated electron-hole pairs,25 enhancing the conversion efficiency in visible light. Besides, the surface plasmon resonance (SPR) effect of noble metals also improves the visible light response of TiO2.26 Owing to the low cost and excellent conductivity, Ag nanoparticles are most widely used to modify TiO2 by electrodeposition,27 ultraviolet irradiation reduction,28 hydrothermal methods29 and electrospinning technique.30 However, there are still some problems left in the above methods, both size and uniformity of


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silver particles are hard to be controlled. Moreover, the loading amount of Ag NPs is restricted on the substrate due to the limited active sites, and the photocatalytic activity will be suppressed once Ag NPs tending to aggregate to form big clusters. Thus, the TiO2 nanotubes with over-length and hollow structure is an excellent framework with more area to load more Ag NPs and also provide fast channels for charge transfer on the interface of Ag NPs and TiO2 NTs. Fan and co-workers synthesized silicon pyramid and nanowire/Ag nanoparticle nanocomposites by Ag-assisted electroless etching method.31 In addition, Jiang et al. prepared Ag NPs/SnO2 nanoflowers via a traditional silver mirror reaction.32 It reveals that there still have the inhomogeneity of size and distribution of Ag NPs in these two articles. Moreover, Ag NPs are easily oxidized during experimental process and the preparation process is complex. Therefore, it is urgent to seek for a simple reduction method to make the Ag NPs anchor uniformly and provide the protective layer for Ag NPs. Inspired by the bioadhesive proteins of marine mussels,33 dopamine, an important hormone and neurotransmitter in most animals, can self-polymerize at weakly alkaline aqueous in ambient temperature to form polydopamine (PDA) with adhesiveness and reducibility.34,35 The resulting PDA which is rich in catechol and amine functional groups like mussel adhesive proteins exhibits distinctive biocompatibility and adhesive properties, attracting massive interest in cell adhesion,36 anti-bacterial,37 drug delivery,38 and energy storage39 as a surface modification agent for diverse materials. Therefore, PDA serves as a reducing agent to reduce different metal nanoparticles in situ onto various substrates firmly without any adhesion agent and controls the uniformity and size of metal nanoparticles by surface regulation.40,41 Moreover, PDA could act as an effective carbon source for carbon-coated materials after thermal annealing,42 which not only functions as a protector with high chemical/thermal stability to prevent metal nanoparticles from coalescing and


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oxidizing, but also increases electrical conductivity for fast photogenerated charge carriers transfer. 43

Hence, we designed a facile synthesis of Ag@C@TiO2 NTs core-shell nanocomposite by using polydopamine as a reductant and precursor of carbon shell, which anchored and reduced Ag NPs in situ on TiO2 NTs uniformly. Figure1 depicted the fabrication process of Ag@C@TiO2 NTs core-shell nanocomposite. Firstly, dopamine could spontaneously self-polymerize into polydopamine and encapsulate the elongated TiO2 NTs. Secondly the silver nitrate was reduced to Ag NPs and immobilized on the surface of PDA@TiO2 NTs owing to the reducibility and adhesion of PDA. Finally, after the calcination process, PDA transforms into an amorphous carbon layer to wrap silver nanoparticles, which provided electronic transfer channels to increase the conductivity of the composite. Furthermore, the carbon-shell protects the silver nanoparticles from oxidizing quickly in order to increase lifetime effectively. In addition, owing to effectively stable Ag NPs against secondary crystallite, the thermal cracking process of PDA also prevents Ag NPs aggregation.44 Therefore, Ag@C@TiO2 NTs core-shell nanocomposite exhibited efficient separation of photogenerated electron/hole pairs, narrowed band gap, improved conductivity for charge carriers transfer, and high utilization efficiency of visible light due to SPR effect. Impressively, under the synergistic effect of Ag NPs and carbon layer, the hydrogen generation rate and degradation efficiency of RhB and is 4.8 times and 2.5 times than pure TiO2 NTs.


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Figure 1. Schematic illustration for the fabrication of Ag@C@TiO2 NTs core-shell nanocomposite.

Experimental section Fabrication of core-shell Ag@C@TiO2 NTs composites The synthesis of long TiO2 NTs based on the previous reports by a stirring hydrothermal strategy.45 A certain amount of NaOH was dissolved in 35 mL of deionized water, and P25 was added to this solution. Then the reaction mixture transferred into the Teflon-lined stainless-steel autoclave with a magnetic stirrer and heated at 130 °C for 24 h in an oil bath with stirring to form a white suspension. The suspension was rinsed with deionized water by filtering until pH = 7.0, the precipitate was obtained and dipped in 0.1 M nitric acid for 24 h to displace sodium ions. And repeat the process of filtering and soaking for three times to ensure the complete replacement of Na+. After this procedure, the sediment was soluble in anhydrous ethanol, achieving the elongated TiO2 NTs. TiO2 NTs were dispersed in a beaker containing 50 mL Tris buffer solution consisting of Tris (hydroxymethyl) aminomethane, deionized water and anhydrous ethanol (pH = 8.5). After mixing


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uniformly, 0.1 g of dopamine hydrochloride was added into with stirring for 24 h. During this period, the color of the solution changed from yellow to black due to the polymerization of dopamine (DA). Following that, the solution was centrifuged, washed with deionized water and absolute ethanol three times to obtain PDA@TiO2 NTs. In order to achieve Ag@C@TiO2 NTs, in a typical produce, firstly, 0.75 g PVP-K30 added into 0.01 mol L-1 AgNO3 solution slowly to guarantee the uniform dispersion of solute. Secondly, PDA@TiO2 NTs were dispersed in the solution with vigorous stirring for 5 min at room temperature. Further dropping a few drops of ammonia into the above solution. After the reaction, the precipitant was rinsed and was collected by centrifuging with deionized water and ethanol in three times to obtain Ag@PDA@TiO2 NTs-5 min. Following the same produces as that for Ag@PDA@ TiO2 NTs-5 min composite, Ag@PDA@ TiO2 NTs-30 min, Ag@PDA@TiO2 NTs-2 h were synthesized when the reduction time was adjusted to 30 min and 2 h, respectively. Finally, the PDA@TiO2 NTs and Ag@PDA@TiO2 NTs was dried at 60 °C in a vacuum oven, subsequently C@TiO2 NTs and Ag@C@TiO2 NTs was obtained by calcination at 600 °C in argon atmosphere. Characterization The morphology and structure of Ag@C@TiO2 NTs were investigated by the field emission scanning electron microscopy (FESEM, Hitachi S-4800) at 5.0 kV. The presence of carbon layer and the microstructure of Ag@C@TiO2 NTs were further observed by transmission electronic microscope (TEM, FEI TecnalG-20 operated at 200 kV). Moreover, the elemental distribution was analyzed with an energy dispersive X-ray spectrometer fitted to the TEM. An X-ray diffractometer with Cu-Kα radiation (XRD, Philips, X’pert-Pro MRD) was used to confirm the crystal phases of the samples. The chemical composition of the Ag@C@TiO2 NTs was analyzed using an X-ray photoelectron spectroscopy with a 100W Al-Kα X-ray source (XPS, KRATOS, Axis-Ultra HAS).


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The binding energies were normalized to the signal for C 1s at 284.5 eV. UV-Vis diffuse reflection spectroscopy (UV-DRS) was carried out at room temperature in the range of 200-800 nm using UV-3600. Photoluminescence measurements (PL) were operated at room temperature by a fluorescence spectroscopy with a HOKIBA JOBIN YVON FM4P-TCSPC and a xenon lamps as excitation source (λex = 370 nm). Photoelectrochemical measurements The tests including photocurrent measurements and electrochemical impedance spectroscopy (EIS) were carried out by using a CHI 660D electrochemical workstation with a standard three electrode system. A glassy carbon electrode (GCE, 4 mm in diameter) loaded with different samples was used as working electrode, Ag/AgCl (in 1 M KCl) electrode and a Pt mesh served as reference and counter electrode, respectively. 0.1 M sodium sulfate (pH = 7.0) was used as the electrolyte solution and the focused incident light intensity on the samples was 100 mW cm -2. To prepare the working electrode, 5 mg samples were dispersed into 0.2 ml 5 wt% Nafion solution and 0.8 ml anhydrous ethanol and the mixed solution was scattered fully by ultrasonic for one hour. Then 8 μL solution was dripped onto the glassy carbon electrode, and dried at room temperature. Electrochemical impedance spectra (EIS) of the samples was tested in the dark and under a 150 W xenon lamp illumination. Photocatalytic activity measurements The photocatalytic performance of samples was measured through the degradation efficiency of RhB with an initial concentration of 50 mg L-1 in a PS-GHX photochemical reactor. It has a circulating water system and agitator to keep the temperature of the solution and a 300 W Xenon with UV light cutoff filters (λ ≥ 420 nm) was served as the visible light sources. The distance between the light source and the sample was 60 mm. Before light irradiation, 50 mg samples were


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dissolved in 50 ml RhB in quartz tubes with stirring to make the samples mixed with the organic dye fully for 1 h to achieve the adsorption-desorption equilibrium at room temperature. The concentration of RhB was analyzed with a 30 min interval using a UV-Vis spectrophotometer (Hitachi, UV-1080, Japan) at a wavelength of 554 nm. Photocatalytic H2 generation reactions was tested in a high-throughput photoreactor and the solution was irradiated from the bottom by visible-light, which was a 500 W xenon lamp (Zolix LSP-X500) with UV light cutoff filters (λ ≥ 420 nm). 50 mg Ag@C@TiO2 NTs was added into the solution containing 20 vol% anhydrous methanol as the sacrificial agent and then placed in the sealed bottle. The solution was put into the reactor and then been completely evacuated to eliminate air. During the reaction period, a flow of cooling water and draught fan used to maintain the solution at room temperature. After that, the produced gases were analyzed of quantitative and qualitative in a volumetric device with a vacuum line after 4 h irradiation under visible light.

Results and discussion As shown in Figure 2a, the diameter and length of TiO2 nanotubes fabricated by stirring hydrothermal method is around 100 nm and ~10 μm, which is longer than traditional hydrothermal method,46 And Figure 2b-d display the SEM images of C@Ag@TiO2 NTs with different reduction time at 5 min, 30 min and 2 h, respectively. Only a little of Ag particles were reduced and deposited on TiO2 NTs at 5 min, as displayed in Figure 2b. With the increase of reduction time, the silver nanoparticles gradually increased. After 30 min reduction, the size of silver particles became ~20 nm and distributed uniformly on the nanotubes. As the reduction time increased to 2 h, the silver particles began to aggregate, forming nanoclusters of about 100 nm that attached to the surface of the nanotube. Figure S1 shows


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the EDS spectrum of Ag@C@TiO2 NTs-5 min and 2 h, which confirmed that the composites consist of Ti, O, C, N, and Ag elements, demonstrating that the atomic ratio of silver nanoparticles is 9.933% and 18.781%, respectively. And, the N element of Ag@C@TiO2 NTs comes from decomposition of polydopamine. The factors affecting the size of silver particles are Ag+ concentration, reduction time and reaction environment. When the concentration of silver ions and the reaction conditions is constant, the size and distribution of silver particles is determined by the reduction times. When the initial silver nanoparticles began to reduce via PDA that is also contributing to increase the dispersion and diminish the dimension of Ag NPs,47 there is no conspicuous particles on the surface of TiO2 NTs in 5 min reduction time. As the time increases, the silver particles gradually form and distribute uniformly. When the load exceeds the loading capacity of the nanotube surface, most of the particles begin to aggregate to form clusters, which may act as the recombination center of electron/hole pairs and decreased the photocatalytic performance.

Figure 2. SEM images of the elongated TiO2 NTs (a), Ag@C@TiO2 NTs with different reduction time of 5 min (b), 30 min (c) and 2 h (d) after annealing.


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Figure 3. XRD patterns of pristine TiO2 NTs, C@TiO2 NTs and Ag@C@TiO2 NTs-30 min. The phase composition and crystallinity of the resulting composites was prepared with X-ray diffraction (XRD) analysis. Figure 3 shows the XRD patterns of pristine TiO2 NTs, C@TiO2 NTs and Ag@C@TiO2 NTs-30 min. The diffraction peaks at 25.2°, 37.7°, 47.9°, 53.8° and 62.5° correspond to the (101), (004), (200), (105) and (204) crystal planes of anatase TiO2 phase (JCPDS 21-1272). It is obvious that the peaks of anatase TiO2 decreases in C@TiO2 NTs, owing to the wrapping carbon layer over the nanotube in the core-shell structure. After loading Ag with nanoparticles, except for anatase TiO2 phase, the obvious peaks at 38.1°, 44.2°, 64.42°, 77.3° and 81.4° could be indexed to the (111), (200), (220), (311) and (222) diffraction planes of Ag NPs (JCPDS 04-0783). The prominent peaks of Ag (111) at 38.1° might be in coincidence with the (004) crystal plane of anatase TiO2 at 37.7° and the dominant signal Ti at 38.4°. In addition, the patterns of TiO2 NTs and Ag@C@TiO2 NTs acquired at diverse reduction times can be observed in Figure S2, which may be concluded that the density of Ag peak for Ag@C@TiO2 NTs-30 min is larger compared with other specimens thanks to the uniformly distributed Ag NPs relatively and the suitable size of particles controlled appropriately. Transmission electron microscopy (TEM) was applied to reveal the micromorphology and composition of Ag@C@TiO2 NTs-30 min. The TEM images of PDA@TiO2 NTs,


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Ag@PDA@TiO2 NTs and the samples after calcination is shown in Figure S3, which is consistent with our vision about the evolution processes from TiO2 NTs to Ag@C@TiO2 NTs. Dopamine (DA) is polymerized to form polydopamine (PDA) coated TiO2 NTs. During the reduction of silvers, due to the reduction of PDA, Ag+ are directly reduced to Ag nanoparticles on the surface of PDA@TiO2 NTs. Finally, the PDA is converted to carbon layer by calcination, which limits the aggregation of Ag NPs and encapsulates it partially. According to the inset images of Figure S3, it could be clearly seen that the carbon shell is uniformly coated on TiO2 NTs about 12 nm thick and the core-shell Ag@C@TiO2 NTs nanocomposites with numerous uniform Ag NPs loaded on the ultra-long TiO2 NTs tightly and the diameter is about 20 nm. The high-resolution transmission electron microscope (HRTEM) equipped with the selected area electron diffraction (SAED) (inset) is shown in Figure 4b. The image indicates that Ag nanoparticles are packaged by the carbon shell partly and the presence of two lattice fringe spacing of 0.234 nm and 0.35 nm, corresponding to the (111) lattice plane of Ag NPs and the (101) lattice plane of anatase TiO2, respectively. The result is consistent with the analysis of SAED, EDX and element mapping (Figure 4c, d), which are further verified by the existence of Ag nanoparticles and element distribution of Ag@C@TiO2 NTs nanocomposite. The Ag NPs occupied about 12.953% of the atomic percentage and uniformly anchored on the elongated TiO2 NTs, when compared to the well distribution of Ti and O element of TiO2 NTs.


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Figure 4. (a) TEM images at low-magnification (inset at high-magnification) of Ag@C@TiO2 NTs-30 min, (b) HRTEM image and SAED pattern (inset), and element mappings (c) and the corresponding EDS spectrum (d) of Ti element, O element and Ag element. To understand the surface valance states and element bonding composition of the as-prepared samples, especially for the synergistic effects between TiO2 and Ag, we have employed X-ray photoelectron spectroscopy to investigate. The main element composition TiO2 NTs, C@TiO2 NTs and Ag@C@TiO2 NTs were shown in Figure 5a, which displayed the binding energy peaks at 283.9 eV, 458.9 eV and 530.4 eV, representing C 1s, Ti 2p and O 1s respectively. The outstanding N 1s peak exists in C@TiO2 NTs and a faint peak of N 1s in Ag@C@TiO2 NTs as a result the carbon layer that came from calcination of polydopamine. The high-resolution XPS spectra for N 1s region of C@TiO2 NTs and Ag@C@TiO2 NTs located at 400.13 eV and 398.54 eV (Figure S4), which can be deemed to -N-NH- and aromatic N in dopamine. More unambiguously, the distinct peaks of Ag 3d emerged in Ag@C@TiO2 NTs indicates Ag nanoparticles were


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successfully deposited onto the surface of core-shell C@TiO2 NTs. The high-resolution spectra was fitted into two specific peaks at 374.0 eV and 368.0 eV, which could be assigned to Ag 3d 3/2 and Ag 3d5/2, as shown in Figure 5b. The distance of 6.0 eV approximately reveals that Ag is in the form of Ag0. Figure 5c-d shows the Ti 2p and O 1s XPS spectra, which further gained insight into the mechanism of the interaction and charge transfer in this synergistic effect.

Figure 5. XPS spectra of samples: survey spectrum of the pristine TiO2 NTs, C@TiO2 NTs and Ag@C@TiO2 NTs. (a) High-resolution XPS spectra of the Ag 3d in the pristine TiO2 NTs and Ag@C@ TiO2 NTs. Further High-resolution XPS spectra of the pristine TiO2 NTs and Ag@C@ TiO2 NTs: (a) Ti 2p and (b) O 1s XPS spectrum. For the Ti 2p spectrum of pristine TiO2 NTs, it is deconvoluted by the Ti 2p1/2, Ti 2p3/2 peaks at 464.45 eV and 458.75 eV, respectively. And the binding energies gap of them demonstrated the Ti4+ state. After the incorporation of Ag NPs, these two peaks decrease to 464.20 eV and 458.46


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eV. Moreover, Figure 5d shows the O 1s spectra of the TiO2 NTs, which is fitting into two peaks at 531.85 eV and 530.19 eV, corresponding to the surface absorbed OH group and the bond Ti-OTi in the lattice of TiO2. A negative shift of O 1s peaks is observed in Ag@C@TiO2 NTs, except for the two peaks diminish to 531.26 eV and 529.92 eV, respectively. The peak centered at 533.09 eV is accompanied with the associative C-O in the existence of carbon layer. The negative shift of the binding energies in both Ti 2p and O 1s spectrum represents an increase in the electron density of TiO2. This can be considered into the surface binding interaction between Ag NPs and C@TiO2 NTs, which induces an electron transfer from Ag NPs to TiO2 NTs.48,49 The optical properties of these samples were investigated by photoluminescence (PL) spectra and UV-Vis diffuse reflectance spectroscopy. As shown in Figure 6a, TiO2 NTs exhibit an absorption edge at 370 nm approximately in the UV light region. Because of the uniform coating of carbon-shell C@TiO2 NTs, exhibit remarkably enhanced absorption in visible light region. After the deposition of Ag NPs, Ag@C@TiO2 NTs presents a remarkable improvement of the absorbance in the visible and ultraviolet regions. The introduction of Ag NPs and carbon layer brings the increased absorption in the visible light region, owing to the photosensitizing and surface plasmon resonance (SPR) effect of Ag. Furthermore, the absorption of Ag@C@TiO2 NTs30 min is superior to others with the outstanding performance, leading to the best improvement of solar light utilization efficiency. The band gap energy (Eg) is calculated by Tauc plot of (αhv)1/n versus (hv) as formula (1) (Figure 6b): (αhv)1/n = A (hv - Eg)

(1)

Where “α” is the optical absorption coefficient, “hv” is photonic energy and “A” is a constant, Since the anatase crystal of TiO2 is an indirect band gap semiconductor, therefore n value is 2.0.50,51 It can be calculated that the band gap of TiO2 NTs is determined to 3.26 eV and reduced to 3.17


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eV after carbon coating. In addition, the Eg of Ag@C@TiO2 NTs-5 min, 30 min and 2 h is calculated to be 3.07 eV, 2.57 eV and 2.65 eV correspondingly. Both decreased energy band and enhancement in visible light absorption are ascribed to the excellent absorption of carbon shell and the outstanding SPR effect of uniform Ag NPs.

Figure 6. (a) UV-DRS absorption spectra and (b) the corresponding band gap of TiO2 NTs, C@TiO2 NTs and Ag@C@TiO2 NTs-5 min, 30 min and 2 h. (c) PL spectra of TiO2 NTs, C@TiO2 NTs and Ag@C@TiO2 NTs-5 min, 30 min and 2 h. The correlation of light absorption and photocurrent conversion will be further reflected in PL emission spectra, which originates from the transformation of photo-generated charge carriers in semiconductors and the recombination of photo-induced electron-hole pairs. Figure 6c shows the PL spectra of TiO2 NTs, C@TiO2 NTs, and Ag@C@TiO2 NTs-5 min, 30 min and 2 h in the range of 410-650 nm. The lower intensity indicates a lower recombination of electron-hole pairs.52 The PL emission peaks located at about 447 nm, 466 nm, 505 nm and 609 nm are attributed to the band gap transition of TiO2 NTs and the charge transformation of oxygen vacancy trapped electron.53 The PL intensity of C@TiO2 NTs decreases sharply when compared to pristine TiO2 NTs and further declines after the incorporation of Ag NPs that could restrain the recombination of the electron-hole pairs in the ternary Ag@C@TiO2 NTs system. The intensity of Ag@C@TiO2 NTs increased in the order of Ag@C@TiO2 NTs-30 min < Ag@C@TiO2 NTs-2 h < Ag@C@TiO2


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NTs-5 min, indicates a significant suppression of photo-generated charge carriers owing to Ag decoration. The optimized size and uniform distribution of Ag NPs reveals the outstanding separation of photo-induced electron-hole pairs for Ag@C@TiO2 NTs-30 min. The results of PL spectra, which in consistent with the observation of UV-Vis, shows that the carbon-shell acts as the absorbance layer and provides an electronic transmission channel. Moreover, the synergism of Ag NPs and carbon-shell exerts a significant influence on the optical properties of Ag@C@TiO2 NTs. Moreover, the smaller and the more uniform dispersion of Ag NPs has the lowest PL intensity, resulting in the faster of electron transfer and more efficient charge separation.

Figure 7. Transient photocurrent responses curves (a) under Xenon-lamp irradiation in 0.1 M Na2SO4 solution recorded at 0.3 V vs Ag/AgCl and EIS Nyquist plots (b) of the TiO2 NTs, C@TiO2 NTs and Ag@C@TiO2 NTs-5 min, 30 min and 2 h. To investigate the separation and transport characteristics of electron-hole pairs among these samples, transient photocurrent measurement and electrochemical impedance spectroscopy (EIS) with light on and off were carried out. The TiO2 NTs, C@TiO2 NTs and Ag@C@TiO2 NTs samples, displayed an immediate and reproducible photocurrent response with respect to the ON-OFF


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cycles of the irradiation signal. Furthermore, transient photocurrent increases sharply upon light irradiation and decreases gradually when the irradiation is stopping due to the over-length nanotubes or independent particles probably in Figure 7a. The photocurrent intensity of these electrodes under illumination follows an ascending order as Ag@C@TiO2 NTs-30 min (0.94 μA cm-2) > Ag@C@TiO2 NTs-2 h (0.79 μA cm-2) > Ag@C@TiO2 NTs-5 min (0.50 μA cm-2) > C@TiO2 NTs (0.49 μA cm-2), all of them are higher than pristine TiO2 NTs (0.04 μA cm-2). Clearly, the Ag@C@TiO2 NTs-30 min exhibits the highest photocurrent intensity among these samples, indicating the uniform deposition of Ag NPs is significant for the separation of photo-induced electrons and holes. EIS measurement under solar light irradiation can be invoked to investigate the charge transport efficiency of the as-prepared photoanodes in Figure 7b. EIS plots reveals the reaction rate on electrode surface and a smaller semicircle diameter corresponding to a smaller charge transfer resistance.54 The Ag@C@TiO2 NTs-30 min is observed with the smallest arc radius both in dark (Figure S5) and under solar light irradiation in comparison with TiO2 NTs and other composites. In addition, a larger slope of the straight line is present in Ag@C@TiO2 NTs-30 min, demonstrating that Ag NPs with proper size can efficiently facilitate charge transport kinetics based on the interfacial interaction between Ag NPs and TiO2 NTs. Besides, the carbon-shell provides rapid charge carriers transport channel, core-shell structure for protection of Ag NPs. The photocatalytic performance of as-prepared photocatalyst were evaluated by means of the degradation of rhodamine B (RhB, 50 mg L-1) under xenon lamp irradiation with a cut-off filter (λ ≥ 420 nm). As shown in Figure 8a, the adsorption of RhB with a concentration of 10 mg L-1, 20 mg L-1, 30 mg L-1 and 40 mg L-1 on the Ag@C@TiO2 NTs-30 min reaches more than 60% within 60 min. Therefore, 50 mg L-1 RhB serves as initial concentration for absorption and degradation. As seen in Figure 8b, where C0 is the concentration of 50 mg L-1 RhB and the C is the concentration


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after a reaction time of RhB. The curve of original RhB almost has no degradation after 120 min, indicating the sufficient stability of RhB under visible light and the good photocatalytic degradation ability of the Ag@C@TiO2 NTs samples. And the photodegradation performance comparison of the current work with others is summarized in Table S1.

Figure 8. (a) Adsorption capacity of RhB of Ag@C@TiO2 NTs-30 min in different RhB concentration within 60 min. (b) Photocatalytic degradation of 50 mg L-1 RhB with 60 min adsorption and 120 min visible light irradiation: (a) original 50 mg L-1 RhB, (b) TiO2 NTs, (c) C@TiO2 NTs, (d) Ag@C@TiO2 NTs-5 min, (e) Ag@C@TiO2 NTs-30 min and (f) Ag@C@TiO2 NTs-2 h. Photodegradation efficiency (c) and kinetic curves (d) fit for the RhB degradation efficiency under visible-light irradiation in 120 min.


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However, there is nearly 41% and 50% of the RhB removal with the same illumination time in the TiO2 NTs and C@TiO2 NTs, and the RhB degradation rate with Ag@C@TiO2 NTs-5 min, 30 min and 2 h reached to 63.1%, 84.7% and 86.6%, respectively. Nevertheless, 67% of RhB adsorption was obtained in Ag@C@TiO2 NTs-2 h, showing a higher absorption than other samples owing to the more content of Ag NPs. The degradation rate of the samples under visible light irradiation was shown in Figure 8c, where C, Ct is the concentration after adsorption and a period of degradation of RhB. Obviously, the degradation rate of Ag@C@TiO2 NTs-30 min is the highest than other as-prepared composites after removal of adsorption efficiency. Subsequently, the repeatability of Photocatalytic degradation of RhB using the Ag@C@TiO2 NTs was shown in Figure S6a. To further compare the photocatalytic performance of these photocatalysts under visible light illumination, the reaction kinetics of RhB degradation is following the pseudo-firstorder kinetics is calculated by formula (2): Ln(C/Ct) = kt

(2)

Where C is the concentration after 60 min absorption and Ct is the concentration after a certain reaction time of RhB, k, t is the kinetic constant and the reaction time, respectively. Based on Figure 8d, the result shows the Ag@C@TiO2 NTs-30 min has the maximal kinetic constants of 0.00962 min-1, which is almost 3.6 times higher than TiO2 NTs (0.00263 min-1). Moreover, the k values of original RhB, C@TiO2 NTs and Ag@C@TiO2 NTs-5 min and 2 h were 0.000286, 0.00283, 0.00391 and 0.00745 min-1, respectively. The photocatalytic performance is associated with Ag NPs size and uniformity controlling. So Ag@C@TiO2 NTs-30 min with the diameter of 20 nm and uniform dispersion contributes to the best photo activity based on all of the researches results, indicating the uniformity of Ag NPs is effective for the charge transfer and depress the recombination of electron-hole pairs.


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Figure 9. (a) Photoelectrocatalytic hydrogen production rate of pristine TiO2 NTs, C@TiO2 NTs and Ag@C@TiO2 NTs-5 min, 30 min and 2 h in distilled water containing 20 vol% methanol under a 500 W xenon lamp (80 mW cm-2) with a UV light cutoff filter within 4 h. (b) a schematic illustration of the hydrogen generation and dye degradation process by using the Ag@C@TiO2 NTs core-shell nanocomposite catalyst under simulated visible light irradiation. In order to validate the photocatalytic properties of Ag@C@TiO2 NTs-30 min, photocatalytic H2 evolution under visible light were carried out with 20 vol% methanol as sacrificial agent. The H2 generation rate of TiO2 NTs, C@TiO2 NTs and Ag@C@TiO2 NTs-5 min, 30 min and 2 h after 4 h irradiation was shown in Figure 9a. It is obvious that the Ag@C@TiO2 NTs-30 min samples display the highest H2 evolution of 72 μmol h-1 g-1, which is 4.8, 2.8 times as large as the TiO2 NTs (15 μmol h-1 g-1) and C@TiO2 NTs (25 μmol h-1 g-1), respectively. The comparison of hydrogen evolution rate with other TiO2 based photocatalysts under different light source irradiation is also summarized in Table S2. Moreover, cycling measurements of hydrogen evolution using Ag@C@TiO2 NTs was shown in Figure S6b. The result proves again that the best photocatalytic performance can be obtained by the interaction between the uniform Ag NPs and the core-shell structure of TiO2 NTs-carbon layer. Based on these above measurements, both C@TiO2 NTs and Ag@C@TiO2 NTs exhibit


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excellent photocatalytic performance compared to TiO2 NTs, which is ascribed to the introduction of carbon-shell and Ag nanoparticles. The carbon-shell increased electrical conductivity by providing rapid charge carriers transfer channels and suppressed the recombination of electronsholes, and had sufficient area to deposit a relatively large amount of Ag NPs. Meanwhile, the introduction of Ag nanoparticles makes the photocatalysts better performance due to the synergistic effects among over-length TiO2 NTs, carbon-shell and Ag NPs. The possible mechanism about energy band structure and path of electron-hole pairs shown in Figure 9b. The electron can be excited from Ag nanoparticles on its surface under visible light illumination and the photogenerated holes are distributed at the interface mainly, owing to the SPR effect of Ag. The photogenerated electrons transfer through carbon shell from the surface of Ag NPs to the conduction band of TiO2 NTs, resulting in the inhibition of photo-induced electron-hole pairs and the increase of photocatalytic properties. The electron on the CB of TiO2 NTs functions as active sites for H2 generation, and the methanol as the sacrificial regent to capture the holes to reduce the recombination of electron-hole in the hydrogen production process.

Conclusion In summary, a facile and well-designed polydopamine-inspired synthesis of Ag NPs@C@ elongated TiO2 NTs core-shell nanocomposite was prepared in this work. As encapsulated with carbon-shell and loaded with satellite plasmonic Ag NPs, this work shows that the photocatalytic activity can be strengthened as an efficient photocatalysts for the degradation of RhB and photocatalytic hydrogen evolution. The carbon layer inspired by polydopmine, a binding agent to adhere Ag NPs on TiO2 NTs, is conformably coated on the surface of the elongated TiO2 NTs to form core-shell structure. Especially, the carbon-shell provide the channel for efficient charge


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transfer from Ag nanoparticles to the conduction band of TiO2 NTs more easily, and has the superiority of structural stability of the as prepared photocatalyst. In addition, Ag NPs anchored on C@TiO2 NTs with optimized deposition time shows the excellent photocatalytic performance in the processes of RhB degradation and hydrogen generation, which is 2.5 times and 4.8 times for comparison to pristine TiO2 NTs. The synergistic effect of Ag nanoparticles and carbon-shell benefits to narrow the bandgap of TiO2 NTs, suppress the electron-hole pairs recombination and improve the utilization efficiency of solar light. Consequently, the work that utilizes the elongated TiO2 NTs as composite substrate, and introduces the polydopamine-derived carbon-shell and Ag NPs confirms that Ag@C@TiO2 NTs core-shell photo-induced photocatalyst exhibits the outstanding photocatalytic and chemical activities in environmental and energy harvesting applications.

Acknowledgment The authors thank the National Natural Science Foundation of China (21501127 and 51502185). We also acknowledge the funds from the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Project for Jiangsu Scientific and Technological Innovation Team (2013).

Supporting Information EDS spectrum, XRD patterns and TEM images of Ag@C@TiO2 NTs; High-resolution XPS spectra of the N 1s; EIS spectra of TiO2 NTs, C@TiO2 NTs and Ag@C@TiO2 NTs; Photocatalytic degradation of RhB; Cycling measurements of hydrogen evolution; Table of the comparison of degradation reaction and degradation rate; Table of the comparison of the hydrogen evolution rate.


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TOC

To develop a visible-light induced efficient photocatalyst for environmental pollution treatment and energy harvesting applications, Ag NPs @C@ elongated TiO2 NTs core-shell nanocomposite was fabricated via a facile stirring hydrothermal and polydopamine-inspired strategy with high absorption efficiency and low recombination rate of electron-hole for outstanding water remediation and sustainable hydrogen evolution.


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Polydopamine-Inspired Design and Synthesis of Visible-Light-Driven

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