Multiwalled carbon nanotubes (MWCNTs) have been synthesized on thin gold (Au) films using thermal chemical vapor deposition (CVD). The films were evolved to catalytic Au nanoparticles (Au NPs) by plasma argon (Ar) ion bombardment with a direct current (DC) power of 216 W. The characteristics of the MWCNTs grown on Au catalysts are strongly dependent on the growth temperature in thermal CVD process. The MWCNTs were then purified by oxidation (550°C) and acid treatments (3 : 1 H2SO4/HNO3). After purifying the MWCNTs, they were dispersed in deionized water (DI water) under continuous sonication. The MWCNT solution was then ultrasonically dissolved in a conducting polymer mixture of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) to prepare for an electronic ink. The ink was deposited onto the flexible and transparent plastic substrates such as polyethylene terephthalate (PET) with fabricated silver interdigitated electrode using two methods such as drop-casting and inkjet printing to compare in the detection of ammonia (NH3) and other volatile organic compounds (VOCs) at room temperature. Based on the results, the gas response, sensitivity, and selectivity properties of MWCNT-PEDOT:PSS gas sensor for NH3 detection are significantly enhanced by using inkjet printing technique. The sensing mechanism of fabricated gas sensor exposed to NH3 has been also proposed based on the swelling behaviour of polymer due to the diffusion of NH3 molecules into the polymer matrix. For the MWCNTs, they were mentioned as the conductive pathways for the enhancement of gas-sensing signals.
Carbon nanotubes (CNTs) and their composites have attracted increasing attention in various applications for several years [
In this work, the MWCNTs were grown on plasma ion-bombarded thin Au films by thermal CVD. The effects of growth temperature on the MWCNT morphologies and their crystalline qualities were demonstrated. After growing the MWCNTs, they were then prepared to a sensing film in form of an electronic ink by using purified MWCNT dispersion in poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) conducting polymer. The novel method in deposition of sensing film onto the plastic substrates with fabricated silver interdigitated electrodes by using inkjet printing technique for the enhancement of NH3-sensing properties at room temperature was also evaluated. In addition, the sensing mechanism of fabricated NH3 gas sensor has been proposed based on the swelling of the PEDOT:PSS polymer matrix together with the enhancement of sensing signals by MWCNTs.
Copper foils (∼50
AFM topographical images of (a) as-received Cu foil and (b) polished Cu foil.
Sputtered conditions for deposition of Al2O3 and Au films.
Parameters | Al2O3 film | Au film |
---|---|---|
Base pressure (mbar) | 5 × 10−5 | 5 × 10−3 |
Working pressure (mbar) | 3 × 10−3 | 1 × 10−1 |
Substrate temperature (°C) | R.T | R.T |
Distance between the target and the substrate (cm) | 10 | 5 |
Ar flow rate (sccm) | 5 | 5 |
O2 flow rate (sccm) | 1 | — |
DC power (W) | 216 | 20 |
Sputtering time (s) | 60 | 30 |
The Au/Al2O3 films deposited on Cu foils as substrates were ultrasonically cleaned with methanol and dried with N2. For modifying the substrate, the sample was fixed with two screws on Cu target as a sample holder within a reactor chamber. Schematic illustration and photograph of chamber for plasma ion bombardment are shown in Figures
(a) Schematic illustration and (b) photograph of reactor chamber for plasma ion bombardment.
Again, the modified substrates were ultrasonically cleaned with methanol and dried with N2 before inserting them into a horizontal quartz chamber of a home-built thermal CVD system. The details of this system were reported by the previous work of the first author [
The topography of Cu substrates was examined by an atomic force microscope (AFM, AR MFP-3D). After growing the MWCNTs, the samples were characterized in their morphologies by using a Quanta 450 FEI scanning electron microscope (SEM) working at 30 kV and 10
After growing the MWCNTs, the samples were purified by oxidation treatment at 550°C for 30 min followed by the acid-treated process using a mixture of sulfuric acid and nitric acid (3 : 1 H2SO4/HNO3) under continuous sonication for 2 h. The purified MWCNTs were rinsed several times with distilled water and dried at 60°C in an oven. For preparing the precursor inks, 0.5 g of purified MWCNT powder was dispersed in 80 ml of deionized water (DI water) under continuous sonication for 2 h. The MWCNT solution was then ultrasonically dissolved in a polymer mixture of PEDOT:PSS with a weight ratio of 10% MWCNT solution to 90% PEDOT:PSS for 45 min. The inks were deposited onto the plastic substrates such as polyethylene terephthalate (PET) by two methods for comparison in their gas-sensing properties at room temperature. One was a simple method, i.e., drop-casting, and the other was an applied method, i.e., inkjet printing. For the drop-casted method, the ink with a volume of 20
Figure
AFM topographical images of sputtered Au/Al2O3 films deposited on polished Cu foil (a) after bombarding with Ar ion and (b) after annealing processes.
Schematic illustration of filling mechanism for Au films deposited onto the Al2O3/Cu foils (a) after bombarding with Ar ion and (b) after annealing processes.
Figure
SEM images of MWCNTs grown using different growth temperatures with Au nanoparticles as catalysts: (a) 880°C, (b) 900°C, and (c) 950°C. (d) EDS signals of MWCNT with an Au catalyst particle under TEM.
The crystalline qualities of MWCNTs grown on substrates have been identified using intensity ratio of D to G bands (
Raman spectra of MWCNTs grown using different growth temperatures of (a) 900°C and (b) 950°C.
The poor dispersion of MWCNTs within aqueous solution is still a main problem for the preparation of sensing ink in gas sensor applications. Therefore, surface modification and functionalization of MWCNTs with some organic compounds are required for their enhancements of solubility and compatibility properties. In this work, the MWCNTs were purified by oxidation treatment followed by the acid-treated process using a mixture of sulfuric acid and nitric acid (3 : 1 H2SO4/HNO3) under continuous sonication. These processes have been claimed in the removal of carbonaceous impurities and attachment of carboxylic (COOH) organic compounds on the CNT surface [
To understand the effect of purified process on the dispersion quality of MWCNTs, 0.5 g of purified MWCNTs was immersed in 80 ml DI water under continuous sonication for 2 h. Figure
(a) Photographs of purified MWCNTs immersed in DI water after storage in 1 day and 30 days. (b) Schematic diagram of a fabricated gas sensor.
Figure
SEM images of MWCNT-PEDOT:PSS sensing film on silver interdigitated electrode using (a-b) drop-casting and (c-d) inkjet printing.
The performance of our fabricated gas sensors was evaluated using gas response and sensitivity and selectivity properties. The gas response was defined by equation (
Figure
(a) Resistance changes of the fabricated gas sensors prepared by drop-casting and inkjet printing exposed to NH3 with various concentrations. (b) Gas response of sensor exposed to various VOCs with a fixed concentration of 1000 ppm.
In case of drop-casted condition, the gas responses of MWCNT-PEDOT:PSS gas sensor exposed to NH3 in the concentrations of 100, 200, 500, and 1000 ppm are found to be 0.2, 0.3, 0.8, and 2.1%, respectively. In the same NH3 concentrations, the gas responses of MWCNT-PEDOT:PSS gas sensor obtained from inkjet-printed condition are also found to be 8.5, 23.3, 40.7, and 73.7%, respectively. The selectivity of MWCNT-PEDOT:PSS gas sensors was further investigated by using different VOC vapors including methanol, acetone, and DMF as the test gases as shown in Figure
It is well known that both PEDOT:PSS and MWCNTs are p-type semiconductors [
Resistance change of inkjet-printed MWCNT-PEDOT:PSS gas sensor exposed to 1000 ppm NO2 at room temperature.
To understand the effects of MWCNT content on the gas-sensing performance of fabricated gas sensor, the bare PEDOT:PSS and the PEDOT:PSS with the low content of MWCNTs were further more investigated. Although the MWCNT contents are up to 2, 5, and 8 wt.%, the gas responses of the sensors are similar to the case of bare PEDOT:PSS polymer. This is due to the lack of MWCNTs within the precursor inks. The MWCNTs were mentioned as the conductive pathways for the enhancement of sensing signals.
For the carbon-based gas sensor in literature data, the working range of baseline resistance in response to NH3 was indicated to be in the order of kΩ [
There have been some papers reporting that the PEDOT:PSS polymer can be used as a sensing film for NH3 detection [
Resistance changes of the gas sensors prepared by inkjet printing exposed to 1000 ppm NH3 with (a) bare PEDOT:PSS and (b) MWCNT-PEDOT:PSS as sensing films.
Figure
Gas responses of fabricated gas sensors prepared by drop-casting and inkjet printing as a function of NH3 concentration at room temperature.
The schematic illustration of conductive pathways in electron transports for inkjet-printed MWCNT-PEDOT:PSS and drop-casted MWCNT-PEDOT:PSS networks is shown in Figures
Schematic illustration of conductive pathways in electron transports for (a) inkjet-printed MWCNT-PEDOT:PSS networks and (b) drop-casted MWCNT-PEDOT:PSS networks.
The response time was defined as the time of resistance change for the sensor after a gas-sensing cycle. The response time of sensors from all experiments was measured for ∼10 min. However, the resistance of all fabricated sensors does not perfectly return to its baseline resistance, although the NH3 flow is stopped. Therefore, the recovery time of all fabricated sensors cannot be indicated due to the fact that the NH3 molecules will diffuse slowly throughout the polymer chains by dry air purging at room temperature. The sensing mechanism of the PEDOT:PSS polymer-based gas sensor can be mentioned by using a swelling process [
The NH3-sensing measurements were further repeated every week for 30 days. It has been found that the inkjet-printed MWCNT-PEDOT:PSS gas sensor presents the good stability with only ∼5% of letdown from its initial response under room temperature storage. Baseline drift is a vital performance parameter of gas sensor. It appears when the sensor response has changed over time. In this study, the baseline resistances of sensors are higher after detecting a high NH3 concentration and shift upward from the initial baseline resistance after 4 sensing cycles at room temperature (Figure
The MWCNTs have been successfully grown on plasma ion-bombarded thin gold films using thermal CVD process. The optimum temperature for the growth of effective MWCNTs on the films is 950°C. The purified MWCNT solution was mixed together with a conducting polymer of PEDOT:PSS for preparing an electronic ink. The inkjet printing of gas-sensing ink from an ordinary inkjet office printer has been presented as a good method for enhancement of NH3 gas-sensing properties at room temperature. The inkjet-printed MWCNT-PEDOT:PSS gas sensor presents the p-type semiconductor behaviour under NH3 and NO2 gases. The enhanced sensing properties of NH3 gas sensor were attributed to homogeneous gain effect of sensing films to improve the MWCNT conductive pathways for the electron transports. The dominant sensing mechanism of fabricated NH3 gas sensor has been further presented based on the swelling of the polymer due to the diffusion of NH3 molecules into the chains of polymer matrix. This finding can be beneficial for application in printable or wearable NH3 gas-sensing technology.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This work was funded by the Research and Development Institute, Rajamangala University of Technology Krungthep, Thailand. The authors are thankful to Mr. Gun Chaloeipote, one of the Ph.D. students from Department of Physics, Faculty of Science, Kasetsart University, Thailand, for fruitful cooperation.