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Selective detection of chlorine at room temperature utilizing single-walled carbon nanotubes functionalized with platinum nanoparticles synthesized via ultraviolet irradiation
Sensors and Actuators B 249 (2017) 414–422
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Selective detection of chlorine at room temperature utilizing single-walled carbon nanotubes functionalized with platinum nanoparticles synthesized via ultraviolet irradiation Sun-Woo Choi a,c , Byung-Moon Kim b , Sang-Hyub Oh b , Young Tae Byun a,∗ a
Sensor System Research Center, Korea Institute of Science and Technology (KIST), 5 Hwarangro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea Center of Gas Analysis, Korea Research Institute of Standards and Science (KRISS), 267 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea c Department of Materials and Metallurgical Engineering, Kangwon National University, 346 Joongang-ro, Samcheok, Gangwon-do 25913, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 14 December 2016 Received in revised form 14 April 2017 Accepted 19 April 2017 Keywords: Single-walled carbon nanotubes Platinum Nanoparticles Cl2 sensors Ultraviolet
a b s t r a c t In this study, photoreduction by an ultraviolet (UV) irradiation method was applied to synthesizing platinum (Pt) nanoparticles into networked single-walled carbon nanotubes (SWCNTs). To investigate the growth behaviour of Pt nanoparticles, we systematically controlled the UV irradiation intensity and exposure time. These processing factors significantly influenced the formation behaviour of Pt nanoparticles regarding diameter and density. Utilizing the photoreduction process, the sidewalls of SWCNTs were uniformly functionalized with Pt nanoparticles synthesized under optimal UV conditions. For application as practical chlorine (Cl2 ) sensors, the sensing performances of Pt nanoparticle-functionalized SWCNTs for Cl2 were compared against the injection of other gases such as nitrogen dioxide, ammonia and carbon monoxide. The results indicate that UV irradiation is an effective way to functionalize the sidewalls of SWCNTs with catalytic Pt nanoparticles. In addition, the Cl2 selectivity and response of SWCNT-based gas sensors were enhanced by functionalization with catalytic Pt nanoparticles. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The Cl2 gas is a major toxic pollutant at room temperature and generally used in waste water treatment, pharmaceutical production, pesticide, and paper manufacture technologies [1,2]. The threshold concentration for detection of Cl2 by the human nose ranges from 0.1 to 0.3 ppm, whereas the exposure limit of Cl2 recommended by US National Institute for Occupational Safety and Health (NIOSH) is 0.5 ppm [3]. Therefore, development of Cl2 sensors, which is highly sensitive and selective, is necessary for detecting sub-ppm Cl2 molecules. Chemiresistive gas sensors based on semiconductors have attracted increasing attention because of their low power consumption, simple in operation and light weight compared to classical analytical instruments for the detection of hazardous, flammable, poisonous and toxic gases in diverse fields such as those related to the environment, civil life, security industries and health care [4,5]. In particular, the detection and monitoring of far-ranging
∗ Corresponding author. E-mail address: [email protected] (Y.T. Byun). http://dx.doi.org/10.1016/j.snb.2017.04.119 0925-4005/© 2017 Elsevier B.V. All rights reserved.
gases and vapours are essential to applications of chemiresistive gas sensors. Various sensing materials used in chemiresistive gas sensors, including semiconducting metal oxides, polymers, and carbon nanotubes (CNTs), have been demonstrated for gas sensing applications [6–11]. Among semiconductor-based chemiresistive gas sensors, including metal oxides and CNTs, CNT-based sensors offered advantageous properties such as their low cost, small size, simple fabrication, and good compatibility with electronic circuits [12,13]. Accordingly, in the past decade, various types of CNT-based sensors have been developed mostly by employing different structures of CNTs: single-walled (SWCNTs), double-walled (DWCNTs), and multi-walled CNTs (MWCNTs) [14–17]. In spite of these efforts, CNT-based chemiresistive gas sensors have shown limited performance in gas response and response/recovery time due to weak interactions between the CNTbased materials and the gas molecules [17]. Therefore, improving these properties in CNT-based materials is necessary for their practical application in gas sensing. To improve the interaction between the CNTs and some certain gas molecules, several functionalization techniques have been employed to functionalize metallic catalysts such as Pd, Ag, and Au,
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however, at the expense of high-cost and non-uniform functionalization. Dai et al. [18] used Pd nanoparticle to modify SWCNTs for H2 detection at room temperature, which exhibited significant response to H2 (2 at 40 ppm). In addition, Abdelhalim et al. [19] reported that the NH3 sensing capability of CNT-based sensors could be improved by decorating them with Ag and Au nanoparticles. As a result, a number of research groups have attempted to functionalize the sidewall of CNT-based materials with nano-sized catalytic metal nanoparticles using physical vapour deposition methods such as electron-beam evaporation deposition or sputtering techniques [20,21]. However, such physical modification of CNT-based sensors resulted in increasing device costs, the need for special instrumentation, and additional heat-treatment processes. In addition, the above-mentioned sensors exhibited poor sensing performance as a result of non-uniform dispersion of the metal nanoparticles owing to the chemical inertness of the CNTs [22]. To address the existing issues such as non-uniform dispersion, random control of metal ionization degrees and high-temperature process, UV irradiation can provide an alternative method. Although the incorporation of metallic catalysts into CNT-based sensors to improve the interaction between the carbon-based materials and the gas molecules is well-known, to our knowledge, no study has been carried out on the functionalization of sidewalls of SWCNTs with Pt nanoparticles via UV irradiation. Among the various methods for synthesis of metallic nanoparticles, UV irradiation has substantial advantages compared to conventional chemical technique: (1) The metal ions can be easily reduced to the metal species without using excessive reducing agents. (2) The neutral metal atoms uniformly generated in the solution. (3) The process performed at room temperature. In this study, we report the introduction of UV irradiation as a room-temperature, uniform, controlled functionalization method for various CNT surfaces to enhance the gas interaction and thus the sensor response. We fabricated the room temperature response of SWCNT-based sensors, chemically functionalized by Pt nanoparticles, to chlorine (Cl2 ). For practical application of Cl2 sensors, the sensing characteristic of Pt-functionalized SWCNTs for nitrogen dioxide (NO2 ), ammonia (NH3 ), and carbon monoxide (CO) was also investigated. An exceptionally high response and selectivity to Cl2 were obtained for the Pt-functionalized SWCNTs. Our experimental results exhibit a simple method of fabricating a sensitive, selective, and reversible gas sensor with improved Cl2 sensing at room temperature. Furthermore, we propose a mechanism for the enhanced Cl2 sensing characteristic based on the catalytic role of Pt nanoparticles.
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ment were as follows: irradiation intensity: 0.063–0.769 mW/cm2 for 5 min; irradiation time: 5–60 min at 0.769 mW/cm2 . 2.2. Preparation of Pt-functionalized SWCNT composites The Pt-functionalized SWCNT composites were synthesized utilizing spray method and successive UV irradiation technique. For preparation of a SWCNT suspension, 1 mg of SWCNTs with diameters in the range of 1.2–1.7 nm and lengths of 100 nm–4 m was purchased from NanoIntergris and then uniformly dispersed in 50 mL of 1,2–dichlorobenzene (C6 H4 Cl2 , Sigma-Aldrich Corp.) by ultrasonication at room temperature for 4 h. Subsequently, Si substrates with a grown 300 nm-thick SiO2 layer were prepared using a typical cleaning process that was described in our previous report [11]. In order to form networked SWCNTs, 4 mL of the SWCNT suspension was sprayed onto the SiO2 /Si substrate using an air spray gun equipped with a 0.18 mm nozzle. The networked SWCNTs successfully adsorbed onto the SiO2 /Si substrate that was placed on a hot plate heated to 180 ◦ C. To investigate contact condition between SiO2 /Si substrate and SWCNTs depending on adsorption methods (drop casting and spray), we have measured the resistances of the electrodes deposited on the SWCNTs. As shown in Fig. S1, the resistance of electrode deposited on SWCNTs using the drop casting method is about 3 orders of magnitude lower than that of the SWCNTs using the spray method. It can be speculated that the number of junctions between the SWCNTs has influenced considerably on the resistance modulation of the working electrodes. For the functionalization of the sidewalls of SWCNTs with catalytic Pt nanoparticles, the fabricated networked SWCNTs were immersed into a Pt precursor solution (6 mL of a mixed solution). The solution was then irradiated with a 254 nm wavelength UV lamp, schematically depicted in Fig. 1. After UV irradiation, the sample was removed from the Pt precursor solution and then annealed at 300 ◦ C for 1 h in ambient argon (Ar) to remove the remaining solvent. 2.3. Characterization of Pt-functionalized SWCNT composites To analyze the microstructure and phase of synthesized Pt nanoparticles, field-emission scanning electron microscopy (FE–SEM, FEI Nova–SEM), transmission electron microscopy (TEM, JEOL TEM 2100F) equipped with an energy-dispersive X-ray spectrometer (EDS), and X–ray diffraction (XRD, Rigaku D/max–2500 PC) were performed. The elemental composition analysis was performed by X-ray photoelectron spectroscopy (XPS, Thermo K–Alpha) with focused monochromatized Al K␣ (1486.6 eV) radiation.
2. Experimental
2.4. Sensor measurements
2.1. Synthesis of Pt nanoparticles by UV irradiation
The sensing characteristic of the Pt-functionalized SWCNTs for diverse gases (Cl2 , NO2 , CO, and NH3 ) was measured in a quartz chamber at room temperature using a home-made sensing system. For sensing measurement, an electrode was prepared by sequential deposition of Ni (50 nm thickness) and Au (200 nm thickness) on the prepared samples with an interdigitated electrode (IDE) mask consisting of 4 fingers utilizing a sputtering technique. The 50 nm–thick Ni layer was used to enable good adhesion between the Au layer and substrate. For the IDE mask, each electrode was 6.85 mm long and 0.5 mm wide and the spacing between the neighbouring electrodes was 150 m. However, in order to obtain the best sensing performance, a more optimized process for making the electrode is required. Because deposition of the electrode metal layer of 250 nm thickness on a porous structure such as SWCNTs mat is not likely to provide high quality contact. The gas concentrations were controlled at a ratio of calibrated target gas (buffered with dry air) to
For the synthesis of Pt nanoparticles by UV irradiation, precursor solutions were prepared. Briefly, 0.05 g of chloroplatinic acid hexahydrate (H2 PtCl6 ·H2 O, Sigma-Aldrich Corp.) was dissolved in 1 mL of methanol (J.T. Baker). Then, 1 mL of the methanol solution was added to 49 mL of a methanol–deionized (DI) water mixture (90 vol.% CH3 OH–10 vol.% DI water). The prepared solutions were stirred for 4 h, after which they were irradiated with a UV lamp (VILBER, VL–4.LC) under ambient air at room temperature (at 25 ◦ C). To systematically investigate the growth behaviour of Pt nanoparticles by UV irradiation, the processing parameters (i.e. UV irradiation conditions), including the UV intensity and irradiation time, were varied while other conditions were fixed. Si wafers and copper grids were used to collect the Pt nanoparticles reduced in the mixed solutions. The UV irradiation conditions used in this experi-
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Fig. 1. Schematic illustration of the procedure used in this study for the preparation of Pt-functionalized SWCNT sensors.
dry air balance gas by utilizing accurate mass flow controllers. In addition, a total flow rate of 500 sccm was used to avoid possible variation during sensing measurements. The resistance of fabricated sensors was measured at a direct current bias voltage of 1 V using a source measurement instrument (Keithley 2400). Additional details on the sensing measurements can be found in our previous publication [23]. 3. Results and discussion First, to determine the optimal UV irradiation conditions, the growth behaviour of Pt nanoparticles was investigated. Fig. S2(a)–(e) shows high-resolution TEM (HR-TEM) images and histograms of Pt nanoparticles synthesized at different UV intensities (0.063, 0.168, 0.282, 0.491, and 0.769 mW/cm2 ) for 5 min. From the HR-TEM images, the average diameter of Pt nanoparticles was estimated. As shown in histograms, the average diameter of synthesized Pt nanoparticles was 147.0, 55.6, 55.3, 21.8, and 6.3 nm, respectively. This result exhibited that the diameter of Pt nanopar-
ticles decreased with an increase in UV intensity. Fig. S3 exhibits the results for Pt nanoparticles synthesized at various UV irradiation times at a constant intensity of 0.769 mW/cm2 . The Pt nanoparticles gradually increased in diameter with an increase in UV irradiation time. The density of Pt nanoparticles, however, shows the opposite behaviour, as shown in Fig. S3. To understand the growth behaviour of Pt nanoparticles synthesized in this study, we proposed a mechanism for the formation of Pt nanoparticles by UV irradiation. First, Pt metallic phases are formed in the water/methanol solvent by the photoreduction of H2 PtCl6 . According to the previous literature [24], the reduction of hexachloroplatinate(IV) ions (PtCl6 2− ) did not occur under dark condition even in the presence of methanol. In fact, reduction process of Pt ions does not occur in the absence of methanol. In this study, water and methanol were used as solvents, and methanol worked as reducing agents under UV irradiation. Therefore, the PtCl6 2− ions dissolved in the water/methanol solvent are easily reduced to PtCl4 2− and subsequent reduction of PtCl4 2− to Pt metal species takes place. It is widely accepted that nucleation and growth
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Fig. 2. (a) An XRD pattern of Pt nanoparticles. For comparison, an XRD pattern of Pt thin film is included. (b) Typical TEM image of the Pt nanoparticles synthesized at 0.769 mW/cm2 for 5 min.
mechanism of Pt nanoparticles include two steps [25]. The reduction of PtCl6 2− ions results in isolated Pt metal atoms. Next, the reduced neutral Pt atoms occur autocatalytic surface growth and consequently aggregate to form Pt nanoparticles by Ostwald ripening [25]. Our experimental results can be explained on the basis of this proposed photoreduction-based formation process. Hornebecq et al. [26] reported similar experimental results for the synthesis of Ag nanoparticles utilizing ␥-ray radiolysis. In addition, using a chemical reduction method, a correlation between the reduction rate and particle size in nanoparticle synthesis, similar to the general trend observed in the current study, was also reported by Mayer et al. [27]. The phase and crystal structure of Pt nanoparticles synthesized in this experiment was performed using XRD analysis. Fig. 2(a) shows a typical XRD diffraction pattern of sample synthesized at 0.769 mW/cm2 for 5 min. As shown in Fig. 2(a), the XRD pattern of a Pt film deposited on a Si substrate by sputtering technique is also included to compare with that of Pt nanoparticles. The XRD pattern from the Pt nanoparticle is consistent with that from the Pt film, revealing that the nanoparticles synthesized by utilizing UV
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irradiation are Pt phase. For further investigation, the Pt nanoparticles were characterized by TEM analysis as shown in Fig. 2(b). The HR-TEM lattice image reveals lattice fringes, which is consistent with the (111), (200) and (220) planes of cubic Pt, identified by lattice distances of 0.23, 0.2 and 0.14 nm, respectively. These XRD and TEM results indicate that UV irradiation is an effective method to synthesize Pt nanoparticles of high crystalline quality. Fig. 3(a) and (b) shows representative FE-SEM images of the pristine and Pt-functionalized SWCNTs, respectively. As shown in Fig. 3(b), sidewalls of the SWCNTs are uniformly covered with small-sized and discrete Pt nanoparticles. The sidewalls of SWCNTs were functionalized with Pt nanoparticles using the optimal UV irradiation conditions as determined in this study (UV intensity: 0.769 mW/cm2 ; UV exposure time: 5 min). They were grown uniformly on the sidewall of SWCNTs by nucleation and aggregative growth. After the photoreduction process, the presence of uniformly distributed Pt nanoparticles on the SWCNTs mat is evident, as shown in Fig. 3(c). The high-magnification image in Fig. 3(d) evidently shows the surface morphology of SWCNTs functionalized with Pt nanoparticles. The black dots in Fig. 3(c) and (d) were identified as Pt phase in the HR-TEM image shown in Fig. 3(d). The HR-TEM lattice image shows the (111) and (200) planes of Pt with interplanar spacing of 0.226 and 0.196 nm, respectively. Furthermore, to analyze the composition of the synthesized samples, EDS element mapping regarding C and Pt was performed. As shown in Fig. 2(f) and (g), the corresponding elemental maps for C and Pt in the Pt-functionalized SWCNTs are observed. Furthermore, Fig. 4(a) shows XRD patterns of the samples of Fig. 3(b) after heat-treatment for the solvent removal. All diffraction peaks could be attributed to the hexagonal phase of graphite (JCPDS Card No. 75-1621) or cubic structure of Pt (JCPDS Card No. 04-0802). Based on the TEM and the XRD results mentioned above, it is reasonable to conclude that the sidewalls of SWCNTs were successfully functionalized with well-crystallized Pt nanoparticles synthesized by UV irradiation. To confirm the chemical state and surface composition of the Pt nanoparticles and SWCNTs, XPS analysis was performed. The survey-scan XPS spectrum of Pt-functionalized SWCNTs is shown in Fig. 4(b) with all major elements including C, O and Pt. The chemical state of the Pt nanoparticles was accurately investigated by deconvolution of the Pt 4f spectra, shown in Fig. 4(c). These spectra exhibited sharp and wide peaks resulting from metallic Pt and semiconducting PtO, respectively. The spinorbit coupling energy gap between Pt 4f5/2 and 4f7/2 was similar to a previously-reported value (3.3 eV) [28]. However, the binding energy of PtO shifted to a value higher than previously reported [28]. This slight shift indicates charge transfer from PtO to the SWCNTs. In addition, the coexistence of Pt and PtO nanoparticles may play an important role in the enhancement of gas responses through catalytic effects, which will be discussed in detail in a later section. The absence of PtO peaks in XRD analysis (see Fig. 4(a)) is attributed to the low fraction of PtO and its low level of crystallinity. In order to select the optimal Cl2 sensor, the sensing performances of Pt-functionalized SWCNT sensors synthesized at various UV conditions were measured for 10 ppm Cl2 gas. As shown in Fig. S4, the Pt-functionalized SWCNT sensors that were synthesized under optimal UV irradiation condition (UV intensity: 0.769 mW/cm2 ; UV exposure time: 5 min) showed the best Cl2 response. Therefore, all sensing measurements were carried out using these sensors in this study. In addition, it is of note here that the uniformity and the size of the Pt nanoparticles greatly influenced the Cl2 sensing performance. Detailed results pertaining to this will be reported at a later time. To compare selectivity and response, we investigated the sensing capability of the Pt-functionalized SWCNT sensors at room temperature for Cl2 , NO2 , CO, and NH3 . The dynamic sensing curves of the sensors
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Fig. 3. Microstructures of Pt-functionalized SWCNTs; typical SEM images of (a) pristine and (b) Pt-functionalized SWCNTs, (c) a TEM image of Pt-functionalized SWCNTs, (d) a high-resolution lattice image of Pt-functionalized SWCNTS. (e–g) TEM image of the Pt-functionalized SWCNTs and EDS elemental maps: (e) TEM image, (f and g) C and Pt elemental maps.
are shown in Fig. 5(a) and Fig. S5. The sensing behaviours for the various target gases can be explained by the p-type sensing materials effect. The linearity of the responses as a function of Cl2 concentration and the detection limit (DL) of the sensors are shown in the inset of Fig. 5(a). We calculated the DL value of Pt-functionalized SWCNT sensors for Cl2 from experimental data via simple linear extrapolation [29]. The DL value for Cl2 gas was approximately 65 ppb, a reasonable response value. Thus, Pt-functionalized SWCNT sensors could clearly be used to detect Cl2 gas at concentrations as low as sub-ppm levels at room temperature. The noise level and DL of the sensor could also be theoretically derived from its signal processing performance, as suggested by Li et al. [14], and is shown in Fig. S6 and Table S1. A more detailed explanation of the theoretical DL value can be described in the supplementary material sections. In fact, the Cl2 sensing measurement was carried out in the range of 0.1-100 ppm to verify an optimal linear response range for the Pt-functionalized SWCNT sensors, and the results were shown in Fig. S7. As a result,
the response of fabricated sensors exhibited optimal linearity in the range from 0.1 to 10 ppm for Cl2 concentration. Fig. 5(b) summarizes the response values of fabricated sensors at room temperature for various gas species at 1 ppm, revealing that the Pt-functionalized SWCNT sensors exhibited the highest response value for 1 ppm Cl2 (R/R0 × 100 = 33.8%), which was 14.7, 9.9, and 12.5 times higher than that for 1 ppm CO (R/R0 × 100 = 2.3%), NO2 (R/R0 × 100 = 3.4%) and NH3 (R/R0 × 100 = 2.7%), respectively. These results show that the Pt-functionalized SWCNT sensor exhibits an extraordinarily high Cl2 -sensing selectivity. According to previous reports [30], pristine SWCNT-based sensors had no significant response to Cl2 gas, but functionalization with Pt nanoparticles considerably improved their Cl2 -sensing properties [31]. The long-term stability of a sensor’s performance is one of the most important factors for its practical application in gas sensing. Therefore, to demonstrate repeatable sensor responses, the same sensor was exposed to 1 ppm Cl2 in four successive cycles about 6 months later, shown in Fig. 5(c). Moreover, to investigate the
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Fig. 5. (a) Response curve of the Pt-functionalized SWCNT sensors to 0.1–10 ppm Cl2 (inset: responses of the Pt-functionalized SWCNT sensors as a function of Cl2 concentration). (b) Response values to 1 ppm target gases. (c) Long-term stability of the Pt-functionalized SWCNT sensor.
Fig. 4. (a) X-ray diffraction pattern of Pt-functionalized SWCNTs, and X-ray photoelectron spectroscopy: (b) survey-scanned XPS spectra and (c) Pt 4f XPS spectra of Pt-functionalized SWCNTs.
effect of humidity on sensing performances of the Pt-functionalized SWCNT sensors, we preserved the initially fabricated sensor in the humid condition for about 6 months. After about 6 months, sensing responses with a response of 10.3% were observed with a small error range (±3%), indicating substantial deterioration of our sensor. This result shows that CNT-based sensors can be significantly degraded with increasing ambient humidity, which is a normal behaviour for adsorption-based sensors. Although the response of the fabricated sensor was deteriorated under humid condition after
6 months, the initial response value (10.3%) was maintained with a slight difference (±3%) for 1 ppm Cl2 . Therefore, taking 6 months aging period under humid condition into account, it is reasonable to claim that our sensor shows a good repeatability. Furthermore, as shown in Fig. 5(a) and (c), the initial resistance values of sensors showed the significant drift behaviour, which is a normal behaviour for semiconductor sensors. In particular, for carbon-based gas sensors operating at room temperature, the baseline drift of sensors usually occurs during sensing measurement [29,32,33]. The competition between slow kinetics of target gas at room temperature and interference of water molecules in atmosphere is likely to be responsible for this behaviour.
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Fig. 6. Comparison of the Cl2 -response curves of the sensor, which was measured in RH 60%, RH 80% humid air and dry air, respectively.
To investigate the effect of relative humidity on the Cl2 -response in air, we have also measured the Cl2 -sensing capabilities of the sensor in humid air (RH 60% and 80%) and in dry air as shown in Fig. 6. The responses of the sensor for 1 ppm Cl2 are decreased to 15% and 5% in RH 60% and RH 80% humid air, respectively, compared to that in dry air. It clearly shows that the water molecules in humid air significantly decrease the Cl2 -response of the Pt-functionalized SWCNT sensor. In other words, the chemisorption of oxygen and Cl2 molecules on the SWCNT surfaces will be hindered by the preadsorbed water molecules on the SWCNT surfaces, resulting in the decrease of the sensor response in humid air. A comparison of Cl2 -sensing performances of Pt-functionalized SWCNT sensors with those of other sensors previously reported in the literatures is summarized in Table S2. As shown in Table S2, very high responses was attained by use of various materials systems, such as Cr2 O3 thin films, Au-decorated CdIn2 O4 crystals, ␣-Fe2 O3 -doped In2 O3 thin sheets, Zn-doped In2 O3 hollow spheres, Zn phthalocyanine naobelts, Cu phthalocyanine nanowires, WO3 nanorods and SnO2 nanowires. However, except for phthalocyanine-based sensors, the other sensors used the oxide-based materials system, which cannot be realized in actual sensor devices in the near future due to operating temperature and detection limit. Therefore, taking into account operating temperature (room temperature), the sensor response of ∼32% at 0.1 ppm of Cl2 gas obtained in this study is an unprecedented result. In addition, it is reasonable to claim that the Pt-functionalized SWCNT sensors fabricated in this study have very high performance for practical sensor application. The above-mentioned results confirm that Pt functionalization leads to an improved Cl2 -sensing characteristic of SWCNT-based sensors with regard to selectivity and response. However, it should be noted that it is still unclear why the Cl2 -sensing characteristic of the SWCNT-based sensors improved relative to sensing of other gases. Here, we suggest that the associated sensing mechanisms may explain this phenomenon. The sensing mechanism of pristine SWCNTs is related to the gas oxidation effect that occurs between adsorbed molecular ions and the sidewalls of SWCNTs. The direct charge (electrons or holes) transfer between gas molecules and individual SWCNTs leads to a hole accumulation layer established underneath the SWCNT surfaces, resulting in the extraction of electrons by the adsorbed ions (Fig. 7(a)). A second sensing mechanism of pristine SWCNTs is related to the SWCNT-molecule-SWCNT junctions occurring in the interstitial space between SWCNTs. The formation of SWCNT-molecule-SWCNT junctions in a networked SWCNTs structure can promote charge transfers between SWCNTs by a hopping mechanism [29]. In case of non-polar molecules such as Cl2 , F2 , and CO2 , the gas detection mechanism can be mainly ascribed to the latter mechanism, because the gas molecule adsorp-
tion at interstitial sites of the SWCNT mat (or bundle) is stronger than on individual SWCNTs [34]. The Cl2 -sensing characteristic of SWCNT-based sensors, with regard to response and selectivity, can be enhanced by Pt functionalization and sequential heat-treatment processing, as shown in Fig. 5. First, hetero-interfaces between Pt/PtO and SWCNTs are created. It is expected that the coexistence of metallic Pt and ptype semiconducting PtO (p-PtO) [35] nanoparticles on the SWCNT surfaces leads to modulation of the conduction channels (hole accumulation layer) by an electronic effect. The second effect (i.e. the catalytic effect) contributes to the improved Cl2 selectivity (Fig. 7(b)) of the SWCNT sensors. Through the functionalization of SWCNTs with Pt nanoparticles, electron transfer occurs from the SWCNTs to the Pt nanoparticles, and Cl2 gas molecules can be easily adsorbed on the sidewall of SWCNTs due to a process well known as the spill-over effect [36]. In the case of a Pt-functionalized SWCNTs mat, Cl2 molecules easily dissociate onto the Pt nanoparticles and migrate to the interstitial sites of SWCNTs, forming additional SWCNT-Cl2 molecule-SWCNT junctions. Fig. 7(b) schematically illustrates modulation of the conduction channels due to a change in width of the hole accumulation layer underneath the SWCNT surface. The conduction channels will expand and contract by the supply and removal of Cl2 gas, respectively. From the above, on the basis of conduction channel (hole accumulation layer) modulation by SWCNT functionalization with Pt species such as metallic Pt and p-PtO, we propose the following sensing mechanisms for enhanced Cl2 response and selectivity. As a first possibility, the expansion of SWCNT conduction channels by adsorption of Cl2 molecules will cause a decrease in resistance compared to pristine SWCNTs. However, metallic Pt and p-PtO nanoparticles coexist at room temperature (see Fig. 4(c)). Therefore, the electron transfer between SWCNTs and Pt species (Pt and p-PtO) should be considered as well. The XPS results (Fig. 4(c)) indicate that regions below p-PtO nanoparticles contract due to the electron flow from p-PtO to the SWCNTs. Fig. 7(b) shows a conduction channel in a SWCNT established due to the coexistence of metallic Pt and p-PtO. At this stage, since conduction channels contract by the reverse flow of electrons, the conduction channel volume in SWCNTs will decrease, contributing to the sensitivity improvement. As mentioned above, since Pt nanoparticles are very efficient in Cl2 dissociation due to their lower reaction barrier compared to other metals, Pt functionalization will result in a high response to Cl2 molecules. As a second possibility, smallsized (≤5 nm) Pt nanoparticles are interconnected on the surface of SWCNTs, providing a continuous electrical connection between the SWCNTs. Because of this electrical connection, the efficiency of electron transfer between the SWCNTs mat and Cl2 gas molecules could be improved [31]. In addition, catalytic Pt is known to play a role of securing the selective detection of Cl2 gas among various gases [31], which is consistent with the present results. However, further detailed investigation for role of catalytic Pt nanoparticles should be needed in the field of gas sensors. For the highly selective detection of Cl2 gas at room temperature, this study has demonstrated the importance of functionalization of the sidewalls of SWCNTs with metallic Pt and p-PtO nanoparticles. However, the detection of gaseous species at ppb-levels with significant response and selectivity remains challenging. The combination of Pt-functionalized SWCNTs with metal oxides or polymers may be effective to achieve this, but this needs further investigation.
4. Conclusions Pt nanoparticles were synthesized by a UV irradiation technique. The diameter and growth behaviour of Pt nanoparticles
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Fig. 7. Schematic illustration of the proposed sensing mechanism of (a) pristine SWCNT sensors and (b) Pt-functionalized SWCNT sensors.
greatly varied depending on the exact processing factors, with a shorter irradiation time and a higher UV intensity producing larger Pt nanoparticles. A SWCNT mat was uniformly functionalized with Pt nanoparticles by a photoreduction process. The sensing characteristic of the Pt nanoparticle-functionalized SWCNTs was investigated for Cl2 , NO2 , CO, and NH3 , showing enhanced Cl2 response and selectivity. A mechanism for the enhanced Cl2 sensing performance was proposed based on charge hopping between the SWCNTs and the special catalytic role of Pt in the dissociation of Cl2 into more active chemical species.
Acknowledgements This subject is supported by the Korea Ministry of Environment (MOE) as the “Chemical Accident Prevention Technology Develop-
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Biographies Sun-Woo Choi received his B.S. and M.S. degrees from Inha University, Republic of Korea in 2008 and 2010, respectively. He received his Ph.D. degree from Inha University in Materials Science and Engineering in 2014. He has been a post-doctoral researcher at Korea Institute of Science and Technology (KIST) from 2015 to 2017. He joind the Department of Materials and Metallurgical Engineering at Kangwon National University as an assistant professor in 2017. His research interests include the synthesis of low-dimensional nanomaterials and nanomaterials-based gas sensors. Byung-Moon Kim received M.S. degrees in Environmental Engineering in 2012 from Chungnam National University, Korea. He is currently a principal research scientist in Korea Research Institute of Standards and Science (KRISS). His research interests include development of standard gases and analysis method for toxic and reactive gases. Sang-Hyub Oh received Ph.D. in Chemistry in 2000 from Kyungpook National University, Korea. He is currently a principal scientist in Korea Research Institute of Standards and Science (KRISS). His research interests include development of gas analysis methods for toxic and reactive gases and CNT based gas sensor. Young Tae Byun received his Ph.D. in 1995 form Department of Physics, Korea University, Republic of Korea. He is currently a principal research scientist in Korea Institute of Science and Technology (KIST). He is interested in sensor platforms including gas sensors based on nanostructured materials with both high sensitivity and selectivity at the same time.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Selective detection of chlorine at room temperature utilizing single-walled carbon nanotubes functionalized with platinum nanoparticles synthesized via ultraviolet irradiation Sun-Woo Choi a,c , Byung-Moon Kim b , Sang-Hyub Oh b , Young Tae Byun a,∗ a
Sensor System Research Center, Korea Institute of Science and Technology (KIST), 5 Hwarangro 14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea Center of Gas Analysis, Korea Research Institute of Standards and Science (KRISS), 267 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea c Department of Materials and Metallurgical Engineering, Kangwon National University, 346 Joongang-ro, Samcheok, Gangwon-do 25913, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 14 December 2016 Received in revised form 14 April 2017 Accepted 19 April 2017 Keywords: Single-walled carbon nanotubes Platinum Nanoparticles Cl2 sensors Ultraviolet
a b s t r a c t In this study, photoreduction by an ultraviolet (UV) irradiation method was applied to synthesizing platinum (Pt) nanoparticles into networked single-walled carbon nanotubes (SWCNTs). To investigate the growth behaviour of Pt nanoparticles, we systematically controlled the UV irradiation intensity and exposure time. These processing factors significantly influenced the formation behaviour of Pt nanoparticles regarding diameter and density. Utilizing the photoreduction process, the sidewalls of SWCNTs were uniformly functionalized with Pt nanoparticles synthesized under optimal UV conditions. For application as practical chlorine (Cl2 ) sensors, the sensing performances of Pt nanoparticle-functionalized SWCNTs for Cl2 were compared against the injection of other gases such as nitrogen dioxide, ammonia and carbon monoxide. The results indicate that UV irradiation is an effective way to functionalize the sidewalls of SWCNTs with catalytic Pt nanoparticles. In addition, the Cl2 selectivity and response of SWCNT-based gas sensors were enhanced by functionalization with catalytic Pt nanoparticles. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The Cl2 gas is a major toxic pollutant at room temperature and generally used in waste water treatment, pharmaceutical production, pesticide, and paper manufacture technologies [1,2]. The threshold concentration for detection of Cl2 by the human nose ranges from 0.1 to 0.3 ppm, whereas the exposure limit of Cl2 recommended by US National Institute for Occupational Safety and Health (NIOSH) is 0.5 ppm [3]. Therefore, development of Cl2 sensors, which is highly sensitive and selective, is necessary for detecting sub-ppm Cl2 molecules. Chemiresistive gas sensors based on semiconductors have attracted increasing attention because of their low power consumption, simple in operation and light weight compared to classical analytical instruments for the detection of hazardous, flammable, poisonous and toxic gases in diverse fields such as those related to the environment, civil life, security industries and health care [4,5]. In particular, the detection and monitoring of far-ranging
∗ Corresponding author. E-mail address: [email protected] (Y.T. Byun). http://dx.doi.org/10.1016/j.snb.2017.04.119 0925-4005/© 2017 Elsevier B.V. All rights reserved.
gases and vapours are essential to applications of chemiresistive gas sensors. Various sensing materials used in chemiresistive gas sensors, including semiconducting metal oxides, polymers, and carbon nanotubes (CNTs), have been demonstrated for gas sensing applications [6–11]. Among semiconductor-based chemiresistive gas sensors, including metal oxides and CNTs, CNT-based sensors offered advantageous properties such as their low cost, small size, simple fabrication, and good compatibility with electronic circuits [12,13]. Accordingly, in the past decade, various types of CNT-based sensors have been developed mostly by employing different structures of CNTs: single-walled (SWCNTs), double-walled (DWCNTs), and multi-walled CNTs (MWCNTs) [14–17]. In spite of these efforts, CNT-based chemiresistive gas sensors have shown limited performance in gas response and response/recovery time due to weak interactions between the CNTbased materials and the gas molecules [17]. Therefore, improving these properties in CNT-based materials is necessary for their practical application in gas sensing. To improve the interaction between the CNTs and some certain gas molecules, several functionalization techniques have been employed to functionalize metallic catalysts such as Pd, Ag, and Au,
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however, at the expense of high-cost and non-uniform functionalization. Dai et al. [18] used Pd nanoparticle to modify SWCNTs for H2 detection at room temperature, which exhibited significant response to H2 (2 at 40 ppm). In addition, Abdelhalim et al. [19] reported that the NH3 sensing capability of CNT-based sensors could be improved by decorating them with Ag and Au nanoparticles. As a result, a number of research groups have attempted to functionalize the sidewall of CNT-based materials with nano-sized catalytic metal nanoparticles using physical vapour deposition methods such as electron-beam evaporation deposition or sputtering techniques [20,21]. However, such physical modification of CNT-based sensors resulted in increasing device costs, the need for special instrumentation, and additional heat-treatment processes. In addition, the above-mentioned sensors exhibited poor sensing performance as a result of non-uniform dispersion of the metal nanoparticles owing to the chemical inertness of the CNTs [22]. To address the existing issues such as non-uniform dispersion, random control of metal ionization degrees and high-temperature process, UV irradiation can provide an alternative method. Although the incorporation of metallic catalysts into CNT-based sensors to improve the interaction between the carbon-based materials and the gas molecules is well-known, to our knowledge, no study has been carried out on the functionalization of sidewalls of SWCNTs with Pt nanoparticles via UV irradiation. Among the various methods for synthesis of metallic nanoparticles, UV irradiation has substantial advantages compared to conventional chemical technique: (1) The metal ions can be easily reduced to the metal species without using excessive reducing agents. (2) The neutral metal atoms uniformly generated in the solution. (3) The process performed at room temperature. In this study, we report the introduction of UV irradiation as a room-temperature, uniform, controlled functionalization method for various CNT surfaces to enhance the gas interaction and thus the sensor response. We fabricated the room temperature response of SWCNT-based sensors, chemically functionalized by Pt nanoparticles, to chlorine (Cl2 ). For practical application of Cl2 sensors, the sensing characteristic of Pt-functionalized SWCNTs for nitrogen dioxide (NO2 ), ammonia (NH3 ), and carbon monoxide (CO) was also investigated. An exceptionally high response and selectivity to Cl2 were obtained for the Pt-functionalized SWCNTs. Our experimental results exhibit a simple method of fabricating a sensitive, selective, and reversible gas sensor with improved Cl2 sensing at room temperature. Furthermore, we propose a mechanism for the enhanced Cl2 sensing characteristic based on the catalytic role of Pt nanoparticles.
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ment were as follows: irradiation intensity: 0.063–0.769 mW/cm2 for 5 min; irradiation time: 5–60 min at 0.769 mW/cm2 . 2.2. Preparation of Pt-functionalized SWCNT composites The Pt-functionalized SWCNT composites were synthesized utilizing spray method and successive UV irradiation technique. For preparation of a SWCNT suspension, 1 mg of SWCNTs with diameters in the range of 1.2–1.7 nm and lengths of 100 nm–4 m was purchased from NanoIntergris and then uniformly dispersed in 50 mL of 1,2–dichlorobenzene (C6 H4 Cl2 , Sigma-Aldrich Corp.) by ultrasonication at room temperature for 4 h. Subsequently, Si substrates with a grown 300 nm-thick SiO2 layer were prepared using a typical cleaning process that was described in our previous report [11]. In order to form networked SWCNTs, 4 mL of the SWCNT suspension was sprayed onto the SiO2 /Si substrate using an air spray gun equipped with a 0.18 mm nozzle. The networked SWCNTs successfully adsorbed onto the SiO2 /Si substrate that was placed on a hot plate heated to 180 ◦ C. To investigate contact condition between SiO2 /Si substrate and SWCNTs depending on adsorption methods (drop casting and spray), we have measured the resistances of the electrodes deposited on the SWCNTs. As shown in Fig. S1, the resistance of electrode deposited on SWCNTs using the drop casting method is about 3 orders of magnitude lower than that of the SWCNTs using the spray method. It can be speculated that the number of junctions between the SWCNTs has influenced considerably on the resistance modulation of the working electrodes. For the functionalization of the sidewalls of SWCNTs with catalytic Pt nanoparticles, the fabricated networked SWCNTs were immersed into a Pt precursor solution (6 mL of a mixed solution). The solution was then irradiated with a 254 nm wavelength UV lamp, schematically depicted in Fig. 1. After UV irradiation, the sample was removed from the Pt precursor solution and then annealed at 300 ◦ C for 1 h in ambient argon (Ar) to remove the remaining solvent. 2.3. Characterization of Pt-functionalized SWCNT composites To analyze the microstructure and phase of synthesized Pt nanoparticles, field-emission scanning electron microscopy (FE–SEM, FEI Nova–SEM), transmission electron microscopy (TEM, JEOL TEM 2100F) equipped with an energy-dispersive X-ray spectrometer (EDS), and X–ray diffraction (XRD, Rigaku D/max–2500 PC) were performed. The elemental composition analysis was performed by X-ray photoelectron spectroscopy (XPS, Thermo K–Alpha) with focused monochromatized Al K␣ (1486.6 eV) radiation.
2. Experimental
2.4. Sensor measurements
2.1. Synthesis of Pt nanoparticles by UV irradiation
The sensing characteristic of the Pt-functionalized SWCNTs for diverse gases (Cl2 , NO2 , CO, and NH3 ) was measured in a quartz chamber at room temperature using a home-made sensing system. For sensing measurement, an electrode was prepared by sequential deposition of Ni (50 nm thickness) and Au (200 nm thickness) on the prepared samples with an interdigitated electrode (IDE) mask consisting of 4 fingers utilizing a sputtering technique. The 50 nm–thick Ni layer was used to enable good adhesion between the Au layer and substrate. For the IDE mask, each electrode was 6.85 mm long and 0.5 mm wide and the spacing between the neighbouring electrodes was 150 m. However, in order to obtain the best sensing performance, a more optimized process for making the electrode is required. Because deposition of the electrode metal layer of 250 nm thickness on a porous structure such as SWCNTs mat is not likely to provide high quality contact. The gas concentrations were controlled at a ratio of calibrated target gas (buffered with dry air) to
For the synthesis of Pt nanoparticles by UV irradiation, precursor solutions were prepared. Briefly, 0.05 g of chloroplatinic acid hexahydrate (H2 PtCl6 ·H2 O, Sigma-Aldrich Corp.) was dissolved in 1 mL of methanol (J.T. Baker). Then, 1 mL of the methanol solution was added to 49 mL of a methanol–deionized (DI) water mixture (90 vol.% CH3 OH–10 vol.% DI water). The prepared solutions were stirred for 4 h, after which they were irradiated with a UV lamp (VILBER, VL–4.LC) under ambient air at room temperature (at 25 ◦ C). To systematically investigate the growth behaviour of Pt nanoparticles by UV irradiation, the processing parameters (i.e. UV irradiation conditions), including the UV intensity and irradiation time, were varied while other conditions were fixed. Si wafers and copper grids were used to collect the Pt nanoparticles reduced in the mixed solutions. The UV irradiation conditions used in this experi-
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Fig. 1. Schematic illustration of the procedure used in this study for the preparation of Pt-functionalized SWCNT sensors.
dry air balance gas by utilizing accurate mass flow controllers. In addition, a total flow rate of 500 sccm was used to avoid possible variation during sensing measurements. The resistance of fabricated sensors was measured at a direct current bias voltage of 1 V using a source measurement instrument (Keithley 2400). Additional details on the sensing measurements can be found in our previous publication [23]. 3. Results and discussion First, to determine the optimal UV irradiation conditions, the growth behaviour of Pt nanoparticles was investigated. Fig. S2(a)–(e) shows high-resolution TEM (HR-TEM) images and histograms of Pt nanoparticles synthesized at different UV intensities (0.063, 0.168, 0.282, 0.491, and 0.769 mW/cm2 ) for 5 min. From the HR-TEM images, the average diameter of Pt nanoparticles was estimated. As shown in histograms, the average diameter of synthesized Pt nanoparticles was 147.0, 55.6, 55.3, 21.8, and 6.3 nm, respectively. This result exhibited that the diameter of Pt nanopar-
ticles decreased with an increase in UV intensity. Fig. S3 exhibits the results for Pt nanoparticles synthesized at various UV irradiation times at a constant intensity of 0.769 mW/cm2 . The Pt nanoparticles gradually increased in diameter with an increase in UV irradiation time. The density of Pt nanoparticles, however, shows the opposite behaviour, as shown in Fig. S3. To understand the growth behaviour of Pt nanoparticles synthesized in this study, we proposed a mechanism for the formation of Pt nanoparticles by UV irradiation. First, Pt metallic phases are formed in the water/methanol solvent by the photoreduction of H2 PtCl6 . According to the previous literature [24], the reduction of hexachloroplatinate(IV) ions (PtCl6 2− ) did not occur under dark condition even in the presence of methanol. In fact, reduction process of Pt ions does not occur in the absence of methanol. In this study, water and methanol were used as solvents, and methanol worked as reducing agents under UV irradiation. Therefore, the PtCl6 2− ions dissolved in the water/methanol solvent are easily reduced to PtCl4 2− and subsequent reduction of PtCl4 2− to Pt metal species takes place. It is widely accepted that nucleation and growth
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Fig. 2. (a) An XRD pattern of Pt nanoparticles. For comparison, an XRD pattern of Pt thin film is included. (b) Typical TEM image of the Pt nanoparticles synthesized at 0.769 mW/cm2 for 5 min.
mechanism of Pt nanoparticles include two steps [25]. The reduction of PtCl6 2− ions results in isolated Pt metal atoms. Next, the reduced neutral Pt atoms occur autocatalytic surface growth and consequently aggregate to form Pt nanoparticles by Ostwald ripening [25]. Our experimental results can be explained on the basis of this proposed photoreduction-based formation process. Hornebecq et al. [26] reported similar experimental results for the synthesis of Ag nanoparticles utilizing ␥-ray radiolysis. In addition, using a chemical reduction method, a correlation between the reduction rate and particle size in nanoparticle synthesis, similar to the general trend observed in the current study, was also reported by Mayer et al. [27]. The phase and crystal structure of Pt nanoparticles synthesized in this experiment was performed using XRD analysis. Fig. 2(a) shows a typical XRD diffraction pattern of sample synthesized at 0.769 mW/cm2 for 5 min. As shown in Fig. 2(a), the XRD pattern of a Pt film deposited on a Si substrate by sputtering technique is also included to compare with that of Pt nanoparticles. The XRD pattern from the Pt nanoparticle is consistent with that from the Pt film, revealing that the nanoparticles synthesized by utilizing UV
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irradiation are Pt phase. For further investigation, the Pt nanoparticles were characterized by TEM analysis as shown in Fig. 2(b). The HR-TEM lattice image reveals lattice fringes, which is consistent with the (111), (200) and (220) planes of cubic Pt, identified by lattice distances of 0.23, 0.2 and 0.14 nm, respectively. These XRD and TEM results indicate that UV irradiation is an effective method to synthesize Pt nanoparticles of high crystalline quality. Fig. 3(a) and (b) shows representative FE-SEM images of the pristine and Pt-functionalized SWCNTs, respectively. As shown in Fig. 3(b), sidewalls of the SWCNTs are uniformly covered with small-sized and discrete Pt nanoparticles. The sidewalls of SWCNTs were functionalized with Pt nanoparticles using the optimal UV irradiation conditions as determined in this study (UV intensity: 0.769 mW/cm2 ; UV exposure time: 5 min). They were grown uniformly on the sidewall of SWCNTs by nucleation and aggregative growth. After the photoreduction process, the presence of uniformly distributed Pt nanoparticles on the SWCNTs mat is evident, as shown in Fig. 3(c). The high-magnification image in Fig. 3(d) evidently shows the surface morphology of SWCNTs functionalized with Pt nanoparticles. The black dots in Fig. 3(c) and (d) were identified as Pt phase in the HR-TEM image shown in Fig. 3(d). The HR-TEM lattice image shows the (111) and (200) planes of Pt with interplanar spacing of 0.226 and 0.196 nm, respectively. Furthermore, to analyze the composition of the synthesized samples, EDS element mapping regarding C and Pt was performed. As shown in Fig. 2(f) and (g), the corresponding elemental maps for C and Pt in the Pt-functionalized SWCNTs are observed. Furthermore, Fig. 4(a) shows XRD patterns of the samples of Fig. 3(b) after heat-treatment for the solvent removal. All diffraction peaks could be attributed to the hexagonal phase of graphite (JCPDS Card No. 75-1621) or cubic structure of Pt (JCPDS Card No. 04-0802). Based on the TEM and the XRD results mentioned above, it is reasonable to conclude that the sidewalls of SWCNTs were successfully functionalized with well-crystallized Pt nanoparticles synthesized by UV irradiation. To confirm the chemical state and surface composition of the Pt nanoparticles and SWCNTs, XPS analysis was performed. The survey-scan XPS spectrum of Pt-functionalized SWCNTs is shown in Fig. 4(b) with all major elements including C, O and Pt. The chemical state of the Pt nanoparticles was accurately investigated by deconvolution of the Pt 4f spectra, shown in Fig. 4(c). These spectra exhibited sharp and wide peaks resulting from metallic Pt and semiconducting PtO, respectively. The spinorbit coupling energy gap between Pt 4f5/2 and 4f7/2 was similar to a previously-reported value (3.3 eV) [28]. However, the binding energy of PtO shifted to a value higher than previously reported [28]. This slight shift indicates charge transfer from PtO to the SWCNTs. In addition, the coexistence of Pt and PtO nanoparticles may play an important role in the enhancement of gas responses through catalytic effects, which will be discussed in detail in a later section. The absence of PtO peaks in XRD analysis (see Fig. 4(a)) is attributed to the low fraction of PtO and its low level of crystallinity. In order to select the optimal Cl2 sensor, the sensing performances of Pt-functionalized SWCNT sensors synthesized at various UV conditions were measured for 10 ppm Cl2 gas. As shown in Fig. S4, the Pt-functionalized SWCNT sensors that were synthesized under optimal UV irradiation condition (UV intensity: 0.769 mW/cm2 ; UV exposure time: 5 min) showed the best Cl2 response. Therefore, all sensing measurements were carried out using these sensors in this study. In addition, it is of note here that the uniformity and the size of the Pt nanoparticles greatly influenced the Cl2 sensing performance. Detailed results pertaining to this will be reported at a later time. To compare selectivity and response, we investigated the sensing capability of the Pt-functionalized SWCNT sensors at room temperature for Cl2 , NO2 , CO, and NH3 . The dynamic sensing curves of the sensors
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Fig. 3. Microstructures of Pt-functionalized SWCNTs; typical SEM images of (a) pristine and (b) Pt-functionalized SWCNTs, (c) a TEM image of Pt-functionalized SWCNTs, (d) a high-resolution lattice image of Pt-functionalized SWCNTS. (e–g) TEM image of the Pt-functionalized SWCNTs and EDS elemental maps: (e) TEM image, (f and g) C and Pt elemental maps.
are shown in Fig. 5(a) and Fig. S5. The sensing behaviours for the various target gases can be explained by the p-type sensing materials effect. The linearity of the responses as a function of Cl2 concentration and the detection limit (DL) of the sensors are shown in the inset of Fig. 5(a). We calculated the DL value of Pt-functionalized SWCNT sensors for Cl2 from experimental data via simple linear extrapolation [29]. The DL value for Cl2 gas was approximately 65 ppb, a reasonable response value. Thus, Pt-functionalized SWCNT sensors could clearly be used to detect Cl2 gas at concentrations as low as sub-ppm levels at room temperature. The noise level and DL of the sensor could also be theoretically derived from its signal processing performance, as suggested by Li et al. [14], and is shown in Fig. S6 and Table S1. A more detailed explanation of the theoretical DL value can be described in the supplementary material sections. In fact, the Cl2 sensing measurement was carried out in the range of 0.1-100 ppm to verify an optimal linear response range for the Pt-functionalized SWCNT sensors, and the results were shown in Fig. S7. As a result,
the response of fabricated sensors exhibited optimal linearity in the range from 0.1 to 10 ppm for Cl2 concentration. Fig. 5(b) summarizes the response values of fabricated sensors at room temperature for various gas species at 1 ppm, revealing that the Pt-functionalized SWCNT sensors exhibited the highest response value for 1 ppm Cl2 (R/R0 × 100 = 33.8%), which was 14.7, 9.9, and 12.5 times higher than that for 1 ppm CO (R/R0 × 100 = 2.3%), NO2 (R/R0 × 100 = 3.4%) and NH3 (R/R0 × 100 = 2.7%), respectively. These results show that the Pt-functionalized SWCNT sensor exhibits an extraordinarily high Cl2 -sensing selectivity. According to previous reports [30], pristine SWCNT-based sensors had no significant response to Cl2 gas, but functionalization with Pt nanoparticles considerably improved their Cl2 -sensing properties [31]. The long-term stability of a sensor’s performance is one of the most important factors for its practical application in gas sensing. Therefore, to demonstrate repeatable sensor responses, the same sensor was exposed to 1 ppm Cl2 in four successive cycles about 6 months later, shown in Fig. 5(c). Moreover, to investigate the
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Fig. 5. (a) Response curve of the Pt-functionalized SWCNT sensors to 0.1–10 ppm Cl2 (inset: responses of the Pt-functionalized SWCNT sensors as a function of Cl2 concentration). (b) Response values to 1 ppm target gases. (c) Long-term stability of the Pt-functionalized SWCNT sensor.
Fig. 4. (a) X-ray diffraction pattern of Pt-functionalized SWCNTs, and X-ray photoelectron spectroscopy: (b) survey-scanned XPS spectra and (c) Pt 4f XPS spectra of Pt-functionalized SWCNTs.
effect of humidity on sensing performances of the Pt-functionalized SWCNT sensors, we preserved the initially fabricated sensor in the humid condition for about 6 months. After about 6 months, sensing responses with a response of 10.3% were observed with a small error range (±3%), indicating substantial deterioration of our sensor. This result shows that CNT-based sensors can be significantly degraded with increasing ambient humidity, which is a normal behaviour for adsorption-based sensors. Although the response of the fabricated sensor was deteriorated under humid condition after
6 months, the initial response value (10.3%) was maintained with a slight difference (±3%) for 1 ppm Cl2 . Therefore, taking 6 months aging period under humid condition into account, it is reasonable to claim that our sensor shows a good repeatability. Furthermore, as shown in Fig. 5(a) and (c), the initial resistance values of sensors showed the significant drift behaviour, which is a normal behaviour for semiconductor sensors. In particular, for carbon-based gas sensors operating at room temperature, the baseline drift of sensors usually occurs during sensing measurement [29,32,33]. The competition between slow kinetics of target gas at room temperature and interference of water molecules in atmosphere is likely to be responsible for this behaviour.
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Fig. 6. Comparison of the Cl2 -response curves of the sensor, which was measured in RH 60%, RH 80% humid air and dry air, respectively.
To investigate the effect of relative humidity on the Cl2 -response in air, we have also measured the Cl2 -sensing capabilities of the sensor in humid air (RH 60% and 80%) and in dry air as shown in Fig. 6. The responses of the sensor for 1 ppm Cl2 are decreased to 15% and 5% in RH 60% and RH 80% humid air, respectively, compared to that in dry air. It clearly shows that the water molecules in humid air significantly decrease the Cl2 -response of the Pt-functionalized SWCNT sensor. In other words, the chemisorption of oxygen and Cl2 molecules on the SWCNT surfaces will be hindered by the preadsorbed water molecules on the SWCNT surfaces, resulting in the decrease of the sensor response in humid air. A comparison of Cl2 -sensing performances of Pt-functionalized SWCNT sensors with those of other sensors previously reported in the literatures is summarized in Table S2. As shown in Table S2, very high responses was attained by use of various materials systems, such as Cr2 O3 thin films, Au-decorated CdIn2 O4 crystals, ␣-Fe2 O3 -doped In2 O3 thin sheets, Zn-doped In2 O3 hollow spheres, Zn phthalocyanine naobelts, Cu phthalocyanine nanowires, WO3 nanorods and SnO2 nanowires. However, except for phthalocyanine-based sensors, the other sensors used the oxide-based materials system, which cannot be realized in actual sensor devices in the near future due to operating temperature and detection limit. Therefore, taking into account operating temperature (room temperature), the sensor response of ∼32% at 0.1 ppm of Cl2 gas obtained in this study is an unprecedented result. In addition, it is reasonable to claim that the Pt-functionalized SWCNT sensors fabricated in this study have very high performance for practical sensor application. The above-mentioned results confirm that Pt functionalization leads to an improved Cl2 -sensing characteristic of SWCNT-based sensors with regard to selectivity and response. However, it should be noted that it is still unclear why the Cl2 -sensing characteristic of the SWCNT-based sensors improved relative to sensing of other gases. Here, we suggest that the associated sensing mechanisms may explain this phenomenon. The sensing mechanism of pristine SWCNTs is related to the gas oxidation effect that occurs between adsorbed molecular ions and the sidewalls of SWCNTs. The direct charge (electrons or holes) transfer between gas molecules and individual SWCNTs leads to a hole accumulation layer established underneath the SWCNT surfaces, resulting in the extraction of electrons by the adsorbed ions (Fig. 7(a)). A second sensing mechanism of pristine SWCNTs is related to the SWCNT-molecule-SWCNT junctions occurring in the interstitial space between SWCNTs. The formation of SWCNT-molecule-SWCNT junctions in a networked SWCNTs structure can promote charge transfers between SWCNTs by a hopping mechanism [29]. In case of non-polar molecules such as Cl2 , F2 , and CO2 , the gas detection mechanism can be mainly ascribed to the latter mechanism, because the gas molecule adsorp-
tion at interstitial sites of the SWCNT mat (or bundle) is stronger than on individual SWCNTs [34]. The Cl2 -sensing characteristic of SWCNT-based sensors, with regard to response and selectivity, can be enhanced by Pt functionalization and sequential heat-treatment processing, as shown in Fig. 5. First, hetero-interfaces between Pt/PtO and SWCNTs are created. It is expected that the coexistence of metallic Pt and ptype semiconducting PtO (p-PtO) [35] nanoparticles on the SWCNT surfaces leads to modulation of the conduction channels (hole accumulation layer) by an electronic effect. The second effect (i.e. the catalytic effect) contributes to the improved Cl2 selectivity (Fig. 7(b)) of the SWCNT sensors. Through the functionalization of SWCNTs with Pt nanoparticles, electron transfer occurs from the SWCNTs to the Pt nanoparticles, and Cl2 gas molecules can be easily adsorbed on the sidewall of SWCNTs due to a process well known as the spill-over effect [36]. In the case of a Pt-functionalized SWCNTs mat, Cl2 molecules easily dissociate onto the Pt nanoparticles and migrate to the interstitial sites of SWCNTs, forming additional SWCNT-Cl2 molecule-SWCNT junctions. Fig. 7(b) schematically illustrates modulation of the conduction channels due to a change in width of the hole accumulation layer underneath the SWCNT surface. The conduction channels will expand and contract by the supply and removal of Cl2 gas, respectively. From the above, on the basis of conduction channel (hole accumulation layer) modulation by SWCNT functionalization with Pt species such as metallic Pt and p-PtO, we propose the following sensing mechanisms for enhanced Cl2 response and selectivity. As a first possibility, the expansion of SWCNT conduction channels by adsorption of Cl2 molecules will cause a decrease in resistance compared to pristine SWCNTs. However, metallic Pt and p-PtO nanoparticles coexist at room temperature (see Fig. 4(c)). Therefore, the electron transfer between SWCNTs and Pt species (Pt and p-PtO) should be considered as well. The XPS results (Fig. 4(c)) indicate that regions below p-PtO nanoparticles contract due to the electron flow from p-PtO to the SWCNTs. Fig. 7(b) shows a conduction channel in a SWCNT established due to the coexistence of metallic Pt and p-PtO. At this stage, since conduction channels contract by the reverse flow of electrons, the conduction channel volume in SWCNTs will decrease, contributing to the sensitivity improvement. As mentioned above, since Pt nanoparticles are very efficient in Cl2 dissociation due to their lower reaction barrier compared to other metals, Pt functionalization will result in a high response to Cl2 molecules. As a second possibility, smallsized (≤5 nm) Pt nanoparticles are interconnected on the surface of SWCNTs, providing a continuous electrical connection between the SWCNTs. Because of this electrical connection, the efficiency of electron transfer between the SWCNTs mat and Cl2 gas molecules could be improved [31]. In addition, catalytic Pt is known to play a role of securing the selective detection of Cl2 gas among various gases [31], which is consistent with the present results. However, further detailed investigation for role of catalytic Pt nanoparticles should be needed in the field of gas sensors. For the highly selective detection of Cl2 gas at room temperature, this study has demonstrated the importance of functionalization of the sidewalls of SWCNTs with metallic Pt and p-PtO nanoparticles. However, the detection of gaseous species at ppb-levels with significant response and selectivity remains challenging. The combination of Pt-functionalized SWCNTs with metal oxides or polymers may be effective to achieve this, but this needs further investigation.
4. Conclusions Pt nanoparticles were synthesized by a UV irradiation technique. The diameter and growth behaviour of Pt nanoparticles
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Fig. 7. Schematic illustration of the proposed sensing mechanism of (a) pristine SWCNT sensors and (b) Pt-functionalized SWCNT sensors.
greatly varied depending on the exact processing factors, with a shorter irradiation time and a higher UV intensity producing larger Pt nanoparticles. A SWCNT mat was uniformly functionalized with Pt nanoparticles by a photoreduction process. The sensing characteristic of the Pt nanoparticle-functionalized SWCNTs was investigated for Cl2 , NO2 , CO, and NH3 , showing enhanced Cl2 response and selectivity. A mechanism for the enhanced Cl2 sensing performance was proposed based on charge hopping between the SWCNTs and the special catalytic role of Pt in the dissociation of Cl2 into more active chemical species.
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Biographies Sun-Woo Choi received his B.S. and M.S. degrees from Inha University, Republic of Korea in 2008 and 2010, respectively. He received his Ph.D. degree from Inha University in Materials Science and Engineering in 2014. He has been a post-doctoral researcher at Korea Institute of Science and Technology (KIST) from 2015 to 2017. He joind the Department of Materials and Metallurgical Engineering at Kangwon National University as an assistant professor in 2017. His research interests include the synthesis of low-dimensional nanomaterials and nanomaterials-based gas sensors. Byung-Moon Kim received M.S. degrees in Environmental Engineering in 2012 from Chungnam National University, Korea. He is currently a principal research scientist in Korea Research Institute of Standards and Science (KRISS). His research interests include development of standard gases and analysis method for toxic and reactive gases. Sang-Hyub Oh received Ph.D. in Chemistry in 2000 from Kyungpook National University, Korea. He is currently a principal scientist in Korea Research Institute of Standards and Science (KRISS). His research interests include development of gas analysis methods for toxic and reactive gases and CNT based gas sensor. Young Tae Byun received his Ph.D. in 1995 form Department of Physics, Korea University, Republic of Korea. He is currently a principal research scientist in Korea Institute of Science and Technology (KIST). He is interested in sensor platforms including gas sensors based on nanostructured materials with both high sensitivity and selectivity at the same time.