1. Fabrication and Characterization of Chemical Vapor Deposition Buckypapers for Use in Air Sampling of Volatile Organic Compounds
Jacob S. Shedd, Jonghwa Oh, Claudiu T. Lungu
Department of Environmental Health Sciences, School of Public Health, University of Alabama at Birmingham
Air sampling is important for assessing exposure to volatile organic compounds (VOCs), a group of chemicals with recognized adverse health effects
VOC samples are traditionally collected through active sampling using a pump and sorbent media; however, diffusive sampling is rising as a popular alternative
Diffusive samplers are compact and do not require a pump, but their low mass-uptake rate limits usability for short term and/or low concentration samples
The limitations of diffusive samplers can be avoided by adopting a modified analytical technique, such as photothermal desorption (PTD), which quickly (>1 sec) desorbs analytes by exposing thermally conductive sorbent to pulses of light, and analyzes the sample using a photoionization detector (PID)1
In this study, the applicability of chemical vapor deposition (CVD) single-walled carbon nanotubes (SWNT) as a sorbent for the photothermal desorption (PTD) technique was investigated for use in air sampling.
INTRODUCTION
RESULTS
Figure 1: Toluene adsorption isotherm generated by diffusive adsorption isotherm chamber; Inset image – Diagram of DAIC
Figure 2: TGA curves for two BP samples (CV108 and CV110); Inset image – CVD BP
Summary of results:
BET surface area ranged between m2/g (n = 3)
Pore size ranged between nm (n=3)
Toluene adsorption capacity was found to be 64.7 mg/g (n = 1) at 800 ppm
Thermogravimetric analysis showed that CVD BPs 10% mass loss between °C (n = 2)
Table 1: Analytical Data
Characterization
Data
BET Surface Area
346 ± 6 m2/g
Pore Size
9.2 ± 0.6 nm
Toluene Adsorption Capacity
64.7 mg/g
Thermogravimetry
10% mass 501℃
Fabrication of Buckypaper (BP)2:
50 mg CVD SWNTs was suspended in 400 mL methanol via sonication (2 h)
Suspension was vacuum filtered using a 47 mm diameter PTFE membrane filter (5 µm pore size)
Resulting BP was allowed to vacuum dry for 30 minutes followed by turning the vacuum off and allowing for 2 h of rest
BP is delaminated from PTFE filter and stored at 100°C in an oven oven
BP samples are named CV###; numbers are assigned in order of fabrication (as seen in Figures 1 and 2)
Characterization:
CVD BPs were characterized to determine their efficacy as sorbents to be used in conjunction with the PTD technique. The following methods were used to quantify the adsorptive properties and thermal stability of the material.
Brunauer-Emmitt-Teller (BET) surface area and pore size were determined to characterize the adsorption efficiency of BPs
Performed by physisorption analysis with N2 adsorption at 77K
Toluene adsorption capacity at 800 ppm was obtained from adsorption isotherms at 30­°C
Toluene adsorption isotherms were generated using a diffusive adsorption isotherm chamber (DAIC) system by diffusing toluene onto BP and constantly measuring toluene flux via photoionization detector
Toluene capacity is determined from the mass adsorbed at the saturation concentration on the adsorption isotherm (Figure 1)
Thermogravimetric analysis was conducted to determine the material’s pattern of decomposition
Degassed at 120°C for 20 min then heated to 800°C (ramping rate: 10°C/min) for 45 min in air environment (Figure 2)
METHODS
Average BET surface area was lower than activated charcoal (AC) and 3M OVM 3500/3520 pad (charcoal pad) by 72% and 61% respectively3
Average pore size is 5.4 times larger than that AC and charcoal pad3
Though AC and the 3M charcoal pad were seen to have better adsorbent properties, there is promise of improving CVD BPs by heat treating to remove amorphous carbons and suspension solvents from BP adsorption sites
Future works will explore the use of BPs, made from different SWNTs, for adsorption of VOCs with varying polarities.
CONCLUSION
ACKNOWLEDGEMENTS
This study was supported by The Deep South Center for Occupational Health and Safety (Grant #2T42OH from NIOSH). Its contents are solely the responsibility of the authors and do not necessarily represent the official views of NIOSH.
Special thanks to Clint Anderson (Safeware inc.) for donation of PPE, and to Virginia A. Davis, Phd (Auburn University) for use of thermogravimetry instrumentation.
References 4 4
Floyd, E. L., Sapag, K., Oh, J. & Lungu, C. T. Photothermal desorption of single-walled carbon nanotubes and coconut shell-activated carbons using a continuous light source for application in air sampling. Ann. Occup. Hyg. 58, 877–888 (2014).
Oh, J., Floyd, E. L., Watson, T. C. & Lungu, C. T. Fabrication and adsorption characterization of single-walled carbon nanotubes (SWNT) buckypaper (BP) for use in air samples. Anal. Methods 8, 4197–4203 (2016).
Oh, J. Fabrication and Characterization of Buckypapers for Use in Air Sampling. (Thesis, University of Alabama at Birmingham, 2016).