High response and selectivity toward hydrogen gas detection by In₂O₃ doped Pd@ZnO core-shell nanoparticles

Highlights

• For the first time, we prepared an effective hydrogen sensor by doping In₂O₃ into Pd@ZnO (Pd@ZnO–In₂O₃) core-shell nanoparticles.

• Pd@ZnO–In₂O₃ material presented a higher BET surface area compared to Pd@ZnO and pure ZnO.

• Obtained sensor not only exhibited high response and fast response/recovery times, but also increased the selectivity towards hydrogen gas.

• The improvements could be attributed to its high BET surface area and synergistic effects between Pd, ZnO and In₂O₃ parts.

Abstract

An efficient hydrogen gas sensor comprising 5 wt% In₂O₃ doped in Pd@ZnO core-shell nanoparticles (Pd@ZnO–In₂O₃ CSNPs) was synthesized via a facile hydrothermal approach. The obtained material has a higher Brunauer-Emmett-Teller surface area (80 m² g⁻¹) compared to Pd@ZnO (56 m² g⁻¹) and pure ZnO (40 m² g⁻¹). The Pd@ZnO–In₂O₃ sensor achieved the maximal response (42) to 100 ppm hydrogen at 300 °C. Whereas, Pd@ZnO and pure ZnO sensors exhibited lower responses (17 and 9) to 100 ppm hydrogen at a higher optimal temperature (350 °C). It also demonstrated faster response and recovery time (0.4 and 4.0 min) than those obtained from Pd@ZnO (1.4 and 14 min) and pure ZnO (6.0 and 18.0 min) sensors. The hydrogen sensing enhancement of Pd@ZnO–In₂O₃ materials could be largely attributed to the synergistic electronic and chemical activities of Pd, ZnO and In₂O₃ parts, and its large surface area. Especially, due to the ability to adsorb hydrogen of the core, Pd based sensors exhibited high selectivity to hydrogen with respect to Pd-free sensors.

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 PdH_x 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.