ZnO Nanoarray Nanogenerator as a Self

Article pubs.acs.org/JPCC

Core−Shell In2O3/ZnO Nanoarray Nanogenerator as a Self-Powered Active Gas Sensor with High H2S Sensitivity and Selectivity at Room Temperature Weili Zang, Yuxin Nie, Dan Zhu, Ping Deng, Lili Xing,* and Xinyu Xue* College of Sciences, Northeastern University, Shenyang 110004, China ABSTRACT: Self-powered active gas sensing has been realized from core−shell In2O3/ZnO nanoarray nanogenerator, and extremely high H2S sensitivity and selectivity at room temperature have been obtained. In2O3/ZnO nanoarrays have two functions: one is an energy source because In2O3/ZnO nanoarrays can produce piezoelectric output power; the other is a H2S sensor because the piezoelectric output of In2O3/ZnO nanoarrays varies with the concentration of H2S. Upon exposure to 700 ppm of H2S at room temperature, the piezoelectric output of In2O3/ZnO nanoarrays decreases from 0.902 V (in air) to 0.088 V, and the sensitivity is up to 925, much higher than that of bare ZnO nanoarrays. The heterostructure conversion of In2O3/ZnO to In2S3/ZnO, in which the electron depletion layer on the surface of ZnO changes to the accumulation layer, has stronger adjustment on the free-carrier piezo-screening effect of ZnO than the gas adsorption. Our results demonstrate that a heterostructured nanoarray nanogenerator has potential applications in actively detecting toxic gas without using any external electricity power.

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INTRODUCTION In the past several decades, functional nanodevices have attracted worldwide attention due to their low power consumption, portable operation, and high performance, such as nanoscale sensors, LEDs, and transistors.1−6 At the same time, the development of portable, small-size, and sustainable power sources for driving these functional nanodevices is becoming more and more important. Recently, a self-powered nanosystem that integrates nanoscale energy generators and functional nanodevices has been proposed, aiming at harvesting energy from the environment to power the functional nanodevice, such as self-powered pH sensors, UV detectors, liquid crystal displays, etc.7−11 Most recently, a new type of selfpowered nanosystem, named as an active sensor, has been demonstrated by treating the output from piezoelectric nanogenerators as either a power source or sensing signal in response to the change in environment, such as self-powered wind-velocity detectors, automobile speedometers, gas sensors, and magnetic sensors.12−15 In our previous work, by coupling the piezoelectric and gas sensing characteristics of ZnO nanowire (NW) arrays, an unpackaged ZnO NW piezonanogenerator (NG) as a self-powered active gas sensor has been first demonstrated.12 The piezoelectric effect of ZnO NW can be influenced by the free carriers within it (piezotronics effect); reducing or oxidizing gas molecules adsorbed on the surface of ZnO NW can change the free-carrier density, and thus the piezoelectric output of ZnO NW contains the gas sensing information. After the establishment of the new piezogas-sensing mechanism, further work needs to be done on clarifying this novel physical−chemical process. Also, more efforts need to be made to improve the sensing performance of © 2014 American Chemical Society

this new device, such as enhancing the sensitivity and selectivity. For traditional resistance-type gas sensors, introducing heterostructures is an effective way to enhance the sensing performance.16−18 Thus, it is expected that introducing heterostructure conversion can probably improve the selfpowered active gas sensing. In this paper, In2O3/ZnO heterostructured nanoarrays have been used to fabricate self-powered active gas sensors, and high H2S sensitivity and selectivity have been achieved. The selfpowered H2S sensing performance of In2O3/ZnO nanoarrays is much higher than that of bare ZnO nanoarrays. The conversion of In2O3/ZnO to In2S3/ZnO has stronger adjustment on the free-carrier piezo-screening effect than the gas adsorption. Our present results can stimulate a research trend on exploring the novel mechanism and designing new materials for self-powered active gas sensors.

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EXPERIMENTAL SECTION The brief fabrication process and the final device structure of the self-powered active gas sensor based on In2O3/ZnO NW arrays are shown in Figure 1. Uniformly loaded In2O3/ZnO NW arrays were synthesized by a two-step method. First, ZnO NW arrays were synthesized via a hydrothermal route.19 A piece of titanium (Ti) foil as the substrate was cleaned in advance with acetone, deionized water, and ethanol and dried at 60 °C, as shown in Figure 1a. 0.5 g of Zn(NO3)2·6H2O was dissolved in 38 mL of deionized water, stirring for 10 min at Received: January 16, 2014 Revised: April 3, 2014 Published: April 9, 2014 9209

dx.doi.org/10.1021/jp500516t | J. Phys. Chem. C 2014, 118, 9209−9216

The Journal of Physical Chemistry C

Article

Figure 1. Fabrication of the self-powered active gas sensor based on In2O3/ZnO NW arrays. (a) A precleaned Ti foil is used as the substrate. (b) Vertically aligned ZnO NW arrays are grown on the Ti foil. (c) In2O3 nanoparticles are uniformly loaded on the whole surface of ZnO NW arrays. (d) Design of the self-powered active gas sensor. (e) Schematic image showing the device actively detecting H2S at room temperature. (f) Optical image of the flexible device showing that it can be easily bent.

Figure 2. (a) SEM image of In2O3/ZnO NW arrays on the top view. The inset is an enlarged view of the tip. (b) SEM image of In2O3/ZnO NW arrays on the side view. The inset is an enlarged view of one single NW. (c) TEM image of In2O3/ZnO NW. (d) HRTEM and SAED pattern of the tip region of In2O3/ZnO NW.

room temperature. 2 mL of NH3·H2O was added into the solution drop by drop under continuous stirring. After that, the Ti foil was immersed into the above solution. The reaction autoclave was sealed and maintained at 70 °C for 24 h. After cooling to room temperature, the Ti substrate coated with vertically aligned ZnO NW arrays was removed from the solution, washed with deionized water and ethanol, and dried at 60 °C (Figure 1b). Second, ZnO NW arrays were coated with In2O3 nanoparticles by a simple wet-chemical method, as shown in Figure 1c. 0.018 g of In(NO3)3·4.5H2O was

ultrasonically dispersed in 60 mL of deionized water. The Ti substrate coated with ZnO NW arrays was immersed into the above solution for 15 min. The Ti substrate was dried at 70 °C and annealed at 600 °C in air for 2.5 h. Figure 1d shows the final device structure of the self-powered active gas sensor, which is composed of three major components: In2O3/ZnO NW arrays on Ti foil, Al layer, and Kapton boards. Such a device structure is similar to a typical NG without packaging.7 Ti foil acted as both the substrate for In2O3/ZnO NW arrays and the conductive electrode. Al layer 9210



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Figure 3. (a) EDS spectra of In2O3/ZnO NW arrays before and after being exposed to H2S. (b) XRD pattern of In2O3/ZnO NW arrays. (c) XRD patterns of In2O3 before and after being exposed to H2S at room temperature.

inset of Figure 2a, showing the typical hexagonal shape of ZnO NW. It can also be seen that In2O3 nanoparticles are uniformly distributed on the tip region of ZnO NW. Figure 2b is a typical SEM image of In2O3/ZnO NW arrays on the side view, showing that the nanoarrays have an average length of ∼2.2 μm. It further confirms that In2O3/ZnO NWs are vertically aligned on the substrate. The inset of Figure 2b is an enlarged view of one single NW, obviously showing that In 2O3 nanoparticles are uniformly loaded on the whole surface of ZnO NWs. From the TEM image of one single In2O3/ZnO NW (Figure 2c), it can be seen that the whole surface of ZnO NW is uniformly coated with a large amount of In2O3 nanoparticles. Figure 2d is a high-resolution TEM (HRTEM) image of the tip region of In2O3/ZnO NW, and the inset is the corresponding SAED pattern. The lattice spacing of 0.26 nm corresponds to the (002) plane of ZnO, and the lattice spacing of 0.41 nm corresponds to the (211) plane of In2O3.20 Figure 3a shows the EDS spectra of In2O3/ZnO NW arrays before and after being exposed to H2S. Before being exposed to H2S, three elements (In, Zn, and O) exist at this area, and no peaks can be indexed to the S element. After exposure to H2S, a new peak corresponding to S element can be observed, confirming the formation of In2S3 after the sulfuration reaction. Figure 3b shows the XRD pattern of In2O3/ZnO NW arrays, in which the sharp diffraction peaks can be indexed to three kinds of crystals. The peaks marked by pentagram can be indexed to Ti (JCPDS file No. 44-1294) arising from the Ti foil substrate, the peaks marked by a diamond can be indexed to ZnO (JCPDS file No. 36-1451), and the peaks marked by inverted triangle can be indexed to In2O3 (JCPDS file No. 06-0416). There are no other clear sharp peaks coincident with other impurities. Figure 3c shows the XRD patterns of In2O3 before and after being exposed to H2S. In the XRD pattern after being exposed to H2S, the diffraction peaks around 14.2°, 23.3°, 27.4°, 28.7°, 33.2°, 43.6°, 56.6°, and 59.4° can be perfectly indexed to (111), (220), (311), (222), (400), (511), (622), and

as the counter electrode was positioned on the top of In2O3/ ZnO NW arrays. Two terminal copper leads were glued on the two electrodes with silver paste for electrical measurements. To ensure the stability of the device, the finished device was fixed between two sheets of Kapton boards as the frame. Finally, the device was connected to the outside electronic circuit, which independently monitored the change of their piezoelectric output voltage upon exposure to various concentrations of H2S in a sealed chamber, as show in Figure 1e. An optical image of the device is shown in Figure 1f, indicating that the device is flexible and can be easily bent by the human hand. A manipulator producing constant strain was fixed inside of the chamber. The compressive force applied on the device is 34 N at the frequency of 0.5 Hz. All the measurements are conducted under the pressure of 1.01 × 105 Pa. A low-noise preamplifier (Model SR560, Stanford Research Systems) was used to measure the piezoelectric output voltage, and a gas flow system controlled the concentration of the gas in the chamber. The measurements were not continuous, in which the exposure time was long enough for the reaction with H2S. The crystal phase of the nanocomposites was characterized by X-ray powder diffraction (XRD; D/max 2500 V, Cu Kα radiation, λ = 1.5405 Å). The morphology of the nanocomposites was investigated by a scanning electron microscope (SEM, Hitachi S4800) with an energy dispersive X-ray spectrometer (EDS). The microstructures of the nanocomposites were investigated by transmission electron microscopy (TEM, JEOL JEM-2100F) and select area electron diffraction (SAED).

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RESULTS AND DISCUSSION Figure 2a is a typical SEM image of In2O3/ZnO NW arrays on the top view. It can be seen that In2O3/ZnO NW arrays are densely grown on Ti substrate along a consistent growth direction. The average diameter of In2O3/ZnO NWs is about 200 nm, and the tip region of one single NW is enlarged in the 9211



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Figure 4. (a) Piezoelectric output voltage of the device in dry air and upon exposure to H2S at room temperature. The inset is the relationship between the piezoelectric output voltage and H2S concentration. (b), (c), (d), (e), (f), and (g) are enlarged views of the piezoelectric output.

(444) planes of the In2S3 crystal given by the standard data file (JCPDS file No. 65-0459), respectively. Among them, the diffraction peaks around 14.2°, 23.3°, 27.4°, and 33.2° are newly observed compared with the XRD pattern before exposing to H2S, and the peaks around 28.7°, 43.6°, 56.6°, and 59.4° are similar to those in the XRD pattern before exposing to H2S because In2O3 and In2S3 have similar diffraction peaks at these degrees. Figure 4a shows the piezoelectric output voltage of In2O3/ ZnO NW arrays in dry air and upon exposure to various concentrations of H2S at room temperature. The compressive force in these measurements keeps the same (34 N, 0.5 Hz). Figures 4b−g are enlarged views of the piezoelectric output. When the device is in dry air, the piezoelectric output voltage generated by the compressive force is ∼0.902 V. Upon exposure to 100, 200, 300, 500, and 700 ppm of H2S, the piezoelectric output voltage of the device is about 0.706, 0.489, 0.301, 0.138, and 0.088 V, respectively. The piezoelectric output voltage of the device is dependent on the concentration of the test gas. The piezoelectric voltage dramatically decreases as the H2S concentration increases. Figure 5a shows a continuous responding process of the piezoelectric output voltage against 100 ppm of H2S under the same compressive force. After the atmosphere of the test chamber changing to H2S, the piezoelectric output voltage decreases and then maintains stable at 0.746 V in about 87 s.

The response time of the device is several tens of seconds. The recovery of the self-powered active gas sensor after H2S sensing is shown in Figure 5b. Natural recovery in air for 1 h is not enough for the complete oxidation of In2S3, and after 12 h, the device completely recovers. Thus, further work needs to be done on accelerating the recovery of the device. Similar to the traditional definition of the sensitivity of resistance-type gas sensors (S% = (|Ra − Rg|)/(Rg) × 100%, where Ra and Rg represent the resistance of the sensor in dry air and in the test gas, respectively),21 the sensitivity of our device against H2S in the same deformation conditions can be simply presented as S% =

|Va − Vg| Vg

× 100%

where Va and Vg are the piezoelectric output voltage of the device in dry air and test gas, respectively. Figure 6 shows the sensitivity−concentration curves of In2O3/ZnO NW arrays and bare ZnO NW arrays. The sensitivity of In2O3/ZnO NW arrays is much higher than that of bare ZnO NW arrays. Against 700 ppm of H2S, the sensitivity of In2O3/ZnO and bare ZnO NW arrays is 925 and 134, respectively. The sensitivity of In2O3/ ZnO NWs is ∼7 times higher than that of bare ZnO NWs. The detection limitation of In2O3/ZnO NWs as a self-powered active H2S sensor is 20 ppm, and the sensitivity is about 4.9. 9212



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Figure 5. (a) A continuous responding process of the piezoelectric output voltage against 100 ppm of H2S, showing the response time. (b) The recovery of the self-powered active gas sensor after H2S sensing.

In practical application, selectivity is an important parameter for gas sensors to distinguish different kinds of detected gases. It can be seen from Figure 7a that the sensitivity of In2O3/ZnO NW arrays against H2S at room temperature is much higher than that against carbon disulfide, ethanol, methanol, formaldehyde, and acetone gases (the concentrations of these test gases are all 700 ppm). Figure 7b is the sensitivity of In2O3/ ZnO NW arrays upon exposure to the mixed gases, including H2S/ethanol, H2S/formaldehyde, ethanol/formaldehyde, and methanol/formaldehyde. The concentration of each component in the mixed gases is 350 ppm. The sensitivity of the sensors is high only if the mixed gas contains H2S. These results suggest that In2O3/ZnO NW arrays as a self-powered active gas sensor have very good selectivity to H2S. It is well-known that ZnO NWs have a high density of point defect (oxygen vacancies), which provide n-type carriers (electrons) for their conductivity.24 Also, ZnO NWs have high piezoelectric output under applied deformation and have widely been used to fabricate piezoelectric nanogenerators.25 When the c-axis of ZnO NW is under external strain, a piezoelectric field can be created on the surface that can drive the electrons in the external circuit flowing forward and back (the output of NG); at the same time, the free electrons in ZnO NWs will transfer and partially screen this piezoelectric field and decrease the piezoelectric output (piezotronics effect).26,27 Previous reports have confirmed that the change of free-carrier density within ZnO NW can affect the piezoelectric output.28 For example, ZnO NWs after annealing in oxygen have low free-carrier density; thus, the screening effect is weak and the piezoelectric output is high.27 Our previous work on the selfpowered active gas sensing of ZnO NWs has shown that oxidizing or reducing gas adsorbed on the surface can change the free-carrier density at the surfaces of ZnO NWs, thus affecting the piezoelectric output through the changes of the screening effect.12 In this work, the heterostructure conversion

Figure 6. Sensitivity of In2O3/ZnO NW arrays and bare ZnO NW arrays upon exposure to different concentrations of H2S.

The sensitivity of bare ZnO NW arrays upon exposure to 20 ppm of H2S is very small and negligible. Table 1 is the Table 1. Comparison of the Sensing Performance of Various Nanosensors to H2S Gas materials

sensing type

T (°C)

C (ppm)

S (%)

ref

ZnO nanorods In2O3 nanocubes ZnO NWs In2O3/ZnO NWs

resistance resistance piezoelectric piezoelectric

100 125 RT RT

100 50 700 700

∼150 ∼1100 134 925

22 23 this work this work

comparison of the sensing performance between In2O3/ZnO heterostructured nanoarrays and other H2S sensing materials. It can be seen that the work temperature of self-powered active gas sensors is lower than other traditional H2S sensors. The self-powered active gas sensor can work at room temperature. These results demonstrate a very feasible direction for the development of self-powered active gas sensors. 9213



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Figure 7. Selectivity of the sensor. (a) Sensitivity of In2O3/ZnO NW arrays upon exposure to 700 ppm of H2S, carbon disulfide, ethanol, methanol, formaldehyde, and acetone at room temperature. (b) Sensitivity of In2O3/ZnO NW arrays upon exposure to mixed gas. The concentration of each component is 350 ppm.

output. It has been demonstrated by many literatures that both ZnO and In2O3 are n-type semiconductors.29 The electron affinity (χ) of ZnO and In2O3 is −4.3 and −4.45 eV, respectively. The band gap of ZnO and In2O3 is 3.37 and 2.00 eV, respectively.30,31 The electrons flow from ZnO into In2O3, and the n−n heterojunction is formed at the ZnO− In2O3 interface with a depletion layer building up on the ZnO side and an accumulation layer building up on the In2O3 side. When the device is under a compressive strain (Figure 8b), a piezoelectric field is created along In2O3/ZnO NWs. The freeelectron density of ZnO NWs is low; thus, the screening effect is weak and the piezoelectric output is high. When the device is exposed to H2S at room temperature without any applied force (Figure 8c), In2O3 can be partially sulfurated by H2S. Such a sulfuration reaction has been confirmed by the EDS spectra (Figure 3a) and the XRD patterns (Figure 3c) of In2O3 before and after being exposed to H2S. As shown in Figure 3a, the peak corresponding to the S element can be observed after being exposed to H2S. As shown in Figure 3c, some new diffraction peaks perfectly corresponding to In2S3 can be observed. These results demonstrate that In2O3 can convert into In2S3 upon exposure to H2S at room temperature. Additionally, a previous report has also confirmed that at relatively low temperature range (25−160 °C), the dominant mechanism is the sulfuration of In2O3.32 The reaction is as follows:

of In2O3/ZnO to In2S3/ZnO has been introduced into the piezo-gas sensing, which has stronger adjustment on the freecarrier piezo-screening effect of ZnO NW than the gas adsorption. The detailed work mechanism is as follows: The detailed H2S sensing mechanism of the self-powered active gas sensor based on In2O3/ZnO heterostructured nanoarrays is shown in Figure 8. In2O3/ZnO NW arrays have

In2O3(s) + 3H 2S(g) → In2S3(s) + 3H 2O(g)

Since Ec of ZnO is lower than that of In2S3 (χ = −4.1 eV) and the band gap of ZnO is wider than that of In2S3 (2.0 eV),31 the free electrons can transfer from In2S3 into ZnO, leading to an accumulation layer on the ZnO side. Additionally, In2S3 has been reported to have high conductance and high free-carrier density.33 When the device in H2S is under a same compressive strain (Figure 8d), a piezoelectric field is created along ZnO NWs. The large amount of free electrons in ZnO NWs can migrate and screen the piezoelectric field, and such a screening effect is very strong. At the same time, high-conductance In2S3 can also contribute to the screening effect. Thus, the piezoelectric output is very low. With the increasing concentration of H2S, more In2O3 nanoparticles can be sulfurated and more electrons can be injected into ZnO NWs, resulting in lower piezoelectric output. The high selectivity can also be attributed to the heterostructure conversion. The heterostructure conversion of In2O3/ZnO to In2S3/ZnO has a stronger adjustment on the free-carrier piezoscreening effect of ZnO than the gas adsorption. The gases with

Figure 8. Working mechanism of the self-powered active gas sensor based on In2O3/ZnO NWs. (a) Schematic illustration showing the band diagram of In2O3/ZnO heterostructure in dry air without applied force. (b) Piezoelectric output of the device in dry air under mechanical compressive deformation. (c) Band diagram of In2S3/ZnO heterostructure in H2S without applied force. (d) Piezoelectric output of the device in H2S under mechanical compressive deformation.

two functions: one is an energy source because the In2O3/ZnO nanoarrays can produce piezoelectric output power; the other is a H2S sensor because the piezoelectric output of In2O3/ZnO nanoarrays varies with the concentration of H2S. When the device is in dry air without any compressive force (Figure 8a), the device is in the natural state without any piezoelectric 9214



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low reducibility, such as ethanol, methanol, acetone, formaldehyde, and carbon disulfide, can only react with the adsorbed oxygen ions on the surface of the nanocomposites, leading to a limited increase of carrier density.

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CONCLUSION In summary, high H2S sensitivity and selectivity at room temperature were obtained from In2O3/ZnO nanoarrays NG as a self-powered active gas sensor. The sensitivity of In2O3/ZnO NWs is ∼7 times higher than that of bare ZnO NWs. The heterostructure conversion of In2O3/ZnO to In2S3/ZnO had stronger adjustment on the free-carrier piezo-screening effect than the gas adsorption. The present work was an important step for the practical applications of self-powered active gas sensors.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (L.X.). *E-mail [email protected] (X.X.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51102041 and 11104025), the Fundamental Research Funds for the Central Universities (N120205001 and N120405010), and Program for New Century Excellent Talents in University (NCET-13-0112).

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ZnO Nanoarray Nanogenerator as a Self

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