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Parts per Billion-Level H2S Detection at Room Temperature Based on Self-Assembled In2O3 Nanoparticles Kun Yao, Daniela Caruntu, Zhongming Zeng, Jiajun Chen, Charles J. O’Connor, and Weilie Zhou* AdVanced Materials Research Institute, UniVersity of New Orleans, New Orleans, Louisiana 70148 ReceiVed: June 2, 2009; ReVised Manuscript ReceiVed: July 7, 2009
Homogenous In2O3 nanoparticles (NPs) with diameters in the range of 6-10 nm were synthesized via a chemical solution method, and nanoparticles were assembled on Si substrate for H2S detection. The sensor demonstrates ultrahigh sensitivity to H2S to part per billion (ppb) levels at room temperature. Different sensor parameters were then investigated in terms of electrode gap, In2O3 NP film thickness, and surface modification by noble metals. It was found that sensitivity was slightly improved by reducing the electrode gap or increasing NP film thickness. However, sensitivity was dramatically enhanced (from 20 ppm to 20 ppb) after surface modification with 0.3 nm Au deposition. In addition, the sensor also shows a certain selective detection to H2S in comparison with other gases such as CO and NO2. Chemical sensing based on various nanostructures has been recognized as one of the most promising applications for nanomaterials.1,2 Metal oxide nanostructures have been widely studied for highly sensitive detection in different chemical environments.3-9 As one of the most promising metal oxide nanomaterials, In2O3 has demonstrated highly sensitive capabilities to various target gases such as NH3,7 CO,10 NO2,5 etc. These promising applications have pushed researchers to synthesize In2O3 nanostructures in different forms such as nanoparticles (NP),11-13 nanowires,14,15 nanorods,16 etc. Compared to one-dimensional (1D) nanostructures such as nanowires or nanorods, nanoparticles are easier to synthesize and assemble into thin film structures with large area coverage on a substrate for practical sensor fabrication, unlike nanowires or nanorods with alignment difficulties.17 The properties of easier assembly, larger area, more homogeneity, structure stability, and higher surface/volume ratio of NPs could lead to direct integration of sensors with low cost, if the sensing capabilities are comparable or even better than those of nanowire- or nanorod-based sensors. As one of the most toxic gases, H2S can damage the human nerve and respiratory systems, causing people to lose consciousness or even die at very low concentrations (ppm levels).18 Therefore, the detection of H2S is of great importance for environmental and safety concerns. For several typical harmful target gases such as NO2 and NH3, many efforts have been made to improve the detection limits to part per billion (ppb) levels,5,19 but few reports were presented to achieve detection limits to ppb levels at room temperature for H2S sensors. Several groups have been focused on H2S detection. Our group once demonstrated H2S detection to 500 ppb at 160 °C using vertically aligned CuO nanowire array sensors.20 Kaur et.al21 obtained a In2O3 whisker-based H2S sensor with a detection limit as low as 200 ppb at room temperature. Very recently, Shirsat et. al22 reported H2S detection as low as 0.1 ppb using polyaniline nanowires at room temperature; however, the mechanism is still unknown. * To whom correspondence should be addressed. E-mail:
[email protected]. Telephone: (504) 280-1068. Fax: (504) 280-3185.
Exploring different forms of nanostructures, seeking different sensor architectures, and improving sensitivity and selectivity, especially at room temperature, are still very important issues to pursue for nanomaterial scientists. In this work, we demonstrate a highly sensitive H2S sensor with a detection limit as low as 20 ppb at room temperature using assembled In2O3 nanoparticles. Selectivity, compared with reducing gas CO and oxidizing gas NO2, was also investigated. The In2O3 nanoparticles used in this work were prepared via a chemical solution method, involving thermal decomposition of metal-organic precursors in fatty alcohol solutions at 320 °C.23 Size distribution (about 6-10 nm in diameter) of the as-synthesized nanoparticles (Figure 1a) was investigated by transmission electron microscopy (JEOL 2010 TEM) and is plotted in the lower left inset of Figure 1a. The nanoparticles were then dispersed onto the Si/SiO2 substrate forming a short-range ordered thin film, followed a 1 h annealing at 350 °C in air to remove the remaining organic molecules and make the nanoparticles have better contacts among each other without increasing the size (Figure 1b).24 It is shown that the nanoparticles stay homogeneous in size and form a self-assembled film with a fairly smooth surface after annealing. For sensor fabrication, we fabricated a simple mask to define the size and position of electrodes. Au electrodes with a 40 nm film thickness (Figure 1c) were then deposited by an electron beam evaporator (Lesker PVD 75) or a magnetron sputtering machine (Cressington 308R). The thickness and homogeneity were monitored by cross-sectional SEM (Leo 1530 VP FESEM) observation (Figure 1d), where layers of the nanoparticles and thicknesses of SiO2 can be clearly distinguished. For gas-sensing measurements, the sensors were mounted in a custom-designed sealed chamber with an electrical feedthrough and gas inlet/outlet, and the data were collected by a Keithley 2400 source meter. Figure 2a illustrates the current-voltage (I-V) curves of a sensor with a NP thickness of about 400 nm at different temperatures (25, 50, 75, 100, and 125 °C from curve 1 to 5, respectively), showing typical semiconductor behavior, where the inset is the magnified I-V curve at 25 °C. Conductance of the In2O3 nanoparticle sensor is similar to that of the multiple
10.1021/jp905189f CCC: $40.75 2009 American Chemical Society Published on Web 07/28/2009
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Figure 1. (a) TEM image of well-dispersed In2O3 nanoparticles. Top right inset is a HRTEM image of one single nanoparticle, showing good crystalline structure. Lower left inset is a size distribution of In2O3 nanoparticles. (b) SEM image of the assembled In2O3 NP film. (c) Top view of the sensor. (d) Cross-sectional view of the sensor.
In2O3 nanowire device,5 reflecting good contact among nanoparticles. The perfect linear curve indicates the good ohmic contact between the gold electrodes and In2O3 nanoparticles. The ultraviolet (UV) response of the sensor is also shown in Figure 2b, where the current increases by at least 4 orders of magnitude under UV illumination. In previous studies, research was focused on current changes when introduced into certain target gases and their recoveries under air flow flushing. It was seldom reported that introducing only air flow could cause current variation when sensors had already been in static air. However, in our work, we clearly observed an air flow effect during the timedependent current measurements, plotted in Figure 2c. The NP sensor current kept decreasing until nearly the entire open circuit status when continuously exposed in an air flow, and the current could be recovered after the air flow was stopped (such loops were repeated many times). If measurement was suspended during the recovery period (Figure 2c, inset, red dashed line), the sensor current was found to continuously increase after the measurement continued (Figure 2c, inset, black line.). This means that the conductivity of the In2O3 NP film continuously increases during this period, even if there is no current flow though the film, which excluded the possible effect of current heating. The current response to air flow with different flow speeds was also investigated. It was found that the faster the air flowed, the quicker the current decreased as shown in Figure 2d, which implied that the In2O3 nanoparticle sensors could be used for air flow speed monitoring. Introducing N2 gas instead of the air, we found the sensor current could keep stable at a much higher value without a large decrease in comparison to the air flow. Furthermore, the variation of the sensor current was similar in dry air and humid air environments, which excludes the effect from water vapor as well. Therefore, all of the above results are fully attributed to adsorbed oxygen as depicted in the mechanism sketch shown in Figure 2e. It is well-known that oxygen molecules adsorb onto the surface of n-type
semiconductors in the form of O2- or O22- as the following equilibriums
O2(g) + e- T O2-(ad)
(1)
2O2 (ad) + e T O2 (ad)
(2)
The electron carriers are immobilized due to the chemisorption of oxygen, leading to a surface depletion region and decreasing the current. When there is air flow (Figure 2e, left), oxygen molecules are more likely to enter the inside nanoparticle layers and adsorb onto the In2O3 NPs, which makes the equilibrium shift toward adsorption, enhancing the depletion region and decreasing the current. Stopping the air flow can make the equilibrium as well as the current slowly recover, while the depletion region decreases (Figure 2e, right). This air flow effect was not observed in the In2O3 nanowire sensor25 or other nanowire sensors,26 which implies that the large surface-to-volume ratio and the self-assembled layer structure of the smaller In2O3 nanoparticles greatly contribute to this air flow effect. The sensor is further tested with H2S. After the sensor was stabilized in the air flow recycling loops, diluted H2S (mixed with air) was introduced each time after stopping the air flow. Figure 3a demonstrates the current response to H2S (current increasing) with different concentrations of 0 ppb, 20 ppb, 200 ppb, 2 ppm, 20 ppm, 200 ppm, and 2000 ppm at 0.2 V bias (Au electrodes were deposited by sputter deposition, with a gap of about 100 µm). Here, the nearly entire open circuit status was defined as the initial point of each sensing process (Figure 3a, arrow), and the recovery (current decreasing) was realized by introducing air flow. For higher concentration H2S detection (>ppm), we found a response time of several minutes is enough to get a clear sensing signal. For ppb-level detection, we found tens
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Figure 2. (a) I-V curves of a sensor at different temperatures (25, 50, 75, 100, and 125 °C from curve 1 to 5, respectively). Inset is the magnified I-V curve at 25 °C. (b) UV response of an In2O3 NP sensor with 0.5 V voltage bias. (c) Current verse time when air flow is turned on and off. (d) Current response to air flow with different flow speeds. (e) Sketch of the mechanism of air flow effect.
of minutes is needed to get a clear sensing signal, which is longer than those working at high temperatures and comparable to those working at room temperature as previously reported. Therefore, 30 min was used as the response time to ensure the distinction of currents with different H2S concentrations, and less than 5 min were recorded for each recovery procedure. The plot in Figure 3b shows the sensitivities of the sensor to H2S with different concentrations. Here, the sensitivity (S) is defined as S ) (Ggas - Gair)/Gair × 100%, where Ggas and Gair are the conductance in diluted H2S and air, respectively. The detection limit is as low as 20 ppb at room temperature with a sensitivity of about 50%, which is the lowest value for a H2S sensor based on metal oxide nanomaterials reported so far. The sensing mechanism is attributed to the interaction between H2S and chemisorbed oxygen in such a way that H2S molecules react with chemisorbed oxygen, speed reduction of the depletion region, release electron carriers back to the nanoparticles, and increase current.25 In general, sensor performance depends on many factors. To better realize the mechanism and optimized structure of the sensor, we investigated several factors such as the gap between electrodes, electrode width, thickness of the In2O3
film, and metal surface modification. First, electrode gap and electrode width were studied. Figure 4a shows the I-V curves of three sensors on the same In2O3 NP film with different electrode gaps (l) and electrode width (d). It is shown that resistance of the In2O3 NP sensor is proportional to the electrode gap (Figure 4a, black and red lines) but in the inverse ratio to the electrode width (deduced from blue line in Figure 4a). The almost linear relationship as shown in Figure 4b more clearly illustrates the fairly homogeneous electrical property of the In2O3 NP film using the resistance in dependence with the electrode gap/width ratio by studying another four sensors. The sensitivity to H2S was further investigated by using two sensors from Figure 4b. It is shown that sensitivity to H2S of sensor 4 (with narrower electrode gap) is higher than that in sensor 1 (Figure 4c,d), which might be because of gap distance of the Au electrodes. It has been reported that Au as a noble metal improves the sensing performance by playing a catalytic role in gas sensors.27,28 It might be easier for the adsorbed H2S molecules on the electrodes to diffuse to the In2O3 NP film, which is called “spillover”,28 causing a more distinct current enhancement (higher sensitivity) of sensor 4 with a narrower electrode gap. In addition, the current responses of sensor 4 and sensor 1
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Figure 3. (a) Current response to different concentrations of H2S. (b) Sensitivities to different H2S concentrations.
Figure 5. (a) I-V curves of three sensors with exactly the same electrode gap and electrode width via electron beam deposition, where the thickness of the In2O3 NP film is 40, 200, and 400 nm. (b) Sensitivities to H2S of these three sensors.
are shown in Figure 4c, in which the current of sensor 4 is larger than that of sensor 1 under the same voltage bias, implying a higher signal-to-noise ratio for sensor 4 (narrower gap). Besides electrode gap and width, film thickness is another important factor to determine sensor performance.28 Figure 5a gives the I-V curves of three sensors with exactly the same electrode gap and electrode width but different NP film thicknesses of 40, 200, and 400 nm. It is shown that the thicker film has a lower resistance due to a larger crosssectional area supplying more conducting channels. Meanwhile, it also clearly illuminates that the sensor with the thicker film has a higher sensitivity to H2S as shown in Figure 5b. As mentioned before, the variation of the equilibrium
between oxygen adsorption and desorption determines the sensor current. For thicker films, oxygen molecules could enter the deeper layers of the NP film and lead to more discrepancy of the current change, showing higher sensitivity. We further increased the thickness of the NP film to 800 nm; however, the sensitivity was not improved much compared with that of the 400 nm thick sensor. There might be two reasons for this phenomenon: (1) It is hard for oxygen to penetrate the deeper NP layer causing further current change. (2) It is hard to control the homogeneity of the film once the NP layer becomes thicker. Further investigation is needed to find an approach to obtain more homogeneous NP film as the film becomes thicker. It should be noted the sensor performance fabricated via the sputtering method (Figure 3) is much better (about three
Figure 4. (a) I-V curves of three sensors on the same In2O3 NP film with different electrode gaps and electrode widths. (b) Relationship between the resistance and electrode gap/width ratio from four sensors on a NP film. (c) Current response to H2S of two sensors in Figure 4b. (d) Sensitivities to different H2S concentrations of the two sensors in Figure 4b.
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Figure 6. (a) Current response to H2S of sensor 1 in Figure 4 after Au surface modification, measured two times at each concentration repeatedly. (b) Sensitivity comparison before and after 0.3 nm Au deposition of sensor 1 in Figure 4. Inset shows the I-V curves before and after 0.3 nm Au deposition.
Yao et al. suggest Au surface modification plays a significant role to improve the sensitivity in our sensors, which was not observed by modifying using other noble metals (Pt and Ag), implying the formation of Au-S bonding enhances sensitivity in our sesnors.29 The selectivity of the sensors was also evaluated. Panel a and b of Figure 7 demonstrate the current response to CO and NO2, respectively, using the sensor in Figure 3. It is shown that the sensor has a weaker interaction between the chemisorbed oxygen and CO-reducing molecules (only 14% to 2000 ppm CO) at room temperature, while for NO2 oxidizing gas, the current response can reach 40 ppb as shown in Figure 7b. However, the response curve is opposite to that of the H2S reducing gas. Thus, our In2O3 NP sensors also show a certain selective detection to CO and NO2. In summary, highly sensitive H2S sensors (to the 20 ppb level at room temperature) were studied by using In2O3 NP films and different sensing parameters such as electrode gap, In2O3 NP film thickness, and surface modification by noble metals. Sensitivity can be slightly improved by fabricating narrower gaps and thicker films but dramatically enhanced by Au metal surface modification. High sensitivity is mainly attributed to homogeneity of the well-assembled small In2O3 nanoparticles (about 6-10 nm) and Au surface modification. In addition, the sensors also show certain selectivity to H2S in comparison to other gases (CO and NO2). The excellent sensing performance and easy fabrication make In2O3 NP sensors extremely important for future applications. Acknowledgment. We thank Dr. Z. X. Zhang at Rice University for discussions. This work was supported by Defense Advanced Research Projects Agency (DARPA) Grant HR001107-1-0032 and research grants from the Louisiana Board of Regents Contracts LEQSF(2007-12)-ENH-PKSFI-PRS-04 and LEQSF(2008-11)-RD-B-10. W.L. Zhou acknowledges partial support from the Research Fund of Key Laboratory for Nanomaterials, Ministry of Education (2007-1). References and Notes
Figure 7. (a) Current response to CO. (b) Current response to NO2.
orders of magnitude higher) than those fabricated through electron beam evaporation (Figures 4 and 5). We think more Au nanoparticles were spread onto the In2O3 NP film during sputter deposition than electron beam evaporation when the electrodes were deposited, forming an effective surface modification. Therefore, one of the sensors measured in Figure 4c was further deposited with about 0.3 nm of Au directly onto the In2O3 NP film surface, and surprisingly, the current response to H2S after Au surface modification (with double tests at each H2S concentration) reached the 20 ppb level, which was dramatically improved by several orders of great sensitivity as shown in Figure 6. The current enhancement before and after the 0.3 nm Au deposition is also shown in the inset of Figure 6b. These results strongly
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