Synthesis of AG@AgCl Core–Shell Structure Nanowires and Its Photocatalytic Oxidation of Arsenic (III) Under Visible Light
© The Author(s). 2017
Received: 10 December 2016
Accepted: 21 March 2017
Published: 4 April 2017
Ag@AgCl core–shell nanowires were synthesized by oxidation of Ag nanowires with moderate FeCl3, which exhibited excellent photocatalytic activity for As(III) oxidation under visible light. It was proved that the photocatalytic oxidation efficiency was significantly dependent on the mole ratio of Ag:AgCl. The oxidation rate of As(III) over Ag@AgCl core–shell nanowires first increased with the decrease of Ag0 percentage, up until the optimized synthesis mole ratio of Ag nanowires:FeCl3 was 2.32:2.20, with 0.023 mg L−1 min−1 As(III) oxidation rate; subsequently, the oxidation rate dropped with the further decrease of Ag0 percentage. Effects of the pH, ionic strength, and concentration of humic acid on Ag@AgCl photocatalytic ability were also studied. Trapping experiments using radical scavengers confirmed that h+ and ·O2 − acted as the main active species during the visible-light-driven photocatalytic process for As(III) oxidation. The recycling experiments validated that Ag@AgCl core–shell nanowires were a kind of efficient and stable photocatalyst for As(III) oxidation under visible-light irradiation.
As(III) is prevalent in anoxic groundwater, more harmful to human health, more mobile, and less efficiently removed than As(V); thus, pre-treatment by transforming As(III) to As(V) using different types of oxidation technologies is highly desirable for enhancing the immobilization of arsenic and is a prerequisite step for the use of sequent arsenic removal technologies such as coagulation, sorption, and membrane filtration. In recent years, photocatalytic oxidation technology has been considered as a potential and environmentally acceptable technique for As(III) oxidation . Up to now, the photocatalyst TiO2 has been reported to be widely used for the oxidation from As(III) to As(V) in many literatures . Rapid oxidation from As(III) to As(V) can be realized in TiO2 suspensions under UV irradiation . The photocatalysis mechanism of TiO2 involves the generation of valence band holes and conduction band electrons under UV illumination and the subsequent generation of hydroxyl (HO·) and superoxide radical (·O2 −) . However, as we know, TiO2 crystalline phase can only be photoexcited by UV light (λ < 388 nm, a small fraction of solar spectrum) which only accounts for about 5% of the solar light energy owing to its large band gap (3.2 eV). This practically rules out the use of sunlight as a light source.
In order to expand the absorption band of TiO2-based photocatalysts, numerous attempts such as deposition with metal cations or nonmetal ions , decoration with another semiconductor [6, 7], and coating with organic matters  have been devoted to modify the photocatalyst TiO2. By far, TiO2-based nanoparticles functionalized with noble metal Pt nanoparticles  and sensitized with ruthenium dye [10, 11] have been used for As(III) oxidation, and all exhibited excellent photocatalytic oxidation performance for As(III) under visible light. Recently, visible-light driving photocatalysts were firstly used in As(III) oxidation by Hu et al. ; they found BiOI showed its great potential application value in photocatalytic oxidation of As(III). Later, Kim et al. firstly verified that tungsten trioxide (WO3)  was also active under visible light and could successfully oxidize As(III) to As(V).
Considering energy utilization and saving, the investigation of the development of visible-light-driven photocatalysts has currently been considered to be a hot topic. Recently, various Ag-based compounds such as silver/silver halides (Ag/AgX, X = Cl, Br, and I), Ag3PO4 , Ag2CO3, Ag3AsO4, AgxSiyOz , and Agx(SiO4)y(NO3)z  were demonstrated to be a new family of highly efficient visible-light photocatalytic materials. Among the Ag-based visible-light-energized photocatalysts, silver/silver chloride (Ag/AgCl) is one of the most attractive candidates that can meet the requirements of excellent visible-light absorption, high photocatalytic efficiency, and stability, owing to the surface plasmon resonance (SPR) characteristics of metallic silver nanoparticles (NPs) which can promote charge separation/transfer efficiently . It is mainly used in water photolysis, water disinfection, pollutant degradation , carbon dioxide reduction, etc. Core–shell structured nanomaterials have aroused considerable attention in recent years because of their unique and tunable properties by changing either the constituting materials or the ratio of core to shell. Such adjustable structures with suitable shell materials enable these functional materials to meet application requirements in protection, modification, and functionalization of core particles .
One-dimensional (1-D) nanostructure-based catalysts such as wires, rods, and tubes  have been the focus of many recent studies due to their intrinsic large aspect ratio which favors a directional charge transport with a reduced grain boundary. The superior charge transport property of the catalytic species generally plays an important role for the enhancement of photocatalytic performances . Among them, 1-D core–shell nanowires (NW) have recently become of particular interest because the function of the fabricated core–shell nanostructure can be further tuned or enhanced by coating the nanowires with a thin layer of another material and adjusting their core and shell with different materials . As the consequence, considering the specific and excellent properties of 1-D core–shell nanowires, Ag nanowires were first used as chemical templates for the synthesis of 1-D Ag@AgCl core–shell nanowires by Bi and Ye  through an in situ oxidation reaction between Ag nanowires and FeCl3 solution, which exhibited excellent photocatalytic performance for the decomposition of methyl orange (MO) dye under visible-light irradiation. Ag@AgCl core–shell nanowires were also synthesized by etching Ag wires using ionic liquid [Bmim]FeCl4 IL and showed high visible-light photocatalytic ability . Zhu et al. added Ag nanowires into H2O2 solution and then dropped hydrochloric acid into the above aqueous solution to synthesize Ag@AgCl core–shell nanowires . AgCl nanowires decorated with Au nanoparticles by reducing Au precursors with Fe2+ ions could efficiently decompose methylene blue molecules under illumination of white light . All these works illustrated that Ag@AgCl core–shell nanowires exhibited unique photocatalytic ability.
While As(III) photocatalytic oxidation performance using Ag@AgCl core–shell nanowires with different ratios of Ag nanowires:Ag+ was not researched, in this work, Ag@AgCl core–shell nanowires with different ratios of Ag: AgCl were synthesized to realize the rapid oxidation of As(III) under visible-light irradiation. Photocatalytic mechanism in this oxidation process for As(III) and recycling tests of Ag@AgCl core–shell nanowires were also studied.
Twenty milligrams per liter Ag nanowire dispersion was purchased from Xianfeng (China). Polyvinylpyrrolidone (PVP) was purchased from Sigma-Aldrich (USA), FeCl3 and EDTA were purchased from Aladdin (USA), high-performance liquid chromatography (HPLC) grade methanol was purchased from Fisher (USA) for HPLC analysis, and tetrabutylammonium hydroxide was purchased from PerkinElmer (America). One thousand milliliters of As(III) standard solution (1000 mg/L) was prepared by dissolving 1.3203 g of As2O3 in the minimum amount of 4.0 M NaOH solution and then adjusting pH to 3.0 with 1.0 M H2SO4 solution. All solutions and subsequent dilutions were prepared using deionized water from a scientific nanopure water purifier (Thermo fisher, America) with a resistivity of less than 0.055 μS/cm.
Morphological analysis was performed on a Hitachi SU8010 field-emission scanning electron microscope (FE-SEM) (Japan) with an acceleration voltage of 2 kV.
The X-ray diffraction (XRD) data was detected via a AXS D8-Focus X-ray diffractometer using Cu Kα radiation (Bruker, Germany). The operated conditions were controlled at 40 kV and 40 mA with a scan step width of 0.01°, and the scan range was 20°–90°.
X-ray photoelectron spectroscopy (XPS) measurements were carried out on a RBD upgraded PHI-5000C ESCA system (PerkinElmer, America) with Al X-ray source operating at 250 W. All the binding energies were referenced to the C 1 s peak at 284.8 eV of the surface adventitious carbon.
To detect concentration of As(III) and As(V), an ELAN DRC II inductively coupled plasma mass spectrometry (ICP-MS) (PerkinElmer, America) equipped with an atomizer and a spray chamber was used. The ICP-MS normal operating parameters were as follows: RF power 1100 W, lens voltage 7.25 V, nebulizer gas flow rate 0.98 L/min, auxiliary gas flow rate1.2 L/min, and plasma gas flow rate 15.00 L/min. Arsenic species were separated by Series 200 HPLC (PerkinElmer, America) with an automatic sample injector and directly introduced into ICP-MS. A C8 chromatographic column (PerkinElmer, America) was used with the mobile phase containing 1 mM tetrabutylammonium hydroxide, 0.05 mM dipotassium EDTA, and 0.05% methanol (pH 6.8).
Preparation of Ag@AgCl Core–Shell Nanowires
Ag@AgCl core–shell nanowire dispersions with different mole ratios of Ag:AgCl were synthesized using an in situ oxidation method. In a typical procedure, 250 μL dispersion of 20 mg/L Ag nanowires was added to 10 mL deionized water in every 25-mL beaker. One hundred fifty microliters 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 different volume of 20 mM FeCl3 solution was dropwise injected into the dispersion. The adding volume of 20 mM FeCl3 solution which was freshly prepared in order to avoid hydrolysis was 0.5, 1, 1.5, 2.0, 2.2, and 4.0 mL, respectively. The reaction solution was stirred for 1 h until the color of the solution became stable. The reaction of the Ag nanowires with different volume FeCl3 transformed them to Ag@AgCl core–shell nanowires with different mole ratios of Ag:AgCl. The resulting Ag@AgCl core–shell nanowires were centrifuged to remove excess FeCl3 and PVP after reaction. Every kind of Ag@AgCl core–shell nanowires was dispersed in 0.5 mL of deionized water after being washed by water and alcohol. The synthesis was carried out at room temperature and could be scaled up.
The quality of Ag@AgCl core–shell nanowires synthesized with 250 μL Ag nanowire dispersion and 2.2 mL FeCl3 solution was equal to 6 mg, and the mole ratio of Ag nanowires:FeCl3 used in the preparation of this kind of Ag@AgCl core–shell nanowires was 2.32:2.20. Subsequently, this kind of synthesized Ag@AgCl core–shell nanowire dispersion was prepared in a large scale according to the above process, then dried in the air at 50 °C for 8 h, and grinded to obtain the solid powder of Ag@AgCl core–shell nanowires with the mole ratio of Ag nanowires:FeCl3 = 2.32:2.20. The dry solid was used in the subsequent batch tests for influence factors, mechanism, and recycling.
Photocatalytic Oxidation for As(III)
To study the effect of the mole ratio of Ag:AgCl on photocatalytic oxidation for As(III), 0.5 mL suspension of Ag@AgCl core–shell nanowires with different mole ratios of Ag:AgCl for As(III) was respectively added in the 20 mL aqueous solution of 2.0 mg/L As(III).
In the subsequent batch experiments for influence factors, mechanism, and recycling, 6 mg solid of the Ag@AgCl core–shell nanowires synthesized with the mole ratio of Ag nanowires:FeCl3 = 2.32:2.20 was added in the 20 mL aqueous solution of 2.0 mg/L As(III) in every test.
Results and Discussion
Physiochemical Characterization of Ag@AgCl Core–Shell Nanowires
Morphology Study (SEM)
Phase and Compositional Study (XRD)
X-ray Photoelectron Spectroscopy (XPS)
The XPS spectrum of Ag 3d in Fig. 4b showed that the 3d5/2 and 3d3/2 signals were located at 373.151 and 367.051 eV, respectively, and the splitting of the 3d doublet was about 6.0 eV. However, the two bands could not be divided into different peaks and were both attributed to the Ag+ of AgCl, which indicated that the peaks of metallic Ag centered at 368.73 and 374.91 eV were not contained in this XPS spectra, which meant that only AgCl crystals existed on the surface of Ag@AgCl core–shell nanowires.
Photocatalytic Oxidation Experiments for As(III)
Photocatalytic Oxidation of Ag@AgCl Core–Shell Nanowires Synthesized with Different Mole Ratios of Ag Nanowires:FeCl3 for As(III)
Because of the strong photosensitive property of AgCl phase without Ag, under the sunlight, the photogenerated electron combines with an Ag+ ion to form an Ag0 atom; ultimately, a cluster of silver atoms is formed within a AgCl particle upon the repeated absorption of photons. So AgCl is instable under sunlight and seldom used as a photocatalyst. However, when a certain amount of Ag nanoparticles are deposited onto AgCl particles, electron–hole separation occurs smoothly in the presence of Ag nanoparticles, so metallic Ag plays an important role on the photoinduced stability of Ag/AgCl composite .
From the SEM images (Fig. 2), it could be observed that the surface area of Ag@AgCl core–shell nanowires became larger and larger with the mole ratio of AgCl. As we know, a large specific surface area as a photocatalytic material is conducive to adsorption of pollutants . The investigation of Matsui et al. clearly demonstrated that the catalytic activity of ZrO2/carbon cluster composite materials increased with the increase of their surface areas . Generally, the performances of semiconductor oxide photocatalysts are dependent on the surface area, mesoporosity, crystallinity, morphology , and active facets exposed . The exposed high-index facets (1 1 1) of the AgCl crystals in the Ag@AgCl core–shell nanowires also caused high photocatalytic oxidation efficiency for As(III).
Kinetic Study of As(III)
In this case, C 0 = 2.669 × 10−5 mol/L.
Pseudo-first-order kinetic parameters of As(III) photocatalytic oxidation
Quantity of FeCl3 (mL)
Effects of pH and Ionic Strength
When NaSO4 concentrations in As(III) solution was 0.02 and 0.06 M, the conversion percentages of As(III) were 75.55 and 66.75%, respectively. It is known that Na+ is an alkaline metal ion and at its maximum oxidation state; therefore, it will not compete as hole scavenger and does not show distinct effect on photocatalytic reaction .
Effect of Humic Acids
Where E vb is the VB edge potential, X is the absolute electronegativity of the semiconductor, E 0 is the energy of free electrons on the hydrogen scale (ca. 4.5 eV), and E g is the band gap energy of the semiconductor. Thus, the CB and VB potentials of AgCl are estimated to be −0.06 and 3.19 eV, respectively .
To better understand the decrease reason of photocatalytic oxidation efficiency during recirculation process, the samples of Ag@AgCl core–shell nanowires after 20 cycles of photocatalytic oxidation for As(III) were tested by XRD and SEM. It was found that the diffraction peak of Ag0 at 38.2° with enhanced intensity could be observed in the XRD pattern of the sample after the twentieth cycle (Fig. 9b). This revealed the continuous transformation from AgCl into Ag0 with the leaching of chloridion (Cl−) during the recycling process. Such kind of transformation led to a deviation from the optimum ratio of AgCl to Ag with the best photocatalysis performance, which should be responsible for the continuous and slight decrease of photocatalysis activity in the recycle runs. What is more, the formation of Ag nanoparticles resulted in the decrease of the interface area between Ag and AgCl, which was another reason for the decrease of photocatalytic activity. This phenomenon was also found in other Ag@AgCl nanocomposites . The SEM image (Fig. 9c) showed that the core–shell nanowires cracked during the photocatalytic activity.
The 20th recycled Ag@AgCl nanowires were collected and reacted with FeCl3 solution again; the SEM image showed that Cl-afresh core–shell nanowires structure was rebuilt, but cracked more seriously which might be caused in the process of ultrasonic dispersion, and became thicker because of the agglomeration (Fig. 9d). The The XRD patterns showed that the mole ratio of Ag:AgCl in regenerated Ag@AgCl core–shell nanowires returned the optimum again (Fig. 9b). The As(III) oxidation rate of regenerated Ag@AgCl core–shell nanowires was increased to 40.2% (Fig. 9a), which proved Cl-afresh of recycled Ag@AgCl core–shell nanowires could efficiently prolong the lifetime of prepared photocatalysts.
In order to exploit more visible-light-responding photocatalysts for As(III) oxidation, Ag@AgCl core–shell nanowires synthesized via a controllable oxidation reaction with different mole ratios of Ag nanowires:FeCl3 in solution were selected to conduct the photocatalytic oxidation experiment for As(III). Ag@AgCl core–shell nanowires showed excellent photocatalytic activity toward As(III) oxidation and the mole ratio of Ag nanowires:FeCl3 had obvious influence on its photocatalytic ability. Photocatalytic oxidation rate of As(III) was favored at high pH and could be promoted by humus acid. High concentration of Na2SO4 in solution will slightly inhibit the photocatalytic oxidation reaction. With quenching agent, the holes and ·O2 − were proved to be the main active materials in the photocatalytic oxidation process for As (III). The photocatalytic ability of Ag@AgCl core–shell nanowires gradually decreased with recycle times, and Ag@AgCl core–shell nanowires after recycling could be efficiently regenerated by Cl-afresh. The prepared Ag@AgCl core–shell nanowires were proved to be an efficient and relatively stable visible-light-induced photocatalyst for As(III) oxidation in water with high humic substance.
This work was financially supported by National Natural Science Foundation of China (No: 40902070), Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (2017, No: CUG110414), Nature Science Foundation of Hubei Province (No: 2016CFB609), and Experimental Technology Program of China University of Geosciences (Wuhan) (No: SJ-201509).
YQ prepared the nanomaterials, carried out the structural analyses of the samples, conducted the photocatalytic experiments, and drafted the manuscript. ZT and YW took part in the experimental section. YC and YL proposed, designed, and guided the experiments. YC edited the manuscript. All the authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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