Fabrication and Characterization of Visible-Light-Driven Plasmonic

Aug 24, 2009 - Reduced Ensemble Plasmon Line Widths and Enhanced Two-Photon Luminescence in Anodically Formed High Surface Area Au–TiO2 3D Nanocompo...
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J. Phys. Chem. C 2009, 113, 16394–16401

Fabrication and Characterization of Visible-Light-Driven Plasmonic Photocatalyst Ag/AgCl/ TiO2 Nanotube Arrays Jiaguo Yu,*,† Gaopeng Dai,† and Baibiao Huang‡ State Key Laboratory of AdVanced Technology for Materials Synthesis and Processing, Wuhan UniVersity of Technology, Wuhan 430070, People’s Republic of China, and State Key Laboratory of Crystal Materials, Shandong UniVersity, Jinan 250100, People’s Republic of China ReceiVed: June 4, 2009; ReVised Manuscript ReceiVed: July 30, 2009

Conventional TiO2 photocatalyst possesses excellent activities and stabilities, but requires near-ultraviolet (UV) irradiation (about 4% of the solar spectrum) for effective photocatalysis, thereby severely limiting its practical application. It is highly desirable to develop a photocatalyst that can use visible light in high efficiency under sunlight irradiation. In this work, we prepare new visible-light-driven plasmonic photocatalyst Ag/ AgCl/TiO2 nanotube arrays (NTs) by depositing AgCl nanoparticles (NPs) into the self-organized TiO2 NTs, and then reducing partial Ag+ ions in the surface region of the AgCl particles to Ag0 species under xenon lamp irradiation. The prepared metal-semiconductor nanocomposite plasmonic photocatalyst exhibits a highly visible-light photocatalytic activity for photocatalytic degradation of methyl orange in water and stability. A new plasmonic photocatalytic mechanism, which is proposed on the basis of the fact that the Ag NPs are photoexcited due to plasmon resonance and charge separation, is accomplished by the transfer of photoexcited electrons from the Ag NPs to the TiO2 conduction band and the simultaneous transfer of compensative electrons from a donor (Cl-) to the Ag NPs. The proposed mechanism is further confirmed by the experiments of hydroxyl radical and transient photocurrent response. The prepared photocatalysts are also of great interest in solar cell, catalysis, separation technology, biomedical engineering, and nanotechnology. This study may provide new insight into the design and preparation of advanced visible-light photocatalytic materials. 1. Introduction 1

Since the discovery of carbon nanotubes in 1991, onedimensional (1D) nanostructured materials have been extensively studied because of their distinctive geometrical morphologies, novel physical and chemical properties, and potential applications in nanoscale optical and electric devices.2 Among the various oxide and nonoxide 1D nanostructured materials, TiO2 nanotubes have gained more and more attention because of their enhanced properties, cheap fabrication, high specific surface area and pore volume, and wide applications in photocatalysis,3 gas sensing,4 photoelectrolysis,5 and photovoltaic cells.6 Titania nanotubes and nanotube arrays have been fabricated by many methods including nanoporous alumina template methods,7 sol-gel transcription using organo-gelators as templates,8 seeded growth,9 and hydrothermal processes.10 However, of these nanotube fabrication routes, the fabrication of highly ordered nanotube arrays by anodization of titanium in fluoride-based electrolytes has received considerable attention;11 also, their dimensions can be precisely controlled. Uniform TiO2 NTs of various pore sizes (22-110 nm), lengths (200-1 000 000 nm), and wall thicknesses (7-34 nm) are easily obtained by tailoring electrochemical conditions. A variety of reports in the literature give evidence of the unique properties this material architecture possesses,4-6 making it of considerable scientific interest as well as practical importance.12 Considering their large specific surface area, high pore volume, and unique morphology, the obtained NTs will offer new chances to design * Corresponding author. Tel.: 0086-27-87871029. Fax: 0086-2787879468. E-mail: [email protected]. † Wuhan University of Technology. ‡ Shandong University.

various 1D TiO2-related photocatalytic materials by doping, deposition, and sensitization.13 TiO2 has been extensively used and investigated as an excellent photocatalyst because of its exceptional properties such as nontoxicity, low cost, and long-term stability against photo and chemical corrosion.14 However, a large intrinsic band gap of TiO2 (3.2 eV for anatase and 3.0 eV for rutile) allows only a small portion of solar spectrum in the ultraviolet (UV) light region to be absorbed. Therefore, the effective utilization of visible light has become one of the most important goals in photocatalytic applications. Various methods have been developed to reduce the band gap of TiO2 via, for example, substitutional doping (N, C, F, etc.)15 and combing TiO2 with organic dyes or narrow-gap semiconductors quantum dots (QDs), such as CdS, CdSe, InP, and PbS QDs.13b,16 Noble-metal nanoparticles (NPs) show strong visible-light absorption because of size- and shape-dependent plasmon resonance, which has a wide variety of applications such as colorimetric sensors,17 photovoltaic devices,18 photochromic devices,19 and photocatalysts.20 Plasmon-based photochemical and photothermal reactions have been used for preparation of Ag nanoprisms and Au nanorods and irreversible and reversible photoimaging.18a In particular, silver NPs show efficient plasmon resonance in the visible region, which has been utilized to develop a plasmonic photocatalyst.20 Recently, Huang and co-worker reported fabrication of a highly efficient and stable plasmonic photocatalyst Ag@AgCl by first treating Ag2MoO4 with HCl to form AgCl powder and then reducing some Ag+ ions in the AgCl particles to Ag0 species.20b However, little work has been done on the fabrication of TiO2 NTs and Ag/AgCl NPs-based nanocomposite structures and their photocatalytic activity.

10.1021/jp905247j CCC: $40.75  2009 American Chemical Society Published on Web 08/24/2009

Photocatalyst Ag/AgCl/TiO2 Nanotube Arrays

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TABLE 1: Experimental Conditions for the Preparation of Samplesa no.

compositions

phase

xenon illum.

anneal

activity

1 2 3 4 5 6 7 8

TiO2 TiO2 AgCl/TiO2 Ag/AgCl/TiO2 AgCl/TiO2 Ag/AgCl/TiO2 Ag/TiO2 TiO2-xNx

Am A Am Am A A A A

no no no yes no yes no no

no yes no no yes yes yes yes

no no no medium no high low low

a

A and Am denote anatase and amorphous, respectively.

In this work, new visible-light-driven plasmonic photocatalyst Ag/AgCl/TiO2 NTs are prepared by AgCl NPs deposited into the self-organized TiO2 NTs, and then reducing partial Ag+ ions in the AgCl particles to Ag0 species under xenon lamp irradiation. The prepared samples show high visible-light photocatalytic activity for the photocatalytic degradation of methyl orange (MO) aqueous solution and stability. To the best of our knowledge, this is the first report on the preparation and visible-light photocatalytic activity of plasmonic photocatalyst Ag/AgCl/TiO2 NTs. 2. Experimental Details 2.1. Preparation. The self-organized TiO2 NTs were grown by anodization of Ti foils in an aqueous solution containing 0.5 M H3PO4 and 0.14 M NaF at pH ca. 1.5, similarly to the method described by Misra et al.21 Prior to anodization, Ti foils (0.25 mm, 99% purity) were respectively degreased by sonication in acetone, isopropanol, and methanol, rinsed with deionized water (DI), and finally dried in a nitrogen stream. Anodization was performed in a two-electrode configuration connected to a DC power supply with titanium foil as the working electrode and platinum foil as the counter electrode under a constant 20 V anodic potential for 1 h at room temperature. After anodic oxidation, the samples were rinsed with deionized water and dried in a N2 stream. The resulting amorphous titania NTs were annealed at 450 °C for 2 h with heating and cooling rates of 2 °C/min in air to crystallize the tube walls and improve their stoichiometry and crystallization. Ag/AgCl NPs were deposited into the amorphous and crystallized TiO2 NTs by an impregnating-precipitationphotoreduction method. Typically, the TiO2 NTs samples were successively immersed in four different beakers for about 30 min in each beaker. One beaker contained 1 M HCl aqueous solution, another contained 0.1 M AgNO3 aqueous solution, and the other two contained distilled water to rinse the excess of each precursor solution from the samples. Such an immersion cycle was repeated several times, typically between 1 and 3 cycles. After several cycles, the sample became a little canary due to AgCl NPs deposited into the TiO2 NTs. Finally, the asprepared samples were irradiated with a 300 W xenon lamp for 10 min to reduce partial Ag+ ions in the AgCl particles to Ag0 species by photochemical decomposition of AgCl or TiO2 photocatalytic reduction. The experimental conditions for the preparation of samples are listed in Table 1. 2.2. Characterization. The morphology observation was performed on a S-4800 field emission scanning electron microscope (SEM, Hitachi, Japan). Transmission electron microscopy (TEM) analyses were conducted with a JEM-2100F electron microscope (JEOL, Japan), using a 200 kV accelerating voltage. X-ray diffraction (XRD) patterns obtained on a D/MAX-RB X-ray diffractometer (Rigaku, Japan) using Cu KR radiation at a scan rate (2θ) of 0.05° s-1 were used to determine

the identity of any phase present and their crystallite size. The accelerating voltage and the applied current were 40 kV and 80 mA, respectively. The average crystallite sizes were determined according to the Scherrer equation using the full-width half-maximum data after correcting the instrumental broadening. X-ray photoelectron spectroscopy (XPS) measurements were performed on a VG ESCALAB 210 XPS system with Mg KR (1253.6 eV) source. All of the binding energies were referenced to the C1s peak at 284.8 eV of the surface adventitious carbon. UV-visible diffuse reflectance spectra were obtained on a UV-visible spectrophotometer (UV-2550, Shimadzu, Japan). BaSO4 was used as a reflectance standard. 2.3. Photocatalytic Activity. The photocatalytic activity of the samples was evaluated by the photocatalytic decolorization of MO aqueous solution at ambient temperature,22 because MO is a kind of chemically stable and persistent containing-nitrogen dye pollutant. Experiments were as follows: 2 × 2 cm2 Ag/ AgCl/TiO2 NTs sample was placed in a 15 mL MO aqueous solution with a concentration of 1 × 10-5 M in a rectangle cell (20 W × 50 L × 20 H mm). The solution was allowed to reach an adsorption-desorption equilibrium among the photocatalyst, MO, and water before visible light irradiation. A 300 W xenon lamp through a UV-cutoff filter (e400 nm), which was positioned 10 cm away from the cell, was used as a visible light source to trigger the photocatalytic reaction. The average light intensity striking on the surface of the reaction solution was about 25 mW cm-2. The concentration of MO was determined by an UV-visible spectrophotometer (UV-2550). After visible light irradiation for some time (every 10 min), the reaction solution was taken out to measure the concentration change of MO. To further determine the mineralization of MO, changes in total organic carbon (TOC) were determined using a total organic carbon analyzer (model TOC-Aopllo 9000). The visible-light photocatalytic activity of other samples (see Table 1) was also measured in the same conditions. 2.4. Analysis of Hydroxyl Radical ( · OH). The formation of hydroxyl radicals ( · OH) at the photoilluminated samples/ water interface was detected by the PL technique using terephthalic acid as a probe molecule. Terephthalic acid readily reacts with · OH to produce highly fluorescent product, 2-hydroxyterephthalic acid.23 This technique has been used in radiation chemistry, sonochemistry, and biochemistry for the detection of · OH generated in water. This method relies on the PL signal at 425 nm of the hydroxylation of terephthalic acid with · OH generated at the water/catalyst interface. The PL intensity of 2-hydroxyterephtalic acid is proportional to the amount of · OH radicals produced in water.23 The method is rapid, sensitive, and specific and needs only a simple standard PL instrumentation. Experimental procedures are similar to the measurement of photocatalytic activity except that MO aqueous solution is replaced by the 5 × 10-4 M terephthalic acid aqueous solution with a concentration of 2 × 10-3 M NaOH. PL spectra of generated 2-hydroxyterephthalic acid were measured on a Hitachi F-7000 fluorescence spectrophotometer. After visiblelight irradiation for every 10 min, the reaction solution was used to measure the increase of the PL intensity at 425 nm excited by 315 nm light. 2.5. Photoelectrochemical Measurements. Photocurrents were measured using an electrochemical analyzer (CHI660C Instruments) in a standard three-electrode system with the prepared samples as the working electrodes with an active area of ca. 0.8 cm2, a Pt wire as the counter electrode, and Ag/AgCl (saturates KCl) as a reference electrode. A 300 W Xe arc lamp equipped with an ultraviolet cutoff filter to provide visible light

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Figure 2. XRD patterns of samples 2 (a), 5 (b), and 6 (c).

Figure 1. SEM (a and b) and TEM (c and d) images of self-organized TiO2 NTs: (a) a typical top view SEM image of sample 2, the inset in (a) showing the cross-sectional SEM image of sample 2; (b) SEM image of sample 6; (c) TEM images of sample 2; (d) TEM images of sample 6; and (e) corresponding EDX pattern of sample 6.

with >400 nm served as the visible light source. The integrated visible-light intensity measured with a visible-light radiometer (FZ-A) was 25 mW/cm2 with the wavelength range of 400-1000 nm. A 1 M Na2SO4 aqueous solution was used as the electrolyte. 3. Results and Discussion 3.1. Morphology and Phase Structures. The self-organized TiO2 NTs were synthesized by anodic oxidation of Ti foils and then annealed in air at 450 °C for 2 h to obtain the anatasephase tube walls. AgCl NPs were deposited into the crystallized or amorphous TiO2 NTs by a simple impregnating precipitation method, and then the partial Ag+ ions of AgCl NPs were reduced to Ag0 species under xenon lamp irradiation, to form visible-light-driven plasmonic photocatalyst Ag/AgCl/TiO2 NTs (sample 6 in Table 1). The morphologies of the TiO2 NTs were observed using SEM. Figure 1a presents a typical SEM image of the anatase-phase TiO2 NTs (sample 2), which shows a regularly arranged pore structure of the film. These pores have a relative uniform size distribution around 120 nm. The inset in Figure 1a is a cross-sectional view of the film showing that the film is composed of well-aligned nanotubes of about 550 nm in length, which grow vertically from a Ti substrate. Shown in Figure 1b is a SEM image of the Ag/AgCl NPs deposited into TiO2 NTs (sample 6), indicating that well-ordered pores structure still exists, suggesting that the Ag/AgCl NPs deposition process does not damage the ordered TiO2 NTs structure. Closer observation indicates that the surface of sample 6 is not clean

and some NPs (marked with arrows) have still deposited on the surface of TiO2 NTs. Ag/AgCl NPs deposited into TiO2 NTs were further examined using TEM. Figure 1c is a TEM image of the pristine nanotube sample (sample 2), showing clearly that the sample has an ordered array tubular structure. Figure 1d is a TEM image of sample 6, showing that the NPs have deposited into the pore of the TiO2 nanotubes. These NPs have a uniform size distribution around 20 nm. The composition of the NPs was determined by energy dispersive X-ray spectroscopy (EDX) experiments, which were carried out in the SEM. Figure 1e shows a typical EDX spectrum obtained from sample 6. In spectrum peaks associated with Ti, O, Ag, and Cl are observed. Ti and O peaks result from TiO2 nanotubes. Quantitative analysis of these spectra gives the deposited materials a possible composition of AgCl, which agrees with the fact that these NPs were formed during AgCl deposition experiment. XRD was used to determine the phase structure of the samples. Figure 2a shows the XRD pattern of sample 2. Quantitative analysis of this pattern shows that all peaks in the pattern can be indexed using the TiO2 anatase phase (JCPDS file no.: 21-1272) and the Ti metal phase (JCPDS file no. 441294), which are marked with A and T in the figure, respectively. The peaks of the anatase phase are from the TiO2 NTs, and that of the metal Ti phase is from Ti substrate. The XRD pattern of sample 5 (Figure 2b) indicates that, as compared to Figure 2a, additional peaks appear, which can be attributed to the cubic phase of AgCl with lattice constant a ) 5.5491 Å (JCPDS file no.: 31-1238) marked with C in Figure 2b, suggesting that the deposited AgCl has a crystalline cubic phase. The broad diffraction peaks suggest that the sizes of the AgCl NPs are small (about 20.0 nm), and this is consistent with the above SEM and TEM observations. After xenon lamp irradiation of sample 5, the diffraction peaks of metallic Ag (JCPDS file no.: 65-2871) appear in sample 6 (see Figure 2c) due to the following photochemical or photocatalytic reduction reaction of AgCl under xenon lamp light in the presence of TiO2:

AgCl f Ag + Cl

(1)

Ag atoms produced by eq 1 tend to aggregate to form small silver nanocrystals (about 3.5 nm determined by the Scherrer equation), and then deposit on the surface of AgCl particles.24

nAg0 f (Ag0)n

(2)

Photocatalyst Ag/AgCl/TiO2 Nanotube Arrays

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Figure 3. High-resolution XPS spectra of Ag 3d of samples 5 (a) and 6 (b).

Figure 5. (A) Comparison of photocatalytic activity of samples 2 (a), 4 (b), 6 (c), and 8 (d) for the photocatalytic decomposition of MO in water. (B) Cycling degradation curve for sample 6. Figure 4. UV-visible diffuse reflectance spectra of samples 5 (a) and 6 (b).

Therefore, it is not too surprising that sample 6 contains a small amount of metallic silver coexisting with AgCl. 3.2. Chemical Composition. The elemental compositions and chemical status of samples 5 and 6 were further analyzed by XPS. Before xenon lamp irradiation, XPS results (not shown here) indicate that sample 5 contains not only Ti and O elements, but also some C, Ag, and Cl elements. The XPS peak for C1s (284.8 eV) is due to the adventitious hydrocarbon from the XPS instrument itself. Ag (368 eV) and Cl peaks (199 eV) are from the deposited AgCl NPs within the nanotubes. After xenon lamp irradiation, the elemental compositions of sample 6 have no great change. Figure 3 shows the high-resolution XPS spectra of Ag of samples 5 and 6. Before and after xenon lamp irradiation, the Ag 3d5/2 peaks appear at binding energies of 367.6 and 367.9 eV, respectively. The difference of binding energy indicates that silver is of metallic nature in sample 6.24-26 This is also confirmed by the UV/vis spectra. The UV/vis diffuse-reflectance spectra of samples 5 and 6 are compared in Figure 4. In contrast to nonirradiated sample 5, xenon lamp-irradiated sample 6 has a strong plasmon resonance adsorption peak (peak at 442 nm) observed in the visible region. This also further indicates sample 6 contains silver NPs due to photochemical decomposition or photocatalytic reduction of AgCl. On the basis of the above XRD, EDX, XPS, UV/vis results, and extensive electron microscope observations, we verify that NPs deposited inside TiO2 nanotubes are Ag/ AgCl composite NPs, and these Ag/AgCl NPs are dispersedly deposited onto the inner walls. 3.3. Photocatalytic Activity and Mechanism. The photocatalytic activity of the samples was evaluated by photocatalytic

degradation decolorization of MO aqueous solution under visible-light irradiation. Figure 5A and Table 1 show the comparison of photocatalytic activities of the samples prepared at different conditions. For pristine amorphous and anatase TiO2 nanotube array films (samples 1 and 2), no photocatalytic activity is observed because they are not excited by visible light. Furthermore, AgCl-TiO2 composite films show neglectable visible-light photocatalytic activity (samples 3 and 5). This is not surprising because AgCl and anatase TiO2 have great indirect band gaps of 3.25 and 3.2 eV, respectively. After xenon lamp irradiation, a small amount of metallic Ag was deposited on the surface of AgCl NPs. The as-formed Ag/AgCl/TiO2 NTs (samples 4 and 6) show obvious visible-light photocatalytic activity (as shown in Figure 5A). With increasing irradiation time, the decomposition of MO progresses steadily, and decomposition over sample 6 is completed within 60 min. However, the TOC results indicated that the rate of TOC reduction was significantly slower than that of photocatalytic decolorization of MO. After 1 h of irradiation for sample 6, the decrease in the TOC of solution is about 13%. This is not difficult to understand because the complete mineralization of MO requires a long time.22c Provided that the photocatalytic reaction follows a pseudo-first-order reaction, the rate constant of MO decomposition over sample 6 is estimated to be about 0.051, faster than those over sample 4 (0.025) and sample 8 (0.015, N-doped TiO2 NTs, used as reference photocatalyst) by a factor of 2 and 3.4. We also found that other organic pollutants such as rhodamine B, methylene blue, and phenol are quickly decomposed by sample 6 under visible-light irradiation. However, no photocatalytic decomposition of MO is observed in the absence of visible-light irradiation (in the dark) or sample 6, indicating that sample 6 is an efficient visible-light photocatalyst.

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The photoactivity of sample 6 can be understood by the following suggested mechanism (as shown in Figure 6), which is similar to Tatsuma et al.’s mechanism of plasmon-induced charge separation and Huang et al.’s mechanism of Ag@AgCl plasmonic photocatalysis.20b,27 Under visible-light irradiation, photogenerated electron-hole pairs are formed in Ag NPs due to surface plasmon resonance. The photoexcited electrons at the silver NPs are injected into the TiO2 conduction band (Figure 6),27 and the injected electrons can be transferred to the ubiquitously present molecular oxygen to form first the superoxide radical anions, O2- · , then on protonation yields the HOO · radicals, and HOO · radicals and the trapped electrons combine to produce H2O2, finally forming HO · radicals.28 These active species will result in the degradation and mineralization of MO. Meanwhile, the holes transfer to the surface of the AgCl particles because the surface of AgCl particles is negatively charged and most likely terminated by Cl- ions due to the xenon lampinduced reduction of partial Ag+ ions. The transferred holes will cause the oxidation of Cl- ions to Cl0 atoms.20b As chlorine atoms are reactive radical species, they are able to oxidize MO and become reduced to chloride ions again,20b so the Ag NPs can be rapidly regenerated and the Ag/AgCl/TiO2 system remains stable. The major reaction steps in this plasmonic photocatalytic mechanism under visible light irradiation are summarized by eqs 3-10.

Ag-NPs + hν f Ag-NPs*

(3)

Ag-NPs* + TiO2 f Ag-NPs+ · + TiO2(e)

(4)

· TiO2(e) + O2 f TiO2 + O2

(5)

O2- · + H+ f · OOH

(6)

· OOH + TiO2(e) + H+ f H2O2 + TiO2

(7)

H2O2 + TiO2(e) f · OH + OH- + TiO2

(8)

Ag-NPs+ · + Cl- f Ag-NPs + Cl0

(9)

· organic pullutant + Cl0 (or · OH or O2 or H2O2) f f degraded or mineralized products (10)

Further investigation shows that the phase structures of TiO2 nanotubes also influence the photocatalytic activity of the Ag/ AgCl/TiO2 NTs. Sample 6 shows higher photocatalytic activity than does sample 4. This is due to anatase TiO2 NTs containing fewer surface and inner defects. In this study, an important problem is why the photoexcited electrons at the silver NPs can inject into the TiO2 conduction band. This is because a Schottky barrier is formed at the Ag/TiO2 interface due to the larger work function of silver (Figure 6) and the electric field in the space-charge layer could promote transport of excited electrons at the Ag surface to the walls of TiO2 nanotubes;27b observation of the electron transfer from the photoexcited silver to nonexcited TiO2 is not surprising.20a A similar process has been observed for electron transfer from a photoexcited dye to TiO2 bulk in dye-sensitized solar cells. It should be noted that

Figure 6. Schematic diagram for the charge separation in a visiblelight irradiated Ag/AgCl/TiO2 system.

this Ag/AgCl/TiO2 NTs plasmonic photocatalyst is different from the plasmonic photocatalyst prepared by Awazu et al. and Huang et al.20 The former is to deposit TiO2 on NPs comprising an Ag cores covered with a silica (SiO2) shell to prevent oxidation of Ag by direct contact with TiO2.20a The latter is to treat Ag2MoO4 with HCl to form AgCl powders and then reduce partial Ag+ ions in the surface region of the AgCl particles to Ag0 species.20b On the contrary, our photocatalyst is beneficial to the transfer and separation of photogenerated electrons and holes due to heterojunction of TiO2 nanotubes and metal titanium.24 As a photocatalyst, its stability is very important for its application. The N-doped TiO2 and sulfide photocatalysts sometimes suffer from instability in practical application.20b So, the stability of plasmonic photocatalyst Ag/AgCl/TiO2 NTs is further investigated by the recycle experiments of photocatalyst (see Figure 5B). After four recycles for the photodegradation of MO, the catalyst does not exhibit any significant loss of activity, indicating that the catalysts are stabile during the photocatalytic oxidation of the pollutant molecules. Further observation shows that after the first cycle, the activity slightly decreases probably due to a drop of a small amount of Ag/ AgCl NPs from the walls of TiO2 nanotubes, and then the activity almost keeps stable. The XRD pattern of sample 6 at the end of the repeated photocatalytic experiment is almost identical to that of the as-prepared sample (not shown here). This indicates that sample 6 is a highly efficient and stable photocatalyst under visible-light irradiation. This is also in agreement with the above suggested mechanism. The stability of the Ag/AgCl/TiO2 NTs plasmonic photocatalyst under visiblelight irradiation arises most likely from the fact that a photon is absorbed by the silver NPs, and an electron separated from an absorbed photon remains on the surface of TiO2 nanotubes rather than being transferred to the Ag+ ions of the AgCl lattice.20b However, sample 7 (Ag/TiO2 system) shows a low photocatalytic activity and instability due to the dissolving of oxidized Ag+ ions from Ag NPs. Furthermore, the prepared Ag/ AgCl/TiO2 NTs (sample 6) can be regarded as an ideal photocatalyst for environmental purification at the industrial scale because they can be more readily separated from the slurry system after photocatalytic reaction and reused than can conventional nanosized powder photocatalytic materials. Considering their large specific surface area, high pore volume, unique morphology, and high photocatalytic activity, sample 6 is also of great interest in sensor, solar cell, catalysis, separation technology, biomedical engineering, and nanotechnology. 3.4. Hydroxyl Radical Analysis. The proposed plasmonic photocatalytic mechanism was further confirmed by the detection of · OH. Figure 7A shows the changes of PL spectra of

Photocatalyst Ag/AgCl/TiO2 Nanotube Arrays

Figure 7. (A) PL spectral changes with visible-light irradiation time on sample 6 in a 5 × 10-4 M basic solution of terephthalic acid and (B) PL spectra of samples 2 (a), 3 (b), 4 (c), 5 (d), and 6 (e) in a 5 × 10-4 M basic solution of terephthalic acid under visible-light irradiation at a fixed 30 min.

terephthalic acid solution under visible-light irradiation with irradiation time. A gradual increase in PL intensity at about 425 nm is observed with time for sample 6. However, no PL increase is observed in the absence of visible-light irradiation or sample 6. This suggests that the fluorescence is from the chemical reactions between terephthalic acid and · OH formed during photocatalytic reactions.23 Figure 7B shows the comparison of PL intensity for different samples at 30 min. Usually, PL intensity is proportional to the amount of produced hydroxyl radicals. It can be easily seen that at a fixed time (30 min), the amount of OH radicals produced on sample 6 is larger than that of OH radicals produced on sample 4. This implies that the former has higher photocatalytic activity than the latter. Further observation shows that no PL signals are observed for the other three samples (2, 3, and 5) because they are not activated by visible light, implying no photocatalytic activity for these samples. Hydroxyl radical experiments further confirm that hydroxyl radicals are active species and indeed participate in photocatalytic reactions, and the suggested mechanism is correct. 3.5. Transient Photocurrent Response. To give further evidence to support the above suggested plasmonic photocatalytic mechanism, the transient photocurrent responses of TiO2, AgCl/TiO2, and Ag/AgCl/TiO2 NTs electrodes were recorded via several on-off cycles of irradiation. Several representative traces are shown in Figure 8A for comparing transient photocurrent responses of the different samples. For sample 6, a schematic transient photocurrent curve is shown in Figure 8B. The initial anodic photocurrent spike is denoted by Iph,in. This current is due to separation of plasmon-induced electron-hole pairs at the TiO2/Ag/AgCl interface: holes move to the AgCl surface, where they are trapped or captured by reduced species in the electrolyte, while the electrons are transported to the back

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Figure 8. (A) Comparison of transient photocurrent response of the samples 2 (a), 4 (b), and 6 (c) in 1 M Na2SO4 aqueous solutions under visible-light irradiation at 0.4 V vs Ag/AgCl. (B) Schematic representation of a transient photocurrent response curve; the parameters used in the text to describe the transients are indicated.

contact via TiO2. After Iph,in has been attained, a continuous decrease of the photocurrent with time can be observed until a steady-state photocurrent, Iph,st, is reached (Figure 8B). The photocurrent decay indicates that recombination processes are occurring. Holes reaching the AgCl surface may, instead of capturing electrons from the electrolyte, accumulate at the surface and recombine with electrons from TiO2 conduction band; that is, the decay is determined by the rate at which minority carriers trapped at surface states capture majority carriers. Another recombination process giving rise to a decay of the photocurrent is that conduction band electrons start to reduce photogenerated oxidized species in the electrolyte. When the light is switched off, a cathodic spike, lct,in, can be observed due to the back reaction of TiO2 conduction band electrons with holes trapped at the AgCl surface.29a On the contrary, for sample 4 (as shown in b of Figure 8A), the transient photocurrent shows a relatively slow response when the light is switched on and off. The electrons trapped in the Ti3+ surface states of TiO2 nanotubes are responsible for the slow photocurrent response. This surface state locates below the conduction band edge of TiO2 and is named as the “shallow surface state”, in which the trapping of conduction band electrons is a very rapid event.15c The electrons trapped in this shallow surface state are expected to be constantly released from and retrapped into “deep surface state”, becoming more difficult to release. The deep surface states usually locate in the middle of the band gap. The rate of photoelectrons trapped in this deep surface state is much slower than that of photoelectrons trapped in shallow traps. Therefore, upon irradiation, the shallow traps lying close to the conduction band are first filled by the electrons, and the trapped electrons then moved progressively down to the deep surface states. During this time, only a part of the

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photoelectrons, including the released electrons from the shallow surface states, can transport to the back contact electrode titanium until the deep surface states are filled, which causes the slowly increased Iph,in response. Similarly, the slow response of the Iph,st decaying to zero upon being switched off is controlled by charge carriers released from the traps.29b So, this suggests that sample 4 contains more surface and inner defects due to TiO2 NTs being amorphous. It can be seen from Figure 8A that a prompt generation of photocurrents is observed and with good reproducibility when sample 6 is irradiated by visible light. While the light is off, the value of photocurrent for sample 6 is instantaneously close to zero. This indicates that under visible-light irradiation, most of the photoexcited electrons at the Ag surface due to plasmon resonance are transported to the walls of TiO2 nanotubes, and then transfer to titanium substrate to produce photocurrent. Sample 6 shows the biggest photocurrent due to anatase TiO2 nanotubes walls with fewer defects. Contrarily, sample 4 shows less photocurrent and slower response due to amorphous TiO2 nanotubes walls with more defects, which inhibit the transfer of photogenerated electrons. Of course, it is not surprising that sample 2 exhibits very weak photocurrent because of their big band gaps. 4. Conclusions Novel visible-light-driven plasmonic photocatalyst Ag/AgCl/ TiO2 NTs can be easily prepared by AgCl NPs deposited into the self-organized TiO2 NTs, and then reducing partial Ag+ ions of the AgCl particles to Ag0 species under xenon lamp irradiation. The prepared plasmonic photocatalyst exhibits a highly visible-light photocatalytic activity and stability. This arises from the surface plasmon resonance absorption of silver NPs under visible light irradiation and the charge separation at the silver NPs, which includes electrons transferring from the plasmon-excited silver to TiO2 and that from a donor (Cl-) to the silver NPs. The PL and photocurrent response results further confirm that the suggested mechanism is correct. The prepared Ag/AgCl/TiO2 NTs can be regarded as an ideal photocatalyst for industrial application because they can be more readily separated from the slurry system after photocatalytic reaction and reused than can conventional powder photocatalyst. Considering their large specific surface area, high pore volume, unique morphology, and high photocatalytic activity, the new photocatalyst is also of great interest in sensor, solar cell, catalysis, separation technology, biomedical engineering, and nanotechnology. Acknowledgment. This work was partially supported by the National Natural Science Foundation of China (50625208, 20773097, and 20877061). This work was also financially supported by the National Basic Research Program of China (2007CB613302 and 2009CB939704). References and Notes (1) Iijima, S. Nature 1991, 354, 56–58. (2) (a) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947– 1949. (b) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208–209. (c) Martin, C. R. Science 1994, 226, 1961–1966. (d) Yu, J. G.; Fan, F.-R. F.; Pan, S. L.; Lynch, V. M.; Omer, K. M.; Bard, A. L. J. Am. Chem. Soc. 2008, 130, 7196–7197. (3) (a) Adachi, M.; Murata, Y.; Harada, M.; Yoshikawa, Y. Chem. Lett. 2000, 29, 942–943. (b) Chu, S. Z.; Inoue, S.; Wada, K.; Li, D.; Haneda, H.; Awatsu, S. J. Phys. Chem. B 2003, 107, 6586–6589. (4) (a) Varghese, O. K.; Gong, D.; Paulose, M.; Ong, K. G.; Dickey, E. C.; Grimes, C. A. AdV. Mater. 2003, 15, 624–627. (b) Mor, G. K.; Carvalho, M. A.; Varghese, O. K.; Pishko, M. V.; Grimes, C. A. J. Mater. Res. 2004, 19, 628–634.

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