Nanosensors Based on a Single ZnO:Eu Nanowire for Hydrogen Gas Sensing

Abstract

Fast detection of hydrogen gas leakage or its release in different environments, especially in large electric vehicle batteries, is a major challenge for sensing applications. In this study, the morphological, structural, chemical, optical, and electronic characterizations of ZnO:Eu nanowire arrays are reported and discussed in detail. In particular, the influence of different Eu concentrations during electrochemical deposition was investigated together with the sensing properties and mechanism. Surprisingly, by using only 10 μM Eu ions during deposition, the value of the gas response increased by a factor of nearly 130 compared to an undoped ZnO nanowire and we found an H₂ gas response of ∼7860 for a single ZnO:Eu nanowire device. Further, the synthesized nanowire sensors were tested with ultraviolet (UV) light and a range of test gases, showing a UV responsiveness of ∼12.8 and a good selectivity to 100 ppm H₂ gas. A dual-mode nanosensor is shown to detect UV/H₂ gas simultaneously for selective detection of H₂ during UV irradiation and its effect on the sensing mechanism. The nanowire sensing approach here demonstrates the feasibility of using such small devices to detect hydrogen leaks in harsh, small-scale environments, for example, stacked battery packs in mobile applications. In addition, the results obtained are supported through density functional theory-based simulations, which highlight the importance of rare earth nanoparticles on the oxide surface for improved sensitivity and selectivity of gas sensors, even at room temperature, thereby allowing, for instance, lower power consumption and denser deployment.

Keywords: Eu2O3; ZnO; electrochemical deposition; hydrogen; sensor.

Figure 1

SEM images of ZnO:Eu nanowire arrays grown in electrolyte solution with EuCl₃ concentrations of (a–c) −3 μM, (d–f) −5 μM, and (g–i) −10 μM. The magnification increases from left to right.

Figure 2

XRD pattern of ZnO:Eu nanowires grown by electrochemical deposition with different Eu concentrations: 6 and 10 μM EuCl₃ content in the electrolyte (samples: ZnO:Eu6 and ZnO:Eu10). Peaks from the used FTO substrate (SnO₂:F) are indicated by red circles.

Figure 3

(a) TEM image of needle-shaped ZnO:Eu nanowires decorated with amorphous Eu-oxide species. (b) High-magnification micrograph showing the amorphous network covering the surface of the single crystalline ZnO nanowire. (c) Zero energy-loss filtered and EFTEM image recorded using the Eu N_4,5-edge. Electron energy-loss spectra recorded on Eu-networks intendity the fingerprint of Eu³⁺ according to the (d) N_4,5 and (e) M_4,5-edges.

Figure 4

XPS spectra of ZnO:Eu nanowire samples: (a) overview spectrum indicating the presence of Eu, Zn, O, and C; (b) high resolution spectra of the C 1s line, Zn 2p_3/2 line, and Eu 3d_5/2 line. (1) 3, (2) 6, and (3) 10 μM EuCl₃ content in the electrolyte.

Figure 5

(a) Raman spectra of ZnO:Eu nanowires deposited with different Eu contents: (1) 8 and (2) 10 μM. (b) Transmission spectra of ZnO:Eu nanowires with different Eu contents: (1) 6, (2) 8, and (3) 10 μM. (c) Plot of (αhν)² vs (hν) for ZnO:Eu nanowires with different Eu contents: (1) 6, (2) 8, and (3) 10 μM.

Figure 6

(a) Dynamic response of the ZnO:Eu5 nanosensor to 100 ppm of hydrogen gas at room temperature. (b) Gas response of the ZnO:Eu5 nanosensor to 100 ppm of various gases at room temperature. (c) UV response for the ZnO:Eu5 nanosensor at 100 °C operating temperature.

Figure 7

(a) SEM of the sensor based on the sample for ZnO:Eu10 (10 μM EuCl₃ content in the electrolyte); (b) gas response for 100 ppm hydrogen of a ZnO:Eu10 nanosensor at 150 °C operating temperature; (c) gas response for 100 ppm of different gases at 20, 100, 125, 150, and 175 °C operating temperatures for the ZnO:Eu10 nanosensor.

Figure 8

Gas response for 100 ppm hydrogen at 150 °C operating temperature for ZnO:Eu nanowires with different Eu contents: (1) 0, (2) 5, (3) 6, (4) 7, (5) 10, and (6) 20 μM EuCl₃ concentration in the electrolyte.

Figure 9

DFT relaxed structure of one H₂ molecule at (a) the Eu:ZnO(101̅0) surface and (b) the Eu₂:ZnO(101̅0) surface. O, Zn, and Eu atoms are denoted by red-, gray-, and purple-colored balls, respectively, while small pink-colored balls denote H atoms.

Figure 10

(a) Charge density difference plot iso-surfaces of the H₂ molecule interaction at the Eu₂:ZnO (101̅0) surface, where the H₂ molecule gains 0.22 e^– in Bader charge. Blue and yellow colored iso-surfaces indicate negative and positive changes in charges, respectively. (b) H₂O and H₂ molecules over the Eu₂:ZnO (101̅0) surface.