Protecting the Nanoscale Properties of Ag Nanowires with a Solution

Article Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Protecting the Nanoscale Properties of Ag Nanowires with a Solution-Grown SnO2 Monolayer as Corrosion Inhibitor Yang Zhao,†,‡,¶ Xijun Wang,§,¶ Shize Yang,∥ Elisabeth Kuttner,⊥ Aidan A. Taylor,# Reza Salemmilani,∇ Xin Liu,○ Martin Moskovits,† Binghui Wu,† Ahmad Dehestani,◆ Jian-Feng Li,‡ Matthew F. Chisholm,∥ Zhong-Qun Tian,‡ Feng-Ru Fan,*,† Jun Jiang,*,§ and Galen D. Stucky*,†,# †

Department of Chemistry and Biochemistry, University of California Santa Barbara, Santa Barbara, California 93106, United States State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, P. R. China § Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), CAS Center for Excellence in Nanoscience, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230026, P. R. China ∥ Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, United States ⊥ BASF SE, Ludwigshafen am Rhein 67063, Germany # Materials Department, University of California Santa Barbara, Santa Barbara, California United States ∇ Department of Mechanical Engineering, University of California Santa Barbara, Santa Barbara, California 93106, United States ○ School of Resources and Materials, Northeastern University at Qinhuangdao, Qinhuangdao 066004, P. R. China ◆ California Research Alliance (CARA), BASF Corporation, Berkeley, California 94720-1460, United States

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‡

S Supporting Information *

ABSTRACT: The chemical reactivity and/or the diffusion of Ag atoms or ions during thermal processing can cause irreversible structural damage, hindering the application of Ag nanowires (NWs) in transparent conducting films and other applications that make use of the material’s nanoscale properties. Here, we describe a simple and effective method for growing monolayer SnO2 on the surface of Ag nanowires under ambient conditions, which protects the Ag nanowires from chemical and structural damage. Our results show that Sn2+ and Ag atoms undergo a redox reaction in the presence of water. First-principle simulations suggest a reasonable mechanism for SnO2 formation, showing that the interfacial polarization of the silver by the SnO2 can significantly reduce the affinity of Ag to O2, thereby greatly reducing the oxidation of the silver. The corresponding values (for example, before coating: 17.2 Ω/sq at 86.4%, after coating: 19.0 Ω/sq at 86.6%) show that the deposition of monolayer SnO2 enables the preservation of high transparency and conductivity of Ag. In sharp contrast to the large-scale degradation of pure Ag-NW films including the significant reduction of its electrical conductivity when subjected to a series of harsh corrosion environments, monolayer SnO2 coated Ag-NW films survive structurally and retain their electrical conductivity. Consequently, the thermal, electrical, and chemical stability properties we report here, and the simplicity of the technology used to achieve them, are among the very best reported for transparent conductor materials to date.

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INTRODUCTION

have emerged as highly promising constituents of nextgeneration transparent electrodes, especially for flexible and wearable electronics.9−12 However, rapid surface oxidation and

Transparent conductors with low resistance and high flexibility are indispensable in many optoelectronics devices such as organic light-emitting diodes (OLED), photovoltaic devices, and displays.1−8 Metal nanowires (NWs), especially Ag NWs, exhibiting superior optoelectrical performance and flexibility, © XXXX American Chemical Society

Received: July 6, 2019

A

DOI: 10.1021/jacs.9b07172 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Journal of the American Chemical Society

brownish-red Ag seed solution and cooled at room temperature to facilitate additional growth. Five mL EG, 5 mL 25 mg/mL AgNO3, and 5 mL 20 mg/mL PVP (MW = 360 000) were added into a 50 mL Teflon liner. 0.5 mL 100 mM NaCl and 0.5 mL Ag seed solution were added. The solution was mixed thoroughly then loosely capped and held in the preheated 130 °C ovens for 6 h, then quenched by immersing the Teflon liner in an ice water bath. The synthesized pristine silver nanowire has a diameter of 50 ± 10 nm, the number is given by measuring 200 single nanowire which are randomly selected from SEM images. Preparation of SnO2 Coated Ag NWs. 2.5 mL of pristine Ag nanowire solution was precipitated using 30 mL of acetone. The precipitated wires aggregated at the bottom, the supernatant was discarded and the aggregated wires redispersed in 10.0 mL ethanol. 0.5 mL ammonium hydroxide (28%) was added to dissolve the byproduct AgCl particles in the solution and settled for 2 min. The solution was added to 40 mL ethanol and centrifuged at 3600 rpm for 10 min, followed by washing with ethanol twice to remove excess EG and PVP. Finally, the Ag NWs was collected and suspended in 40 mL of ethanol. Next, 300−1000 μL T2E or SnF2 was added to the Ag NWs solution with gentle shaking for even dispersion and left at room temperature overnight undisturbed. Finally, the Ag@SnO2 solution was centrifuged and washed with ethanol twice before the material was further characterized. Fabrication and Characterization of Transparent Films and Stability Test under Harsh Environment. To make a transparent conductive film, a dilute suspension of nanowires in ethanol was made via sonication. The thin film was constructed by filtering down the nanowire dispersion onto a hydrophobic polytetrafluorethylene (PTFE) membrane filter (Sartorius 11806−47-N, pore size 450 nm, diameter 47 mm) via vacuum filtration. The film was transferred to a glass slide (Corning 2947−75 × 50) by pressing the back side of the membrane and then removing the membrane gently. Then resulting transparent film on glass was annealed in a conventional oven (Thermo Lindberg Blue M) at 140 °C (in ambient air) for 30 min to improve junction contact before measurements. The transmittance measurement was carried out at 550 nm on an Agilent 8453 UV−vis Spectrometer. 4-point resistance measurements were performed to measure the sheet resistance of the nanowire-based films using a Keithley 2400 SourceMeter. For each film, the quoted resistance is the average of 9 measurements at 9 random points, which yielded a relative standard deviation of ≤5.0%. For thermal stability testing, the freshly made nanowire films were heated in air using a Muffle furnace with temperature of 300 °C for 30 min. For antioxidation testing, the prepared films were placed in a sealed tube furnace (Carbolite 3216) under an extreme oxidation environment made up of an O2/O3 mixture (O3 conc. = 5.0%, V:V) with 150 °C heating for 1 h. An ozone generator connected to an ultrahigh pure oxygen tank was used to generate a continuous ozone stream. For antisulfidation testing, the prepared films were placed in a sealed glass desiccator with an internal pressure of less than 0.1 Torr by vacuuming. Then H2S/N2 gas mixture (50% v/v) were introduced into the desiccator until a standard atmospheric pressure and the films exposed to H2S atmospheres under room temperature for 5−10 h. Structural Characterization and Spectral Analysis. Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS)-Scanning transmission electron microscopy (STEM) elemental mapping were performed on a Thermo Scientific Talos transmission electron microscope. High-resolution annular dark-field (ADF) imaging and electron energy-loss spectroscopy (EELS) analyses were carried out in aberration-corrected scanning transmission electron microscope Nion Ultra STEM100 at 100 kV and Nion Ultra STEM200 at 200 keV in Oak Ridge National Laboratory. The beam convergent semiangle was 30 mrad and EELS collection semiangle was 48 mrad. The energy scale in EELS spectrum is carefully calibrated to get correct quantification results. The microstructures of the samples were characterized by scanning electron microscopy (SEM, FEI Nova Nano 650 FEG) and X-ray diffraction (XRD, Empyrean). X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD XPS equipped with a nonmonochromated Mg X-ray anode, and a

decomposition of the Ag NWs reduce their conductivity dramatically, preventing their use in many applications. To resolve this issue, several strategies have been developed to improve the thermal and chemical stability of Ag NW-based thin films. Controllable growth or deposition of a layer of a heterogeneous material on the surface of Ag NWs is a promising alternative. Metallic oxides and metals such as TiO2,13,14 ZnO,15 Au,9 and Ni,16 which possess promising thermal and/or electrical stability, have been used as protective passivation layers. Traditional semiconductor technology utilizes atomic layer deposition (ALD) to grow epitaxial monolayers of heterogeneous materials. Recently, this technique has been used to coat ZnO15 and Al2O317 layers on Ag NW-based films. But ALD is expensive and time-consuming. Solution-phase overlayer growth methods, such as colloidal syntheses and underpotential deposition (UPD)18 have also been developed for the overlayer growth of various hybrid nanostructures, for example, metal−metal and metal−metal oxide. However, accurately controlling layer thickness, especially when growing monolayers of oxide on metal in solution, remains a significant challenge. Here, we describe a straightforward solution synthesis of monolayer SnO2 coated, large Ag nanowire arrays under ambient conditions. By introducing trace amounts of Sn2+ to an Ag NW solution, the Ag atoms on the outermost layers undergo a redox reaction with Sn2+ to form a monolayer of oxide. This protective layer bestows excellent thermal and chemical stability to the Ag NWs enabling them to resist oxidation and sulfurization without significantly reducing their transmittance and conductivity. Through systematic characterization, we have identified the protective Sn species to be SnO2 and explain the growth mechanism in term of a redox reaction taking place between surface active Ag atoms and Sn2+. Firstprinciples simulations were carried out to explore the formation of monolayer SnO2 and the ability of the nanowires to resist oxidation. The trace Sn2+ treatment benefits from its low cost, ease of fabrication, and scale-up capability. It outperforms both in terms of thermal and chemical stability, such procedures as ALD and postprocessing (Figure S1 of the Supporting Information, SI). We believe this to be the first promising technique for protecting Ag NWs using an all-solution-based method that can be directly and easily upscaled to a device fabrication method with commercial application potential.

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EXPERIMENTAL SECTION

Materials. All the syntheses were carried out using commercially available reagents without further purification. Silver nitrate (AgNO3), polyvinylpyrrolidone (PVP, MW = 40 000), Polyvinylpyrrolidone (PVP, MW = 360 000), Sodium bromide (NaBr), Tin(II) 2-ethyl hexanoate (92.5−100.0%), Tin(II) fluoride (99%) were purchased from SigmaAldrich. Sodium Chloride (NaCl), acetone (Certified ACS), and absolute ethanol (Gold Shield, 200 proof) were purchased from Fisher Scientific. Ethylene glycol (EG, ≥ 99%) and Sodium hydroxide (NaOH) were purchased from VWR. Ammonium hydroxide (NH3· H2O, 28.0−30.0%) was purchased from EMD Millipore. Synthesis of Ag Nanowires (Ag NWs). Highly uniform Ag NWs were prepared in high yield and satisfactory reproducibility using an established seed-mediated synthesis. The procedures were inspired by a previously reported method,19 which was used with major modification. All solutions were prepared in EG as solvent. Five mL EG, 1 mL 100 mM PVP (MW = 40 000), 136 μL 25 mg/mL AgNO3, and 50 μL 220 mM NaBr were first thoroughly mixed in a 20 mL glass vial and tightly capped. Each of the solutions was then placed in a preheated 130 °C oven. After 15 min, the vial was removed from the oven containing B


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Figure 1. Morphological characterizations of trace amounts of Sn2+-treated Ag NWs. a, Schematic of the step followed in the wet-chemical Sn2+ treatment and photograph of as prepared Ag@SnO2 dispersion. b, c, Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of Sn2+-treated Ag NWs, respectively. A transparent layer of approximate 3 nm is covered uniformly on the surface of Ag NWs. Ag NWs have an average diameter of 50 nm and length of 50 μm. d, e, High-resolution TEM (HRTEM) annular dark field (ADF) image and bright field image (BF) respectively. The Sn2+-treated Ag nanowire is clearly seen to have two surface layers (I&II) and Ag wire core (III). f, Electron energy loss spectroscopy (EELS) spectra were collected from three regions, in which region I showed only carbon K edge signal, region II showed a mixture of Sn M edge spectra, and region III showed primarily the Ag M edge. Insert: The EELS region of the Sn M edge is compared to the Sn metal EELS signal.

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monochromated Al anode) was used to analyze the binding energy of the samples. The surface bonding information were provided by Raman spectroscopy (LabRam Aramis Raman microscope, Horiba, Kyoto, Japan). Mö ssbauer spectra were collected on Topologic 500A spectrometer at room temperature using 119Sn in the BaSnO3 as an emitter. Calculation Methodology. Calculations of geometric and electronic structures were performed at the density functional theory (DFT) level implemented by the Vienna ab initio Simulation Package (VASP),20−23 with the frozen-core all-electron projector augmented wave (PAW) model24 and Perdew−Burke−Ernzerhof (PBE) functions.25 The kinetic energy cutoff for the plane-wave expansion of the electronic wave function was set to 400 eV. The force and energy convergence criterion were set to 0.01 eV/Å and 10−5 eV, respectively. A Gaussian smearing of 0.1 eV was applied for optimization. A k-point grid with 8 × 8 × 8 Gamma-centered mesh was chosen for the Ag bulk unit structure. For all the supercells, a corresponding number of kpoints was used to keep the k-mesh spacing constant across different structures. Charge distribution analysis was examined with the Bader charge analysis model.26 The climbing image nudged elastic band (CINEB) method was applied to search the transition state structures.27 To avoid artificial interaction between adjacent molecules due to the periodic boundary conditions, the 3 × 3 × 1 supercell of Ag(100) surface model with a vacuum spacing of ∼15 Å was used for all the simulations. All possible adsorption configurations with corresponding adsorption energies of SnF2, Sn(OH)2, O2, and O3 molecules are listed in Figures S9−12, respectively, and the most stable ones were used for subsequent calculations. The SnO2/Ag interfacial model was built by combining the SnO2 monolayer cluster from the most stable facet of SnO2(110)28 and three Ag(100) layers. The peripheral oxygen atoms of SnO2 are all saturated by hydrogen atoms.

RESULTS AND DISCUSSION

Preparation of Uniform SnO2-Coated Ag NWs. The Ag NWs used in this work were synthesized in a two-step seedmediated growth method (methods), in which the preprepared Ag seeds selectively grow into nanowires at a relatively low temperature. Uniform Ag NWs with an average diameter of 50 nm and length of 50 μm (Figure S2a) were obtained and subjected to the Sn2+ treatment. Figure 1a shows a schematic of the coating procedure in which a trace quantity of Sn2+ was added to the Ag NW suspension, yielding an atomically thin SnO2 layer on the surfaces of the Ag NWs. As shown in Figure 1b, c, and Figure S2b, 2c, SEM and TEM images reveal no discernible morphological change in the nanowires following the Sn2+ treatment. At this point, the surface of each Sn2+-treated Ag NW is covered with a 2 nm thick amorphous polymer layer, the residual polyvidone (PVP) used as a surfactant during the synthesis of the Ag NWs. To better understand the layer structure and determine the location and thickness of the SnO2, we used electron microscopy imaging and high-resolution element analysis to characterize the surface layer of the core− shell structure of the Ag NWs in detail. The comparisons between the high-resolution scanning transmission electron microscopy (HRSTEM) annular dark field (ADF) image (Figure 1d) and bright field image (BF) (Figure 1e) clearly show the three-layer structure of the Sn2+treated Ag NW, since the STEM-ADF image contrast is directly related to the average atomic Z number of the material.29−31 The lowest contrast (region I) corresponds to the surface polymer residue. The medium contrast (region II) corresponds to the Sn layer; while the highest contrast corresponds to the pure inner Ag nanowire. We used electron energy loss (EELS) spectrosC


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Figure 2. Identification of the Sn species formed on the surface of Ag after Sn2+ treatment. a, Raman spectra of untreated, Tin(II)2-ethylhaxanoate (T2E)-treated and SnF2-treated Ag NWs. The vertical dashed lines indicate the positions of 4 major Raman peaks offering useful information. b, O 1s XPS data of pristine Ag NWs and SnF2-treated Ag NWs, showing some new oxygen-containing bonds following the Sn2+ treatment. c, Experimental and fitted room-temperature 119Sn Mössbauer spectra of as-prepared SnF2-treated Ag NWs; the reference is BaSnO3. The vertical dashed line indicates the position of isomer shift.

treated Ag NWs used here were all covered with a monolayer of SnO2 by precisely controlling the concentration of reactants in the solution. Identification of Sn Species on the Surface. Raman spectroscopy was used to analyze both pristine and Sn2+-treated Ag NWs to gain a deeper understanding of the composition of Sn species formed on the surface. To determine if the product formed depended on the source of tin, Sn2+ was introduced using one of two compounds, SnF2, and Tin(II) 2-ethylhexanoate (T2E). Both compounds produced similar results (Figure 2a). Compared with pristine Ag NW, the Ag−O stretching vibration (233 cm−1) is substantially reduced after the Sn2+ treatment. Prior to the tin treatment, this stretching mode arose from the coordination of the PVP molecules to the Ag surface through the nonbonding electrons of the oxygen atom in the carbonyl.33 Following the tin treatment, most of these Ag−O bonds are broken and likely replaced by Ag bonding to tin species leaving other PVP vibrational modes, such as those at 1760 and 2922 cm−1 attributed to the CO groups34 and CH2 asymmetric stretching vibration33 in PVP almost unchanged, indicating that PVP molecules remain on the surface, but not directly coordinated to silver, consistent with the results of electron microscopy. (Figure S5). Meanwhile, a new and intense peak at 580 cm−1 appears after the Sn2+ treatment (Figure 2a). These peaks appeared identically after using the two Sn2+ sources, likely suggesting that this new vibration originates from the Sn-containing species. The O 1s X-ray photoelectron spectroscopy (XPS) spectra of pristine Ag and Sn2+-treated Ag NWs are fitted to peaks at

copy to analyze the components of the three different regions, as shown in Figure 1f and Figure S3a, b; these spectra results agree with the ADF contrast interpretation. While region I shows only carbon K edge signal, region II shows a mixture of Sn M edge and O K edge, region III shows the dominating Ag M edge. The EELS signal consisting primarily of the Sn M edge is extracted and compared with the Sn metal EELS signal in Figure 1f (inset). The splitting of the top peak indicates the presence of oxygen.32 HAADF-STEM coupled with energy-dispersive X-ray (EDX) spectroscopy was further used to reveal the distribution of Ag, Sn, and O in the Sn2+-treated Ag NW. The signal distribution obtained by EDX line intensity mapping acquired across a representative single nanowire (Figure S4) indicates that Sn is distributed in the coating of the inner Ag nanowire, being dominant in the inner layer closest to the Ag NW core, and showing a lower concentration in the sub-1 nm region exterior to the edge of the inner Ag nanowire; whereas O and C arising from the residual PVP layer are present more or less homogeneously ranging from the outer exterior portion of the coated nanowire to within a few nanometers of the Ag surface. As a control experiment, STEM ADF and BF images of the pristine Ag nanowire without Sn2+ treatment were also collected (Figure S3c, d), showing the two-layer structure consisting of an Ag metal region and the residual polymer region. By comparing the results just described, we confirm that elemental Sn is not present in, or dispersed on, the PVP polymer layer, but exists only on the Ag surface. Furthermore, we calculate that the trace amount of Sn2+ added to the Ag surface, is not more than a monolayer thick (Note S1). Unless otherwise stated, the Sn2+D


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Figure 3. Investigation of the mechanism of forming SnO2 monolayers on the Ag. a, Raman spectra of pristine Ag NWs and Sn2+-treated Ag NWs processed in solvents containing various controlled amounts of H2O. The vertical dashed line indicates the position of the representative Sn−O vibration peak in SnO2 monolayers. b, Raman spectra of pristine, SnF2-treated, and two Sn4+ sources-treated Ag NWs; Tin(IV) I: Tin(IV) isopropoxide, Tin(IV)II: Tin(IV) 2-ethylhexanoate diisopropoxide. c, Schematic showing the redox mechanism of Sn2+ treatment process, leading to protective SnO2 monolayers formed on the surface of Ag. d, Computed energy potential profiles of each elementary step for the formation of threecoordinate oxygen from Sn(OH)2, showing the possible reaction pathways of the SnO2 network formation on the surface of Ag.

various binding energies (Figure 2b). After Sn2+ treatment, the peak at 532 eV assigned to the interaction between the carbonyl oxygen of PVP chain and Ag is obviously weakened.35 This suggests that the adsorption of PVP on the Ag surface is likely hindered by other species thereby reducing the bonding between the CO group and the silver. Peaks and features that belong to metallic Ag, including Auger peaks and loss features, are not altered before and after Sn2+ treatment (Figure S6). A chemically distinct O 1s XPS peak appears at 530 eV after Sn2+ treatment. Its very low binding energy suggests an oxygen atom bonded uniquely to a metal other than Ag. The survey scan (Figure S6a) shows that the oxygen coverage increased from 15.9% to 26.2% after Sn2+ treatment, which again strongly suggests that O is bonded to Sn, and the calculated ratio between O and Sn is 2:1 (Figure S6e). Therefore, the peak at 580 cm−1 in the Raman spectrum can reasonably be assigned to a vibration of Sn−O, as also observed in SnO2 nanoparticles, and contributes to the Raman surface phonon mode that dominates at the nanoscale.36,37 The strong Sn−O vibrational Raman band is undoubtedly benefiting from the close presence of the nanosilver surface, i.e. it is surface-enhanced.38 We, therefore, conclude that Sn2+ replaces oxygen adsorbed on the surface of Ag after penetrating the PVP layer, eventually forming a SnO2 monolayer. Elemental Sn on the surface of Ag NWs was also investigated using synchrotron X-ray absorption fine structure (XAFS). However, the strong Ag signal overwhelmed the much weaker signal from the trace amount of Sn which, consequently, was not observed. The local environment and oxidation state of monolayer Sn was also investigated using synchrotron 119Sn Mössbauer

spectroscopy. The 119Sn Mössbauer spectrum (Figure 2c) obtained from Sn2+-treated Ag NWs could be fitted by a single Lorentzian, indicating that all of the Sn was equivalent. Two quantities were obtained using MS: the isomer shift (IS) and the quadruple splitting (QS). The former is sensitive to the valence state and atomic spacing, while the latter is sensitive to the charge symmetry surrounding the target atoms.39,40 The IS values of Sn4+ (4d105s05p0) and Sn2+(4d105s25p0) lie in the range: − 0.4 to 2.0 mm/s, and 2.3 to 4.5 mm/s respectively (relative to BaSnO3).41,42 The Sn2+-treated Ag showed an IS shift at 0.12 mm/s, which is characteristic of Sn4+. Peaks associated with Sn2+ were not observed. A QS parameter ∼0.65 mm/s, suggests octahedrally coordinated Sn4+−O6, rather than a defective Sn4+−O5 possessing an oxygen vacancy.39 These observations suggest that coating with Sn2+ to produce a monolayer of SnO2 on the surface of the Ag NWs, resulted in a valence change in the tin atom. Mechanism of the Spontaneous Formation of Monolayer SnO2. A series of experiments were carried out to better understand the mechanism of the SnO2 monolayer formation. The intense Raman peaks at 580 cm−1 attributed to the Sn−O vibration were observed on various Ag surfaces treated with Sn2+ (Figure S7), showing that PVP or other surfactants do not significantly affect the outcome. We believe that the formation of SnO2 occurs as a result of the hydrolysis of trace quantities of water by the ethanol solvent. To confirm this, we precisely controlled the concentration of water in the ethanol and noted the resulting intensities of the Raman peaks. And, indeed, no Sn−O Raman peaks (580 cm−1) were observed on using anhydrous ethanol. Moreover, the Raman signal is observed to E


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First-principles simulations were carried out to elucidate this mechanism. The formation process of SnO2 was modeled at the density functional theory (DFT) level. We considered the transformation from SnF2 to Sn(OH)2 via SnF2 + H2O → Sn(OH)2 + 2HF (Figure S14). The computed energy barrier of the replacement of the first F− by OH− (0.53 eV) is not prohibitively high at room temperature. Considering the accelerating effect of the spontaneous hydrolysis process of SnF2 (SnF2 ↔ SnF+ + F−),45 the actual barrier is likely even lower. The barrier for replacing the second F− by OH− (0.31 eV) can be easily overcome at room temperature. Subsequently, taking Sn(OH)2 molecule as the basic unit, we explored the possible reaction pathways of the SnO2 network formation (Figure 3d). Two adsorbed Sn(OH)2 molecules can easily bond by sharing one hydroxyl ion as a connecting bridge with a negligible barrier of 0.09 eV, followed by proton transfer from the hydroxyl ion to the adjacent hydroxyl to form a water molecule and a Sn−O−Sn connection bridge with a relatively low barrier of 0.36 eV. The subsequent binding of another Sn(OH)2 molecule to the bridge oxygen is an exothermic process without energy barrier. Thus far, we suggest the formation of a three-coordinate O bridge, which is the basic constituent unit of the SnO2 network. The formation of subsequent three-coordinate bridge oxygens eventually create the SnO2 network, leading to its ability to physically shield the silver metal nanowire surface from oxygen and other reactive molecules. As mentioned above, only Ag atoms with unsaturated surface coordination can oxidize Sn2+ to form monolayer SnO2 without forming a multilayer structure. In order to test this conclusion theoretically, we computed the Sn (OH)2 binding energy on monolayer and bilayer SnO2 (Figure S15). The binding of an extra Sn(OH)2 at the surface of monolayer SnO2 (ΔEbind = 5.75 eV) has a much higher binding energy than that on the bilayer (ΔEbind = 5.04 eV), suggesting that the binding of additional Sn(OH)2 becomes progressively more difficult with increasing SnO2 thickness. Antidegradation Performance of Protected Ag. A series of transparent conducting films were produced on thin glass slides using vacuum filtration (Figure S16). Importantly, the corresponding values (before coating 17.2 Ω/sq, after coating 19.0 Ω/sq at 86%) show that the deposition of monolayer SnO2 enables the preservation of the high transparency and conductivity of Ag. Considering the good conductivity of SnO246 and the untrathin thickness of monolayer coating, the slight influence on resistance by the Sn2+ treatment is reasonable. We further investigated the stability of the Sn2+-treated Ag NWs in various harsh environments (Figure S17). As shown by SEM images and Xray powder diffraction (XRD) characterization (Figure S18), the pristine Ag NWs were completely melted into nanoparticles when heated to 300 °C for 30 min. In contrast, Sn2+-treated Ag NWs survived undamaged under the same condition. Moreover, after annealing, the transparent film exhibits a lower sheet resistance. Since the silver is unlikely to sinter after being coated with SnO2 and the decrease in the sheet resistance is rather small, we suggest that the most likely reason for this lower sheet resistance is a decrease in the junction resistance due to the degradation of the PVP polymer layer during the annealing at 300 °C. In addition to exhibiting thermal stability, the treated nanowires, unlike the pristine Ag NWs, also exhibit good chemical stability. For example, films were exposed to a 5% (v/ v) O3/O2 atmosphere at 150 °C for 60 min. After such an oxidation treatment, the surfaces of the pristine Ag-NWs were

increase gradually on adding increasingly greater quantities of water (Figure 3a) to the anhydrous ethanol. Specifically, these results rule out O2 in the solvent as a contributor to the formation of the tin oxide associated with the 580 cm−1 spectrum. This was also the conclusion reached in a control experiment in which O2 was scrupulously removed from ethanol, and performing all operations in an Ar-filled glovebox. This approach yielded the same Raman spectrum (Figure S8). The requirement of Sn2+ was also confirmed by using two Sn4+ precursors. As shown by Raman spectra and thermal stability comparison (Figure 3b and Figure S18c), Sn4+ does not form a protective layer with the observed Raman spectrum, confirming the importance of Sn2+ in the coating process. On the basis of these results, we propose the following mechanism (Figure 3c). The Sn2+ is hydrolyzed to form −Sn− OH and Sn(OH)2, which, when adsorbed onto the Ag surface, undergo a redox reaction to form a SnO2 layer. The corresponding half-reaction reduction electrode potentials of Ag+ and SnO2 are, therefore, as follows,43 SnO2 + 4H+ + 2e− → Sn 2 + + 2H 2O

Ag + + e− → Ag

E 0 = −0.094 V

E 0 = 0.80 V

(1) (2)

yielding an overall potential of 1.6872 V as shown in eq 3, confirming the spontaneity of the overall redox reaction of Ag+ ions, supporting the possible electron transfer between unsaturated silver and Sn2+. 2Ag + + Sn 2 + + 2H 2O → 2Ag + SnO2 + 4H+

E 0 = 1.6872 V

(3)

We first considered the possibility that Ag2O might be formed on the surface of silver and then take part in the reaction. However, as shown previously, the peaks and features in core level XPS Ag 3d belong to metallic Ag (Figure S6b), and not Ag+, showing no evidence for the existence of Ag oxides before the Sn coating. Moreover, as included in the nanowire washing procedures, we use ammonium hydroxide to dissolve AgCl nanoparticles, during which, other Ag+ species such as Ag2O are also easily removed from the surface. That suggests the reasonable hypothesis that, it is the coordinatively unsaturated surface Ag atoms, i.e., those that do not have complete nearest neighbor Ag atom coordination in the case of surfactant-capped Ag nanowires, that act as the oxidizing agent. In this model, the role of surface Sn2+ is primarily to reduce surface silver active atoms to zerovalent silver. A trace quantity of H2O in the solvent provides the oxygen for Sn to form SnO2, which results in monolayer SnO2 that is formed on the surface of the Ag nanowire. In order to demonstrate the reactivity of Ag surface, Bader charge analysis,44 which is an intuitive method to qualitatively partition a charge density grid into the volumes of each atom, was performed on a Ag(100) model containing 5 layers of Ag atoms. In our case, the number of valence electrons of Ag atom is 11, and the other electrons are treated by using projector augmented wave (PAW) pseudopotentials. The computed average electron number of the first-layer Ag is 11.03, indicating that the net electron number should be 11.03− 11 = 0.03, which is much larger than other layers (Figure S13). The net electron number of the second layer is much lower because the electrons are captured by the first layer. These findings indicate that the outermost surface Ag atoms have a stronger ability to capture electrons compared with inner atoms due to their unsaturation. On the basis of this model, the outmost two layers of Ag atoms on the silver nanowire could likely serve as the oxidant that oxidizes Sn2+ to Sn4+. F


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Figure 4. Thermal annealing, antioxidation, and antisulfurization tests of untreated and treated Ag NWs. a, Ag 3d XPS data of pristine and Sn2+-treated Ag before and after O3/O2 oxidizing process (5% O3/O2 treatment at 150 °C for 1h). b, Sheet resistance variation for pristine Ag-NW films and Sn2+treated Ag-NW films before and after different stability tests. BN, AN: Before and after annealing at 300 °C, 30 min in air. BO, AO: Before and after oxidation in 5% O3/O2 at 150 °C for 1h. BS, AS: Before and after sulfurization in 50% H2S, 50% N2 at room temperature for 5−10h. c, Computed charge density differences of SnO2(110)/Ag(100) interface structure. Differential charges were computed as Bader charge differences between the hybrid and the isolated SnO2 (110) and Ag(100) structures. Yellow and blue bubbles represent electron and hole charge distribution. d, e, Computed O2 adsorption energy and dissociation energy barrier at Ag(100) surface with neutral or one extra electron/hole charge injection, respectively.

completely oxidized and converted to AgO nanoparticles,47 thereby losing electrical conductivity (Figure 4a, b, and Figure S19). In contrast, Sn2+-treated Ag-NWs exhibited excellent antioxidant activity with only a 27% increase in membrane resistance (R/Ro). No significant changes were observed in the surface morphology of nanowires. As evidenced by the XPS data (Figure 4a), even after oxidation, the Ag surface of the NWs remained metallic Ag with high conductivity. The results presented above summarize the remarkable ability of the SnO2 monolayer to protect Ag nanowires. In addition to the isolation effect caused by forming the SnO2 network, further simulations suggest that the charge polarization at the SnO2/Ag(100) interface can also help prevent oxidation of the Ag NW surface. Driven by the work function (WF) difference between SnO2 (6.98 eV) and Ag(100) (4.20 eV) (Figure S21), electrons tend to flow from the Ag (higher potential level) to SnO2 (lower potential level) (Figure S22) with holes remaining on Ag surface, resulting in the effective interfacial polarizations illustrated in Figure 4c. The injection of extra holes further influences O2 adsorption and dissociation as shown in Figure 4d and 4e, respectively. An extra hole charge decreases the O2 adsorption energy from 0.48 at neutral to 0.32 eV, while the O2 dissociation barrier is increased from 0.95 to 1.17 eV (Figure S23). These indicate that the polarized charge can significantly reduce the affinity of Ag to O2 and thereby prevent Ag from oxidation. Since the experiment is carried out by using O3 instead of O2, we also considered the adsorption and dissociation of O3. Results show that the interfacial polarization

can also decrease the adsorption of O3 on Ag from 2.19 at neutral to 2.08 eV (Figure S24) and increase the dissociation barrier from 0.59 to 0.81 eV (Figure S25). It is worth noting that the induced charge polarization is due mainly to the redistribution of electron−hole density at the interface, which changes to the atomic coordination very little. To illustrate this point, we compared the average Bader charge of the Sn ion in SnOH2 molecule (Sn2+: + 1.19) and SnO2 bulk structures (Sn4+: + 2.28), with the charge of Sn ion in SnO2(110)/Ag(100) structure (+2.18). It was found that the oxidation state of Sn ion in SnO2 over Ag(100) surface remained +4. Evaluation of the antisulfidation capacity of nanowires was carried out by exposing the films to a gas mixture (H2S/N2, 1:1 v/v) for several hours. Under such conditions, the pristine Ag NWs reacts with H2S to form Ag2S with rapid degradation of the Ag NWs. The pristine Ag NW film membrane resistance is increased 18-fold after 5 h of exposure, and it becomes completely nonconductive after 10 h. An identical experiment was performed on the Sn2+-treated Ag NWs. The membrane resistance increased slightly 1.8-fold after 5 h and 2.7-fold after 10 h (Figure 4b, Table S1, and Figure S18). Following the development of an understanding of the mechanism based on the first set of experimental results, we tested the proposed mechanism further by carrying out additional experiments in which the thickness of the SnO2 layer was varied. As described in Note S1 in the SI (Manual calculation of the quantity of monolayer Sn 2+ coverage on the surface of Ag), our stoichiometry calculation determined that 0.3 μL of T2E is G


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Journal of the American Chemical Society needed to form one monolayer. When 0.1−0.2 μL T2E is introduced, the silver nanowires did not survive the thermal annealing test, as shown in Figure S20a, b, implying that a submonolayer SnO2 did not protect the silver surface effectively. When 0.9 μL T2E is used, no large-scale changes were observed in the TEM image (Figure S20c) except for the appearance of small particles in the PVP layer. We believe that after appropriately coating the Ag surface, excess Sn2+ reduces residual Ag+ in the PVP layer left over from the nanowire synthesis, forming these small Ag nanoparticles. This is a reasonable conclusion since only the surface Ag atoms on the Ag NW have the oxidizing ability required to form the SnO2 coat. This also agrees with our simulation, as mentioned previously, which suggests that binding additional Sn(OH)2 becomes progressively more difficult with increasing SnO2 thickness. From the above, the self-limiting reaction between the surface silver atoms and Sn 2+ inherently limits layer growth. Accordingly, the trade-off between the preservation of optoelectronic performance and effective protection suggests that a monolayer of Sn2+ achieves the desired balance of properties. Additionally, a series of experiments were carried out with various Sn2+ compounds to evaluate the protective effect (Table S2). In general, the Sn2+ compounds dissolved in ethanol were found to protect Ag NWs by the same mechanism, indicating that the method is universal with promising commercial applications.

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CONCLUSIONS In summary, we report a reliable, low-cost, and scalable approach for fabricating monolayer SnO2 protected Ag NWs. By using a series of characterization techniques and firstprinciples simulations, we showed that Sn2+ is oxidized by unsaturated coordination of active Ag on the surface of nanowires in the presence of water. Unlike pristine Ag NWs that are easily oxidized and sulfurized, coated nanowires can survive in various harsh environments, exhibiting excellent thermal and chemical stability. We believe this methodology and fabrication approach to be broadly applicable to various Ag surfaces. Further investigation of the application of this wetchemistry coating process can be expected to impact favorably on the development of commercial metal nanowire-based transparent and flexible electronics. Moreover, as Ag-SnO2 is the first example of the overlayer growth of monolayer oxides in a liquid phase based on the principle of a redox reaction, the present results inspire future exploration and developments of new stable metal catalysts and two-dimensionally coated materials with better performance efficiencies and lifetimes.

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ASSOCIATED CONTENT

S Supporting Information *

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.9b07172. Calculation of the quantity of monolayer Sn2+ coverage on the surface of Ag; schematic showing protected Ag NW against corrosion and other damage as transparent films after Sn2+ treatment, indicating the relative effectiveness of various methods; SEM and TEM images of pristine and Sn2+ treated Ag NW; STEM ADF and BF images showing the regions of collected EELS signal and pristine Ag nanowires without Sn2+ treatment; the signal distribution obtained by EDX line intensity mapping acquired across a

representative single nanowire; Raman spectra in longer wavenumber of pristine and Sn2+ treated Ag NWs; further investigation by XPS regarding the bond modes at the interface; Raman spectra of Sn2+ treatment on various agents capped Ag surface; Raman spectra of Ag and Sn2+treated Ag using O2 -removed solvent; computed adsorption configurations for SnF2 to the top, bridge, and hollow sites of Ag(100) surface together with corresponding adsorption energies; computed adsorption configurations for Sn(OH)2 to the top, bridge, and hollow sites of Ag(100) surface together with corresponding adsorption energies; computed adsorption configurations for O2 to the top, bridge, and hollow sites of Ag(100) surface together with corresponding adsorption energies; computed adsorption configurations for O3 to Ag(100) surface together with corresponding adsorption energies; model structure of Ag(100) with the average net number of electrons per atom of each layer obtained from Bader charge analysis; computed energy potential profiles of each elementary step for the replacement of first and second F− by OH− on SnF2 at Ag(100) surface; computed binding configuration and binding energy of Sn(OH)2 on the monolayer and bilayer SnO2; fabrication of Ag NWs to transparent conductive films and sheet resistance vs transparency of Ag-NW films with different loadings; schematic showing the harsh treatment apparatus of AgNW based films; SEM images and XRD patterns of pristine and Sn2+-treated Ag NWs before and after annealing; SEM images of pristine and Sn2+-treated Ag NWs after oxidation, sulfurization tests; SEM images of 0.1−0.2 μL of T2E treated Ag NWs after 300 °C annealing and TEM image of 0.9 μL Sn2+ treated Ag NW; computed potential energy surfaces of SnO2(110) and Ag(100) along the z-axis; schematic illustration of the charge transfer mechanism driven by the work function difference between SnO 2 (110) and Ag(100) surfaces; computed energy potential profiles of each elementary step for O2 dissociation at Ag(100) surface with neutral or one extra electron/hole charge injection; adsorption energy and dissociation energy barrier of O3 at Ag(100) surface with neutral or one extra electron/hole charge injection; computed energy potential profiles of each elementary step for O3 dissociation at Ag(100) surface with neutral or one extra electron/hole charge injection; sheet resistance of pristine and Sn2+-treated Ag-NW based films before and after stability tests; list of Sn (II) compounds that may have protective effects; comparison of various methods to fabricate protected Ag NWs as transparent conductors, in terms of production atmosphere, cost advantages, scale-up capability, thermal and chemical stability, and optical performance preservation after coating (PDF)

AUTHOR INFORMATION

Corresponding Authors

*[email protected], *[email protected], *[email protected] ORCID

Xijun Wang: 0000-0001-9155-7653 Shize Yang: 0000-0002-0421-006X Reza Salemmilani: 0000-0003-1152-6828 H


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Martin Moskovits: 0000-0002-0212-108X Binghui Wu: 0000-0003-4015-9991 Jian-Feng Li: 0000-0003-1598-6856 Zhong-Qun Tian: 0000-0002-9775-8189 Feng-Ru Fan: 0000-0001-6474-471X Jun Jiang: 0000-0002-6116-5605 Galen D. Stucky: 0000-0002-0837-5961 Author Contributions ¶

These authors contributed equally to this work.

Notes

The authors declare the following competing financial interest(s): A relevant patent application is pending, listing inventors E.K., G.D.S., F.R.F., Y.Z., L.Y.C., and B.H.W. The other authors declare no competing interests.

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ACKNOWLEDGMENTS We thank N. F. Zheng, W. Z. Wu, and T. Mates for helpful discussion. This work was financially supported by the BASF Corporation through the California Research Alliance Program (CARA), grant No. 040532. The work of X.W. and J.J. was supported by the National Key Research and Development Program of China (2018YFA0208603) and the National Science Foundation of China (21633006). The work of S.Z.Y. and M.F.C. was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences, and Engineering Division, and performed in part as a user project at the ORNL Center. The MRL Shared Experimental Facilities are supported by the MRSEC Program of the NSF under Award No. DMR 1720256; a member of the NSF-funded Materials Research Facilities Network (www.mrfn.org). The work at Xiamen University was supported by the National Science Foundation of China through grant number 21522508. Y.Z. acknowledges financial support from the China Scholarship Council (No. 201706310206).

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Protecting the Nanoscale Properties of Ag Nanowires with a Solution

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