Layer-by-Layer Self-Assembly Construction of Highly Ordered Metal

Jul 11, 2012 - Layer-by-Layer Self-Assembly Construction of Highly Ordered Metal-TiO2 Nanotube Arrays Heterostructures (M/TNTs, M = Au, Ag, Pt) with ...
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Layer-by-Layer Self-Assembly Construction of Highly Ordered MetalTiO2 Nanotube Arrays Heterostructures (M/TNTs, M = Au, Ag, Pt) with Tunable Catalytic Activities Fangxing Xiao* State Key Laboratory Breeding Base of Photocatalysis & College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou, 350002, P. R. China S Supporting Information *

ABSTRACT: We have developed a facile and easily accessible layer-by-layer (LBL) self-assembly route to synthesize hierarchically ordered M/TNTs (M = Au, Ag, Pt) heterostructures. These integrated heterostructures show remarkably enhanced photoactivity and outstanding photostability; photoelectrochemical exploitations substantiated the contribution role of metal NPs acting as “electron reservoir” in prolonging the lifetime of photogenerated electron−hole charge carriers. In addition, these well-defined self-assembled hybrid systems can also be used as a promising catalyst for recycled selective catalytic reduction of 4-nitrophenol toward 4-aminophenol. The integration of high photoactivity and efficient catalytic reduction properties of the heterostructures lies crucially on the LBL self-assembly-induced monodispersivity of metal NPs on the framework of TNTs, and, particularly, the intimate interfacial contact between metal NPs and TNTs substrate arising from the pronounced electrostatic attractive interaction afforded by polyelectrolytes multilayering. Our results show that the design and utilization of highly ordered metal/1-D semiconductor hybrid nanostructures based on the facile LBL selfassembly strategy can find diverse catalytic applications.

1. INTRODUCTION One-dimensional (1-D) semiconducting nanostructures, such as nanotubes, nanorods, and nanowires, have received tremendous attention in view of their unique shape-dependent optoelectronic and surface properties in a wide range of technological fields,1−4 including solar energy conversion, catalysis, lithium batteries, photodetectors, and photocatalysis.5−9 Among copious 1-D inorganic nanostructures, titania (TiO2) has been extensively explored as a photocatalyst for degradation of pollutants in air or water and H2 generation.10,11 Nanotubular TiO2 exhibits unique benefits in comparison with commonly used TiO2 nanoparticles (NPs) or bulk materials due to novel chemical structure and properties, more specifically, that is, (1) excellent fast and long-distance electron-transport capability; (2) larger specific surface area and pore volume; and (3) enhanced light absorption and scattering stemming from the high length-to-diameter ratio.12−15 Alternatively, the well-defined internal voids of 1-D TiO2 are subject to afford the introduction of other matter into the cavities, leading to intriguing properties different from the bulk materials.16−18 Therefore, it is of prime importance to probe the potential applications of nanotubular TiO2-based nanomaterials in the field of heterogeneous catalysis. Metal NPs are generally utilized as effective catalysts because of their large percentage of active surface atoms and unusual electronic structure.19 In particular, metal NPs immobilized on © 2012 American Chemical Society

the solid supports exhibit prominent catalytic potency; for example, metal NPs highly dispersed on an active oxide are a classic example of a high-performance heterojunction catalyst.20−22 Accordingly, integration of 1-D TiO2 with metal NPs in an appropriate fashion could combine their respective advantages, by which hybrid heterogeneous nanostructures with outstanding catalytic activities could thus be attained. To date, diverse approaches have been adopted to prepare 1D TiO2, for example, deposition with templates,23−25 hydrothermal treatment of TiO2 particles with alkali solutions,26−28 and electrochemical means.29,30 Among them, anodization of titanium foil leading to hierarchically self-organized TiO2 nanotube arrays (TNTs) has attracted burgeoning interest because it represents a unique combination of highly functional features of TiO2 with a regular and controllable nanoscale geometry.31 It can thus be speculated that intrinsic morphological features coupled to precise physicochemical parameter control of TNTs render the TNTs motif remarkably attractive as starting nanobuilding blocks toward functional nanomaterials. There has been a cornucopia toward the fabrication of metal/TiO2 hybrid nanocomposites; nevertheless, conventional Received: April 11, 2012 Revised: June 11, 2012 Published: July 11, 2012 16487

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mm × 0.1 mm, 99.6%, Xin RuiGe), ethylene glycol (CH2OH)2, ammonium fluoride (NH4F), hydrogen fluoride (HF), nitric acid (HNO3), hydrochloric acid (HCl), poly(acrylic acid) (PAA, 25 wt % soln. in water, Mw ≈ 240 000 g mol−1), and poly(allylamine hydrochloride) (PAH, Mw ≈ 15 000 g mol−1) were used. 2.2. Preparation of TiO2 Nanotube Arrays (TNTs) via Two-Step Anodization Approach. Titanium sheets (50 mm × 20 mm × 0.1 mm, 99.9%, Xin RuiGe) were polished by abrasive paper and ultrasonically washed by acetone, ethanol, and ddH2O for 15 min, respectively. Afterward, the titanium sheets were immersed in a mixed solution of HF−HNO3−H2O with volume ratio of 1:4:5 for 30 s, washed by ddH2O, and dried in a N2 stream. Anodization of Ti foil was conducted in an organic electrolyte of ethylene glycol containing 0.5% (w/v) NH4F and 2% (v/v) H2O, where the titanium sheet was used as a working electrode with a graphite sheet as a counter electrode. The anodization of titanium foil was first carried out at 40 V for 3 h, after which the thus obtained samples were washed by ddH2O, dried in a N2 stream, and subsequently sonicated for 5 min to remove the surface layer; the second anodization was subsequently carried out under the same experimental conditions. Finally, the TNTs samples were gently sonicated and annealed at 450 °C in air for 3 h to transform the amorphous phase of TiO2 to the anatase phase. 2.3. Preparation of Citrate-Stabilized Metal Colloidal NPs. (a). Synthesis of Citrate-Stabilized Au NPs. Citratestabilized Au NPs were prepared by Dotzauer’s method.47 In brief, all glassware was thoroughly cleaned with aqua regia (3 parts HCl, 1 part HNO3) for 12 h and rinsed with ddH2O. In a 100 mL Erlenmeyer flask, 25 mL of aqueous 1 mM HAuCl4·3H2O was heated to a rolling boil under vigorous stirring (2500 rpm). Aqueous sodium citrate (2.5 mL, 38.8 mM) was also heated to a rolling boil and rapidly added to the above gold precursor solution. After 20 s, the mixture became dark and then burgundy; heating was continued with vigorous stirring for 10 min. Finally, the mixture was stirred without heating for an additional 15 min to fulfill the synthesis. The asprepared Au NPs were stored in a refrigerator at 4 °C in a dark area and used within 2 weeks. (b). Synthesis of Citrate-Stabilized Ag NPs. Citratestabilized Ag NPs were prepared from the NaBH4 reduction of AgNO3 in aqueous solutions following the procedures of Lee.48 In brief, 100 mL of a 1 mM aqueous AgNO3 solution was mixed with 8 mL of a 40 mM aqueous sodium citrate solution used as stabilizer. A total of 2 mL of a 112 mM aqueous NaBH4 solution was then added dropwise under vigorous stirring (2500 rpm) at ambient temperature, immediately yielding a yellowish brown Ag hydrosol. The Ag hydrosol was stocked in a refrigerator at 4 °C and aged for 24 h to decompose the residual NaBH4 before it was used in subsequent steps. (c). Synthesis of Citrate-Stabilized Pt NPs. Citrate-stabilized Pt NPs were synthesized according to Elliott’s method.49 In brief, a total of 26 mL of 2.8 mM aqueous sodium citrate solution was added to 50 mL of 0.4 mM aqueous hydrogen hexachloro-platinate solution at room temperature. Subsequently, 5 mL of 12 mM NaBH4 was introduced dropwise with vigorous stirring (2500 rpm), and the pale-yellow solution turned dark-brown in 5 min. Finally, the dark-brown colloidal solution was stirred for 4 h and stored in a refrigerator at 4 °C until ready for further use.

preparative routes are principally confined to deposition− precipitation (DP),32−34 impregnation,35 sol−gel process,36 and photoreduction,37,38 by which a secondary metal phase was formed on the surface of TiO2. It is noteworthy that these strategies normally involve either additional thermal treatment or delicate manipulation. Owing to the post-thermal treatment, unfortunately, size control and distribution of the metal constituents have been demonstrated to be a primary difficulty to circumvent if well-defined nanostructures are desired, thereby resulting in metal clusters of nonuniform size and shape. With regard to the preparation of metal/TNTs hybrid nanocomposites, the same technological challenges of synthetically controlling the monodispersivity and homogeneous site distribution of metal NPs are still met with limited success. In particular, uniform deposition of metal NPs on the internal surfaces of TNTs has been evidenced to be rather unfavorable compared with the case on the pore entrance.39 Meanwhile, conventional synthetic methods such as complicated photoreduction40−42 or chemical reduced approach43 continued to plague the construction of well-defined metal/TNTs nanomaterials owing to poor repeatability. On account of these perennial problems tailor-made metal NPs of active components were deposited directly to the nanotubular framework of TiO2 via a facile and easily accessible layer-by-layer (LBL) selfassembly approach without post heat treatment to explore the general technique for the fabrication of highly ordered metal/ TNTs heterostructures. It is worth noting that LBL selfassembly absorption technique is a versatile bottom-up nanofabrication method, exhibiting prominent advantage over conventional approaches on its versatility and simplicity and furnishing molecular-level control over the thickness, structure, and composition of multilayer films with rather simple benchmark operation.44−46 In this work, we predominantly focused on developing a facile, precisely controlled, and repeatable LBL self-assembly route to fabricate hierarchically ordered metal/TNTs heterostructures, M/TNTs (M = Au, Ag, Pt), for extensive catalytic investigations. The tailor-made metal (Au, Ag, Pt) colloidal NPs were uniformly deposited to the framework of TNTs through the self-assembly monolayer (SAM) of LBL buildup, which is afforded by substantial electrostatic attractive interaction between metal NPs and polyelectrolytes. It has been found that the LBL self-assembly strategy provides a far superior and efficient route to achieve readily the functionalization of TNTs with metal NPs. Additionally, combined studies on the versatile photocatalytic and selective catalytic reduction activities of these self-assembled systems have revealed that the as-prepared well-aligned M/TNTs (M = Au, Ag, Pt) heterostructures exhibit a significant advance in facilitating great potential for tangible applications. Furthermore, photoactivity and catalytic reduction capability of the well-defined heterostructures can be exquisitely tuned by deposition cycles in the LBL process.

2. EXPERIMENTAL SECTION 2.1. Materials. Chloroauric acid (HAuCl4·3H2O), silver nitrate (AgNO3), chloroplatinic acid (H2PtCl6·6H2O), sodium borohydride (NaBH 4 ), trisodium citrate dehydrate (C6H5Na3O7·2H2O), ethanol (C2H6O), 4-nitrophenol (4-NP, C6H5O3N), titanium sheets (50 mm × 20 mm × 0.1 mm, 99.9%, Xin RuiGe, Beijing, China), deionized water (ddH2O, Milipore, 18.2 MΩ·cm resistivity), graphite sheet (50 mm × 20 16488

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2.4. Zeta Potential (ξ) Measurement. Zeta potentials (ξ) of the citrate-stabilized Au, Ag, and Pt colloidal NPs were determined by dynamic light scattering analysis (Zeta sizer 3000HSA) by taking advantage of five measurements at room temperature of 25 °C. In brief, 200 μL of citrate-stabilized Au, Ag, and Pt NPs was diluted to 30 mL of ddH2O to obtain a concentration of ca. 500 mg/L in an aqueous solution. The pH adjustment was achieved by diluted 0.1 N HCl or NaOH aqueous solution when the zeta potential of the sample was measured as a function of pH value. 2.5. Immobilization of Metal Colloidal NPs on the Framework of TNTs. The chemical structures of polyelectrolytes (PEs, i.e., PAA and PAH) and citrate-stabilized Au, Ag, and Pt NPs, together with procedures for preparing the M/ TNTs (M = Au, Ag, Pt) hybrid nanostructures via the LBL deposition technique, are vividly illustrated in Chart S1 and Scheme S1 in the Supporting Information, respectively. In principle, a single cationic PE layer or bilayer consisting of a cationic inner and an anionic outer layer was absorbed in the LBL buildup. Both of the PE solutions (1.0 mg mL−1) used for the whole experiments contain 0.5 M NaCl aqueous solution with pH value of 4.5. The specific LBL self-assembly process was illustrated in the following: First, TNTs were completely dipped in an aqueous HCl solution (pH 2.5) for 20 min, washed three times with ddH2O for 2 min, followed by drying with a gentle stream of nitrogen, which yields positively charged TNTs substrate (TNTs+). Second, a negatively charged PAA layer was deposited on the TNTs+ matrix (TNTs+/PAA−) using the same washing and drying procedures. Third, the positively charged PAH layer was subsequently deposited to the TNTs+/ PAA− layer to acquire the positively charged layer (TNTs+/ PAA−/PAH+). Finally, the thus-obtained TNTs+/PAA−/PAH+ multilayer films were dipped in the presynthesized metal (Au, Ag, Pt) colloidal NPs to achieve the well-defined TNTs/PAA/ PAH/Au (or Ag, Pt) heterostructures, that is, M/TNTs (M = Au, Ag, Pt). It should be emphasized that the outermost layer of the PE multilayer films fabricated was always PAH, a positively charged layer, thus, negatively charged metal colloidal NPs facilitate the periodic deposition to the PE multilayer films. The deposition cycle was repeated until the desired TNTs/ PAA/PAH/(Ag/PAH)n (n = 1, 2, 4, 8, 10) multilayer thin films were attained. 2.6. Characterization. The phase composition of the sample was determined by X-ray diffraction (XRD) on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation. The accelerating voltage and applied current were 40 kV and 40 mA, respectively. Transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) spectrometer, and highresolution transmission electron microscopy (HRTEM) images were obtained by a JEOL model JEM 2010 EX instrument at an accelerating voltage of 200 kV. The UV−vis diffuse reflectance spectra (DRS) were recorded on a Varian Cary 500 Scan UV− vis-NIR spectrometer, in which BaSO4 was used as the background between the 200 and 800 nm regime. X-ray photoelectron spectroscopy (XPS) measurements were conducted on an ESCALAB 250 photoelectron spectrometer (Thermo Fisher Scientific) at 2.4 × 10−10 mbar using a monochromatic Al Kα X-ray beam (1486.60 eV). Binding energy (BE) of the element was calibrated to the carbon BE of 284.60 eV. The morphologies of the samples were measured by field-emission scanning electron microscopy (FESEM/EDX, FEI Nova NanoSEM 230). The photoluminescence (PL)

spectra for solid samples were investigated on an Edinburgh FL/FS900 spectrometer. 2.7. Photocatalytic Activity. Photocatalytic activities of the samples were evaluated by using methyl orange (MO) as a model organic dye pollutant compound. In a typical test, blank TNTs or TNTs/PAA/PAH/(Ag/PAH)n (n = 1, 2, 4, 8, 10) sample with the same area of 3 cm2 was soaked into 3 mL of MO solution (5 mg/L) at pH value of 7 in a quartz cuvette. Before irradiation, the mixtures were kept in the dark for 1 h to reach the equilibrium of adsorption−desorption at room temperature. A 300 W Xe arc lamp (PLS-SXE 300C) equipped with a cutoff filter (λ = 365 ± 15 nm) was applied as the UV light source. The temperature of the reaction system was controlled to be around 25 °C by a commercial electric fan at ambient conditions. The irradiation time ranged from 5 min to 2.5 h. Under ambient conditions, all the samples in the quartz cuvette were placed ca. 21 cm away from the UV source with the same light intensity of ca. 5 mW/cm2. At each time interval of 30 min the light absorption of the reaction solution was measured by a Cary 500 scan UV−vis spectrophotometer. The concentration of MO was determined by the absorption of MO at 464 nm. The degradation ratio of MO at each time interval was calculated from the difference of the light absorbance of irradiated to the nonirradiated solution. 2.8. Photoelectrochemical Measurement. Photoelectrochemical measurements were performed on a CHI 600D electrochemical system (Chenhua Instruments, Shanghai, China). The system consisted of three electrodes, a singlecompartment quartz cell, which was filled with 0.1 M Na2SO4 electrolyte (30 mL), and a potentiostat. A platinum black sheet was used as a counter electrode with Hg/Hg2Cl2/KCl as a reference electrode. A thin film of TNTs or TNTs/PAA/PAH/ (Ag/PAH)n (n = 1, 2, 4, 8, 10) sample (30 mm ×10 mm) with impregnated area of 3 cm2 was employed as a working electrode. A 300 W Xe arc lamp (PLS-SXE 300C) equipped with a band-pass light filter (λ = 365 ± 15 nm) was used as the exciting light source for UV light irradiation. The intensity of light was controlled to be ∼5 mW/cm2. 2.9. Catalytic Reduction Activity. In a typical reaction, blank TNTs or TNTs/PAA/PAH/(Ag/PAH)n (n = 1, 2, 4, 8, 10) sample (30 mm × 10 mm) with an area of 3 cm2 was dipped into an aqueous solution in a quartz cuvette containing aqueous solution of 2 mM (40 μL) 4-NP, 100 mM (400 μL) NaBH4, and 2 mL of ddH2O. Afterward, the mixture was stirred at room temperature for 30 min to generate uniform aqueous solution (200 rpm). The use of a high excess of NaBH4 ensures that its concentration remains essentially constant during the whole reaction, which allows the assumption of pseudo-firstorder kinetics with respect to the nitro compound. Samples of the reaction mixture were collected at a specific time interval (10 min) for UV−vis spectroscopy analysis. The light absorbance of the characteristic peak of 4-NP at 400 nm was monitored.

3. RESULTS AND DISCUSSION 3.1. Analysis of the LBL Self-Assembly Mechanism. 3.1.1. Preparation of Citrate-Stabilized Metal Colloidal NP (Au, Ag, Pt NPs). Metal NPs were obtained by citrate reduction of gold, silver, and platinum precursors, and thus citrate anions of starting reagents are intrinsically adsorbed on the surface. These citrate-capped metal NPs present a carboxylic acid derivatized surface that opens up the possibility of exploiting electrostatic attractive force for surface attachment in the LBL 16489

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Figure 1. (a) UV−vis absorption spectra of citrate-stabilized Au, Ag, and Pt NPs with corresponding photograph in the inset and zeta potential of (b) Au, (c) Ag, and (d) Pt NPs as a function of pH value.

being stable exclusively by electrostatic repulsion.54 Therefore, metal NPs used in our work (pH 7) possessing the most negative ξ value demonstrate the highest stability. The excellent stability of colloidal NPs can also be verified by the photograph in the inset of Figure 1a, in which the metal NPs stored beyond more than 4 months still remain homogeneous. On the contrary, NaBH4-reduced Au, Ag, and Pt NPs are prone to precipitate in a few hours on account of weak negative charge on the surface of metal NPs.55 As a result, pronounced repulsive force is attained between citrate-stabilized metal NPs to counteract the van der Waals interaction in a dispersing medium. A prerequisite for implementation of LBL self-assembly strategy for multilayer construction concentrates on the charge reversal attained with each adsorption step, in which anionic polyelectrolyte of PAA with a pKa of 4.556 and cationic PAH with a pKa of 8.557 were used as the primary building blocks. Normally, PAH and PAA are expected to be fully protonated and deprotonated, respectively, at lower pH values (pH ≤4.5) according to their pKa values. Moreover, the positively charged layer of the TNTs substrate is readily achieved by immersing in an acid medium with a pH value of 2.5 much lower than the isoelectric point (IEP) of TiO2 (4.5−6.8).58−61 Consequently, spontaneous LBL self-assembly of cationic TNTs substrate between the anionic and cationic polyelectrolytes (i.e., PAA, PAH) is initiated based on the pronounced ionic electrostatic attractive force afforded by the oppositely charged species. The synthetic flowchart of the LBL self-assembly process is vividly illustrated in Scheme S1 in the Supporting Information. 3.2. LBL Self-Assembly of M/TNTs (M = Au, Ag, Pt) Heterostructures. Scanning electron microscopy (SEM) images are taken to analyze directly the morphologies of the self-aligned TNT substrate and metal NP-deposited TNTs

self-assembly process. Figure S1(a−c) in the Supporting Information shows the TEM images of the synthesized Au, Ag, and Pt NPs having average diameter of 14.7 ± 1.2 (Figure S1d), 6.33 ± 0.4 (Figure S1e), and 3.14 ± 0.2 nm (Figure S1f), respectively. It is demonstrated that the metal NPs exhibit excellent monodispersivity and uniform size distribution resulting from the substantial electrostatic repulsion between NPs. The EDX spectra in Figure S1(g−i) show the presence of Au, Ag, and Pt signals from bulk colloidal NPs as well as C and Cu signals from carbon grid used for TEM measurement. The selective area electron diffraction (SAED) patterns in the inset of Figure S1(a−c) all unambiguously exhibit the polycrystalline nature of the metal NPs. Formations of citrate-stabilized Au, Ag, and Pt NPs were further ascertained by ultraviolet−visible (UV−vis) absorption spectra. Figure 1a displays the surface plasmon resonance (SPR) absorption bands presenting at 395 and 526 nm, which correspond to Ag and Au NPs, respectively. In the case of Pt NPs, no adsorption peak at λ > 200 nm was observed apart from a weak absorption tail stretching across the UV region, which is in faithful agreement with the previous investigations.50−52 3.1.2. Elucidation of Electrostatic Interaction Force between Constituents for LBL Self-Assembly Buildup. The stability of metal NPs was evaluated by the measurement of zeta potential (ξ), which is normally utilized as an index of the magnitude of electrostatic attractive interaction between colloidal NPs.53 As revealed in Figure 1b−d, metal NPs were stable over a wide range of pH value (2−12) due to substantial negative zeta potential. Notably, zeta potential (ξ) of the aqueous metal NPs at pH 7 shows pronounced negative values of −51.5, −47.5, and −36.8 mV, for Au, Ag, and Pt NPs, respectively. In general, zeta potential larger than +30 mV or more negative than −30 mV is regarded as the criterion for NPs 16490

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Figure 2. (a) Panoramic, (b) bottom, and (c) magnified cross-sectional SEM images of TNTs substrate post-treated by calcination at 450 °C in air for 3 h with corresponding overall cross-sectional view in the inset of panel c. Top-view SEM images of (d) Au/TNTs, (e) Ag/TNTs, and (f) Pt/ TNTs hybrid nanostructures prepared via the LBL self-assembly approach.

part of TiO2 nanotube very difficult to initiate as desired. To this end, strategy concerning two-step anodization in conjunction with sonication treatment was harnessed during the synthetic process for attaining the smooth TNTs surface. The sporadic small fragments attached to the top layer of the TNTs framework (Figure 2d) are ascribed to the sonicationinduced removal of patterns left on the foil surface after the first-step anodization, and these fragments have no influence on the deposition of metal NPs to the TNTs support, which will be corroborated by the TEM results. To obtain further the microscopic structure information on the changes in morphology, composition, and wall thickness of the blank TNTs and M/TNTs (M = Au, Ag, Pt) heterostructures, we have performed TEM and EDX spectrum analysis, as displayed in Figure 3. TEM images of the TNTs substrate are shown in Figure 3a; it is apparent that the tube wall is uniform in thickness along the length of tube, and these nanotubes are open at the top (Figure 3a, inset) and closed at the bottom (Figure 2b). It can be seen from Figure 3b and inset views in Figure 3c,d that Au, Ag, and Pt NPs ingredients are uniformly distributed on the framework of TNTs, which is consistent with the SEM results. High-resolution TEM (HRTEM) images in Figure 3b−d unambiguously show the characteristic lattice fringe of 0.352 nm for TNTs and 0.233, 0.231, and 0.237 nm for Au, Ag, and Pt NPs, which corresponds to the (101) crystal plane of anatase TiO2 and (111) crystallographic planes of face-centered cubic (fcc) structures of metal NPs, respectively. EDX result (Figure S3 in the Supporting Information) gives the signals of Ti, O, from TNTs matrix and Au, Ag, and Pt elements from bulk metal

heterostructures. The M/TNTs (M = Au, Ag, Pt) hereafter are defined as hybrid nanostructures with one deposition cycle other than special illustration in the text. It is shown in Figure 2a that the as-prepared self-organized TNTs exhibit a regularly arranged pore structure of the film with uniform size distribution around 76 nm. Figure 2b,c reveals that the film is composed of well-aligned nanotube arrays growing vertically from a Ti metal plate. The inset in Figure 2c displays the film possessing thickness profile of ca. 6 μm. Notably, the highly ordered morphology of the TNTs substrate was retained postheat treatment, which is conducive to the transformation of the amorphous phase of TiO2 to the anatase phase leading to superior photocatalytic performance of TNTs. As reflected in Figure 2d−f, when the metal NPs were deposited on the TNTs scaffold through the LBL self-assembly approach, the morphologies of the M/TNTs (M = Au, Ag, Pt) heterostructures are distinctly different from those of plain TNTs. More specifically, in regards to Au/TNTs hybrid nanostructure (Figure 2d), Au NPs spread uniformly on the internal void space and exterior surfaces of the TNTs with substantial interfacial adhesion. Enlarged images in Figure 2e,f clearly show that Ag and Pt NPs are evenly tethered on the framework of TNTs, whereas it is a little difficult to discern directly from the panoramic views owing to their small diameter and sporadic TiO2 fragments (Figure 2d) retained on the top layer of TNTs after gentle sonication treatment. Fundamentally, conventional TiO2 nanotube arrays prepared directly via one-step anodization are usually bundled at the top layer and have large outer tube spacing (Figure S2 in the Supporting Information),62 thus making monodisperse deposition of metal NPs to the inner 16491

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respectively. As marked in Figure 4c−e, M/TNTs (M = Au, Ag, Pt) heterostructures exhibit featured crystallographic planes of fcc Pt (JCPDS file no. 65-2868), Ag (JCPDS file no. 04-0783), and Au (JCPDS file no. 65-2870) which agrees well with the HRTEM results. The broad diffraction peaks strongly indicate the nanocrystalline nature of the samples. UV−vis DRS are utilized to determine the optical properties of the samples. Figure 5 shows that all samples display an

Figure 3. TEM images of (a) TNTs substrate with interior morphology in the inset and HRTEM image of (b) Au/TNTs, (c) Ag/TNTs heterostructure with panoramic view in the inset, and (d) Pt/TNTs hybrid nanostructure with top-view in the inset.

Figure 5. UV−vis diffuse reflectance spectra (DRS) of TNTs and M/ TNTs (M = Au, Ag, Pt) hybrid nanostructures.

intense adsorption band in the region spanning from 200 to 400 nm, which is ascribed to electron promotion of TiO2 from the valence band (VB) to the conduction band (CB).10,63 Besides, Au/TNTs and Ag/TNTs hybrid nanostructures both exhibit pronounced visible-light adsorption peaks at the wavelength of 520 and 400 nm, resulting from the SPR absorption peaks of Au and Ag NPs, respectively, in the visiblelight regime. Noteworthily, DRS result of the Pt/TNTs heterostructure is nearly shielded by that of blank TNTs owing to the absent absorption of Pt NPs in the visible-light regime. As a result, DRS result demonstrates successful incorporation of metal NPs to the scaffold of TNTs and substantial UV light photosensitity of the hybrid heterostructures. XPS is used for surface analysis of chemical changes induced by LBL self-assembly adsorption of polyelectrolytes and metal NPs. The survey spectra of the M/TNTs (M = Au, Ag, Pt) heterostructures (Figure 6a) present pronounced featured signals of Au 4f, Ag 3d, and Pt 4f, indicating successful deposition of metal NPs to the framework of TNTs. Alternatively, to differentiate the specific chemical bond species and elemental chemical state of the samples, high-resolution spectra were correspondingly deconvoluted. High-resolution C 1s spectrum of the M/TNTs heterostructures (Figure 6b) was deconvoluted to three peaks showing BE of 284.60, 286.26, and 288.47 eV, which are attributed to C−C/C−H species from the alphatic hydrocarbon chains of PAA and PAH and adventitious carbon of CO2, C−OH/C−O−C species from the oxidized carbon species of aliphatic hydrocarbon of PAA and PAH and adventitious carbon, and CO 3 2− species from PAA, 64 respectively. High-resolution O 1s spectrum (Figure 6c) can be deconvoluted to three peaks; the peak at BE of 529.90 eV is assigned to lattice oxygen of TiO2, and the other two peaks at BE of 531.42 and 532.52 eV are attributable to oxygen species of Ti−OH and C−OH/C−O−C species corresponding to the C 1s peak at 286.26 eV (Figure 6b),65 respectively. The detailed

NPs. Furthermore, EDX results also demonstrate that the atomic ratios of Ti to O are equal to 1:2, which agrees with the stoichiometric ratio of TiO2. The crystallographic structure and phase purity of the TNTs substrate and M/TNTs (M = Au, Ag, Pt) heterostructures were examined by XRD. The TNTs matrix and M/TNTs (M = Au, Ag, Pt) heterostructures exhibit similar diffraction peaks in terms of TiO2 framework and Ti matrix (Figure 4), in which

Figure 4. XRD patterns of TNTs substrate (a) before and (b) after calcination at 450 °C in air for 3 h, (c) Pt/TNTs, (d) Ag/TNTs, and (e) Au/TNTs heterostructures.

the peaks at 2θ values of 25.3, 37.8, 48.0, 53.9, 62.7, and 75.0 can be attributed to (101), (004), (200), (105), (204), and (215) crystallographic planes of anatase TiO2 (JCPDS file no. 21-1272), and featured peaks at 2θ values of 38.4, 40.3, 53.1, and 70.8 are indexed to (002), (101), (102), and (103) crystal planes of metal Ti phase (JCPDS file no. 44-1294) from the Ti foil, respectively. The characteristic diffraction peaks of anatase TiO2 and Ti phase are marked with A and T in Figure 4, 16492

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under the UV-light irradiation (365 ± 15 nm) under ambient conditions. Blank experiments (without photocatalyst or UV light) reveal negligible photocatalytic activities, verifying that the degradation reaction is truly driven by a photocatalytic process. It has been well-established that the photodegradation process of organic dye pollutant can be ascribed to a pseudofirst-order reaction that follows the simplified Langmuir− Hinshelwood model, that is, ln(C0/C) = kat, when C0 is very small, where ka is the apparent first-order rate constant.63,73 Significantly, M/TNTs (M = Au, Ag, Pt) hybrid nanostructures exhibit superior photocatalytic performances to that of plain TNTs. From comparison, kinetic rate constants (Figure 7a) of

Figure 6. (a) XPS survey spectra of the M/TNTs (M = Au, Ag, Pt) heterostructures and high-resolution spectra of (b) C 1s, (c) O 1s, (d) Ti 2p, (e) Au 4f, (f) Ag 3d, and (g) Pt 4f. The Gaussian deconvolution of C 1s and O 1s spectra is displayed in color curves.

chemical bond species versus BE for the M/TNTs heterostructures are tabulated in Table S1 in the Supporting Information. Therefore, high-resolution spectra of C 1s and O 1s for the M/TNTs (M = Au, Ag, Pt) heterostructures suggest the effective adsorption of polyelectrolytes (PAA/ PAH) on the TiO2 matrix via LBL self-assembly approach. As displayed in Figure 6d, the 2p3/2 and 2p1/2 core-levels of Ti signal that occurred at 458.81 and 464.42 eV correspond to anatase TiO2 in the literature.66,67 This indicates that LBL selfassembly-induced attachment of metal NPs does not alter the textual property of the TNTs scaffold. The featured Au 4f7/2 and Au 4f5/2 peaks (Figure 6e) with BE of 84.00 and 87.45 eV faithfully agree with metallic Au0.68,69 Similarly, characteristic peaks at 368.23 and 374.26 eV in the Ag 3d spectrum (Figure 6f) should be assigned to the BE of Ag 3d5/2 and Ag 3d3/2 of metallic Ag0,70,71 and the doublet with BE of 71.50 (Pt 4f7/2) and 74.60 eV (Pt 4f5/2) (Figure 6g) can be assigned to metallic Pt0,72 respectively. As a consequence, high-resolution spectra of Au 4f, Ag 3d, and Pt 4f unambiguously reveal that the metal NPs attached to the framework of TiO2 via substantial electrostatic force retain the chemical state of metallic Au, Ag, and Pt during the self-assembly process. Hence, joint data of SEM, TEM, XRD, DRS, and XPS suggest that incorporation of metal colloidal NPs into the nanotubular microstructure of TNTs has been readily achieved via the facile LBL self-assembly route, leading to hierarchically ordered M/TNTs (Au, Ag, Pt) heterostructures. In this regard, it is reasonably speculated that these hybrid nanomaterials will find promising catalytic applications in view of excellent monodispersivity of metal NPs ingredients on the framework of TNTs. 3.3. Photocatalytic Performances of the M/TNTs (M = Au, Ag, Pt) Heterostructures. The photocatalytic performances of the M/TNTs (M = Au, Ag, Pt) heterostructures were evaluated by liquid-phase degradation of aqueous MO solution

Figure 7. (a) Photocatalytic performances of TNTs and M/TNTs (M = Au, Ag, Pt) heterostructures toward degradation of organic dye pollutant (MO) under the irradiation of UV light (365 ± 15 nm), (b) photocatalytic activities of the Ag/TNTs hybrid nanostructures with varied deposition cycles (n = 0, 1, 2, 4, 8, 10).

the samples follow the order: Ag/TNTs > Pt/TNTs > Au/ TNTs > TNTs. The result suggests that direct deposition of metal NPs to the framework of TNTs via LBL self-assembly approach is beneficial for yielding remarkably enhanced photocatalytic performances, which may be ascribed to the well-dispersed Au, Ag, and Pt NPs ingredients acting as “electron trap” through intimate interfacial contact between metal NPs and TNTs afforded by conducting polyelectrolytes. Therefore, the lifetime of charge carriers photogenerated from TNTs upon light irradiation could be prolonged much more effectively as compared with blank TNTs. To probe the influence of loading amount of metal NPs on the photocatalytic activities of the M/TNTs (M = Au, Ag, Pt) heterostructures, typically, photocatalytic activities of the Ag/TNTs hybrid nanostructures with varied deposition cycles (n = 0, 1, 2, 4, 8, 10) were tailored. The formation of multilayers of the Ag/ 16493

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Figure 8. (a) Transient photocurrent responses and (b) electrochemical impedance spectroscopy (EIS) Nynquist plots of TNTs and M/TNTs (M = Au, Ag, Pt) heterostructures in 0.1 M Na2SO4 aqueous solution without bias versus Pt counter electrode under the irradiation of UV light (365 ± 15 nm). The amplitude of the sinusoidal wave was set at 10 mV, and the frequency varied from 100 kHz to 0.05 Hz. (c) Transient photocurrent responses and (d) electrochemical impedance spectroscopy (EIS) Nynquist plots of the Ag/TNTs hybrid nanomaterials with varied deposition cycles (n = 0, 1, 2, 4, 8, 10) under the irradiation of UV light (365 ± 15 nm).

the result of electrochemical impedance spectroscopy (EIS), as shown in Figure 8b. EIS has been established as an effective tool for studying the interface properties of surface-modified electrodes.75 In each case, there was only one arc/semicircle on the EIS plane display, suggesting that only the surface chargetransfer step is involved in the photocatalytic reaction.76 Normally, the smaller arc radius on the EIS plot indicates an effective separation of photogenerated electron−hole pairs and a fast interfacial charge transfer to the electron donor and/or electron acceptors.64 As displayed in Figure 8b, under UV light irradiation, impedance arc radius of TNTs electrode is much larger than that of M/TNTs (M = Au, Ag, Pt) heterostructures electrodes, indicating greater separation efficiency of photogenerated electron−hole pairs and fast charge transfer of hybrid nanocomposites than blank TNTs at the solid−liquid interface. The order of arc radius of electrodes is consistent with the order of the photocatalytic activities of the samples (Figure 7). Accordingly, EIS results further evidence that deposition of metal NPs significantly prolong the separation lifetime of photogenerated electron−hole pairs, leading to enhanced photocatalytic performances of the hybrid nanostructures. Photocurrent responses and EIS results of the Ag/TNTs heterostructures with different deposition cycles (n = 1, 2, 4, 8, 10) were probed to assess the influence of loading amount of metal NPs on the separation efficiency of photogenerated charge carriers for hybrid nanostructures. Figure 8c shows that photocurrent responses of the Ag/TNTs heterostructures are strongly dependent on the deposition cycle of Ag NPs, in which photocurrent of the Ag/TNTs (n = 1) electrode exhibits the maximum intensity and subsequently readily decreases with the

TNTs heterostructures was characterized by monitoring the linear increment concerning the peak intensity of Ag 3d5/2 signal (Figure S4 in the Supporting Information). As demonstrated in Figure 7b, photocatalytic performances of the Ag/TNTs heterostructures vary with the deposition cycle, among which Ag/TNTs hybrid nanostructure (n = 1) exhibits the optimal photocatalytic activity and subsequently decreases with deposition cycle processing. Nonetheless, it is noted that all Ag/TNTs hybrid nanostructures samples with varied deposition cycles (n = 1, 2, 4, 8, 10) conformably exhibit enhanced photocatalytic performance as compared with blank TNTs. In addition, the pronounced separation efficiency of photogenerated electrons and holes for the M/TNTs (M = Au, Ag, Pt) hybrid nanomaterials can also be revealed by their photoelectrochemical behaviors, as illustrated in Figure 8. Figure 8a exhibits the transient photocurrent responses of blank TNTs and M/TNTs (M = Au, Ag, Pt) heterostructures under intermittent UV-light illumination (365 ± 15 nm). It shows that incorporation of metal NPs is favored to enhance the photocurrent significantly and the photocurrent rapidly decreases to zero when the light is switched off. It is well known that the photocurrent is formed mainly by the diffusion of the photogenerated electrons to the back contact and meanwhile the photoinduced holes are taken by the hole acceptor in the electrolyte.74 Consequently, the enhanced photocurrent responses over the M/TNTs (M = Au, Ag, Pt) hybrid systems reveal a more efficient separation of the photoexcited electron− hole pairs and longer lifetime of the photogenerated charge carriers as compared with blank TNTs. The prolonged lifetime of the photogenerated charge carriers can also be reflected by 16494

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spectroscopy, by which the characteristic peak at 400 nm corresponding to 4-NP decreased, whereas a new peak at 298 nm assigned to 4-AP appeared (Figure 9b). Figure 9a

deposition cycle increasing, eventually achieving saturation when the deposition cycle reaches up to eight. This indicates active sites on the TNTs scaffold are wholly occupied by Ag NPs or aggregates of Ag NPs, thereby resulting in relatively low separation efficiency of photoexcited electron−hole pairs. Nevertheless, it is worth noting that Ag/TNTs hybrid nanostructures (n = 1, 2, 4, 8, 10) exhibit remarkably enhanced photocurrent and smaller arc radius on the EIS panel compared with blank TNTs (Figure 8d). Furthermore, PL intensity of the Ag/TNTs hybrid nanostructure is lower than that for blank TNTs (Figure S5 in the Supporting Information). Therefore, all experimental facts corroborate the enhanced separation efficiency of photogenerated electrons and holes for the Ag/ TNTs heterostructure. A possible mechanism for photodegradation of organic dye pollutant over the M/TNTs (M = Au, Ag, Pt) heterostructures is tentatively proposed, as vividly delineated in Scheme 1. When Scheme 1. Pictorial Representation of the Proposed Mechanism for Photocatalytic Degradation of Dye Pollutant over the M/TNTs (M = Au, Ag, Pt) Heterostructures

Figure 9. (a) Catalytic reduction of 4-nitrophenol (4-NP) to 4aminophenol (4-AP) over TNTs and M/TNTs (M = Au, Ag, Pt) heterostructures with an excess amount of NaBH4 in aqueous media at ambient temperature by monitoring the decrease in the absorption intensity of 4-NP for the peak at 400 nm. (b) UV−vis absorption spectra of a mixed 4-NP (2 mM) and NaBH4 (100 mM) aqueous solution taken at regular time interval over the Ag/TNTs hybrid nanostructure. The inset in panel b shows the enlarged image for the peak at 298 nm corresponding to the increased intensity of 4-AP as the reduction reaction proceeded.

the framework of TNTs is irradiated by light with photon energy surpassing its band gap energy, electrons can be photoexcited from the VB to the CB, leaving holes in the VB. On account of the monodisperse deposition of metal NPs to the TNTs, photoinduced electrons intrinsically transfer to metal NPs due to the lower energy level of the top of the CB relative to that of newly formed Fermi energy level of the M/ TNTs (M = Au, Ag, Pt) hybrid systems.77 Moreover, the first layer of coated polyelectrolyte is anionically charged PAA, and thus the photogenerated holes upon UV light illumination might be trapped by the negatively charged sites at the surface, reducing the electron−hole recombination rates in the TiO2 matrix and increasing the carrier lifetime.78 Furthermore, intimate interfacial contact between well-dispersed metal NPs and TNTs matrix attained from LBL self-assembly buildup also contributes to making metal NPs ideal photoelectron sinks for recombination of photoexcited electron−hole charge carriers. 3.4. Selective Catalytic Reduction Activities of the M/ TNTs (M = Au, Ag, Pt) Heterostructures. Selective catalytic reduction properties of the M/TNTs (M = Au, Ag, Pt) heterostructures were evaluated via chemical reduction of 4-NP to 4-aminophenol (4-AP) by NaBH4 as a model reaction, which was demonstrated to be useful for the analysis of the catalytic activities of noble-metal nanocrystal-deposited nanocomposites.79−81 NaBH4 was added to the reaction in excess as compared with 4-NP so that the reduction rate could be assumed to be independent of the concentration of NaBH4.82 The progress of the reaction could be monitored by UV−vis

demonstrates that M/TNTs (M = Au, Ag, Pt) hybrid nanostructures exhibit efficient catalytic reduction activities of 4-NP toward 4-AP, whereas no reduction phenomenon was observed for blank TNTs, indicating that the reduction of 4-NP was authentically driven by the surface-active atoms of metal nanocrystals deposited on the framework of TNTs. In addition, catalytic reduction performance of the Ag/TNTs nanocomposite is comparable to that of Au/TNTs nanocomposite, whereas both of which are significantly superior to Pt/TNTs under the same ambient conditions. The detailed catalytic reduction process of the M/TNTs (M = Au, Ag, Pt) heterostructures is schematically depicted in Scheme 2. Catalytic reduction capabilities of the Ag/TNTs hybrid nanostructures with varied deposition cycles (n = 1, 2, 4, 8, 10) were further investigated to clarify the relationship between deposition cycle and reduction performances of the hybrid nanomaterials. Figure 10 reveals the catalytic reduction performances of the Ag/TNTs heterostructures with varied deposition cycles, which are optimized when the deposition cycle is confined to four and further decrease with more deposition cycles processing. The result suggests that catalytic reduction capabilities of the M/TNTs (M = Au, Ag, Pt) hybrid systems can be exquisitely tuned by deposition cycle. The inset in Figure 10 discloses the detailed process concerning the catalytic reduction of 4-NP over the Ag/ 16495

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Scheme 2. Schematic Diagram of the Catalytic Reduction of 4-Nitrophenol (4-NP) toward 4-Aminophenol (4-AP) over the Hierarchically Ordered M/TNTs (M = Au, Ag, Pt) Heterostructures

Figure 11. (a) Recycled photodegradation of MO aqueous solution under the irradiation of UV light (365 ± 15 nm) and (b) recycled catalytic reaction of 4-NP toward 4-AP in aqueous media over the Ag/ TNTs hybrid nanostructure at ambient temperature. (c) Highresolution XPS spectra of Ti 2p before and after recycled photocatalytic reactions over the Ag/TNTs hybrid nanostructure and (d) high-resolution XPS spectra of Ag 3d before and after recycled catalytic reduction of 4-NP over the Ag/TNTs hybrid nanostructure.

corresponding to anatase TiO2 (Ti4+) before and after successive photocatalytic reactions exhibit no BE shift. Additionally, XRD patterns (Figure S6a in the Supporting Information) and high-resolution O 1s spectra (Figure S6b in the Supporting Information) of the fresh and used samples revealed intact crystalline of the TNTs scaffold after recycled photocatalytic reactions, which further verifies the photostability of the Ag/TNTs nanocomposite. In addition, catalytic stability of the Ag/TNTs hybrid nanostructure in recycled chemical reduction of 4-NP was also investigated. As shown in Figure 11b, Ag/TNTs nanocomposite retains extraordinary catalytic stability, and featured high-resolution XPS peaks of Ag 3d5/2 and Ag 3d3/2 (Figure 11d) before and after the cycling reduction reactions exhibit no BE shift and correspond to the same elemental chemical state of metallic Ag0. Hence, recycled experiments disclose that the Ag/TNTs heterostructures exhibit prominent photostability and catalytic reduction stability, making them versatile a candidate for a wide range of catalytic applications.

Figure 10. Catalytic reduction of 4-NP over the Ag/TNTs hybrid nanostructures with different deposition cycles (n = 0, 1, 2, 4, 8, 10) by monitoring the peak adsorption intensity at wavelength of 400 nm. The inset displays the detailed catalytic reduction process over the Ag/ TNTs hybrid nanostructures at typical reaction time of 10, 20, and 30 min.

TNTs heterostructures with different deposition cycles (n = 1, 2, 4, 8, 10) at reaction time of 10, 20, and 30 min, which reflects analogous trend and establishes the optimal deposition cycle of four. This indicates that there is still a wide scope to tailor the catalytic reduction activities of the M/TNTs (M = Au, Ag, Pt) heterostructures by tuning the deposition cycle of metal NPs for achieving the optimal catalytic potency. 3.5. Photostability and Catalytic Stability of the M/ TNTs (M = Au, Ag, Pt) Heterostructures. Figure 11a displays the cycling experiments of the Ag/TNTs nanocomposite for photodegradation of MO aqueous solution under the irradiation of UV light (365 ± 15 nm). The result suggests that the photocatalyst did not exhibit pronounced deactivation even going through four successive recycles, indicating the Ag/TNTs hybrid nanostructure is sufficiently stable during the photocatalytic process. It should be emphasized that the excellent photostability of the M/TNTs (M = Au, Ag, Pt) heterostructures has significant implications for practical applications, especially in a slurry system, because no separation of photocatalyst is needed by virtue of the direct up-growth property of TNTs on the Ti foil, thus establishing inimitable advantages over its counterpart of particulate or power photocatalyst. As mirrored in Figure 11c, the characteristic high-resolution XPS peaks of Ti 2p3/2 and Ti 2p1/2

4. CONCLUSIONS In summary, well-defined M/TNTs (M = Au, Ag, Pt) heterostructures have been fabricated based on a facile LBL self-assembly approach, by which tailor-made metal colloidal NPs used as starting building blocks were uniformly deposited to the framework of TNTs. The combined studies have revealed that the hierarchical M/TNTs (M = Au, Ag, Pt) nanomaterials can serve as versatile catalysts for photodegradation of organic dye pollutants and catalytic reduction of 4-NP toward 4-AP under mild conditions by virtue of processing simplicity and architectural flexibility. The promising catalytic performances of these hierarchically self-assembled systems stem out of two cumulative contributions, that is, welldispersed metal NPs nutrients on the scaffold of TNTs acting as “electron reservoir” or highly active catalytic centers and substantial interfacial adhesion between metal NPs and TNTs substrate afforded by LBL self-assembly buildup. In addition, versatile catalytic performances of the hybrid nanostructures can be tuned by deposition cycles, along with excellent catalytic 16496

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stability. It is hoped that our work could furnish new examples concerning the explorations on the metal/1-D semiconductor hybrid nanocomposites based on the facile LBL self-assembly strategy.

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ASSOCIATED CONTENT

S Supporting Information *

Characterization of citrate-stabilized metal NPs, SEM images of the TiO2 nanotube arrays fabricated via conventional one-step anodization, and EDX and PL results of the hybrid nanomaterials. Detailed chemical bond species versus BE for the samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Program for Changjiang Scholarship and Innovative Research Team in University (PCSIRT0818), National Basic Research Program of China (973 Program: 2007CB613306) is greatly acknowledged.

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dx.doi.org/10.1021/jp3034984 | J. Phys. Chem. C 2012, 116, 16487−16498