Enhancement of the Humidity Sensing Performance in Mg-Doped Hexagonal ZnO Microspheres at Room Temperature

Abstract

In this paper, Mg-doped black ZnO microspheres with the characteristics of large surface area and surface oxygen vacancies were synthesized using the sol-gel method. The humidity sensing behavior of the Mg-doped ZnO for relative humidity (RH) from 11% to 95% was measured at room temperature. The superior humidity sensing performance recorded for Mg-doped ZnO microspheres (1.5 mol%) exhibits a dramatic change of impedance of about four orders of magnitude, excellent sensing linearity, small hysteresis (4.1%), a fast sensing response time of as low as 24 s, and a recovery time of 12 s. Our studies demonstrate that Mg dopant can significantly enhance the humidity sensing performance of black ZnO. This benefits from the Mg-doped ZnO (1.5 mol%) microspheres having a high level of surface adsorption and the abundant oxygen vacancies on the surface. Such a new material could be very useful to develop simple and high-performance humidity sensors for future applications in varying commercial fields and industries.

Mg-Doped ZnO Nanoparticles with Tunable Band Gaps for Surface-Enhanced Raman Scattering (SERS)-Based Sensing

“…Figure 2 b depicts the scan of Zn 2P with peaks around 1020.3 eV and 1043.4 eV, which can be attributed to the binding energy lines of Zn 2p 3/2 and Zn 2p 1/2 , respectively [ 26 ]. The 23.1 eV peak separation indicates that the Zn atom is in the +2 oxidation state [ 27 ]. These energy levels are ~0.3 eV and ~0.2 eV lower than the peak positions of the Mg-doped ZnO nanoparticles (1020.6 eV and 1043.6 eV).…”

“…The O 1s peak of the high-resolution spectra of pure and Mg-doped ZnO nanoparticles was fitted into two Gaussian peaks ( Figure 2 c,d). The lower-energy peak is centered around 529.03 eV and corresponds to 0 2− on the wurtzite structure of Zn 2+ [ 16 , 26 , 27 ]. In comparison, the higher-energy peak is around 530.39 eV and can be attributed to oxygen vacancies [ 16 ].…”

“…For instance, Mg has been used to dope ZnO due to its high solubility in the ZnO lattice [ 16 ] and comparable radius to Zn, which could prevent lattice distortion. The bandgap shift resulting from doping ZnO with Mg has been studied in different experiments exploring various applications; while an increased band gap was reported in some experiments [ 21 , 22 , 23 , 24 ], others reported a decrease in the band gap [ 25 , 26 , 27 ]. From this, we can deduce that doping ZnO with different concentrations of Mg can be used as a mechanism to tune the band gap and tailor it for better performance, as demonstrated by research on humidity sensors [ 27 ], antibacterial applications [ 25 ], and photocatalytic and photodegradation activities [ 24 , 26 ].…”

“…The bandgap shift resulting from doping ZnO with Mg has been studied in different experiments exploring various applications; while an increased band gap was reported in some experiments [ 21 , 22 , 23 , 24 ], others reported a decrease in the band gap [ 25 , 26 , 27 ]. From this, we can deduce that doping ZnO with different concentrations of Mg can be used as a mechanism to tune the band gap and tailor it for better performance, as demonstrated by research on humidity sensors [ 27 ], antibacterial applications [ 25 ], and photocatalytic and photodegradation activities [ 24 , 26 ]. For cellular applications, particularly live-cell imaging, acquiring the SERS spectrum of analytes directly from a colloidal substrate is important and could pave the way for SERS in clinical applications.…”

Highly Sensitive Magnesium-Doped ZnO Nanorod pH Sensors Based on Electrolyte–Insulator–Semiconductor (EIS) Sensors

“…No Mg metal or Mg oxide peaks were found even when the concentration of Mg was increased to 5.0%, which indicates that the dopant was fully incorporated into the host lattice and the wurtzite structure of ZnO remains unaltered after Mg-doping [ 41 , 42 ]. The Mg substitution caused a slight shift in the Bragg angle’s position (34.47°–34.90°) as presented in the inset image in Figure 1 , which might because of the Mg 2+ dopant creating distortions in the ZnO lattice, producing crystal defects around the dopants [ 34 , 43 ]. The peaks’ intensity decreased as the Mg concentrations increased from 1 to 5%, except for Mg doped at 3%, which increased the most, confirming that this sample had the best crystallization quality and the Mg species occupied interstitial sites and substitution lattice sites [ 42 , 44 ].…”