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Fabrication of comb-like ZnO nanostructures for room-temperature CO gas sensing application
Vacuum 101 (2014) 113e117
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Fabrication of comb-like ZnO nanostructures for room-temperature CO gas sensing application Hong-Di Zhang a, b, *, Yun-Ze Long a, b, c, *, Zhao-Jian Li d, Bin Sun a, b a
College of Physics, Qingdao University, Qingdao 266071, China Key Laboratory of Photonics Materials and Technology in Universities of Shandong, Qingdao University, Qingdao 266071, China c State Key Laboratory Cultivation Base of New Fiber Materials and Modern Textile, Qingdao University, Qingdao 266071, China d Neurosurgery Department, Affiliated Hospital of Medical College, Qingdao University, Qingdao 266003, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 January 2013 Received in revised form 28 July 2013 Accepted 31 July 2013
Comb-like ZnO nanostructures are prepared by chemical vapor deposition in a quartz vial and characterized by a scanning electron microscope. The comb-like ZnO structures are several tens of micrometers. The lengths of the branches (nanowires) gradually decrease along the growth direction of the comb-like ZnO structures. The branches have uniform diameters and are evenly distributed on the belt-stem. Electrical measurements indicate that the comb-like ZnO is intrinsic n-type semiconductor. Moreover, the nanostructures have good sensing properties. It is found that an individual ZnO comb exhibits fast and large response to ppm-level CO gas even at room temperature. The sensing mechanism is also discussed briefly. These results demonstrate that comb-like ZnO nanostructures have potential application in gas sensors without an external heater. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Zinc oxide Nanostructures Gas sensor Comb-like structure
1. Introduction Quasi-one-dimensional (quasi-1D) semiconductor nanostructures have attracted great interest in the fabrication of nanodevices due to their high surfaceevolume ratio. Zinc oxide (ZnO), an important semiconductor with a large direct band gap (3.35 eV at room temperature) has been suggested for its broad applications, including electronics, optoelectronics, electrochemistry, photovoltaics and gas sensors because of their special geometry [1e5]. For example, nanofibers, nanorods, nanotubes, nanopropeller, nanorings, nanocombs have been reported [6e14]. Among all these nanostructures, the comb-like ZnO nanostructures assembled by the aligned nanowires and back-bone nanobelts have been demonstrated to function as ultraviolet laser arrays and grating bean-divider. More applications are expected to develop on optical, electrical and gas sensing properties. Nanowires generally were synthesized by vaporeliquidesolid (VLS) or vaporesolid (VS) growth processes. And metal catalysts are important to assist the growth of nanowires especially Au catalyst
* Corresponding authors. College of Physics, Qingdao University, Qingdao 266071, China. Tel./fax: þ86 (0) 53285955977. E-mail addresses: [email protected] (H.-D. Zhang), [email protected] (Y.-Z. Long). 0042-207X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.vacuum.2013.07.046
[15e17]. In the VLS growth of nanowires, the catalyst is in the liquid phase and precursors in the vapor phase can adsorb and condense to form the nanowire. In the VS process, instead, the nanowire crystallization originates from the direct condensation from the vapor phase. ZnO nanowires have been synthesized by a range of techniques, such as chemical vapor deposition (CVD) [18], template-assisted growth [19], electrospinning [20], metal-organic chemical vapor deposition (MOCVD) [21] and pulsed laser deposition [22] methods, etc. Compared with above methods, CVD methods can yield high purity ZnO and be of low cost, less hazardous, simply methods for producing nanowires. ZnO has been proved to be a kind of highly sensitive material for the flammable or toxic gas detection, such as H2, CO, NO, or ethanol [23,24]. Since quasi-1D nanomaterials have higher sensitivity, faster response, and better capability to detect low concentration gases compared to the corresponding bulk materials. Oxides geometrical morphology is an important issue for gas sensing applications, such as branched, flower-like, porous nanowires et al. due to their high surfaceevolume ratio [25,26]. The operating temperature is another important issue for gas sensors. To improving the sensing performance of the gas sensors, usually the ZnO nanowires were doped with metal or metal-oxide to enhance the sensing properties [27e29]. However, the work temperature for most of the gas sensors is about 200e400 C, a heater must be added which lead to the sensor complex and large power
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consumption [30]. Hence, it’s necessary to construct a novel nanostructure for gas sensor to satisfy normal working at room temperature. A room-temperature sensor system could simplify device design by eliminating the heater component, save electrical power, and most importantly, avoid triggering an explosion in an explosive environment. Groups have made tremendous effort towards the development of room temperature oxide gas sensors in various ways [31,32]. In this paper, ZnO nanostructures were grown by a modified CVD method and comb-like structures were obtained. Particularly, a quartz vial was used in the quartz tube of the furnace to control the different pressure ratio of oxygen and zinc vapor qualitatively. With this method, a spatial variation of synthesis/vacuum conditions such as partial pressures of Zn and oxygen was created, and thus different ZnO nanostructures including nanowires, comb- and belt-like structures were obtained at different locations of the collector. The electrical characteristics of an individual ZnO comb were examined to demonstrate a high sensitivity to CO gas attributing to the comb structure at room temperature. 2. Experimental details In the process of fabricating ZnO nanocombs/nanowires, generally the ZnO source is a mixture of graphite powder, ZnO powder or other metal oxide such as CuO and SnO2 as catalysts [1,29,33]. Comparison to above reports, the ZnO source in this study was pure zinc powder and no other dopant. ZnO nanostructures were fabricated on Si wafer by chemical vapor deposition. Fig. 1 shows the experimental apparatus for fabrication of ZnO nanostructures. The CVD furnace included a horizontal quartz tube of 1-inch diameter and a small quartz vial inside the quartz tube. Firstly, Au thin film with thickness of about 10 nm was deposited on Si wafer N and then annealed at 700 C for 30 min to get Au nanoparticles as catalyst. Pure zinc powder (99.9%, as the raw source material) was placed close to the bottom of the small quartz vial. Wafer N was placed on the quartz vial’s neck to grow zinc oxide nanostructures. Secondly, the quartz tube was evacuated to 4 102 Torr and was purged with Ar gas under 1 atm, then rapidly elevated to 710 C in 30 min with a constant Ar flow of 90 sccm (standard cubic centimeters per minute). Thirdly, the temperature (700 C) of the furnace was maintained for 30 min with oxygen/argon (O2/Ar) (2% O2 concentration mixed with 98% Ar). The total flow rate of oxygen/argon was 250 sccm. The Au nanoparticles on silicon wafer N was formed liquid droplets. When the furnace temperature went above the melting point of zinc metal (420 C), zinc would gradually vaporize to fill the quartz vial. And a high zinc concentration condition with dilute oxygen content created and ZnO nanostructures formed on wafer N at the bottleneck of the quartz vial. After reaction, Wafer N was taken out from the furnace into air for rapid cooling. White products were deposited on the Si wafer N. The surface morphologies of the nanostructures were characterized by a scanning electron microscope (SEM, JEOL JSM-6390).
Fig. 1. Experimental apparatus for fabrication of ZnO nanostructures.
The phase purity of the resultant ZnO nanostructures on wafer N was analyzed by X-ray diffractometer (XRD, Bruker D8 Advance) at room temperature with Cu-Ka1 radiation. The nanocombs were removed from the growth wafer N and then dispersed on a new SiO2/Si chip with pre-patterned Au electrode arrays via traditional lithography technology. The square Au electrodes have a side length of 50 mm, and comb-like ZnO nanostructures can be bridged across the electrodes. The electrical property was performed by using two-metal-microprobe testing platform. And the illumination was performed by a LED lamp with tunable color temperature of Philips. The gas sensing properties of the fibers were measured by a Keithley 6517 high resistance meter and a home-made gas sensing system at room temperature. The sample was placed in a sealed quartz chamber and the apparatus were connected. The gas chamber contains one inlet port and one outlet port. Moreover, the inlet port links up with a pipeline which connects the air and standard gas of CO balanced by air. And all the vents were controlled by valves, respectively. The standard gas of CO balanced by air with the concentration of 250 ppm and the air were stored in different gas cylinders. Then the air and the sensing gas were introduced with the flow rate of 250 sccm by independent flow controller, respectively. The electrical resistance of the element was measured in air and in the presence of the CO gas in air. Firstly the measurement system was filled with air and the test began after the electric current of the sample was stable. The measurement included two processes of gas-on and gas-off. Gas-on was to fill standard gas of CO balanced by air for 5 min, meanwhile the air was cut off. Gas-off was to cut off standard gas of CO balanced by air and fill with air for 5 min. Both the gas flow rates were controlled to be 250 sccm. Then the two processes circulated couple periods and the change in resistance was recorded automatically by a computer. 3. Results and discussion 3.1. Characterization of ZnO nanostructures The morphologies of the white products at different positions on wafer N were characterized by SEM. The different positions were labeled as a1, a2, a3, a4 and a5 from the inner edge to the outer edge of the wafer N, respectively (as shown in Fig. 2a) in the quartz vial. Fig. 2a1ea5 shows the SEM images of different structures at locations a1, a2, a3, a4 and a5. It can be seen that the surface morphology of the resultant ZnO strongly depends on the location. There exist several self-assembled nanostructures of ZnO with various geometrical shapes on wafer N. Though the microstructure differences of ZnO have been well-characterized [29], the various nanostructures within such a small area on the silicon substrate with one experimental run has few reported. From position a1 to position a5, the ZnO nanostructures grow up gradually and evolve from particles to rods, combs and belts. Fig. 2a1 shows that only hexagonal Au particles uniformly distributed on the wafer and ZnO just nucleated and ready to grow against Au particle, which can be confirmed by the structure at the next location a2. According to the crystal growth theory, when the Au droplets reached a critical radius, ZnO began to precipitate. Fig. 2a2 shows the short nanorods grown under the hexagonal Au nanoparticle, which indicates that Au nanoparticles assist the ZnO nucleating as catalyst. And the nanorods continue to grow longer (Fig. 2a3). Contrasting with VS growth, VLS growth requires the existence of a liquid. VLS growth mechanism means the presence of the catalyst particle at the top of the nanowire, [15] locations a1 and a2 are from the region on the substrate where Au catalysts were tiptop, and therefore it was indicated that the growth of the synthesized ZnO nanostructures was controlled by VLS mechanism. Fig. 2a4 shows the SEM image of
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Fig. 2. (a) Five locations on wafer N were characterized by SEM, labeled as a1, a2, a3, a4 and a5 from the inner edge to the outer edge of the wafer N, respectively. (a1) Only hexagonal Au particles uniformly distributed on the wafer and ZnO just nucleated and ready to grow. (a2) Short nanorods grew under the hexagonal Au particle. (a3) The nanorods continued to grow longer under the hexagonal Au particle. (a4) One-side teeth comb of ZnO nanowires formed. (a5) ZnO nanobelt could be found at the outer edge.
the single-side ZnO combs. These comb-like ZnO structures are several tens of micrometers and may be suitable for gas sensors. The lengths of the comb teeth gradually decrease along the growth direction of ZnO comb-like structures. The branches have uniform diameters and are evenly distributed on the belt-like stem. The diameters of the teeth are about 250 nm and the lengths range from 0 to 3 mm. In addition, thicker needle-like ZnO structures are also observed at outside of the wafer N because of the higher oxygen concentration, as shown in Fig. 2a5. Fig. 3 shows the X-ray diffraction profile of the ZnO nanocombs. A clear diffraction pattern is shown, and nine reflection peaks appear at 2q ¼ 31.9 (100), 34.6 (002), 36.5 (101), 47.7 (102), 56.8 (110), 63.1 (103), 66.5 (200), 68.1 (112), and 69.2 (201). It agrees well with the JCPDS card, No. 89-1397 for pure ZnO, confirming that only ZnO phase are formed without diffraction peaks due to secondary phases. This confirmed that only pure ZnO nanostructures were formed by this CVD method. Furthermore, it also implies that the nanostructured ZnO is highly crystallized. The formation of ZnO nanostructures with different geometrical shapes could be attributed to the ratio change of Zn and O2 vapor pressure inside the neck of the quartz vial. According to the theory
of crystal growth, zinc molecules are easy to be adsorbed on the Au nanoparticles and nucleate. Moreover, the crystal growth rate and the nucleation rate are determined by the concentration ratio of oxygen and zinc vapor. At the inner part of the vial, the Zn concentration is very high and the oxygen concentration is relatively very low. So the ZnO grows slowly at inner locations (a1 and a2). When close to the bottleneck the oxygen concentration increases gradually, the ZnO nanostructures grow rapidly at outer positions (a4 and a5). Then the investigations through SEM and XRD indicated that nothing except the ZnO crystal occurred at the tip of tooth or around the structures. In zinc oxide crystal with hexagonal structure, ½0001; ½2110 and ½0110 are preferential growth directions. The forming mechanism of comb-like ZnO structure can be understood as follows [34,35]: First is a faster growth along ½0110 direction forming the nanobelt stems, and they were enclosed by the ð0001Þ side facets or ð2110Þ top/bottom facets; second is the nucleation and growth of the nanorod branches along [0001] direction synchronously, which may be related to the structure characteristic of wurtzite family. Once it began to grow on the surface of individual nanorods, ZnO dendrite nanostructures were formed. As shown in Fig. 2a4 comb-like ZnO was created. Moreover, from Fig. 2 one can see that there existed different morphologies of ZnO nanostructures at different locations of wafer N. The pressure of oxygen was smaller and the pressure of Zn atoms was larger in the inner part of the quartz vial, while the pressure of oxygen increased when close to the bottleneck, as shown in Fig. 1. So the pressure ratio of oxygen and zinc vapor plays a key role to the growth of ZnO combs, although accurate values (or direct measurements) of partial pressures of Zn and oxygen are not available in the quartz vial. Therefore, to obtain the required ZnO nanostructures such as comb-like structures, it is necessary to adjust proper concentration ratio of zinc and oxygen vapor. 3.2. Gas sensing properties
Fig. 3. X-ray diffraction profile of the ZnO nanostructures.
To fabricate ZnO nanostructure-based gas sensor device, SiO2/ Si chip with pre-patterned Au electrode arrays via traditional lithography technology was used as substrate to collect Zn
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Fig. 4. Optical image of an individual ZnO comb on SiO2/Si wafer with pre-patterned Au electrode array. Its electrical property was measured by a two-probe method.
nanostructures. As shown in Fig. 4, the square Au electrodes have a side length of 50 mm. The ZnO nanocomb was dispersed between two Au contact grids and no other processing was made which formed not ohmic contacts. The two ends of ZnO micro-comb on the electrodes formed natural Schottky contacts. Fig. 4 shows the optical picture of the measurement that two metal microprobes are pressing directly on the square Au electrodes. The two ends of ZnO micro-comb on the electrodes formed natural Schottky contacts. Fig. 5 shows the IeV curves of the single ZnO micro-comb at dark and under illustration, which testified a typical Schottky contact device. The IeV curves are in agreement with previous observations on Schottky contact device [36] although the values of current changed obviously at about 10 V not 1 V in the previous ones. This difference might be due to the different experimental conditions. The scanning voltage was applied from 20 V to 20 V with a step of 0.1 V. At the same time the current corresponding to each applied voltage was recorded. It is evident that the ZnO has a very little current (2.5 mA, 20 V bias) at dark. But the current increases sharply under illustration. The current can reach 60 mA (20 V bias) when the color temperature of illustration is 2900 K, which attributes to photo-generated electronehole pair resulting to the photocurrent. In addition, the Si substrate was used as a back gate to measure gate effect of this comb-like ZnO device. Fig. 6 depicts the time-dependent current of periodic exposure to 250 ppm CO ambient. The applied constant voltage was 20 V. The sensor exhibits rapid, apparent response upon exposure and removal of CO even at room temperature. The current reached 15 mA under gas-on and decreased sharply when gas-off. Since the gas response can be defined as the ratio: S z jRair Rgasj/Rair, where Rair and Rgas are the resistance of the sensor in dry air and in the test gas, respectively. From Fig. 6 it indicates that the resistance of sensor was dropped by about 82% in 5 min towards 250 ppm CO.
Fig. 5. Currentevoltage (IeV) characteristic curves of the individual ZnO comb at dark and under illustration.
Fig. 6. Response of the gas sensor based on single ZnO comb.
Consequently, this ZnO sensor is suitable and effective in detecting CO gas even at room temperature. 3.3. Gas sensing mechanism It is well known that when zinc oxide semiconductors are exposed to air, oxygen molecules will be absorbed onto the ZnO surface and form negative ions such as O1 or O2 ions by attracting electrons from the conduction band of ZnO. Under carbon monoxide atmosphere, the CO gas reacts with oxygen molecule on the surface and gives back electrons into the conduction band, thereby increasing the electron carriers and lowering the resistance of ZnO sensors. Furthermore, ZnO nanostructures exhibit higher response than the bulk even at room temperature without assembling a heater. It can be simply explained by the effect of the surface-tovolume ratio. In the present case, the fast and large response/recovery of ZnO nanocomb to CO at room temperature is mainly based on the nanostructure. Fig. 7 shows the sensing mechanism of comb-like ZnO. Compared with smooth nanowire, the teeth of comb enlarge the contact area for CO molecules. Moreover, the ordered teeth of the comb constituted parallel circuit and all the partial current in each tooth gathered into the straight primary ribbon, which enhanced the total current density. On the other hand, the high sensitivity in room temperature is attributed to the Au/ZnO junction at the interface between the nanocomb and the contacts. The Schottky-contact device greatly improved the sensor performance for CO sensing [37], because the contact resistance belongs to semiconductor resistance which is much important for a single nanowire. Au over the ZnO surface can speed up surface reactions and improve selectivity to target gas, which acts as catalyst to be active near the grain boundaries where carrier transport takes place [16]. The contact area between the nanocomb and the Au electrode is rather small, and then the adsorption of a few molecules at the junction could significantly
Fig. 7. Schematic illustration of comb-like ZnO gas sensing mechanism.
H.-D. Zhang et al. / Vacuum 101 (2014) 113e117
change the local barrier height. So the total conductance is dominated by the contact inside the contact area and the areas near the outer surface. Consequently the comb-like ZnO displays high sensitivity to CO even at room temperature and has potential application in CO gas sensor without an external heater. 4. Conclusion In summary, comb-like ZnO nanostructures are successfully prepared by CVD method in a quartz vial. The SEM shows that the teeth of comb grown out of one straight primary ribbon. The vapor pressure change of Zn/O2 ratio results in the formation of ZnO nanostructures with different morphologies inside the neck of the quartz vial. The electrical measurements of a single ZnO comb indicate that the comb-like ZnO is n-type semiconductor, and sensitive to illumination and CO gas. The single ZnO comb exhibits fast and large response to 250 ppm CO even at room temperature. These results demonstrate that the comb-like ZnO nanostructures have potential application in gas sensors without an external heater. Acknowledgments This work was supported by National Natural Science Foundation of China (Grant Nos. 11004114 and 11074138), Natural Science Foundation of Shandong Province (China) for Distinguished Young Scholars (Grant No. JQ201103), Taishan Scholars Program of Shandong Province (China), National Key Basic Research Development Program of China (973 special preliminary study plan, grant No. 2012CB722705), Project of Shandong Province Higher Educational Science and Technology Program (Grant No. J13LJ07), and Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province, China. References [1] Huang MH, Mao S, Feik H, Yan H, Wu Y, Kind H, et al. Science 2001;292:1897.
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Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Fabrication of comb-like ZnO nanostructures for room-temperature CO gas sensing application Hong-Di Zhang a, b, *, Yun-Ze Long a, b, c, *, Zhao-Jian Li d, Bin Sun a, b a
College of Physics, Qingdao University, Qingdao 266071, China Key Laboratory of Photonics Materials and Technology in Universities of Shandong, Qingdao University, Qingdao 266071, China c State Key Laboratory Cultivation Base of New Fiber Materials and Modern Textile, Qingdao University, Qingdao 266071, China d Neurosurgery Department, Affiliated Hospital of Medical College, Qingdao University, Qingdao 266003, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 January 2013 Received in revised form 28 July 2013 Accepted 31 July 2013
Comb-like ZnO nanostructures are prepared by chemical vapor deposition in a quartz vial and characterized by a scanning electron microscope. The comb-like ZnO structures are several tens of micrometers. The lengths of the branches (nanowires) gradually decrease along the growth direction of the comb-like ZnO structures. The branches have uniform diameters and are evenly distributed on the belt-stem. Electrical measurements indicate that the comb-like ZnO is intrinsic n-type semiconductor. Moreover, the nanostructures have good sensing properties. It is found that an individual ZnO comb exhibits fast and large response to ppm-level CO gas even at room temperature. The sensing mechanism is also discussed briefly. These results demonstrate that comb-like ZnO nanostructures have potential application in gas sensors without an external heater. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Zinc oxide Nanostructures Gas sensor Comb-like structure
1. Introduction Quasi-one-dimensional (quasi-1D) semiconductor nanostructures have attracted great interest in the fabrication of nanodevices due to their high surfaceevolume ratio. Zinc oxide (ZnO), an important semiconductor with a large direct band gap (3.35 eV at room temperature) has been suggested for its broad applications, including electronics, optoelectronics, electrochemistry, photovoltaics and gas sensors because of their special geometry [1e5]. For example, nanofibers, nanorods, nanotubes, nanopropeller, nanorings, nanocombs have been reported [6e14]. Among all these nanostructures, the comb-like ZnO nanostructures assembled by the aligned nanowires and back-bone nanobelts have been demonstrated to function as ultraviolet laser arrays and grating bean-divider. More applications are expected to develop on optical, electrical and gas sensing properties. Nanowires generally were synthesized by vaporeliquidesolid (VLS) or vaporesolid (VS) growth processes. And metal catalysts are important to assist the growth of nanowires especially Au catalyst
* Corresponding authors. College of Physics, Qingdao University, Qingdao 266071, China. Tel./fax: þ86 (0) 53285955977. E-mail addresses: [email protected] (H.-D. Zhang), [email protected] (Y.-Z. Long). 0042-207X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.vacuum.2013.07.046
[15e17]. In the VLS growth of nanowires, the catalyst is in the liquid phase and precursors in the vapor phase can adsorb and condense to form the nanowire. In the VS process, instead, the nanowire crystallization originates from the direct condensation from the vapor phase. ZnO nanowires have been synthesized by a range of techniques, such as chemical vapor deposition (CVD) [18], template-assisted growth [19], electrospinning [20], metal-organic chemical vapor deposition (MOCVD) [21] and pulsed laser deposition [22] methods, etc. Compared with above methods, CVD methods can yield high purity ZnO and be of low cost, less hazardous, simply methods for producing nanowires. ZnO has been proved to be a kind of highly sensitive material for the flammable or toxic gas detection, such as H2, CO, NO, or ethanol [23,24]. Since quasi-1D nanomaterials have higher sensitivity, faster response, and better capability to detect low concentration gases compared to the corresponding bulk materials. Oxides geometrical morphology is an important issue for gas sensing applications, such as branched, flower-like, porous nanowires et al. due to their high surfaceevolume ratio [25,26]. The operating temperature is another important issue for gas sensors. To improving the sensing performance of the gas sensors, usually the ZnO nanowires were doped with metal or metal-oxide to enhance the sensing properties [27e29]. However, the work temperature for most of the gas sensors is about 200e400 C, a heater must be added which lead to the sensor complex and large power
114
H.-D. Zhang et al. / Vacuum 101 (2014) 113e117
consumption [30]. Hence, it’s necessary to construct a novel nanostructure for gas sensor to satisfy normal working at room temperature. A room-temperature sensor system could simplify device design by eliminating the heater component, save electrical power, and most importantly, avoid triggering an explosion in an explosive environment. Groups have made tremendous effort towards the development of room temperature oxide gas sensors in various ways [31,32]. In this paper, ZnO nanostructures were grown by a modified CVD method and comb-like structures were obtained. Particularly, a quartz vial was used in the quartz tube of the furnace to control the different pressure ratio of oxygen and zinc vapor qualitatively. With this method, a spatial variation of synthesis/vacuum conditions such as partial pressures of Zn and oxygen was created, and thus different ZnO nanostructures including nanowires, comb- and belt-like structures were obtained at different locations of the collector. The electrical characteristics of an individual ZnO comb were examined to demonstrate a high sensitivity to CO gas attributing to the comb structure at room temperature. 2. Experimental details In the process of fabricating ZnO nanocombs/nanowires, generally the ZnO source is a mixture of graphite powder, ZnO powder or other metal oxide such as CuO and SnO2 as catalysts [1,29,33]. Comparison to above reports, the ZnO source in this study was pure zinc powder and no other dopant. ZnO nanostructures were fabricated on Si wafer by chemical vapor deposition. Fig. 1 shows the experimental apparatus for fabrication of ZnO nanostructures. The CVD furnace included a horizontal quartz tube of 1-inch diameter and a small quartz vial inside the quartz tube. Firstly, Au thin film with thickness of about 10 nm was deposited on Si wafer N and then annealed at 700 C for 30 min to get Au nanoparticles as catalyst. Pure zinc powder (99.9%, as the raw source material) was placed close to the bottom of the small quartz vial. Wafer N was placed on the quartz vial’s neck to grow zinc oxide nanostructures. Secondly, the quartz tube was evacuated to 4 102 Torr and was purged with Ar gas under 1 atm, then rapidly elevated to 710 C in 30 min with a constant Ar flow of 90 sccm (standard cubic centimeters per minute). Thirdly, the temperature (700 C) of the furnace was maintained for 30 min with oxygen/argon (O2/Ar) (2% O2 concentration mixed with 98% Ar). The total flow rate of oxygen/argon was 250 sccm. The Au nanoparticles on silicon wafer N was formed liquid droplets. When the furnace temperature went above the melting point of zinc metal (420 C), zinc would gradually vaporize to fill the quartz vial. And a high zinc concentration condition with dilute oxygen content created and ZnO nanostructures formed on wafer N at the bottleneck of the quartz vial. After reaction, Wafer N was taken out from the furnace into air for rapid cooling. White products were deposited on the Si wafer N. The surface morphologies of the nanostructures were characterized by a scanning electron microscope (SEM, JEOL JSM-6390).
Fig. 1. Experimental apparatus for fabrication of ZnO nanostructures.
The phase purity of the resultant ZnO nanostructures on wafer N was analyzed by X-ray diffractometer (XRD, Bruker D8 Advance) at room temperature with Cu-Ka1 radiation. The nanocombs were removed from the growth wafer N and then dispersed on a new SiO2/Si chip with pre-patterned Au electrode arrays via traditional lithography technology. The square Au electrodes have a side length of 50 mm, and comb-like ZnO nanostructures can be bridged across the electrodes. The electrical property was performed by using two-metal-microprobe testing platform. And the illumination was performed by a LED lamp with tunable color temperature of Philips. The gas sensing properties of the fibers were measured by a Keithley 6517 high resistance meter and a home-made gas sensing system at room temperature. The sample was placed in a sealed quartz chamber and the apparatus were connected. The gas chamber contains one inlet port and one outlet port. Moreover, the inlet port links up with a pipeline which connects the air and standard gas of CO balanced by air. And all the vents were controlled by valves, respectively. The standard gas of CO balanced by air with the concentration of 250 ppm and the air were stored in different gas cylinders. Then the air and the sensing gas were introduced with the flow rate of 250 sccm by independent flow controller, respectively. The electrical resistance of the element was measured in air and in the presence of the CO gas in air. Firstly the measurement system was filled with air and the test began after the electric current of the sample was stable. The measurement included two processes of gas-on and gas-off. Gas-on was to fill standard gas of CO balanced by air for 5 min, meanwhile the air was cut off. Gas-off was to cut off standard gas of CO balanced by air and fill with air for 5 min. Both the gas flow rates were controlled to be 250 sccm. Then the two processes circulated couple periods and the change in resistance was recorded automatically by a computer. 3. Results and discussion 3.1. Characterization of ZnO nanostructures The morphologies of the white products at different positions on wafer N were characterized by SEM. The different positions were labeled as a1, a2, a3, a4 and a5 from the inner edge to the outer edge of the wafer N, respectively (as shown in Fig. 2a) in the quartz vial. Fig. 2a1ea5 shows the SEM images of different structures at locations a1, a2, a3, a4 and a5. It can be seen that the surface morphology of the resultant ZnO strongly depends on the location. There exist several self-assembled nanostructures of ZnO with various geometrical shapes on wafer N. Though the microstructure differences of ZnO have been well-characterized [29], the various nanostructures within such a small area on the silicon substrate with one experimental run has few reported. From position a1 to position a5, the ZnO nanostructures grow up gradually and evolve from particles to rods, combs and belts. Fig. 2a1 shows that only hexagonal Au particles uniformly distributed on the wafer and ZnO just nucleated and ready to grow against Au particle, which can be confirmed by the structure at the next location a2. According to the crystal growth theory, when the Au droplets reached a critical radius, ZnO began to precipitate. Fig. 2a2 shows the short nanorods grown under the hexagonal Au nanoparticle, which indicates that Au nanoparticles assist the ZnO nucleating as catalyst. And the nanorods continue to grow longer (Fig. 2a3). Contrasting with VS growth, VLS growth requires the existence of a liquid. VLS growth mechanism means the presence of the catalyst particle at the top of the nanowire, [15] locations a1 and a2 are from the region on the substrate where Au catalysts were tiptop, and therefore it was indicated that the growth of the synthesized ZnO nanostructures was controlled by VLS mechanism. Fig. 2a4 shows the SEM image of
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Fig. 2. (a) Five locations on wafer N were characterized by SEM, labeled as a1, a2, a3, a4 and a5 from the inner edge to the outer edge of the wafer N, respectively. (a1) Only hexagonal Au particles uniformly distributed on the wafer and ZnO just nucleated and ready to grow. (a2) Short nanorods grew under the hexagonal Au particle. (a3) The nanorods continued to grow longer under the hexagonal Au particle. (a4) One-side teeth comb of ZnO nanowires formed. (a5) ZnO nanobelt could be found at the outer edge.
the single-side ZnO combs. These comb-like ZnO structures are several tens of micrometers and may be suitable for gas sensors. The lengths of the comb teeth gradually decrease along the growth direction of ZnO comb-like structures. The branches have uniform diameters and are evenly distributed on the belt-like stem. The diameters of the teeth are about 250 nm and the lengths range from 0 to 3 mm. In addition, thicker needle-like ZnO structures are also observed at outside of the wafer N because of the higher oxygen concentration, as shown in Fig. 2a5. Fig. 3 shows the X-ray diffraction profile of the ZnO nanocombs. A clear diffraction pattern is shown, and nine reflection peaks appear at 2q ¼ 31.9 (100), 34.6 (002), 36.5 (101), 47.7 (102), 56.8 (110), 63.1 (103), 66.5 (200), 68.1 (112), and 69.2 (201). It agrees well with the JCPDS card, No. 89-1397 for pure ZnO, confirming that only ZnO phase are formed without diffraction peaks due to secondary phases. This confirmed that only pure ZnO nanostructures were formed by this CVD method. Furthermore, it also implies that the nanostructured ZnO is highly crystallized. The formation of ZnO nanostructures with different geometrical shapes could be attributed to the ratio change of Zn and O2 vapor pressure inside the neck of the quartz vial. According to the theory
of crystal growth, zinc molecules are easy to be adsorbed on the Au nanoparticles and nucleate. Moreover, the crystal growth rate and the nucleation rate are determined by the concentration ratio of oxygen and zinc vapor. At the inner part of the vial, the Zn concentration is very high and the oxygen concentration is relatively very low. So the ZnO grows slowly at inner locations (a1 and a2). When close to the bottleneck the oxygen concentration increases gradually, the ZnO nanostructures grow rapidly at outer positions (a4 and a5). Then the investigations through SEM and XRD indicated that nothing except the ZnO crystal occurred at the tip of tooth or around the structures. In zinc oxide crystal with hexagonal structure, ½0001; ½2110 and ½0110 are preferential growth directions. The forming mechanism of comb-like ZnO structure can be understood as follows [34,35]: First is a faster growth along ½0110 direction forming the nanobelt stems, and they were enclosed by the ð0001Þ side facets or ð2110Þ top/bottom facets; second is the nucleation and growth of the nanorod branches along [0001] direction synchronously, which may be related to the structure characteristic of wurtzite family. Once it began to grow on the surface of individual nanorods, ZnO dendrite nanostructures were formed. As shown in Fig. 2a4 comb-like ZnO was created. Moreover, from Fig. 2 one can see that there existed different morphologies of ZnO nanostructures at different locations of wafer N. The pressure of oxygen was smaller and the pressure of Zn atoms was larger in the inner part of the quartz vial, while the pressure of oxygen increased when close to the bottleneck, as shown in Fig. 1. So the pressure ratio of oxygen and zinc vapor plays a key role to the growth of ZnO combs, although accurate values (or direct measurements) of partial pressures of Zn and oxygen are not available in the quartz vial. Therefore, to obtain the required ZnO nanostructures such as comb-like structures, it is necessary to adjust proper concentration ratio of zinc and oxygen vapor. 3.2. Gas sensing properties
Fig. 3. X-ray diffraction profile of the ZnO nanostructures.
To fabricate ZnO nanostructure-based gas sensor device, SiO2/ Si chip with pre-patterned Au electrode arrays via traditional lithography technology was used as substrate to collect Zn
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Fig. 4. Optical image of an individual ZnO comb on SiO2/Si wafer with pre-patterned Au electrode array. Its electrical property was measured by a two-probe method.
nanostructures. As shown in Fig. 4, the square Au electrodes have a side length of 50 mm. The ZnO nanocomb was dispersed between two Au contact grids and no other processing was made which formed not ohmic contacts. The two ends of ZnO micro-comb on the electrodes formed natural Schottky contacts. Fig. 4 shows the optical picture of the measurement that two metal microprobes are pressing directly on the square Au electrodes. The two ends of ZnO micro-comb on the electrodes formed natural Schottky contacts. Fig. 5 shows the IeV curves of the single ZnO micro-comb at dark and under illustration, which testified a typical Schottky contact device. The IeV curves are in agreement with previous observations on Schottky contact device [36] although the values of current changed obviously at about 10 V not 1 V in the previous ones. This difference might be due to the different experimental conditions. The scanning voltage was applied from 20 V to 20 V with a step of 0.1 V. At the same time the current corresponding to each applied voltage was recorded. It is evident that the ZnO has a very little current (2.5 mA, 20 V bias) at dark. But the current increases sharply under illustration. The current can reach 60 mA (20 V bias) when the color temperature of illustration is 2900 K, which attributes to photo-generated electronehole pair resulting to the photocurrent. In addition, the Si substrate was used as a back gate to measure gate effect of this comb-like ZnO device. Fig. 6 depicts the time-dependent current of periodic exposure to 250 ppm CO ambient. The applied constant voltage was 20 V. The sensor exhibits rapid, apparent response upon exposure and removal of CO even at room temperature. The current reached 15 mA under gas-on and decreased sharply when gas-off. Since the gas response can be defined as the ratio: S z jRair Rgasj/Rair, where Rair and Rgas are the resistance of the sensor in dry air and in the test gas, respectively. From Fig. 6 it indicates that the resistance of sensor was dropped by about 82% in 5 min towards 250 ppm CO.
Fig. 5. Currentevoltage (IeV) characteristic curves of the individual ZnO comb at dark and under illustration.
Fig. 6. Response of the gas sensor based on single ZnO comb.
Consequently, this ZnO sensor is suitable and effective in detecting CO gas even at room temperature. 3.3. Gas sensing mechanism It is well known that when zinc oxide semiconductors are exposed to air, oxygen molecules will be absorbed onto the ZnO surface and form negative ions such as O1 or O2 ions by attracting electrons from the conduction band of ZnO. Under carbon monoxide atmosphere, the CO gas reacts with oxygen molecule on the surface and gives back electrons into the conduction band, thereby increasing the electron carriers and lowering the resistance of ZnO sensors. Furthermore, ZnO nanostructures exhibit higher response than the bulk even at room temperature without assembling a heater. It can be simply explained by the effect of the surface-tovolume ratio. In the present case, the fast and large response/recovery of ZnO nanocomb to CO at room temperature is mainly based on the nanostructure. Fig. 7 shows the sensing mechanism of comb-like ZnO. Compared with smooth nanowire, the teeth of comb enlarge the contact area for CO molecules. Moreover, the ordered teeth of the comb constituted parallel circuit and all the partial current in each tooth gathered into the straight primary ribbon, which enhanced the total current density. On the other hand, the high sensitivity in room temperature is attributed to the Au/ZnO junction at the interface between the nanocomb and the contacts. The Schottky-contact device greatly improved the sensor performance for CO sensing [37], because the contact resistance belongs to semiconductor resistance which is much important for a single nanowire. Au over the ZnO surface can speed up surface reactions and improve selectivity to target gas, which acts as catalyst to be active near the grain boundaries where carrier transport takes place [16]. The contact area between the nanocomb and the Au electrode is rather small, and then the adsorption of a few molecules at the junction could significantly
Fig. 7. Schematic illustration of comb-like ZnO gas sensing mechanism.
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change the local barrier height. So the total conductance is dominated by the contact inside the contact area and the areas near the outer surface. Consequently the comb-like ZnO displays high sensitivity to CO even at room temperature and has potential application in CO gas sensor without an external heater. 4. Conclusion In summary, comb-like ZnO nanostructures are successfully prepared by CVD method in a quartz vial. The SEM shows that the teeth of comb grown out of one straight primary ribbon. The vapor pressure change of Zn/O2 ratio results in the formation of ZnO nanostructures with different morphologies inside the neck of the quartz vial. The electrical measurements of a single ZnO comb indicate that the comb-like ZnO is n-type semiconductor, and sensitive to illumination and CO gas. The single ZnO comb exhibits fast and large response to 250 ppm CO even at room temperature. These results demonstrate that the comb-like ZnO nanostructures have potential application in gas sensors without an external heater. Acknowledgments This work was supported by National Natural Science Foundation of China (Grant Nos. 11004114 and 11074138), Natural Science Foundation of Shandong Province (China) for Distinguished Young Scholars (Grant No. JQ201103), Taishan Scholars Program of Shandong Province (China), National Key Basic Research Development Program of China (973 special preliminary study plan, grant No. 2012CB722705), Project of Shandong Province Higher Educational Science and Technology Program (Grant No. J13LJ07), and Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province, China. References [1] Huang MH, Mao S, Feik H, Yan H, Wu Y, Kind H, et al. Science 2001;292:1897.
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