Sol-gel preparation of Ag-silica nanocomposite with high electrical conductivity

Accepted Manuscript Title: Sol-gel Preparation of Ag-Silica Nanocomposite with High Electrical Conductivity Authors: Zhijun Ma, Yuwei Jiang, Huisi Xiao, Bofan Jiang, Hao Zhang, Mingying Peng, Guoping Dong, Xiang Yu, Jian Yang PII: DOI: Reference:

S0169-4332(17)33689-9 https://doi.org/10.1016/j.apsusc.2017.12.101 APSUSC 37971

To appear in:

APSUSC

Please cite this article as: Ma Z, Jiang Y, Xiao H, Jiang B, Zhang H, Peng M, Dong G, Xiang Y, Yang J, Sol-gel Preparation of Ag-Silica Nanocomposite with High Electrical Conductivity, Applied Surface Science (2010), https://doi.org/10.1016/j.apsusc.2017.12.101 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Sol-gel Preparation of Ag-Silica Nanocomposite with High Electrical Conductivity

a,*,

Yuwei Jiang b, Huisi Xiao a, Bofan Jiang a, Hao Zhang a, Mingying Peng a, Guoping Dong a, Xiang

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Zhijun Ma

a State

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Yu c, * Jian Yang d

Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Fiber

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Laser Materials and Applications Technology, School of Materials Science and Engineering, South China

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University of Technology, Guangzhou 510640, Guangdong, P. R China.

Environmental Studies Program 4312 Bren Hall, University of California, Santa Barbara 93106-4160, US.

c

Analytical and Testing Center, Jinan University, Guangzhou, Guangdong 510632, China.

d

Institute of Process Equipment, Zhejiang University, Hangzhou 310027, Zhejiang, P. R. China.

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b

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Correspondence: [email protected], [email protected]

Highlights



Abstract:

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 

Acetonitrile is used bi-functionally as solvent and Ag+ stabilizer in sol-gel preparation of Ag-silica nanocomposites. The as prepared Ag-silica nanocomposites have conductivity as high as 850 S/cm. The as prepared Ag-silica nanocomposites possess tunable conductivity-temperature variation activity. The method here is applicable to co-polymer directed synthesis of mesoporous composite, spin-coating of film, and electrospinning of fiber.

Sol-gel derived noble-metal-silica nanocomposites are very useful in

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

many applications. Due to relatively low price, higher conductivity, and higher

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chemical stability of silver (Ag) compared with copper (Cu), Ag-silica has gained

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much more research interest. However, it remains a significant challenge to realize

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high loading of Ag content in sol-gel Ag-silica composite with high structural

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controllability and nanoparticles’ dispersity. Different from previous works by using

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multifunctional silicon alkoxide to anchor metal ions, here we report the synthesis of Ag-silica nanocomposite with high loading of Ag nanoparticles by employing

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acetonitrile bi-functionally as solvent and metal ions stabilizer. The electrical

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conductivity of the Ag-silica nanocomposite reached higher than 6800 S/cm. In addition, the Ag-silica nanocomposite could simultaneously possess high electrical conductivity and positive conductivity-temperature coefficient by properly controlling

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the loading content of Ag. Such behavior is potentially advantageous for high-temperature devices (like phosphoric acid fuel cells) and inhibiting the thermal-induced increase of devices’ internal resistance. The strategy proposed here is also compatible with block-copolymer directed self-assembly of mesoporous material,

spin-coating of film and electrospinning of nanofiber, making it more charming in various practical applications.

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Keywords: sol-gel, silica, Ag nanoparticle, electrical conductivity

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1. Introduction

The versatile sol-gel chemistry of silica accompanied by the abundance of

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diverse silicon organics makes it highly feasible for synthesizing a wide range of

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advanced materials with high designing flexibility and structural controllability. Due

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to the chemical inertness and dielectric nature of silica, usually, active materials are introduced in a sol-gel process to impart functionalities [1-5]. For example, metal

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nanoparticles or carbon materials are frequently used as activators to impart electrical

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conductivity [6-11]. Generally, the loading of conductive components in sol-gel silica is performed through post functionalization or doping of metal/carbon nanomaterials

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or their precursors in silica sol before gelation. Compared with the post functionalization method, the doping strategy is more advantageous for structural

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control and heavy loading of active components [12]. For the doping strategy, the surface chemistry of the additives should be carefully designed to yield high dispersity and colloidal stability when nanoparticles are introduced directly, or elaborate molecular design is required to stabilize metal ions or organic molecules to avoid precipitation or phase separation when precursors are introduced.

Sol-gel derived noble-metal-silica nanocomposites are among the most frequently investigated silica based composites, due to their wide range of applications. In some applications, for example the application as electrode materials for catalysis or cells, high loading of metal content is required [13-19]. For such

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applications, Ag is superior to other noble-metal-silica nanocomposites, due to its highest conductivity, much lower price (compared to gold (Au), platinum (Pt),

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palladium (Pd), et al), and higher chemical stability (compare to Cu). The biggest

challenge in high loading of Ag nanoparticles in sol-gel silica is to avoid the

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precipitation or phase separation of Ag nanoparticles or the precursors before gelation

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[12]. It is more challenging when silver salts (typically silver nitrite (Ag(NO3)3)) are

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employed as the metal source, because of their high reducibility, which causes

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uncontrolled growth and precipitation of Ag crystals in silica sol [20]. To overcome

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this challenge, silicon alkoxide with various multifunctional moieties were applied to anchor silver ions [6, 21-25]. Yet, the high price of the multifunctional silicon

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alkoxide, together with the tedious synthesis process, pose obstacles to the practical

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application of this strategy. Silica wet gel is commonly composed of two phases, namely the solid phase

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composed of the Si-O network and the reciprocal liquid phase composed of nanopores fulfilled by solvent, wherein the liquid phase can take a volume fraction of over 90%, depending on the preparation details. Previous works on high loading of metal nanoparticles in sol-gel silica mostly focused on tethering metal ions to multifunctional silicon alkoxide (as schematically illustrated in Fig. 1(a)) [6, 21-25].

Consequently, metal ions were mostly hosted in the solid phase, and the potential of the liquid phase has not been fully exploited. Acetonitrile can be used as solvent in the silica sol-gel synthesis, meanwhile, it is an efficient stabilizer to transition metal ions [26-33]. Previous investigations have reported the fabrication of noble-metal-silica

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composites using acetonitrile as ligand [20, 34, 35]. Nevertheless, the reported works rarely refer to high loading of metal nanoparticles, and the electrical property of the

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noble-metal-silica composites was not investigated.

Here, we employ acetonitrile bi-functionally as solvent and metal ligand to host

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high content of Ag nanoparticles in sol-gel derived silica. Therefore, the potential of

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the liquid phase for hosting metal ions in silica sol was fully exploited (as

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schematically illustrated in Fig. 1(b)). By this strategy, porous Ag-silica nanocomposites with high metal loading content were successfully synthesized. In

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addition, we also demonstrate that the strategy here is compatible with copolymer

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directed synthesis of mesoporous material, spin-coating of film, and electrospinning of fiber, further demonstrating its versatility.

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2. Experimental

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2.1.

Chemicals

For the synthesis of Ag-silica nanocomposites, Tetraethyl orthosilicate (TEOS,

98%) was used as silica source, AgNO3 was used as metal source. Acetonitrile (99.9%) was employed as solvent for the sol-gel process, and at the same time act as stabilizer toward silver ions. Distilled water was used for the hydrolysis reaction, while nitric

acid

(HNO3,

65wt%)

was

used

as

catalyst.

Pluronic

poly(ethylene

glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (P123, Mw=5800) was employed for block copolymer directed synthesis of mesoporous Ag-silica nanocomposite. All the reagents were used without further purification. Synthesis of bulky Ag-silica nanocomposites

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2.2.

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Metal salts were dissolved in acetonitrile by magnetic stirring at room temperature,

then stoichiometric TEOS, distilled water and HNO3 were sequentially dripped into the above solution accompanied by fast stirring. For Block copolymer directed

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synthesis of mesoporous Ag-silica nanocomposite, P123 was added and dissolved

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after the dissolution of AgNO3. The molar ratio of TEO: H2O was controlled at 1:4.

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The solution was sealed in Teflon beaker by polyethylene film and aged at the temperature of 333 K for two days to achieve complete gelation. Then the

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polyethylene film was peeled off, and the wet gels were dried at 373 K to form dry

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xerogels. Pyrolysis in air at the temperature of 773 K was performed to convert the

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xerogels to final Ag-silica nanocomposites. 2.3.

Spin-coating of Ag-silica nanocomposite film

The preparation of the precursor sol was almost the same as that for bulky

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sample, except that the molar ratio of H2O: TEOS was controlled below 2.5 in order to facilitate preferential formation of linear molecules. The aging time was shortened substantially to guarantee relatively low viscosity. The glass slide was ultrasonically washed with distilled water for 5 minutes, acetone for two cycles (5 minutes per

cycle), then HNO3 (5 wt%) for 5 minutes, and finally dried at 373 K for 5 h. In the spin-coating process, 200 µL of the sol was dripped onto the glass slide, and the spinning speed and time was set as 400 rpm-3 s and 2000 rpm-30 s. The film sample was dried at 353 K for 5 h in an oven, then transferred to a muffle furnace for

Electrospinning of Ag-silica composite fibers

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2.4.

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annealing (773 K for 3 h).

The preparation of the precursor sol was the same as that for spin-coating of film. The precursor sol was magnetically stirred in an oil bath at 343 K for ~3 h to get

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spinnable sol. For electrospinning, the silica sol was sucked into a plastic syringe

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gauged 20 mL, which was connected to a positively biased steel capillary by a

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polyethylene pipe. After the silica sol was feed into the capillary with a feeding rate of 2 mL/h and formed suspended droplet at the capillary end, a positive bias of 12 kV

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was applied to produce fibers, which were collected at an aluminum foil and dried at

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353 K. Thoroughly dried fibers were calcined in a muffle furnace at 773 K for 3 h to

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get final Ag-silica nanocomposite fibers. 2.5.

Characterization

Scanning Electron Microscopy (SEM, Nova NanoSEM 430, FEI) and

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transmission electron microscopy (TEM, JEOL 2100F, JEOL) were employed to observe the micro structure of the Ag-silica nanocomposites. For SEM, the Ag-silica nanocomposites were coated with a thin layer of platinum by sputtering to achieve enough electrical conductivity. For TEM, the bulky samples were ground to powders,

which was dispersed in ethanol to form suspension. After 30 minutes of deposition, 10 µL of the supernatant was sucked by a glass capillary, then transferred on carbon membrane supported by copper grids and left at ambient environment for drying. For the Ag-silica fibers, the sample preparation for TEM was similar, but free of the

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grinding process step. XRD (X-ray diffraction, AXS D8 Advance, Bruker) and energy dispersive spectrometer (EDS, Model Inca350, Oxford, equipped on SEM) were

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performed to investigate the crystalline property and chemical composition of the

Ag-silica nanocomposites. The porous nature of the Ag-silica nanocomposites was

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studied by N2 physisorption. For this test, the samples were also processed to powders

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by 10 mintutes of grinding. The Van der Pauw method was employed to measure the

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electrical conductivity of the Ag-silica nanocomposites. The samples were carefully

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cut to platelet-shaped pieces with the thickness of 1.1~3.5 mm by a wire cutting

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machine, then washed by aqua regia for 10 seconds and deionized water for 1 mintues, finally dried in vacuum at 353 K overnight. The wiring of the samples for Van der

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Pauw test is schematically illustrated in Fig. S1, and the detailed recorded data is

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listed in table S1-S4. PS: every sample was performed by 5 times measurement, and their average value was used to calculate the electrical conductivity.

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3. Results and discussion For the preparation of Ag-silica nanocomposites, the initial sol-gel solution was

prepared by mixing AgNO3, TEOS, acetonitrile, H2O and HNO3 homogeneously, then sealed and reacted to form wet gel. Highly transparent hydrogel with well integrity was shown in Fig. S2(a1). The wet gel was baked in an oven mildly, generating a

flavescent integral xerogel (Fig. S2(a2)). EDS mapping (Fig. S2(b1-b4) and point-gathered EDS spectra (Fig. S2(c)) indicate that the distribution of silver in the xerogel matrix was homogeneous. After pyrolysis at 773 K in air for 1 h, Ag-silica nanocomposite with yellowish green color was finally prepared (Fig. 2(a)) (designed

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molar ratio of Ag was 40 mol%, termed as Ag-silica-3). Though the doping concentration was very high compared to some similar works, especially those that

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didn’t use multifunctional silicon alkoxide for tethering metal ions, distinct macro phase separation of silver didn’t happen to the nanocomposite. The sample

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Ag-silica-3 is an optimized example with maximal doping concentration without

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macro phase separation. The TEM image in Fig. 2(b) provides a more intuitive vision

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of the sample’s microstructure. The black Ag nanoparticles in the gray silica matrix

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present a very high population density, and inter-particle conglutination was observed

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(Fig. 2(c)). The average size of the Ag nanoparticles was measured to be ~8.6 nm. XRD spectrum reveals the cubic crystalline phase of Ag nanoparticles (Fig. 2(d)).

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According to the EDS result (Fig. 2(e)), molar ratio of Ag in the Ag-silica composite

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is calculated to be ~38.8 mol%, being slightly lower than the designed stoichiometry, which we assume should be ascribed to the slight evaporation of silver source (as testified in Fig. S9). The sample demonstrated a gas physisorption behavior closer to

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type-IV isotherms with approximately type-H3 hysteresis loop (Fig. 2(f)), implying a slit-shaped mesoporous structure. Such mesoporous structure is advantageous for mass transport and hosting active materials in applications as high-performance electrode [36, 37]. We speculate the formation of the mesoporous structure should be

ascribed to the evaporation of the solvent and the released gas (NO2 and O2) in the thermal decomposition of AgNO3. An investigation on the influence of the doping concentration of AgNO3 to the size and population density of Ag nanoparticles, and the microstructure of the composite was performed. The corresponding results and

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discussion are provided in Fig. S4 and Fig. S5. Quantitative evaluation of the samples’ conductivity was performed using Van

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der Pauw method. Due to the contribution of silver’s very high intrinsic conductivity (6.3×105 S/cm), electrical conductivity (at 303 K) of the Ag-silica-3 composite was

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measured as high as ~850 S/cm. The highest conductivity of sol-gel derived

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metal-silica nanocomposites achieved higher than 1000S/cm, which exceeds most

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porous carbons by a factor of ~100-1000 and exceeds the highest performing porous silica sol-gel nanocomposite by a factor of ~2000 [6]. Although the conductivity of

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the Ag-silica nanocomposite synthesized in this work is not so high as the highest one

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in sol-gel derived metal-silica composites reported yet, the result is still persuasive considering the simple and cost-effective strategy employed here. Very interestingly,

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the conductivity of the sample increased with the testing temperature (Fig. 2(g) and (h)), implying that the percolation network in Ag-Silica-3 has not been formed yet. By

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plotting the conductivity-temperature curve (Fig. 2(h)), we found Ag-Silica-3 exhibited a conduction behavior close to the Arrhenius-form, being characteristic of activated charging of closely neighbored metallic particles in dielectric matrix [38-40]. An inspiring value of ~2096 S/cm was achieved by Ag-Silica-3 at the temperature of 393 K. The conduction property of Ag-Silica-3 was very stable during 30 h test at the

temperature of 393 K (Fig. 2(i)). To the best of our knowledge, this is the first time that porous metal-insulator nanocomposite with conductivity near the 103 S/cm level of positive conductivity-temperature coefficient and high thermal stability was reported. The increase of internal resistance in battery or fuel-cell caused by heat

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accumulation is a big problem that deteriorates the device performance. Consequently, for electrode material positive conductivity-temperature coefficient of the Ag-silica

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nanocomposite is very advantageous in practical device applications. By using the

sample as a connector in an electrical circuit, the white LED can be lightened at a

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driving voltage of 2.5 V (inset in Fig. 2(i)), intuitively indicating the high conductivity

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of the material.

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Further increasing the doping concentration of AgNO3 (to 52 mol%) was conducted to further enhance the conductivity of the Ag-silica nanocomposite (the

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sample was nominated as Ag-Silica-4). Macro-domain phase separation of silver from

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the silica matrix happened in this case. As shown in Fig. 3(a1), the front side of Ag-Silica-4 presents an overall yellowish green color interspersed by some bright

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yellow flecks with metallic lusters, which is just due to the formation of the continuous metal network. Even a big area of such fleck appeared as marked by the

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green box in Fig. 3(a1) and demonstrated by the magnified image in Fig. 3(a2). The whole sample presents a fluffy structure like a sponge, which is produced by the bubbling effect caused by the released gas in the decomposition of AgNO 3 (as proved by Fig. S6). Its structure is revealed more intuitively by the digital photo of the back side of the sample in Fig. 3(a3) and the SEM image in Fig. 3(b). EDS mapping

gathered within an area illustrated by the purple box in Fig. 3(c) was employed to microscopically analyze the elemental distribution of Ag-Silica-4. The distribution maps of the elements O and Si overlap neatly, while the one of Ag exhibits a reciprocal pattern to O and Si with a feature size of several micrometers (Fig.

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3(d1)-(d3)), implying the macro-domain phase separation of Ag in the silica matrix. The average molar ratio of Ag within the tested area was measured to be ~50.8 mol%,

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also slightly lower than the predesigned stoichiometry (52 mol%). Amazingly, besides

the representative structure where Ag nanoparticles with the size of 2 nm~6 nm

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distributing homogeneously in the silica matrix (Fig. 3(e)), a small number of platelet-

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or branch-shaped Ag nanostructures with a feature size of tens to hundreds of

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nanometers were also observed (Fig. 3(f)). N2 physisorption isotherms of Ag-Silica-4

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almost remained the same type as Ag-Silica-3 (Fig. 3(g)), but its specific area

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decreased slightly.

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The conductivity of this sample was measured to be as high as 2890 S/cm at 303 K, equivalent to more than three times of enhancement compared with Ag-Silica-3.

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Temperature-dependent variation of Ag-Silica-4 exhibits a quite different style from one of the Ag-Silica-3 (Fig. 3(h)). In the beginning, the conductivity decreased

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slightly from 2890 S/cm at 303 K to 2633 S/cm at 323 K. A sudden drop (to 1027 S/cm) was detected when the testing temperature increased to 343 k, then it re-increased gradually along with the rise of temperature. Such electrical behavior, we speculate,

may

be

ascribed

to

the

synergistic

effect

of

the

positive

conductivity-temperature coefficient of isolated Ag nanoparticles and the negative

conductivity-temperature coefficient of continuous Ag macro-domain phase. Another charming property of Ag-Silica-4 is its improved mechanical property. The samples Ag-Silica-1 to Ag-Silica-3 are relatively brittle and difficult to endure Van der Pauw measurement and mechanical test. Interestingly, Ag-Silica-4 exhibited a certain

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degree of ductility, behaving in a certain degree like bulky silver. Some dents appeared on the front side of Ag-Silica-4 after being pressed hard by a steel ruler, but

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the sample remained integral without apparent cracking (Fig. S7). The enhanced

mechanical strength may be ascribed to the formation of continuous Ag

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macro-domain phase, and the Ag nanoplates and nanobranches. Samples with

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higher doping concentration of AgNO3 (57 mol%, 62 mol%, corresponding samples

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were nominated as Ag-silica-5 and Ag-silica-6) were also prepared, and their

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conductivity was measured to be ~6190 S/cm and ~6870 S/cm, respectively. Fig. 3(i)

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presents the variation rule of the samples’ (Ag-silica-3 to Ag-silica-6) conductivity as a function of AgNO3’s initial doping concentration. Saturation of the increase of

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conductivity seems to happen when the doping concentration of AgNO3 increased

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higher, which may due to intensified evaporation of silver source (Fig. S8). Therefore, a balance between optimization of materials’ performance and cost should be

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seriously considered in practical use. The versatility of the pH dependent sol-gel process of silica enables the synthesis

of various materials with simultaneous control over chemical composition and microstructure. Here we demonstrate the versatility of the proposed strategy by performing the block copolymer directed assembly of mesoporous composite,

spin-coating of film, and electrospinning of nanofiber. For the mesoporous composite (nominated as Ag-silica-m), Pluronic P123 was employed as the pore-forming director. The molecule of triblock copolymer P123 is composed of hydrophilic block poly(ethylene oxide) (PEO) and hydrophobic block poly(propylene oxide) (PPO). In

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the silica precursor sol, the P123 molecules can self assemble into micells with hydrophobic core composed of PPO blocks and hydrophilic water swollen corona

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composed of PEO blocks. After gelation of the sol, the P123 micells will be dispersed

in the gel matrix, which can be removed by post thermal annealing and generating

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porous structure [41, 42]. In our work, both high loading of Ag nanoparticles and

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mesoporous structure can be produced in the silica matrix, as identified by the TEM

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image (Fig. 4(a1)) and N2 physisorption (Fig. 4(a2) and Fig. S9). Specific surface area

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of the sample was calculated to be ~86 m2/g. The EDS mapping (Fig. S10) indicates

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the homogenous distribution of Ag nanoparticles in the silica matrix, implying that the introduction of block copolymer didn’t cause macro-domain phase separation. For

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the preparation of thin films and fibers, the molar ratio of water to TEOS was

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controlled below 2.5 to promote the preferential formation of linear molecules. By spin-coating and post thermal annealing in air, a yellow-colored film without macro crack was prepared (Fig. 4(b1)). The cross-sectional SEM image indicates the

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uniformity of the thickness of the film (Fig. 4(b2)). Under the illumination of xenon light, the electrical resistance of the film changed remarkably along with the variation of light power (Fig. S11), making the film potentially applicable as sensitive photo detector [40, 43]. Fibers loading high content of metal nanocrystals have been

drawing increasing research interest in many applications [15, 44, 45]. Although it is possible to deposit metal nanocrystals on the surface of fibers with post sputtering or adsorption [46-48], heavy doping of metal nanocrystals in the interior of glass or ceramic fibers still remains a big challenge. By employing the sol-gel strategy

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proposed here and the electrospinning technique, we demonstrate the successful fabrication of silica fibers with heavy doping of Ag nanocrystals. After thermal

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annealing at 573 K in air for 1 hour, fibrous mat with gray yellow color was obtained

(Fig. S12(a)). Most of the Ag-silica composite fibers are continuously long with the

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diameter ranging from 250 nm to 4.7 µm (Fig. 4(c1)). The distribution density of Ag

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nanoparticles in the silica fibers is very high, even direct contact of neighboring

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nanoparticles happened in a large percentage (Fig. 4(c2) and Fig. S12(b)). The XRD

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spectrum of the fibers exhibits a very sharp pattern of the characteristic peaks from

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diffraction of cubic Ag nanoparticles (Fig. S12(C)). According to the EDS result (Fig. S12(d)), the content of Ag nanoparticles in the fibers was measured to be ~34 mol%.

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The Ag-silica composite fiber prepared here is promising in many applications, such

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as efficient catalyst, antibacterial agent, SERS substrate, etc.

4. Conclusion

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In summary, we report the sol-gel fabrication of Ag-silica nanocomposites by

using acetonitrile as solvent and metal ions stabilizer. The loading content of Ag nanoparticles in Ag-silica nanocomposite without macro phase separation reached up to 38.8 mol%, yielding a conductivity as high as ~850 S/cm, close to the highest value ever achieved in sol-gel derived metal-silica composite. Meanwhile it demonstrated a

positive conductivity-temperature coefficient. A conductivity over 2000 S/cm with long-term operation reliability was achieved at the temperature of 393 K. The Ag-silica nanocomposite synthesized here concurrently embraced the merits of high conductivity, high thermal stability, and positive conductivity-temperature coefficient,

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making it potentially promising as high-performance electrode materials. Higher conductivity (6870 S/cm) was achieved in the Ag-silica nanocomposite by further

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increasing the AgNO3 concentration in the silica sol-gel. The strategy proposed here is compatible to block copolymer directed synthesis of mesoporous composite,

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spin-coting of film and electrospinning of nanofiber loading high content of Ag

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nanoparticles.

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Acknowledgement

This work was financially supported by the National Natural Science Foundation of

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China (Grant No. 51302087, 51375444), the Pearl River S&T Nova Program of

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Guangzhou (Grant No. 201610010119), and the Xiangjiang Scholar (Grant No.

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XJ2016051).

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Figure captions

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Fig. 1. Schematic illustration for structures of silica hydrogels loading high content of metal ions. a: the strategy by employing multifunctional silicon alkoxide to tether metal ions; b: the strategy

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by using bi-functional solvent to stabilize metal ions (proposed in this investigation).

Fig. 2. a: digital photo of the Ag-silica-3 nanocomposite; b, c: TEM images of the Ag-silica-3

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nanocomposite at low and high magnification; d-f: XRD pattern, EDS spectrum and physisorption

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isotherms of the Ag-silica-3 nanocomposite; g: the I-V curves of the Ag-silica-3 nanocomposite at

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different testing temperature; h: calculated electrical conductivity of the Ag-silica-3

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nanocomposite at different testing temperature; i: the conductivity variation as a function of

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baking time tested at the temperature of 393K; the inset in I shows an electrical circuit using the Ag-silica-3 nanocomposite as a conductive connector, the white LED was lightened by a power

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supplier at the voltage of 2.5V.

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Fig. 3. a1, a3: digital photos of the Ag-silica-4 nanocomposite at the front (a1) and back (a3) side; a2: a locally magnified image of a domain in a1 (as marked by the green box); b: SEM image of

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the Ag-silica-4 nanocomposite; c: a locally magnified image of b (the purple box in c marked an area where EDS mapping was performed); d1-d4: EDS mapping patterns of the Ag-silica-4 nanocomposite gathered in the area marked in c; e, f: TEM images showing different parts of the Ag-silica-4 nanocomposite; g: physisorption isotherms of the Ag-silica-4 nanocomposite; h: the conductivity variation of the Ag-silica-4 nanocomposite as a function of testing temperature; i: the

conductivity variation of the Ag-silica-4 nanocomposite as a function of the initial doping concentration of AgNO3 in the silica sol.

Fig. 4. a1: TEM image of the Ag-silica-m nanocomposite; a2: N2 physisorption isotherms of the Ag-silica-m nanocomposite; b1: digital photo of the Ag-silica nanocomposite film; b2: side-view

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SEM image of the Ag-silica nanocomposite film; c1: SEM image of the electrospun Ag-silica

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fibers; c2: TEM image of single electrospun Ag-silica fiber

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Figures

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Fig. 1.

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Fig. 2.

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Fig. 4.