Shell PN

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Synthesis and H2S Sensing Properties of CuO-SnO2 Core/Shell PN-Junction Nanorods Xinyu Xue,*,† Lili Xing,† Yujin Chen,‡ Songlin Shi,§ Yanguo Wang,§ and Taihong Wang§ College of Science, Northeastern UniVersity, Shenyang 110004, China, College of Science, Harbin Engineering UniVersity, Harbin 150001, China and Micro-Nano Technologies Research Center, Hunan UniVersity, Changsha 410082, China ReceiVed: April 30, 2008; ReVised Manuscript ReceiVed: May 28, 2008

One-dimensional nanosized core/shell PN-junctions are formed from N-type SnO2 nanorods (synthesized via a hydrothermal method; diameter ∼10 nm, length ∼100 nm) uniformly coated with P-type CuO nanoparticles (diameter ∼4 nm). Gas sensors are realized from these PN-junction nanorods, and their resistances greatly decrease upon exposed to H2S at room temperature. The sensitivity against 10 ppm H2S at 60 °C is up to 9.4 × 106. At the same time, the sensors have very good selectivity against H2S. Such good performances are probably attributed to the destruction of PN-junctions and the small size effect of nanostructures. Our results imply that one-dimensional heterostructured nanomaterials are promising candidates for high-performance gas sensors. Introduction Recently, one-dimensional (1D) heterostructured nanomaterials have attracted great attention due to their novel and distinct properties, and more effort has been taken to explore highperformance nanodevices based on them.1–9 Nowadays, nanowires, nanotubes, and nanorods with transverse or core/shell heterostructures have been reported, and their applications are mainly focused on the electronic and optoelectronic nanodevices, such as light-emitting diodes, photodetectors, and transistors.4–9 However, 1D heterostructured nanomaterials used in gas sensors have rarely been investigated. In the past years, PN-junctions between P-type CuO and N-type SnO2 bulk materials have been reported to have high H2S sensing.10–12 By considering the high surface-to-volume ratio and the small size effect of 1D nanostructures,13–17 CuO-SnO2 1D PN-junction nanomaterials should be promising candidates for high-performance H2S sensors with low work temperature, high sensitivity, and good selectivity. In this work, 1D nanosized core/shell PN-junctions are formed from N-type SnO2 nanorods coated with P-type CuO nanoparticles. The SnO2 nanorods, which are synthesized by a hydrothermal method, have a diameter of ∼10 nm and length of ∼100 nm. The PN-junctions are uniformly distributed along the nanorods via coating with Cu(NO3)2 and then annealing. The sensors realized from these PN-junction nanorods have extremely high sensitivity against H2S at room temperature. At 60 °C, the sensitivity upon exposure to 10 ppm H2S is up to 9.4 × 106. And both the sensitivity and recovery time decrease as the temperature increases. At the same time, the sensors show very good selectivity against H2S. These behaviors are probably attributed to the conversion of CuO-SnO2 PN-junction to CuS-SnO2 Ohmic contact and the small size effect of nanorods. Our results imply that 1D heterostructured nanomaterials have potential applications in gas sensors. * To whom correspondence should be addressed. E-mail: xuexinyu@ mail.neu.edu.cn. Phone: +86-024-8367 8326. † Northeastern University. ‡ Harbin Engineering University. § Hunan University.

Figure 1. (a) SEM image of SnO2 nanorods. (b) High-resolution TEM image of one single SnO2 nanorod.

Experimental Section SnO2 nanorods were synthesized under hydrothermal conditions, similar to synthesis strategy we reported in refs 13 and 14. Here, 7 mL of aqueous solution containing SnCl4 (0.45 g) and NaOH (0.35 g) was prepared in a beaker, and 30 mL of water-ethanol mixture was added. After being ultrasonically dispersed for 5 min, the mixture was transferred into a 50 mL

10.1021/jp8037818 CCC: $40.75  2008 American Chemical Society Published on Web 07/22/2008

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Figure 2. (a) SEM image of CuO-SnO2 PN-junction nanorods. (b) EDS spectrum of CuO-SnO2 PN-junction nanorods. (c) XRD pattern of CuO-SnO2 PN-junction nanorods. (d) High-resolution TEM image of CuO-SnO2 PN-junction nanorods. The inset is the enlarged view of the junction region.

Teflon-lined autoclave that was heated to 190 °C and kept for 24 h. The collected products were purified with water and ethanol. Nontoxic reagents and mass production make this synthesis method very attractive. The general morphology, crystal structure, and microstructure of the products were characterized by scanning electron microscope (SEM; JEOLJSM-6700F), X-ray powder diffraction (XRD; D/max 2550V, Cu KR radiation), and transmission electron microscope (TEM; TECNAI F20). CuO-SnO2 PN-junction nanorods were obtained by a simple chemical method. SnO2 nanorods (0.3g) and 0.05 g of Cu(NO3)2 · 3H2O were added in 50 mL of ethanol. After ultrasonic treatment for 30 min, the solution was stirred for 2 h. The resulting green products were filtered and dried at 75 °C. Then, the products were loaded in an alumina boat, which was placed at the center of a horizontal tube furnace. The furnace was slowly heated to 800 °C at the rate of 5 deg/min, and kept for 2 h. After the furnace had slowly cooled to room temperature, the black products were collected. SEM, energy dispersive X-ray spectrometer (EDS), XRD, and TEM were used to characterize the PN-junction nanorods. These CuO-SnO2 PN-junction nanorods were then used to fabricate H2S sensors. First, the PN-junction nanorods were ultrasonically dispersed in ethanol, and dried at 75 °C for 1 h. The paste was then laid uniformly on the ceramic tubes of the sensors. Two Pt electrodes were placed on the surface of the ceramic tube, and a heating resistance coil was inserted into the tube.13–16 The paste should completely cover on the two Pt electrodes. The sensors were dried at 100 °C for 1 h, and a 6 h aging process at 300 °C was performed. Then, the sensors were connected to the outside electronics, which monitored the

voltage on the load resistance (4.7 MΩ). The voltage applied to the whole circuit was 5 V. The H2S sensing properties of the sensors are investigated in a gas flow chamber. The sensitivity S is defined as S (%) ) Ra/Rg × 100, where Ra is the resistance of the sensors in air ambience, and Rg is the resistance of the sensors in H2S gas.10–20 Results and Discussion Figure 1a is a typical SEM image of SnO2 nanorods. It can be seen that the products are highly dominated by the nanorods with the average length of 100 nm and average diameter of 10 nm. The XRD pattern of the products completely coincides with the results we reported in refs 13 and 14 (JCPDS file No. 411445), which confirms that the products are SnO2 nanorods with rutile crystal structures. A high-resolution TEM image of one single SnO2 nanorod is shown in Figure 1b. It can be seen that SnO2 nanorods have single-crystal structure and a clean surface. Figure 2a is a typical SEM image of CuO-SnO2 PN-junction nanorods. It can be seen that the diameter of the PN-junction nanorods is larger than that of pure SnO2 nanorods. Figure 2b is the EDS spectrum of CuO-SnO2 PN-junction nanorods, in which the peak of Cu can be clearly observed. The presence of the Si peaks in the spectra comes from Si substrates in the SEM experiment. Figure 2c is a typical XRD pattern of CuO-SnO2 PN-junction nanorods. The asterisked peaks are indexed to CuO with tenorite crystal structures (JCPDS file No. 41-0254). Figure 2d is the high-resolution TEM image of one single CuO-SnO2 PN-junction nanorod. It can be seen that PN-junctions are uniformly distributing along the nanorods and the average diameter of CuO nanoparticles is 4 nm. The inset of Figure 2d is the enlarged view of the junction region. Different crystal

CuO-SnO2 Core/Shell PN-Junction Nanorods

J. Phys. Chem. C, Vol. 112, No. 32, 2008 12159 TABLE 1: H2S Sensors Based on CuO-SnO2 PN-Junction Materials Reported before and in This Work materials thick film thin film nanoparticles nanoribbons nanorods

Figure 3. (a) Response and recovery characteristics of CuO-SnO2 PN-junction nanorod sensors upon exposed to 10 ppm H2S at 18, 60, 95, and 180 °C, respectively. (b) The sensitivity of the sensors at various temperatures. The inset is the response of the sensors at 60 °C, and 5 V heating voltage (300 °C) is used to quicken the recovery process.

structures with the lattice fringe spacing of 0.25 and 0.33 nm are observed on the two sides of the boundary, which coincide with {002} planes of CuO and {100} planes of SnO2, respectively. These results indicate that the PN-junctions are well formed between CuO nanoparticles and SnO2 nanorods. Figure 3a shows the response and recovery of CuO-SnO2 PN-junction nanorod sensors upon exposure to 10 ppm H2S at 18, 60, 95, and 180 °C, respectively. It is obvious that the sensors show switch-like response after H2S exposure and the response is extremely high. At a work temperature of 60 °C, the voltage on the load resistance greatly increases from 0.044 to 4.994 V. The response time is about 30 s and the recovery time is several hours. Raising the work temperature can

size 12 cm 320 nm 90 nm 0.1 × 100 µm 10 × 100 nm

concn, ppm 100 150 20 3 10

temp, °C

sensitivity

ref

200 140 150 50 60

7.5 × 6.5 × 106 7.3 × 105 2 × 104 9.4 × 106

10 11 12 20 this work

105

greatly shorten both the recovery and response time. But the response of the sensors also decreases as the work temperature increases. At 180 °C, the change of voltage is merely from 0.097 to 4.885 V. The sensitivity of the sensors against 10 ppm H2S at different temperatures is shown in Figure 3b. The sensitivity at 60 °C is the highest (9.4 × 106), and it decreases with increasing temperature. As a result, the optimum work temperature of the sensors is 60 °C. Detecting H2S at such a low temperature is very helpful for chemical industries and research laboratories. However, the recovery time at 60 °C is too long, which limits their application at the industrial level. To quicken the recovery process, heating voltage can be applied to the heating coil of the sensors. As shown in the inset of Figure 3b, a 5 V heating voltage (300 °C) can reduce the recovery time to about 30 s. The selectivity of the sensors has been investigated at the same condition, and the experimental gases include ethanol, SO2, and H2. At room temperature, the changes of the resistance of the sensors against these gases are all less than 1 order of magnitude, much smaller than that against H2S. For example, the resistance of the sensors upon being exposed to 200 ppm of ethanol gas at room temperature merely decreases by a fact of 2. Good selectivity of CuO-SnO2 PN-junction nanorods facilitates their application in industry for detecting H2S. Table 1 is the comparison of sizes, work temperatures, and sensitivities between the H2S sensors reported before and in this work. It can be seen that the sensitivity increases as the size of the materials decreases, and the 1D nanostructure can lower the work temperature. Compared with the other H2S sensors reported so far, the highest sensitivity at low temperature is achieved in this work. Such extremely high sensitivity at low temperature is probably attributed to both the destruction of PNjunctions and the small size effect of 1D nanostructures. By only considering the case of two PN-junction nanorods, an energy band diagram is simply shown in Figure 4 to describe the response and recovery process of CuO-SnO2 PN-junction nanorod sensors against H2S. Numerous PN-junctions between SnO2 nanorods and CuO nanoparticles are uniformly distributed along the nanorods, and the two nanorods are connected to each other via the PN-junctions. In air, the barriers at the PN-junctions

Figure 4. Energy band diagram of CuO-SnO2 PN-junction nanorod sensors showing the H2S sensing process.

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greatly block the electrons transporting through the nanorods, and the resistance of the sensors is very high. After putting the sensors in H2S, the CuO nanoparticles are converted to CuS nanoparitcles by the following chemical reaction:10–12,18–20

CuO + H2S f CuS + H2O

(1)

It is reported that CuS is metallic,10–12,18–20 thus the PN-junctions are destroyed. As discussed in ref 10, since the work function of metallic CuS is lower than that of SnO2, the band bends downward and no barrier exists between them, which makes for easy flow of the electrons. Thus, the resistance of the sensors in H2S is much lower than that in air. After taking the sensors out of the chamber, CuS is oxidized by the oxygen in air and converted back to CuO. The CuO-SnO2 PN-junctions recover, and the resistance of the sensors recovers. The conversion of CuS to CuO is slow at low temperature, and the rate of oxidation increases with increasing temperature.11 This is the reason why the recovery rate of the sensors is slower at low temperature than that at high temperature. CuS is converted to Cu2S at temperatures above 103 °C and the conductance of Cu2S is lower than that of CuS,19,20 which leads to the decrease of sensitivity with increasing temperature. The small sizes of the PN-junction nanorods make the amount of PN-junctions in the devices much larger than that of bulk material devices. Therefore, the sensitivity of 1D nanosized PN-junctions is larger than that of bulk materials. The small size effect of the SnO2 nanorods also does contribute to such high sensitivity, and should be considered in the energy band diagram. The diameter of SnO2 nanorods (D) is 10 nm, and it is close to the depletion width (for SnO2, Ld is 3 nm) caused by the adsorbed oxygen on the surface.13,14 Thus, the conductive channel (D - 2Ld) of SnO2 nanorods in air is very narrow. And the depletion layer dominates the resistance of SnO2 nanorods, which usually lead to high gas sensing. The energy band bends upward at the depletion layer, and the barrier width is wider than that of bulk materials and large-sized nanoribbons, as shown in Figure 4. H2S is a kind of reducing gas, which can reduce the amount of the adsorbed oxygen, decreasing both the depletion width and the barrier width. Compared with bulk materials and large-sized nanoribbons, the depletion layer of small-sized nanorods has more contribution to the H2S sensing and cannot be neglected. Conclusions In summary, 1D nanosized core/shell PN-junctions were formed from N-type SnO2 nanorods coated with P-type CuO

nanoparticles. On the basis of them, extremely high sensitivity and selectivity against H2S at low temperature were achieved. Such good performances were probably attributed to the destruction of PN-junctions and the small size effect of nanostructures. Our results suggested that 1D heterostructured nanomaterials have potential applications in gas sensors. Acknowledgment. This work was partly supported by the Northestern University Elite Foundation (No. 28720606), the Special Funds for Major State Basic Research Project (No. 2007CB310500), and the National Natural Science Foundation of China (Nos. 90606009, 60571044, 10774174, and 50772025). References and Notes (1) Park, W. I.; Yi, G. C.; Kim, M. Y.; Pennycook, S. J. AdV. Mater. 2003, 15, 526. (2) Hu, J. Q.; Bando, Y.; Liu, Z. W.; Sekiguchi, T.; Golberg, D.; Zhan, J. H. J. Am. Chem. Soc. 2003, 125, 11306. (3) Banerjee, S.; Wong, S. S. Nano Lett. 2002, 2, 195. (4) Cassell, A. M.; Li, J.; Stevens, R. M. D.; Koehne, J. E.; Delzeit, L.; Ng, H. T.; Ye, Q.; Han, J.; Meyyappan, M. Appl. Phys. Lett. 2004, 85, 2364. (5) Thelander, C.; Martensson, T.; Bjork, M. T.; Ohlsson, B. J.; Larsson, M. W.; Wallenberg, L. R.; Samuelson, L. Appl. Phys. Lett. 2003, 83, 2052. (6) Bjork, M. T.; Ohlsson, B. J.; Sass, T.; Persson, A. I.; Thelander, C.; Magnusson, M. H.; Deppert, K.; Wallenberg, L. R.; Samuelson, L. Nano Lett. 2002, 2, 87. (7) Qian, F.; Li, Y.; Gradecak, S.; Wang, D.; Barrelet, C. J.; Lieber, C. M. Nano Lett 2004, 4, 1975. (8) Peng, K. Q.; Huang, Z. P.; Zhu, J. AdV. Mater. 2004, 16, 73. (9) Qian, F.; Gradecak, S.; Li, Y.; Wen, C. Y.; Lieber, C. M. Nano Lett. 2005, 5, 2287. (10) Manorama, S.; Devi, G. S.; Rao, V. J. Appl. Phys. Lett. 1994, 64, 3163. (11) Khanna, A.; Kumar, R.; Bhatti, S. S. Appl. Phys. Lett. 2003, 82, 4388. (12) Chowdhuri, A.; Gupta, V.; Sreenivas, K.; Kumar, R.; Mozumdar, S.; Patanjali, P. K. Appl. Phys. Lett. 2004, 84, 1180. (13) Chen, Y. J.; Xue, X. Y.; Wang, Y. G.; Wang, T. H. Appl. Phys. Lett. 2005, 87, 233503. (14) Chen, Y. J.; Nie, L.; Xue, X. Y.; Wang, Y. G.; Wang, H. Appl. Phys. Lett. 2006, 88, 083105. (15) Xue, X. Y.; Chen, Y. J.; Wang, Y. G.; Wang, T. H. Appl. Phys. Lett. 2005, 86, 233101. (16) Xue, X. Y.; Chen, Y. J.; Liu, Y. G.; Shi, S. L.; Wang, T. H. Appl. Phys. Lett. 2006, 88, 201907. (17) Wan, Q.; Li, Q. H.; Chen, Y. J.; Wang, T. H.; He, X. L.; Li, J. P.; Lin, C. L. Appl. Phys. Lett. 2004, 84, 3654. (18) Katti, V. R.; Debnath, A. K.; Muthe, K. P.; Kaur, M.; Dua, A. K.; Gadkari, S. C.; Gupta, S. K.; Sahni, V. C. Sens. Actuators B 2003, 96, 245. (19) Zhou, X. H.; Cao, Q. X.; Huang, H.; Yang, P.; Hu, Y. Mater. Sci. Eng., B 2003, 99, 44. (20) Kong, X.; Li, Y. Sens. Actuators B 2005, 105, 449.

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