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

Abstract

Development of sensor materials based on metal oxide semiconductors (MOS) for selective gas sensors is challenging for the tasks of air quality monitoring, early fire detection, gas leaks search, breath analysis, etc. An extensive range of sensor materials has been elaborated, but no consistent guidelines can be found for choosing a material composition targeting the selective detection of specific gases. Fundamental relations between material composition and sensing behavior have not been unambiguously established. In the present review, we summarize our recent works on the research of active sites and gas sensing behavior of n-type semiconductor metal oxides with different composition (simple oxides ZnO, In₂O₃, SnO₂, WO₃; mixed-metal oxides BaSnO₃, Bi₂WO₆), and functionalized by catalytic noble metals (Ru, Pd, Au). The materials were variously characterized. The composition, metal-oxygen bonding, microstructure, active sites, sensing behavior, and interaction routes with gases (CO, NH₃, SO₂, VOC, NO₂) were examined. The key role of active sites in determining the selectivity of sensor materials is substantiated. It was shown that the metal-oxygen bond energy of the MOS correlates with the surface acidity and the concentration of surface oxygen species and oxygen vacancies, which control the adsorption and redox conversion of analyte gas molecules. The effects of cations in mixed-metal oxides on the sensitivity and selectivity of BaSnO₃ and Bi₂WO₆ to SO₂ and VOCs, respectively, are rationalized. The determining role of catalytic noble metals in oxidation of reducing analyte gases and the impact of acid sites of MOS to gas adsorption are demonstrated.

Keywords: active sites; gas sensor; gas-solid interaction; metal oxide semiconductor; nanoparticles; selectivity.

Figure 5

Types of active sites at the surface of tin oxide: noble metal cluster (NM), chemisorbed oxygen species (O_2(ads), O₂⁻, O⁻), charged oxygen vacancies (V_O⁻), partially reduced cations (Sn³⁺), coordinately unsaturated cations (Sn⁴⁺_cus), hydroxyl species (OH, Sn-OH), surface oxygen anions (O²⁻). Adapted with permission from ref. [79]. Copyright 2014 American Chemical Society.

Figure 17

Sensitivity of pristine and RuO₂-functionalized nanocrystalline n-type MOS to 20 ppm NH₃ at temperature 150–250 °C in relation to metal-oxygen bond energy in MOS (a) and acid sites concentration at the MOS surfaces (b). Adapted with permissions from ref. [75] copyright 2018 Elsevier; ref. [98] copyright 2018 Elsevier; copyright [116] 2019 John Wiley and sons; ref. [117] copyright 2012 Elsevier.

Figure 18

Sensitivity of pristine and PdO_x-functionalized nanocrystalline n-type MOS to 20 ppm CO at temperature 250–300 °C in relation to metal-oxygen bond energy in MOS (a) and acid sites concentration at the MOS surfaces (b). Adapted with permissions from ref. [75] copyright 2018 Elsevier; ref. [76] copyright 2019 by the authors (BB BY); ref. [87] copyright 2010 Elsevier; ref. [97] copyright 2018 by the authors (CC BY); ref. [120] copyright 2015 by the authors (CC BY).

Figure 19

Sensitivity of pristine and Au-functionalized nanocrystalline n-type MOS to 20 ppm acetone (a,c) and 20 ppm methanol (b,d) at temperature 300 °C in relation to metal-oxygen bond energy in MOS (a,b) and sum of Lewis acid sites and oxidizing sites concentration at the MOS surfaces (c,d). Adapted with permission from ref. [78]. Copyright 2021 Elsevier.

Figure 20

Sensitivity of nanocrystalline n-type MOS to 1 ppm NO₂ at temperature 100–150 °C in relation to metal-oxygen bond energy (a) and donor sites concentration (b). Adapted with permissions from ref. [42] copyright 2019 by the authors (CC BY); ref. [63] copyright 2021 Elsevier; ref. [102] copyright 2013 Elsevier; ref. [123] copyright 2015 by the authors (CC BY).

Figure 1

Schematic of representation (top) of the surface of MOS possessing oxygen vacancies (V_O) before (a) and after oxygen ionosorption (b); and the corresponding modulation of band energy levels (bottom): vacuum level (E_vac), conduction band bottom (E_C), Fermi level (E_F), donor states level (E_D), valence band top (E_V), potential energy surface barrier (−eV_S).

Figure 2

Schematic representation of the surface of n-type MOS with ionosorbed oxygen species (b) and after its interaction with reducing gas CO (c) and oxidizing gas NO₂ (a) within the chemisorption model of sensor response (top); and the corresponding modulation of band energy levels (bottom).

Figure 3

Schematic representation of the n-type MOS with oxygen vacancies and lattice oxygen anions (b), and after its interaction with reducing gas CO (c) and oxidizing gas NO₂ (a) within the oxygen vacancy model of sensor response (top); and the assumed changes in donor states population (E_D) and Fermi level positions (E_F) (bottom).

Figure 4

Positions of conduction band minima (E_c), valence band maxima (E_v) and electronegativity (χ) of n-type MOS, respective to vacuum level (E_vac). Adapted with permission using numeric data from ref. [54]. Copyright 2011 Elsevier. The oxides electronegativity corresponds to middle bandgap position. The levels of atomic orbitals for metal cations (Mⁿ⁺ s⁰/d⁰) and oxygen anions (O²⁻ 2p⁶) are shown schematically.

Figure 6

Scheme of water molecule dissociative adsorption at metal oxide surface with the formation of bridging and terminal OH-groups. Reprinted with permission from ref. [84]. Copyright 2018 Springer.

Figure 7

FTIR spectra of nanocrystalline n-type MOS. Adapted with permission from ref. [75] copyright 2018 Elsevier; ref. [76] copyright 2019 Marikutsa, Rumyantseva, Gaskov, Batuk, Hadermann, Sarmadian, Saniz, Partoens and Lamoen Creative Commons Attribution License (CC BY); ref. [78] copyright 2021 Elsevier; ref. [79] copyright 2014 American Chemical Society.

Figure 8

XP-spectra of O 1s state for nanocrystalline n-type MOS. Adapted with permissions from ref. [75] copyright 2018 Elsevier; ref. [78] copyright 2021 Elsevier; [79] copyright 2014 American Chemical Society; ref. [87] copyright 2010 Elsevier.

Figure 9

HRTEM images of nanocrystalline ZnO, In₂O₃, and SnO₂, and TEM image of WO₃; the sample were annealed at 300 °C. The insets show electron diffraction patterns. Adapted with permissions from ref. [42] copyright 2019 by the authors (CC BY); ref. [76] copyright 2019 by the authors (CC BY); ref. [94] copyright 2015 Elsevier; ref. [95] copyright 2013 American Chemical Society.

Figure 10

SEM images of nanocrystalline TiO₂, BaSnO₃, and Bi₂WO₆. Adapted with permissions from ref. [63] copyright 2021 Elsevier; ref. [77] copyright 2015 by the authors (CC BY); ref. [96] copyright 2021 Springer Nature.

Figure 11

Temperature plots of ammonia desorption rate from the surface of n-type MOS (a). Adapted with permissions from ref. [63] copyright 2021 Elsevier; ref. [75] copyright 2018 Elsevier; ref. [78] copyright 2021 Elsevier; ref. [79] copyright 2014 American Chemical Society. TPD pattern of TiO₂ compared with mass-spectral (MS) analysis of desorbed gas (b).

Figure 12

Concentration of Brønsted and Lewis acid sites at the surface of nanocrystalline n-type MOS synthesized at 300 °C (ZnO, In₂O₃, SnO₂, WO₃) and 700 °C (TiO₂) in relation to metal-oxygen bond energy. Adapted with permission from reference [78]. Copyright 2021 Elsevier.

Figure 13

EPR spectra of nanocrystalline n-type MOS. Adapted with permissions from ref. [79] copyright 2014 American Chemical Society; ref. [95] copyright 2013 American Chemical Society; ref. [96] copyright 2021 Springer Nature; ref. [101] copyright by the authors; ref. [102] copyright 2013 Elsevier.

Figure 14

Concentration of active sites at the surface of nanocrystalline n-type MOS in relation to metal-oxygen bond energy: oxidizing sites estimated from H₂ consumption in TPR at temperature below 300 °C (a), ionosorbed oxygen O₂⁻ determined by EPR (b), and donor sites (V_O⁻) determined by EPR (c). The values are taken from Table 2.

Figure 15

Temperature plots of hydrogen consumption rate during TPR of nanocrystalline n-type MOS. Adapted with permissions from ref. [63] copyright 2021 Elsevier; ref. [75] copyright 2018 Elsevier; ref. [76] copyright by the authors (CC BY); ref. [78] copyright 2021 Elsevier; ref. [79] copyright 2014 American Chemical Society.

Figure 16

Unit cells of rutile-like tetragonal SnO₂, perovskite-like cubic BaSnO₃ (a), monoclinic WO₃ and Aurivillius structure of Bi₂WO₆ (b). Adapted with permissions from references [61,63,113]. Copyrights 2013 American Physical Society, 2021 Elsevier, 2014 Elsevier.

Figure 21

Sensitivity of nanocrystalline SnO₂ and BaSnO₃ to NO₂ (2 ppm) at 100 °C, CO (50 ppm), NH₃ (20 ppm), H₂S (2 ppm), H₂ (100 ppm), SO₂ (10 ppm), ethanol (20 ppm), methanol (20 ppm) at 300 °C (a) [77]; and sensitivity of nanocrystalline WO₃ and Bi₂WO₆ to NO₂ (1 ppm) at 100 °C, ethanol (20 ppm) at 150 °C, SO₂ (2 ppm), formaldehyde (400 ppb), H₂S (2 ppm) at 250 °C, CO (20 ppm), H₂ (50 ppm), NH₃ (20 ppm), acetone (2 ppm), benzene (2 ppm) at 300 °C (b) [63].

Figure 22

TEM micrographs, STEM images and EDX elemental maps of nanocrystalline SnO₂ and WO₃ functionalized by PdO_x (a) and RuO₂ (b); and TEM micrographs with electron diffraction patterns of In₂O₃ and TiO₂ functionalized by Au (c). Adapted with permissions from ref. [75] copyright 2018 Elsevier; ref. [78] copyright 2021 Elsevier; ref. [95] copyright 2013 American Chemical Society.

Figure 23

DRIFT spectra of pristine and functionalized by noble metal additives nanocrystalline SnO₂ (a) and WO₃ (b) exposed to CO (100 ppm (a); 200 ppm (b)) at room temperature for 1 h. Adapted with permissions from ref. [75] copyright 2018 Elsevier; ref. [118] copyright 2015 American Chemical Society.

Figure 23

DRIFT spectra of pristine and functionalized by noble metal additives nanocrystalline SnO₂ (a) and WO₃ (b) exposed to CO (100 ppm (a); 200 ppm (b)) at room temperature for 1 h. Adapted with permissions from ref. [75] copyright 2018 Elsevier; ref. [118] copyright 2015 American Chemical Society.

Figure 24

DRIFT spectra of pristine and functionalized by noble metal additives nanocrystalline SnO₂ (a) and WO₃ (b) exposed to NH₃ (100 ppm (a); 200 ppm (b)) at 200 °C for 1 h. Adapted with permissions from ref. [75] copyright 2018 Elsevier; ref. [118] copyright 2015 American Chemical Society.

Figure 25

Concentration of oxidizing sites at the surface of pristine and Au-functionalized n-type MOS estimated from H₂ consumption in TPR at temperature below 300 °C (a) in comparison with the sensitivity to 20 ppm of acetone (b) and 20 ppm of methanol (c) as a function of metal-oxygen bond energy in MOS. Sensitivity of Au-functionalized MOS to 20 ppm of acetone (d) and 20 ppm of methanol (e) in relation to of oxidizing sites concentration (d,e). Operation temperature of sensors was 250–300 °C for pristine MOS and 150–225 °C for Au-functionalized MOS. Adapted with permission from ref. [78]. Copyright 2021 Elsevier.