On-chip hydrothermal growth of ZnO nanorods at low temperature for highly selective NO2 gas sensor

Author’s Accepted Manuscript On-chip Hydrothermal growth of ZnO nanorods at low temperature for highly selective NO2 gas sensor Mingzhi Jiao, Nguyen Viet Chien, Nguyen Van Duy, Nguyen Duc Hoa, Nguyen Van Hieu, Klas Hjort, Hugo Nguyen www.elsevier.com

PII: DOI: Reference:

S0167-577X(16)30123-9 http://dx.doi.org/10.1016/j.matlet.2016.01.123 MLBLUE20251

To appear in: Materials Letters Received date: 8 December 2015 Accepted date: 26 January 2016 Cite this article as: Mingzhi Jiao, Nguyen Viet Chien, Nguyen Van Duy, Nguyen Duc Hoa, Nguyen Van Hieu, Klas Hjort and Hugo Nguyen, On-chip Hydrothermal growth of ZnO nanorods at low temperature for highly selective NO2 gas sensor, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.01.123 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

On-chip hydrothermal growth of ZnO nanorods at low temperature for highly selective NO2 gas sensor Mingzhi Jiao1, Nguyen Viet Chien2, Nguyen Van Duy2, Nguyen Duc Hoa2, Nguyen Van Hieu2, Klas Hjort1, Hugo Nguyen1,* 1

Uppsala University, Department of Engineering Sciences, Lägerhyddsvägen 1, 751 21 Uppsala, Sweden 2 International training Institute for Materials Science, Hanoi University of Science and Technology No 1 Dai Co Viet, Hanoi, Vietnam E-mail: [email protected]

Abstract ZnO nanorods were selectively grown on-chip with a two-step low-temperature hydrothermal method and their gas sensing properties were investigated. Small zinc islands were deposited by sputtering on a glass substrate and used as nucleation sites for the ZnO nanorod growth. An equimolar aqueous solution of 0.005 M Zn(NO3)2•6H2O and (CH2)6N4 at 85 °C was used in two steps. The first step was used for nucleation and growth of short ZnO nanorods for 4 hours, whereas the second step was used for elongation of the nanorods for 36 hours. Long porous nanorods from neighboring islands connected to each other and formed nanorod junctions. A gas sensor with such nanorods was evaluated towards NO2, ethanol, hydrogen, and ammonia to characterize its sensing properties. It showed that the gas sensor has the highest sensitivity to NO2, and a very high selectivity to this gas when measured at 450 °C.

Keywords: crystal growth; deposition; nanocrystalline materials; sensors; thin films

1. Introduction ZnO thin film was used for detection of gaseous components at about 400 °C for the first time in 1962 [1]. Since then, ZnO has been widely investigated for its good and stable gas sensing properties. Different morphologies provide different sensing performances [2-5]. ZnO nanopetals have a sensitivity (defined as Rgas/Rair, where Rgas and Rair are electrical resistance of the sensor in target gas and in dry air, respectively) of 119 to NO2 gas at concentration of 20 ppm at room temperature [2]. Monodisperse ZnO hollow sixsided pyramids have a sensitivity of about 187 to ethanol and 15 to dimetylformamid (DMF), both at concentration of 200 ppm and at 240 °C [3]. Among the other ZnO nanostructures, such as rod-like,

flower-like and caddice clew-like, the flower-like showed the highest response, of 144.38, and excellent selectivity to ethanol at concentration of 500 ppm at 360 °C [4]. Most of the nanostructures were grown and then harvested and mixed with water or ethanol to make a slurry for coating [5]. This so called post coating technique is simple and often used to bring the sensing material onto the surface between the electrodes of a gas sensor. However, it may cause considerable variation of density and position of the sensing material on the sensors. Therefore, the performance of such sensors may differ from each other and from batch to batch. Furthermore, the function of these sensors relies on adhesion of the nanomaterial to the electrodes. Gas sensors with on-chip grown ZnO structures with a chemical bond to the electrodes, and controlled density are more reliable and robust. Among the nanostructures of ZnO, nanorods (NRs) and nanowires (NWs) have the advantage that they can be controllably grown to form junctions between electrodes of a sensor, making them a part of the electrical path. As an example, it can be mentioned that an on-chip grown ZnO nanowire gas sensor showed a sensitivity of 62 to NO2 at concentration of 5 ppm at 250 °C [7, 8]. However, these ZnO NWs were synthesized with chemical vapor deposition (CVD) at a temperature of 850 °C, which required a vacuum system and substrate and electrode materials that can withstand an oxidizing environment at this high temperature. An on-chip hydrothermally grown ZnO NR gas sensor reported in [9] was realized at 110 °C so that a Teflon-lined autoclave was required. In 2013, other researchers reported on wafer-scale, planar ZnO NR sensors that were realized by on-chip hydrothermal growth at 90 °C and at atmospheric pressure [10]. Unfortunately, the sensitivity was not very high because of a too high density of NRs, and the continuous seed layer at the bottom of NR arrays resulted in short circuits in the electrical path. Recently, ZnO nanowires were grown on small catalytic Au islands for planar-type micro gas sensors, but these were also made by a high temperature CVD method [11]. Here, we use similar islands but of oxidized zinc, and a hydrothermal method to grow ZnO NRs at 85 °C, and study the sensitivity of these to NO2 gas.

2. Experimental detail

The formation process of ZnO NR junctions by two-step hydrothermal growth is shown in Figure 1. Zn(NO3)2•6H2O and (CH2)6N4 (HMTA) powder (both with purity ≥99.0%, Sigma-Aldrich, Sweden), and a Zn target (purity 99.99%, Mateck, Germany) were used in the experiments. On a glass substrate, electrodes of Pt on Cr, 80 and 10 nm thick, respectively, were created by sputter deposition and lift-off technique, Figure 1(A). Thereafter, small circular islands of 15 nm zinc on a 13 nm silicon adhesion layer were created, also by sputtering and lift-off, and used as nucleation sites for the hydrothermal growth of ZnO NRs. The islands had a diameter of 5 m and were distributed at a distance of 2.5 m to each other, Figure 1(B). NR junctions, i.e., connections of NRs from neighboring islands, were created by the twostep hydrothermal growth method. A chip from the glass substrate with zinc islands was put face down in a bottle with a 100 ml precursor solution of 0.005 M Zn(NO3)2 and 0.005 M HMTA at room temperature. The bottle was then put in an oven at 85 °C for 4 hours; thereafter the chip was taken out from the bottle for cooling in air for around 10 minutes. By this, arrays of ZnO NRs were obtained, but the NRs were too short to make contact to each together, Figure 1(C). Next, the chip was put in another bottle with a fresh precursor solution of the same amount and the same concentration of Zn(NO3)2 and HMTA at room temperature and kept in the oven at 85 °C for 36 hours, Figure 1(D). Thereafter, the chip was taken out and immersed in DI water three times. Shortly afterwards, the chip was put into the oven to dry for 6 hours at 85 °C. The morphology of ZnO NRs was characterized by scanning electron microscopy (LEO 1550, Carl Zeiss SMT, Germany). The structure of the ZnO NRs was verified by X-ray diffraction (Parallel Beam Geometry with X-ray mirrors, SIEMENS D5000, Germany). The gas sensing properties were characterized by a homemade measurement system [11]. The sensitivity of the gas sensor to NO2 oxidizing gas was defined as Rgas/Rair, whereas to other reductive gas (ethanol, hydrogen, ammonia) was defined as Rair/Rgas.

Figure 1. Formation of ZnO NRs junctions by two-step hydrothermal growth: (A) deposition of Pt electrodes; (B) deposition of Zn seed islands; (C) growth of dense ZnO nanorods (first step); (D) growth of long porous ZnO nanorods (second step).

3. Results and discussion An SEM image of the sensing part of the sensor is shown in Figure 2(A). The design of the isolated islands as nucleation sites was made to create as many NR junctions as possible within the conducting path from one electrode to the other of the gas sensor, Figure 2(A) and (B). As can be seen in Figure 2(C), the surface of ZnO NRs is very rough and is rather a porous material formed from nanocrystals. XRD data of the NRs is shown in Figure 2(D). The two peaks not indexed belong to the Pt electrodes. The other peaks are all indexed to hexagonal ZnO (reference code: 01-070-2551). The lattice parameter is a =b = 3.2490 Å, c = 5.2070 Å.

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Figure 2. Material characterization of ZnO NRs synthesized with hydrothermal method at 85 °C: (A) SEM image of ZnO NRs within Pt electrodes, (B) higher magnification of ZnO NRs grown from different islands making NR junctions, (C) image of a single ZnO NR, (D) X-ray diffraction of the ZnO NRs.

Figures 3(A-C) show the dynamic changes of resistance of the ZnO NRs during 4 cycles of gas in and gas out of NO2 with concentration of 5, 10, 25, and 50 ppm, respectively, at temperatures of 350, 400, and 450 °C. The data are summarized and shown as sensitivity of the sensor in Figure 3(D). The sensitivity increases with temperature and reaches the highest value at 450 °C. The sensitivity to 50 ppm NO2 is as high as 24.1 at 450 °C, whereas it is only 6.7 to 50 ppm NO2 at 350 °C. This may be due to the fact that high temperature facilitates diffusion of NO2 into a porous structure so that number of electrons near the surface of ZnO NRs decreases much more at 450 °C than that at 350 °C.

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Figure 3. Sensitivity of ZnO NR junctions to NO2 with concentration of 5, 10, 25, and 50 ppm at different temperatures: (A) 350 °C, (B) 400 °C, (C) 450 °C. (D) Comparison of sensitivity to NO 2 at different temperature.

The sensitivity of the gas sensor to ethanol with concentration of 100, 250, 500, and 1000 ppm is shown in Figure 4(A). To 1000 ppm ethanol, the sensitivity is merely 1.38, 1.49, and 2 at 350, 400, and 450 °C, respectively; whereas it is much lower at lower concentrations. Figure 4(B) shows the sensitivity of the

gas sensor to hydrogen with the same concentrations as in the measurement with ethanol. Here, the sensitivity to 1000 ppm hydrogen is generally lower, of 1.49, 1.15, and 1.24 at 350, 400, and 450 °C, respectively. Figure 4(C) shows the sensitivity of the gas sensor to ammonia. To the concentration of 1000 ppm, the sensitivity is 1.3, 1.21, and 1.17 at 350, 400, and 450 °C, respectively, i.e., slightly lower than that to hydrogen. Because of the low sensitivity, a higher concentration of 2000 ppm was used, but at 450 °C the sensor produced an unexpected value. Technically, this measurement can be regarded as out of range. The drop of sensitivity, caused by the resistance increase of the ZnO NRs in this case, is not fully understood, because due to the gas sensing mechanism based on depletion the sensitivity is expected to reach a maximum value and level out as the ammonia concentration increases. Figure 4(D) shows comparison of the sensitivities of the gas sensor to all four gases measured at 450 °C, where concentration of NO2 is only 1% of that of the other gases. Assuming a linearly scaling sensitivity, the sensitivity to 10 ppm NO2 would be about 300, 490, and 520 times higher than that of ethanol, hydrogen, and ammonia. This shows that the selectivity to NO2 is very high for the sensor. 2.5

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4. Conclusions A gas sensor of planar type with on-chip selectively grown ZnO NRs has been realized by a two-step hydrothermal method at low temperature. Two steps with fresh precursor solution are needed to promote the growth of long NRs and thus to create NR junctions between small growth islands. The sensor design with the small island between electrodes helps to increase the number of NR junctions and to force the conducting path to go through the junctions. A low growth temperature is believed to be the main cause of the NR porosity, due to low energy of the precursors in the solution so that solid NR crystals cannot be formed. Tests with NO2, ethanol, hydrogen, and ammonia showed that the gas sensor had a very high sensitivity, of 6.1, to 10 ppm NO2 at 450 °C, and a very high selectivity to NO2 compared with the measured gases.

Acknowledgements The authors thank Chinese Scholarship Council (CSC) and Uppsala University for financial support of Mingzhi Jiao’s PhD studies, and the Vietnam National Foundation for Science and Technology Development (NAFOSTED) for Grant No. 103.02-2014.18.

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[5] X.B. Li, S.Y.Ma, F.M.Li, et al. Porous spheres-like ZnO nanostructure as sensitive gas sensors for acetone detection, Materials Letters 100 (2013) 119-123 [6] Abu Z. Sadek, Supab Choopun, Wojtek Wlodarski, Samuel J. Ippolito, and Kourosh Kalantar-zadeh. Characterization of ZnO Nanobelt-Based Gas Sensor for H2, NO2, and Hydrocarbon Sensing, IEEE SENSORS JOURNAL 7 (2007) 919-924 [7] M.-W. Ahn, K.-S. Park, J.-H. Heo, et al. On-chip fabrication of ZnO-nanowire gas sensor with high gas sensitivity, Sensors and Actuators B 138 (2009) 168-173 [8] M.-W. Ahn, K.-S. Park, et al. Gas sensing properties of defect-controlled ZnO-nanowire gas sensor, Applied Physics Letters 93 (2008) 263103 [9] Fang-Tso Liu, Shiang-Fu Gao, Shao-Kai Pei, Shih-Cheng Tseng, Chin-Hsin J. Liu, ZnO nanorod gas sensor for NO2 detection, Journal of the Taiwan Institute of Chemical Engineers 40 (2009) 528-532 [10] Nguyen Duc Khoang, Hoang Si Hong, Nguyen Van Duy, Nguyen Duc Hoa, Nguyen Van Hieu, et al. On-chip growth of wafer-scale planar-type ZnO nanorod sensors for effective detection of CO gas, Sensors and Actuators B 181 (2013) 529-536 [11] Hugo Nguyen, Chu Thi Quy, Nguyen Duc Hoa, Nguyen Van Duy, Nguyen Van Hieu, et al. Controllable growth of ZnO nanowires grown on discrete islands of Au catalyst for realization of planartype micro gas sensors, Sensors and Actuators B 193 (2014) 888-894

Highlights -

ZnO nanorods were synthesized on-chip by hydrothermal method at 85 °C.

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Gas sensor based on these ZnO nanorods showed a very good sensitivity to NO2.

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The selectivity to NO2 was very high compared with ethanol, hydrogen, and ammonia.