Wooyoung Kima,
Jun Seop Lee*b and
Jyongsik Jang*a
aSchool of Chemical and Biological Engineering, College of Engineering, Seoul National University, 599 Gwanangno, Gwanakgu, Seoul, 151-742 Korea. E-mail: jsjang@plaza.snu.ac.kr; Fax: +82-2-888-1604; Tel: +82-2-880-7069
bDepartment of Nanochemistry, College of Bionano, Gachon University, 1342 Seongnamdaero, Sujeong-gu, Seongnam-Si, Gyeonggi-Do, 13120 Korea. E-mail: junseop@gachon.ac.kr; Fax: +82-31-750-5389; Tel: +82-31-750-5814
First published on 12th September 2018
Hydrogen sulfide (H2S) is one of the most plentiful toxic gases in a real-life and causes a collapse of the nervous system and a disturbance of the cellular respiration. Therefore, highly sensitive and selective H2S gas sensor systems are becoming increasingly important in environmental monitoring and safety. In this report, we suggest the facile synthesis method of the Fe2O3 particles uniformly decorated on carbon nanotubes (Fe2O3@CNT) to detect H2S gas using oxidative co-polymerization (pyrrole and 3-carboxylated pyrrole) and heat treatment. The as prepared Fe2O3@CNT-based sensor electrode is highly sensitive (as low as 1 ppm), selective and stable to H2S gas at 25 °C, which shows promise for operating in medical diagnosis and environment monitoring. Excellent performance of the Fe2O3@CNT is due to the unique morphology of the nanocomposites made from uniformly dispersed Fe2O3 nanoparticles on the carbon surface without aggregation.
Recently, the concern over the environmental protection and the demand to control hazardous chemicals in industry have caused comprehensive interest in developing sensor systems for various toxic gases.14–18 Hydrogen sulfide (H2S), originating from microbial breakdown of plants and animals, is one of the most abundant toxic gases found in coal mines, manholes and semiconducting device industries.19–23 H2S is broadly hazardous in the human body causing collapse of the nervous system and disturbance of the cellular respiration due to binding iron in the mitochondrial cytochrome enzymes.24,25 A threshold limit value (TLV) and recommended acceptable ambient levels of H2S are, respectively, 10 ppm for 8 h and lower than 0.1 ppm by the Occupational Safety and Health Administration (OSHA).26 Accordingly, highly sensitive (<10 ppm) and selective detection of H2S is emerging importance issue in the field of environmental monitoring, disease diagnosis, and food safety. However, conventional H2S gas sensor has several limitations such as slow response/recovery times and high operating temperature that cause complex sensing platform and lead to difficult placement in the confined area.27
Metal oxide semiconductors (MOS) have been conducted in developing H2S sensor systems due to low operating power, stability, and ease of incorporation into the microelectronic devices.28–32 Among them, Fe2O3 displays critical advantages (i.e. ease synthesis, low cost, nontoxicity and abundance in the earth crust) to apply into the H2S sensor system.33–36 In spite of these benefits, Fe2O3 has several limitations such as a long recovery time and a requirement of heating (200–400 °C) to promote sensing reaction to apply into the real sensing system. Therefore, there are a lot of studies about improving H2S detecting performance of the Fe2O3-based sensor systems through loading with noble metals, doping with catalytic oxides, and composing composites with carbon materials.37–39 For example, Sun et al. fabricated α-Fe2O3 nanotubes on the carbon nanotube templates using a hydrothermal synthetic method and a following calcination.40 Jiang et al. synthesized paper like Fe2O3/graphene nanosheets by means of a super critical CO2-assisted thermal process and a controlled magnetic field step.41 However, it is difficult to control the size of these materials, particularly on the nanoscale; thus, material fabrication process can be very complicated.
Herein, we describe a facile synthesis method for Fe2O3 particle uniformly decorated carbon nanotubes (Fe2O3@CNT) using functional group (–COOH) incorporated polypyrrole nanotubes. First, 3-carboxylated polypyrrole nanotubes (CPPyNT) were fabricated using oxidative co-polymerization between monomers (pyrrole and 3-carboxylated pyrrole) and initiator (iron cation (Fe3+) and methyl orange complex). Then, Fe2O3 nanoparticles were formed onto the nanotube structure with phase change of polypyrrole to carbon through simple heat treatment. The Fe2O3@CNT was then applied as a hydrogen sulfide (H2S) gas sensor transducer. The Fe2O3@CNT electrode displays enhanced performances (such as rapid response/recovery time, superb cycle stability, low working temperature and highly sensitive to H2S molecule) than other conventional H2S sensors owing to uniformly dispersed Fe2O3 nanoparticles on the carbon surface. In particular, a minimum detectable level (MDL) of the sensor system is as low as 1 ppm that is much lower than that of other sensors based on Fe2O3 contained nanomaterials.38–42
During the heating procedure, a carboxyl group (–COOH) acts as a nucleation site to form Fe2O3 nanoparticles on the carbon surface. For the without carboxyl group based composites, a small number of Fe2O3 particles is randomly decorated on the carbon surface with different sizes from 4 nm to 11 nm (Fig. 2a). On the other hand, a size and a population of the Fe2O3 particles are uniformly reduced and enhanced, respectively, with increasing concentration of the carboxyl group on the surface (from 0.2 wt% to 3.0 wt%) due to improve amount of nucleate sites (–COOH) and iron cations (Fe3+). The Fe2O3@CNT with 0.2 wt%, 1.0 wt%, and 3.0 wt% carboxyl group concentrations are denoted as Fe2O3@CNT_0.2, Fe2O3@CNT_1.0, and Fe2O3@CNT_3.0. Fig. 2b–d display that an average diameter of the particles decreases from 7 nm (0.2 wt%) to 4 nm (3.0 wt%) and a population of the particles dramatically increases up to 3.0 wt%. However, a diameter of the Fe2O3 particles increases from more than 3.0 wt% concentration of a carboxyl group owing to a large amount of iron cations is caused aggregation of the Fe2O3 during phase transfer process. Fig. 2e shows Fe2O3@CNT at 5.0 wt% of a carboxyl group (denoted as Fe2O3@CNT_5.0) contained large diameter (more than 10 nm) of the particles on the surface. In addition, a high resolution transmission microscopy (HR-TEM) image of the particles indicates interlayer spacing of 0.25 nm and 0.27 nm for (110) and (104) of hematite Fe2O3 and confirms that α-Fe2O3 phase is generated after a heat treatment step (Fig. 2f).
Fig. 3b presents complete spectra over the range over of 0–1200 eV. These overview spectra illustrate that the C, N, O and Fe atoms are existed in the Fe2O3@CNT, whereas only C, N and O atoms are suggested in the CPPyNT. The peak of N 1s is attributed to nitrogen atoms from the pyrrole component. High-resolution XPS spectra for the C 1s around 285 eV are shown in Fig. 3c; this peak is deconvoluted into four components. The peaks of the CPPyNT are attributed as follows: the 284.3 eV peak to CC bonds, the 285.3 eV peak to C–C bonds, the 286.6 eV peak to C–O bonds, and the 284.9 eV peak to C–N bonds. But, Fe2O3@CNT suggests increase peak at 284.6 eV, which is attributed to graphitic sp2 hybridization, and decrease peaks at 285.5 and 287.9 eV, assigned to C–O and C–N, after heat treatment. A high resolution spectrum for the Fe 2p of the Fe2O3@CNT is also suggested in Fig. 3d. In the Fe 2p spectrum, spin–orbit components of 2p3/2 and 2p1/2 are shown near 710.2 and 722.8 eV, indicating that the valence state of Fe is +3. Consequently, the final product (Fe2O3@CNT) is composed of Fe2O3 nanoparticle decorated amorphous carbon nanotubes as confirmed by HR-TEM, XRD, and XPS.
Fig. 4 I–V curves of the different nanotubes (black: CPPyNT; red: Fe2O3@CNT_0.2; blue: Fe2O3@CNT_1.0; green: Fe2O3@CNT_3.0; pink: Fe2O3@CNT_5.0). |
The uniformly dispersed Fe2O3 particles on the carbon surface rapidly detect H2S gas at room temperature by a chemical reaction between adsorbed oxygen species and H2S gas (Fig. 5). Sensing mechanism of the Fe2O3@CNT is illustrated as below. Initially, an electron depletion layer, composed of negatively charged adsorbing oxygen species (O2−, O−, and O2−), is formed on the Fe2O3 particle surface after exposing of the sensor electrode to air.39 Chemical reactions of adsorbed oxygen species are suggested as follows:
O2(g) → O2(ads) | (1) |
O2(ads) + αe− → O2α−(ads). | (2) |
When the sensor electrode is exposed to H2S gas, the chemisorbed oxygen species on the Fe2O3 particles oxidize H2S gas. In detail, the oxygen species dissemble H2S gas into H2O and SO2 gases with transferring electrons to the Fe2O3 and the carbon nanotube structure.39 Oxidation reaction of H2S gas on the Fe2O3 particles is described by
2H2S(g) + 3O2α−(ads) ⇄ 2H2O(g) + 2SO2(g) + 3αe−. | (3) |
A resistance of the sensor electrode increases with electron transfer due to decrease a population of charge carriers (hole) through recombination of the transferred electron and the hole in the carbon structure. Then, H2O and SO2 gases from the oxidation of H2S are desorbed by exposure air contained gas (N2 80 vol% and O2 20 vol%) again. Consequently, the amount of chemisorbed oxygen species on the Fe2O3 surface effects on the sensing ability of the sensor electrode. Thus, a large population of the small sized-Fe2O3 provides an increase in specific surface area to H2S gas and efficient electron transfer to the carbon structure.
To investigate the sensing abilities of the Fe2O3@CNT-based sensor electrode, real-time responsive resistance changes are evaluated for different concentrations of H2S gas at room temperature. Fig. 6a presents a real time response of the sensor electrodes with different amount of Fe2O3 decoration as a function of H2S concentration. Upon various concentrations of H2S exposure, the Fe2O3@CNT based electrodes display a rapid resistance improvement less than 30 s before reaching a saturated value. A minimum detectable level (MDL) of the each electrode presents as following values: 20 ppm for the Fe2O3@CNT_0.2; 5 ppm for the Fe2O3@CNT_1.0; 1 ppm for the Fe2O3@CNT_3.0; 1 ppm for Fe2O3@CNT_5.0 (Fig. S2†). The MDL value of the Fe2O3@CNT based electrode suggests much lower than that of other conventional Fe2O3 sensors at low working temperature (25 °C) (Table S1†). Thus, a better sensitive response is attained concomitant with a high density of the Fe2O3 active site that causes enhancement of activity to H2S gas. However, even though the Fe2O3@CNT_5.0 contains more Fe2O3 particles than the Fe2O3@CNT_3.0, a value of MDL and a sensitive response of the Fe2O3@CNT_5.0 are lower than that of the Fe2O3@CNT_3.0 because an excess amount of Fe2O3 component generates aggregation rather than small particles and then reduces active surface area to H2S gas. Moreover, the improved active surface area of the Fe2O3 particles cause rapid diffusion of H2S and increase sensitivity with providing more active sites (chemisorbed oxygen species) to detect H2S.
Fig. 6b represents sensitivity changes of the electrode as a function of an amount of Fe2O3 particles, with respect to H2S concentration. The sensitivity (S) is determined from the saturation point of the normalized resistance change after 30 s of H2S exposure. As lower than 1 ppm, the sensor electrodes present nonlinear changes in sensitivity. On the other hand, linear tendency is observed over a wide range of H2S concentration (from 1 ppm to 100 ppm). Accordingly, the Fe2O3@CNT based sensor electrodes illustrate reversible and reproducible responses to various concentration of the target analyte (H2S), and sensing ability is more pronounced as enhancing concentration of the analyte.
To apply practical application into the sensor systems, splendid cycle stability is desired for sensor transducer materials. Fig. 6c shows the electrical responses of the sensor electrodes upon periodic disclosure to 20 ppm of H2S at room temperature. The different Fe2O3@CNT nanotubes show similar responses for an each time without retardation of the response and recovery times owing to small size of the Fe2O3 nanoparticles transfer electrons quickly and generate the uniform electron depletion layer on the surface. Moreover, sensing ability of the Fe2O3@CNT sensor electrode also remains sensitivity more than 95% of initial value after 4 weeks that is higher than that of other conventional metal oxide based sensor electrode (Fig. S3†).19–23 Therefore, the Fe2O3@CNT based sensor electrode suggests high stability to H2S gas detection.
The selectivity of the sensor electrodes is also one of the important issues to apply them into practical application. In other words, it is essential to specific detect the target analyte among various chemicals. To evaluate selectivity, Fe2O3@CNT_3.0 based electrode is exposed to different reducing (H2S and NH3) and oxidizing (NO2, MeOH and Et–OH) gases at concentrations of 20 ppm. As shown in Fig. 7, the sensor electrode displays much higher response to H2S than that of other gases due to strong interaction between H2S and the chemisorbed oxygen species and low bonding energy (381 kJ mol−1) of H2S.44 A faster release of trapped electrons in the Fe2O3 particles is also one of important constituents of the selectivity. Thus, H2S can be classified from other chemicals based on the extent and direction of the resistance changes upon analyte disclosures.
Fig. 7 Normalized resistance changes of Fe2O3@CNT_3.0 based sensor electrode to different analytes: concentration of the chemicals is maintained at 20 ppm. |
Footnote |
† Electronic supplementary information (ESI) available: Raman spectra of nanocomposites; sensing ability of nanocomposites; comparison H2S sensing performance of different chemical sensors; stability test to H2S gas. See DOI: 10.1039/c8ra06464d |
This journal is © The Royal Society of Chemistry 2018 |