Individual Bi2S3 Nanowire-Based Room-Temperature H2 Sensor

May 20, 2008 - Single Bi2S3 nanowire devices were fabricated using the focused ion beam technique to deposit Pt electrodes. Repeatable and reliable oh...
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J. Phys. Chem. C 2008, 112, 8721–8724

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Individual Bi2S3 Nanowire-Based Room-Temperature H2 Sensor K. Yao, W. W. Gong, Y. F. Hu, X. L. Liang, Q. Chen,* and L.-M. Peng Key Laboratory for the Physics and Chemistry of NanodeVices and Department of Electronics, Peking UniVersity, Beijing 100871, China ReceiVed: March 14, 2008; ReVised Manuscript ReceiVed: April 8, 2008

Single Bi2S3 nanowire devices were fabricated using the focused ion beam technique to deposit Pt electrodes. Repeatable and reliable ohmic contacts were obtained. The devices exhibit outstanding H2 sensing performance that can detect H2 (in N2) at room temperature with a sensitivity of 22% at 10 ppm (corresponds to 4.0 × 10-7 mol/L). Decorating Pd nanoparticles on the surface of the nanowire does not improve the sensing performance, contrary to most of the existing nanowire (or nanotube) H2 sensors. The sensing mechanism of Bi2S3 nanowire devices is found to be different from that of other sensors based on nanowires or nanotubes. Experiments on different H2 concentrations and gas pressures suggest that the sensing response is determined mainly by the amount of H2 molecules instead of the proportion of H2 in the mixed gas. 1. Introduction

2. Experimental Section

Chemical sensing based on various nanostructures has attracted enormous attention and is regarded as one of the most promising applications of nanoscience and nanotechnology.1–5 Being a very important chemical, H2 is used widely in many areas, for example, the chemistry industry, the semiconductor industry, and clean energy. However, H2 is a dangerous gas that can explode in air in certain concentrations. Detection of H2 is of great importance in both application and safety aspects. Numerous developments have been made to detect H2 in low concentrations, but few H2 sensors can reach the detection limit down to 10 ppm at room temperature. A Pd-InP Schottky diode has been reported to detect H2 down to 15 ppm.6 A heterojuction field-effect transistor (FET) has been reported to detect H2 down to 4.3 ppm.7 Because of the large surface to volume ratio of nanostructures, H2 sensors based on nanotubes and nanowires are expected to have higher sensitivities and lower detection limits than those of traditional sensors. Pd nanowires have been used to detect H2 in the percentage range based on the dilation of Pd grains undergoing H2 absorption.8–10 Sensors fabricated from Pd-functionalized Si nanowires have shown better sensitivity and faster responding time than the macroscopic Pd wire hydrogen sensor.11 A titanium dioxide nanotube array has been demonstrated to detect H2 down to 100 ppm at 290 K.12 Pdcoated ZnO nanorods array have been demonstrated to detect H2 down to 10 ppm at room temperature.13 Carbon nanotubes with Pd decoration have been found can detect H2 less than 10 ppm at room temperature.14 However, almost all of the highperformance H2 sensors based on nanowires and nanotubes reported so far rely on Pd decoration. The search for new materials, new structures, and new sensing mechanisms is desired to pursue H2 sensors with higher sensitivities, lower detection limits, and better convenience. In this work, we successfully fabricated H2 sensors with high sensitivity using individual Bi2S3 nanowires. The sensing mechanism is found to be different from other existing nanowire- (or nanotube)based H2 sensors.

Bi2S3 nanowires used in this work were fabricated by a simple hydrothermal method.15 Bi2S3 nanowires were first dispersed onto a Si substrate covered by a SiO2 layer with predefined electrodes on the top. A focused ion beam (FIB, STRATA DB235 system from FEI Company) was used to deposit Pt electrodes to connect Bi2S3 nanowires with predefined microsized electrodes. A 30 keV Ga+ ion beam was used with 27 pA current aperture. Current-voltage (I-V) curves were measured from the devices using Keithley 4200. Scanning electron microscopy (SEM) observation was done on a FEI XL30F microscope. Sensing studies were performed using a vacuum chamber (with 2L volume) having electrical feedthrough, pumping system, gas flow inlet and outlet, and a special gas injector (MIS-Controller 1.4 from Kleindiek Nanotechnik) to accurately control the quantity of injecting gas. The chamber was first pumped to a vacuum of 0.1 Pa followed by filling with N2 several times to flush the chamber and get a stable initial state for the sensing study.

* Corresponding author. E-mail: [email protected].

3. Results and Discussion In our previous work, n-type Bi2S3 nanowire field-effect transistors (FETs) have been fabricated using electron-beam lithography together with metal film deposition and a lift-off process. H2 has been found to increase current through enhancing carrier density and electronic mobility within the nanowires.16 However, it was observed that the contact and the device performance are not easy to control, probably because of the unknown surface condition of the nanowires. To solve the problem, here we used FIB to deposit Pt electrodes to connect Bi2S3 nanowires with predefined microsized electrodes. Figure 1a shows a SEM image of a Bi2S3 nanowire connected to two FIB-deposited Pt electrodes. The large electrode on the left is an Au electrode predefined by optical lithography together with thin film deposition. Perfect linear I-V curves were obtained from the nanowire device (shown in Figure 1b), indicating that the contacts between the nanowire and the Pt electrodes are ohmic. FIB is an effective technique to fabricate ohmic-contact nanowire devices,17 but a small amount of deposition outside the defined area is often observed. To evaluate the effect, we

10.1021/jp8022293 CCC: $40.75  2008 American Chemical Society Published on Web 05/20/2008

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Figure 1. (a) SEM image of a Bi2S3 nanowire connecting two Pt electrodes. (b) I-V curves measured from the nanowire and the substrate (without nanowire) between Pt electrodes separated by about 10 µm. (c) I-V curves of the nanowire measured in air, in vacuum, and in N2. (d) I-V curves of the nanowire measured at different times.

made several electrodes on the SiO2 substrate without connecting any nanowires. The current through two of these electrodes was then measured. We observed that the current could be on the level of 0.1 µA at 1 V bias when the two electrodes are separated by only a couple of micrometers. However, if the two electrodes are separated by about 10 µm, then the current between the two electrodes is less than 50 pA at 1 V bias, which is on the same level as that from the SiO2 substrate. Therefore, to avoid the effect of out-boundary deposition, here the electrodes in the nanowire devices were defined to separate about 10 µm. Figure 1b compares the I-V curves measured from the nanowire and the substrate (without nanowire) between Pt electrodes separated by 10 µm. The current measured from the substrate is at least 2 orders of magnitude smaller than that from the nanowire, indicating that the effect of out-boundary deposition can be ignored. I-V curves measured from the present nanowire devices are all linear and repeatable, implying stable and reliable ohmic contact. During FIB deposition, there is a short-time ion beam etching process before Pt deposition. This etching process may move away a thin surface layer of the nanowires and let the deposited Pt contact the nanowires directly and form a repeatable contact. Ion doping due to the ion beam exposure may also help to form the ohmic contact. I-V curves measured from present devices are almost the same in air, in vacuum (0.1 Pa), and in pure N2 (Figure 1c); they are also almost invariable in more than 2 weeks (Figure 1d), indicating that the Bi2S3 nanowire devices fabricated by FIB have excellent stability in the custom conditions. To study the H2 sensing performance, an individual nanowire device was placed in a vacuum chamber having a special gas injector to accurately control the quantity of injecting gas. H2 (mixed with N2) was injected into the chamber (filled with N2), and the current passing through the nanowires was measured at a fixed voltage. The total amount of the mixed gas injected each time is several milliliters at 1 atm. It was observed that when H2 was introduced into the chamber the current of the device increased. Figure 2a shows that the current of the device increases when H2 is introduced. It also shows that the current increases along with the time of exposure in mixed H2 and N2, while ohmic contact was maintained during the process. No current increase was observed with H2 when only substrate was measured for comparison, verifying that the current increase is

Yao et al.

Figure 2. (a) I-V curves of the Bi2S3 nanowire measured in vacuum and in 1% H2. (b) Current in the nanowire at 0.5 V bias vs time. (c) Current in the nanowire at 2 V bias vs time measured when different amounts of H2 or air were introduced into the chamber. (d) Sensitivities of five nanowire devices that respond to different H2 concentrations. The diameters of the five nanowires are also listed.

due to the influence of H2 to the nanowire device. Figure 2b plots the change of current in the nanowire at 0.5 V bias as a function of time. The current increased quickly by 2 times in the first 10 min after 1% H2 was introduced, and continuously increased with time although no more H2 was introduced. It takes about 2 h to reach the maximum current value at a fixed voltage. The current decreased quickly when the pump was restarted and the gas inside the chamber was pumped out. However, after 1 h of pumping (the pressure was kept at 0.1 Pa), the current still could not get back to the level before H2 injection. Flushing with N2 by filling the chamber with N2 could not let the current decrease to the initial value either. The complete recovery of the device was realized by introducing air. However, as shown in Figure 1c, air does not directly affect the electrical transport characteristics of Bi2S3 nanowires. The complete recovery of the device in air is probably due to the reaction between O2 and hydrogen.14,18 The above processes are repeatable. The sensing ability of five devices fabricated by FIB was studied by injecting different amounts of H2. Figure 2c shows the current response of a typical device to H2 with concentrations of 1000, 100, and 10 ppm at 2 V bias. The recovery was realized by introducing air. The current increases even for 10 ppm H2. All five sensors show similar sensing abilities. The sensitivities of the five devices were also calculated and shown in Figure 2d. Here, the sensitivity (S) is defined as S ) (GH - GN)/GN × 100%, where GH and GN are the conductance in diluted H2 and in N2, respectively. The sensitivities range from 3.2% to 22% for 10 ppm H2 and from 8.5% to 30.4% for 100 ppm H2. Such sensitivities are better than those of Pd-decorated ZnO nanorod arrays, which are 2.6% at 10 ppm H2 and 4.2% at 500 ppm H2.13 The present devices also have very low operating powers (∼10 nW), which is attractive for long-term hydrogen sensing applications. The present H2 sensors can be used for many times. We measured the sensing ability of one of the sensors in 1000 ppm H2 for four continuous times. We observed that the current got back to the original low level whenever air was introduced, while the current increase in H2 did not decrease for different sensing tests. The present H2 sensors are also stable in air for long time. We measured some sensors after one month and

Bi2S3 Nanowire-Based Room-Temperature H2 Sensor

Figure 3. (a) Current response curves of a nanowire sensor before and after Pd deposition. (b) Sensitivities of the nanowire sensor to different H2 concentrations before and after Pd deposition.

observed the same sensitivity in 10 ppm H2 as that obtained one month ago. Pd nanoparticles have been demonstrated to improve the sensing performance of H2 sensors made from nanowires or nanotubes.11,13,14 To the best of our knowledge, before our work, all of the nanowire- (or nanotube)-based H2 sensors with detection limits down to 10 ppm have Pd decoration.13,14 To study the effect of Pd to the present Bi2S3 sensors, we deposited 0.2-nm-thick Pd on the nanowires using electron beam deposition. The deposited Pd forms discontinuous nanoparticles (with diameter about 1 nm), verified by transmission electron microscopy. Figure 3a shows the current response of the same Bi2S3 nanowire device to H2 before and after Pd deposition. The current at the same bias increases about 30% after Pd deposition. However, the sensitivities of the sensor do not change significantly after Pd deposition (as shown in Figure 3b). It is known that hydrogen molecules dissociate into atomic hydrogen on Pd surfaces and atomic hydrogen dissolved into Pd and lowered the work function of Pd.14,18 The decrease of the work function of Pd is believed to cause electron transfer from Pd to the nanowire or the nanotube and change the current in the device enormously.14 However, such a mechanism cannot explain what we observed for Bi2S3 nanowire sensors. The sensing mechanism of Bi2S3 nanowire H2 sensors is different from that of other nanowire or nanotube sensors. The present FIB-made devices have linear I-V curves in the air and in H2, indicating that the contacts between FIB-deposited Pt and the Bi2S3 nanowires are ohmic. Our previous work shows that H2 increases the current inside Bi2S3 nanowires by increasing carrier density and mobility instead of changing the contact between Pd and the Bi2S3 nanowire.16 We believe that the main sensing mechanism of the present FIB-made Bi2S3 nanowire H2 sensors is hydrogen-induced carrier density increase and mobility increase in the Bi2S3 nanowire. The current increase after Pd deposition indicates that there could be electron transfer from Pd to Bi2S3 nanowires even before H2 injection, which means the Fermi level of Pd is higher than that of Bi2S3 before the two materials contact each other. That H2 increases the electron density in Bi2S3 nanowires may imply that H2 also increases the Fermi level of Bi2S3 nanowires (or maybe of the nanowire surface). The fact that the sensitivity of Pd-decorated Bi2S3 nanowires to H2 is roughly the same as that of pure Bi2S3 nanowires indicates that the amount of Fermi level increase of Pd due to H2 might be roughly the same as that of the amount of Fermi level increase of Bi2S3 nanowires at least in the H2 concentration range we studied. We noticed that our devices take a relatively longer time to reach their highest current compared with previously reported nanowire- (or nanotube)-based H2 sensors. One of the reasons could be that in the present experiments, instead of continuously blowing the diluted H2 gas, we just injected a fixed amount of H2 gas into the chamber once for one test. The concentration

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Figure 4. Current response curves of the same nanowire sensor measured when H2 was introduced into different amounts of N2.

of H2 decreases with time because of the adsorption by the nanowires and by the surface of the chamber. Another reason could be that besides being absorbed by the surface H2 may also diffuse into the nanowires and further increase the current. The latter process normally takes a long time. Electrochemical hydrogen storage of Bi2S3 nanostructures has been reported,19 supporting that H2 can diffuse into Bi2S3. Our experiments show that pumping the chamber only cannot get rid of the hydrogen thoroughly and air plays an important role in decreasing the current completely. However, as shown in Figure 1c, air and N2 do not directly affect the electrical transport character of Bi2S3 nanowires. The complete recovery of the device in air might be due to the reaction between oxygen and hydrogen. The dissolved atomic hydrogen combines with oxygen in air, departing the nanowire in the form of water and thus recovering the sample’s electrical characteristics.14,18 The unit “ppm” is used widely in the gas sensor field. It is normally defined as the volume proportion (or mole proportion) between two gases. Here, we observed that “ppm” could not exactly describe the sensing ability of the Bi2S3 nanowire sensor. Figure 4 shows the current response of the same nanowire sensor to H2. Line A was measured when the mole concentrations of the injected H2 in N2 were 1000, 100, and 10 ppm. Line B was measured when the same amount of H2 was injected but the amount of N2 was about 2 orders of magnitude smaller. As a result, the mole proportion (with the unit of ppm) of H2 in N2 is 2 orders of magnitude larger than that in process A. However, Figure 4 shows that the current responses are almost the same in the two processes, indicating that it is the absolute amount of H2 that determines the sensing response of the nanowire sensor. The units such as “g/L” or “mol/L” may be more suitable and more accurate to describe the sensing ability of our devices. In the present case, 10 ppm H2 corresponds to 4.0 × 10-7 mol/L. 4. Conclusions Bi2S3 nanowire devices were fabricated with reliable and repeatable ohmic contacts using the FIB technique to deposit Pt electrodes. Single nanowire devices were found to detect H2 (in N2) at room temperature with a sensitivity of 22% at 10 ppm (corresponds to 4.0 × 10-7 mol/L). The sensing mechanism of the Bi2S3 nanowire device is mainly due to the increase of electron density and mobility caused by H2, while diffusion of H2 into the nanowires may also be involved. This mechanism is different from that of other existing H2 sensors based on nanowires and nanotubes. Contrary to most of the existing nanowire- (or nanotube)-based H2 sensors, Pd decoration on the surface of the Bi2S3 nanowire does not improve the sensing performance significantly at least in the concentration range we studied, although the current increases somewhat at the same bias. The mechanism is discussed based on the effect of H2 on Pd and on Bi2S3. Experiments on different H2 concentrations

8724 J. Phys. Chem. C, Vol. 112, No. 23, 2008 and gas pressures suggest that the sensing response is determined mainly by the amount of H2 molecules instead of the proportion of H2 in the mixed gas, indicating that units like “g/L” or “mol/ L” are more suitable than ppm to describe the performance of the gas sensor. Acknowledgment. We thank Dr. Y. Yu for supplying the specimen and Mr. J. Xu for the FIB operation. We also thank Dr. H. Y. Pan, Mr. X. L. Wei, and Mr. Y. Liu for the TEM operation. This work was supported by NSF of China (60771005, 60728102, 60571002, 10434010) and the MOST (2006CB932401). References and Notes (1) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. Science 2000, 287, 622. (2) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Science 2001, 293, 1289. (3) Law, M.; Kind, H.; Messer, B.; Kim, F.; Yang, P. Angew. Chem., Int. Ed. 2002, 41, 2405. (4) Zhang, D.; Liu, Z.; Li, C.; Tang, T.; Liu, X.; Han, S.; Lei, B.; Zhou, C. Nano Lett. 2004, 4, 1919. (5) Ponzoni, A.; Comini, E.; Sberveglieri, G.; Zhou, J.; Deng, S. Z.; Xu, N. S.; Ding, Y.; Wang, Z. L. Appl. Phys. Lett. 2006, 88, 203101. (6) Chou, Y. I.; Chen, C. M.; Liu, W. C.; Chen, H. I. IEEE Electron DeVice Lett. 2005, 26, 62.

Yao et al. (7) Huang, C. W.; Chang, H. C.; Tsai, Y. Y.; Lai, P. H.; Fu, S. I.; Chen, T. P.; Chen, H. I.; Liu, W. C. IEEE Electron DeVice Lett. 2007, 52, 1224. (8) Favier, F.; Walter, E. C.; Zach, M. P.; Benter, T.; Penner, R. M. Science 2001, 293, 2227. (9) Im, Y.; Lee, C.; Vasquez, R. P.; Bangar, M. A.; Myung, N. V.; Menke, E. J.; Penner, R. M.; Yun, M. Small 2006, 2, 356. (10) Atashbar, M. Z.; Banerji, D.; Singamaneni, S. IEEE Sens. J. 2005, 5, 792. (11) Chen, Z. H.; Jie, J. S.; Luo, L. B.; Wang, H.; Lee, C. S.; Lee, S. T. Nanotechnology 2007, 18, 45502. (12) Varghese, O. K.; Gong, D.; Paulose, M.; Ong, K. G.; Grimes, C. A. Sens. Actuators, B 2003, 93, 338. (13) Wang, H. T.; Kang, B. S.; Ren, F.; Tien, L. C.; Sadik, P. W.; Norton, D. P.; Pearton, S. J.; Lin, J. Appl. Phys. Lett. 2005, 86, 243503. (14) Kong, J.; Chapline, M. G.; Dai, H. J. AdV. Mater. 2001, 13, 1384. (15) Yu, Y.; Jin, C. H.; Wang, R. H.; Chen, Q.; Peng, L.-M. J. Phys. Chem. B 2005, 109, 18772. (16) Yao, K.; Zhang, Z. Y.; Liang, X. L.; Chen, Q.; Peng, L.-M.; Yu, Y. J. Phys. Chem. B 2006, 110, 21408. (17) Long, Y.; Chen, Z.; Wang, W.; Bai, F.; Jin, A.; Gu, C. Appl. Phys. Lett. 2005, 86, 153102. (18) Mandelis, A.; Christofides, C. Physics, Chemistry and Technology of Solid State Gas Sensor DeVices; Wiley: New York, 1993. (19) Zhang, B.; Ye, X.; Hou, W.; Zhao, Y.; Xie, Y. J. Phys. Chem. B 2006, 110, 8978.

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