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Amorphous europium hexaboride: a potential room temperature formaldehyde sensing material Da Wei, Jia Xie, and Dong-Ge Tong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13234 • Publication Date (Web): 04 Oct 2018 Downloaded from http://pubs.acs.org on October 5, 2018
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ACS Applied Materials & Interfaces
Amorphous europium hexaboride: a potential room temperature formaldehyde sensing material Da Wei, a,b Jia Xie,a,b Dong Ge Tong a,b* a
State Key Laboratory of Geohazard Prevention and Geoenvironment Protection, Chengdu University of
Technology, Chengdu 610059, China. E-mail:
[email protected]; Fax: +86 28 8407 3193. b
Collaborative Innovation Center of Panxi Strategic Mineral Resources Multi-purpose Utilization, College of Materials and Chemistry & Chemical Engineering, Chengdu University of Technology, Chengdu 610059, China. ABSTRACT: Amorphous EuB6 was successfully prepared having a high specific surface area (221.3 m2g−1), via the reaction between EuCl3 and B2H6 in the presence of liquid plasma in an ionic liquid environment. The material exhibits an immediate, lasting and highly-selective response toward formaldehyde at room temperature, with a detection limit of 50ppb. The good sensing performance of the amorphous EuB6 material is attributed to the strong interaction between formaldehyde and the increased number of accessible electron-rich surface Eu sites.
Formaldehyde, as a carcinogen, is a major air pollutant of concern, and which can be easily released into the environ1–7 ment. Hence, there is a real need to develop formaldehyde gas sensors with excellent detection levels, so as to limit exposure. A wealth of interdependent research has led to mul1 2–4 5,6 tiple systems such as CuO, SnO2, In2O3, and Ga-In bime7 tallic oxide nanofibers, being investigated to detect the concentration levels of formaldehyde at high temperature. However, for practical applications, the development of highly sensitive and selective sensing materials at room temperature is desired. Recently, europium hexaboride (EuB6)-a rare-earth hexaborides-has attracted significant attention as a result of the interesting properties displayed, specifically related to a sta8 ble specific resistance (730 μΩcm) and a low expansion coef−6 −1 9 ficient (5.34 × 10 K ) , making this compound a suitable 10,11 material as a high-resolution sensor. Sensing materials that exhibit such properties can reduce the intensity of any signals associated with the volume or resistance change of the sensor material. Additionally, EuB6 is a rare sample possessing an extremely low number of intrinsic charge carriers 20 −3 10,11 (10 cm ) that allows for improved sensitivity, even for minute changes in the concentration of conduction electrons—providing a basis to explore the formaldehydesensing performance of EuB6. 11-17 EuB6 (Table S1) , is typically prepared by carbothermal reduction of europium oxide (Eu2O3) and boron carbide (B4C) 11 at 1650℃, in vacuo, or by reduction of EuO with B in vacu12 um at 1400℃. However, the extremely high reaction temperatures required makes these methods highly energyintensive, and hence, there is a need to develop lowtemperature methods for preparation of EuB6. Recently, liquid plasma technique (LPT), in which plasma is introduced in an liquid environment containing chemical 18-20 agents, has attracted a great deal of attention. Some amorphous metal–boride have been successfully fabricated
possessing excellent performance because of their isotropic structure and high concentration of coordinative unsaturated 20 sites. Herein, we have extended on the recent literature results by synthesizing, for the first time, amorphous EuB6 with a high specific surface area at room temperature via the reduction of EuCl3 with B2H6 with the presence of plasma in 1-butyl-3methylimidazolium chloride ionic liquid environment ([BMIM]Cl). The as-prepared sample displays an immediate, lasting, and highly selective response to formaldehyde, at room temperature, with a detection limit of 50ppb. The x-ray diffraction pattern (XRD) at room temperature (Fig.1a), displays only one broad peak at ~30.1° (2θ) suggesting an amorphous or weakly crystalline material. However, after annealing under an Ar atmosphere at high temperature (973 K and 1173 K), for 2 h, a crystalline EuB6 material (JCPDS-65-5933) was obtained (Fig.1a). The transmission electron microscopy micrograph (TEM) in Fig.1b shows that the sample possesses an irregular morphology having an average particle diameter of ~6 nm. The areas marked in the selected area electron diffraction pattern (Fig.1b inset) further confirm the amorphous structure. The B/Eu ratio of the as-made sample, determined from elemental analysis, is six. Moreover, the specific surface area 2 −1 was measured to be 221.3 m g , significantly greater than 2 -1 that of a commercial EuB6 (14.6m g ) material having an average diameter of ~5-8 µm (Fig.S1). The Eu3d x-ray photoelectron spectroscopy (XPS) peaks at 1156.5 and 1128.8eV, 0 0 respectively, correspond to Eu 3d3/2 and Eu 3d5/2, indicating 21 the presence of metallic Eu. Additionally, XPS also con0 firmed the presence of B having a lower binding energy of 21 188.3 eV. The binding energy shifts of the Eu3d (adversely shifted by 0.5eV for Eu3d3/2 and 0.8eV for Eu3d5/2), and the B1s species (positively shifted by 1.2eV) confirms a transfer of 21 electrons from B to Eu, indicating that EuB6 is a Eu electron
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Figure 1. (a) XRD patterns; (b) TEM image (the inset is the SAED pattern；the scale bar is 50 nm); (c) Eu 3d XPS spectra; and (d) B 1s XPS spectra of the sample. -enriched material—similar to the majority of previously 20 reported amorphous metal–boron alloys. Herein, the presence of the plasma field is key to fabricating −1 EuB6. If the plasma is
