Sensors and Actuators B 195 (2014) 657–666 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical jo ur nal home page: www.elsevier.com/locate/snb Sensing performance and mechanism of Fe-doped ZnO microflowers Shouli Bai a , Teng Guo a , Yangbo Zhao a , Jianhua Sun a , Dianqing Li a,∗ , Aifan Chen a,∗ , Chung Chiun Liu b a State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China b Department of Chemical Engineering, Case Western Reserve University, Cleveland, OH 44106, USA a r t i c l e i n f o Article history: Received 11 September 2013 Received in revised form 13 January 2014 Accepted 21 January 2014 Available online 29 January 2014 Keywords: Fe-doped ZnO microflowers Gas sensing NO2 Langmuir–Hinshelwood reaction mechanism First-principle calculations a b s t r a c t Fe-doped ZnO microflowers have been hydrothermally synthesized without any surfactant at 120 ◦ C for 10 h. The characteristics of products were examined by XRD, SEM, TG–DTA and XPS. The sensing tests reveal that the response is significantly enhanced by Fe doping, and the 3.0 wt%-Fe doped sample exhibits the highest response of 604 to 10 ppm NO 2 at lower operating temperature of 125 ◦ C. The intrin- sic sensing characteristic is attributed to be native defects in ZnO, which has been confirmed by room temperature photoluminescence PL and XPS analysis. The response time is reduced disproportionately with the increase in NO 2 concentration by modeling transient responses of the sensor using L–H reaction mechanism. The band structures and densities of states for undoped ZnO and two Fe-doped supercells of Zn 0.9815 Fe 0.0185 O and Zn 0.9583 Fe 0.0417 O have been calculated using the first-principles based on the density functional theory (DFT). The calculated results show that the band gap is significantly narrowed and the conductance is increased by Fe doping, which coincide with that of experimental results of gas sensing. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Zinc oxide (ZnO), as an important semiconductor oxide, is an interesting sensing material due to its high mobility of electrons and good chemical and thermal stability under the operating con- ditions of sensor. Doping of metal elements or surface modifying of oxides already has been proved to be one of the most facial and effective methods in improving the sensing performances of semiconductor metal oxides. Because doping can effectively mod- ulates the parameters of crystal cell and band structures of ZnO nanocrystals. For example, Li et al. reported that doping of Sn remarkably shortened the response time and recovery time of the sensor [1]. Liu et al. reported that Mg-doped ZnO film showed much higher H 2 sensing performance than undoped ZnO film did [2]. Hardan et al. reported the optimum operating temperature of the undoped ZnO from 400 ◦ C shifted to 300 ◦ C for the Cr-doped ZnO under the acetone vapor [3]. According to reports of litera- ture, when group III elements such as Al, In and Ga were doped into the ZnO nanostructures, it is expected that the dopants act as singly charged donors and supply the excess carriers to con- ductance band, which will increases the conductance of ZnO and provide a route to improve the sensing properties of metal oxides [4–6]. Many methods have been used to synthesize nanostructure ∗ Corresponding authors. Tel.: +86 010 64436992. E-mail address: lidq@mail.buct.edu.cn (A. Chen). ZnO with various elements doping, such as radio frequency sput- tering, chemical vapor deposition and hydrothermal method, etc. [7–9]. Among them, the hydrothermal method is recognized to be the most facile and successful method for preparing well-defined morphology and highly crystalline products under relatively mild conditions. Herein, Fe-doped ZnO microflowers were prepared via a facile hydrothermal method without surfactant at lower temperature of 120 ◦ C for 10 h. The responses of undoped and Fe-doped ZnO sensing materials to NO 2 were examined, respectively, and the key factors influencing performance of gas sensing were revealed by PL analysis. The response transients for these sensing elements under different gas concentrations were measured and illustrated using L–H reaction mechanism. The band structures and density of states for undoped and Fe-doped ZnO nanocrystalline have been calculated using the first-principles based on the density functional theory, which is in agreement with the experimental results. 2. Experiment 2.1. Preparation of Fe-doped ZnO microflowers All chemicals are analytical-grade reagents without further purification. Fe-doped ZnO microflowers were prepared by follow hydrothermal procedure: Zinc nitrate (2.9 g) and urea (1.7 g) were dissolved in 25 mL deionized water under constant stirring, proper amount of Fe(NO 3 ) 3 ·9H 2 O (Fe/ZnO = 1.0, 3.0, 5.0 wt% respectively) 0925-4005/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2014.01.083