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.

1. Introduction

1.1. Issues in the Design of Metal Oxide Sensors

Nanomaterials based on metal oxide semiconductors (MOS) have been extensively investigated for use in resistive gas sensors. The electronic properties of MOS are sensitive to chemical processes occurring at the surface, which enables the detection of gas molecules in the ppb-ppm concentration range in the environment [1,2,3]. Thermal stability in air, miniaturization, low cost and ability of integration into wireless devices are the main advantages of resistive sensors. The main drawback is the low selectivity, which is due to the sensing principle. Redox conversion of reducing or oxidizing gas molecules at the surface of MOS controls the thickness of a space charge region and electric conduction in the semiconductor [2,4]. Therefore, gas molecules with distinct compositions but similar redox properties are indistinguishable by the sensor response. A number of approaches to improve the selectivity of MOS sensors were elaborated. Those include technological ones, e.g., using preconcentration or filtering membranes [5,6], modulation of the operation temperature regime, use of multisensor arrays with appropriate data treatment algorithms (“electronic nose”) [7,8,9]. The fundamental approach is modification of the chemical composition of the sensor material. It can be performed by functionalization of MOS surfaces [9,10,11,12], bulk doping of MOS [13,14], use of p-type MOS and mixed-metal oxides instead of conventionally employed n-type MOS (SnO₂, ZnO, WO₃, TiO₂, Ga₂O₃, In₂O₃, etc.) [15,16,17], or MOS-based composites with different additives, e.g., noble metals, metal oxides, graphene and carbon nanotubes, polymers [18,19,20,21]. Although there are numerous reviews available on MOS-based materials for gas sensors, a comprehensive materials design concept for the creation of sensors with predetermined selectivity for a desired analyte gas or a group of gases can hardly be found. In a recent review by Korotcenkov, et.al., the criteria for choosing the chemical composition of a gas-sensitive MOS were proposed [22]. For example, simple metal oxides conventionally used in sensors combine thermal stability (high melting and decomposition points above 1200 °C), and electronic configuration d⁰ for transition metal cations (in TiO₂, WO₃) or d¹⁰ for post-transition metal cations (in In₂O₃, ZnO, SnO₂) that conditions the wide bandgap n-type semiconductor behavior. These cations may be partially reduced through the formation of oxygen vacancies, and the concentration of point defects in these metal oxides falls in the 10¹⁶–10¹⁹ cm⁻³ range, which is appropriate for the sensitivity of bulk conduction of localized charges at the surface [22]. However, no guidelines can be found for choosing the chemical composition of a sensor material. Even the choice between the listed simple MOS is an issue when the selectivity of analyte gas adsorption and redox interaction with the surface is considered. A more sophisticated problem is an appropriate selection of constituents when gas sensitive mixed-metal oxides or composites are elaborated. Besides the predictable effect on electronic properties of the semiconductor, the additives influence the adsorption capacity and chemical reactivity of the material surface, and the latter has often been disregarded in the gas sensor studies.

1.2. Gas Sensing Principles of Metal Oxide Sensors

The mechanism of gas sensing consists of three principle steps: (i) adsorption and redox interaction of gas molecules with the MOS surface (“reception”); (ii) transduction of the electric signal produced by the redox reaction from the surface to the bulk of MOS; and (iii) transduction of the electric signal through the network of contacting MOS particles [1,2,23]. The reception is a chemical process involving the active sites at MOS surface and is discussed in detail below.

1.2.1. Oxygen Vacancy Formation and Oxygen Chemisorption

Oxygen vacancies are formed due to partial oxygen loss from the surface of oxides in which metals have the highest oxidation states for the respective elements group:

O²⁻_(bulk) ↔ ½ O_2(g) + V_O²⁻ ↔ ½ O_2(g) + V_O⁻ + e⁻ ↔ ½ O_2(g) + V_O + 2 e⁻

(1)

Electron donor states associated with oxygen vacancies (or, reduced metal cations) are available for oxygen chemisorption in air yielding chemisorbed species (Equation (2)), or restoring the lattice O²⁻ anions on the oxide surface (Equation (3)):

O_2(g) + 4 e⁻ ↔ O₂⁻_(ads) + 3 e⁻ ↔ 2 O⁻_(ads) + 2 e⁻ ↔ 2 O²⁻_(ads)

(2)

O_2(g) + 2 V_O + 4 e⁻ ↔ 2 O²⁻_(bulk)

(3)

As a result, in the subsurface region the donor states become less occupied by electrons, negative charge is localized at the chemisorbed oxygen and surface potential barrier arises leading to reduced conductivity of the n-type MOS decrease (Figure 1). The chemisorption equilibria described in Equation (2) shift to the right at increasing temperature, so that predominant chemisorbed oxygen species changes from molecular O₂⁻ (160–200 °C) to atomic O⁻ (200–350 °C) and O²⁻ ones (above 350 °C). It was established for polycrystalline SnO₂ using numerous spectroscopic techniques and indirect measurements [5,24,25], and the agreeing conclusions were done for other n-type MOS (ZnO, In₂O₃, WO₃) using the conduction measurements under variable oxygen pressure [26,27,28].

There are two models describing the sensor response as the interaction of analyte gases with: (i) chemisorbed oxygen species (ionosorption model), and (ii) lattice oxygen anions at the oxide surface (oxygen vacancy model) [29,30].

1.2.2. The Ionosorption Model of Sensor Response

Within the ionosorption model, sensor response to a reducing gas (Red) is due to oxidation of gas molecules by chemisorbed oxygen species (O_n^m−) [4,6,7,8,29,30]:

Red_(g) + O_n^m−_(ads) → RO_n(g) + m e⁻

(4)

where RO_n is the oxidation product. The removal of chemisorbed oxygen species releases the trapped electrons back to the semiconductor and decreases the surface potential barrier (Figure 2), which results in increasing conductivity. For example, the reducing gases H₂, CO, H₂S, NH₃, and VOCs are commonly considered to be oxidized to H₂O, CO₂, H₂O + SO₂, H₂O + N₂, and H₂O + CO₂, respectively [2,4,5,31,32,33,34,35]. The surface reaction is described similarly as the mechanisms of heterogeneous oxidation catalysis within the Langmuir-Hinshelwood model (reaction between co-adsorbed Red molecule and oxygen) or Iley-Rideal model (reaction of gas-phase Red and adsorbed oxygen) [36,37]. Unlike the mechanisms of catalysis which have been the subject for careful kinetic studies, the interaction of reducing gases with chemisorbed oxygen at the sensors surfaces has hardly been experimentally proven. The ionosorption model, i.e., oxidation by ionosorbed oxygen species, has often been speculatively used to describe the sensing mechanisms of reducing gases [32,33,34,35,38,39,40]. The major issue is the lack of techniques for the observation of chemisorbed oxygen species, especially under in situ reaction conditions [25]. The response to an oxidizing gas (Ox) is attributed to the competitive chemisorption of target molecules (e.g., NO₂, O₃, Cl₂) which are stronger electron acceptors than O₂:

Ox_(g) + m e⁻ → Ox^m−_(ads)

(5)

This leads to trapping of more electrons in the surface states and a higher potential barrier at the MOS surface, resulting in decreased conductivity in the presence of oxidizing gases (Figure 2). The sensing mechanism related to chemisorption of oxidizing gases (Equation (5)) was experimentally proven by spectroscopy studies, e.g., NO₂ adsorption on the MOS surfaces with the formation of NO₂⁻ was observed by in situ Raman measurements [41] and diffuse reflectance infrared Fourier-transform (DRIFT) spectroscopy [42].

1.2.3. The Oxygen Vacancy Model of Sensor Response

The oxygen vacancy model describes the sensor response of n-type MOS in terms of reversible shift of equilibria between bulk oxygen anions and oxygen vacancy formation (Equation (1)) depending on the ambient gas composition [29,30]. The reducing gas (Red) favors the formation of oxygen vacancies via the binding with lattice O²⁻ anions at the oxide surface and desorption of the oxidation product (RO_n):

Red_(g) + n O²⁻_(lat) → RO_n(g) + n V_O²⁻ ↔ RO_n(g) + n V_O⁻ + n e⁻ ↔ RO_n(g) + n V_O + 2n e⁻

(6)

As a result, the concentration of donor states with loosely bound electrons increases (Figure 3), and the conductivity of n-type MOS rises. The reaction of target gas with lattice O²⁻ anions (Equation (6)) with simultaneous cleavage of metal-oxygen bonds is considered within the Mars-van Krevelen model of catalytic oxidation over metal oxide catalysts [36]. It has been accepted that the oxygen vacancy mechanism of sensor response takes place under oxygen-lean conditions, e.g., when the reducing gases (CO, hydrocarbons, H₂) are detected in inert gases [43,44]. Such conditions are unfavorable for oxygen chemisorption at the MOS surface, and the ionosorption model of sensor response is unsuitable. Response to an oxidizing gas (Ox) is attributed to the fulfilment of an oxygen vacancy by an oxygen atom and restoration of lattice O²⁻ anions and metal-oxygen bonds:

Ox_(g) + V_O + 2 e⁻ → R_(g) + O²⁻_(lat)

(7)

where the Ox molecule contains oxygen atoms and R is the reduction product. The participation of oxide anions in the interaction of MOS with analyte gases was confirmed experimentally in a number of works. The evolution of Sn²⁺ cations was observed by Mossbauer spectroscopy as a result of partial lattice oxygen reduction in SnO₂ during the interaction with CO in inert atmosphere [45]. By DRIFT spectroscopy it was established that the W-O bonds were cleaved on the surface of WO₃ in presence of CO in air [46]. On the contrary, the W-O bonds restored when WO₃ was oxidized by NO₂ or even H₂O molecules in humid air [46,47]. Thus, the sensor response of tungsten oxide has been appropriately described by the oxygen vacancy model.

1.3. The Concerns on Sensor Materilals from Heterogeneous Catalysis

The reception of gas molecules at the sensor surface consists of same processes as on the surface of heterogeneous catalysts: adsorption and redox conversion of gas molecules. The correlations of sensitivity with catalytic activity of MOS have been observed experimentally [48,49,50]. Typical studies of heterogeneous catalysts consist of in-depth materials characterization, including the determination of active surface sites and the investigation of gas-solid interactions (adsorption, redox conversion) which are controlled by the surface sites [36,51]. On the contrary, in studies of sensors the greater attention has been paid to electrophysical phenomena (charge transfer, surface charging, band bending, conduction paths); and the sensitivity was successfully modelled depending on semiconductor type, donor states concentration, particle size, materials morphology, heterojunctions etc. [1,2,3,4,8,23,52,53]. Characterization of active sites at sensors surfaces was hardly performed, therefore, the chemical routes of analyte gas adsorption and redox interaction with the surface was incompletely understood. Catalytic activity of MOS and additives used in the composites is often referred to when gas sensing is discussed; however, the characterization of the surface sites and the direct experimental evidences for the catalytic interaction with gases was most often missing. The additives utilized in MOS-based sensors have traditionally been grouped as the catalytic (chemical) and the electronic ones [4,8,21]. The disadvantage of such classification is that the origin and mechanism of the catalytic activity in the process of target gas conversion is disregarded, and, consequently, the improvement of selectivity is not substantiated from the viewpoint of surface chemistry. Moreover, the additives (noble metals, metal oxides, carbon nanomaterials, polymers) can simultaneously exhibit the catalytic behavior and electronic interaction, i.e., heterojunction formation with MOS.

1.4. The Comparison of Energetic Parameters of n-Type Metal Oxide Semiconductors

In this review, we summarize our recent works aimed at reveling the active sites at the surface of gas sensitive MOS-based materials and the impact of surface sites on the adsorption and redox interactions with gas molecules which influence the sensing behavior. We show that the paradigm of heterogeneous catalysts research is worth applying for the investigation of sensor materials, since the reasons for selectivity in gas sensing may be fundamentally rationalized in terms of surface chemical reactivity. The approaches conventionally used for catalysts research were successfully transferred to the studies of gas sensors, including the qualitative and quantitative determination of active sites by surface science techniques such as temperature-programmed reduction and temperature-programmed desorption of probe molecules. Using in situ infrared spectroscopy the roles of adsorption sites and redox sites were revealed in the processes of gas molecules interaction with the sensors surfaces. The dependence of materials functional behavior on gas-solid interaction is the common point for heterogeneous catalysts and sensors. The sensitivity of sensors is much influenced also by the transduction of charge carriers between the surface and bulk of semiconductor and through the network of conduction paths. However, in the present review we propose that the selectivity of sensor response is mainly controlled by gas molecules reception, which in turn depends on active sites at the sensor surface. The nature and concentration of active sites depend on the chemical composition of sensing materials. Revealing these correlations will be useful for predictable control of sensors selectivity through appropriate tailoring the materials composition. Herein, the effect of chemical composition of simple n-type MOS (In₂O₃, ZnO, SnO₂, TiO₂, WO₃) and mixed-metal oxides (BaSnO₃, Bi₂WO₆) on the acid sites, oxidizing sites (surface oxygen species) and donor sites (oxygen vacancies) is discussed. The materials are wide-bandgap n-type semiconductors and differ in energetic parameters of the oxides and constituent cations (

Table 1. Parameters of n-type metal oxide semiconductors (MOS): cationic radius r, electronegativity χ and ionic-covalent parameter ICP; metal-oxygen bond energy E_M-O and bond distance d_M-O; negative formation enthalpy per oxygen atom −1/x·Δ_fH; bandgap width E_g.

MOS	r, Å [59]	χ, 1 P.u. [54]	ICP 2 [54]	EM-O3, eV [55]	dM-O, Å	−1/x·ΔfH [55], eV	Eg [54], eV
ZnO	0.74	1.65	0.45	10.4	1.96–1.98 [57]	3.6	3.4
In2O3	0.79	1.78	0.53	12.4	2.18 [60]	3.2	2.8
SnO2	0.69	1.96	0.61	15.5	2.02 [61]	6.0	3.6
TiO2	0.61	2.01	0.52	20.7	1.95 (rutile) 1.96 (anatase) [62]	9.8	3.4
WO3	0.58	2.36	0.49	36.6	1.74–2.17 [63]	8.7	2.8

¹ P.u. is Pauling units; ² Ionic-covalent parameter (dimensionless) correlates to Pearson’s hardness of cations [54]; ³ Bond energy in metal oxides MO_x estimated from thermodynamic parameters [55]: E_M-O = {−Δ_fH°(MO_x) + Δ_subH°(M) + x/2·Δ_disH°(O₂) + x·Δ_el.af.H°(O) + ΣΔ_ion.H°(M)}/(x·CN_O), where Δ_subH°(M)—sublimation enthalpy of metal, Δ_disH°(O₂)—dissociation enthalpy of oxygen molecule, Δ_el.af.H°(O)—electron affinity of oxygen atom, ΣΔ_ion.H°(M)—sum of ionization potentials of metal cation, CN_O—coordination number of oxygen in the oxide.

) [54,55]. Electronegativity of the cations increases in the order Zn²⁺, In³⁺, Sn⁴⁺, Ti⁴⁺, W⁶⁺; it agrees with the increment of charge/radius ratio. Accordingly, the metal-oxygen bond energy (E_M-O) increases in respective metal oxides (Table 1) [54,56], and electron energy band positions (valence band top E_v, conduction band bottom E_c, middle of bandgap χ) decrease (Figure 4). The conduction band minima are mainly contributed by empty cationic s-orbitals (In³⁺, Zn²⁺, Sn⁴⁺) or d-orbitals (Ti⁴⁺, W⁶⁺), and the valence band maxima—by filled oxygen 2p-orbitals [57,58]. Energy levels of the orbitals shift deeper below the vacuum level as the electronegativity of cations increases (Figure 4). Bandgap width of oxides is related to cationic Pearson’s hardness (or, ionic-covalent parameter), which is close for the mentioned cations (intermediately hard) and, therefore, the oxides have wide bandgaps (E_g = 2.8–3.6 eV) [54]. In the following, we use M-O bond energy as the fundamental descriptor parameter of the active sites and gas sensitivity of metal oxides. The roles of acid sites, chemisorbed oxygen and oxygen vacancies in adsorption and redox conversion of gas molecules NH₃, SO₂, H₂S, volatile organic compounds (VOCs), and NO₂ is discussed in relation to sensing behavior. The key role of noble metal clusters (Pd, Ru) in specific catalytic oxidation of analyte molecules (CO, NH₃) improving the selectivity of sensor response of catalytically functionalized MOS is discussed.

2. Types of Active Sites at the MOS Surface

In the literature there is no strict definition of an active site at the metal oxide surface; rather, it is conditional according to the materials and processes under consideration. In catalysts, the proper representation of an active site is the surface atom or ensemble which binds to a foreign molecule, thus loosening the intramolecular chemical bonds and forming the intermediates and/or final products of the catalytic reaction [36,64]. Yet, such defined active sites are hardly observable experimentally and unmeasurable due to their low fraction in the overall number of surface atoms, the specificity of catalytic reaction and the dynamic lability of active species in course of reaction. The other way demonstrated in some works was to regard the active sites as the adsorption sites for the specific reactant or probe molecule (e.g., isopropanol [65], aromatics [66,67]). Such defined active sites, i.e., adsorption sites, were quantified by the adsorption measurements and their concentration correlated with the catalytic activity; yet, no observations under in situ reaction conditions were available.

Herein, we use the conditional term of an active site as the surface species that possess a definite chemical behavior, like acidity/basicity, redox reactivity, electron donor/acceptor behavior, specific chemisorption capacity for probe molecules [64,65,66,67,68]. The 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 general, the active sites at a metal oxide surface can be constituted by coordinately unsaturated cations and oxygen anions, atomic ensembles, molecular and atomic adsorbates [69,70]. Defects are also considered as important active sites in catalytic reactions; including the point defects (cationic or anionic vacancies, atomic interstitials), and the extended defects like steps, corners, edges, dislocations, perimeter between metal oxide support and catalytic clusters [71,72,73]. The adsorption capacity and reactivity of surface sites is due to dangling bonds and unsaturated coordination of the peripheral atoms. The surface sites can form localized electron energy states and influence the band energy levels in MOS, thus influencing the electric response to surface processes. Coordinately unsaturated cations, oxygen anions and defects (vacancies, interstitials, dislocations, etc.) are the possible intrinsic active sites. Adsorbed oxygen, OH-groups, hydrogen adatoms resulting from spontaneous adsorption of oxygen and humidity represent the intrinsic adsorbate sites at the surface of pristine metal oxides. The extrinsic active sites can be created via the introduction of additives: atoms, atomic groups, clusters, nanoparticles of metals or metal oxides. In accordance with the chemical reactivity, the intrinsic active sites can be grouped into acid/base and reducing/oxidizing (or, donor/acceptor) ones [68]. The distinct chemical reactivity allows for the experimental determination of these species using appropriate techniques. In Figure 5 the types of active sites at the surface of tin oxide are shown schematically.

The concentration of different active sites depends on the nature of metal oxide (cationic charge and size, metal-oxygen bond energy), the synthesis conditions, microstructure, and additives. 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].

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 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.

2.1. Acid/Base Sites

Surface metal cations are the Lewis acid sites, since the positive charge and unsaturated coordination favor the acceptation of lone electron pairs from the adsorbate to the cationic empty orbitals. From the electrostatic attraction law, Lewis acidity is expected to increase with increasing charge/radius ratio of the cation. On the contrary, the covalent contribution to cation-adsorbate bonding can be crucial for the acid-base bond strength. For example, it was shown that Sn²⁺ cations at the surface of partially reduced SnO₂ are formally stronger Lewis acid sites than Sn⁴⁺ due to more covalent binding with adsorbed ammonia [80]. An alternative approach to quantitative scaling of the Lewis acidity is based on the concept of optical basicity (Λ); it is related to the ability of cations to shift electron density from oxygen anions in metal oxides [81].

Adsorption of H₂O molecules (Lewis base) and strong binding to surface cation (Lewis acid) results in water molecule dissociation producing the OH-groups (Figure 6). In case of binding to strongly Lewis acidic cations and/or binding to more than one cation in a bridging conformation, the OH-group possesses an acidic proton and behaves as Brønsted acid site. Hydroxyl groups which are loosely bound to weakly acidic cations in a terminal conformation can behave as Brønsted base or acid sites depending on the counterpart interacting species. The OH-groups can be distinguished using FTIR spectroscopy: the terminal hydroxyls give rise to separate O-H peaks with wavenumbers above 3600 cm⁻¹ [82,83]. The broad O-H bands centered at about 3400 cm⁻¹ are indicative of the families of bridging hydroxyls associated via hydrogen bonds (Figure 7).

Surface oxygen anions O²⁻ are the intrinsic base sites at a metal oxide surface, and can act as the Lewis or Brønsted base depending on the adsorbate. The oxide anions can be determined by X-ray photoelectron spectroscopy (XPS). The asymmetric O 1s signal of nanocrystalline metal oxides can be simulated by two components (Figure 8): the major signal with binding energy 530.0–530.5 eV due to bulk O²⁻ anions and the lower peak of surface oxygen species at higher binding energy (531.0–533.0 eV) [34,85,86].

2.2. Oxidizing Sites

The oxidizing surface sites are most often associated with adsorbed oxygen [2,8,12,85,86,88,89]. There are several types of adsorbed oxygen species on a metal oxide surface:

• physisorbed O₂ molecules held at the surface via van der Waals attraction, the physisoption dominates at temperature below −70 °C [90];
• chemisorbed O₂ molecules covalently bound with surface cations via a local redistribution of electron density, the chemisorbed oxygen was found to dominate at the surface of tin oxide at temperature below 200 °C [91];
• ionosorbed species O₂⁻, O⁻, and O²⁻ are formed by molecular and dissociative oxygen adsorption and acceptation of delocalized electrons from the bulk of MOS (Equation (2)) [5].

Besides, hydroxyl species were found to possess oxidizing activity to reducing gases: CO, H₂ [82,83,92]. Metal cations in high oxidation states (Sn⁴⁺, Ti⁴⁺, V⁵⁺, Mo⁶⁺, W⁶⁺) represent the other type of oxidation sites at the surface of respective metal oxides. Of particular interest are the double M=O bonds which end up the metal-oxygen framework at the surface of V₂O₅, MoO₃, WO₃, and have been considered as the active oxidizing sites on these metal oxides and composites [93]. The reduction of surface metal cations to lower oxidation state proceeds through the cleavage of metal-oxygen bonds leaving oxygen vacancies in the surface structure of metal oxide. It is the so-called Mars-van Krevelen mechanism in heterogeneous oxidation catalysis.

2.3. Electron Donor Sites

Oxygen vacancies (V_O²⁻, V_O⁻) and reduced metal cations possess loosely bound electrons and behave like donor sites [4]. Oxygen vacancies which diffuse into the oxide bulk form donor states and account for the n-type conductivity of MOS: In₂O₃, ZnO, SnO₂, TiO₂, WO₃ [22,58]. In these oxides, metals have the highest oxidation states for the respective elements group and, hence, can be reduced to a lower oxidation state by trapping electrons liberated from oxygen desorption and ionization of oxygen vacancies (Equation (1)).

3. Active Sites Concentrations at the Surface of Nanocrystalline n-Type MOS

In

Table 2. Concentration of active sites at the surface of n-type MOS obtained at different annealing temperature (T_anneal) and having different mean crystallite size (d_XRD) and specific surface area (SSA) [63,76,77,78,79,94,96].

MOS	Tanneal, °C	dXRD, nm	SSA, m2/g	Active Sites Concentration, 10−6 mole/m2
				Acid Sites 1		Oxidizing Sites		Donor Sites VO− 3
				Weak (Broensted)	Strong (Lewis)	Total n (H2,TPR) 2	O2− 3
ZnO	300	11–13	45	0.4 ± 0.1	1.8 ± 0.4	6.6 ± 0.5	4.4 × 10−5	8 × 10−6
In2O3	300	7–8	100	n.d. 4	n.d. 4	5.8 ± 0.4	1.3 × 10−2	-
	500	16–19	35	0.5 ± 0.1	1.7 ± 0.4	4.1 ± 0.8	1.5 × 10−2
SnO2	300	3–5	95	0.6 ± 0.1	2.4 ± 0.5	23.6 ± 0.8	1.4 × 10−5	3.5 × 10−4
	500	10–12	25	0.5 ± 0.2	1.5 ± 0.4	14.0 ± 1.2
	700	16–20	10	0.5 ± 0.2	1.2 ± 0.5	12.6 ± 2.6
BaSnO3	500	20–22	8	0.4 ± 0.2	0.7 ± 0.3	12.2 ± 2.8	2.5 × 10−4	-
TiO2	700	27–30 (rutile), 38–46 (anatase)	5	3.3 ± 0.8	8.2 ± 1.9	20.8 ± 3.1	6 × 10−6	6.8 × 10−4
WO3	300	7–9	35	6.1 ± 0.2	21.7 ± 0.9	5.4 ± 0.6	-	3.1 × 10−3
	450	19–22	9	5.1 ± 0.9	15.6 ± 1.2	-
	600	23–25	4	3.1 ± 0.6	8.9 ± 2.0	-
Bi2WO6	300	14–15	9	0.8 ± 0.3	1.7 ± 0.6	16.4 ± 2.0	-	-

¹ Evaluated by TPD of ammonia. ² Evaluated by TPR with hydrogen. ³ Evaluated by EPR. ⁴ No data for TPD from In₂O₃ annealed at 300 °C because of ammonia oxidation caused by residual nitrate species [78,97].

, we summarized the concentrations of active sites determined in our works on n-type simple MOS (ZnO, SnO₂, TiO₂, WO₃) with different phase compositions and microstructure parameters, and mixed-metal oxides (BaSnO₃, Bi₂WO₆). 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 SnO₂, monoclinic γ-WO₃, cubic perovskite-like BaSnO₃, and Aurivillius phase Bi₂WO₆ with the crystallite size in the range d_XRD = 3–50 nm and specific surface area 4–100 m²/g (Table 2) [63,74,75,76,77,78]. Mixed-phase TiO₂ with the rutile:anatase ratio of 3:2 was obtained after calcination at 700 °C [78]. TEM and SEM micrographs of the materials in Figure 9 and Figure 10 demonstrate the random shaped morphology of the nanoparticles of materials. Active sites of different types were determined by electron paramagnetic resonance (EPR) and probe molecules techniques: temperature programmed desorption of ammonia (TPD) and temperature programmed reduction by hydrogen (TPR). The principles and procedures of the active sites determination are described in the Appendix A.

3.1. Acid Sites

Acid sites were determined by TPD of ammonia [63,75,78,79,98]. Desorption patterns are shown in Figure 11a. The low-temperature bands at 50–200 °C were due to desorption of weakly bound probe molecules. Molecular NH₃ evolved at this temperature, as was confirmed by mass-spectrometry (Figure 11b). These bands were ascribed to weak (Brønsted) acid sites, i.e., acidic OH-groups. At higher temperature (above 200 °C, Figure 11a) ammonia desorbed from stronger (Lewis) acid sites, i.e., coordinately unsaturated cations at the oxides surfaces. Concentration of Brønsted and Lewis acid sites increased in the order ZnO ≈ In₂O₃ < SnO₂ < TiO₂ < WO₃ (Figure 12). It agrees with the increment of metal-oxygen bond energy, electronegativity and charge/radius ratio of cations (Table 1). In the same order does the optical basicity of oxides decrease [81]. The coincident trends of surface acidity, E_M-O and electronegativity should be due to increasing charge/size ratio for the cations Zn²⁺, In³⁺, Sn⁴⁺, Ti⁴⁺, W⁶⁺. The higher positive charge density at a cation (Lewis acid) favors stronger attraction of lone electron pair donor (Lewis base).

3.2. Donor Sites (Oxygen Vacancies)

Donor paramagnetic sites attributed to single charged oxygen vacancies V_O⁻ were recognized on the EPR spectra of ZnO, SnO₂, TiO₂, WO₃, by the signals with g-factor below 2.00 (Figure 13). The signal intensity was highly sensitive to temperature because of relaxation of excited V_O⁻ spin states on lattice phonon vibrations [79]. Concentration of V_O⁻ in MOS increases with increasing E_M-O (Figure 14a). The concentration was calculated per unit surface area (Table 2), although the sensitivity to phonon vibrations suggests that oxygen vacancies are mainly inside the oxide nanoparticles. Nevertheless, the values normalized per 1 mole of MOS followed the same tendency as in Figure 14a. Noteworthy, oxygen vacancies were detected in oxides under ambient conditions. The increasing concentration of V_O⁻ in SnO₂, TiO₂ and WO₃ agrees with the renowned oxygen deficiency of these compounds, e.g., the existence of Magneli phases (Ti_nO_2n−1) and numerous WO_3-x phases [99]. The stability of oxygen vacancies in MOS with high E_M-O can be explained by the relatively high electronegativity of the cations which can trap the loosely bound electrons, and, thus, be reduced to Sn²⁺ (and Sn³⁺ [100]), Ti³⁺, W⁵⁺ (and W⁴⁺), respectively. The formation of oxygen vacancy reduces the coordination number of cations, which is favorable for stronger (more covalent) bonding of electronegative cations with oxygen anions. On the other hand, the more stable are oxygen vacancies, the less readily should they donate electrons to chemisorbed acceptor molecules like O₂. Therefore, the concentration of chemisorbed oxygen (including ionosorbates O₂⁻) is lower on the surface of these oxides (Figure 14b). Oxidation of gas molecules (e.g., H₂ in TPR) on the surface of oxides which are prone to oxygen deficiency is likely limited by the interaction with lattice oxygen anions (Equation (6)), i.e., described by Mars-van Krevelen mechanism in heterogeneous catalysis or oxygen vacancy model of sensor response.

3.3. Oxidizing Sites (Chemisorbed Oxygen)

The paramagnetic ionosorbates O₂⁻ with the triplet EPR signal (g₁ = 2.00, g₂ = 2.01, g₃ = 2.02–2.03 [25,103]) were observed on EPR spectra of nanocrystalline In₂O₃ [94,97], ZnO [102], SnO₂ and TiO₂ [96,101,104] (Figure 13). The concentration of O₂⁻ at the oxides surfaces (10⁻²–10⁻⁵ micromole/m²) was by 2–6 orders of magnitude lower than that of oxidizing sites evaluated by TPR (Table 2). Hence, the active sites responsible for probe molecules oxidation were mainly constituted by other adsorbed and/or oxygen species and OH-groups, rather than O₂⁻. The small concentration of O₂⁻ 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 × 10⁻²–1.7 × 10⁻¹ micromole/m² assuming that O₂⁻ has normal orientation and radius close to r(O⁻) = 1.76 Å [8]. The concentration of O₂⁻ determined by EPR decreased as the metal-oxygen bond energy increased (Figure 14b). As an assumption, from the oxides with lower E_M-O (ZnO, In₂O₃) lattice oxygen anions are desorbed into gas phase (Equation (1)). Thus formed oxygen vacancies are unstable due to low electronegativity of the cations In³⁺, Zn²⁺ and readily adsorb oxygen, including the formation of O₂⁻ (Equation (2)). Noteworthy, the fraction of O₂⁻ in oxidizing sites concentration was larger in In₂O₃: n(O₂⁻):2n(H_2(TPR)) ≈ 4 × 10⁻². Thus, the donor sites (oxygen vacancies) at the surface of oxides with low E_M-O are mostly blocked by chemisorbed oxygen species from air; it limits the surface population by other adsorbates (e.g., neutral adsorbed O₂, OH-groups) which could serve as the oxidizing surface sites.

Oxidizing surface sites were evaluated using TPR with hydrogen [63,75,76,78,79]. TPR profiles are shown in Figure 15. The oxides SnO₂, In₂O₃ and WO₃ were reduced to metals at temperature 400–900 °C. Temperature of reduction rates maxima (T_m) shifted from T_m = 500–660 °C for In₂O₃ and SnO₂ to T_m = 700–900 °C for WO₃, which agrees with increasing thermodynamic stability of the oxides, as follows from increasing E_M-O and decreasing formation enthalpy (Table 1). The maxima of bulk reduction rates of SnO₂ and WO₃ also shifted to higher temperature with the increase of particle size. It can be due to higher thermodynamic stability with increasing crystallinity, as well as due slower reduction kinetics of larger MOS nanoparticles. TiO₂ cannot be reduced completely by hydrogen, and the reduction of ZnO was not completed under the TPR conditions to prevent the evolution of Zn vapor. Low intense hydrogen consumption bands below 300 °C were attributed to the reduction of oxidizing surface sites like chemisorbed oxygen (O_2(ads), O₂⁻, O⁻), surface oxygen anions (O²⁻) and hydroxyl species:

O_n^m−_(surf) + 2n H_2(g) = 2n H₂O_(g) + m e⁻_, (n = 1, 2; m = 0, 1, 2)

OH⁻_(surf) + 1/2 H_2(g) = H₂O_(g) + e⁻

(8)

Concentration of oxidizing surface sites n(H_2,TPR) in Table 2 was calculated equivalent to micromoles of H₂ consumed per 1 m² of surface area. It was shown to be affected by the microstructure parameters. On the examples of SnO₂ or TiO₂ it can be seen that the increment of particle size and decrease of BET area results in the drop of n(H_2,TPR) (Table 2). It can be assumed that smaller nanoparticles possess more defect-site atoms (at the edges, steps, corners, etc.) which hold the adsorbed oxygen and OH-groups more strongly (or, the smaller particles are more easily reducible) than regular atoms on a crystal surface, and the number of adsorbates per unit surface area increases. To estimate the effect of the metal oxide composition on the surface reducibility, the n(H_2(TPR)) values were plotted in relation to metal-oxygen bond energies in Figure 14c. Noteworthy, the higher concentration of oxidizing sites was found for the oxides with intermediate E_M-O (SnO₂, TiO₂), in comparison to In₂O₃ (lower E_In-O) and WO₃ (higher E_W-O). The origin of the volcano-shape dependence may be deduced from the trends of descending O₂⁻ (Figure 14b) and growing V_O⁻ concentrations with increasing E_M-O (Figure 14a). The enhanced surface reducibility of MOS with intermediate metal-oxygen bond energy might result from a proper balance between oxygen vacancy formation (Equation (1)) and oxygen chemisorption (Equations (2) and (3)). On the one hand, the oxides ZnO, SnO₂, TiO₂ possess oxygen vacancies which are persistent in ambient air (in contrast to those in In₂O₃) and may serve as adsorption sites for oxygen species (including O₂⁻ observed by EPR) and OH-groups. On the other hand, oxygen vacancies associated with the cations Zn²⁺, Sn⁴⁺, Ti⁴⁺ should be rather unstable and adsorptive, in comparison to those associated by electronegative W⁶⁺ cations in oxygen-deficient WO₃.

4. From Simple to Mixed-Metal Oxides: Metal-Oxygen Bond Energy and Active Sites

The complication of chemical composition of MOS is a powerful tool for tailoring the functional properties, including gas sensing behavior. The impurities may be introduced into the bulk of metal oxides or deposited onto the surface. Bulk additives can be grouped into dopants which do not alter the phase composition (e.g., Sb-doped SnO₂ [105]; Sn-doped In₂O₃ [106]; Nb-doped TiO₂ [107], etc.), and second components which condition the formation of new phases. Being incorporated into crystal lattice, dopants have minor effect on surface reactivity and are used for regulation of electronic properties of MOS. In this section, we focus on the examples of mixed-metal oxides which can be considered as derivatives of simple MOS, but exists in distinct crystalline phases: BaSnO₃ in relation to SnO₂, and Bi₂WO₆ in relation to WO₃. The mixed-metal oxides were synthesized under hydrothermal conditions starting from freshly deposited SnO₂∙xH₂O and H₂WO₄. The former was mixed with Ba(OH)₂ to obtain BaSnO₃, and the latter—with Bi(OH)₃ to obtain Bi₂WO₆ [63,77].

4.1. Crystal Structure and Metal-Oxygen Bonding

BaSnO₃ has a perovskite-like cubic structure, which differs from the tetragonal rutile structure of SnO₂ (Figure 16a). The structure of Aurivillius phase Bi₂WO₆ consists of alternating fluorite-like (Bi₂O₂)²⁺ and the perovskite-like (WO₄)²⁻ layers. The latter are comprised by corner-shared octahedra {WO₆}, which is relative to the structure of WO₃ (Figure 16b). Semiconductor properties of the pairs of compounds BaSnO₃–SnO₂ and Bi₂WO₆–WO₃ are similar: the simple and mixed-metal oxides are n-type semiconductors with close bandgap widths of E_g = 3.4–3.6 eV (BaSnO₃, SnO₂) [61,108] and E_g = 2.7–2.8 eV (Bi₂WO₆, WO₃) [109,110]. Thus, the introduction of cations Ba²⁺ or Bi³⁺ and the accordant change of crystalline phases when the mixed-metal oxides are formed from SnO₂ and WO₃, respectively, do not affect semiconductor properties. The latter are determined by the framework of interconnected octahedra {SnO₆} or {WO₆}, which exist in the structures of simple oxides and mixed-metal oxides (Figure 16). Actually, DFT simulations showed that valence band maxima (VBM) in these compounds are mainly contributed by filled non-bonding O 2p-states [63,111,112]. The conduction band maximum (CBM) in SnO₂ is mainly due to antibonding Sn 5s and O 2p states; and they are slightly contributed by Ba 6s states in the CBM of BaSnO₃. The antibonding Sn 5s- and Ba 6s-orbitals are empty in both cases, which does not deteriorate metal-oxygen bonding in SnO₂ and BaSnO₃. Nevertheless, a slightly weaker Sn-O bonding in BaSnO₃ can be deduced, respective to SnO₂. Firstly, Sn-O distance is a little longer in BaSnO₃ (2.06 Å [113]), respective to SnO₂ (2.02 Å [61]). Secondly, from the TPR patterns (Figure 15) it can be inferred that bulk reduction of BaSnO₃ started at a lower temperature (about 390 °C), than that of SnO₂ (430 °C), judging by the data for materials with comparable particles sizes. Tin oxide was completely reduced in one step with the maximum rate of H₂ consumption at T_m = 600–660 °C:

SnO_2(s) + 2 H_2(g) = Sn_(l) + 2 H₂O_(g)

(9)

Although BaSnO₃ was incompletely reduced during TPR, the 1st hydrogen consumption peak centered at T_m = 500 °C was due to partial reduction of the oxide and, thus, corresponded to the cleavage of Sn-O bonds in the crystal lattice:

BaSnO₃(s) + ½ H_2(g) = ½ Ba₂SnO_4(s) + ½ SnO_(s) + ½ H₂O_(s)

(10)

The 2nd peak on the TPR pattern of BaSnO₃ with the maximum at 890 °C was due to reduction of residual SnO.

In WO₃ and Bi₂WO₆, the CBM is equally impacted by W 5d- and O 2p-states [63]. The empty Bi 6p-states do not contribute to CBM, but the filled Bi 6s states do affect the VMB. The Bi 6s–O 2p interaction is antibonding, which decreases the energetic stability of Bi₂WO₆ and diminishes W-O bond energy in the structure, respective to WO₃. It follows from the comparison of integral partial crystal orbital Hamilton population (-IpCOHP) as a measure of bond energy, bond lengths and partial charges (Bader charge analysis) of atoms. Bi₂WO₆ possesses lower W-O bond energy (lower-IpCOHP), longer average W-O distance and increased ionicity of W-O bonds (higher partial atomic charges), in comparison to WO₃ [63]. In agreement with the first-principles calculations, TPR data support the lower W-O bond stability in Bi₂WO₆ than in WO₃ (Figure 15). Reduction of WO₃ proceeded sequentially [63,75]:

WO_3(s) + H_2(g) = WO_2(s) + H₂O_(g), T_m = 700 °C

(11)

WO_2(s) + 2 H_2(g) = W_(s) + 2 H₂O_(g), T_m = 880 °C

(12)

In Bi₂WO₆, bismuth was first reduced to metallic Bi at temperature T_m = 550 °C:

Bi₂WO_6(s) + 3 H_2(g) = 2 Bi_(l) + WO_3(s) + 3 H₂O_(g)

(13)

The peaks of two-step reduction of residual WO₃ were observed at T_m = 680 °C and T_m = 780 °C, i.e., at lower temperature than the respective ones for pristine WO₃ (Figure 15). It indicates on the lower W-O bonds stability in the structure of Bi₂WO₆, in comparison to WO₃ [63].

4.2. Concentration of Active Sites

The concentration of oxidizing surface sites estimated by hydrogen consumption in the lower temperature intervals (below 300 °C) on TPR patterns (Table 2) was close for SnO₂ and BaSnO₃, if samples with similar particle sizes were compared. The higher surface reducibility was observed for Bi₂WO₆, respective to WO₃, which agrees with lower metal-oxygen bonds energy in the former.

Surface acidity of MOS is strongly influenced by the constituent cations. The concentration of Brønsted and Lewis acid sites evaluated by TPD was lower at the surfaces of BaSnO₃ and Bi₂WO₆, in comparison to SnO₂ and WO₃, respectively (Table 2). It was shown that microstructure had minor impact on the difference of surface acidity between Bi₂WO₆ and WO₃; it was mainly due to distinct chemical composition [63]. The lower acidity of mixed-metal oxides was attributed to the presence of Ba²⁺ or Bi³⁺ cations at the surfaces of BaSnO₃ and Bi₂WO₆, provided that these cations have lower charge/radius ratio and, hence, lower electronegativity and weaker Lewis acidity than Sn⁴⁺ in pristine SnO₂ or, moreover, W⁶⁺ in pristine WO₃.

5. Impact of Active Sites on Gas Sensitivity of Nanocrystalline n-Type MOS

In this section, the correlations in active sites concentrations and the sensitivities of nanocrystalline n-type MOS to four analyte gases (NH₃, CO, VOCs, NO₂) are discussed based on our previously published research works. These correlations were deduced from the plots of sensitivity vs. metal-oxygen bond energy (Figure 17, Figure 18, Figure 19 and Figure 20) compared with the relations of active sites concentrations and E_M-O (Figure 12 and Figure 14). The specific cases of improved selectivity of mixed-metal oxides to SO₂ and H₂S are outlined.

The sensitivity (S) was defined as the relative change of resistance in presence of reducing (Red) or oxidizing (Ox) gases:

S_Red = (R_air−R_Red)/R_Red,

S_Ox = (R_Ox−R_air)/R_air

(14)

where R_air is sensor resistance in air, R_Red—resistance in presence of reducing gases (NH₃, CO, VOCs, SO₂, H₂S, etc.), R_Ox—resistance in presence of oxidizing gas (NO₂). It known that the sensitivity in general increases with the increment of specific surface area and the decrease of particle size [22,23,114], and we also observed it on the particular examples of nanocrystalline MOS and analyte gases [42,75]. In the syntheses of nanocrystalline MOS, the microstructural parameters could not be finely matched between different materials due to distinct energetics and kinetics of oxides crystallization. Therefore, in order to uniformly compare the sensitivities we defined an effective sensitivity (S_eff) as the sensitivity normalized per 50 m²/g of the specific surface area of each sensor material [78]:

S_eff = S∙50/SSA

(15)

where SSA is the main value of the specific surface area of materials (Table 2). The value of 50 m²/g was chosen as an average expected SSA of nanocrystalline MOS obtained by the aqueous deposition or hydrothermal routes.

The effective sensitivities of MOS to the fixed concentration of analyte gases were compared in dry air conditions and measured at optimal operation temperature, i.e., at temperature of maximum sensitivity of the sensors to the given analyte gas. The large scattering of the S_eff data in Figure 17, Figure 18, Figure 19 and Figure 20 arouse from the sets of data compared, e.g., samples with same composition and different microstructure parameters, as well as errors in BET area evaluation.

5.1. Sensitivity to Ammonia and Surface Acidity

The sensitivity of different MOS to ammonia tends to increase for the MOS with increasing metal-oxygen bond energy (Figure 17a). Ammonia is a reducing gas, which is oxidized on the surface of pristine MOS mainly to N₂ [31,115,116,117]:

NH_3(ads) + 3/2n O_n^m−_(surf) = ½ N_2(g) + 3/2 H₂O_(g) + 3m/2n e⁻

(16)

although NO_x species were observed by mass-spectral analysis of desorbed gas in TPD of NH₃ from NO₃-contaminated In₂O₃ [78] or doped SnO₂ [116].

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 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.

DRIFT studies showed that basic NH₃ molecules were chemisorbed on the acid sites and the binding with Lewis acid sites (surface cations) was strong enough, so that molecular adsorbates were predominant at the surfaces of MOS exposed to ammonia gas at elevated temperature [75,116,118]. Thus, the oxidation reaction (Equation (16)) which determines the sensor response is preceded by NH₃ chemisorption on surface acid sites. The concentration of acid sites increases with the increment of metal-oxygen bond energy (Figure 12), which agrees with the increasing sensitivity to NH₃ (Figure 17b). Thus, the sensitivity of MOS to NH₃ is contributed by surface acidity that controls the chemisorption of analyte gas molecules.

5.2. Sensitivity to CO and VOCs Impacted by Oxidizing Sites and Acid Sites

The relation of CO sensitivity to metal-oxygen bond energy in MOS (Figure 18a) resembles that of oxidizing sites concentrations estimated by TPR (Figure 14c). Sensor response to CO is due its oxidation by surface oxygen and hydroxyl species at the surface of MOS [75,82,87]:

CO_(g) + 1/n O_n^m−_(surf) = CO_2(g) + m/n e⁻

(17)

CO_(g) + OH_(surf) = CO_2(g) + H⁺_(surf) + e⁻

(18)

Besides CO₂, carbonate species were detected by infrared spectroscopy on the surface of ZnO, In₂O₃ and SnO₂ exposed to CO under certain conditions [5,76,119]. Carbon monoxide has no acid-base properties; hence, its adsorption is not influenced by surface acidity of metal oxides. Thus, it is reasonable that the sensitivity to CO is dependent on the surface reducibility of MOS which reflects the concentration of active species for CO oxidation (Equations (17) and (18)). In agreement with this is the increasing sensitivity with the concentration of oxidizing sites at the MOS surface (Figure 18b).

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 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 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.

The sensitivity to VOCs (methanol, acetone) increased with metal-oxygen bond energy and reached a plateau for SnO₂, TiO₂ and WO₃ (Figure 19) [78]. On the one hand, it agrees with increasing concentration of oxidizing surface sites (Figure 14c). However, in contrast to the case of CO, the sensitivity of WO₃ to VOCs did not drop, in spite of lower concentration of oxidizing sites. DRIFT study of MOS interaction with methanol and acetone vapors indicated that VOCs molecules adsorbed on surface cations and the adsorption enhanced with increasing surface acidity of MOS. It is likely due to Lewis basicity of oxygen atoms in CH₃OH and CH₃COCH₃. At raised temperature that corresponds to sensors operation temperature, the adsorbed VOCs molecules were oxidized with the formation of carboxylate species [78,121]:

CH₃OH_(ads) + 5/2 O_n^m−_(surf) = HCOO⁻_(surf) + 3/2 H₂O_(g) + 5m/2n e⁻

(19)

CH₃COCH_3(ads) + 9/2 O_n^m−_(surf) = CH₃COO⁻_(surf) + CO_2(g) + 3/2 H₂O + 9m/2n e⁻

(20)

Thus, the sensitivity of MOS to oxygen-containing VOCs should be contributed by both processes: adsorption on Lewis acid sites and the adsorbates oxidation by oxidizing surface species (Equations (19) and (20)). The optimal sensitivity to VOCs could be expected for the oxides that have relatively high surface acidity balanced by surface reducibility, as it was observed for SnO₂ and TiO₂ (Figure 19). Then, the persistence of sensitivity in case of WO₃ may be explained by its stronger surface acidity (Figure 12). The lower concentration of oxidizing sites at the surface of WO₃ is compensated by the abundance of acid sites, which enforces the adsorption of VOCs and thus enhances the sensitivity. To illustrate this conclusion, the sensitivity to VOCs was plotted against the sum of Lewis acid sites and oxidizing sites n(H_2,TPR) concentrations in Figure 19c,d.

5.3. Sensitivity to NO₂ Determined by Donor Sites

The sensitivity to NO₂ was found to enhance with increasing metal-oxygen bond energy of MOS (Figure 20), i.e., following the same trend as the sensitivity to NH₃ (Figure 17). It is not an issue of selectivity, since the sensor resistance changes in opposite directions when exposed to oxidizing gas NO₂ or reducing gas NH₃. It has been established that sensing mechanism of NO₂ relies on analyte molecules ionosorption with the formation of NO₂⁻:

NO_2(g) + e⁻ = NO₂⁻_(ads)

(21)

which undergoes disproportionation and/or conversion resulting in NO₃⁻, NO⁺, etc. [41,42,122]. Loosely bound electrons in n-type MOS are associated with oxygen vacancies (Equation (1)), the concentration of which (as suggested for V_O⁻ by EPR) increases in MOS with increasing E_M-O (Figure 14a). Thus, the trend of increasing sensitivity to NO₂ is due to higher concentration of electron donor sites in metal oxides with increasing E_M-O.

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 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).

5.4. Effect of Cations on Sensitivity and Selectivity of Mixed-Metal Oxides

The sensitivities of mixed-metal oxides and simple MOS to sets of various analyte gases are compared in Figure 21 for the pairs BaSnO₃–SnO₂ and Bi₂WO₆–WO₃ [63,77]. Comparing the sensitivities of SnO₂ to definite concentrations of analyte gases (Figure 21a), it may noticed that tin oxide was unselective in the detection of reducing gases and the difference in sensor signals might be attributed to different concentrations of analyte gases (e.g., higher sensitivity to 100 ppm H₂ and lower sensitivities to 2–20 ppm CO, NH₃, H₂S, etc.). On the contrary, BaSnO₃ demonstrated an improved sensitivity selectively to SO₂ and, to a lesser extent, to alcohols; the sensitivity of BaSnO₃ to other reducing gases was comparable with that of SnO₂ (Figure 21a).

DRIFT spectroscopy revealed that the selectivity of BaSnO₃ to SO₂ was related with specific enhancement of target molecules oxidation due to binding the produced sulfate species with the Lewis basic sites (lattice O²⁻ anions) associated with Ba²⁺ cations [77]:

SO_2(g) + 1/n O_n^m−_(surf) + Ba²⁺_(lat) + O²⁻_(lat) = BaSO_4(surf) + m/n e⁻

(22)

Comparative sensing tests of Bi₂WO₆ and WO₃ demonstrated the commonly higher sensitivity of bismuth tungstate to reducing gases [63]. The improved sensitivity of Bi₂WO₆ to VOCs, including formaldehyde and benzene, may be outlined (Figure 21b). Based on DFT simulations of W-O bonding in these structures, the higher sensitivity of Bi₂WO₆ to reducing gases was ascribed to the lower W-O bond strength and, hence, easier W-O cleavage on the surface during the oxidation of analyte gas molecules. On the other hand, the lack of sensitivity to NO₂ was in line with the decreased concentration of donor sites (oxygen vacancies) in Bi₂WO₆, in comparison to WO₃. It was evidenced by XPS evaluations that showed lower content of reduced W⁵⁺ cations in Bi₂WO₆ than in WO₃ [63]. Also, the sensitivity of BaSnO₃ to NO₂ was lower than that of SnO₂ (Figure 21a); it could be due to lower concentration of oxygen vacancies in the mixed-metal oxide. Indirectly, it was evidenced by the higher energy of oxygen vacancy V_O formation in BaSnO₃ (above 4 eV [113]) than that in SnO₂ (3.5 eV [124]). The other evidence for lower concentration of donor states in mixed-metal oxides is the higher baseline sensors resistance, in comparison to simple MOS [63,77]. Aside from the high sensitivity of WO₃ to NO₂, tungsten oxide displayed poor selectivity to reducing gases, as inferred from the comparable sensor signals in the range S_Red = 1–10 for the WO₃ sensor (Figure 21b). On the contrary, Bi₂WO₆ displayed the selectively high sensitivity to H₂S, which was also in the spotlight of other works [39,40]. It should be due to specific H₂S-Bi³⁺ binding between the soft Pearson’s acid (bismuth cation) and base (sulfur atom in H₂S).

6. Crucial Role of Catalytic Clusters in Sensitivity and Selectivity of Functionalized n-Type MOS

Surface functionalization by catalytic noble metal (NM) clusters (Figure 22) is a powerful approach to improve sensor characteristics of MOS. Well-documented advantages of catalytically modified MOS sensors include the enhanced sensitivity, decreased operation temperature, shortened response and recovery times, and improved selectivity to analyte gases [8,9,18,19]. Selectivity is a major issue in the sensing of reducing gases, since most of toxic, pyrogenic, flammable or explosive impurities in air belong to this group. The oxidizing pollutants are few; these molecules that have stronger electron affinity than oxygen, e.g., NO₂, ozone, halogens. Therefore, the search for selective oxidation catalysts is challenging for the functionalization of MOS sensors to improve the sensing behavior in the detection of reducing gases. Based on the Sabatier principle, a proper oxidation catalyst enables the adsorption of reactants (reducing gas and oxygen) and intermediates, as well as desorption of reaction products [64]. The optimal activity and specificity is anticipated when the binding energies with the catalyst surface are balanced between these species [125]. Platinum group metals have moderate adsorption energies for various gases, since almost completed d-shell enables chemical bonding through electron acceptation from adsorbate as well as back-donation to the adsorbate [37]. Elements with less occupied d-orbitals as well as s-, p-, and f-elements easier donate electrons and too strongly adsorb oxygen (resulting in oxide formation) which deteriorates its oxidation activity. The adsorption capacity of elements with fully occupied d-orbitals (Ag, Au) is relatively low and is limited by the formation of electron vacancies in d-shell and nano-size effect [125,126,127].

The additives of NM form new (extrinsic) active sites at the surface of MOS (Figure 5), as well as influence its intrinsic active sites. The immobilized catalyst may impose the route of analyte gas conversion at the surface of MOS, which results in improved selectivity and sensitivity. The sensing mechanism in this case is dominated by the catalytic sites at the functionalized MOS surface [49]. In this section, we summarize our research on the effects of catalytic additives of PdO_x, RuO₂, and Au on the active sites and sensitivity of n-type MOS to CO, NH₃, and VOCs. The concentration of noble metals was fixed at 1 wt.%, and the additives were introduced by impregnation of MOS with alcohol solutions of Pd(acac)₂ or Ru(acac)₃. The impregnated materials were annealed at minimum temperature required for the decomposition of acetylacetonates to maintain the dispersion of NM clusters and prevent aggregation [74,75,117]. Gold was introduced via the colloid adsorption technique in aqueous suspensions of MOS [78].

6.1. Role of PdO_x in Room-Temperature Sensitivity to CO

Palladium- and platinum-based catalysts have long been known as efficient catalysts for CO oxidation. The adsorption energy of oxygen on these metals (340–360 kJ/mole) is comparable to that of CO which is favorable for co-adsorption of reacting species [128]. The additives of Pd and Pt have often been recognized to improve sensing behavior of MOS to CO [33,75,87,129,130].

In our works, the nanocomposites MOS/PdO_x obtained after decomposition of deposited Pd(acac)₂ consisted of agglomerated MOS nanocrystals covered by amorphous PdO_x clusters and nanoparticles with the size 1–15 nm (Figure 22a). Palladium was observed in mixed oxidation states PdO + Pd, with the dominance of the oxidized form [75,87,95]. Investigation of the active sites by surface science techniques suggested that PdO_x induced an increased concentration of aqueous species (adsorbed H₂O, OH-groups, OH∙spin centers) at the surface of SnO₂ [79]. However, no such effect was observed for WO₃-based materials [75].

The notable effect of PdO_x clusters was the increased sensitivity of functionalized MOS sensors to CO (Figure 18). Valuably, the higher sensitivity was observed at room temperature [75,120]. Conventional MOS sensors require raised operation temperature (200–500 °C) [2,4,8,12], because of the activation barrier for analyte gas conversion at the oxide surface (Equations (4) and (6)). At room temperature the sensitivity of MOS-based sensors is usually vanished. Therefore, the outstanding room-temperature sensitivity of MOS/PdO_x can be utilized to improve the selectivity of CO detection and, by the way, to diminish the power consumption of the sensor device. DRIFT studies revealed that CO was chemisorbed specifically on Pd-sites at room temperature (Figure 23). It was deduced that Pd-CO binding loosened the interatomic bond in the adsorbate and thus catalyzed its oxidation by surface oxygen and hydroxyl species [75,118]:

Pd-CO_(ads) + 1/n O_n^m−_(surf) = Pd_(surf) + CO_2(g) + m/n e⁻,
Pd-CO_(ads) + OH_(surf) = Pd_(surf) + CO_2(g) + H⁺_(surf) + e

(23)

Raising temperature suppressed CO chemisorption, which explained the drop of sensitivity to CO with increasing temperature [74,75,120].

6.2. Impact of RuO₂ on Sensitivity and Selectivity to NH₃

Ruthenium in the metallic Ru or oxidized RuO₂ state is an efficient active phase of catalysts for selective processing of ammonia: synthesis, decomposition or oxidation [125,131,132]. DFT simulations suggested that due to moderate Ru-N bond energy, the chemisorption of NH₃ is not too strong and its conversion to NH_x-intermediates and final products (N₂, NO_x) proceeds with low activation barriers [125,133]. The catalytic effect of ruthenium in the oxidation of ammonia was found promising for the improvement of sensitivity and selectivity of Ru-modified MOS sensors [134,135,136].

In the nanocomposites MOS/RuO₂ obtained via thermal decomposition of MOS-impregnated Ru(acac)₃ at 265 °C, the additive was observed in the form of randomly shaped clusters and nanoparticles with the size ranging from 2–20 nm (Figure 22b) to few hundred nm [75,95]. Chemical state analysis indicated on the predominantly RuO₂ state of ruthenium with a fraction of Ru³⁺ [74,75,95,117]. Agreeing results were obtained by TPR of SnO₂- and WO₃-based materials that the concentration of oxidizing sites increased at the surface of Ru-modified MOS [75,79].

Ru-modified MOS demonstrated substantially improved sensitivity and selectivity to NH₃ at raised operation temperature (150–250 °C) [75,117,135,136]. It was demonstrated that SnO₂/RuO₂ was capable to discriminate NH₃ traces in presence of interfering CO gas in air [120]. Comparing different ruthenated metal oxides, an increasing sensitivity to NH₃ was found with increasing E_M-O and surface acidity of MOS (Figure 17). Similarly to the case of pristine MOS, this trend is explained by the increasing adsorption of NH₃ on Lewis acid sites at the oxides surfaces. By DRIFT spectroscopy, the specific route of NH₃ oxidation catalyzed by RuO₂ was evidenced [75,118]. Although NH₃ adsorption was almost unaffected by the NM additives, it was only on the Ru-modified MOS that Ru-bound NO species evolved from the interaction with ammonia (Figure 24) [75,118,136]. Nitrosyl species were further oxidized to NO₂ gas [117]. Thus, the reason for improved sensitivity and selectivity to NH₃ is that supported RuO₂ nanoparticles specifically catalyze deep oxidation of adsorbed ammonia at the sensors surfaces, while in the absence of ruthenium the oxidation of ammonia likely stopped at the stage of N₂ formation:

NH_3(ads) + 5/2n O_n^m−_(surf) + Ru⁴⁺_(surf) = Ru⁴⁺-NO_(ads) + 3/2 H₂O_(g) + 5m/2n e⁻,
Ru⁴⁺-NO_(ads) + 1/n O_n^m−_{(surf) =} NO_2(g) + m/n e⁻

(24)

The distinction between the surface sites of NH₃ adsorption (acid sites) and oxidation (catalytic RuO₂ sites) suggested an idea of independent co-functionalization of MOS surface by acidic and catalytic species for tailoring the sensor characteristics. For this purpose, SnO₂ was loaded by different amounts of sulfate SO₄-groups and RuO₂ clusters [98]. It was demonstrated that introducing the comparable amounts (1–3 wt.%) of the acidic SO₄- and catalytic RuO₂ additives properly enhanced the adsorption of NH₃ balanced by its oxidation, resulting in the optimum sensitivity.

6.3. Au-Promoted Oxidizing Sites and Sensitivity to VOCs

Gold nanoparticles supported on metal oxides have been in focus of extensive research in catalysis [37,72,126]. Although Au itself is inactive in the bulk state, it was reported that the catalytic activity of nanosized gold exceeded that of platinum group metals in low-temperature CO oxidation [126]. Gold nano-catalysts were also shown to be efficient for total oxidation of VOCs [137,138]. Functionalization of MOS sensors by Au resulted in improved sensitivity to CO [139] and VOCs [139,140], which was also attributed to catalytic activity of Au, electronic cluster-support interaction and spillover of oxygen. Catalytic activity of Au strongly depends on cluster size and on the support. Reducible metal oxides, including MOS, are the proper supports enabling the adhesion of Au clusters and active oxygen species for the catalytic reaction cycle [141]. Among the various metal oxides, TiO₂-supported Au catalysts have attracted the great interest [72,92,126,142,143]. The higher extent of surface oxygen activation on TiO₂/Au was ascribed to the proper Ti-O bond energy favorable for facile interplay between oxygen vacancy formation and oxygen chemisorption from gas phase [144]. However, the origin of active sites in oxide-supported Au catalysts has still been questionable: periphery Au and O atoms, charged Au+ species, Au-OH groups, etc., were suggested [92,126,145].

Functionalization of MOS surfaces by small gold clusters is challenging, since the mobility of Au atoms induces the aggregation of gold in larger nanoparticles at raised operation temperature of sensors. Furthermore, chloride ions evolved from the precursor HAuCl₄ decomposition promote the aggregation of gold. To minimize it, the colloid adsorption technique of MOS modification by freshly deposited Au(OH)₃ is preferable, rather than the impregnation by HAuCl₄ [126]. Recently, we reported on a comparative study of Au-modified MOS sensors obtained by colloid adsorption and thermal decomposition of Au(OH)₃ at above 200 °C [78]. The additive was observed in the metallic state Au⁰ in the form of nanoparticles with the size 5–30 nm. Investigation of active sites showed that in presence of Au the concentration of oxidizing sites increased at the surfaces of n-type MOS, and the relation n(H_2,TPR –E_M-O had the volcano shape with the maximum for TiO₂/Au (Figure 25a), similarly to that for pristine MOS (Figure 14c).

The Au-functionalized sensors displayed higher sensitivity to methanol and acetone, respective to pristine MOS (Figure 25b,c). Noteworthy, the dependence of VOCs sensitivity on E_M-O in MOS was similar to that of oxidizing sites concentration (Figure 25a), i.e., with the maximum for TiO₂/Au. On the other hand, it was dissimilar to the relation VOCs sensitivity–E_M-O for pristine MOS which had a plateau for SnO₂, TiO₂, and WO₃ (Figure 19a,b). The plateau could be ascribed to increased adsorption of VOCs at the abundant acid sites of WO₃ which compensated the lower oxidizing sites concentration at its surface. The sensitivity of Au-functionalized MOS to VOCs was less dependent on surface acidity and to a larger extent determined by the concentration of oxidizing sites (Figure 25d,e). DRIFT measurements showed no significant changes in the oxidation routes of CH₃OH and CH₃COCH₃ on Au-functionalized MOS, respective to pristine oxides (Equations (19) and (20)) [78]. Thus, in presence of Au the oxidation of VOCs by surface oxygen was enhanced leading to higher sensitivity, but the sensing mechanism was not altered. The volcano-shaped relation with the maximum for TiO₂/Au allows assuming that the oxidation reactions (Equations (19) and (20)) influence the sensitivity of MOS/Au to a larger extent, than the adsorption on the acid sites of MOS. It is in line with the concept of Au-promoted surface oxygen activation, which was most noticeable on titania-supported catalysts [144].

7. Conclusions

In this review of the authors’ research on sensor materials based on nanocrystalline n-type MOS (In₂O₃, ZnO, SnO₂, TiO₂, WO₃) with particle sizes in the 3–50 nm range, the direct correlations between material composition, concentrations of active sites at the surface, and gas sensing behavior was substantiated. The active sites were classified as acid sites of Brønsted type (acidic OH-groups) and Lewis type (coordinately unsaturated cations), oxidizing sites (adsorbed oxygen, OH-species, and/or surface oxygen anions), and donor sites (oxygen vacancies and/or reduced cations). In line with the increment of charge/radius ratio and electronegativity of the constituent cations, the metal-oxygen bond energy reported in literature and surface acidity probed by TPD of ammonia increase in the order: In₂O₃, ZnO, SnO₂, TiO₂, WO₃. Concentration of oxidizing sites evaluated by TPR was higher at the surface of the oxides with moderate E_M-O (SnO₂, TiO₂). Probably, it was due to interplay of two opposite trends: enhancing oxygen vacancies formation and decreasing oxygen chemisorption with the increase of E_M-O, as was shown by the concentrations of paramagnetic sites V_O⁻ and O₂⁻ measured by EPR. The extrinsic catalytically active sites were formed via the functionalization of MOS surfaces by PdO_x and RuO₂ clusters with the size 1–20 nm, and Au nanoparticles with the size 5–30 nm surfaces. Mixed-metal oxides BaSnO₃ and Bi₂WO₆ obtained by hydrothermal synthesis were considered in comparison with SnO₂ and WO₃, respectively. In the pairs BaSnO₃–SnO₂ and Bi₂WO₆–WO₃ there are similar structural units and semiconductor properties determined by the Sn-O and W-O networks, respectively. The introduction of Ba²⁺ slightly elongated Sn-O bond in BaSnO₃, respective to SnO₂. First-principles analysis showed that the filled 6s-level of Bi³⁺ deteriorates the stability of crystal structure and W-O bonding in Bi₂WO₆, in comparison to WO₃. Due to the relatively low charge/radius ratio and electronegativity of Ba²⁺ and Bi³⁺ cations, the concertation of acid sites was lower at the surfaces of mixed-metal oxides, compared with SnO₂ and WO₃. Due to lower W-O bond energy, Bi₂WO₆ had higher oxidizing surface reactivity than WO₃.

The trends in active sites concentration and sensitivity to different analyte gases (NH₃, NO₂, CO, VOCs) were compared for MOS using metal-oxygen bond energy as a descriptor parameter. It was shown that sensitivity of pristine and RuO₂-modified MOS to NH₃ increased with surface acidity reaching maximum for WO₃-based sensors. Diffuse-reflectance infrared spectroscopy suggested that it was due to enhanced adsorption of NH₃ on Lewis acid sites that favors the oxidation of adsorbed molecules and sensor response formation. The sensitivity to CO which has no acid-base behavior is likely determined by the oxidation reaction at the surface. The sensitivity of pristine and PdO_x-modified MOS to CO correlated with the concentration of oxidizing sites which was higher for SnO₂ and TiO₂. The sensitivity to VOCs molecules possessing Lewis-basic O atoms (methanol, acetone) was contributed by the processes of adsorption on acid sites and the conversion on oxidizing sites. For pristine MOS, the sensitivity to VOCs reached a plateau for SnO₂, TiO₂, WO₃. The higher surface acidity of WO₃ and enhanced adsorption of VOCs could compensate its lower oxidizing sites concentration, respective to SnO₂ and TiO₂. The sensitivity to oxidizing gas NO₂ increased with the increment of donor sites concentration (oxygen vacancies) in MOS with the maximum for pristine WO₃.

Changing the composition of sensor materials modifies the metal-oxygen bonding and intrinsic active sites, as well as results in the formation of specific active sites and improved selectivity. Mixed-metal oxides BaSnO₃ and Bi₂WO₆ possessed lower concentration of donor sites and decreased sensitivity to NO₂, respective to SnO₂ and WO₃. The presence of Ba²⁺ cations at the surface of BaSnO₃ favored the oxidation of sulfur dioxide to sulfate-species, which provided the selectively high sensitivity to SO₂. The lower W-O bond energy and higher surface reducibility of Bi₂WO₆ correlated with its improved sensitivity to reducing gases including VOCs, in comparison to WO₃. The selectivity of Bi₂WO₆ to H₂S should be due to specific binding of hydrogen sulfide to Bi³⁺ cations.

Clusters and nanoparticles of noble metals (or noble metal oxides) with specific catalytic activity in the oxidation of certain gas molecules can significantly change the sensing mechanism. Pd sites specifically bind CO which imposes the sensitivity and selectivity of Pd-functionalized MOS to carbon monoxide at room temperature. Moreover, the lack of target molecules adsorption at raised temperature determines the drop of MOS/PdO_x sensitivity to CO. RuO₂ clusters specifically catalyze oxidation of NH₃ adsorbed at the surface of MOS producing NO_x. It leads to a pronounced improvement in the selectivity to ammonia for RuO₂-modified sensors, and the sensitivity increases with the surface acidity of MOS reaching maximum for WO₃/RuO₂. Gold nanoparticles promote oxidizing sites at the surface of MOS, likely due to activation of surface oxygen species. It results in improved sensitivity to reducing VOCs molecules. Although the route of methanol and acetone conversion did not change in presence of Au, the sensitivity of Au-functionalized sensors was mainly determined by the oxidation reaction and was higher for the SnO₂/Au and TiO₂/Au nanocomposites.

Further research is necessary to improve fundamental understanding of the interrelation “materials composition—active sites at the surface—sensing behavior”. We believe that surface science techniques like TPR, TPD, infrared spectroscopy (FTIR, DRIFT) are worth being included into the portfolio of characterization tools for sensing materials. Finding out the correlations between sensitivities to analyte gases and concentrations of adsorptive (acid/base) and reactive (redox) surface sites is informative on the processes which determine the sensitivity and selectivity, and DRIFT spectroscopy sheds the light on sensing mechanisms. The possible directions of future research on the active sites and gas sensitivity of nanocrystalline MOS include but are not limited to the following ones. (i) Design of new sensor materials based on simple MOS, mixed-metal oxides, and composites possessing specific active sites at the surface. The parameters that could be varied are the chemical composition, crystal structure and facets, morphology, concentration of additives. Experimental approaches for creating and controlling the concentrations of acid sites, reactive surface oxygen and oxygen vacancies should be developed, e.g., thermal quenching, chemical etching, surface modification, etc. It is challenging to stabilize the active sites in air under heating, so as to maintain the sensors stability during long-term operation. (ii) Extending the research of mixed-metal oxides, evaluation of the effects of cationic composition on metal-oxygen bonding, active sites, semiconductor properties and selectivity of gas—solid interaction. (iii) The search for new catalytic additives with specific reactivity towards redox conversion of definite analyte gases is challenging for theoretical and experimental studies, since the catalytic functionalization of MOS sensors is a powerful approach to improve selectivity and specify the temperature regime of gas sensing. Besides the conventionally used additives of noble metals and transition metal oxides, the potentially efficient catalysts may be found among bimetallic clusters, organically hybridized complexes, metal-organic frameworks, chalcogenides, carbon-based compounds, etc. The main issue would be stability of the catalytic additives under sensors operation conditions. Metal-oxygen bonding and active sites at the surface of MOS influence the cluster/support interplay which conditions the successive catalytic functionalization of sensors. Hence, different MOS should be comparatively investigated as the gas sensitive composites with a certain catalyst. (iv) Development of in situ and operando techniques for the revelation of kinetics and energetics of gas molecules interaction which with the active sites at the MOS sensors surfaces.