Virji et al.
synthesized using an aqueous interfacial or rapid mixing process7,8
and purified by filtration. The nanofibers were dried and subse-
quently redispersed in water to form a final concentration of 1 g/L.
Polyaniline nanofibers were modified with different transition metal
salts in solution, deposited on sensor array substrates, and dried in
air to produce films with thicknesses of ∼0.4 µm. The final
concentration of metal salt in the polyaniline nanofiber dispersion
that gave the optimum response was 0.0075 M. Metal salt films
were deposited on the electrodes by simple drop casting from the
above solutions and dried under ambient conditions overnight.
6
9
The experimental techniques have been described previously.
Briefly, the sensor arrays consist of six separate interdigitated
electrode sensors fabricated on one substrate using standard
photolithographic methods. The electrode geometry is comprised
of 50 pairs of digits, with each digit having dimensions of 10 µm
Figure 1. Electrical response of dedoped polyaniline nanofibers (dedoped
Pani-NF, green), copper chloride (CuCl2, magenta), copper chloride/
polyaniline (CuCl2/Pani, dark blue) nanofibers, copper acetate/polyaniline
(Cu(CH3COO)2/Pani, black) nanofibers, and copper acetate (Cu(CH3COO)2,
red) films exposed to 10 ppm of hydrogen sulfide.
×
3200 µm × 0.18 µm (width × length × height) and a 10 µm
gap between digits.
A certified gas mixture of 200 ppm of hydrogen sulfide in
nitrogen (Scott Specialty Gases) was diluted with humidified
nitrogen for gas exposures. All gas exposures were carried out with
a concentration of 10 ppm of hydrogen sulfide. Mass flow
controllers were used to meter separate flows of nitrogen buffer
gas and the calibrated gas mixture. All the gas flow experiments
were performed using 45% relative humidity in the nitrogen gas
flow. The humidity was generated using a bubbler and measured
in the nitrogen flow with a humidity sensor (Vaisala). The final
humidity levels were calculated from the flow ratios. Typically,
only one relative humidity value (45%) was used throughout these
experiments. Upon varying the humidity, we observed that the
resistance of copper acetate remained constant in the range of
attributed to the direct conversion of a very insulating copper
acetate starting material (high initial resistance) to a highly
conductive copper sulfide product (low final resistance).
Analysis of the copper acetate films before and after exposure
to hydrogen sulfide using energy-dispersive X-ray analysis
(EDX) and X-ray diffraction (XRD) clearly shows the
formation of copper sulfide (CuS).
Copper acetate and related salts are known to react with
hydrogen sulfide both in solution and in the solid state.
Copper acetate has been reported to react with hydrogen
sulfide in water to produce an insoluble black copper sulfide
0-80% relative humidity. At greater humidity levels, the resistance
decreases by falling about 1 order of magnitude at 100% relative
humidity. This may be due to enhanced ionic conductivity with
higher water loading in the films.
10
precipitate or, more recently, in organic solutions to produce
11
organosols. Copper sulfide films have been deposited using
atomic layer deposition from the surface reaction of a copper
Electrical resistances (dc) were measured with a programmable
electrometer (Keithley 617) with a resistance measurement range
12
â-diketonate and hydrogen sulfide. Copper acetate films
11
from 2 × 10 to 0.1 Ω. A low-current scanner card and switch
system (Keithley 7158 and 7001) were used to multiplex measure-
ments over 10 sensors from two sensor arrays. All instruments were
controlled and read by a computer using a GPIB interface and
LabView software.
have also been shown to react directly with hydrogen sulfide
13
to form copper sulfide, but this conversion has yet to be
examined electrically. Copper acetate films are highly
insulating, and the ability to measure such high resistances
has been a limitation in the past. The use of an electrometer
with a very large dynamic range and interdigitated electrodes
allows us to monitor the resistance change associated with
the conversion of copper acetate to copper sulfide, a small
band gap semiconductor with a conductivity of 10 S/cm.14
Copper acetate has a much larger change in conductivity
than neat copper chloride or composites of copper acetate
or copper chloride with polyaniline nanofibers (Figure 1).
This is likely due to the high reactivity of copper acetate
films with hydrogen sulfide and their very high initial
resistance. Also, acetate ligands are labile enough to dis-
sociate from the metal center more freely upon interaction
with hydrogen sulfide. Copper chloride films, on the other
hand, do not react with hydrogen sulfide to produce any
Results and Discussion
The exposure of copper acetate to hydrogen sulfide is
shown in Figure 1. As can be seen from Figure 1, a film of
copper acetate changes resistance by over 8 orders of
magnitude upon exposure to 10 ppm of hydrogen sulfide at
room temperature. Note that other hydrogen sulfide sensors
often require high operating temperatures.2,3 The change
observed here is rapid with a time response of about τ90
)
3.8 s (τ90 is the response to 90% of full scale). At 100 ppb
of hydrogen sulfide, copper acetate responds with an over 5
orders of magnitude decrease in resistance and a time
response of less than 1 min. This large change can be
(
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1
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10468 Inorganic Chemistry, Vol. 45, No. 26, 2006