Effect of Zinc Oxide Modification by Indium Oxide on Microstructure, Adsorbed Surface Species, and Sensitivity to CO

Abstract

Additives in semiconductor metal oxides are commonly used to improve sensing behavior of gas sensors. Due to complicated effects of additives on the materials microstructure, adsorption sites and reactivity to target gases the sensing mechanism with modified metal oxides is a matter of thorough research. Herein, we establish the promoting effect of nanocrystalline zinc oxide modification by 1-7 at.% of indium on the sensitivity to CO gas due to improved nanostructure dispersion and concentration of active sites. The sensing materials were synthesized via an aqueous coprecipitation route. Materials composition, particle size and BET area were evaluated using X-ray diffraction, nitrogen adsorption isotherms, high-resolution electron microscopy techniques and EDX-mapping. Surface species of chemisorbed oxygen, OH-groups, and acid sites were characterized by probe molecule techniques and infrared spectroscopy. It was found that particle size of zinc oxide decreased and the BET area increased with the amount of indium oxide. The additive was observed as amorphous indium oxide segregated on agglomerated ZnO nanocrystals. The measured concentration of surface species was higher on In 2 O 3-modified zinc oxide. With the increase of indium oxide content, the sensor response of ZnO/In 2 O 3 to CO was improved. Using in situ infrared spectroscopy, it was shown that oxidation of CO molecules was enhanced on the modified zinc oxide surface. The effect of modifier was attributed to promotion of surface OH-groups and enhancement of CO oxidation on the segregated indium ions, as suggested by DFT in previous work.

The Key Role of Active Sites in the Development of Selective Metal Oxide Sensor Materials

“…Hence, the active sites responsible for probe molecules oxidation were mainly constituted by other adsorbed and/or oxygen species and OH-groups, rather than O 2 − . The small concentration of O 2 − is in line with Weisz limitation, which states that maximum coverage of ionosorbates at the semiconductor surface cannot exceed 10 −3 -10 −2 monolayer, i.e., 1.7 × 10 Oxidizing surface sites were evaluated using TPR with hydrogen [63,75,76,78,79]. TPR profiles are shown in Figure 15.…”

“…In the following sections, these relationships are reviewed based on our experimental studies of nanocrystalline n-type MOS: ZnO, In 2 O 3 , SnO 2 , BaSnO 3 , TiO 2 , WO 3 , Bi 2 WO 6 . The materials were synthesized via the standardized aqueous precipitation of metal hydroxides followed by calcination at temperature 300-700 • C [63,[74][75][76][77][78]. observation of actual reactivity of surface sites under in situ reaction conditions is an unresolved problem; and the above definition refers to a potential participation of certain types of active sites in real-life chemical reactions that determine the functionality of catalytic and sensing materials.…”

“…In the following sections, these relationships are reviewed based on our experimental studies of nanocrystalline n-type MOS: ZnO, In2O3, SnO2, BaSnO3, TiO2, WO3, Bi2WO6. The materials were synthesized via the standardized aqueous precipitation of metal hydroxides followed by calcination at temperature 300-700 °C [63,[74][75][76][77][78]. ).…”

“…In Table 2, we summarized the concentrations of active sites determined in our works on n-type simple MOS (ZnO, SnO 2 , TiO 2 , WO 3 ) with different phase compositions and microstructure parameters, and mixed-metal oxides (BaSnO 3 , Bi 2 WO 6 ). The simple MOS were synthesized by aqueous deposition of metal hydroxides with subsequent annealing at temperature 300-700 • C. Mixed-metal oxides were obtained by co-precipitation of metal hydroxides and hydrothermal treatment with subsequent annealing at 300-500 • C. The materials consisted of phase-pure nanocrystalline wurtzite-like ZnO, rutile-like SnO 2 , monoclinic γ-WO 3 , cubic perovskite-like BaSnO 3 , and Aurivillius phase Bi 2 WO 6 with the crystallite size in the range d XRD = 3-50 nm and specific surface area 4-100 m 2 /g (Table 2) [63,[74][75][76][77][78]. Mixed-phase TiO 2 with the rutile:anatase ratio of 3:2 was obtained after calcination at 700 • C [78].…”

Electrospun ZnO/Pd Nanofibers: CO Sensing and Humidity Effect

“…The nature of the two negative bands in the 1600–1300 cm −1 region is not entirely clear [ 50 , 51 ]. However, it is worth noting that the positive absorption intensity in this region increases with the increase in the partial pressure of oxygen in the surrounding atmosphere [ 52 ], while the negative intensity is observed in the presence of reducing gases CO and H 2 [ 50 ] and increases due to continuous heating in high vacuum at T = 370 °C [ 52 ]. Thus, the nature of these bands is related to defects in the crystal structure of ZnO [ 51 ].…”