AlGaN/GaN heterojunction hydrogen sensor using ZnO-nanoparticles/Pd dual catalyst layer

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Highlights

  • We report a hydrogen sensor fabricated on an AlGaN/GaN heterojunction platform with a dual catalyst layer of ZnO nanoparticles on Pd.

  • The ZnO nanoparticles of diameter ∼10 nm were dispersed in a co-solvent of chloroform/ethanol and spin coated on the Pd surface.

  • The addition of the ZnO nanoparticles on the Pd catalyst enhanced hydrogen absorption and drastically improved the sensor response.

Abstract

ZnO-nanoparticles (NPs) were investigated for improving the response characteristics of a Pd-functionalized AlGaN/GaN heterojunction-based hydrogen gas sensor. The ZnO-NPs of diameter ∼10 nm were dispersed in a co-solvent of chloroform/ethanol (chloroform 75 %, v/v), which was found to be an optimal solution for uniform dispersion, and spin coated on the surface of the fabricated Pd/AlGaN/GaN hydrogen sensor. The ZnO-NP synthesis and dispersion methods were optimized carefully before being applied to sensor fabrication. It was observed that the addition of ZnO-NPs to the Pd catalyst enhanced the hydrogen absorption so that the response of the hydrogen sensor was improved. Compared with the single Pd catalyst layer, the ZnO-NPs/Pd double layer exhibited superior response characteristics—a response of ∼210 % and response time of ∼2 s were achieved at 200 °C with a bias voltage of 3 V.

Introduction

In recent years, hydrogen has been intensively researched as a clean source of renewable energy to replace the existing petroleum-based fuels [1]. Therefore, hydrogen sensor development is rapidly becoming important for issues concerning industrial safety and environmental protection because the hydrogen gas tends to ignite and explode easily [2,3]. It is therefore necessary to develop a hydrogen sensor capable of quick and accurate detection of hydrogen gas concentrations with high response characteristics.

Gallium nitride (GaN) has an extremely low intrinsic carrier concentration owing to its wide energy bandgap property [4]; this low intrinsic carrier concentration allows the semiconductor properties to be maintained at much higher temperatures than those of conventional semiconductor materials, such as Si and GaAs. This would also result in low leakage currents and stable operation at elevated temperatures, which are typically required for hydrogen catalytic reactions. When an AlGaN/GaN heterojunction is employed in a sensor platform, the polarization induced two-dimensional electron gas (2DEG) channel formed at the interface between the AlGaN and GaN provides the additional benefit of high sensitivity owing to the thin AlGaN barrier layer and high electron mobility [5].

Various catalyst materials have been reported for hydrogen sensors, including Pt, Pd, and oxide semiconductors (e.g., ZnO, SnO2, TiO2) [[6], [7], [8], [9], [10], [11]]. It was reported that the hydrogen solubility in Pd was about three orders of magnitude higher than that in Pt even though the hydrogen diffusion coefficients are similar for both materials [12]. In oxide semiconductors, the gas molecules are absorbed in the oxygen vacancies [13]. However, the reduction mechanisms by the oxygen vacancies make it difficult to differentiate hydrogen from other gases in the sensing environment. Recently, the incorporation of metal-oxide semiconductors (e.g., ZnO, SnO2, and TiO2) to noble metals (Pt, Pd, and Au) (i.e., as dual catalysts) has been demonstrated as an efficient and promising strategy to enhance the performances of gas sensors. In this regard, Jung et al. demonstrated enhanced H2 detection with Pd-decorated ZnO nanorods [14]. In addition, the effects of Ni, Pd, and Pt deposition on a TiO2 nanotube-based H2 sensor were thoroughly studied by David et al. [15]. The use of bimetallic nanoparticles for H2 sensing was also shown to be a good approach to improve sensing performance [16]. Furthermore, enhanced NO2 sensing results were reported for the use of Au nanoparticles grown on SnO2 nanowires [17]. However, it is not easy to obtain such dual catalysts with simple fabrication methods. The deposition of metal-oxide nanoparticles (NPs) on noble metals via spin coating may thus be a simple and an inexpensive approach to realize dual catalysts. In this approach, the synthesis of homogeneous metal-oxide NPs and their dispersion in an appropriate solvent are important to acquire uniform metal-oxide NPs coated on the noble metals.

In this study, a hydrogen gas sensor was fabricated on an AlGaN/GaN heterojunction platform using the dual catalyst layers of ZnO-nanoparticles (NPs) on a Pd layer. The ZnO-NP synthesis and dispersion processes were carefully optimized for sensor fabrication. A simple spin-coating method was used to form the ZnO-NP layer on the Pd catalyst layer. The ZnO-NPs increase the available surface area, thereby enhancing hydrogen molecule absorption. The ionized hydrogen molecules in ZnO-NPs accelerate the catalytic reaction with Pd.

Section snippets

Synthesis of ZnO-NPs and their dispersion

ZnO-NPs were synthesized by the hydrolysis of Zn(CH3COO)2 2H2O (Sigma-Aldrich) with KOH (Samchun, 95 %) in methanol (Sigma-Aldrich, 99.9 %). Briefly, the KOH solution (23 mM in 65 ml methanol) was added dropwise into the Zn(CH3COO)2 2H2O solution (13.4 mM in 125 ml methanol) under vigorous agitation. After the completion of reaction (2 h), ZnO-NPs were formed in the mixed solution. These ZnO-NPs were then separated by centrifugation (4000 rpm for 20 min, Lagogene, 124BR). To remove any

Characterization of ZnO-NPs powder and film optimization

The crystal structure the of as-synthesized ZnO-NPs was characterized by powder XRD (Fig. 2a). The peaks at 31.8°, 34.5°, 36.2°, 47.5°, 56.6°, 62.9°, and 68° corresponding to the (100), (002), (101), (102), (110), (103), and (112) planes confirm the crystalline ZnO (PDF#36-1451). Fig. 2b and c show the microscopic morphologies of the ZnO-NPs by HRTEM. The as-prepared ZnO displayed crystallites with lattice spacings of 0.26 nm and 0.25 nm (corresponding to the (002) and (101) planes,

Conclusions

ZnO-NPs spin coated onto a Pd catalyst layer enhanced the hydrogen catalytic reaction process in an AlGaN/GaN heterojunction hydrogen sensor. The coated ZnO-NP layer was 170 nm thick, with NP diameters of approximately 5–10 nm. It was determined that ZnO-NPs dispersed in a chloroform/ethanol (chloroform 75 %, v/v) co-solvent was an appropriate choice to achieve a uniform coating. The addition of the ZnO-NPs increased the sensing responses significantly. It is suggested that the hydrogen

CRediT authorship contribution statement

June-Heang Choi: Conceptualization, Data curation, Formal analysis, Writing - original draft. Taehyun Park: Data curation, Formal analysis. Jaehyun Hur: Investigation, Methodology, Writing - review & editing, Supervision, Funding acquisition. Ho-Young Cha: Conceptualization, Investigation, Methodology, Writing - review & editing, Supervision, Funding acquisition.

Declaration of Competing Interest

The authors reported no declarations of interest.

Acknowledgements

This work was supported by the Korea Electric Power Corporation (Grant No. R18XA02) and the Basic Science Research Program (Grant Nos. 2015R1A6A1A03031833, 2019R1A2C1008894) through the National Research Foundation of Korea (NRF).

June-Heang Choi received B.S. degree in Materials Science & Engineering / Department of Chemical Engineering from Hongik University in Seoul, Korea, in 2015. He received M.S. degree in Electronic and Electrical Engineering from Hongik University, Seoul, Korea, in 2018, respectively. He is currently pursuing the Ph.D. degree at Hongik University. His research interest is wide-bandgap semiconductor devices.

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    June-Heang Choi received B.S. degree in Materials Science & Engineering / Department of Chemical Engineering from Hongik University in Seoul, Korea, in 2015. He received M.S. degree in Electronic and Electrical Engineering from Hongik University, Seoul, Korea, in 2018, respectively. He is currently pursuing the Ph.D. degree at Hongik University. His research interest is wide-bandgap semiconductor devices.

    Taehyun Park received B.S. degree in Chemical & Biological Engineering from Gachon University, Seongnam, Republic of Korea in 2018. He is currently pursuing the M.S. degree in Department of Nano science and Technology at Gachon University. His research interest is synthesis of nanomaterials for sensor application.

    Jaehyun Hur received his B.S. in Applied Chemical Engineering from Seoul National University, Seoul, Republic of Korea in 2000. He received the Ph.D. in Department of Chemical Engineering in 2008 from Purdue University, West Lafayette, IN, USA in 2008. He was a research scientist in Samsung Advanced Institute of Technology (SAIT) until 2014 where he developed organic electronic devices including organic field-effect-transistors and organic photovoltaics. He is currently an associate professor in the department of Chemical and Biological Engineering at Gachon University. His researches are focused on the synthesis of various nanomaterials toward the development of solution-processed photodetectors. He has published over 100 papers in his research area.

    Ho-Young Cha received his B.S. and M.S. in Electrical Engineering from Seoul National University, Seoul, Republic of Korea, in 1996 and 1999, respectively, and his Ph.D. in Electrical and Computer Engineering from Cornell University, Ithaca, NY, U.S.A. in 2004. He was a Postdoctoral Research Associate with Cornell University until 2005 where he focused on the design and fabrication of wide-bandgap semiconductor devices. He was with the General Electric Global Research Center, Niskayuna, NY, from 2005 to 2007, developing wide-bandgap semiconductor sensors and high-power devices. Since 2007, he has been a professor of in the School of Electronic and Electrical Engineering. His research interests include wide-bandgap semiconductor devices. He has authored over 130 publications in his research area.

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    These authors contributed equally.

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