Strategies to improve the electrical conductivity of nanoparticle-based antimony-doped tin oxide aerogels

Felix Rechberger, Roman Städler, Elena Tervoort and Markus Niederberger*

Laboratory for Multifunctional Materials, Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, CH-8093 Zurich, Switzerland. E-mail:

Electronic Supplementary Information

Experimental

Antimony(III) acetate (99.99 %), tin(IV) chloride (99.99% metals basis), toluene (anhydrous, 99.8 %), benzyl alcohol (puriss., 99 – 100.5% GC), acetone (≥99.99 %) and chloroform (≥99.8 %) were purchased from Sigma-Aldrich. Liquid carbon dioxide (CO2, ≥99 %), nitrogen (N2, 99,999 %), helium (He, 99,999 %) and oxygen (O2, 99,999 %) were provided by PanGas AG, Switzerland. All chemicals were used as received without further purification.

Preparation of nanoparticles

The synthesis of antimony-doped (SbSb+Sn= 0, 5, 10, 15 and 20 at %) tin oxide nanoparticles was performed according to a procedure reported by Müller et al. [1], which was later adapted to microwave-assisted method[2] and published by us previously for the formation of antimony-doped aerogels [3]. The reaction mixture was prepared in a humidity and oxygen free glove box with argon atmosphere for all samples containing antimony. The corresponding molar amount of antimony(III) acetate was dissolved in 10 mL of toluene and stirred until completely dissolved. 12.3 mmol (1.44 mL) of tin(IV) chloride and 30 mL of benzyl alcohol were added to the solution outside the glove box. Subsequently, 20mL of this clear solution was transferred to a 35mL glass vessel sealed with a Teflon cap. The reaction mixture was heated in a microwave reactor (CEM Discover, 2.45GHz) at 150 °C for 8 minutes. The resulting particles were extracted as precipitate by centrifugation and washed three times in acetone.

Dispersions, gelling and supercritical drying

The procedure to prepare the aerogels follows our previously published method [3]. In order to prepare the gels, 400mg of washed, wet nanoparticles were stirred in 1 mL of deionized water resulting in a turbid dispersion. The solution was placed in a round-bottom glass flask and a weak vacuum was applied under slow stirring to remove excess acetone and further volatile solvents. The dispersion was then transferred into Teflon cups and heated for 1h at 90 °C in a saturated water atmosphere to induce gelation. The gels were immediately covered by water to prevent drying and cracking, followed by solvent exchange to pure acetone in 10 vol % steps, each lasting ≥12 h. Finally, supercritical drying (CPD) was performed in carbon dioxide with a SPI-DRY Critical Point Dryer 13200 and a Tousimis 931 GL.

Aerogel treatment

Annealing in air (Nabertherm P 330 furnace) and oxygen atmosphere (Carbolite MTF 12/38/250 tube furnace) was performed by placing the aerogels in alumina based crucibles and heating in the respective furnaces to 400–650°C at heating rates of 1 K min-1. After holding for 2 h, the samples were slowly cooled to room temperature inside the furnace.

In order to remove organics from the surface of the aerogel, the samples irradiated with UV−vis (wavelength range in 200−600 nm at 43 mW cm-2 for 10 – 30 h) before annealing, using a Honlegroup UVACUBE 2000 equipped with an H-Strahler lamp (emission maximum at ~250 nm).

Characterization

For the analysis of organics, attenuated total reflectance infrared (ATR-IR) spectra of the aerogels were recorded in the range of 375– 4000 cm-1 by a Bruker ALPHA FT-IR spectrophotometer. Additionally, elemental analyses were carried out on a device of LECO for quantification of the carbon content.

Powder X-ray diffraction (XRD) was measured on a PANalytical Empyrean equipped with a PIXcel 1D detector and a Cu Kα X-ray tube. By using the Scherrer equation on the (110) peak broadening, the average crystal size could be calculated.

The microstructure was investigated by scanning electron microscopy (SEM) on a Leo Gemini 1530 FEG, Zeiss. The samples were placed on a polished aluminium sample holder and silver paste was used to fix and contact the aerogel sample. High resolution transmission electron microscopy (HRTEM) was carried out on a Philipps Tecnai F30 microscope operated at 300 kV. The samples for TEM characterization were dispersed in chloroform and transferred onto a lacey carbon copper grid on a filter paper and subsequently dried in air at room temperature.

For the gas sorption analyses, the samples were outgassed at 100 °C for at least 24 h and nitrogen gas sorption measurements were carried out on a Quantachrome Autosorb iQ at 77 K. The surface area was determined via the Brunauer-Emmet-Teller (BET) method and the pore size and pore volume were determined by a density functional theory (DFT) analysis using a Non Local DFT (NLDFT) calculation model for nitrogen at 77 K based on silica cylindrical pores [4].

Resistivity measurements were performed with a Keithley 237 high voltage source measuring unit using a custom built Jandel 4-point probe head (tip spacing s = 1 mm, tip radius = 500 µm) from MDC Switzerland (Fig.S1). The probes were gently pressed onto the aerogels with forces in the range of 20 to 150mN, corresponding to 2 to 15g weight measured by a balance and tuned with a z-axis stage. Currents in the range of nA to mA were applied between the outer pins while measuring the potential difference between the two central pins. The electrical resistivity was calculated by the formula ρel=G ∙ VI with voltage V, current I and geometric factor G=2π ∙s ∙0.951=0.598using the approximation for an infinite plane of thickness t = 2.5mm [5]. For statistics, at least 6 measurements were carried out for each sample.

Figures and table

Supplementary Fig. S 1 Setup with a four-point probe head for the resistivity measurement of porous and fragile aerogel samples on a gravimetric balance. The applied force on the sample can be tuned by a z-axis stage.

Supplementary Fig. S 2 Resistivity of ATO aerogels before and after heat treatment in different atmospheres and in combination with UV treatment and subsequent heating in air as a function of annealing temperature.

Supplementary Fig. S 3 Complete N2 gas adsorption-desorption isotherms for ATO aerogels before and after heat treatment at different temperatures for 2 h in air. Inset: BET surface area as a function of annealing temperature and atmosphere.

Supplementary Fig. S 4 ATR-IR spectra of ATO particles and aerogels for different UV treatment times. Characteristic bands are indicated at different wavenumbers corresponding to surface adsorbed organics originating from the synthesis solvents benzyl alcohol and toluene

Supplementary Fig. S 5 Carbon content for ATO particles and aerogels as a function of heat treatment temperature (2 h) and in combination with UV treatment

Supplementary Fig. S 6 BET surface area (right ordinate, open symbols) and crystal size (left ordinate, filled symbols) for as-prepared and heat treated ATO aerogels as a function of UV treatment time

Supplementary Fig. S 7 BET surface area (right ordinate, open symbols) and crystal size (left ordinate, filled symbols) for ATO aerogels before and after UV treatment as a function of heat treatment temperature for 2 h in air.

Supplementary Fig. S 8 Complete N2 gas adsorption-desorption isotherms for ATO aerogels for different UV treatments prior to annealing in air at 500 °C for 2 h. Inset: Corresponding pore size distribution

Supplementary Fig. S 9 Resistivity of ATO aerogels (10 % Sb) before and after heat treatment for 2 h in air at 500 °C and 650 °C as a function of UV treatment time.

Supplementary Fig. S 10 BET surface area as a function of antimony content for ATO aerogels before and after heat treatment at 500°C for 2 h in air.

Supplementary Table S 1 Summary of the crystal size, BET surface area and resistivity of the ATO aerogel samples for different dopant concentrations and treatment conditions.

Sb (at %) / Treatment / Crystal size (nm) / BET surface area (m2g-1) / Resistivity (kΩ cm)
0 / CPD / 3.3 / 298.2 / 151623
0 / 650 °C 2h / 14.1 / - / 10432
5 / CPD / 3 / - / 11054
5 / 400 °C 2h / 5 / - / 0.956
5 / 500 °C 2h / 6.7 / 93.5 / 0.0164
5 / 600 °C 2h / 8.3 / - / 0.0064
5 / 650 °C 2h / 8.8 / - / 0.0045
10 / CPD / 3 / 344 / 151620
10 / CPD + 30 h UV / 2.9 / 344.2 / 7828
10 / 400 °C 2h / 4.2 / 148.8 / 0.81
10 / 500 °C 2h / 4.9 / 115.7 / 0.088
10 / 30 h UV + 500 °C 2h / 5.1 / 116.7 / 0.036
10 / 600 °C 2h / 6.2 / 87.7 / 0.019
10 / 650 °C 2h / 6.5 / 100.2 / 0.012
10 / 30 h UV + 650 °C 2h / 6.6 / 90.3 / 0.0066
15 / CPD / 2.8 / 362.5 / 116070
15 / 400 °C 2h / 3.4 / - / 2.9
15 / 500 °C 2h / 4.2 / 142.9 / 0.3451
15 / 600 °C 2h / 5.1 / - / 0.0652
15 / 650 °C 2h / 5.7 / - / 0.019
20 / CPD / 2.9 / 343.2 / 173280
20 / 400 °C 2h / 3.4 / - / 4.08
20 / 500 °C 2h / 4 / 149.7 / 0.23
20 / 600 °C 2h / 5.2 / - / 0.048
20 / 650 °C 2h / 5.4 / - / 0.036


References

1.  Müller V, Rasp M, Stefanic G, Ba J, Günther S, Rathousky J, Niederberger M, Fattakhova-Rohlfing D (2009) Highly conducting nanosized monodispersed antimony-doped tin oxide particles synthesized via nonaqueous sol–gel procedure. Chem Mater 21(21):5229–5236. doi:10.1021/cm902189r

2.  Luo L, Bozyigit D, Wood V, Niederberger M (2013) High-quality transparent electrodes spin-cast from preformed antimony-doped tin oxide nanocrystals for thin film optoelectronics. Chem Mater 25(24):4901–4907. doi:10.1021/cm4030149

3.  Rechberger F, Ilari G, Niederberger M (2014) Assembly of antimony doped tin oxide nanocrystals into conducting macroscopic aerogel monoliths. Chem Commun 50(86):13138–13141. doi:10.1039/C4CC05648E

4.  Landers J, Gor GY, Neimark AV (2013) Density functional theory methods for characterization of porous materials. Colloids Surf A 437:3-32. doi:10.1016/j.colsurfa.2013.01.007

5.  Topsoe H (1968) Geometric Factors in Four Point Resistivity Measurement. Semiconductor Devision, 2 edn.