TiO2 Nanotube Heterojunction with Improved Visible

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Synthesis of Ag/TiO2 Nanotube Heterojunction with Improved Visible-light Photocatalytic Performance Inspired by Bioadhesion Dong Yang, Yuanyuan Sun, Zhenwei Tong, Yao Tian, Yuanbing Li, and Zhongyi Jiang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp511948p • Publication Date (Web): 27 Feb 2015 Downloaded from http://pubs.acs.org on March 3, 2015

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The Journal of Physical Chemistry

Synthesis of Ag/TiO2 Nanotube Heterojunction with Improved Visible-light Photocatalytic Performance Inspired by Bioadhesion

Dong Yang, a b Yuanyuan Sun, a Zhenwei Tong, c Yao Tian, a Yuanbing Li, a Zhongyi Jiang*, c, d a

Key Laboratory of Systems Bioengineering of Ministry of Education, School of Chemical

Engineering and Technology, Tianjin University, Tianjin 30072, China; b

c

School of Environmental Science and Engineering, Tianjin University, Tianjin 30072, China; Key Laboratory for Green Technology, School of Chemical Engineering and Technology, Tianjin

University, Tianjin 300072, China; d

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072,

China) *Corresponding author. Tel.: +86 22 2350 0086; Fax: +86 22 2350 0086; E-mail: [email protected]

ACS Paragon Plus Environment


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ABSTRACT：：Inspired by the bioadhesion mechanism found in mussel, a catechol derivative, 3-(3, 4-dihydroxyphenyl) propionic acid (diHPP), is employed as both linker and reducer of Ag+ to synthesize the Ag/TiO2 nanotube (Ag/TNT) heterojunction under ambient conditions in this study. In the prepared Ag/TNT composite, Ag nanocrystals about 3.8 nm in diameter distribute over the TNT surface uniformly and form the heterojunction structure with TNT. The diHPP firstly links to the TNT surface through the bi-dentate chelation of catechol group with Ti4+, and then acts as both anchor and reducer to in situ nucleate and grow Ag nanocrystals on the TNT surface. By adjusting the AgNO3 concentration, the loading amount of Ag nanocrystals on the TNT surface can be controlled easily, and the visible-light absorption ability of Ag/TNT heterojunctions enhances with increasing the Ag loading amount. Moreover, their photocatalytic activity was evaluated by the degradation capability of Rhodamine B (RhB) under visible light. The Ag/TNT heterojunctions exhibit the high visible-light photocatalytic activity, which can almost degrade 100% RhB within 2 hours. This excellent performance can be attributed to the local electric field caused by the surface plasmon resonance (SPR) of Ag nanocrystals and the high adsorption capability of TNTs with large specific surface area.

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1. INTRODUCTION Solar energy, as a clean and inexhaustible resource, has intrigued many researchers to explore its utilization for several decades. The semiconductor photocatalysis, in which the solar energy is used to drive thermodynamic uphill reaction to generate highly energetic chemical fuels or remove inorganic and organic pollutants, is one of the most important approaches to harness solar energy.

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TiO2 nanotube (TNT), a typical one-dimensional

nanostructure, possesses unique geometrical characteristics and advantages in the photocatalytic application due to its large surface-to-volume ratio, direct pathways for charge transport, good mechanical stability and rich activity sites for pollutant degradation. 8, 9

However, it can only utilize no more than 5% of total solar energy irradiating on the earth

surface due to its wide bandgap, and exhibits high recombination rate of photogenerated electron/hole pairs, which restrict the wide application of TNT in the semiconductor photocatalysis. Recently, the noble metal/semiconductor heterojunctions, such as Au/TiO2, M/TNT (M=Au, Ag, Pt) and Ag/WO3, offer a new opportunity to overcome above two problems simultaneously. 10-14 The noble metal plays a dual role in the photocatalytic process: on one hand, the surface plasmon resonance (SPR) photosensitization 15 caused by noble metal can be utilized to harvest the visible light (accounting for 43% of solar spectrum); on the other hand, the formation of Schottky barrier between semiconductor and noble metal is obviously beneficial to the separation of electron-hole pairs. Among all the noble metals, Ag is promising for extensive application because of its lower cost, inherent antibacterial activity and facile preparation.

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Till now, several strategies including chemical reduction,

UV irradiation, sol–gel and hydrothermal method have been developed to synthesize the Ag/TNT heterojunction. However, these methods usually need either rigorous synthetic condition or poisonous reducer, and Ag nanoparticles in the prepared Ag/TNT 2



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heterojunction hardly have high crystallinity and uniform size.

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Thus, it is desirable to

develop a facile and generic method to synthesize the Ag/TNT heterojunction with well-defined Ag nanocrystals. Compared with traditional physical and chemical methods, the bio-inspired approach which can synthesize inorganic nanomaterials under mild conditions has distinct green feature. To date, biomolecules including proteins/peptides and polysaccharides and bio-inspired polymers like polyamines have been employed as the catalyst and/or template to induce the formation of inorganic nanomaterials,

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however, these

molecules are usually too expensive for the large-scale preparation of inorganic nanomaterials. More recently, mussels have aroused dramatic interesting of researchers in the fields including materials science, biomineralization and protein immobilization etc., because of their rapid, strong and tough adhesive ability to solid surfaces, so-called bioadhesion. 23, 24 3, 4-Dihydroxy-L-phenylalanine (DOPA) in mussels’ proteins near the plaque-substrate interface was found to be the major contributor of bioadhesive versatility, which can form metal bidentate coordination, hydrogen bonding, or π-π stacking with solid surfaces, via its catechol moiety. Inspired by the bioadhesion mechanism, catechols and their polymers have been widely applied for the functionalization of materials surface under ambient conditions.

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Moreover, catechols

have a low reduction potential, which could be used to reduce noble metal ions, such as Ag+ and AuCl4-. 26 Therefore, catechols, as a kind of low-cost small-molecule inducers, may be exploited for the bio-inspired synthesis of inorganic materials doped with noble metal nanoparticles.

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Here, a kind of small organic molecule, 3-(3, 4-dihydroxyphenyl) propionic acid (diHPP), has been employed to synthesize the Ag/TNT heterojunction. The catechol group in diHPP may chelate with Ti4+ on the TNT surface, and then reduce Ag+ to Ag nanocrystal during the synthetic process. Furthermore, the visible-light photocatalytic activity of as-prepared Ag/TNT heterojunction was evaluated by the degradation capability of Rhodamine B (RhB). This study may afford a general green platform to synthesize the noble metal/TNT heterojunction with well-defined nanocrystaline structure.

2. EXPERIMENTAL 2.1 Materials. 3-(3, 4-Dihydroxyphenyl) propionic acid (diHPP) was supplied by Alfa Aesar China Co., Ltd. (Tianjin, China). Sodium hydroxide (NaOH) and silver nitrate (AgNO3) were purchased from Guangfu Company (Tianjin, China). Rutile TiO2 powder (99.8%, about 60 nm in diameter) was obtained from Aladdin Chemistry Co., Ltd. (Shanghai, China). All chemicals were of analytical grade, and used without further purification. Deionized water was used in all experiments. 2.2 Synthesis of TNTs. TNTs were fabricated by a hydrothermal method as reported in literature. 27 In a typical synthesis, 2 g rutile TiO2 powders were added to 85 mL of 10 mol L-1 NaOH aqueous solution, and dispersed by ultrasonication for 15 min. The suspension was then heated, and maintained at 130 oC for 72 h with a heating rate of 5 o

C min-1 in a closed Teflon-lined autoclave. After the hydrothermal treatment, the

precipitates were collected by centrifugation, and washed with deionized water until pH7 was reached. Subsequently, they were immersed in 100 mL of 0.1 mol L-1 HCl solution overnight, and then treated repeatedly with distilled water until pH value was 4



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about 7.0. After dried in air at 60 oC for 24 h, titanate nanotubes were obtained finally. In order to obtain anatase TNTs, titanate nanotubes were annealed at 400 oC for 1 h with a heating rate of 5 oC min-1. 2.3 Synthesis of Ag/TNT heterojunctions. DiHPP was utilized both as a linker and a reducer for the synthesis of Ag/TNT heterojunctions. In a typical procedure, 10 mL of 1 mg mL-1 diHPP solution was first added to 10 mL of aqueous suspension containing 2 mg mL-1 TNT under vigorous stirring. After stirring for 30 min, the precipitate was collected by centrifugation, and washed with deionized water for three times. Thereafter, TNT modified by diHPP was re-dispersed by ultrasonication in 20 mL AgNO3 solution in a beaker wrapped with tinfoil, and the suspension was kept at room temperature under gentle stirring for 24 h in darkness. The product was collected by centrifugation, and washed with deionized water for three times. At last, the Ag/TNT heterojunction was obtained after dried in air at 60 oC for 24 h. Six samples were prepared in the AgNO3 solution with different concentrations (0.5, 1.0, 2.0, 3.0, 5.0 and 10.0 mmol L-1), which are denoted as Ag/TNT-X (X = 0.5, 1, 2, 3 5 and 10), respectively. When the AgNO3 concentration exceeded 2.0 mmol L-1, Ag nanocrystals in the Ag/TNT heterojunction would agglomerate seriously. For comparison, two Ag/TNT nanocomposites were also prepared by a photochemical reduction (500 W mercury lamp for 1 h) without diHPP under the similar conditions, in which the AgNO3 concentration was 1.0 and 3.0 mmol L-1, respectively. 2.4 Characterization. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) images were taken on a Tecnai G2 F-20 instrument at an accelerating voltage of 200 kV. The X-ray powder diffraction (XRD) patterns were collected by a Philips X’Pert pro MPD diffractometer (Cu Kα, λ = 0.154 nm, 40 kV, 200 mA). Nitrogen adsorption/desorption isotherm 5


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measurements were conducted on a Tri-Star 3000 gas adsorption analyzer at 77 K. The diffuse-reflectance spectroscopy (DRS) spectra of the samples were obtained by utilizing a Hitachi U-3010 UV-vis spectrophotometer equipped with an integration sphere and using BaSO4 as the reference. The X-ray photoelectron spectroscopy (XPS) measurement was carried out on a Perkin-Elmer PHI 1600 ESCA X-ray photoelectron spectroscope using a monochromatic Mg Kα radiation with photon energy of 1253.6 eV. Fourier transform infrared (FTIR) spectra of the samples were recorded by using a Nicolet-560 Fourier transform infrared spectroscope. Thermogravimetric analysis (TGA) was carried out on a Q600 SDT Thermo-gravimetric analyzer using air as the carrier gas at a heating rate of 10 oC min-1. 2.5 Measurement of photocatalytic activity. The photocatalytic activity of Ag/TNT heterojunctions was evaluated by the degradation of RhB solution under visible light irradiation at ambient temperature and pressure. In a typical experiment, 30 mg photocatalysts were dispersed in 30 mL of 10 mg L-1 RhB solution, and the suspension was then stirred for 1 h in the dark to reach the adsorption equilibrium. Thereafter, the mixture was irradiated for 4 h under a 500 W Xenon arc lamp with a 420 nm cutoff filter, and at a defined time interval, the RhB concentration was analyzed at 553 nm using a Hitachi U3100 UV-Vis spectrophotometer. The photocatalytic activity of TNT and P25 were also determined under the same conditions as the control. The trapping experiment was designed to investigate the specific reactive species involved in the photocatalytic degradation of RhB by the Ag/TNT heterojunction. In comparison with the measurement of photocatalytic activity, the only difference is that the RhB solution was replaced by the RhB solution mixed with trapping agents. The used trapping agents include disodium ethylenediaminetetraacetate (EDTA, 10 6



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mmol L-1), benzoquinone (1 mmol L-1) and methanol (1: 15 / V: V), which can capture the photogenerated hole (h+), superoxide anion radical (·O2-) and hydroxyl radical (·OH), respectively.

3. RESULTS AND DISCUSSION 3.1 Morphology and structure of Ag/TNT heterojunctions. The TNTs were fabricated by an alkaline hydrothermal method followed by an annealing process. Under high pressure, titanate nanosheets were first formed from rutile TiO2 nanoparticles with gradual dissolution by the strong base, and then easily dehydrated to produce titanate nanotubes since they were unstable under high temperature. 27 The driving force for transforming from nanosheet to nanotube was suggested to be the asymmetrical chemical environment between two sides of titanate nanosheets. 28 After titanate nanotubes were annealed at 400 oC for 1 h, the TNTs were prepared. As shown in Fig. 1a, the TNTs are typical nanotube structure with the outer and inner diameter of 8-10 nm and 4-5 nm, respectively. Their wall consists of two or three layers with an interlayer space of 0.8 nm, in consistence with the result reported by literature.

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Their high anatase phase purity is confirmed by the corresponding XRD

pattern (Fig. 2) and their specific surface area is as high as 248.8 m2 g-1 based on the nitrogen adsorption/desorption measurement. Scheme 1 illustrates the synthetic procedure of Ag/TNT heterojunctions assisted by diHPP. Firstly, the TNT is suspended in a diHPP solution, and then its color changes from white to orange gradually, directly indicating the successful adsorption of diHPP on the TNT surface. It is deduced that diHPP links to the TNT surface through the bi-dentate chelation of catechol group with Ti4+.

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Subsequently, the diHPP-functionalized TNT is

dispersed in the AgNO3 solution, in which the diHPP acts as both anchor and reducer 7


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to in situ nucleate and grow Ag nanocrystals on the TNT surface. After stirring for 24 h, the suspension color changes from orange to gray, suggesting the generation of Ag nanocrystals. Figure 1b shows the typical TEM image of Ag/TNT-1.0 heterojunctions. Clearly, Ag nanocrystals are uniformly distributed over the TNT surface, the diameter of which is about 3.8 + 0.7 nm as observed in Fig. 1e. A closer observation by HRTEM (Fig. 1c) demonstrates that the Ag nanocrystal becomes a nearly hemispherical with clear lattice fringes about 0.24 nm, which is attributed to the (111) plane of the face centered cubic (fcc) Ag crystal. It is noted that the lattice fusion occurs apparently between Ag and TNT, indicating the formation of Ag/TNT hetrojunction. The EDX spectrum of Ag/TNT heterojunctions (Fig. 1f) exhibits two Ag peaks at 3.00 and 3.19 keV besides the peaks belonging to Ti, O and C elements, further confirming the Ag formation. Ag/TNT composites (CAg+=1.0 mmol L-1) without using diHPP as the reducer were also synthesized by the photochemical approach (Fig. 1d), in which the Ag nanocrystals are about 50-60 nm in diameter, and mix with TNTs disorderly. It is inferred that the diHPP can serve not only as a reducing agent for Ag+, but also a capping agent to stabilize the growth of Ag nanocrystals during the synthetic process of Ag/TNT heterojunctions. Thus, the Ag/TNT heterojunctions with well-defined Ag nanocrystals can be in situ fabricated facilely by using diHPP. XRD were conducted to investigate the crystallographic structure of Ag/TNT heterojunctions. As illustrated in Fig. 2, the TNT belongs to the pure anatase,

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and

its crystal structure doesn't almost change after loading diHPP and Ag nanocrystals. It is worth noting that compared with pure TNT, the diffraction peak at about 37.8° (peak B) of Ag/TNT heterojunctions becomes higher, which can be attributed to the overlapping of TiO2 (004) and Ag (110) peak. In order to further analyze 8



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quantificationally the intensity of this peak, the area ratio of peak A (TiO2 (101)) to peak B is calculated. The area ratio of peak A to peak B decreases from 8.63 (Ag/TNT-0.5) to 7.58 (Ag/TNT-2) with increasing the AgNO3 concentration, implying the loading of Ag nanocrystals on the TNT surface enhances. The inductively coupled plasma emission spectrometry (ICP) data demonstrate that the Ag contents of Ag/TNT-0.5, Ag/TNT-1.0 and Ag/TNT-2.0 are 0.16 wt. %, 0.53 wt. % and 0.86 wt. %, respectively. The above results indicate that the content of Ag nanocrystals loaded on the TNT surface can be regulated effectively through adjusting the initial concentration of AgNO3 solution. The FT-IR spectra of TNT, diHPP/TNT and Ag/TNT samples are shown in Fig. 3a. As we can see, a band around 1631 cm-1 presents in the spectrum of TNT, which is assigned to the bending vibration of hydroxyl group, caused by adsorbed water on the TNT surface.

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This peak shifts from 1631 cm-1 to 1619 cm-1 after modified by

diHPP, suggesting the strong interaction between diHPP and TNT. Moreover, a new weak peak at 1164 cm−1 appears in the FT-IR spectrum of diHPP/TNT, which is indicative of the stretching vibration of benzene ring. Based on the TGA curve of diHPP/TNT (Fig. 3b), the loading amount of diHPP on the TNT surface is about 5 wt%. These results confirm that the diHPP molecules can be adsorbed on the TNT surface via simply blending, which may form the bi-dentate chelation with Ti4+.

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After the formation of Ag nanocrystal on the TNT surface, the FT-IR spectrum changes little, which can be attributed to the excessive diHPP. The XPS was carried out to further determine the element composition of as-prepared Ag/TNT heterojunctions and analyze the chemical status of various elements in them. The C, O, Ti and Ag element can be found in the survey XPS spectrum of Ag/TNT (Fig. 4a) and the high resolution XPS spectra of C1s, O1s, Ag 9


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3d and Ti2p were showed in Fig. 4b-4e. The high-resolution C1s XPS curve can be fitted into three peaks at 284.6, 286.6 and 288.2 eV by using Gaussian-Lorentzian peak fitting (Fig. 4b), which are assigned to the occasional carbon, C-O and C=O bond, respectively. Similarly, the high-resolution XPS spectrum of O1s can be fitted into three peaks at 529.8, 531.4 and 532.7 eV (Fig. 4c), which are ascribed to the Ti-O, C-O and C=O bond, respectively.

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These results further confirm that there are still

some diHPP molecules on the surface of Ag/TNT heterojunctions. As exhibited in Fig. 4d, the high-resolution Ag3d XPS spectrum has two peaks at 367.4 eV and 373.4 eV, belonging to the Ag3d5/2 and Ag3d3/2 orbits, respectively. The splitting of Ag3d doublet about 6 eV confirms that the Ag element is present as Ag0 in the Ag/TNT sample. 16 However, compared to those of bulk Ag0 materials (368.3 eV for Ag 3d5/2 and 374.3 eV for Ag 3d3/2), these two peaks exhibit a negative shift. The high-resolution Ti2p XPS spectrum (Fig. 4e) demonstrates two peaks at 465.1 eV and 459.4 eV, which can be attributed to the Ti2p1/2 and Ti2p3/2 orbits, respectively. Similarly, the two peaks shift about 1.0 eV to higher binding energy compared with those of pure anatase TiO2 (464.1 eV for Ti2p1/2 and 458.4 eV for Ti2p3/2).

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It is

deduced that some electrons may transfer from TiO2 to metallic Ag owing to the strong interaction between Ag and TNT, resulting in the binding energy shift of Ag3d and Ti2p. 3.2 Optical property and photocatalytic activity. In order to study the optical response of Ag/TNT heterojunctions, their DRS were measured and illustrated in Fig. 5. It is observed that the Ag/TNT heterojunctions appear a new absorption band at 400-600 nm compared with pure TNT, in consequence of the surface plasmon resonance (SPR) of Ag nanocrystals. This indicates that this kind of heterojunction materials should have the photocatalytic activity under visible light. In addition, the 10



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absorbance intensity in the wavelength range from 400 to 600 nm enhances with the increase Ag content, in accordance with the previous research. 21 The photocatalytic activity of Ag/TNT heterojunctions was evaluated by measuring the decomposition of RhB under visible light, and shown in Fig. 6a. Before light irradiation, the mixed solution of RhB and photocatalyst was stirred for 1 h to reach the adsorption equilibrium. Compared with that of P25 (5.0%), the adsorption capacity of TNT and Ag/TNT heterojunctions can reach up to 18.0% and 20.5%, respectively. This phenomenon can be attributed to the fact that they (STNT=287.8 m2 g-1, SAg/TNT-2.0=266.0 m2 g-1) possess much higher BET specific surface area than P25 (SP25=65.80 m2 g-1). It is noted that though STNT is higher than SAg/TNT-2.0, the adsorption capacity of TNT is lower than that of Ag/TNT heterojunctions. As-prepared Ag nanoparticles on the TNT surface have a small size (3.8 + 0.7 nm in diameter), however, the large density leads to their lower specific surface area than that of TNT with the same weight. Therefore, STNT is a little higher than SAg/TNT. During the adsorption procedure, the inner surface of TNT is not able to effectively adsorb the dye molecules due to the nanotubular structure; while most of Ag nanoparticles expose on the TNT surface, which can enhance the outer surface area of TNT, thus resulting in the higher adsorption capacity of Ag/TNT. The RhB doesn't almost degrade under visible light irradiation for 4 h; while 99% RhB is degraded photocatalytically by Ag/TNT-2.0 heterojunctions under the same conditions, which is much higher than that by P25 (35%) and TNT (38%). Since the reactive conditions are different, the photocatalytic activities of Ag/TiO2 composites reported in literatures are only compared roughly. As listed in Table 1, the Ag/TNT heterojunction prepared in this work has the better or comparative visible-light photocatalytic activity, compared with other Ag/TiO2 composites. 10, 17, 19, 36-40 11


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The photocatalytic activity of P25 and TNT under visible light is assigned to a dye-photosensitized process,

41

in which the photo-induced electrons coming from

RhB molecules can inject into the conduction band of TiO2, and then are captured by the surface-adsorbed O2 to generate active species for the RhB degradation. The photocatalytic activity of Ag/TNT slightly reduces after removing the diHPP remained on the Ag/TNT surface by a NaOH solution with pH=14 (Fig. S1). It is inferred that diHPP can also act the role similar to the dye to sensitize the photocatalyst owing to absorbing the visible light. In addition, the photocatalytic activity of Ag/TNT-3 heterojunction is apparently higher than that of corresponding Ag/TNT nanocomposite (CAg+=3.0 mmol L-1), indicating that the well-defined structure of Ag/TNT heterojunction play an important role during the photocatalytic process (Fig. S1). The corresponding kinetic study demonstrates that the photocatalytic degradation of RhB belongs to the quasi-first order kinetics. The reaction rate constant (k) of Ag/TNT-2.0 is 1.108 h-1, which is about seven and nine times higher than that of TNT (0.117 h-1) and P25 (0.153 h-1), respectively. The effect of Ag content on the photocatalytic activity of Ag/TNT heterojunctions was also investigated. As shown in Fig. 6b, Ag/TNT-5 has the highest photocatalytic activity in all of as-prepared six Ag/TNT samples. Since the Ag+ concentration is lower than 5 mmol L-1, the photocatalytic activity of Ag/TNT increases with the increase of Ag content. When the Ag+ concentration exceeds 5 mmol L-1, the photocatalytic activity of Ag/TNT begins to decline. This phenomenon can be attributed to the balance between the surface plasmon resonance and the recombination of photogenerated holes and electrons. When the Ag content increases, the absorbance intensity of Ag/TNT in the visible region enhances due to the surface plasmon resonance, thus resulting in the increase of photocatalytic activity. However, 12



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excess Ag nanoparticles tend to form the recombination center of photogenerated holes and electrons, thereby lowering the photocatalytic activity. In order to explore the photocatalytic mechanism of Ag/TNT heterojunctions, a radical trapping experiment was performed to investigate the reactive radical species involved in the photocatalytic process. It is well known that three radical groups including h+, •OH and •O2−, can be produced during the photocatalytic process, which play important roles in the degradation of organic pollutants.

3, 42

Thus, three

different chemicals including benzoquinone, EDTA and methanol were employed as the scavenger of •O2−, hole and •OH, respectively,

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As illustrated in Fig. 7, EDTA

and benzoquinone can apparently inhibit the RhB degradation; while the methanol influences the photocatalytic process slightly. This result suggests that the •O2− and hole play an important role during the photocatalytic degradation of RhB; while the •OH is produced only a few during the photocatalytic process. It was further proved by the UV-vis spectrum change of RhB during the photocatalytic degradation process (Fig. 8). The maximum absorbance of RhB exhibits obvious blue shift from 553 to 496 nm with the prolongation of reaction time gradually. This change suggests that the major degradation process of RhB is N-demethylation rather than hydroxylation, further confirming that the •OH is not the main reactive radical species. 41 Moreover, the COD value of RhB solution decreases with the prolongation of reaction time, exhibiting the same changing trend to the RhB concentration. After reaction for 4 h, about 89% COD is degraded, indicating that RhB should be degraded completely into CO2, but not a decoloration process (Fig. S2). Based on above results, a tentative mechanism is suggested and illustrated in Scheme 2. Under visible light illumination, Ag nanocrystals generate intense electric fields on their surface at the plasmon frequency,

44

which induces the electron

13


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transition of TNT from valence band (VB) to conduction band (CB). Then, the electrons react with O2 adsorbed on the TNT surface to produce •O2-, which can oxidize RhB further. Meanwhile, the hole directly oxidizes the RhB molecules adsorbed on the TNT surface. In addition, the presence of the heterojunction between Ag and TNT can also facilitate the electron transfer, thus inhibiting the recombination of photogenerated carriers. During the photocatalytic process, Ag nanocrystals may lose electron and turn to Ag+, and then release to the reactive solution due to the SPR effect. Indeed, the ICP data demonstrate that about 3.3 wt% Ag in the Ag/TNT heterojunction is dissolved in the solution after photocatalytic reaction for 4 h. However, the degradation rate of RhB over Ag/TNT-5 only decreases from 99% to 96% after five recycles, suggesting that the Ag/TNT heterojunction is chemically stable and mechanically robust during the photocatalytic process. Therefore, this conversion of Ag to Ag+ does not have much impact on the stability of Ag/TNT heterojunctions.

4. CONCLUSION In summary, a facile bio-inspired method assisted by diHPP is applied for in situ preparing the Ag/TNT heterojunctions under benign conditions. In the as-prepared Ag/TNT nanocomposites, hemispherical Ag nanocrystals about 3.8 nm in diameter are dispersed uniformly on the TNT surface and form the heterojunction structure with TNT. During the bio-inspired synthetic process, the diHPP molecules serve as the dual functions, i.e. the reducing agent of Ag+ and the capping agent to stabilize Ag nanocrystals and hinder their agglomeration. The Ag/TNT heterojunctions show much higher photocatalytic activity than pure TNT and P25 under visible light, which is attributed to the SPR effect and the inhibiting effect for photogenerated carriers of Ag 14



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nanocrystals. Hopefully, such kind of heterojunction materials may become the promising visible-light photocatalyst for the environmental remediation in the near future.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by National Science Fund for Distinguished Young Scholars (21125627), the National Basic Research Program of China (2009CB724705) and the Program of Introducing Talents of Discipline to Universities (No.B06006)

Supporting Information Additional experimental data of photocatalytic activity of Ag/TNT heterojunctions after removing the remained diHPP and Ag/TNT nanocomposites (Figure S1), COD experiment of Ag/TNT heterojunctions (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.”

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Figure Captions Fig. 1. TEM images of TNT (a), Ag/TNT heterojunction (b) and Ag/TNT nanocomposites prepared without diHPP (d); HRTEM image (c), Ag particle size histogram (e) and EDX spectrum (f) of Ag/TNT heterojunctions. Fig. 2. XRD patterns of TNT, diHPP/TNT and Ag/TNT heterojunction with different Ag loadings. Fig. 3. FTIR spectra of TNT, diHPP/TNT and Ag/TNT heterojunction (a); TGA curve of diHPP/TNT (b). Fig. 4. XPS spectra of Ag/TNT-2.0: survey scan spectrum (a) and high-resolution spectra of C1s (b), O1s (c), Ag3d (d) and Ti2p (e). Fig. 5. DRS spectra of TNT and Ag/TNT heterojunction with different Ag loadings. Fig. 6. Visible-light photocatalytic degradation curves of RhB over P25, TNT, and Ag/TNT-2 (a); the effect of Ag content on the photocatalytic activity of Ag/TNT (b). Fig. 7. Visible-light photocatalytic degradation curves of RhB over Ag/TNT-2 without scavenger and in the presence of EDTA, benzoquinone and methanol, respectively. Fig. 8. UV-visible spectrum change of RhB during the photocatalytic process over Ag/TNT-2. Scheme 1. Schematic illustration of the synthetic procedure of Ag/TNT heterojunctions. Scheme 2. Schematic illustration of the photocatalytic degradation mechanism of RhB over the Ag/TNT heterojunction under visible light.

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Table1. Comparison with other Ag/TiO2 semiconductor photocatalytic activity Light source

150W xenon lamp

Catalysts

Ag/TiO2 nanofibers

Dye ( initial

Irradiation

Decomposition

concentration)

time (h)

Rate (%)

Rhodamine B

6

90

10

5

75

17

2

92

37

6

80

38

6

90

36

6

80

19

1.5

100

40

2

90

39

2

100

This

(10 mg L-1)

Visible light λ >400nm 500W halogen lamp

Ag:TiO2

Methyl Blue

Visible light λ >420nm

thin film

(2 ppm)

Four 8W lamp

Twist-like Ag/TiO2

Phenol

UV light λ=365 nm

−1

composites

(60 mg L )

500W xenon lamp

Ag-TiO2

Methyl Blue


nanocomposites

(1x10-5 mol L-1)

10W UV lamp

Ag/TiO2

Methyl Orange -5

-1

UV light λ =254 nm

nanotube arrays

(4.6 x10 mol L )

Tubular lamp

Ag/TiO2

Methyl Blue

UV light

ref

−1

nanotube arrays

(35 mg L )

120 W mercury lamp

Ag-TiO2

Phthalic Acid

UV light λ >280 nm

nanocomposites

(100ppm )

320 W xenon lamp

Ag/TiO2

Methyl Orange


nanotubes

(10 ppm)

500 W Xenon arc lamp

Ag/TiO2

Rhodamine B


nanotubes

(10 mg L-1)

19


work

Page 21 of 32

(e) Frequency /%

25

3.8 ± 0.7 nm 20 15 10 5 0 2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

Particle size /nm

(f) Ti O

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cu

Ag

C Cu

0

1

Ti

Ag

2

3

4

5

Cu

6

7

8

9

Energy /keV

Figure 1.

20


10


peak A

peak B Ag/TNT-2 A/B=7.58

Intensity /a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Ag/TNT-1 A/B=8.06 Ag/TNT-0.5 A/B=8.63 diHPP/TNT A/B=9.09 TNT

20

30

40

50

A/B=9.00

60

2 Theta /degree

Figure 2.

21


70

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a 1631

Transmission /%

TNT

diHPP/TNT 1619 Ag/TNT 1164

Ti-OH

2000

1800

1600

1400

1200

1000

Wavelength /cm-1

b 100 99

Weight /%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


98 97 96 95 94 93 92

100

200

300

400

500

600

700

800

Temperature /oC

Figure 3.

22


1000

800

600

Ti 3s Ti 3p

C 1s

Ag 3d5-Ag 3d3

Ag 3s Ti 2s

Ti 2p3-Ti2p1

O 1s

OKLL

Intensity/ a.u.

a

400

200

0

Binding energy /eV

b C1s

Intensity / a.u.

C-C/C=C

C-O

282

283

284

285

286

287

C=O

288

289

290

Binding energy / eV

c O1s

Intensity / a.u.

Ti-O

C=O C-O

528

530

532

534

536

Binding energy / eV

d Ag3d

367.4

Intensity /a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Ti LMM


373.4

365.0

367.5

370.0

372.5

375.0

377.5

380.0

Binding energy /eV

23


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e Ti2p

459.4

Intensity /a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


465.1

456

458

460

462

464

466

468

Binding energy /eV

Figure 4.

24



TNT Ag/TNT-0.5 Ag/TNT-1 Ag/TNT-2

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

400

500

600

700

Wavelength /nm

Figure 5.

25


800

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a

1.2

C/C0

Light on

Dark

1.0 0.8 0.6 0.4

Blank P25 TNT Ag/TNT-2

0.2 0.0 -1

0

1

2

3

4

Time /h

b

1.2 Dark

1.0

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Light on

0.8

Ag/TNT-0.5 Ag/TNT-1 Ag/TNT-2 Ag/TNT-3 Ag/TNT-5 Ag/TNT-10

0.6 0.4 0.2 0.0 -1

0

1

2

Time / h

Figure 6.

26



1.2 Dark

1.0

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Light on

0.8 0.6 0.4 No scavenger EDTA Benzoquinone Methanol

0.2 0.0 -1

0

1

2

3

4

Time /h Figure 7.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0h 1h 2h 3h 4h

Absorbance

Page 29 of 32

300

350

400

450

500

550

600

650

700

Wavelength / nm

Figure 8.

28



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Scheme 1.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 2.

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BRIEFS Inspired by the bioadhesion mechanism found from mussel, a catechol derivative, 3-(3, 4-dihydroxyphenyl) propionic acid, is employed as both linker and reducer of Ag+ to synthesize the Ag/TiO2 nanotube heterojunction under ambient conditions in this study. The prepared Ag/TNT heterojunction exhibits the high visible-light photocatalytic activity, which can almost degrade 100% Rhodamine B within 2 hours.

SYNOPSIS

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TiO2 Nanotube Heterojunction with Improved Visible

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