Conversion of Ag Nanowires to AgCl Nanowires Decorated with Au

Jan 19, 2010 - ... Zhang , Cheng Lu , Jingjing Wang , Jian Lv , Liping Ding , and Meng Ju ... Ivor Tan Kian Joo , Teck Hua Lau , Zhili Dong , and Zhon...
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J. Phys. Chem. C 2010, 114, 2127–2133

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Conversion of Ag Nanowires to AgCl Nanowires Decorated with Au Nanoparticles and Their Photocatalytic Activity Yugang Sun* Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass AVenue, Argonne, Illinois 60439 ReceiVed: December 6, 2009; ReVised Manuscript ReceiVed: December 25, 2009

A two-step approach has been developed to synthesize AgCl nanowires decorated with Au nanoparticles by using Ag nanowires as chemical templates. In the first step, the Ag nanowires are oxidized with FeCl3 followed by a simultaneous precipitation reaction between Ag+ and Cl- ions at room temperature, resulting in conversion of the Ag nanowires to AgCl nanowires as well as reduction of Fe3+ to Fe2+ ions. In the second step, the Fe2+ ions generated in the first step reduce Au precursors (e.g., NaAuCl4) to deposit Au nanoparticles on the surfaces of the AgCl nanowires, resulting in the formation of AgCl:Au composite nanowires. Because of strong surface plasmon resonance and chemical inertness of Au nanoparticles, the as-synthesized AgCl:Au nanowires exhibit enhanced absorption coefficient in the visible region and enhanced chemical stability to prevent them from degradation and aggregation. These unique properties enable the AgCl:Au nanowires to be used as a class of promising plasmonic photocatalysts driven by visible light. Preliminary results demonstrate these composite nanowires can efficiently decompose organics, such as methylene blue molecules, under illumination of white light. Introduction Silver nanowires have been extensively studied, and a number of chemical approaches have been developed to be capable of synthesizing high quality Ag nanowires in large quantity.1–8 The as-synthesized Ag nanowires have been used in many applications, for example, transparent conductive electrodes for solar cells and optoelectronic devices,9,10 conductive adhesives,11,12 plasmonic waveguides,13–16 substrates for surface-enhanced Raman scattering spectroscopy (SERS),17–24 due to their unique electrical and optical properties. In addition, the Ag nanowires can serve as templates for synthesizing nanowires with different compositions and complex structures that can expand their applications due to new properties associated with the newly derived nanowires. For instance, Ag nanowires synthesized through a polyol process with the assistance of poly(vinyl pyrrolidone) (PVP) have been successfully used as a class of physical templates for the synthesis of Ag@SiO2 nanocables, which consist of the Ag nanowires coated with uniform SiO2 sheaths.25 Thickness of the SiO2 layers can be easily tuned by controlling reaction conditions including concentration of precursors and reaction time. The good conductivity of the Ag nanowires and the insulating property of the SiO2 sheaths make the Ag@SiO2 core-shell nanowires of ideal connectors for integrating functional components on the nanometer scale.26 In addition, the Ag nanowires can also serve as chemical templates to react with precursors, which can oxidize Ag, for synthesizing nanowires made of other materials. Typical examples include tubular nanowires (i.e., nanotubes) made of Au/Ag, Pd/Ag, and Pt/Ag alloys synthesized through galvanic replacement reactions between the Ag nanowires with appropriate precursors (e.g., HAuCl4 for Au/Ag, Pd(NO3)2 for Pd/Ag, and Pt(CH3COO)2 for Pt/Ag).27–31 These nanotubes exhibit novel properties significantly different from the original Ag nanowires due to the morphological (i.e., from solid nanowires to hollow nanotubes) * To whom correspondence should be addressed. E-mail: [email protected].

and chemical transformation. For example, absorption bands of Au/Ag alloy nanotubes can be tuned in both visible and nearinfrared (NIR) regions, while the Ag nanowires can only absorb violet and blue light with wavelengths less than 450 nm.29,32 Because localized surface plasmon resonance (SPR) of a metal nanoparticle is sensitive to its morphology and composition, Au-Ag alloy nanoparticles,33–35 dielectric/metal core/shell nanoparticles,36–38 nanoparticles with various well-defined morphologies such as nanorods,39–44 nanoplates,45–49 octahedra,50–52 oligomers, and aggregates of nanoparticles,53,54 nanowires decorated with nanoparticles,17,18 etc., also have been synthesized to expand its plasmonic bands covering both visible and NIR regions. In this article, we report the use of Ag nanowires as a class of chemical templates for the synthesis of polycrystalline AgCl nanowires decorated with Au nanoparticles, which are described as AgCl:Au nanowires throughout the manuscript. The synthesis starts with simultaneous oxidation and precipitation reactions between the Ag nanowires and an aqueous solution of FeCl3 at room temperature to transform the Ag nanowires to AgCl nanowires. In the following step, Fe2+ ions derived from the reduction of FeCl3 reduce Au precursor (i.e., NaAuCl4) to deposit Au nanoparticles on the surfaces of the AgCl nanowires. The as-synthesized AgCl:Au nanowires represent a new class of hybrid nanostructures consisting of a semiconductor of AgCl with large bandgaps (a direct bandgap of 5.15 eV (241 nm) and an indirect bandgap of 3.25 eV (382 nm))55 and a relatively stable metal of Au with strong SPR in the visible spectral region.33 Similar to other semiconducting nanoparticles sensitized with plasmonic nanoparticles,56,57 the as-synthesized AgCl: Au nanowires exhibit excellent photocatalytic activity and stability to decompose organic molecules under visible light. Experimental Section Synthesis of Ag Nanowires. Synthesis of Ag nanowires was carried out through the polyol process described elsewhere.58 In a typical synthesis, 5 mL of ethylene glycol (J. T. Baker)

10.1021/jp9115645  2010 American Chemical Society Published on Web 01/19/2010

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was added to a three-neck round flask, which was immersed in an oil bath set at 160 °C. A light nitrogen flow over the heated ethylene glycol for 10 min quickly removed trace water in the reaction system. Heating was then continued under air for another 50 min. Meanwhile, ethylene glycol solutions of 0.10 M AgNO3 (Aldrich) and 0.15 M poly(vinyl pyrrolidone) (PVP, Mw ≈ 55000, Aldrich, the concentration was calculated in terms of the repeating unit) were prepared. Small amounts of NaCl (Fisher) and tris(acetylacetonato)iron(III) (Fe(acac)3, Aldrich) were added to the PVP solution with final concentrations of 0.06 mM and 2.2 µM, respectively. In the next step, 3 mL of each solution (i.e., AgNO3 and PVP with additives) were simultaneously injected into the hot ethylene glycol with a syringe pump (KDS-200, KD Scientific Inc., Holliston, MA) at a rate of 45 mL/h. The reaction was maintained at 160 °C for additional 60 min. Magnetic stirring was applied throughout the entire synthesis. Synthesis of AgCl:Au Nanowires. The synthesis of AgCl: Au composite nanowires includes two sequential steps: conversion of Ag nanowires to AgCl ones and deposition of Au nanoparticles on the AgCl nanowires. In a typical synthesis, 100-µL dispersion of the as-synthesized Ag nanowires was added to 3 mL deionized (DI) water in a 20-mL scintillation vial with a polyethylene cap (VWR). 50 µL aqueous solution of 1 M PVP was added to the dispersion of Ag nanowires. The dispersion was vigorously stirred with a magnetic stirrer for 5 min before an aqueous solution of 20 mM FeCl3 (Aldrich, which was freshly prepared in order to avoid hydrolysis) was added to the dispersion drop by drop. The reaction solution was stirred for 1 h until the color of the solution became stable. Reaction of the Ag nanowires with adequate FeCl3 transformed them to pure AgCl nanowires. In the next step, 50 µL of 0.1 M NaAuCl4 (Aldrich) aqueous solution was added to the reaction solution containing AgCl nanowires. Further reaction led the color to change from brown to opaque red, indicating the deposition of Au nanoparticles on the AgCl nanowires. The resulting AgCl: Au nanowires were centrifuged to remove excess FeCl3 and NaAuCl4 after reaction and redispersed in DI water. The synthesis was carried out at room temperature and could be scaled up. Characterization. For the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies, a drop of the aqueous suspension of nanowires was placed on a piece of silicon wafer or a carbon-coated copper grid (Ted Pella, Redding, CA), respectively. All the samples were dried in the fume hood. SEM images of the as-grown samples were collected on a Quanta 400F (FEI) microscope operated at 12.5 kV under high vacuum mode. TEM images were recorded on a Philips CM30 transmission electron microscope operated at 200 kV. The corresponding energy dispersive X-ray spectroscopy (EDS) was analyzed with an EDS detector (EDAX) equipped on the TEM microscope. Absorption spectra were taken with a Varian Cary-50 spectrophotometer. Photocatalytic Activity of AgCl:Au Nanowires. Photocatalytic activity of the as-synthesized AgCl:Au nanowires was evaluated with a decomposition reaction of methylene blue (MB, Aldrich) under illumination of a 150-W Quartz halogen lamp (Fiber-lite A3000, Dolan-Jenner Industries), which produces cold white light. In a typical reaction, AgCl:Au nanowires (synthesized from 250 µL dispersion of Ag nanowires) in 4 mL of deinoized (DI) water were mixed with 100 µL of 1 M PVP solution in a 20-mL scintillation vial. One mL of an aqueous solution of MB molecules with concentration of 20 mg/L was then added to the dispersion of the AgCl:Au nanowires. The

Sun

Figure 1. Schematic illustration of the major steps involved in the synthesis of AgCl nanowires coated with Au nanoparticles (i.e., AgCl: Au nanowires) by templating against Ag nanowires. The white lines represent grain boundaries in the AgCl nanowire, highlighting its polycrystallinity. The reactions involved in the synthesis are also highlighted in the black frames.

mixture was stored in the dark for 45 min to reach an adsorption-desorption equilibrium of MB molecules on the nanowires. The solution was then exposed to the lamp to trigger decomposition of the MB molecules. During the decomposition reaction, aliquots of 0.45 mL of the solution were taken out from the reaction system at different times. Each sampling solution was centrifuged at 13000 rpm for 8 min to settle the nanowires at the bottom of the centrifuge tube. The top solution was transferred to a quartz cuvette with optical length of 10 mm for measuring absorption spectrum. The reaction was performed under constant magnetic stirring at room temperature. After reaction, the AgCl:Au nanowires were collected through centrifuge and redispersed in 4 mL of DI water with 100 µL of 1 M PVP solution for catalyzing a new reaction. Results and Discussion The strategy for synthesizing AgCl:Au nanowires by templating against Ag nanowires is illustrated in Figure 1. Major steps include chemical transformation of Ag nanowires to polycrystalline AgCl nanowires followed by deposition of Au nanoparticles on the surfaces of the AgCl nanowires. The synthesis starts with reaction of Ag nanowires with FeCl3 at room temperature:

Ag + FeCl3 ) AgCl V + FeCl2

(1)

Because AgCl has a very small solubility product constant (ksp ) 1.8 × 10-10) at 25 °C, the standard reduction potential of silver couples is reduced from 0.80 V (vs standard hydrogen electrode, SHE) for Ag+/Ag to 0.223 V (vs SHE) for AgCl/Ag in the presence of Cl- ions. On the other hand, the standard reduction potential of the redox couple of Fe3+/Fe2+ keeps constant at 0.771 V (vs SHE) regardless of the concentration of Cl- ions. The difference of the standard reduction potentials between AgCl/Ag and Fe3+/Fe2+ couples provides the driving force for reaction 1. In this redox reaction, each Ag nanowire serves as a template to guide the resulting AgCl to nucleate and condense into a nanowire. The resulting AgCl nanowires exhibit polycrystallinity and larger thickness because of larger lattice constant (5.54 Å) of AgCl in comparison with that (4.09 Å) of Ag. The Ag nanowires can be completely transformed to nanowires made of pure AgCl when the amount of FeCl3 is in large excess of the amount of Ag. In the second step, addition

Conversion of Ag Nanowires to AgCl Nanowires

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Figure 2. SEM images of 100-µL dispersion of the as-synthesized Ag nanowires before and after they reacted with different volumes of an aqueous solution of 20 mM FeCl3 at room temperature: (A) 0, (B) 50, (C) 100, and (D) 200 µL.

of NaAuCl4 to the reaction system leads to the redox reaction between NaAuCl4 and FeCl2 (which is generated from reaction 1)

NaAuCl4 + 3FeCl2 ) Au + 3FeCl3 + NaCl

(2)

due to the standard reduction potential (0.99 V vs SHE) of AuCl4-/ Au being higher than that of Fe3+/Fe2+. The resulting Au atoms nucleate and crystallize on the surfaces of the AgCl nanowires to form Au nanoparticles, leading to the formation of composite nanowires of AgCl decorated with Au nanoparticles. Figure 2 presents a series of SEM images taken from samples containing a same amount of Ag nanowires before and after they react with different volumes of an aqueous solution of 20 mM FeCl3, clearly showing the morphological evolution involved in the conversion of Ag nanowires to AgCl nanowires. As shown in Figure 2A, the Ag nanowires exhibit smooth surfaces and have diameters of 40-50 nm and lengths of 5-10 µm. Reaction of the Ag nanowires with a small amount of FeCl3 (i.e., 50 µL) results in that the surface of each nanowire is decorated with a number of individual AgCl nanocrystals (Figure 2B). The formation of necklace-like morphologies indicates that the reaction 1 initiates at some defect sites of a Ag nanowire rather than homogeneously over its whole surface. Crystalline defects (such as twins, steps, dislocations, etc.) usually exhibit higher surface energies than smooth surfaces to provide active spots for reaction. Once the redox reaction 1 starts, Ag atoms around the defects of the Ag nanowire are oxidized to Ag+ ions,

which immediately react with Cl- ions to nucleate and condense as AgCl nanocrystals at these defect sites. The stem of each composite nanowire shown in Figure 2B is still crystalline continuous, similar to the original Ag nanowires shown in Figure 2A. Further reaction of the Ag nanowires with more FeCl3 (e.g., 100 µL) consumes more Ag to enlarge the AgCl nanocrystals as shown in Figure 2B, resulting in that each Ag nanowire becomes a thicker AgCl one with polycrystallinity. Figure 2C shows a typical SEM image of such AgCl nanowires consisting of multiple AgCl particles assembled along one dimension. Close observation reveals that segments of Ag nanowires (i.e., nanorods highlighted by the red arrows) exist because Ag atoms in the Ag nanowires are not completely converted to AgCl. When excess FeCl3 (e.g., 200 µL) reacts with the Ag nanowires, Ag atoms can be completely converted to AgCl, resulting in continuous AgCl nanowires as shown in Figure 2D. Compositional analysis of the resulting nanowires confirms that they are composed of essentially pure AgCl. The reaction between the Ag nanowires and FeCl3 can also be monitored with ultraviolet-visible (UV-vis) absorption spectroscopy because of intense SPR associated with the Ag nanowires.4,59 Figure 3 presents the spectra taken from samples of the same amount of Ag nanowires before and after they react with different amounts of FeCl3. The Ag nanowires synthesized through the polyol process exhibit a strong absorption peak at ∼380 nm, which corresponds to the transverse SPR mode of the nanowires, along with two shoulder peaks at ∼350 nm and ∼410 nm. The spectrum (black curve) shown in Figure 3 is

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Figure 3. Absorption spectra of the same amount of Ag nanowires before and after they reacted with different volumes of an aqueous solution of 20 mM FeCl3 at room temperature. The solutions for spectral measurements were diluted to the same volume of 4.5 mL.

consistent with typical optical features of Ag nanowires synthesized through different methods.59 Reaction of the Ag nanowires with FeCl3 decreases absorbance of these peaks and eventually results in disappearance of these peaks, indicating that the Ag nanowires are continuously converted to AgCl with enough FeCl3. Meanwhile, a new peak around 270 nm starts to appear due to the generation of AgCl. The samples formed through reaction of the Ag nanowires with 100 µL (green curve) and 150 µL (blue curve) of FeCl3 solution exhibit two weak peaks located around 550 and 610 nm, which are ascribed to the longitudinal SPR mode of the Ag nanorods (i.e., segments of Ag nanowires indicated by the red arrows in Figure 2C). The absorption peak (at ∼270 nm) of AgCl continuously increases with increase of the volume of the FeCl3 solution up to 150 µL because more FeCl3 can convert more Ag atoms of the Ag nanowires to AgCl. The two weak peaks at longer wavelengths essentially disappear when the volume of the FeCl3 solution is higher than 200 µL (see the cyan and magenta curves), indicating the complete conversion of the Ag nanowires to the pure AgCl nanowires as shown in Figure 2D. However, the intensity of the absorption peak of AgCl decreases to a constant value. This switchover in variation of AgCl absorption intensity implies that existence of Ag components (i.e., segments of Ag nanowires) may enhance absorption coefficient of AgCl due to coupling with SPR of Ag. When we were preparing the manuscript, we noticed a paper published in the November 21st (2009) issue of Chemical Communications that reports a similar approach for conversion of Ag nanowires to Ag@AgCl core/shell nanowires.60 The core/ shell structures are not observed in our experiments. The difference might be ascribed to the difference of diameters (40-50 nm in our work vs ∼100 nm in ref 60) of Ag nanowires. The Ag@AgCl core/shell nanowires were reported to have catalytic activity to decompose organic molecules, e.g., methylene orange (MO), under irradiation of visible light although their stability and recyclability were not evaluated. The pure AgCl nanowires synthesized in our work have also been tested to decompose organic molecules, i.e., methylene blue (MB), with illumination of cold white light provided by a Quartz halogen lamp. Once the AgCl nanowires are mixed with MB molecules, the MB molecules are quickly adsorbed on the surfaces of the AgCl nanowires. After the adsorption becomes equilibrium for 45 min, 67.2% of MB molecules are adsorbed according to the change of absorption spectra (calculated from the difference between the black and red curves in Figure 4A). The strong adsorption is consistent with previous studies on

Figure 4. Absorption spectra of the MB molecules at different times since the photocatalytic reaction started under continuous illumination of visible light with assistance of the (A) AgCl nanowires and (B) Ag nanowires. The absorption spectrum of a solution of MB molecules without nanowires recorded after the solution was stored in a glass vial for 45 min is also presented as the black curves to serve as the reference for evaluating adsorption of the MB molecules on the nanowires.

adsorption of dye molecules on AgCl crystals.61 Although illumination can drive decomposition of the free MB molecules in solutions with assistance of the AgCl nanowires (Figure 4A), the strong adsorption behavior is not favorable to catalytic efficiency and stability of the catalysts. As a result, the AgCl nanowires are severely aggregated after the photocatalytic reaction has been performed for 10 min under illumination. Strong adsorption of MB molecules on the surfaces of the AgCl nanowires can be confirmed by the fact that the MB molecules can be released after the AgCl nanowires are stored in pure water. It is worthy of note that the ratio of peak intensity at 660 nm (monomers of MB molecules) and 615 nm (dimers of MB molecules) varies during the decomposition reaction, indicating that decomposition rate of the monomers is larger than that of the dimers. On the other hand, adsorption of MB molecules on the Ag nanowires is very weak (Figure 4B). Only 9% of the MB molecules are adsorbed after the mixture of Ag nanowires and MB molecules is stored in the dark for 45 min. Exposure of the mixture to visible light can also slowly decrease the concentration of the MB molecules (Figure 4B). The intensity ratio between the two major peaks keeps essentially constant, which is different from the kinetic behavior catalyzed with the AgCl nanowires. The decrease of free MB molecules in the mixture containing the Ag nanowires under photoillumination might be ascribed to a photocatalytic reaction different from that shown in Figure 4A or continuous adsorption on the surfaces of the Ag nanowires. Although the decomposition efficiency with the Ag nanowires is low, the weak adsorption of MB molecules on the surfaces of the Ag nanowires indicates that covering the AgCl nanowires with metal nanoparticles may decrease adsorption of MB molecules to enhance efficiency of photocatalytic reactions and stability of catalysts. Herein, we

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Figure 5. Characterization of the as-synthesized AgCl:Au nanowires. (A, B) SEM images of the composite nanowires with different magnifications. (C) EDS spectrum taken from an assembly of a number of the nanowires. Assignments of the peaks are highlighted in red. The inset presents the quantitative composition of different elements calculated on the basis of the Mott/Massey model. (D) Absorption spectrum of a solution of the AgCl:Au nanowires (red curve). In comparison, the absorption spectrum of the bare AgCl nanowires shown in Figure 2D is also presented as the black curve.

coat the as-synthesized AgCl nanowires with Au nanoparticles through the procedure of Figure 1. Surface decoration with Au nanoparticles indeed effectively decreases the adsorption of MB molecules down to 17.7% and enhances their stability during photocatalytic reactions. Parts A and B of Figure 5 show typical SEM images of the AgCl:Au nanowires, clearly revealing the deposition of Au nanoparticles (with brighter contrast) on the surfaces of the AgCl nanowires. EDS of the AgCl:Au nanowires exhibits strong peaks of Ag, Cl, and Au as well as C, Cu, and O (Figure 5C). The signals of C and Cu originate from the TEM grid and that of O is ascribed to the surfactant molecules and/or organic contaminations during sample preparation. The presence of strong peak of Au confirms the deposition of Au nanoparticles on the AgCl nanowires. Quantitative analysis shows that the atomic ratio between Ag, Cl, and Au is 43.6:43.2:13.1, which is close to the theoretic ratio of 3:3:1 calculated from reactions 1 and 2. The offset of Au concentration is caused by the fact that some Fe2+ ions generated from reaction 1 are oxidized back to Fe3+ ions by oxygen (from air dissolved in solution) during reaction 1. The ratio (i.e., 43.6:43.2) between Ag and Cl is slightly higher than 1, indicating that trace amount of elemental Ag still exists in the as-synthesized AgCl:Au nanowires. Deposition of Au nanoparticles on the surfaces of the AgCl nanowires increases absorption coefficient of the composite AgCl:Au nanowires in the visible region because of strong SPR of the Au nanoparticles. Figure 5D compares absorption spectra of the AgCl nanowires (as shown in Figure 2D) before and after modification with Au nanoparticles, clearly showing the appearance of a new peak centered at 580 nm. Intensity of the absorption peak (at ∼260 nm) of AgCl also increases with deposition of Au nanoparticles, confirming that coupling AgCl with plasmonic metal nanopar-

Figure 6. Deconvoluted peaks of the absorption spectrum of the AgCl: Au nanowires as shown in Figure 5D. The inset lists the central positions of the deconvoluted peaks.

ticles can enhance absorption coefficient of AgCl. Deconvolution of the spectral curve (i.e., red curve in Figure 5D) of the AgCl: Au nanowires reveals that it consists of 5 sub-bands as shown in Figure 6. The two bands at longer wavelengths correspond to the strong SPR of the Au nanoparticles: the peak at 595 nm is from individual Au nanoparticles and the one at 676 nm is caused by coupling between different Au nanoparticles.53,54 The major band centered at 285 nm originates from the AgCl nanowires. The band around 205 nm is attributed to the highly dispersed Ag+ ions.62 According to the quantitative ratio between Ag and Cl being larger than 1 (Figure 5C), the as-synthesized AgCl:Au nanowires may include Ag nanodomains, which accounts to the band around 385 nm.63 Although absorbance of the AgCl component is enhanced by coupling with the Au

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Figure 8. Degradation kinetics of MB molecules for 3 successive reactions catalyzed with the same batch of AgCl:Au nanowires under visible-light irradiation. The rate constants derived from the fitting lines according to eq 3 are also presented.

that the decomposition reaction of MB molecules follows the first-order kinetics

-

Figure 7. (A) Absorption spectra of the MB molecules at different times since the photocatalytic reaction started under continuous illumination of visible light with assistance of the AgCl:Au nanowires. (B) (9) Plot of the concentration of MB molecules calculated according to the absorbance at 660 nm as a function of the reaction time; (b) relationship between the logarithmic scale of the normalized concentration (against the concentration at the beginning of the reaction) and the reaction time. The decomposition of MB molecules was driven by visible light at room temperature.

nanoparticles, no new peak is observed due to coupling between AgCl nanowires and the Au nanoparticles. The enhanced absorption of the AgCl:Au nanowires in the visible region is favorable to photocatalytic reactions driven by visible light. Figure 7A shows a series of absorption spectra of the MB solution before and after a mixture of AgCl:Au nanowires and MB molecules is illuminated with white light for different times. Incubation of the mixture in the dark for 45 min leads to 17.7% of the MB molecules adsorbed on the surfaces of the AgCl:Au nanowires, which is much less than the amount (67.2%) adsorbed on the pure AgCl nanowires. The resulting weak adsorption along with strong absorption of visible light makes the AgCl:Au nanowires of a promising class of photocatalysts for decomposing the MB molecules under visible illumination. The monotonic decrease of absorbance indicates that the MB molecules are effectively decomposed under visible light with assistance of the as-synthesized AgCl:Au nanowires. Degradation kinetics of the MB molecules is monitored by the change of their concentration, which is approximately calculated according to the absorbance at 660 nm (i.e., position of the major peak of the MB molecules). The black curve in Figure 7B plots the dependence of the concentration (C) of MB molecules on the reaction time (t) under light illumination, confirming that the MB molecules are decomposed steadily with elongation of time. The average decomposition rate in the first 20 min is 63.9 µg/L · min. As shown in the red curve of Figure 7B, ln(C/C0) and the reaction time exhibits a linear relationship, indicating

dC ) kC dt

(3)

where C is concentration of the MB molecules and C0 represents the concentration of the MB molecules at the beginning of the photocatalytic reaction, t is reaction time, and k is the rate constant. The rate constant over 110 min is determined 0.0225 min-1 according to the slope of fitted red line in Figure 7B. The AgCl:Au nanowires remain stable and well dispersed throughout the photocatalytic reaction and can be recovered via centrifuge for catalyzing new reactions. Figure 8 plots the kinetic curves for decomposition of different MB solutions with the use of the same batch of AgCl:Au nanowires. The catalytic efficiency of the as-synthesized hybrid nanowires does not decrease in the three successive reactions. In contrast, the rate constant slightly increases according to the estimation of reactions up to 50 min. These results indicate that the assynthesized AgCl:Au nanowires represents a promising class of visible-light-driven photocatalysts with good chemical stability and recyclability. Conclusion Ag nanowires can be chemically transformed to polycrystalline nanowires made of AgCl through a reaction of the Ag nanowires with an aqueous solution of FeCl3 at room temperature, in which Ag atoms are oxidized by Fe3+ ions followed by immediate precipitation reaction with Cl- ions. The surfaces of the resulting AgCl nanowires are feasibly decorated with Au nanoparticles through reduction of AuCl4- ions with the Fe2+ ions, which are generated from reduction of Fe3+ ions with the Ag nanowires. The composite AgCl:Au nanowires exhibit good photocatalytic activity toward decomposition of organics, such as methylene blue dye molecules, under illumination of visible light. The surfaces of the Au nanopartricles play the critical role for enhancing optical absorption coefficient of the AgCl: Au nanowires in the visible region as well as for enhancing their stability. It is believed that the as-synthesized AgCl:Au nanowires can be of a class of potential candidate photocatalysts in decomposition of organic pollutants and disinfection of water by using the sunlight. The strategy reported in this work can be easily extended to synthesize composite nanowires made of AgCl nanowires decorated with different metal nanoparticles

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