Synthesis of Silver Nanowires with Reduced Diameters Using Benzoin-Derived Radicals to Make Transparent Conductors with High Transparency and Low Haze

Transcription

1 Supporting Information Synthesis of Silver Nanowires with Reduced Diameters Using Benzoin-Derived Radicals to Make Transparent Conductors with High Transparency and Low Haze Zhiqiang Niu,, Fan Cui,, Elisabeth Kuttner, Chenlu Xie, Hong Chen, Yuchun Sun, Ahmad Dehestani, Kerstin Schierle-Arndt, Peidong Yang *,,,, # Department of Chemistry, University of California, Berkeley, California 94720, United States California Research Alliance (CARA), BASF Corporation, Berkeley, California 94720, United States BASF Corporation, Ludwigshafen am Rhein 67056, Germany Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States Department of Materials Science and Engineering, University of California, Berkeley, California 94720, United States # Kavli Energy NanoSciences Institute, Berkeley, California 94720, United States

2 Experimental Section Materials. Ethylene glycol (EG), sodium chloride (NaCl, 99%), sodium bromide (NaBr, 99%) and acetone ( 99.9%) were purchased from Fluka. Polyvinylpyrrolidone (PVP, Mw = g/mol) was purchased from Sigma-Aldrich. Polytetrafluoroethylene porous membrane (25 mm diameter, 450 nm pore size) was purchased from Sartorius Stedim Biotech. All chemicals were used as received without further purification. Characterization. Transmission electron microscopy (TEM) was performed with a Hitachi H-7650 at 100 kv. High-resolution (HR) TEM was recorded on a FEI Tecnai G20 at 200 kv. X-ray diffraction (XRD) was acquired at Stanford Synchrotron Radiation Light Source at Beamline 2-1 by using an X-ray beam with a wavelength of Å. Scanning electron microscope (SEM) images were obtained on a JEOL JSM- 6340F field emission scanning microscope. The transmittance and haze measurement was carried out on a Shimadzu UV-2660 UV vis NIR spectrophotometer with an integrating sphere. Synthesis of silver nanowires. In a typical synthesis of silver nanowires with diameters of 16 nm, PVP (50 mg) and AgNO3 (45 mg) were mixed in EG (9.7 ml) in a 20 ml vial. After the dissolution of the solids, NaCl (12 mg/ml in EG, 0.2 ml) and NaBr (23 mg/ml in EG, 0.1 ml) were added into the solution. The mixture was stirred (1200 rpm) at room temperature (RT) for 30 min. The transparent colorless solution gradually turned opaque white. Then, benzoin (500 mg) was added and mixed thoroughly by vigorous stirring (1800 rpm) for 30 seconds. The magnetic stirring bar

3 was removed and the vial was capped by a sleeve stopper. After N2 bubbling through the reaction mixture for 7 min, the vial was placed in an oil bath at RT and heated up to 150 o C in about 15 min. Thereafter, N2 bubbling was stopped and the reaction mixture was left undisturbed at this temperature for 60 min. During this course, the white reaction solution slowly turned to yellow, reddish brown, and green. Upon the completion of the reaction, the nanowires were crashed down by slowly adding acetone and purified by selective precipitation in a procedure as reported by Wiley and coworkers. To synthesize silver nanowires with diameters of 13 nm, composite halide ions was replaced by bromide ions alone. In specific, NaBr (23 mg/ml in EG, 0.2 ml) was added and the reaction temperature was raised to 160 o C. Time-dependent UV vis absorption spectra. Two reaction mixtures, with and without benzoin, respectively, were placed in a same oil bath at room temperature. Right after the oil bath was set to the desired reaction temperature (150 o C), aliquots (0.2 ml) were concurrently taken from the two reaction solutions by syringes, immediately diluted by 0.8 ml of EG in sample tubes, and placed in an ice-water bath. It took 75 min, including 15 min of heating process and 60 min of growth, to collect all the samples. Prior to spectral measurement, the samples were further diluted thirteen times with EG. Fabrication of transparent conducting films. As-synthesized silver nanowires were diluted in isopropanol and dispersed using vortex mixer. The silver nanowires were deposited onto a polytetrafluoroethylene porous membrane (pore size = 450 nm)

4 by vacuum filtration and then transferred onto a glass slide. The films were then treated with 0.5 M NaBH4 (ethanol : H2O = 1 : 1) for 60 seconds to improve wire-to-wire connection. Transmittance and haze measurement. Characteristic transmittance and haze factors were acquired at 550 nm of wavelength. The haze measurement was carried out by D standard. (Citation: ASTM D , Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics, The American Society for Testing and Materials, West Conshohocken, PA.) Background substrate transmittance and haze have been subtracted from all the data.

5 Figure S1. TEM image of 16 nm silver nanowires. The wire highlighted in green background has a length of about 35 µm.

6 Figure S2. TEM images of the products collected at 3 min (a), 12 min (b), 15 min (c,d), 30 min (e) and 35 min (f) in a standard polyol synthesis with the addition of benzoin.

7 Figure S3. (a) The XRD pattern of the products collected at 12 min in a standard polyol synthesis with the presence of benzoin, which can be identified as silver halides (blue line, Ag(Br, Cl), JCPDS# ). (b) The XRD pattern of the products obtained at 75 min in a reaction performed under the same conditions but without adding benzoin. The weak diffractions (*) can be assigned as silver (red line, Ag, JCPDS# ), reflecting the slow reduction kinetics.

8 Figure S4. TEM images of the products obtained by performing the reaction without benzoin at 150 o C.

9 Figure S5. TEM images of silver nanowires prepared with varied halide concentrations: (a) 4.2 mm NaCl and 0 mm NaBr; (b) 4.2 mm NaCl and 4.4 mm NaBr; (c) 4.2 mm NaCl and 6.6 mm NaBr; (d) 0 mm NaCl and 4.4 mm NaBr.