Impacts of Morphology, Natural Organic Matter, Cations, and Ionic

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Impacts of Morphology, Natural Organic Matter, Cations, and Ionic Strength on Sulfidation of Silver Nanowires Yinqing Zhang, Junchao Xia, Yongliang Liu, Liwen Qiang, and Lingyan Zhu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03034 • Publication Date (Web): 14 Nov 2016 Downloaded from http://pubs.acs.org on November 23, 2016

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Environmental Science & Technology

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Impacts of Morphology, Natural Organic Matter, Cations, and Ionic

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Strength on Sulfidation of Silver Nanowires

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Yinqing Zhang, Junchao Xia, Yongliang Liu, Liwen Qiang, Lingyan Zhu*

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Key Laboratory of Pollution Processes and Environmental Criteria (Ministry of Education), Tianjin

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Key Laboratory of Environmental Remediation and Pollution Control, College of Environmental

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Science and Engineering, Nankai University, Tianjin 300350, P. R. China

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ACS Paragon Plus Environment


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ABSTRACT

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Silver nanowires (AgNWs) are being widely utilized in increasing number of consumer

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products, which could release silver to aquatic environments during the use or washing process, and

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receive growing concerns on their potential risks to bio-organisms and humans. The present study

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demonstrated that AgNWs mainly experienced direct oxysulfidation by reacting with dissolved

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sulfide species (initial S2- concentration at 1.6 mg/L) to produce silver sulfide nanostructures under

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environmentally relevant conditions. Granular Ag2S nanoparticles were formed on the surface of the

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nanowires. The sulfidation rate constant (kAg) of AgNWs was compared with those of silver

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nanoparticles (AgNPs) at different particle sizes. It was found that the kAg positively correlated with

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the specific surface areas of the silver nanomaterials. Natural organic matter (NOM) suppressed the

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sulfidation of AgNWs to different extents depending on its concentration. Divalent cations (Mg2+ and

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Ca2+ ions) substantially accelerated the sulfidation rates of AgNWs compared to monovalent cations

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(Na+ and K+ ions). At the same ionic strengths, Ca2+ ions displayed the highest promoting effect

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among the four metallic ions.

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Keywords: silver nanowires, sulfidation, nanomaterials, natural organic matter, electrolyte type,

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ionic strength

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INTRODUCTION

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Silver nanomaterial is one of the most widely utilized nanomaterials in the past few decades.

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Due to their unique optical, catalytic, sensing properties, and antibacterial effects, nearly one third of

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the products currently registered in the nano-product database claim to contain nanosilver.1,2 All

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these products may release silver to aquatic environments during the use, washing or disposal

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processes.3,4 Growing concerns have been raised on the potential adverse effects of silver

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nanomaterials on organisms and humans. Many studies demonstrated the toxicities of silver

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nanoparticles (AgNPs) to a variety of organisms, such as plants, invertebrates, vertebrates,

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microorganisms, and even human cells.5-8 The toxicities of AgNPs are partly explained by the release

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of Ag+ ions from AgNPs in water, which is dependent on many factors, including physical, chemical

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and biological transformations and water chemistry conditions.7,9 Sulfidation is an important

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transformation process of silver nanomaterials in the aquatic environment because of the extremely

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low solubility of Ag2S (Ksp = 6.31 × 10−50). Sulfidation may severely limit the concentration of

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soluble Ag+ and eventually reduce the overall bioavailability and toxicities of silver nanomaterials to

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aquatic organisms. 10 The products and mechanisms of spherical AgNPs sulfidation were investigated

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under controlled laboratory conditions. 11-13 Kim et al. reported the presence of Ag2S nanoparticles in

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the sludge of a municipal wastewater treatment plant (WWTP),14 and the fate of engineered AgNPs

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within WWTPs has since been investigated in several studies, which demonstrated AgNPs could be

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partly or totally converted to Ag2S during different treatments of the WWTP.15-17

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The physicochemical properties of silver nanomaterials, such as shape,18 size

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and surface

coating,20 would affect their bioavailability and reactivity, but the underlying mechanisms remain 3



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unclear. Current synthesis methods enable the production of differently shaped and sized silver

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nanomaterials. Although AgNPs are mostly produced, silver nanowires (AgNWs) have found

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increasing applications in different technologies, for example, in circuits of nanoelectronic products

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transformations which would affect their fate, transport, and toxicities.23-25 AgNWs are different from

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spherical AgNPs in shape, size and coating, and it is presumed that the sulfidation kinetics of

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AgNWs is different from that of AgNPs. Several recent studies investigated sulfidation of AgNWs in

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atmosphere upon exposure to H2S 26 and in cell culture media.

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in aquatic environments has not been well discovered. Natural organic matter (NOM) is ubiquitous

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in complex aquatic systems. Previous studies indicated that the steric effects provided by the

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adsorbed NOM on AgNPs surface could inhibit the aggregation and help to stabilize AgNPs.29-31 In

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addition, there are abundant electrolytes, especially divalent cations like Ca2+ and Mg2+, in aquatic

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environments, and they could greatly affect the stability and bacterial inactivation of AgNPs.32

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However, it remains unclear if these factors, such as NOM, electrolyte types, and ionic strengths,

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could affect the sulfidation of AgNWs in aquatic environments.

and solar cells.22 Once released to the aquatic environment, AgNWs may also undergo further

27,28

However, sulfidation of AgNWs

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In this study, sulfidation of AgNWs was investigated under different water chemistry conditions.

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The objectives were to understand the impacts of morphology and size of Ag nanomaterials on

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sulfidation, and to disclose the effects of water chemistry, including NOM, electrolyte types, and

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ionic strengths, on sulfidation under environmentally relevant conditions. A combination of X-ray

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powder diffraction (XRD) spectrometry, X-ray photoelectron spectroscopy (XPS), UV−vis

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spectrometry, transmission electron microscopy (TEM) and Energy-filtered transmission electron

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microscopy (EFTEM) were applied to characterize the sulfidation process of AgNWs. We also used

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time-resolved sulfide concentration measurement to monitor the sulfidation process and constructed

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AgNWs sulfidation kinetic model under experimental conditions. The sulfidation kinetics of AgNWs

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was compared to that of AgNPs to understand the impacts of morphology, size and coatings.

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MATERIALS AND METHODS

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Chemicals and Reagents

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AgNO3 (>99.9%), polyvinylpyrrolidone (PVP, MW = 58000), ethylene glycol (EG, anhydrous,

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99.9%), and Na2S·9H2O (99.99%) were purchased from Aladdin Chemistry Co. Ltd. (Shanghai,

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China). Suwannee river humic acid (SRHA) was from the International Humic Substances Society

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(IHSS, St. Paul, MN). SRHA was prepared at 1 g/L in ultrapure water and dissolved overnight on a

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rotator and then filtered through a 0.45 µm membrane filter. The dissolved organic carbon (DOC)

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was determined with a Phoenix 8000 total organic carbon analyzer (Tekmar-Dohrmann, Cincinnati,

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OH). The other reagents were of analytical grade and used as obtained without further purification.

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Ultrapure water (18.3 MΩ) produced with a Milli-Q Gradient system (Millipore, Billerica, MA) was

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used throughout the experiments.

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Synthesis of AgNWs.

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AgNWs were synthesized following a modified polyol process.33 In a typical synthesis, 3.5 mL

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of EG solution containing 11.0 g of AgNO3 was added drop-wisely into 5.5 mL of EG solution

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containing 1.6 g of PVP. The reaction mixture was refluxed at 160 oC for 90 min. After diluting the

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reaction mixture with acetone five times in volume, AgNWs were collected by centrifugal

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ultrafiltration (Amicon Ultra-15 100 kD, Millipore, MA). AgNWs were washed with ethanol and 5



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then deionized water, and this washing process was repeated twice. The stock suspension of AgNWs

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was stored at 4 °C in dark for later use.

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AgNWs Sulfidation Experiments.

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To quantitatively determine the reaction rates, a method was developed to monitor the reaction

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dynamics of AgNWs sulfidation by a time-resolved measurement of free sulfide depletion. At

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predetermined times, soluble sulfide (H2S(aq)/ HS-/ S2-) in the reaction system was measured using a

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modified methylene blue spectrophotometric method.34 In a typical experiment, 0.2 mL of 2.5 mM

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Na2S solution was added to 9.0 mL of ultrapure water in a 10-mL glass tube with screw cap. The

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starting Na2S concentration to initiate the sulfidation of AgNWs was 0.05 mM (pH of the reaction

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solution was 8.7), at which AgNWs could be present at a relatively low concentration to simulate the

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environmental conditions. Then 0.8 mL of AgNWs suspension at a predetermined concentration was

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added to obtain different Ag/S ratios ranging from 0.5 to 4. A series of test tubes containing the

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reaction mixture were rotated at 10 rpm facing an 800 W Xe lamp to simulate sunlight. At each

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predetermined reaction time, one tube was sacrificed for total sulfide measurement. The solution was

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separated from the colloids by centrifugal ultrafiltration, and an aliquot of 2.0 mL of the filtrate was

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added in a 50-mL test tube which contained 10.0 mL of zinc acetate (5.0 wt%) and sodium acetate

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(1.3 wt%) solution (the mixture was solution A). The color development regent (solution B) was

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prepared by mixing 200 mL of 1.0 wt% N, N- dimethyl- p-phenylenediamine monohydrochloride

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with 200 mL of concentrated sulfuric acid, and then cooled down and diluted to 1000 mL in dark

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brown bottle with distilled water. 5.0 mL of solution B and 0.5 mL of 10.0 wt% ammonium iron (III)

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sulfate solution were mixed with solution A immediately. After sterilizing for 10 min, the mixture

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was diluted to 50 mL with distilled water. The absorbance of the testing solution at 665 nm was

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measured on a Shimadzu UV-2600 spectrophotometer. The total sulfide concentration was calculated

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according to a linear calibration curve constructed from freshly prepared Na2S standard solutions.

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The sulfidation kinetic experiment was repeated three times at each concentration of AgNWs.

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To investigate the impacts of NOM on sulfidation rate of AgNWs, SRHA at five different

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concentrations was added in the reaction solution described above at Ag/S=2. To investigate the

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effects of cations on AgNWs sulfidation, solution of NaNO3, KNO3, Ca(NO3)2 and Mg(NO3)2 at 1

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mM were added in the reaction solution at Ag/S=2, respectively. In order to study how ionic strength

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affects sulfidation of AgNWs under environmentally relevant conditions, solutions of NaNO3, KNO3,

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Ca(NO3)2 and Mg(NO3)2 at different concentrations were added in the reaction solutions.

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Accompanying these experiments, control experiments were conducted in which all components

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were added except AgNWs. As shown in Figure S1, only marginal loss of sulfide was observed in

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the blank controls. All the sulfidation rate constants in the presence of the electrolytes were corrected

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by the loss of sulfide in the corresponding blanks. All the sulfidation experiments were conducted on

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the same equipment as described in the previous paragraph.

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Characterization of AgNWs and the Sulfidized Products of AgNWs.

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Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS)

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analysis were performed on a JEOL 2100F microscope (Japan) operating at 200 kV. The sample was

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ultrasonically dispersed in ethanol, dropped subsequently on a copper grid coated with ultrathin

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carbon film, and the solvent was allowed to evaporate. Zeta potential of AgNWs was measured based

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on the electrophoretic mobility obtained using electrophoretic light scattering on a Malvern Zetasizer

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Nano ZS90 analyzer (UK). AgNWs suspension was diluted with ultrapure water to 10 mg/L, and the

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results were reported as the average of three measurements. The compositions and phases of AgNWs

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and the sulfidation products were identified by X-ray powder diffraction (XRD) spectrometry on a

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Bruker D8-Advanced diffractometer (German) with Cu Kα radiation (λ = 1.5418 Å). The samples

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were washed with DI water repeatedly to remove residues of reagents, and blown with Ar overnight

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to obtain dry powder. X-ray photoelectron spectroscopy (XPS) was conducted on a Kratos Axis Ultra

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DLD spectrometer (Japan) with a monochromated Al X-ray source with depth profile and

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angle-resolved capabilities. The spectra were fitted using Gaussian curves. UV−vis spectra from 300

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to 800 nm were obtained using a Shimadzu UV-2600 spectrophotometer (Japan). Energy-filtered

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transmission electron microscopy (EFTEM) was obtained with a FEI Tecnai G2 F20 instrument (OR,

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USA) equipped with a Gatan imaging filter (GIF), and the samples were washed three times with

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deionized water to remove the unreacted sulfide. The concentration of dissolved Ag+ ions from silver

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nanomaterials was measured by inductively coupled plasma mass spectrometry (ICP-MS). All the

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silver nanostructures were removed using Amicon centrifugal ultrafiltration (Ultra-15 3 kD,

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Millipore, MA) at 2050 g for 20 min (detailed information is given in Supporting Information),

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then the clear filtrates were analyzed with Ag standards on an Agilent 7700 ICP-MS (CA, USA).

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Elemental analysis (N and S) was carried out on a EuroEA3000 elemental analyzer (EuroVector,

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Italy).

and

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AgNWs Dissolution Experiments.

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The oxidized dissolution rate of AgNWs was quantified by diluting AgNWs stock suspension

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with ultrapure water to a desired concentration, and then measuring the released Ag+ ions by ICP-MS.

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All the dissolution experiments were conducted on the same equipment as the sulfidation

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experiments at room temperature. The dissolution rate constant was estimated by fitting the AgNWs

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dissolution kinetics to a first-order equation: –d[Ag+]/ dt= kd × [Ag+] , where [Ag+] represents the

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concentration of dissolved silver ions, and kd is the dissolution rate constant of AgNWs.

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RESULTS AND DISCUSSION

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AgNWs Preparation and Characterization.

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X-ray diffraction (XRD) pattern of the synthesized AgNWs (Figure S2A, Supporting

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Information) illustrated that there was only one single crystalline phase. The prominent Bragg

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diffraction can be indexed to fcc silver (JCPDS file 87-0717), but not Ag(I) species such as AgCl and

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Ag2O. The diffraction peaks from the {111} (nanowire ends) and {200} (nanowire body) faces, as

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well as the remaining features, were in consistent with those observed for AgNWs in other

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studies.35,36 The EDS results supported that the nanowires were composed of pure Ag and confirmed

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the absence of impurities in the synthesized products.

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A bright-field TEM image of the as-prepared AgNWs is shown in Figure S2B. It is clear that the

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product was in the form of nanowires. As measured from several representative TEM images, the

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diameter of the AgNWs of 300 nanowires was 40.0 ± 4.6 nm (Figure S2C), and the length was 2.7 ±

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0.9 µm. The zeta potential of the AgNWs was −19.7 ± 2.6 mV, which was consistent with the steric

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stabilization of the AgNWs.

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Characterization of the Sulfidized Products of AgNWs.

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The UV-vis spectrum (curve a in Figure 1A) of the aqueous suspension of pure AgNWs

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exhibited two peaks: one at 380 nm, which was corresponding to the absorbance of the relatively 9



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long AgNWs, and a shoulder peak at ∼355 nm, which was attributed to transverse plasmon

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resonance mode of AgNWs.37 After 1 h of reaction (curve b in Figure 1A), the intensity of the

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absorption peak at 380 nm decreased significantly, while that at 355 nm disappeared, suggesting that

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AgNWs were remarkably consumed during the reaction time. As the reaction continued, the peak

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(λmax) at 380 nm gradually red shifted to 389 nm, while the absorbance decreased at the same time.

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These implied that the concentration of AgNWs decreased while the particle size increased due to the

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transformation of AgNWs. This is similar to previous studies on the change of UV-vis absorption for

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AgNPs.30,38

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To identify the new phase(s) of AgNWs during sulfidation process, XRD analysis was

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performed. The XRD patterns in Figure 1B indicated that Ag2S in acanthite phase (JCPDS file

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09-0422) was formed due to reaction of AgNWs with Na2S. The diffraction peaks of Ag2S increased

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as a function of reaction time, indicating gradual formation of Ag2S, which was accordance with the

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UV-vis results.

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The XPS spectra of the sulfidation products are shown in Figure S3, and the binding energies

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are shown in Table 1. The binding energies of C, N, and O elements were similar to those of the

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original AgNWs, indicating that PVP coating was still on the surface of the nanowires after 8h

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reaction. The binding energies of Ag 3d5/2 and Ag 3d3/2 were 368.0 and 374.0 eV, respectively, while

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those of S 2p3/2 and S 2p1/2 were 161.2 and 162.3 eV. These values were consistent with the reported

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data for Ag2S.39 The XPS data confirmed that no sulfites or sulfates were present, implying that the S

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elements in the products were mainly in the form of Ag2S. The binding energies of the O 1s peaks at

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531.9 and 532.9 eV were significantly higher than that in silver oxide (528.6 eV), suggesting that

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there was negligible silver oxide formed during the reaction.26,39

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Kinetics and Mechanisms of AgNWs Sulfidation.

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As shown in Figure 2A, in the absence of AgNWs, the loss of sulfide was marginal. However,

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in the presence of AgNWs, the soluble sulfide concentration decreased gradually with the reaction

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time at four Ag/S ratios, implying that AgNWs experienced sulfidation.

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All sulfide depletion curves in Figure 2A are suitable for limited quantitative kinetic analysis

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after correcting the sulfide loss obtained in the absence of AgNWs. As shown in Figure S4, there was

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a negative log-linear relationship between sulfide concentration and reaction time, which could be

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described by a rate law: -d[sulfide]/dt = kobs× [sulfide]. Therefore, the observed rate constant kobs

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could be estimated using the following equation:

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ln([sulfide]/[sulfide]0) = - kobs × t

(1)

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where [sulfide]0 and [sulfide] represent the concentration of total sulfide in the solution at reaction

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time 0 h and t h, respectively. The observed rate constant kobs increased from 0.028 to 0.15 h-1 as the

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Ag/S ratio increased from 0.5 to 4. Since the initial concentration of Na2S in the sulfidation solution

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was the same, the results indicated that higher initial AgNWs concentration would favor sulfidation.

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This was consistent with a previous study conducted by Liu et al., who investigated the sulfidation

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process of spherical AgNPs in aqueous solution.12

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Figure 2B shows that kobs was proportional to the initial concentration of AgNWs as expected

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for a heterogeneous reaction. According to the kinetic law reported for the sulfidation of AgNPs by

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Liu et al. 12, the following kinetic law was applied for the sulfidation of AgNWs:

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-d[sulfide]/dt = (kAg × [AgNWs]0 + khomogen) × [sulfide]

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(2)


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where [AgNWs]0 represents the initial concentration of AgNWs, and kAg and khomogen are the rate

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constants for AgNWs in sulfidation and the homogeneous sulfide depletion rate constant,

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respectively. As the initial sulfide concentration was constant, fitting the observed sulfidation rate

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constant with AgNWs concentration using Eqn. (2) gave the results: kAg = 0.56 ± 0.02 mM-1•h-1 and

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khomogen = 0.0082 ± 0.0009 h-1 (R=0.99, p

Impacts of Morphology, Natural Organic Matter, Cations, and Ionic

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