High response and selectivity toward hydrogen gas detection by In2O3 doped Pd@ZnO core-shell nanoparticles
Graphical abstract
Introduction
Hydrogen (H2) is a clean and renewable energy source with considerable potential to replace fossil fuels, which often produce global warming and air contaminants [[1], [2], [3], [4], [5]]. Hydrogen is extremely abundant and has been widely adopted by chemical, petroleum and aerospace industries; fuel cells; and laboratories. Utilizing H2 as a fuel offers high energy efficiency without contributing to greenhouse effects [1]. However, hydrogen is highly flammability with high heat of combustion and flame propagation velocity, low boiling point and low ignition energy [2,[6], [7], [8]]. It is also colorless, odorless and tasteless, increasing the risk for practical applications. So far, there are two type of commercial hydrogen gas sensors. One is an electrochemical type, and another is a semiconductor type. The electrochemical type sensors shows the response in relatively higher concentration range of hydrogen gas, shorter life-time and higher price as compared to semiconductor type sensors. However, the semiconductor type sensors have also demerit, which is no selectivity for hydrogen gas. Thus, there is urgent demand for rapid and selective sensors to detect H2 leakage to ensure safe use.
Metal loaded semiconductor hybrids, where the semiconductor severs as an electron donor and the metal acts as an electron reservoir, offer significant advantages to improve gas sensing properties [[9], [10], [11], [12], [13], [14]]. However, these structures have several limitations, including metal nanoparticle agglomeration and detachment during practical operation [[15], [16], [17]], and sensitive noble metal surfaces can be easily contaminated by sensing byproducts [18]. These drawbacks can considerably reduce these hybrid sensing capabilities. Alternatively, the conversional design may be more suitable for gas sensing, where noble metal is coated with semiconductor to form core-shell structures. These heterostructures propose several important properties for sensing applications, such as high surface area [19,20], controllable chemical composition and synergistic properties [16,[21], [22], [23]]. The noble metal cores are protected by semiconductor shells to improve the stability and agglomeration [[24], [25], [26]]. The core-shell structures also enhance charge transfer between core and shell, reducing electron-hole pair recombination in system [9,27].
The Pd is the best noble metal for H2 sensing purpose due to its particular ability to adsorb H2 molecules [5,28,29]. The H2 is first adsorbed on the Pd surface and is dissociated into the hydrogen atoms, then produces PdHx species [1,30], which can lead to rapid volume growth, reaching 900 times the original volume of Pd precursor to easily absorb H2 [6,31]. Since Pd may be easily delaminated during sensing operation [30,[32], [33], [34]], the ZnO is considered a promising n-type semiconductor shell to protect the sensitive surface of Pd catalyst. The ZnO offers chemical stability, high sensitivity, short response time and low cost materials for practical sensing applications. Although ZnO based sensors provide these interesting advantages, it still remains the drawbacks due to their high working temperature, low selectivity and low sensitivity to H2 [11,35,36]. Doping with a third component, e.g., In2O3 n-type semiconductor, into Pd@ZnO core-shell structures, can greatly improve H2 detection. Incorporating In2O3 in the ZnO shell can promote the electron transfer from In2O3 to ZnO because the difference in work function [37,38], which then leads to strong electrical resistance modulation during sensing operation.
In this study, an efficient hydrogen sensing material was prepared by doping In2O3 (5 wt%) into Pd@ZnO core-shell nanoparticles (CSNPs). The obtained Pd@ZnO–In2O3 CSNPs were characterized and applied for hydrogen gas detection in comparison to Pd@ZnO and pure ZnO materials.
Section snippets
Chemicals
All chemicals were commercially sourced at analytical grade and used without further purification. Hexadecyltrimethylammonium bromide (CTAB, C19H42BrN, 99%), hexadecyltrimethylammonium chloride (CTAC, C19H42ClN, 98%), palladium chloride (PdCl2, 99%), ascorbic acid (C6H8O6, 99%), indium chloride (InCl3, 98%), zinc nitrate (Zn(NO3)2·6H2O, 98%), hexamethylenetetramine (HMTA, C6H12N4, 99%), sodium carbonate (Na2CO3, 99%) were supplied by Sigma Aldrich; and sodium citrate (C6H5Na3O7·2H2O, 98%) was
Characterizations
Fig. 2a presents the XRD pattern of pure ZnO wurtzite structure, where the diffraction peaks observed at 31.6, 34.3, 36.1 and 47.4° correspond to the (100), (002), (101) and (102) crystalline planes (JCPDS No. 36–1451). Fig. 2b and c describes the XRD patterns of Pd@ZnO and Pd@ZnO–In2O3 CSNPs after calcination at 500 °C for 2 h in argon. The presence of Pd cores in both materials are detected at 39.9° for the (111) crystalline planes, consistent with the standard card (JCPDS No. 05–0681). In
Conclusions
We proposed a hydrothermal method to prepare effective hydrogen sensor by doping 5 wt% In2O3 into Pd@ZnO CSNPs. The Pd@ZnO–In2O3 sensor exhibited higher sensing response (42) at a lower optimal testing temperature (300 °C) to 100 ppm hydrogen compared to Pd@ZnO (17 and 350 °C) and pure ZnO (9 and 350 °C). It also achieved faster response and recovery time (0.4 and 4.0 min) to hydrogen than those obtained from Pd@ZnO (1.4 and 14.0 min) and pure ZnO (6.0 and 18.0 min) sensors at optimal working
CRediT authorship contribution statement
Thuy T.D. Nguyen: Conceptualization, Methodology, Software, Formal analysis, Investigation, Resources, Data curation, Writing - original draft. Dung Van Dao: Data curation, Writing - original draft. In-Hwan Lee: Writing - review & editing, Visualization, Supervision, Project administration, Funding acquisition. Yeon-Tae Yu: Writing - review & editing, Visualization, Supervision, Project administration, Funding acquisition. Sang-Yeob Oh: Investigation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by the BK21 Plus program of the Ministry of Education and Human-Resource Development of South Korea and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (BRL No. 2015042417, 2016R1A2B4014090, 2017R1A2B3006141, 2020R1A2B5B03001603).
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