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H2S sensing in the ppb regime with zinc oxide nanowires
Sensors and Actuators B 239 (2017) 358–363
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
H2 S sensing in the ppb regime with zinc oxide nanowires Florian Huber ∗ , Sören Riegert, Manfred Madel 1 , Klaus Thonke Institute of Quantum Matter/Semiconductor Physics Group, Ulm University, Albert-Einstein-Allee 45, 89081 Ulm, Germany
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Article history: Received 27 April 2016 Received in revised form 26 July 2016 Accepted 4 August 2016 Available online 5 August 2016 Keywords: Zinc oxide Nanowires Chemical vapour deposition Gas sensing Hydrogen sulfide
a b s t r a c t In this work, we investigate the hydrogen sulfide (H2 S)-gas-sensing-properties of zinc oxide (ZnO) nanowires. By a common chemical vapour deposition (CVD) process ZnO nanowires were grown on silicon (111) using a mixture of ZnO and graphite powder as source material. Subsequently, the nanowires were separated from the substrate and placed on a metal contact structure using dielectrophoresis. The sensing properties of the nanowires were tested in a temperature stabilized setup. Applying a constant voltage and exposing to H2 S leads to an increase of the current through the nanowires. Especially the role of oxygen in the sensing mechanism has been investigated using different gases for flushing. We demonstrate that oxygen is crucial for the reset of the sensor and multiple sensing cycles with one sensor were realized. The ZnO nanowires show a very high sensitivity and a H2 S concentration of only 50 ppb can be detected. © 2016 Elsevier B.V. All rights reserved.
1. Introduction H2 S plays a crucial role in different medical applications. On the one hand, H2 S acts as biomarker and can indicate lung diseases like asthma [1], on the other hand H2 S has great therapeutic potential [2]. Especially the therapeutic capabilities of so-called “slow releasing H2 S-donors”, which are potential anti-cancer agents [3,4] and used as anti-inflammatory drug [5], are in the focus of recent studies. However, the detection of small concentrations of this gas is still challenging. Especially a fast and sensitive sensing device for analyzing the breath of a patient is needed in order to monitor the therapeutic process and control the H2 S concentration in different medical applications. Several sensing approaches like mid-infrared gas sensors [6], selected ion flow tube mass spectroscopy [7], or sensors based on copper acetate [8] have been presented. A detailed overview of the different systems is given by Pandey et al. [9]. The main problem is the extremely low concentration of only some parts per billion (ppb) which has to be measured in medical applications. For these low concentrations various sensor designs have been presented and successfully tested. Using tin oxide microsensors doped with copper oxide, it was possible to detect H2 S in a
∗ Corresponding author. E-mail address: fl[email protected] (F. Huber). 1 Present address: United Monolithic Semiconductors GmbH, 89081 Ulm, Germany. http://dx.doi.org/10.1016/j.snb.2016.08.023 0925-4005/© 2016 Elsevier B.V. All rights reserved.
concentration range of 100 ppm (parts per million) down to 100 ppb [10]. Tanda et al. presented in 2007 a ZnO-based sulfide monitor, which was already tested in a field study [11]. The system is based on a ZnO film coated with an alumina catalyst layer, and the detection of only 20 ppb H2 S has been reported. Concentrations in the same low magnitude have been detected using WO3 nanoparticles in a temperature modulated measurement (150–250 ◦ C) [12], and by CuO-doped (Ba0.8 Sr0.2 )(Sn0.8 Ti0.2 )O3 (BSST) at room temperature [13]. The detection of even lower concentrations has been reported by Shirsat et al. [14]. Using nanowires made of the conducting polymer polyaniline and functionalized with gold nanoparticles, it was possible to measure a change in resistance even at 0.1 ppb H2 S. Besides these, also ZnO nanowires are a promising candidate for H2 S-gas-sensing in this low concentration regime. They can be easily produced without expensive technical equipment in large amount and are non-toxic. Several groups have already published sensor designs based on ZnO nanomaterials, showing that this material system is sensitive to H2 S. Ramgir et al. detected 1 ppm H2 S with hydrothermally grown ZnO nanowires, and investigated the role of gold as a catalyst for the sensing mechanism [15]. Similar results have been reported for sensors based on ZnO tetrapods [16], nanofibers [17], Mo-doped nanowires [18], and nanorods with flower-like structures [19,20]. Even very low concentrations of H2 S in the ppb regime have been detected [21]. In this work we investigate the H2 S-sensing behaviour of ZnO nanowires grown by gold-catalysed CVD on Si substrates. In most publications so far the sensing mechanism is described by a reaction of the H2 S with oxygen, which is adsorbed on the surface of
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Fig. 2. SEM image of the ZnO nanowires on the contact structure. The nanowires have been separated from the substrate and placed on the contacts using dielectrophoresis. Fig. 1. Scanning electron micrograph of the as-grown ZnO nanowires on the silicon (111) substrate.
8x10-6 6 4
current (A)
the sensor structure. Therefore, oxygen should play a crucial role, and the investigation of its impact on the performance of the sensor is in the focus of this work. In contrast to the majority of earlier publications in this field, we do not use only air as flushing gas after exposing the sensor to H2 S, but also use pure nitrogen. We show that oxygen is necessary to reset the sensor and therefore is essential for realizing multiple reproducible sensing cycles. Besides the investigation of the flushing gas, concentration depended measurements are done, showing that the present structures are sensitive for concentrations as low as 50 ppb H2 S.
2 0 -2 -4
2. Material and methods -6
2.1. Growth of ZnO nanowires The ZnO nanowires used for our sensing experiments were grown in a horizontal three zone tube furnace (Carbolite HZS 12/600). Each heating zone is 20 cm long, and the quartz glass liner tube has a diameter of 40 mm. The source material, 140 mg of a mixture of ZnO powder (Alfa Aesar, purity: 99.99%) and graphite powder (Alfa Aesar, purity: 99.9999%) in a molar ratio of 1:1 is heated up to 1050 ◦ C. The resulting zinc vapour is transported by an argon flux (99.995%) of 160 sccm to the substrate, which is placed 25 cm downstream at 1060 ◦ C. As substrate a 1 cm × 1 cm silicon (111) wafer is used. A gold film with 3 nm thickness, deposited by electron beam evaporation, serves as catalyst. At a distance of 0.5 cm upstream of the sample, an additional oxygen inlet is placed. By introducing pure oxygen (99.999%), the zinc vapour is reoxidized on the substrate starting the VLS growth of ZnO nanowires. A detailed analysis of the chemical reactions has been reported before [22]. The whole process takes place at 900 mbar. Fig. 1 shows a scanning electron micrograph of the nanowires grown on the silicon substrate. The nanowires have a diameter of around 90–150 nm and 10 m length. Further details of the growth process were reported by Li et al. [23]. 2.2. Dielectrophoresis To investigate the sensing behaviour of the ZnO nanowires, they were separated from the substrate and placed in a large amount onto electrical contacts. By pulling a small copper TEM grid over the sample, the nanowires were scratched off the substrate into isopropanol. This suspension of ZnO nanowires and isopropanol is dropped on a titanium-gold contact structure, which was fabricated with conventional optical lithography. The structure consists of two parallel 3 cm long contacts with a 5 m gap
-1.0
-0.5
0.0
0.5
1.0
voltage (V) Fig. 3. Typical I(V)-characteristic of the produced sensors. All sensing experiments were performed applying a constant voltage of 1 V to the nanowires.
in between. While the isopropanol is evaporating, an alternating current (10 kHz, 5 V) is applied and the nanowires are aligned between the contacts by dielecrophoresis [24]. A SEM image of the resultant sample is shown in Fig. 2. The total amount of contacted nanowires can be estimated between 30,000 and 600,000, depending on the concentration of nanowires in the suspension. By this method, several sensor structures have been fabricated. Fig. 3 shows a typical I(V)-characteristic of the samples. All sensing experiments are performed applying a constant voltage of 1 V to the nanowires. The measured current depends on the amount of nanowires on the contact structure and is between 1 and 20 A. 2.3. Gas sensing mechanism and setup As given by Barsan and Weimar [25], the chemisorption of oxygen at the surface of metal oxides can be described by Eq. (1): ˇ O2 (g) + ˛ · e− + ∗ ⇔ O−˛ , ˇ,∗ 2
(1)
where O2 (g) is an oxygen molecule in the ambient air, which is chemisorbed at an unoccupied chemisorption spot * at the surface of the structure, O−˛ is the chemisorbed oxygen species in either ˇ,∗
one of the ionized states ˛ = 1,2 and the atomic or molecular state ˇ = 1,2. This chemisorbed oxygen causes an upward band bending at the surface of the ZnO nanowires, influencing the width of the depletion layer. By exposing the sensor to an ambient containing
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H2 S, the latter interacts with the oxygen at the surface, which is usually described by Eq. (2) [26–28]: − 2H2 S(g) + 3O˛− 2 (ad) ⇔ 2H2 O(g) + 2SO2 (g) + 3˛ · e .
(2)
By removing the chemisorbed oxygen, electrons are released into the material and the band bending is reduced. This leads to a thinner depletion layer and this way lower resistivity of the ZnO nanowires. According to this model, oxygen has a crucial role in the sensing mechanism, because H2 S does not attach directly to the ZnO nanowires, but influences the band structure indirectly via oxygen. To verify this, we performed experiments with and without additional oxygen introduced to the system. Alternatively to the sensing mechanism described above, a direct reaction between H2 S and the nanowires surface could be possible. As possible alternative sensing mechanism the sulfidation of ZnO forming ZnS is often discussed in literature [16,26,29]. The process is described by Eq. (3): ZnO(s) + H2 S(g) = ZnS(s) + H2 O(g).
(3)
As discussed in literature [29–31], this reaction, as well as the resulfidation described by Eq. (4) 2ZnS(s) + 3O2 (g) = 2ZnO(s) + 2SO2 (g),
(4)
are exothermic. Kim and Yong [26] investigated the sensing mechanism of ZnO sensors towards H2 S at different temperatures, and find that the direct reaction of H2 S gas with ZnO dominates the sensing mechanism at temperatures above 300 ◦ C. For lower temperatures, they suggest the sensing mechanism according to Eqs. (1) and (2) to dominate. These findings are consistent with the results published by Zhang et al. [32]. They investigated the sulfidation of ZnO films with H2 S at different temperatures at atmospheric pressure. Their XRD results showed that no ZnS was formed at a process temperature of 200 ◦ C, and a ZnS signal was first observable at a temperature of 300 ◦ C. Considering that all measurements presented in this work are performed at 20 ◦ C, a direct reaction of H2 S with the ZnO nanowires forming ZnS is very unlikely. Instead the reaction of the gas with oxygen adsorbed at the surface of the nanowires likely is the dominating sensing mechanism. The sensing properties of the samples were tested in a closed stainless steel gas chamber with a volume of 25 ml. The small volume guarantees a fast exchange of the atmosphere in it. The temperature of the chamber and the sample is stabilized by a water bath to 20 ± 0.1 ◦ C and monitored with a temperature dependent resistor mounted next to the sample. A computer-controlled dilution stage consisting of four mass flow controllers (MFCs) and one pressure controller allows the mixing of different gases and the controlled intake of the mixture, as well as flushing the chamber with a maximum flow of 450 ml per minute, which equals 18 times the chamber volume. Available gases are pure nitrogen (99.9999%), 1 ppm H2 S (98%) in nitrogen (99.9999%), and air for flushing. For the investigation of the sensitivity of the sensor structures, the H2 S gas can be reproducibly diluted down to some ppb by mixing with nitrogen in the computer-controlled dilution stage. All experiments take place at atmospheric pressure. During the whole experiment a constant voltage of 1 V is applied to the sample, and the current through the nanowires is monitored using a source measure unit (SMU, Keithley Instruments, Model 236). 3. Results and discussion 3.1. H2 S sensing versus pure nitrogen In a first series of measurements, the sensing behaviour of the ZnO nanowires was investigated using pure nitrogen as flushing
Fig. 4. Sensor signal during the exposure to 1000 ppb and 500 ppb H2 S in intervals of 10 min, with flushing intervals with pure nitrogen in between. A clear increase of the current through the nanowires is observable. Flushing with pure nitrogen does not lead to a complete reset of the sensor.
Fig. 5. By flushing with pure nitrogen for 60 min, the initial level could not be reached again. After a short decrease the current remains almost constant.
gas. Hereby no additional oxygen is introduced to the setup, with the objective to obtain a deeper understanding of the role of oxygen in the sensing mechanism. After flushing the chamber with nitrogen, 1 ppm H2 S was added to the nitrogen stream. As one can see in Fig. 4, the current through the nanowires increased almost constantly during the whole 10 min testing phase. In the intermediate intervals of 10 min flushing with pure nitrogen, the signal decreased, but did not reach again the initial current level of about 20 A. This behaviour is also observed in the following two cycles with lower H2 S concentration of 500 ppb. After an initial, less steep linear increase of the current, the signal decreases with the same slope as before while flushing with nitrogen, but the starting level is not reached. To verify this measurement, we repeated the procedure with a second sample. This time the testing and flushing phases were extended to 60 min each. As it is shown in Fig. 5, the signal increases linearly as soon as the H2 S is introduced into the chamber. Switching back to nitrogen leads to a short decrease of the current during the first 10 min, comparable to the results above. Subsequently, the
F. Huber et al. / Sensors and Actuators B 239 (2017) 358–363
signal remains almost constant, and the initial level is not reached again. These results substantiate the above suggested sensing mechanism. Obviously, the conductivity of the ZnO nanowires is influenced by H2 S by a surface reaction with chemisorbed oxygen species on the nanowire surface. By binding the oxygen, the depletion layer in the nanowires is decreased, leading to higher conductivity. Because there is no additional oxygen introduced during the flushing phase, the initial situation can not be recovered, and the conductivity remains at a higher level. The observed minor initial decrease in nitrogen atmosphere is presumably caused by re-absorption of the small amount of residual oxygen, which was not removed by the H2 S in the last interval. As already mentioned in Section 2.3, a direct reaction of ZnO with H2 S resulting in ZnS is unlikely regarding the sensor temperature of 20 ◦ C. In order to check the H2 S sensitivity of the ZnO nanowires we reduced the concentration of the gas by diluting it with pure nitrogen. Starting with only 100 ppb H2 S in the first test interval of 20 min, we increased the concentration in steps of 100 ppb after intermediate flushing intervals of 20 min with pure nitrogen. As shown in Fig. 6, even 100 ppb of H2 S lead to a clear response in the signal. Again, the current remains at a higher level after each period with H2 S, and the intermediate flushing with nitrogen can not set back the sensor to the initial state. Again, the result was verified with other samples, in order to ensure the reproducibility. Some samples showed a clear response even at 50 ppb H2 S (Fig. 7), though it was not possible to detect this low concentration with all samples investigated. 3.2. H2 S sensing versus air To clarify the role of oxygen in the sensing mechanism, a second series of experiments was performed, changing the flushing gas from pure nitrogen to ambient air. The H2 S used is pre-diluted to 1 ppm in nitrogen, and therefore one has to take into account, that there is a certain sensor response between air and nitrogen, which may arise even if no H2 S is present in the incoming gas stream. To ensure that the measured sensor response is caused solely by H2 S, we compared the signal answer to pure nitrogen after flushing with air and to 1 ppm H2 S in nitrogen. As shown in Fig. 8, the current increases by about 0.1 A when switching from air to pure nitrogen, which is significantly less than the increase by about 0.8 A when switching from air to 1 ppm H2 S in nitrogen.
Fig. 7. Some of the samples investigated showed a response even at 50 ppb H2 S versus pure nitrogen. At lower concentrations the current through the nanowires stayed constant.
air
air
air
2.5
current (µA)
Fig. 6. By diluting the H2 S with pure nitrogen, the sensitivity of the ZnO nanowires has been tested. The current vs. time plot shows a clear signal even at 100 ppb H2 S.
361
2.0
1 ppm H2S
1 ppm H2S
1.5
N2
N2
0
50
100
150
time (min) Fig. 8. Comparison of the sensor response to switching between ambient air and pure nitrogen (blue), and switching between ambient air and 1 ppm H2 S in nitrogen (black, for better visualization an offset has been added). The additional H2 S causes a significantly larger change in current. By flushing with air instead of nitrogen, the signal reaches the initial level. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Very important is that the signal decreases during the flushing phase with air to the initial level. As shown in Fig. 9, this full sensor response can be reproduced several times. Exposing the nanowires to the test gas for only 5 min leads to a very large signal change. Compared to the measurements with nitrogen as flushing gas (Fig. 4), one can clearly see the difference between both experiments. Using ambient air the signal can be brought back to the initial level. Obviously, the oxygen in the air must be adsorbed to the nanowire surface during flushing, leading to a thicker depletion layer and higher resistance of the nanowires. To investigate the sensitivity, we diluted the H2 S down to 50 ppb (Fig. 10). Like in the experiment shown above with nitrogen for flushing, a response down to 50 ppb H2 S is clearly observable. Again the change in flushing gas from nitrogen to air leads to an improved reset behaviour of the sensor during flushing. Taking the sensor response of air versus pure nitrogen into account (Fig. 8), it is not possible to measure concentrations lower than 50 ppb at this stage,
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consistence with the sensing mechanism proposed in literature, where H2 S interacts with the nanowires via the chemisorbed oxygen. With the sensor device shown here, multiple sensing cycles have been realized and an extremely low concentration of only 50 ppb H2 S has been detected. Acknowledgements The authors thank M.Sc. M. Dickel and Dipl.-Ing. B. Riedmüller for support in the sample preparation. Furthermore, we thank Prof. B. Mizaikoff for technical support. We thank the Carl Zeiss Foundation and the Baden-Württemberg-Stiftung for financial support.
References
Fig. 9. Sensor response to multiple cycles of gas switching. After each interval of 5 min, the sensor can be reset during the following 5 min of flushing with air.
Fig. 10. H2 S-concentration series with air for flushing. Even 50 ppb H2 S can be detected.
because it can not be distinguished anymore whether the chance in the signal is caused by H2 S or simply by switching from air to nitrogen. 4. Conclusions Long and thin ZnO nanowires were fabricated by a simple CVD process on a silicon wafer substrate using a mixture of ZnO powder and graphite powder as source material, and a thin gold film as catalyst. Subsequently, a large amount of nanowires was placed on a parallel contact structure using dielectrophoresis. In a temperature stabilized setup the sensing behaviour of the ZnO nanowires was investigated. As expected, the exposure of the sample to H2 S led to a decrease of the resistance of the nanowires. A crucial factor for the sensing behaviour is the flushing gas used between measurement intervals to define an initial level of the signal. We found that pure nitrogen leads only to a plateau of the current increase, but not to a complete reset of the sensor. Flushing the sensor system with oxygen causes a decrease of the current signal, and the initial level can be established again. This behaviour is in
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Biographies Florian Huber received his M.Sc. degree in Physics from the University of Ulm in 2014 and is currently a Ph.D. candidate at the Institute of Quantum Matters/Semiconductor Physics Group in Ulm/Germany. His research includes the CVD-growth of ZnO nanowires and layers and the characterization of these structures. Especially the sensing properties of ZnO nanowires are in focus of his work. Sören Riegert has completed his BSc degree in Physics in 2016 and is pursuing his M.Sc. degree at the University of Ulm. His work at the Institute of Quantum Matter at Ulm University was focused on the investigation of the sensing behaviour of ZnO nanostructures. Manfred Madel will finish his Ph.D. thesis in 2016 at Ulm University. During his work he investigated the optical sensing properties of ZnO nanowires grown on different substrates. Since 2015 he is working at United Monolithic Semiconductors GmbH in Ulm/Germany. Klaus Thonke received his Ph.D. in Physics in 1986 at the University of Stuttgart on the topic of optical investigations on defects in silicon. In 1990 he moved to the University of Ulm/Germany, where the new Institute of Semiconductor Physics was found (head: Prof. R. Sauer). In 1995, he finished his habilitation on the topic: optical high resolution spectroscopy on 3d and 4f transition metals in semiconductors. He is specialized on the characterization of defects in a broad variety of semiconductor materials ranging from Si, SiGe, diamond, GaAs, InP, GaSb, InGaAlP, to GaN, AlN, InGaN, ZnO and others. During the last decade, he also worked on the growth of ZnO films and nanostructures, and on their application for sensing devices.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
H2 S sensing in the ppb regime with zinc oxide nanowires Florian Huber ∗ , Sören Riegert, Manfred Madel 1 , Klaus Thonke Institute of Quantum Matter/Semiconductor Physics Group, Ulm University, Albert-Einstein-Allee 45, 89081 Ulm, Germany
a r t i c l e
i n f o
Article history: Received 27 April 2016 Received in revised form 26 July 2016 Accepted 4 August 2016 Available online 5 August 2016 Keywords: Zinc oxide Nanowires Chemical vapour deposition Gas sensing Hydrogen sulfide
a b s t r a c t In this work, we investigate the hydrogen sulfide (H2 S)-gas-sensing-properties of zinc oxide (ZnO) nanowires. By a common chemical vapour deposition (CVD) process ZnO nanowires were grown on silicon (111) using a mixture of ZnO and graphite powder as source material. Subsequently, the nanowires were separated from the substrate and placed on a metal contact structure using dielectrophoresis. The sensing properties of the nanowires were tested in a temperature stabilized setup. Applying a constant voltage and exposing to H2 S leads to an increase of the current through the nanowires. Especially the role of oxygen in the sensing mechanism has been investigated using different gases for flushing. We demonstrate that oxygen is crucial for the reset of the sensor and multiple sensing cycles with one sensor were realized. The ZnO nanowires show a very high sensitivity and a H2 S concentration of only 50 ppb can be detected. © 2016 Elsevier B.V. All rights reserved.
1. Introduction H2 S plays a crucial role in different medical applications. On the one hand, H2 S acts as biomarker and can indicate lung diseases like asthma [1], on the other hand H2 S has great therapeutic potential [2]. Especially the therapeutic capabilities of so-called “slow releasing H2 S-donors”, which are potential anti-cancer agents [3,4] and used as anti-inflammatory drug [5], are in the focus of recent studies. However, the detection of small concentrations of this gas is still challenging. Especially a fast and sensitive sensing device for analyzing the breath of a patient is needed in order to monitor the therapeutic process and control the H2 S concentration in different medical applications. Several sensing approaches like mid-infrared gas sensors [6], selected ion flow tube mass spectroscopy [7], or sensors based on copper acetate [8] have been presented. A detailed overview of the different systems is given by Pandey et al. [9]. The main problem is the extremely low concentration of only some parts per billion (ppb) which has to be measured in medical applications. For these low concentrations various sensor designs have been presented and successfully tested. Using tin oxide microsensors doped with copper oxide, it was possible to detect H2 S in a
∗ Corresponding author. E-mail address: fl[email protected] (F. Huber). 1 Present address: United Monolithic Semiconductors GmbH, 89081 Ulm, Germany. http://dx.doi.org/10.1016/j.snb.2016.08.023 0925-4005/© 2016 Elsevier B.V. All rights reserved.
concentration range of 100 ppm (parts per million) down to 100 ppb [10]. Tanda et al. presented in 2007 a ZnO-based sulfide monitor, which was already tested in a field study [11]. The system is based on a ZnO film coated with an alumina catalyst layer, and the detection of only 20 ppb H2 S has been reported. Concentrations in the same low magnitude have been detected using WO3 nanoparticles in a temperature modulated measurement (150–250 ◦ C) [12], and by CuO-doped (Ba0.8 Sr0.2 )(Sn0.8 Ti0.2 )O3 (BSST) at room temperature [13]. The detection of even lower concentrations has been reported by Shirsat et al. [14]. Using nanowires made of the conducting polymer polyaniline and functionalized with gold nanoparticles, it was possible to measure a change in resistance even at 0.1 ppb H2 S. Besides these, also ZnO nanowires are a promising candidate for H2 S-gas-sensing in this low concentration regime. They can be easily produced without expensive technical equipment in large amount and are non-toxic. Several groups have already published sensor designs based on ZnO nanomaterials, showing that this material system is sensitive to H2 S. Ramgir et al. detected 1 ppm H2 S with hydrothermally grown ZnO nanowires, and investigated the role of gold as a catalyst for the sensing mechanism [15]. Similar results have been reported for sensors based on ZnO tetrapods [16], nanofibers [17], Mo-doped nanowires [18], and nanorods with flower-like structures [19,20]. Even very low concentrations of H2 S in the ppb regime have been detected [21]. In this work we investigate the H2 S-sensing behaviour of ZnO nanowires grown by gold-catalysed CVD on Si substrates. In most publications so far the sensing mechanism is described by a reaction of the H2 S with oxygen, which is adsorbed on the surface of
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Fig. 2. SEM image of the ZnO nanowires on the contact structure. The nanowires have been separated from the substrate and placed on the contacts using dielectrophoresis. Fig. 1. Scanning electron micrograph of the as-grown ZnO nanowires on the silicon (111) substrate.
8x10-6 6 4
current (A)
the sensor structure. Therefore, oxygen should play a crucial role, and the investigation of its impact on the performance of the sensor is in the focus of this work. In contrast to the majority of earlier publications in this field, we do not use only air as flushing gas after exposing the sensor to H2 S, but also use pure nitrogen. We show that oxygen is necessary to reset the sensor and therefore is essential for realizing multiple reproducible sensing cycles. Besides the investigation of the flushing gas, concentration depended measurements are done, showing that the present structures are sensitive for concentrations as low as 50 ppb H2 S.
2 0 -2 -4
2. Material and methods -6
2.1. Growth of ZnO nanowires The ZnO nanowires used for our sensing experiments were grown in a horizontal three zone tube furnace (Carbolite HZS 12/600). Each heating zone is 20 cm long, and the quartz glass liner tube has a diameter of 40 mm. The source material, 140 mg of a mixture of ZnO powder (Alfa Aesar, purity: 99.99%) and graphite powder (Alfa Aesar, purity: 99.9999%) in a molar ratio of 1:1 is heated up to 1050 ◦ C. The resulting zinc vapour is transported by an argon flux (99.995%) of 160 sccm to the substrate, which is placed 25 cm downstream at 1060 ◦ C. As substrate a 1 cm × 1 cm silicon (111) wafer is used. A gold film with 3 nm thickness, deposited by electron beam evaporation, serves as catalyst. At a distance of 0.5 cm upstream of the sample, an additional oxygen inlet is placed. By introducing pure oxygen (99.999%), the zinc vapour is reoxidized on the substrate starting the VLS growth of ZnO nanowires. A detailed analysis of the chemical reactions has been reported before [22]. The whole process takes place at 900 mbar. Fig. 1 shows a scanning electron micrograph of the nanowires grown on the silicon substrate. The nanowires have a diameter of around 90–150 nm and 10 m length. Further details of the growth process were reported by Li et al. [23]. 2.2. Dielectrophoresis To investigate the sensing behaviour of the ZnO nanowires, they were separated from the substrate and placed in a large amount onto electrical contacts. By pulling a small copper TEM grid over the sample, the nanowires were scratched off the substrate into isopropanol. This suspension of ZnO nanowires and isopropanol is dropped on a titanium-gold contact structure, which was fabricated with conventional optical lithography. The structure consists of two parallel 3 cm long contacts with a 5 m gap
-1.0
-0.5
0.0
0.5
1.0
voltage (V) Fig. 3. Typical I(V)-characteristic of the produced sensors. All sensing experiments were performed applying a constant voltage of 1 V to the nanowires.
in between. While the isopropanol is evaporating, an alternating current (10 kHz, 5 V) is applied and the nanowires are aligned between the contacts by dielecrophoresis [24]. A SEM image of the resultant sample is shown in Fig. 2. The total amount of contacted nanowires can be estimated between 30,000 and 600,000, depending on the concentration of nanowires in the suspension. By this method, several sensor structures have been fabricated. Fig. 3 shows a typical I(V)-characteristic of the samples. All sensing experiments are performed applying a constant voltage of 1 V to the nanowires. The measured current depends on the amount of nanowires on the contact structure and is between 1 and 20 A. 2.3. Gas sensing mechanism and setup As given by Barsan and Weimar [25], the chemisorption of oxygen at the surface of metal oxides can be described by Eq. (1): ˇ O2 (g) + ˛ · e− + ∗ ⇔ O−˛ , ˇ,∗ 2
(1)
where O2 (g) is an oxygen molecule in the ambient air, which is chemisorbed at an unoccupied chemisorption spot * at the surface of the structure, O−˛ is the chemisorbed oxygen species in either ˇ,∗
one of the ionized states ˛ = 1,2 and the atomic or molecular state ˇ = 1,2. This chemisorbed oxygen causes an upward band bending at the surface of the ZnO nanowires, influencing the width of the depletion layer. By exposing the sensor to an ambient containing
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H2 S, the latter interacts with the oxygen at the surface, which is usually described by Eq. (2) [26–28]: − 2H2 S(g) + 3O˛− 2 (ad) ⇔ 2H2 O(g) + 2SO2 (g) + 3˛ · e .
(2)
By removing the chemisorbed oxygen, electrons are released into the material and the band bending is reduced. This leads to a thinner depletion layer and this way lower resistivity of the ZnO nanowires. According to this model, oxygen has a crucial role in the sensing mechanism, because H2 S does not attach directly to the ZnO nanowires, but influences the band structure indirectly via oxygen. To verify this, we performed experiments with and without additional oxygen introduced to the system. Alternatively to the sensing mechanism described above, a direct reaction between H2 S and the nanowires surface could be possible. As possible alternative sensing mechanism the sulfidation of ZnO forming ZnS is often discussed in literature [16,26,29]. The process is described by Eq. (3): ZnO(s) + H2 S(g) = ZnS(s) + H2 O(g).
(3)
As discussed in literature [29–31], this reaction, as well as the resulfidation described by Eq. (4) 2ZnS(s) + 3O2 (g) = 2ZnO(s) + 2SO2 (g),
(4)
are exothermic. Kim and Yong [26] investigated the sensing mechanism of ZnO sensors towards H2 S at different temperatures, and find that the direct reaction of H2 S gas with ZnO dominates the sensing mechanism at temperatures above 300 ◦ C. For lower temperatures, they suggest the sensing mechanism according to Eqs. (1) and (2) to dominate. These findings are consistent with the results published by Zhang et al. [32]. They investigated the sulfidation of ZnO films with H2 S at different temperatures at atmospheric pressure. Their XRD results showed that no ZnS was formed at a process temperature of 200 ◦ C, and a ZnS signal was first observable at a temperature of 300 ◦ C. Considering that all measurements presented in this work are performed at 20 ◦ C, a direct reaction of H2 S with the ZnO nanowires forming ZnS is very unlikely. Instead the reaction of the gas with oxygen adsorbed at the surface of the nanowires likely is the dominating sensing mechanism. The sensing properties of the samples were tested in a closed stainless steel gas chamber with a volume of 25 ml. The small volume guarantees a fast exchange of the atmosphere in it. The temperature of the chamber and the sample is stabilized by a water bath to 20 ± 0.1 ◦ C and monitored with a temperature dependent resistor mounted next to the sample. A computer-controlled dilution stage consisting of four mass flow controllers (MFCs) and one pressure controller allows the mixing of different gases and the controlled intake of the mixture, as well as flushing the chamber with a maximum flow of 450 ml per minute, which equals 18 times the chamber volume. Available gases are pure nitrogen (99.9999%), 1 ppm H2 S (98%) in nitrogen (99.9999%), and air for flushing. For the investigation of the sensitivity of the sensor structures, the H2 S gas can be reproducibly diluted down to some ppb by mixing with nitrogen in the computer-controlled dilution stage. All experiments take place at atmospheric pressure. During the whole experiment a constant voltage of 1 V is applied to the sample, and the current through the nanowires is monitored using a source measure unit (SMU, Keithley Instruments, Model 236). 3. Results and discussion 3.1. H2 S sensing versus pure nitrogen In a first series of measurements, the sensing behaviour of the ZnO nanowires was investigated using pure nitrogen as flushing
Fig. 4. Sensor signal during the exposure to 1000 ppb and 500 ppb H2 S in intervals of 10 min, with flushing intervals with pure nitrogen in between. A clear increase of the current through the nanowires is observable. Flushing with pure nitrogen does not lead to a complete reset of the sensor.
Fig. 5. By flushing with pure nitrogen for 60 min, the initial level could not be reached again. After a short decrease the current remains almost constant.
gas. Hereby no additional oxygen is introduced to the setup, with the objective to obtain a deeper understanding of the role of oxygen in the sensing mechanism. After flushing the chamber with nitrogen, 1 ppm H2 S was added to the nitrogen stream. As one can see in Fig. 4, the current through the nanowires increased almost constantly during the whole 10 min testing phase. In the intermediate intervals of 10 min flushing with pure nitrogen, the signal decreased, but did not reach again the initial current level of about 20 A. This behaviour is also observed in the following two cycles with lower H2 S concentration of 500 ppb. After an initial, less steep linear increase of the current, the signal decreases with the same slope as before while flushing with nitrogen, but the starting level is not reached. To verify this measurement, we repeated the procedure with a second sample. This time the testing and flushing phases were extended to 60 min each. As it is shown in Fig. 5, the signal increases linearly as soon as the H2 S is introduced into the chamber. Switching back to nitrogen leads to a short decrease of the current during the first 10 min, comparable to the results above. Subsequently, the
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signal remains almost constant, and the initial level is not reached again. These results substantiate the above suggested sensing mechanism. Obviously, the conductivity of the ZnO nanowires is influenced by H2 S by a surface reaction with chemisorbed oxygen species on the nanowire surface. By binding the oxygen, the depletion layer in the nanowires is decreased, leading to higher conductivity. Because there is no additional oxygen introduced during the flushing phase, the initial situation can not be recovered, and the conductivity remains at a higher level. The observed minor initial decrease in nitrogen atmosphere is presumably caused by re-absorption of the small amount of residual oxygen, which was not removed by the H2 S in the last interval. As already mentioned in Section 2.3, a direct reaction of ZnO with H2 S resulting in ZnS is unlikely regarding the sensor temperature of 20 ◦ C. In order to check the H2 S sensitivity of the ZnO nanowires we reduced the concentration of the gas by diluting it with pure nitrogen. Starting with only 100 ppb H2 S in the first test interval of 20 min, we increased the concentration in steps of 100 ppb after intermediate flushing intervals of 20 min with pure nitrogen. As shown in Fig. 6, even 100 ppb of H2 S lead to a clear response in the signal. Again, the current remains at a higher level after each period with H2 S, and the intermediate flushing with nitrogen can not set back the sensor to the initial state. Again, the result was verified with other samples, in order to ensure the reproducibility. Some samples showed a clear response even at 50 ppb H2 S (Fig. 7), though it was not possible to detect this low concentration with all samples investigated. 3.2. H2 S sensing versus air To clarify the role of oxygen in the sensing mechanism, a second series of experiments was performed, changing the flushing gas from pure nitrogen to ambient air. The H2 S used is pre-diluted to 1 ppm in nitrogen, and therefore one has to take into account, that there is a certain sensor response between air and nitrogen, which may arise even if no H2 S is present in the incoming gas stream. To ensure that the measured sensor response is caused solely by H2 S, we compared the signal answer to pure nitrogen after flushing with air and to 1 ppm H2 S in nitrogen. As shown in Fig. 8, the current increases by about 0.1 A when switching from air to pure nitrogen, which is significantly less than the increase by about 0.8 A when switching from air to 1 ppm H2 S in nitrogen.
Fig. 7. Some of the samples investigated showed a response even at 50 ppb H2 S versus pure nitrogen. At lower concentrations the current through the nanowires stayed constant.
air
air
air
2.5
current (µA)
Fig. 6. By diluting the H2 S with pure nitrogen, the sensitivity of the ZnO nanowires has been tested. The current vs. time plot shows a clear signal even at 100 ppb H2 S.
361
2.0
1 ppm H2S
1 ppm H2S
1.5
N2
N2
0
50
100
150
time (min) Fig. 8. Comparison of the sensor response to switching between ambient air and pure nitrogen (blue), and switching between ambient air and 1 ppm H2 S in nitrogen (black, for better visualization an offset has been added). The additional H2 S causes a significantly larger change in current. By flushing with air instead of nitrogen, the signal reaches the initial level. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Very important is that the signal decreases during the flushing phase with air to the initial level. As shown in Fig. 9, this full sensor response can be reproduced several times. Exposing the nanowires to the test gas for only 5 min leads to a very large signal change. Compared to the measurements with nitrogen as flushing gas (Fig. 4), one can clearly see the difference between both experiments. Using ambient air the signal can be brought back to the initial level. Obviously, the oxygen in the air must be adsorbed to the nanowire surface during flushing, leading to a thicker depletion layer and higher resistance of the nanowires. To investigate the sensitivity, we diluted the H2 S down to 50 ppb (Fig. 10). Like in the experiment shown above with nitrogen for flushing, a response down to 50 ppb H2 S is clearly observable. Again the change in flushing gas from nitrogen to air leads to an improved reset behaviour of the sensor during flushing. Taking the sensor response of air versus pure nitrogen into account (Fig. 8), it is not possible to measure concentrations lower than 50 ppb at this stage,
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consistence with the sensing mechanism proposed in literature, where H2 S interacts with the nanowires via the chemisorbed oxygen. With the sensor device shown here, multiple sensing cycles have been realized and an extremely low concentration of only 50 ppb H2 S has been detected. Acknowledgements The authors thank M.Sc. M. Dickel and Dipl.-Ing. B. Riedmüller for support in the sample preparation. Furthermore, we thank Prof. B. Mizaikoff for technical support. We thank the Carl Zeiss Foundation and the Baden-Württemberg-Stiftung for financial support.
References
Fig. 9. Sensor response to multiple cycles of gas switching. After each interval of 5 min, the sensor can be reset during the following 5 min of flushing with air.
Fig. 10. H2 S-concentration series with air for flushing. Even 50 ppb H2 S can be detected.
because it can not be distinguished anymore whether the chance in the signal is caused by H2 S or simply by switching from air to nitrogen. 4. Conclusions Long and thin ZnO nanowires were fabricated by a simple CVD process on a silicon wafer substrate using a mixture of ZnO powder and graphite powder as source material, and a thin gold film as catalyst. Subsequently, a large amount of nanowires was placed on a parallel contact structure using dielectrophoresis. In a temperature stabilized setup the sensing behaviour of the ZnO nanowires was investigated. As expected, the exposure of the sample to H2 S led to a decrease of the resistance of the nanowires. A crucial factor for the sensing behaviour is the flushing gas used between measurement intervals to define an initial level of the signal. We found that pure nitrogen leads only to a plateau of the current increase, but not to a complete reset of the sensor. Flushing the sensor system with oxygen causes a decrease of the current signal, and the initial level can be established again. This behaviour is in
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Biographies Florian Huber received his M.Sc. degree in Physics from the University of Ulm in 2014 and is currently a Ph.D. candidate at the Institute of Quantum Matters/Semiconductor Physics Group in Ulm/Germany. His research includes the CVD-growth of ZnO nanowires and layers and the characterization of these structures. Especially the sensing properties of ZnO nanowires are in focus of his work. Sören Riegert has completed his BSc degree in Physics in 2016 and is pursuing his M.Sc. degree at the University of Ulm. His work at the Institute of Quantum Matter at Ulm University was focused on the investigation of the sensing behaviour of ZnO nanostructures. Manfred Madel will finish his Ph.D. thesis in 2016 at Ulm University. During his work he investigated the optical sensing properties of ZnO nanowires grown on different substrates. Since 2015 he is working at United Monolithic Semiconductors GmbH in Ulm/Germany. Klaus Thonke received his Ph.D. in Physics in 1986 at the University of Stuttgart on the topic of optical investigations on defects in silicon. In 1990 he moved to the University of Ulm/Germany, where the new Institute of Semiconductor Physics was found (head: Prof. R. Sauer). In 1995, he finished his habilitation on the topic: optical high resolution spectroscopy on 3d and 4f transition metals in semiconductors. He is specialized on the characterization of defects in a broad variety of semiconductor materials ranging from Si, SiGe, diamond, GaAs, InP, GaSb, InGaAlP, to GaN, AlN, InGaN, ZnO and others. During the last decade, he also worked on the growth of ZnO films and nanostructures, and on their application for sensing devices.