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The conversion of PN-junction influencing the piezoelectric output of a CuO/ZnO nanoarray nanogenerator and its application as a room-temperature self-powered active H2S sensor
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 265501 (http://iopscience.iop.org/0957-4484/25/26/265501) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 25 (2014) 265501 (9pp)
doi:10.1088/0957-4484/25/26/265501
The conversion of PN-junction influencing the piezoelectric output of a CuO/ZnO nanoarray nanogenerator and its application as a room-temperature self-powered active H2S sensor Yuxin Nie1, Ping Deng1, Yayu Zhao1, Penglei Wang1, Lili Xing1, Yan Zhang2,3,4 and Xinyu Xue1,2,4 1
College of Sciences, Northeastern University, Shenyang 110004, People’s Republic of China Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100000, People’s Republic of China 3 Institute of Theoretical Physics, Lanzhou University, Lanzhou 730000, People’s Republic of China 2
E-mail: [email protected] and [email protected] Received 26 March 2014, revised 29 April 2014 Accepted for publication 7 May 2014 Published 11 June 2014 Abstract
Room-temperature, high H2S sensing has been realized from a CuO/ZnO nanoarray selfpowered, active gas sensor. The piezoelectric output of CuO/ZnO nanoarrays can act not only as the power source of the device, but also as the H2S sensing signal at room temperature. Upon exposure to 800 ppm H2S at room temperature, the piezoelectric output of the device greatly decreased from 0.738 V (in air) to 0.101 V. The sensitivity increased to 629.8, much higher than bare ZnO nanoarrays. As the device was exposed to H2S, a CuO/ZnO PN-junction was converted into a CuS/ZnO Ohmic contact, which greatly increased the electron density in the nanowire and enhanced the screen effect on the piezoelectric output. Our results can stimulate a research trend on designing new composite piezoelectric material for high-performance selfpowered active gas sensors. Keywords: nanogenerators, self-powered, H2S sensing, PN-junctions, screen effect (Some figures may appear in colour only in the online journal) 1. Introduction
back to the conduction band of the nanostructures, narrow the depletion layer and decrease the resistance of the nanostructures (N-type) [11, 12]. Nowadays, many efforts have been made to reduce their work temperature, such as UV assistance [13] and noble metal decoration [14]. In our previous work, we demonstrated that an unpackaged ZnO nanowire (NW) piezo-nanogenerator (NG) can work as a selfpowered active gas sensor at room temperature by coupling the piezoelectric and gas sensing characteristics of ZnO NWs [15]. The piezoelectric output generated by ZnO NW under applied force acts not only as a power source, but also as a response signal to the test gas [15–17]. This new self-powered active gas sensor is sensitive to H2S at room temperature
Gas sensors for detecting toxic gas, such as H2S, CO and NO, have very important applications in industry and society [1–5]. In recent years, one-dimensional (1D) metal oxide nanostructures have been widely reported to be good candidates for highly sensitive and stable gas sensors due to their high surface-to-volume ratio, such as ZnO, SnO2 and In2O3 nanowires/tubes [6–10]. Upon exposure to reducing gas, the gas molecules will react with the adsorbed oxygen ions on the surface of the nanostructures, which can release free electrons 4
Authors to whom any correspondence should be addressed.
0957-4484/14/265501+09$33.00
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through the variety of the screen effect on the piezoelectric output [15]. This new piezo-gas sensing mechanism opens a new way for the next generation of gas sensors and expands the scope of self-powered nanosystems [18–20]. The selfpowered system integrating NGs and sensors aims at harvesting mechanical energy in the environment to power the sensors. For future practical applications of self-powered active gas sensors, their performance needs to be further improved, especially with regard to enhancing the sensitivity. Heterostructured nanomateirals, such as CuO/ZnO and CuO/SnO2 core-shell nanowires/rods, have been confirmed to have extremely high sensitivity against various gases [12, 21–23]. Upon exposure to H2S, CuO (P-type semiconductor) will react with H2S and convert into CuS (a typical metal with a smaller work function than ZnO or SnO2) [24]. The conversion of PN-junction to Ohmic contact results in an anti-blocking layer at the interface through electrons transferring from CuS into ZnO or SnO2 [12, 25]. This process significantly decreases the resistance of the heterostructured nanomaterials, resulting in extremely high H2S sensitivity. If the PN-heterostructures can be introduced into self-powered active gas sensors, then room-temperature highly sensitive H2S sensing can probably be realized. In this paper, CuO/ZnO PN-junction nanoarrays have been used to fabricate self-powered active gas sensors and room-temperature high H2S gas sensing has been obtained. Upon exposure to 800 ppm H2S at room temperature, their sensitivity is up to 629.8, much higher than bare ZnO NW self-powered active gas sensors. The conversion of CuO/ ZnO PN-junction to CuS/ZnO Ohmic contact greatly increases the electron density in the NW, resulting in a strong screen effect on the piezoelectric output. Our study can stimulate a research trend on designing a new material system for high-performance self-powered active gas sensors.
Figure 1. Fabrication process of the self-powered active gas sensor
based on CuO/ZnO PN-junction nanoarrays. (a) A pre-cleaned Ti foil is used as the substrate. (b) ZnO seeds are deposited on Ti foil. (c) Vertically aligned ZnO NW arrays are grown on Ti foil. (d) CuO nanocones are coated on the surface of ZnO NWs by a wet-chemical method. (e) A sheet of flexible aluminum foil as the counterelectrode is positioned on top of CuO/ZnO PN-junction nanoarrays. (f) Schematic diagram showing the structure of the self-powered active gas sensor with two Kapton boards as the frame.
dried at 60 °C. CuO nanocones were then uniformly coated on the surface of ZnO NW arrays by a simple wet-chemical method followed by an annealing process, as shown in figure 1(d). A 5 mM aqueous solution of Cu(NO3)2 • 3 H2O was prepared. The solution was transferred to a 60 ml Teflon-lined stainless steel autoclave, and the Ti substrate with ZnO NW arrays was placed vertically into the autoclave. After the autoclave was kept at 60 °C for 4 h, CuO/ ZnO PN-junction nanoarrays growing on the Ti foil were removed from the autoclave, washed with deionized water and alcohol, and dried at 60 °C. Finally, an annealing process at 300 °C for 2 h was performed to ensure the formation of PN-junction between CuO and ZnO. As the counter-electrode, a sheet of flexible aluminum foil (thickness = 0.05 mm) was positioned on top of CuO/ZnO PN-junction nanoarrays, as shown in figure 1(e). After that, the device was tightly fixed between two sheets of flexible Kapton board as support frames to ensure the electrical contact between the aluminum foil and the NWs, as shown in figure 1(f). It has been confirmed that flexible Kapton films as substrates can follow the height profiles of the NW arrays and make effective contacts between the tips of NWs and electrodes [26–29]. Under externally applied compressive deformation, the piezoelectric output of the device was very sensitive to the outside atmosphere at room temperature. Compared with the prototype ZnO NW self-powered active gas sensor in our previous report [15], the conversion of CuO/ZnO PNjunction to CuS/ZnO Ohmic contact can more greatly increase the electron density in ZnO NW, resulting in a stronger screen effect on the piezoelectric output.
2. Methods Figure 1 shows the brief fabrication process and the final device structure of the self-powered active gas sensor based on CuO/ZnO PN-junction nanoarrays. CuO/ZnO PNjunction nanoarrays were fabricated using a two-step method. ZnO NW arrays were synthesized by a seedassisted hydrothermal method. A piece of flat Ti foil as both the substrate of ZnO NW arrays and the electrode was cleaned with water/alcohol and dried in a nitrogen stream, as shown in figure 1(a). As shown in figure 1(b), a ZnO seed layer was then deposited on the Ti substrate by a wetchemical-annealing method reported in our previous work [15]. Vertically aligned ZnO NW arrays were grown via a hydrothermal route, as shown in figure 1(c). The Ti substrate coated with ZnO seeds was immersed into 200 ml of equimolar (50 mM) aqueous solution of Zn(NO3)2 • 6 H2O and HMTA in a reaction flask. After keeping at 90 °C for 2 h, the substrate with ZnO NW arrays grown on was removed from the solution, rinsed with deionized water and 2
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Figure 2. (a) SEM image of ZnO NW arrays on Ti foil in a top view. The inset is an enlarged view of a selected area. (b) SEM image of CuO/ ZnO nanoarrays on the top-view. The inset is an enlarged view of a selected area. (c) TEM image of ZnO NW. (d) TEM image of CuO/ZnO NW. (e) HRTEM image of the tip region of a ZnO NW. (f) HRTEM image of the surface of CuO/ZnO NW.
3. Results and discussion
region of a ZnO NW, showing that ZnO NW is of single crystalline with a growth direction along the c-axis. The HRTEM image of a CuO/ZnO PN-junction NW is shown in figure 2(f), and the lattice fringe spacing of 0.23 nm corresponds to {111} atomic spacing of CuO. Figure 3(a) shows the XRD pattern of CuO/ZnO PNjunction nanoarrays on the Ti substrate. The sharp diffraction peaks indicate the good crystalline quality. The peaks marked by a diamond can be indexed to Ti (JCPDS file No. 44–1294) arising from the Ti foil substrate; the peaks marked by a solid pentagram can be indexed to ZnO (JCPDS file No. 36–1451); and the peaks marked by an inverted triangle can be indexed to CuO (JCPDS file No. 45–0937). Figure 3(b) shows the EDS spectrum of CuO/ ZnO PN-junction nanoarrays. Three elements (O, Zn and Cu) exist at the selected region, and similar EDS results have been obtained at several different areas. These results
Figure 2(a) is a typical scanning electron microscopy (SEM) image of the synthesized ZnO NW arrays on the top view. A selected area is enlarged in the inset of figure 2(a), showing the typical hexagonal structure of ZnO NW [30, 31]. The topview SEM image of CuO/ZnO PN-junction nanoarrays is presented in figure 2(b), indicating that CuO nanocones are uniformly distributed on the whole surface of ZnO NW arrays. Such a core-shell structure can be more clearly observed in the inset of figure 2(b), and the thickness of the CuO nanocone layer is about 400 nm. Figures 2(c) and (d) are transmission electron microscopy (TEM) images of ZnO NW and CuO/ZnO PN-junction NW, respectively. It can be seen that cone-shaped CuO nanostructures are evenly covered on the smooth surface of ZnO NW. Figure 2(e) is high-resolution transmission electron microscopy (HRTEM) image of the tip 3
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Figure 3. (a) XRD pattern of CuO/ZnO nanoarrays. (b) EDS spectrum of CuO/ZnO nanoarrays.
further confirm that ZnO NW arrays are uniformly coated with CuO nanocones. The piezoelectric output voltage of one device in different gases is shown in figure 4. All the experimental measurements were conducted at room temperature under the constant applied strain (0.012%, frequency = 0.4 Hz). The compressive force was applied by a hammer. The area of the hammer for applying the compressive force was larger than that of the device. The hammer was controlled by a programed motor, and the force and frequency were constant. Thus, the hammer provided stable and uniform strain on the device for the experimental measurement. When the device was in dry air, the piezoelectric output voltage induced by the compressive strain was 0.738 V, as shown in figure 4(a). To obtain the piezoelectric output response in H2S, a mixed gas of H2S and dry air was injected into the test chamber. At concentrations of H2S of 200, 300, 400, 500, 600 and 800 ppm, the piezoelectric output voltage of the device was 0.461, 0.371, 0.291, 0.181, 0.104 and 0.101 V, respectively, as shown in figures 4(b)–(g). The sensitivity S of the device against H2S under the same deformation conditions can be defined as follows:
S% =
Va − Vg Vg
can be attributed to the competition between the adsorption sites versus the concentration of H2S [32]. Self-powered active gas sensors based on CuO/ZnO PN-junction nanoarrays have many advantages over other traditional H2S sensors, including their ability to sense room temperature with no external electrical power consumption. The self-powered active gas sensors with room-temperature operation can be used in an explosive environment. The lack of external electric power consumption by the self-powered active gas sensors can improve security and portability. Removing the power source from the sensing system can also reduce the size. Figure 5(a) shows a continuous responding process of the piezoelectric output voltage against 200 ppm H2S under the same compressive force. After the atmosphere of the test chamber changes to H2S, the piezoelectric output voltage decreases and then remains stable at 0.46 V in about 250 s (response time). Such a relatively long response time can be attributed to the reaction between H2S and CuO. The reaction gradually takes place with time, and the piezoelectric output keeps decreasing with time. After reacting for ∼250 s, the reaction finishes with the formation of CuS, and the piezoelectric output remains stable. This phenomenon is very common in the traditional CuO-based H2S sensors, which usually need ∼200 s for the sulfuration process of CuO [21, 33, 34]. This relatively long response time may restrict the practical applications of the self-powered H2S sensors because the sensing data cannot be quickly obtained. Thus, more work needs to be done on further accelerating the response process. The recovery of the self-powered active gas sensor after H2S sensing is shown in figure 5(b). The piezoelectric output voltage of the device in 800 ppm H2S is about 0.101 V. After natural recovery in air for 1 h at room temperature, the piezoelectric output of the device is 0.122 V. At room temperature, the increase of piezoelectric voltage is very small and the recovery of the device is very limited. CuS can be oxidized by the oxygen in air at room temperature (forming CuO), but the oxidation process of CuS is very slow at room temperature [21, 33, 35, 36]. An annealing treatment on the device at 100 °C for 10 min can greatly accelerate the recovery process because CuS can convert back to CuO more
× 100%
where Va and Vg are the piezoelectric output voltage in dry air and H2S, respectively. The sensitivity against 200, 300, 400, 500, 600 and 800 ppm H2S at room temperature was 60.4, 99.1, 153.4, 308.2, 604.5 and 629.8, respectively, as shown in figure 4(h). The sensitivity of the bare ZnO nanoarrays against 200, 300, 400, 500, 600 and 800 ppm H2S at room temperature was 36.1, 50.2, 78.6, 107.1, 163.9 and 179.5, respectively. The sensitivity of bare ZnO nanoarrays was tested under the same conditions with CuO/ZnO arrays. The sensitivity of CuO/ZnO PN-junction nanoarrays was much higher than that of bare ZnO nanoarrays. A quasi-linear relationship between the sensitivity and concentration of H2S can be observed with the concentration of H2S lower than 600 ppm. This feature is very helpful for their practical application. As the concentration of H2S is larger than 800 ppm, the saturation of sensitivity can be observed, which 4
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Figure 4. The piezoelectric voltage responses of one device that was repeatedly compressed by a constant applied strain of 0.012% at a
frequency of 0.8 Hz in (a) dry air, (b) 200 ppm H2S (c) 300 ppm H2S, (d) 400 ppm H2S (e) 500 ppm H2S, (f) 600 ppm H2S, (g) 800 ppm H2S. All the measurements are performed at room temperature and under 1.01 × 105 Pa. (h) The sensitivity of the device upon exposure to different concentrations of H2S. 5
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screening of the piezoelectric field, thus decreasing the piezoelectric output of the NG. Introducing CuO/ZnO PNjunction into the device can greatly improve the H2S sensitivity. In the above measurements, CuO/ZnO nanoarrays under the same strain have lower piezoelectric outputs in H2S than in air, and the sensitivity is very high. And with increasing concentration of H2S, the piezoelectric output of the device dramatically decreases. CuO/ZnO nanoarrays have two functions: one is as an energy source because the CuO/ ZnO nanoarrayss can produce piezoelectric output power; the other is a H2S sensor function because the piezoelectric output of CuO/ZnO nanoarrays is a measure of H2S concentration. The conversion of CuO/ZnO PN-junction to CuS/ZnO ohmic-contact has a greater impact on the electron density in ZnO than the gas adsorption in our previous report [15], leading to higher sensitivity. The previous report has also confirmed that the piezoelectric energy output of ZnO NW NGs can be enhanced by passivation of ZnO using p-type polymer to decrease the carrier density and the screen effect [40]. The detailed H2S sensing mechanism of the self-powered active gas sensor based on CuO/ZnO PN-junction nanoarrays is shown in figure 6. When the device is in dry air without any compressive force (figure 6(a)), the device is in the natural state without any piezoelectric output. It is well known that CuO is a p-type semiconductor with a work function of 4.3 eV and ZnO is a n-type semiconductor with a work function of ∼5.3 eV [41, 42]. When ZnO NWs are coated with CuO nanocones, PN-heterojunctions are formed at the interface between them. Near the surface of ZnO, a charge depletion layer appears and the electron density in ZnO NWs is decreased. The corresponding energy band diagram is schematically shown in figure 6(b). The energy band of ZnO bends upward at the interface between ZnO and CuO, leading to a high barrier on the junction region. When the device is under a compressive strain (figure 6(c)), a piezoelectric field is created along the NWs. Under the driving of this piezoelectric field, the residual free electrons in the conduction band of ZnO NWs tend to migrate and screen the positive ionic piezoelectric charges at one end, leaving the negative ionic piezoelectric charges alone [15]. As the electron density is greatly decreased by CuO/ZnO PN-junction, the screen effect is weak and the piezoelectric output is high. Figures 6(d)–(f) schematically show the device upon exposure to H2S. When the device is exposed to H2S without any applied force (figure 6(d)), CuO nanocones can react with H2S and convert to CuS [12]. The reaction can be expressed as follows:
(a) Voltage (V)
0.9 0.6 0.3
in air
in H2S (200 ppm)
0.0 -0.3 -0.6 -0.9 0
100
300
200
400
500
Time (s)
(b) Voltage (V)
0.8 0.4
Natural recovery in air
Annealing at
for 1 h at room
100 ˚C for 10
temperature
mins
0.0 -0.4 -0.8
in air
in H2S (800 ppm ) 0
60
0
60
in air 0
60
Time (s) Figure 5. (a) A continuous responding process of the piezoelectric
output voltage against 200 ppm H2S, showing the response time. (b) The recovery of the self-powered active gas sensor after H2S sensing.
quickly at high temperatures than at room temperature [21, 33, 35, 36]. After cooling down to room temperature, the piezoelectric output voltage of the device is about 0.762 V. After complete recovery, the device can again perform consistent sensing. Previous reports have shown that ZnO NWs have a high density of point defect (oxygen vacancies), which provide ntype carriers (electrons) for their conductivity [37, 38]. When the c-axis of ZnO NW is under external strain, a piezoelectric field can be created on the surface, which can not only drive the electrons in the external circuit flowing forward and back (the output of NG), but also make the carriers in ZnO NWs migrate and partially screen this piezoelectric field (screen effect) [15, 39]. And decreasing the surface defects of ZnO NWs by annealing in pure oxygen can dramatically increase the output of ZnO NG because there are fewer n-type carriers within the NWs, thus reducing the screening effect of the free carriers on the piezoelectric polarization charges [39]. Our previous report also showed that the free-carrier density at the surfaces of ZnO NWs is affected by oxidizing or reducing gas adsorbed on the surface, which greatly changes the screening of the piezoelectric polarization charges at the interface by the free carriers, thus affecting the piezoelectric output of the NG [15]. In the prototype of ZnO NW self-powered active gas sensors, the H2S sensing arises from the change of electron density in ZnO NWs affecting the piezoelectric output [15]. Alternatively, the electron density of ZnO NWs can probably be affected by the conversion of CuO/ZnO PN-junction to CuS/ZnO Ohmic-contact. Metallic CuS can provide extra electrons flowing into ZnO NWs and greatly enhancing the
CuO + H 2 S ↔ CuS + H 2 O CuS is known to be metallic in nature [43] and thus CuO/ ZnO PN-junction as well as charge depletion layer are destroyed and transformed to CuS/ZnO Ohmic contact (the work function of CuS is smaller than that of ZnO) [25, 42]. The conversion of PN-junction to Ohmic-contact causes an anti-blocking layer to form at the interface because extra electrons are transferring from CuS into ZnO. This process significantly increases the electron density of ZnO. The 6
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(a)
(b)
(c)
F
(f)
F
Ec 1.35 eV
Ec EF
Ev 3.44 eV
p-CuO
Kapton AI layer ZnO NWs Depletion layer Ti layer Electron CuO
(d)
n-ZnO
(e)
n-ZnO
Φ CuS
Ev
Ec EF
Ev Kapton Al layer ZnO NWs Depletion layer Ti layer H2S Electron CuS
Φ CuS < Φ ZnO
Figure 6. The H2S-sensing mechanism of the self-powered active gas sensor based on CuO/ZnO PN-junction nanoarrays driven by compressive strain. (a) The charge-carrier density in CuO/ZnO PN-junction nanoarrays without compression in dry air. (b) The schematic band diagram of the CuO/ZnO PN-junction when the device is in dry air. (c) The piezoelectric output of the device in dry air under mechanical deformation. (d) The charge-carrier density in CuO/ZnO nanoarrays in H2S without compression. (e) Band diagram of CuS/ZnO Ohmic contact. (f) The piezoelectric output of the device in H2S under mechanical deformation.
corresponding energy band diagram is schematically shown in figure 6(e). The energy band of ZnO bends downwards at the interface between CuS and ZnO, and there is no barrier on this region. When the device in H2S is under a compressive strain (figure 6(f)), a piezoelectric field is created along ZnO NWs. Under the driving of this piezoelectric field, the large amounts of free electrons in the ZnO NWs tend to migrate and screen the piezoelectric field. As the electron density is greatly increased by CuS/ZnO Ohmic contact, the screen effect is very strong and the piezoelectric output is low. The conversion of CuO to CuS upon exposure to H2S at room temperature has been confirmed by TEM, EDS, XRD and electrical transport measurements, as shown in figure 7. Figure 7(a) is TEM image of CuO/ZnO NW before exposure to 800 ppm H2S, showing that the CuO nanocones on the surface of ZnO NW are not broken. The HRTEM image of CuO nanocone is shown in the inset of figure 7(a), and the lattice fringe spacing of 0.232 nm corresponds to {111} atomic spacing of CuO. Figure 7(b) is TEM image of the sample after exposure to 800 ppm H2S at room temperature, showing that the CuO nanocones are broken due to the latticestructure change during the formation of CuS. The HRTEM image of CuS nanoparticles is shown in the inset of figure 7(b), and the lattice-fringe spacing of 0.281 nm corresponds to {103} atomic spacing of CuS. Figure 7(c) shows EDS spectra of CuO/ZnO nanoarrays before and after exposure to 800 ppm H2S at room temperature. It is clearly indicated that S element appears in the sample after exposure to H2S, confirming the sulfuration reaction of CuO to CuS. Figure 7(d) shows XRD patterns of CuO/ZnO nanoarrays
before and after exposure to 800 ppm H2S at room temperature. After exposure to H2S, some new diffraction peaks can be observed, which can be perfectly indexed to CuS (JCPDS file No. 06–0464). It should also be noted that the diffraction peaks of the sample after exposure to H2S can be indexed to CuO and CuS, and CuO does not completely convert into CuS. Figure 7(e) shows the I–V curves of a device before and after being exposed to 800 ppm H2S. The essential change in the electrical property of the device after exposure to H2S confirms the conversion of CuO/ZnO PN junctions to CuS/ ZnO Ohmic contact. In this device, the tip of ZnO NW is completely covered by CuO nanocones. These results strongly demonstrate that CuO can convert into CuS upon exposure to H2S at room temperature.
4. Conclusions In summary, we demonstrated that CuO/ZnO PN-junction nanoarrays as self-powered active gas sensors could realize room-temperature, high H2S sensing. Upon exposure to 200, 400, 600 and 800 ppm H2S at room temperature, the piezoelectric output of CuO/ZnO nanoarrays greatly decreased from 0.738 V (in air) to 0.461, 0.291, 0.104 and 0.101 V, respectively. The sensitivity was much higher than bare ZnO NWs. The piezoelectric output in H2S dramatically decreased through the enhanced screen effect of the large amount of electrons from the conversion of CuO/ZnO PNjunction to CuS/ZnO Ohmic contact. Our results could 7
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Nanotechnology 25 (2014) 265501
(a)
(b)
(c)
(d)
Cu Zn
100
Before exposed to H2S
300
After exposed to H2S
250
Counts
O Ti
50 25
Cu Zn Cu
0 -25
Before exposed to H2S ZnO After exposed to H2S
200
Intensity ( a.u.)
75
ZnO
CuS
150 CuO
100 50
ZnO
CuOTi
Ti
ZnO Ti
Ti
CuO
ZnO
0 -50
S
-100 -50 0
2
6
4
8
20
10
25
30
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45
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2 Theta ( degree )
Energy ( Kev)
(e)
35
4 Before exposed to H2S After exposed to H2S
Current (mA)
2
0
-2
-4 -2
-1
0
1
2
Voltage (V) Figure 7. (a) TEM and HRTEM image of CuO/ZnO NW before exposed to H2S. (b) TEM and HRTEM image of the sample after exposed to H2S at room temperature, confirming the conversion of CuO to CuS. (c) EDS spectra and (d) XRD patterns of CuO/ZnO nanoarrays before and after exposed to 800 ppm H2S at room temperature. (e) The I–V curves of a device before and after being exposed to 800 ppm H2S.
stimulate a research trend on designing a new material system for high-performance, self-powered active gas sensors.
Note
Author contributions
Acknowledgment
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
This work was supported by the National Natural Science Foundation of China (51102041 and 11104025), the Fundamental Research Funds for the Central Universities
The authors declare no competing financial interest.
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(N120205001, N120405010 and lzujbky-2013–35), and Program for New Century Excellent Talents in University (NCET-13-0112).
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References [1] Sun Z, Yuan H, Liu Z, Han B and Zhang X 2005 Adv. Mater. 17 2993–7 [2] Huang H, Ong C Y, Guo J, White T, Tse M S and Tan O K 2010 Nanoscale 2 1203–7 [3] Yamada T, Zhou H S, Uchida H, Tomita M, Ueno Y, Ichino T, Honma I, Asai K and Katsube T 2002 Adv. Mater. 14 812–5 [4] Singh N, Gupta R K and Lee P S 2011 ACS Appl. Mater. Interfaces 3 2246–52 [5] Mai L, Xu L, Gao Q, Han C, Hu B and Pi Y 2010 Nano Lett. 10 2604–8 [6] Li C C, Du Z F, Li L M, Yu H C, Wan Q and Wang T H 2007 Appl. Phys. Lett. 91 032101 [7] Kolmakov A, Klenov D, Lilach Y, Stemmer S and Moskovits M 2005 Nano Lett. 5 667–73 [8] Gurlo A 2011 Nanoscale 3 154–65 [9] Zhou W J, Du G J, Hu P G, Li G H, Wang D Z, Liu H, Wang J Y, Boughton R I, Liu D and Jiang H D 2011 J. Mater. Chem. 21 7937–45 [10] Chen D, Xu J, Xie Z and Shen G Z 2011 ACS Appl. Mater. Interfaces 3 2112–7 [11] Miura N, Yan Y T, Lu G Y and Yamazoe N 1996 Sensor Actuat. B-Chem. 34 367–72 [12] Kim J, Kim W and Yong K 2012 J. Phys. Chem. C 116 15682–91 [13] Comini E, Cristalli A, Faglia G and Sberveglieri G 2000 Sensor Actuat. B-Chem. 65 260–3 [14] Joshi R K, Hu Q, Alvi F, Joshi N and Kumar A 2009 J. Phys. Chem. C 113 16199–202 [15] Xue X Y, Nie Y X, He B, Xing L L, Zhang Y and Wang Z L 2013 Nanotechnology 24 225501 [16] Starr M B and Wang X D 2013 Sci. Rep. 3 2106 [17] Xu S, Qin Y, Xu C, Wei Y G, Yang R S and Wang Z L 2010 Nat. Nano 5 366–73 [18] Wang Z L and Wu W 2012 Angew. Chem. Int. Edit 51 11700–21 [19] Wang Z L 2012 Adv. Mater. 24 280–5 [20] Hu Y H, Zhang Y, Xu C, Lin L, Snyder R L and Wang Z L 2011 Nano Lett. 11 2572–7
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The conversion of PN-junction influencing the piezoelectric output of a CuO/ZnO nanoarray nanogenerator and its application as a room-temperature self-powered active H2S sensor
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 265501 (http://iopscience.iop.org/0957-4484/25/26/265501) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 25 (2014) 265501 (9pp)
doi:10.1088/0957-4484/25/26/265501
The conversion of PN-junction influencing the piezoelectric output of a CuO/ZnO nanoarray nanogenerator and its application as a room-temperature self-powered active H2S sensor Yuxin Nie1, Ping Deng1, Yayu Zhao1, Penglei Wang1, Lili Xing1, Yan Zhang2,3,4 and Xinyu Xue1,2,4 1
College of Sciences, Northeastern University, Shenyang 110004, People’s Republic of China Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100000, People’s Republic of China 3 Institute of Theoretical Physics, Lanzhou University, Lanzhou 730000, People’s Republic of China 2
E-mail: [email protected] and [email protected] Received 26 March 2014, revised 29 April 2014 Accepted for publication 7 May 2014 Published 11 June 2014 Abstract
Room-temperature, high H2S sensing has been realized from a CuO/ZnO nanoarray selfpowered, active gas sensor. The piezoelectric output of CuO/ZnO nanoarrays can act not only as the power source of the device, but also as the H2S sensing signal at room temperature. Upon exposure to 800 ppm H2S at room temperature, the piezoelectric output of the device greatly decreased from 0.738 V (in air) to 0.101 V. The sensitivity increased to 629.8, much higher than bare ZnO nanoarrays. As the device was exposed to H2S, a CuO/ZnO PN-junction was converted into a CuS/ZnO Ohmic contact, which greatly increased the electron density in the nanowire and enhanced the screen effect on the piezoelectric output. Our results can stimulate a research trend on designing new composite piezoelectric material for high-performance selfpowered active gas sensors. Keywords: nanogenerators, self-powered, H2S sensing, PN-junctions, screen effect (Some figures may appear in colour only in the online journal) 1. Introduction
back to the conduction band of the nanostructures, narrow the depletion layer and decrease the resistance of the nanostructures (N-type) [11, 12]. Nowadays, many efforts have been made to reduce their work temperature, such as UV assistance [13] and noble metal decoration [14]. In our previous work, we demonstrated that an unpackaged ZnO nanowire (NW) piezo-nanogenerator (NG) can work as a selfpowered active gas sensor at room temperature by coupling the piezoelectric and gas sensing characteristics of ZnO NWs [15]. The piezoelectric output generated by ZnO NW under applied force acts not only as a power source, but also as a response signal to the test gas [15–17]. This new self-powered active gas sensor is sensitive to H2S at room temperature
Gas sensors for detecting toxic gas, such as H2S, CO and NO, have very important applications in industry and society [1–5]. In recent years, one-dimensional (1D) metal oxide nanostructures have been widely reported to be good candidates for highly sensitive and stable gas sensors due to their high surface-to-volume ratio, such as ZnO, SnO2 and In2O3 nanowires/tubes [6–10]. Upon exposure to reducing gas, the gas molecules will react with the adsorbed oxygen ions on the surface of the nanostructures, which can release free electrons 4
Authors to whom any correspondence should be addressed.
0957-4484/14/265501+09$33.00
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through the variety of the screen effect on the piezoelectric output [15]. This new piezo-gas sensing mechanism opens a new way for the next generation of gas sensors and expands the scope of self-powered nanosystems [18–20]. The selfpowered system integrating NGs and sensors aims at harvesting mechanical energy in the environment to power the sensors. For future practical applications of self-powered active gas sensors, their performance needs to be further improved, especially with regard to enhancing the sensitivity. Heterostructured nanomateirals, such as CuO/ZnO and CuO/SnO2 core-shell nanowires/rods, have been confirmed to have extremely high sensitivity against various gases [12, 21–23]. Upon exposure to H2S, CuO (P-type semiconductor) will react with H2S and convert into CuS (a typical metal with a smaller work function than ZnO or SnO2) [24]. The conversion of PN-junction to Ohmic contact results in an anti-blocking layer at the interface through electrons transferring from CuS into ZnO or SnO2 [12, 25]. This process significantly decreases the resistance of the heterostructured nanomaterials, resulting in extremely high H2S sensitivity. If the PN-heterostructures can be introduced into self-powered active gas sensors, then room-temperature highly sensitive H2S sensing can probably be realized. In this paper, CuO/ZnO PN-junction nanoarrays have been used to fabricate self-powered active gas sensors and room-temperature high H2S gas sensing has been obtained. Upon exposure to 800 ppm H2S at room temperature, their sensitivity is up to 629.8, much higher than bare ZnO NW self-powered active gas sensors. The conversion of CuO/ ZnO PN-junction to CuS/ZnO Ohmic contact greatly increases the electron density in the NW, resulting in a strong screen effect on the piezoelectric output. Our study can stimulate a research trend on designing a new material system for high-performance self-powered active gas sensors.
Figure 1. Fabrication process of the self-powered active gas sensor
based on CuO/ZnO PN-junction nanoarrays. (a) A pre-cleaned Ti foil is used as the substrate. (b) ZnO seeds are deposited on Ti foil. (c) Vertically aligned ZnO NW arrays are grown on Ti foil. (d) CuO nanocones are coated on the surface of ZnO NWs by a wet-chemical method. (e) A sheet of flexible aluminum foil as the counterelectrode is positioned on top of CuO/ZnO PN-junction nanoarrays. (f) Schematic diagram showing the structure of the self-powered active gas sensor with two Kapton boards as the frame.
dried at 60 °C. CuO nanocones were then uniformly coated on the surface of ZnO NW arrays by a simple wet-chemical method followed by an annealing process, as shown in figure 1(d). A 5 mM aqueous solution of Cu(NO3)2 • 3 H2O was prepared. The solution was transferred to a 60 ml Teflon-lined stainless steel autoclave, and the Ti substrate with ZnO NW arrays was placed vertically into the autoclave. After the autoclave was kept at 60 °C for 4 h, CuO/ ZnO PN-junction nanoarrays growing on the Ti foil were removed from the autoclave, washed with deionized water and alcohol, and dried at 60 °C. Finally, an annealing process at 300 °C for 2 h was performed to ensure the formation of PN-junction between CuO and ZnO. As the counter-electrode, a sheet of flexible aluminum foil (thickness = 0.05 mm) was positioned on top of CuO/ZnO PN-junction nanoarrays, as shown in figure 1(e). After that, the device was tightly fixed between two sheets of flexible Kapton board as support frames to ensure the electrical contact between the aluminum foil and the NWs, as shown in figure 1(f). It has been confirmed that flexible Kapton films as substrates can follow the height profiles of the NW arrays and make effective contacts between the tips of NWs and electrodes [26–29]. Under externally applied compressive deformation, the piezoelectric output of the device was very sensitive to the outside atmosphere at room temperature. Compared with the prototype ZnO NW self-powered active gas sensor in our previous report [15], the conversion of CuO/ZnO PNjunction to CuS/ZnO Ohmic contact can more greatly increase the electron density in ZnO NW, resulting in a stronger screen effect on the piezoelectric output.
2. Methods Figure 1 shows the brief fabrication process and the final device structure of the self-powered active gas sensor based on CuO/ZnO PN-junction nanoarrays. CuO/ZnO PNjunction nanoarrays were fabricated using a two-step method. ZnO NW arrays were synthesized by a seedassisted hydrothermal method. A piece of flat Ti foil as both the substrate of ZnO NW arrays and the electrode was cleaned with water/alcohol and dried in a nitrogen stream, as shown in figure 1(a). As shown in figure 1(b), a ZnO seed layer was then deposited on the Ti substrate by a wetchemical-annealing method reported in our previous work [15]. Vertically aligned ZnO NW arrays were grown via a hydrothermal route, as shown in figure 1(c). The Ti substrate coated with ZnO seeds was immersed into 200 ml of equimolar (50 mM) aqueous solution of Zn(NO3)2 • 6 H2O and HMTA in a reaction flask. After keeping at 90 °C for 2 h, the substrate with ZnO NW arrays grown on was removed from the solution, rinsed with deionized water and 2
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Figure 2. (a) SEM image of ZnO NW arrays on Ti foil in a top view. The inset is an enlarged view of a selected area. (b) SEM image of CuO/ ZnO nanoarrays on the top-view. The inset is an enlarged view of a selected area. (c) TEM image of ZnO NW. (d) TEM image of CuO/ZnO NW. (e) HRTEM image of the tip region of a ZnO NW. (f) HRTEM image of the surface of CuO/ZnO NW.
3. Results and discussion
region of a ZnO NW, showing that ZnO NW is of single crystalline with a growth direction along the c-axis. The HRTEM image of a CuO/ZnO PN-junction NW is shown in figure 2(f), and the lattice fringe spacing of 0.23 nm corresponds to {111} atomic spacing of CuO. Figure 3(a) shows the XRD pattern of CuO/ZnO PNjunction nanoarrays on the Ti substrate. The sharp diffraction peaks indicate the good crystalline quality. The peaks marked by a diamond can be indexed to Ti (JCPDS file No. 44–1294) arising from the Ti foil substrate; the peaks marked by a solid pentagram can be indexed to ZnO (JCPDS file No. 36–1451); and the peaks marked by an inverted triangle can be indexed to CuO (JCPDS file No. 45–0937). Figure 3(b) shows the EDS spectrum of CuO/ ZnO PN-junction nanoarrays. Three elements (O, Zn and Cu) exist at the selected region, and similar EDS results have been obtained at several different areas. These results
Figure 2(a) is a typical scanning electron microscopy (SEM) image of the synthesized ZnO NW arrays on the top view. A selected area is enlarged in the inset of figure 2(a), showing the typical hexagonal structure of ZnO NW [30, 31]. The topview SEM image of CuO/ZnO PN-junction nanoarrays is presented in figure 2(b), indicating that CuO nanocones are uniformly distributed on the whole surface of ZnO NW arrays. Such a core-shell structure can be more clearly observed in the inset of figure 2(b), and the thickness of the CuO nanocone layer is about 400 nm. Figures 2(c) and (d) are transmission electron microscopy (TEM) images of ZnO NW and CuO/ZnO PN-junction NW, respectively. It can be seen that cone-shaped CuO nanostructures are evenly covered on the smooth surface of ZnO NW. Figure 2(e) is high-resolution transmission electron microscopy (HRTEM) image of the tip 3
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Figure 3. (a) XRD pattern of CuO/ZnO nanoarrays. (b) EDS spectrum of CuO/ZnO nanoarrays.
further confirm that ZnO NW arrays are uniformly coated with CuO nanocones. The piezoelectric output voltage of one device in different gases is shown in figure 4. All the experimental measurements were conducted at room temperature under the constant applied strain (0.012%, frequency = 0.4 Hz). The compressive force was applied by a hammer. The area of the hammer for applying the compressive force was larger than that of the device. The hammer was controlled by a programed motor, and the force and frequency were constant. Thus, the hammer provided stable and uniform strain on the device for the experimental measurement. When the device was in dry air, the piezoelectric output voltage induced by the compressive strain was 0.738 V, as shown in figure 4(a). To obtain the piezoelectric output response in H2S, a mixed gas of H2S and dry air was injected into the test chamber. At concentrations of H2S of 200, 300, 400, 500, 600 and 800 ppm, the piezoelectric output voltage of the device was 0.461, 0.371, 0.291, 0.181, 0.104 and 0.101 V, respectively, as shown in figures 4(b)–(g). The sensitivity S of the device against H2S under the same deformation conditions can be defined as follows:
S% =
Va − Vg Vg
can be attributed to the competition between the adsorption sites versus the concentration of H2S [32]. Self-powered active gas sensors based on CuO/ZnO PN-junction nanoarrays have many advantages over other traditional H2S sensors, including their ability to sense room temperature with no external electrical power consumption. The self-powered active gas sensors with room-temperature operation can be used in an explosive environment. The lack of external electric power consumption by the self-powered active gas sensors can improve security and portability. Removing the power source from the sensing system can also reduce the size. Figure 5(a) shows a continuous responding process of the piezoelectric output voltage against 200 ppm H2S under the same compressive force. After the atmosphere of the test chamber changes to H2S, the piezoelectric output voltage decreases and then remains stable at 0.46 V in about 250 s (response time). Such a relatively long response time can be attributed to the reaction between H2S and CuO. The reaction gradually takes place with time, and the piezoelectric output keeps decreasing with time. After reacting for ∼250 s, the reaction finishes with the formation of CuS, and the piezoelectric output remains stable. This phenomenon is very common in the traditional CuO-based H2S sensors, which usually need ∼200 s for the sulfuration process of CuO [21, 33, 34]. This relatively long response time may restrict the practical applications of the self-powered H2S sensors because the sensing data cannot be quickly obtained. Thus, more work needs to be done on further accelerating the response process. The recovery of the self-powered active gas sensor after H2S sensing is shown in figure 5(b). The piezoelectric output voltage of the device in 800 ppm H2S is about 0.101 V. After natural recovery in air for 1 h at room temperature, the piezoelectric output of the device is 0.122 V. At room temperature, the increase of piezoelectric voltage is very small and the recovery of the device is very limited. CuS can be oxidized by the oxygen in air at room temperature (forming CuO), but the oxidation process of CuS is very slow at room temperature [21, 33, 35, 36]. An annealing treatment on the device at 100 °C for 10 min can greatly accelerate the recovery process because CuS can convert back to CuO more
× 100%
where Va and Vg are the piezoelectric output voltage in dry air and H2S, respectively. The sensitivity against 200, 300, 400, 500, 600 and 800 ppm H2S at room temperature was 60.4, 99.1, 153.4, 308.2, 604.5 and 629.8, respectively, as shown in figure 4(h). The sensitivity of the bare ZnO nanoarrays against 200, 300, 400, 500, 600 and 800 ppm H2S at room temperature was 36.1, 50.2, 78.6, 107.1, 163.9 and 179.5, respectively. The sensitivity of bare ZnO nanoarrays was tested under the same conditions with CuO/ZnO arrays. The sensitivity of CuO/ZnO PN-junction nanoarrays was much higher than that of bare ZnO nanoarrays. A quasi-linear relationship between the sensitivity and concentration of H2S can be observed with the concentration of H2S lower than 600 ppm. This feature is very helpful for their practical application. As the concentration of H2S is larger than 800 ppm, the saturation of sensitivity can be observed, which 4
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Nanotechnology 25 (2014) 265501
Figure 4. The piezoelectric voltage responses of one device that was repeatedly compressed by a constant applied strain of 0.012% at a
frequency of 0.8 Hz in (a) dry air, (b) 200 ppm H2S (c) 300 ppm H2S, (d) 400 ppm H2S (e) 500 ppm H2S, (f) 600 ppm H2S, (g) 800 ppm H2S. All the measurements are performed at room temperature and under 1.01 × 105 Pa. (h) The sensitivity of the device upon exposure to different concentrations of H2S. 5
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screening of the piezoelectric field, thus decreasing the piezoelectric output of the NG. Introducing CuO/ZnO PNjunction into the device can greatly improve the H2S sensitivity. In the above measurements, CuO/ZnO nanoarrays under the same strain have lower piezoelectric outputs in H2S than in air, and the sensitivity is very high. And with increasing concentration of H2S, the piezoelectric output of the device dramatically decreases. CuO/ZnO nanoarrays have two functions: one is as an energy source because the CuO/ ZnO nanoarrayss can produce piezoelectric output power; the other is a H2S sensor function because the piezoelectric output of CuO/ZnO nanoarrays is a measure of H2S concentration. The conversion of CuO/ZnO PN-junction to CuS/ZnO ohmic-contact has a greater impact on the electron density in ZnO than the gas adsorption in our previous report [15], leading to higher sensitivity. The previous report has also confirmed that the piezoelectric energy output of ZnO NW NGs can be enhanced by passivation of ZnO using p-type polymer to decrease the carrier density and the screen effect [40]. The detailed H2S sensing mechanism of the self-powered active gas sensor based on CuO/ZnO PN-junction nanoarrays is shown in figure 6. When the device is in dry air without any compressive force (figure 6(a)), the device is in the natural state without any piezoelectric output. It is well known that CuO is a p-type semiconductor with a work function of 4.3 eV and ZnO is a n-type semiconductor with a work function of ∼5.3 eV [41, 42]. When ZnO NWs are coated with CuO nanocones, PN-heterojunctions are formed at the interface between them. Near the surface of ZnO, a charge depletion layer appears and the electron density in ZnO NWs is decreased. The corresponding energy band diagram is schematically shown in figure 6(b). The energy band of ZnO bends upward at the interface between ZnO and CuO, leading to a high barrier on the junction region. When the device is under a compressive strain (figure 6(c)), a piezoelectric field is created along the NWs. Under the driving of this piezoelectric field, the residual free electrons in the conduction band of ZnO NWs tend to migrate and screen the positive ionic piezoelectric charges at one end, leaving the negative ionic piezoelectric charges alone [15]. As the electron density is greatly decreased by CuO/ZnO PN-junction, the screen effect is weak and the piezoelectric output is high. Figures 6(d)–(f) schematically show the device upon exposure to H2S. When the device is exposed to H2S without any applied force (figure 6(d)), CuO nanocones can react with H2S and convert to CuS [12]. The reaction can be expressed as follows:
(a) Voltage (V)
0.9 0.6 0.3
in air
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0
60
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60
Time (s) Figure 5. (a) A continuous responding process of the piezoelectric
output voltage against 200 ppm H2S, showing the response time. (b) The recovery of the self-powered active gas sensor after H2S sensing.
quickly at high temperatures than at room temperature [21, 33, 35, 36]. After cooling down to room temperature, the piezoelectric output voltage of the device is about 0.762 V. After complete recovery, the device can again perform consistent sensing. Previous reports have shown that ZnO NWs have a high density of point defect (oxygen vacancies), which provide ntype carriers (electrons) for their conductivity [37, 38]. When the c-axis of ZnO NW is under external strain, a piezoelectric field can be created on the surface, which can not only drive the electrons in the external circuit flowing forward and back (the output of NG), but also make the carriers in ZnO NWs migrate and partially screen this piezoelectric field (screen effect) [15, 39]. And decreasing the surface defects of ZnO NWs by annealing in pure oxygen can dramatically increase the output of ZnO NG because there are fewer n-type carriers within the NWs, thus reducing the screening effect of the free carriers on the piezoelectric polarization charges [39]. Our previous report also showed that the free-carrier density at the surfaces of ZnO NWs is affected by oxidizing or reducing gas adsorbed on the surface, which greatly changes the screening of the piezoelectric polarization charges at the interface by the free carriers, thus affecting the piezoelectric output of the NG [15]. In the prototype of ZnO NW self-powered active gas sensors, the H2S sensing arises from the change of electron density in ZnO NWs affecting the piezoelectric output [15]. Alternatively, the electron density of ZnO NWs can probably be affected by the conversion of CuO/ZnO PN-junction to CuS/ZnO Ohmic-contact. Metallic CuS can provide extra electrons flowing into ZnO NWs and greatly enhancing the
CuO + H 2 S ↔ CuS + H 2 O CuS is known to be metallic in nature [43] and thus CuO/ ZnO PN-junction as well as charge depletion layer are destroyed and transformed to CuS/ZnO Ohmic contact (the work function of CuS is smaller than that of ZnO) [25, 42]. The conversion of PN-junction to Ohmic-contact causes an anti-blocking layer to form at the interface because extra electrons are transferring from CuS into ZnO. This process significantly increases the electron density of ZnO. The 6
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Nanotechnology 25 (2014) 265501
(a)
(b)
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Figure 6. The H2S-sensing mechanism of the self-powered active gas sensor based on CuO/ZnO PN-junction nanoarrays driven by compressive strain. (a) The charge-carrier density in CuO/ZnO PN-junction nanoarrays without compression in dry air. (b) The schematic band diagram of the CuO/ZnO PN-junction when the device is in dry air. (c) The piezoelectric output of the device in dry air under mechanical deformation. (d) The charge-carrier density in CuO/ZnO nanoarrays in H2S without compression. (e) Band diagram of CuS/ZnO Ohmic contact. (f) The piezoelectric output of the device in H2S under mechanical deformation.
corresponding energy band diagram is schematically shown in figure 6(e). The energy band of ZnO bends downwards at the interface between CuS and ZnO, and there is no barrier on this region. When the device in H2S is under a compressive strain (figure 6(f)), a piezoelectric field is created along ZnO NWs. Under the driving of this piezoelectric field, the large amounts of free electrons in the ZnO NWs tend to migrate and screen the piezoelectric field. As the electron density is greatly increased by CuS/ZnO Ohmic contact, the screen effect is very strong and the piezoelectric output is low. The conversion of CuO to CuS upon exposure to H2S at room temperature has been confirmed by TEM, EDS, XRD and electrical transport measurements, as shown in figure 7. Figure 7(a) is TEM image of CuO/ZnO NW before exposure to 800 ppm H2S, showing that the CuO nanocones on the surface of ZnO NW are not broken. The HRTEM image of CuO nanocone is shown in the inset of figure 7(a), and the lattice fringe spacing of 0.232 nm corresponds to {111} atomic spacing of CuO. Figure 7(b) is TEM image of the sample after exposure to 800 ppm H2S at room temperature, showing that the CuO nanocones are broken due to the latticestructure change during the formation of CuS. The HRTEM image of CuS nanoparticles is shown in the inset of figure 7(b), and the lattice-fringe spacing of 0.281 nm corresponds to {103} atomic spacing of CuS. Figure 7(c) shows EDS spectra of CuO/ZnO nanoarrays before and after exposure to 800 ppm H2S at room temperature. It is clearly indicated that S element appears in the sample after exposure to H2S, confirming the sulfuration reaction of CuO to CuS. Figure 7(d) shows XRD patterns of CuO/ZnO nanoarrays
before and after exposure to 800 ppm H2S at room temperature. After exposure to H2S, some new diffraction peaks can be observed, which can be perfectly indexed to CuS (JCPDS file No. 06–0464). It should also be noted that the diffraction peaks of the sample after exposure to H2S can be indexed to CuO and CuS, and CuO does not completely convert into CuS. Figure 7(e) shows the I–V curves of a device before and after being exposed to 800 ppm H2S. The essential change in the electrical property of the device after exposure to H2S confirms the conversion of CuO/ZnO PN junctions to CuS/ ZnO Ohmic contact. In this device, the tip of ZnO NW is completely covered by CuO nanocones. These results strongly demonstrate that CuO can convert into CuS upon exposure to H2S at room temperature.
4. Conclusions In summary, we demonstrated that CuO/ZnO PN-junction nanoarrays as self-powered active gas sensors could realize room-temperature, high H2S sensing. Upon exposure to 200, 400, 600 and 800 ppm H2S at room temperature, the piezoelectric output of CuO/ZnO nanoarrays greatly decreased from 0.738 V (in air) to 0.461, 0.291, 0.104 and 0.101 V, respectively. The sensitivity was much higher than bare ZnO NWs. The piezoelectric output in H2S dramatically decreased through the enhanced screen effect of the large amount of electrons from the conversion of CuO/ZnO PNjunction to CuS/ZnO Ohmic contact. Our results could 7
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1
2
Voltage (V) Figure 7. (a) TEM and HRTEM image of CuO/ZnO NW before exposed to H2S. (b) TEM and HRTEM image of the sample after exposed to H2S at room temperature, confirming the conversion of CuO to CuS. (c) EDS spectra and (d) XRD patterns of CuO/ZnO nanoarrays before and after exposed to 800 ppm H2S at room temperature. (e) The I–V curves of a device before and after being exposed to 800 ppm H2S.
stimulate a research trend on designing a new material system for high-performance, self-powered active gas sensors.
Note
Author contributions
Acknowledgment
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
This work was supported by the National Natural Science Foundation of China (51102041 and 11104025), the Fundamental Research Funds for the Central Universities
The authors declare no competing financial interest.
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Y Nie et al
Nanotechnology 25 (2014) 265501
(N120205001, N120405010 and lzujbky-2013–35), and Program for New Century Excellent Talents in University (NCET-13-0112).
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