Chemical Gas Sensors Studied at SENSOR Lab, Brescia (Italy): From Conventional to Energy-Efficient and Biocompatible Composite Structures

In this paper, we present the investigations on metal oxide-based gas sensors considering the works performed at SENSOR lab, University of Brescia (Italy). We reported the developments in synthesis techniques for the preparation of doped and functionalized low-dimensional metal oxide materials. Furthermore, we discussed our achievements in the fabrication of heterostructures with unique functional features. In particular, we focused on the strategies to improve the sensing performance of metal oxides at relatively low operating temperatures. We presented our studies on surface photoactivation of sensing structures considering the application of biocompatible materials in the architecture of the functional devices as well.


Introduction
Chemical gas sensors based on oxide materials are among the most studied structures for the manufacturing of high-performance sensing systems. The electrical properties of metal oxides are changed due to the adsorption/desorption processes of different gases on their surface [1]. The aforementioned phenomena are the basis of the operation of conductometric gas sensors [2,3]. The capability of oxide materials to detect a broad range of gaseous compounds and the increasing demand to provide safety in our life ensure continuous investigations carried out by several scientific groups to develop high-performance gas sensors. The studies have been performed on the design and manufacturing of mobile and low power consumption sensing devices. In this regard, nanomaterials with their unique electrical properties are going to replace the traditional thick-film structures in the architecture of gas sensors [4,5]. Therefore, cheap fabrication techniques should be developed to reduce the size and prime-cost of metal oxide gas sensors. The optimization of conventional synthesis methods and the studies on the tuning of oxide materials properties using different strategies have been carried out and up to now.
In this Special Issue dedicated to the State of the art of Sensors in Italy, we report our activities on the fabrication of chemical gas sensors performed at the SENSOR Lab in Brescia. In this section, we briefly presented the working principles and the importance of chemical gas sensors. Then, we presented our strategies for the synthesis and integration of metal oxide nanomaterials in sensing devices considering the choice of the substrate material. In particular, we focused on the achievements Recently, cellulose-based structures have attracted a remarkable interest in the field of functional materials and devices due to their intrinsic properties and environmentally friendly character [24][25][26][27]. Cellulose is the most abundant polymer in nature. It is formed by monomers of glucose bounded by β→1,4 links and may be produced by plants, tunicates, fungi, and bacteria. Bacterial, or microbial, cellulose has different properties from plant-cellulose. It is produced by acetic acid bacteria from the Acetobacteraceae family. These aerobic gram-negative bacteria actively ferment at temperatures between 25 and 30 °C and pH from 3 to 7 using saccharides as carbon sources. In particular, the bacterial cellulose (BC) is composed of three-dimensional hierarchical structures presenting a very high degree of polymerization and crystallinity. This can be attributed to the clenched organization of parallel fiber molecules which width is within 5-50 nm [24,25]. The first level of fiber connection is due to the H-bond interaction, which is quite important since it directly affects the physical and mechanical properties of the final BC layer (Figure 1a). To have specific properties the resulting microbial cellulose can be tailored by controlling synthesis methods (bacterial physiology and the growth condition). In water containing nutrition, the bacteria will create tiny fibers that will be extruded parallel to the long axis of the cell and vowel together to a gelatinous mat at the air/water interface. These films can be used as substrates for multiple applications [26][27][28]. BC was used for the fabrication of UV light-activated gas sensors [28]. Figures 2b and 2c show the ZnO nanomaterial deposited on the BC film and the optical image of the sensor obtained on the BC.
Meanwhile, it is important to consider the thermal stability of BC for its integration in real devices like the smart substrates for chemical sensors [29]. The major advantage of BC is the absence of hemicellulose that degrades at lower temperatures and cannot withstand temperatures above 180 Recently, cellulose-based structures have attracted a remarkable interest in the field of functional materials and devices due to their intrinsic properties and environmentally friendly character [24][25][26][27]. Cellulose is the most abundant polymer in nature. It is formed by monomers of glucose bounded by β→1,4 links and may be produced by plants, tunicates, fungi, and bacteria. Bacterial, or microbial, cellulose has different properties from plant-cellulose. It is produced by acetic acid bacteria from the Acetobacteraceae family. These aerobic gram-negative bacteria actively ferment at temperatures between 25 and 30 • C and pH from 3 to 7 using saccharides as carbon sources. In particular, the bacterial cellulose (BC) is composed of three-dimensional hierarchical structures presenting a very high degree of polymerization and crystallinity. This can be attributed to the clenched organization of parallel fiber molecules which width is within 5-50 nm [24,25]. The first level of fiber connection is due to the H-bond interaction, which is quite important since it directly affects the physical and mechanical properties of the final BC layer (Figure 1a). To have specific properties the resulting microbial cellulose can be tailored by controlling synthesis methods (bacterial physiology and the growth condition). In water containing nutrition, the bacteria will create tiny fibers that will be extruded parallel to the long axis of the cell and vowel together to a gelatinous mat at the air/water interface. These films can be used as substrates for multiple applications [26][27][28]. BC was used for the fabrication of UV light-activated gas sensors [28]. Figure 2b,c show the ZnO nanomaterial deposited on the BC film and the optical image of the sensor obtained on the BC.
°C [30]. The thermal properties including the thermal stability of BC were investigated by thermogravimetric analysis (TGA) [31]. The maximum processing and working temperature for functional devices integrating with BC must be below 300 °C [31]. It is also worth noting the possibility of this material to be functionalized by a broad range of reactions due to the high reactive surface covered by -OH sites [32]. The aforementioned achievement opens new perspectives for the development of novel sensing devices using BC as a substrate material.  [27], (b) ZnO film deposited on the bacterial cellulose (BC) with the interdigitated electrodes over the oxide material surface, (c) a ZnO-based sensor obtained on the BC and mounted on a case (TO39 package from Schott Electronic Packaging Asia Pte. Ltd.) by gold wires.

Synthesis of Low-Dimensional Materials
After one of the earlier works, where the synthesis of SnO2 based on tin rheotaxial growth and its thermal oxidation was reported (RGTO technique) [33], we used different methods to fabricate highly crystalline nanomaterials with the control of their growth process. Our studies have been mainly focused on the synthesis of 1-dimensional (1D) metal oxides, such as nanowires, nanorods, and nanotubes. 1D nanostructures have been recognized widely as materials of importance in chemical gas sensing applications, due to their nanoscale morphology, good physical and chemical properties. The bottom-up approach was the basis of the techniques for the preparation of 1D materials. This approach is based on the assembly of molecular building blocks or chemical synthesis procedures through the vapor phase transport, electrochemical deposition, and template-based growth. Vapor phase growth was one of the main techniques used in our laboratory to fabricate 1D Meanwhile, it is important to consider the thermal stability of BC for its integration in real devices like the smart substrates for chemical sensors [29]. The major advantage of BC is the absence of hemicellulose that degrades at lower temperatures and cannot withstand temperatures above 180 • C [30]. The thermal properties including the thermal stability of BC were investigated by thermogravimetric analysis (TGA) [31]. The maximum processing and working temperature for functional devices integrating with BC must be below 300 • C [31]. It is also worth noting the possibility of this material to be functionalized by a broad range of reactions due to the high reactive surface covered by -OH sites [32]. The aforementioned achievement opens new perspectives for the development of novel sensing devices using BC as a substrate material.

Synthesis of Low-Dimensional Materials
After one of the earlier works, where the synthesis of SnO 2 based on tin rheotaxial growth and its thermal oxidation was reported (RGTO technique) [33], we used different methods to fabricate highly crystalline nanomaterials with the control of their growth process. Our studies have been mainly focused on the synthesis of 1-dimensional (1D) metal oxides, such as nanowires, nanorods, and nanotubes. 1D nanostructures have been recognized widely as materials of importance in Sensors 2020, 20, 579 5 of 23 chemical gas sensing applications, due to their nanoscale morphology, good physical and chemical properties. The bottom-up approach was the basis of the techniques for the preparation of 1D materials. This approach is based on the assembly of molecular building blocks or chemical synthesis procedures through the vapor phase transport, electrochemical deposition, and template-based growth. Vapor phase growth was one of the main techniques used in our laboratory to fabricate 1D metal oxide nanomaterials. In this system, the evaporation of the material is performed in a tubular furnace, where the evaporated source material is transported by a gas carrier toward the colder region and condensates/nucleates on growth sites [8]. The condensation happens according to vapor-solid (VS) or vapor-liquid-solid (VLS) mechanisms. vs. growth may occur without catalytic liquid metals. In this case, vapor and solid phases are involved in the growth procedure forming crystalline nanostructures. Instead, the catalyst materials (mainly noble metals) may be deposited on the substrates to support the VLS growth process. These catalyst particles become liquid substances at elevated temperatures and the reactive vapors of source materials precipitate forming crystalline structures of metal oxides. The dimensions of the deposited catalyst clusters may determine the shape and the size of the produced nanomaterials [8]. This control effect on the morphological parameters by the catalyst material becomes weaker as the clusters dimensions increase [34].
Among the 1D metal oxide nanomaterials, nanowires are one of the most promising structures for the manufacturing of gas sensor devices due to their high-surface-area-to-volume and unique electrical properties [8]. SnO 2 nanowires were successfully obtained with and without depositing platinum (Pt) clusters on the substrates as a catalyst material. SnO 2 powder was used as the source material. The temperature of the substrates was ranging from 430 to 470 • C. The Pt clusters play a crucial role in the synthesis procedure acting as nucleation sites and promoting the growth of SnO 2 nanowires ( Figure 3) [35][36][37]. The nanowires obtained using the Pt as a catalyst material were thinner compared to the once obtained without seed layers. In addition, the size and density of nanowires were possible to control by controlling the dimensions and the density of seeds [35]. The gold (Au) was used as a catalyst for the fabrication of ZnO nanomaterials [38]. The synthesis procedure was performed in the tubular furnace at the temperature range from 400 to 500 • C using a relatively short deposition time (20 min). The morphological analyses showed that ZnO nanowires with high-density were obtained. Single-crystalline In 2 O 3 nanowires were fabricated using Au catalytic layers as well [39]. The nucleation and growth mechanism of In 2 O 3 nanowires was studied. The experimental analysis showed that the VLS and vs. mechanisms affect the elongation of wires depending on the condensation temperature. While the vs. mechanism influences the lateral enlargement of nanowires. Moreover, the Au catalyst was more efficient compared to the Pt and palladium (Pd) to improve the growth process of NiO nanowire [40]. metal oxide nanomaterials. In this system, the evaporation of the material is performed in a tubular furnace, where the evaporated source material is transported by a gas carrier toward the colder region and condensates/nucleates on growth sites [8]. The condensation happens according to vapor-solid (VS) or vapor-liquid-solid (VLS) mechanisms. vs. growth may occur without catalytic liquid metals. In this case, vapor and solid phases are involved in the growth procedure forming crystalline nanostructures. Instead, the catalyst materials (mainly noble metals) may be deposited on the substrates to support the VLS growth process. These catalyst particles become liquid substances at elevated temperatures and the reactive vapors of source materials precipitate forming crystalline structures of metal oxides. The dimensions of the deposited catalyst clusters may determine the shape and the size of the produced nanomaterials [8]. This control effect on the morphological parameters by the catalyst material becomes weaker as the clusters dimensions increase [34]. Among the 1D metal oxide nanomaterials, nanowires are one of the most promising structures for the manufacturing of gas sensor devices due to their high-surface-area-to-volume and unique electrical properties [8]. SnO2 nanowires were successfully obtained with and without depositing platinum (Pt) clusters on the substrates as a catalyst material. SnO2 powder was used as the source material. The temperature of the substrates was ranging from 430 to 470 °C. The Pt clusters play a crucial role in the synthesis procedure acting as nucleation sites and promoting the growth of SnO2 nanowires ( Figure 3) [35][36][37]. The nanowires obtained using the Pt as a catalyst material were thinner compared to the once obtained without seed layers. In addition, the size and density of nanowires were possible to control by controlling the dimensions and the density of seeds [35]. The gold (Au) was used as a catalyst for the fabrication of ZnO nanomaterials [38]. The synthesis procedure was performed in the tubular furnace at the temperature range from 400 to 500 °C using a relatively short deposition time (20 min). The morphological analyses showed that ZnO nanowires with high-density were obtained. Single-crystalline In2O3 nanowires were fabricated using Au catalytic layers as well [39]. The nucleation and growth mechanism of In2O3 nanowires was studied. The experimental analysis showed that the VLS and vs. mechanisms affect the elongation of wires depending on the condensation temperature. While the vs. mechanism influences the lateral enlargement of nanowires. Moreover, the Au catalyst was more efficient compared to the Pt and palladium (Pd) to improve the growth process of NiO nanowire [40]. Further, the vapor phase growth was developed and combined with other techniques to fabricate more complex structures. Single crystalline ZnO nanowires covered by the NiO-shell were obtained using a two-step preparation procedure. In the first step, the ZnO nanowires were obtained via vapor phase growth. Then, the NiO was deposited on ZnO nanowires by means of RF sputtering. Further, the vapor phase growth was developed and combined with other techniques to fabricate more complex structures. Single crystalline ZnO nanowires covered by the NiO-shell were obtained using a two-step preparation procedure. In the first step, the ZnO nanowires were obtained via vapor phase growth. Then, the NiO was deposited on ZnO nanowires by means of RF sputtering. The deposition of NiO by sputtering over the ZnO nanowires in special conditions leads to a columnar growth of polycrystalline nanowires with preferential orientation [41]. A heterojunction of ZnO nanowires/GaN was obtained using a two-step procedure. The investigations showed that the density of ZnO nanowires growth on p-type GaN layer without catalyst was not sufficient. Due to this reason, the surface of the p-GaN was treated in hydrofluoric acid followed by the deposition of Au catalytic nanoparticles on the p-GaN. The ZnO nanowires with high density were prepared on the treated p-GaN layer by the VLS synthesis method [42]. The NiO/ZnO nanowire-based heterostructures were obtained by VLS and vs. procedures separately. At first, NiO nanowires were prepared via the VLS mechanism using Au as a catalyst. The ZnO nanowires were directly synthesized on NiO based on vs. mechanism [43].
Thermal oxidation of metallic thin films is another approach that may be used to grow metal oxide nanowires in a furnace at elevated temperatures. In this case, the metallic films may be deposited on the substrates by radio frequency (RF) magnetron sputtering [44]. Then, the samples are placed in the furnace and the synthesis procedure of nanowires is carried out under the oxygen flow. The WO 3 nanowires were obtained by thermal oxidation of tungsten metallic films at 600 • C for 1 h [22].
A few methods, such as the electrochemical anodization [36][37][38][39], atomic layer deposition (ALD) [40][41][42] and hydrothermal synthesis [43][44][45] have been developed for the preparation of metal oxide nanotubes [45][46][47][48][49][50][51]. ALD is a template-assisted technique for the synthesis of nanotubular arrays and requires post-processing separation of the obtained tubes from the template [52]. The fabrication of well-ordered metal oxide nanotubes with the homogeneous tube-size distribution over the substrate by the hydrothermal growth method is difficult [15,50]. Instead, the electrochemical anodization method was applied to obtain highly ordered metal oxide nanotubes in ambient conditions without using vacuum techniques [53,54]. The anodic formation of nanotubes is based on the oxidation and etching of metallic films in the electrolyte solution, where the water molecules and fluorine ions are involved in the synthesis process. The anodization of metallic films is carried out in an electrochemical cell consisting of two electrodes. The procedure is mainly performed at room temperature (RT). The tubular structures can be prepared on different kinds of substrates for the fabrication of new generation functional devices [55][56][57][58][59]. The detailed description of the anodic formation of metal oxide nanotubes has been reported in previous works [15,53,54].
Due to the aforementioned reasons, we used the electrochemical anodization method in order to obtain highly ordered metal oxide nanotube arrays. We prepared pure TiO 2 nanotubes with different diameters varying the anodization parameters, such as the electrolyte solution, anodization time and voltage [60,61]. This method allows the fabrication of TiO 2 tubular arrays on flexible substrates as well ( Figure 4) [16,17]. Doped and mixed TiO 2 nanotubes were prepared by anodization of metallic alloy films. [6,19,62]. The anodization technique is well developed mainly for the synthesis of porous SnO 2 and TiO 2 nanotubes [63][64][65]. Therefore, we investigated the anodic formation of other metal oxide materials. The studies showed that the electrolyte concentration and the applied voltage have a crucial effect on the transformation of Nb 2 O 5 porous arrays to tubular structures [54].
It is worth mentioning that the vapor phase synthesis of nanowires considering the catalyst assisted growth and the anodic formation of nanotubes are successfully developed methods to control the shape and size of nanomaterials during their formation process. Consequently, these developed approaches open new perspectives for manufacturing more complex materials and heterostructures.

Improvement of Sensing Performance Including the Operating Temperature
The working principle of chemo-resistive (or conductometric) sensors is based on their conductance change mechanism due to the population of oxygen ions over the structure surface, which is modulated by the interaction with gaseous molecules. The adsorption process of oxygen on the sensing material in molecular and atomic forms depends on the operating temperature of the sensor device [66]. The formation of oxygen atomic species dominates at 200 °C and above [67]. These acceptor surface states withdraw the electrons from the material causing a band-bending. The formed surface barrier and its modulation due to exposure of different gaseous compounds determine the gas sensing properties of the oxide material. Thus, the oxygen chemisorption has a fundamental effect to improve the response of sensing layers [1,2]. The working temperature of conductometric sensors ranges from 200 to 500 °C in order to keep donor oxygen vacancies ionized but fixed [68]. However, the operation of gas sensors at elevated temperatures increases the power consumption of the final device. Therefore, reducing the working temperature of gas sensors is a challenging issue for the manufacturing of energy-efficient and portable sensing devices.
In this context, the SENSOR Lab has been working extensively to improve the capabilities of chemical sensors even at relatively low operating temperatures applying different strategies. Different catalytic layers were deposited on the oxide materials to improve their sensing performance. The influence of the silver (Ag) and Pt nanoparticles on the gas sensing parameters and

Improvement of Sensing Performance Including the Operating Temperature
The working principle of chemo-resistive (or conductometric) sensors is based on their conductance change mechanism due to the population of oxygen ions over the structure surface, which is modulated by the interaction with gaseous molecules. The adsorption process of oxygen on the sensing material in molecular and atomic forms depends on the operating temperature of the sensor device [66]. The formation of oxygen atomic species dominates at 200 • C and above [67]. These acceptor surface states withdraw the electrons from the material causing a band-bending. The formed surface barrier and its modulation due to exposure of different gaseous compounds determine the gas sensing properties of the oxide material. Thus, the oxygen chemisorption has a fundamental effect to improve the response of sensing layers [1,2]. The working temperature of conductometric sensors ranges from 200 to 500 • C in order to keep donor oxygen vacancies ionized but fixed [68]. However, the operation of gas sensors at elevated temperatures increases the power consumption of the final device. Therefore, reducing the working temperature of gas sensors is a challenging issue for the manufacturing of energy-efficient and portable sensing devices.
In this context, the SENSOR Lab has been working extensively to improve the capabilities of chemical sensors even at relatively low operating temperatures applying different strategies. Different catalytic layers were deposited on the oxide materials to improve their sensing performance. The influence of the silver (Ag) and Pt nanoparticles on the gas sensing parameters and operating temperature of the β-Fe 2 O 3 nano-systems were studied [69]. For all analytes, Pt/β-Fe 2 O 3 nano-system responses showed a maximum-like behavior at an optimal operating temperature of 300 • C. While, the response values of Ag/β-Fe 2 O 3 sensors underwent a progressive enhancement upon increasing the operating temperature, yielding the best results between 300 and 400 • C. The Ag/β-Fe 2 O 3 showed significantly higher responses to hydrogen and ethanol at 400 • C, indicating a change in the selectivity pattern with respect to Pt/β-Fe 2 O 3 ( Figure 5). This observation highlights the beneficial influence of the Ag and Pt nanoparticles on the functional properties of oxide materials, thanks to their synergistic interactions with Fe 2 O 3 . operating temperature of the β-Fe2O3 nano-systems were studied [69]. For all analytes, Pt/β-Fe2O3 nano-system responses showed a maximum-like behavior at an optimal operating temperature of 300 °C. While, the response values of Ag/β-Fe2O3 sensors underwent a progressive enhancement upon increasing the operating temperature, yielding the best results between 300 and 400 °C. The Ag/β-Fe2O3 showed significantly higher responses to hydrogen and ethanol at 400 °C, indicating a change in the selectivity pattern with respect to Pt/β-Fe2O3 ( Figure 5). This observation highlights the beneficial influence of the Ag and Pt nanoparticles on the functional properties of oxide materials, thanks to their synergistic interactions with Fe2O3. In the next work, the catalytic effect of Au on the sensing behavior of Fe2O3 was investigated varying the concentration of catalyst material. The gas tests were carried out at 25, 100 and 200 °C [70]. In this case, the Au particles not only affect the sensor response but similarly influences the operating temperature and selectivity of Fe2O3 towards nitrogen dioxide (NO2) ( Figure 6). The application of the Au catalytic layer resulted in an appreciable response improvement at 25 °C. Nevertheless, the response of the structure at 25 °C was decreased with the increase in the concentration of Au. While the response of the sample with a higher concentration of Au (Au40/Fe2O3) was enhanced at 100 and 200 °C compared to the one obtained with a lower concentration (Au20/Fe2O3). The investigations indicated that Au mainly affects the gas sensing performances via an electronic, rather than a chemical, mechanism. Consistently, such an effect can also explain the In the next work, the catalytic effect of Au on the sensing behavior of Fe 2 O 3 was investigated varying the concentration of catalyst material. The gas tests were carried out at 25, 100 and 200 • C [70]. In this case, the Au particles not only affect the sensor response but similarly influences the operating temperature and selectivity of Fe 2 O 3 towards nitrogen dioxide (NO 2 ) ( Figure 6). The application of the Au catalytic layer resulted in an appreciable response improvement at 25 • C. Nevertheless, the response of the structure at 25 • C was decreased with the increase in the concentration of Au. While the response of the sample with a higher concentration of Au (Au 40 /Fe 2 O 3 ) was enhanced at 100 and 200 • C compared to the one obtained with a lower concentration (Au 20 /Fe 2 O 3 ). The investigations indicated that Au mainly affects the gas sensing performances via an electronic, rather than a chemical, mechanism. Consistently, such an effect can also explain the improved response of metal-containing samples to NO 2 . Indeed, the relative conductance modulation occurring upon NO 2 exposure is expected to be enhanced for the initially thinner hole accumulation layer of Au/ε-Fe 2 O 3 samples in comparison to the bare ε-Fe 2 O 3 . Recently, the effect of Au clusters on the Mn 3 O 4 sensing properties was studied [71]. A huge boost in the response towards di(propylene glycol) monomethyl ether (DPGME) in the Au-Mn 3 O 4 nanostructures was observed compared to the pristine Mn 3 O 4 and Ag-Mn 3 O 4 (Figure 7). The Au functionalized Mn 3 O 4 showed the best response at an operating temperature of 200 • C, which is lower compared to the working temperature of the other two materials (300 • C). Besides, the response of the Au-Mn 3 O 4 towards DPGME is very selective with respect to other chemical warfare agents (CWAs). The obtained results were explained based on two theoretical models: (i) on the active surface, the Au cluster interacts with the Mn 3 O 4 oxygen, thus supporting at the atomic-scale level the occurrence of an intimate Au/Mn 3 O 4 contact, (ii) both Mn 3 O 4 and Au particles' surfaces are directly involved in the interaction with DPGME, revealing a dual-site contact.
In the meanwhile, the sensing performance of metal oxide nanomaterials can be improved by either doping or preparing solid solutions [6,[72][73][74][75]. The effect of Zn-doping on the sensitivity of In 2 O 3 nanowires was investigated towards different gases, such as carbon monoxide (CO), NO 2 , ethanol, and H 2 [73]. High sensor responses were obtained from Zn-In 2 O 3 for CO, H 2, and ethanol respectively at a relatively lower operating temperature of 300 • C. In contrast, In 2 O 3 nanowires showed lower sensor responses for the same gases. The superior performance of Zn-In 2 O 3 nanowires was attributed to the nanowire network sensors, where the large numbers of inter-nanowire junctions present along the conducting path. At each junction, a potential barrier exists which plays a dominant role in defining the resistance of the path. When reducing gases are brought into contact, the oxygen adsorbates are consumed which return the electrons back to the nanowire, leading to lower the overall channel resistance. In addition, the Zn incorporation into the In 2 O 3 lattice creates more oxygen vacancies and increases the surface defects, which enhance the receptor function for reducing gases. improved response of metal-containing samples to NO2. Indeed, the relative conductance modulation occurring upon NO2 exposure is expected to be enhanced for the initially thinner hole accumulation layer of Au/ε-Fe2O3 samples in comparison to the bare ε-Fe2O3. Recently, the effect of Au clusters on the Mn3O4 sensing properties was studied [71]. A huge boost in the response towards di(propylene glycol) monomethyl ether (DPGME) in the Au-Mn3O4 nanostructures was observed compared to the pristine Mn3O4 and Ag-Mn3O4 (Figure 7). The Au functionalized Mn3O4 showed the best response at an operating temperature of 200 °C, which is lower compared to the working temperature of the other two materials (300 °C). Besides, the response of the Au-Mn3O4 towards DPGME is very selective with respect to other chemical warfare agents (CWAs). The obtained results were explained based on two theoretical models: (i) on the active surface, the Au cluster interacts with the Mn3O4 oxygen, thus supporting at the atomic-scale level the occurrence of an intimate Au/Mn3O4 contact, (ii) both Mn3O4 and Au particles' surfaces are directly involved in the interaction with DPGME, revealing a dual-site contact.
In the meanwhile, the sensing performance of metal oxide nanomaterials can be improved by either doping or preparing solid solutions [6,[72][73][74][75]. The effect of Zn-doping on the sensitivity of In2O3 nanowires was investigated towards different gases, such as carbon monoxide (CO), NO2, ethanol, and H2 [73]. High sensor responses were obtained from Zn-In2O3 for CO, H2, and ethanol respectively at a relatively lower operating temperature of 300 °C. In contrast, In2O3 nanowires showed lower sensor responses for the same gases. The superior performance of Zn-In2O3 nanowires was attributed to the nanowire network sensors, where the large numbers of inter-nanowire junctions present along the conducting path. At each junction, a potential barrier exists which plays a dominant role in defining the resistance of the path. When reducing gases are brought into contact, the oxygen adsorbates are consumed which return the electrons back to the nanowire, leading to lower the overall channel resistance. In addition, the Zn incorporation into the In2O3 lattice creates more oxygen vacancies and increases the surface defects, which enhance the receptor function for reducing gases.  The doping of Co3O4 nanomaterials with fluorine (F) resulted in higher responses towards volatile organic compounds (VOCs) and lowered the working temperature (200 °C) compared to the fluorine-free systems [74]. This behavior was attributed to the high fluorine electronegativity. The F centers attract the electron density from Co ones, enhancing, in turn, the Lewis acidity and further promoting the corresponding catalytic activity [76]. The response towards the analyte gases at a lower temperature was also enhanced due to the fact that the F introduction prevents undesired free carrier annihilation in p-type Co3O4 increasing the concentration of h+ species. The introduction of niobium (Nb) into the structure of TiO2 nanotubes improved their response towards VOCs and CO at 200 °C [19,77]. In this case, Ti atoms can be replaced by Nb in the lattice due to the similar ionic radii of Nb 5+ and Ti 4+ [6]. Thus, the Nb acts as a shallow donor in TiO2 and promotes the gas adsorption on the nanotubes enhancing their sensing response. Moreover, the similarity of Nb 5+ and Ti 4+ ions allows us to tune the properties of the doped material to a large extent through the introduction of high concentrations of Nb in TiO2 [62]. It was recently demonstrated that the presence of higher concentrations of Nb in the TiO2 nanotubes plays a crucial role to improve their response and the selectivity [62]. Unlike the structures doped with the lower concentrations of Nb, the TiO2 nanotubes containing higher concentration showed a very high and selective response towards dimethylamine (DMA) compared to the VOCs and CO at an operating temperature of 300 °C.
The manufacturing of gas sensors based on composite nanomaterials and heterojunctions provides novel opportunities to improve the sensing properties of metal oxides [7,43,78,79]. The The doping of Co 3 O 4 nanomaterials with fluorine (F) resulted in higher responses towards volatile organic compounds (VOCs) and lowered the working temperature (200 • C) compared to the fluorine-free systems [74]. This behavior was attributed to the high fluorine electronegativity. The F centers attract the electron density from Co ones, enhancing, in turn, the Lewis acidity and further promoting the corresponding catalytic activity [76]. The response towards the analyte gases at a lower temperature was also enhanced due to the fact that the F introduction prevents undesired free carrier annihilation in p-type Co 3 O 4 increasing the concentration of h+ species. The introduction of niobium (Nb) into the structure of TiO 2 nanotubes improved their response towards VOCs and CO at 200 • C [19,77]. In this case, Ti atoms can be replaced by Nb in the lattice due to the similar ionic radii of Nb 5+ and Ti 4+ [6]. Thus, the Nb acts as a shallow donor in TiO 2 and promotes the gas adsorption on the nanotubes enhancing their sensing response. Moreover, the similarity of Nb 5+ and Ti 4+ ions allows us to tune the properties of the doped material to a large extent through the introduction of high concentrations of Nb in TiO 2 [62]. It was recently demonstrated that the presence of higher concentrations of Nb in the TiO 2 nanotubes plays a crucial role to improve their response and the selectivity [62]. Unlike the structures doped with the lower concentrations of Nb, the TiO 2 nanotubes containing higher concentration showed a very high and selective response towards dimethylamine (DMA) compared to the VOCs and CO at an operating temperature of 300 • C.
The manufacturing of gas sensors based on composite nanomaterials and heterojunctions provides novel opportunities to improve the sensing properties of metal oxides [7,43,78,79]. The superior gas sensing performance of ZnO-TiO 2 nanocomposites for the detection of VOCs with respect to the pristine ZnO was demonstrated in an early report [78]. The observation indicates that the introduction of TiO 2 induced higher oxygen defect content promoting the adsorption of oxygen on the composite structure which provides better response compared to the pure ZnO. Further studies were focused on the improvement of sensing parameters of the heterostructures modifying their morphology and composition. The conjunction of nanomaterials with different types of electrical conductivity resulting in charge transfer between them and the formation of a charge depletion layer at their interface. This unique effect could be the basis for the manufacturing of high-performance chemical sensors based on nanocomposites.
CuO-TiO 2 -Au structure showed high response values towards reducing and oxidizing gases at 100 and 200 • C [79]. The high sensing response was achieved due to the formation of an interfacial area p-n heterojunction between the p-type CuO and the n-type TiO 2 increasing the lifetime of charge carriers. In addition, the high catalytic activity of TiO 2 and Au nanoparticles synergistically contributed to the enhancement in the response of the composite. Meanwhile, the Au nanoparticles optimized the selectivity of the sensor, since they enhanced the response toward O 3 at 100 • C. The modification of TiO 2 by WO x shows very interesting results for the detection of VOCs [80,81]. The formation of W(VI) species in TiO 2 modified its structural parameters resulting in conductance increase by ionization of the loosely bound extra electrons in the anatase lattice. It is also compelling to notice that, by the addition of only a WO x surface layer, the electrical signal of the structure underwent a dramatic change upon the introduction of VOCs, differently from pure TiO 2 that displayed weak variations (Figure 8). Similarly, the coupling of V 2 O 5 with the anatase TiO 2 enhanced the response of material towards ethanol at 200 • C [82].

Surface Photoactivation
One of the main challenges in the development of sensing systems is the fabrication of energyefficient gas sensors working at RT, without the need for a heating element. This could lead to the complete integration of these devices with conventional microelectronic techniques to produce extremely low-cost devices. Furthermore, working at RT, diffusion of atoms, grain growth, and lattice relaxation may be avoided completely. These are some of the common issues of chemical sensors based on metal oxides: solving them could provide a huge increase in the sensor performances especially regarding drift and reproducibility.
Among the strategies employed to reduce the working temperature, surface photoactivation deserves a special mention. When the light interacts with the semiconductor surface, a multitude of effects arises, depending on the photons' energies. Thanks to heterogeneous photocatalysis studies, it was discovered that many of the metal oxide materials have their principal optical and electronic transitions in the near-UV region of the electromagnetic spectrum (Eλ = 2.5-5 eV). For example, ZnO, Furthermore, it has recently been shown that the incorporation of 1D metal oxides with two-dimensional (2D) graphene oxide (GO) is a promising approach to fabricate composite structures with improved gas sensing performance [7,[83][84][85]. A composite structure based on TiO 2 nanotubes and reduced GO (RGO) was obtained (RGO-TiO 2 ) [84]. The compositional and structural effects of each material on the response of RGO-TiO 2 sensors were systematically studied showing the occurrence of an optimal GO concentration arising from the interplay of these two parameters. The reduction of GO (RGO) platelets improves charge transport through the TiO 2 tubular arrays, enhancing the conductance of composite. The response of the RGO−TiO 2 towards H 2 at 200 • C was much higher than the response of pristine TiO 2 at the same temperature. This fact is related to the depletion layer formed between the n-type TiO 2 and RGO, which creates more active centers for the interaction of the structure with H 2 . Thus, the gas sensing response of the RGO−TiO 2 was determined by modulation of the barrier height and barrier width formed between TiO 2 and RGO, which was further modulated due to the gas adsorption. The introduction of Nb in the RGO−TiO 2 composite material shows a huge increase in hydrogen response and a simultaneous reduction of the response to other interfering gaseous compounds, thus providing an enhancement in the selectivity [83]. The response of ZnO nanomaterials was improved in a similar way. The RGO enhanced the sensing response of ZnO towards VOCs, H 2, and NO 2 at 200−250 • C [7,85].
In the next section, the sensing performances of metal oxide-based nanomaterials are reviewed, considering their operation at RT.

Surface Photoactivation
One of the main challenges in the development of sensing systems is the fabrication of energy-efficient gas sensors working at RT, without the need for a heating element. This could lead to the complete integration of these devices with conventional microelectronic techniques to produce extremely low-cost devices. Furthermore, working at RT, diffusion of atoms, grain growth, and lattice relaxation may be avoided completely. These are some of the common issues of chemical sensors based on metal oxides: solving them could provide a huge increase in the sensor performances especially regarding drift and reproducibility.
Among the strategies employed to reduce the working temperature, surface photoactivation deserves a special mention. When the light interacts with the semiconductor surface, a multitude of effects arises, depending on the photons' energies. Thanks to heterogeneous photocatalysis studies, it was discovered that many of the metal oxide materials have their principal optical and electronic transitions in the near-UV region of the electromagnetic spectrum (E λ = 2.5-5 eV). For example, ZnO, TiO 2 and SnO 2 exhibit a bandgap of 3.5-3.7 eV [86], 3.0-3.2 eV [87], and 3.6 eV [88], respectively. The adsorbed light can affect the electrical properties of semiconducting materials by creating free carriers by either intrinsic or extrinsic optical adsorption [89]. Considering conventional polycrystalline materials, formed by grains with different orientation and shapes and in the presence of oxygen ionosorption, photoactivation can affect the electronic transport in different ways (Figure 9 Therefore, UV irradiation changes the occupancy of the defects by charge carriers (holes or electrons) thus modulating the concentration of adsorption centers and the adsorption capacity of the semiconducting surface. Thus, the current flow in the sensing material increases as reported in Figure  10. However, even if the irradiation does not change the chemical composition of the material, it produces additional disorder into the lattice, which is "memorized" for some time even after the light has been turned off. Gradually, the disorder relaxes with time, and the temperature promotes the the semiconducting surface. Thus, the current flow in the sensing material increases as reported in Figure 10. However, even if the irradiation does not change the chemical composition of the material, it produces additional disorder into the lattice, which is "memorized" for some time even after the light has been turned off. Gradually, the disorder relaxes with time, and the temperature promotes the relaxation rate. Figure 9. Grain representation with the depletion regions, Fermi level (EF), conduction band (Ec) and inter-grains barrier due to the oxygen ionosorption. Therefore, UV irradiation changes the occupancy of the defects by charge carriers (holes or electrons) thus modulating the concentration of adsorption centers and the adsorption capacity of the semiconducting surface. Thus, the current flow in the sensing material increases as reported in Figure  10. However, even if the irradiation does not change the chemical composition of the material, it produces additional disorder into the lattice, which is "memorized" for some time even after the light has been turned off. Gradually, the disorder relaxes with time, and the temperature promotes the relaxation rate. Figure 10. Current flowing in a SnO2 layer in dark and under UV illumination pulses, at RT and 30% of relative humidity. Reproduced with permission [93].
Other important phenomena are photoabsorption or photodesorption, which can take place on the surface of the metal oxide in presence of a reactive atmosphere, depending on the experimental condition of temperature, pressure, and incident power of the light used. These are extremely useful in the case of species (for example, NO2) that can poison the semiconductor surface. UV irradiation can reduce the poisoning of the surface due to the NO2 irreversible adsorption by enhancing the desorption process [93]. As previously reported by our group (Figure 11a), the desorption process under UV light is faster than in dark, leading to a complete recovery of the baseline. In most cases, there is no memory effect at temperatures higher than 250 °C [94]. At RT, the enhancement of the response by UV-irradiation is clearly visible, as reported in Figure 11b. However, a gradual reduction of the ratio between the response under illumination and in dark (RL/RD) was observed with the increase in the sensor operating temperature. The advantageous effect of UV light even disappears above a certain temperature. Therefore, the impact of irradiation is maximized when the device is working close to RT. At the same time, the reaction kinetics becomes faster with increasing the power of UV light and the operating temperature. However, increasing the power of the light source does Other important phenomena are photoabsorption or photodesorption, which can take place on the surface of the metal oxide in presence of a reactive atmosphere, depending on the experimental condition of temperature, pressure, and incident power of the light used. These are extremely useful in the case of species (for example, NO 2 ) that can poison the semiconductor surface. UV irradiation can reduce the poisoning of the surface due to the NO 2 irreversible adsorption by enhancing the desorption process [93]. As previously reported by our group (Figure 11a), the desorption process under UV light is faster than in dark, leading to a complete recovery of the baseline. In most cases, there is no memory effect at temperatures higher than 250 • C [94]. At RT, the enhancement of the response by UV-irradiation is clearly visible, as reported in Figure 11b. However, a gradual reduction of the ratio between the response under illumination and in dark (R L /R D ) was observed with the increase in the sensor operating temperature. The advantageous effect of UV light even disappears above a certain temperature. Therefore, the impact of irradiation is maximized when the device is working close to RT. At the same time, the reaction kinetics becomes faster with increasing the power of UV light and the operating temperature. However, increasing the power of the light source does not always lead to an improvement in the sensor performances. For example, in the case of UV-irradiated SnO 2 RGTO, the best results for the detection of CO were obtained at 15 mW/m 2 . At higher values of applied power, the response is lower or comparable to the dark response, due to the prevalence of the photodesorption effect [94].
The response of the same SnO 2 RGTO samples to CO in the dark condition is maximum at 400 • C. Furthermore, the response of the samples in dark at 400 • C is higher than the irradiated one leading to the lowest R L /R D ratio. In fact, the UV light promotes the desorption process of the oxygen chemisorbed, with the former prevailing on the latter at such high temperatures.
values of applied power, the response is lower or comparable to the dark response, due to the prevalence of the photodesorption effect [94].
The response of the same SnO2 RGTO samples to CO in the dark condition is maximum at 400 °C. Furthermore, the response of the samples in dark at 400 °C is higher than the irradiated one leading to the lowest RL/RD ratio. In fact, the UV light promotes the desorption process of the oxygen chemisorbed, with the former prevailing on the latter at such high temperatures. Figure 11. (a) Dynamic current measurements of the SnO2 layer in presence of 3 ppm NO2, with and without UV exposure, at a working temperature of 200°C and 30% of relative humidity. Reproduced with permission [93]. (b) Response in dark-and UV-irradiation condition for the tin-oxide RGTO (rheotaxial growth thermal oxidation) samples gold-catalyzed as a function of the working temperature toward 100 ppm of CO at 30% of relative humidity. Reproduced with permission [94].
Previously, we discussed the effect of photoactivation for films and polycrystalline grains. However, the advantages of photoactivation hold for other structures as well. For example, the crystalline nanowire mats. In recent work, we have decorated the commercial silicon carbide (SiC)based foams by 1D SnO2 nanobelts using a custom evaporation-condensation technique [95]. Pristine SiC, SnO2, and SiC/SnO2 composite materials were characterized at RT in the presence of ammonia (NH3) and NO2 both with and without UV photoactivation. Not surprisingly, both SiC and SnO2 materials benefit from the presence of the external irradiation, enhancing significantly the response of both gases compared to the dark conditions ( Figure 12). The UV activation also increases the response of SiC/SnO2 composite materials, which was otherwise negligible. Previously, we discussed the effect of photoactivation for films and polycrystalline grains. However, the advantages of photoactivation hold for other structures as well. For example, the crystalline nanowire mats. In recent work, we have decorated the commercial silicon carbide (SiC)-based foams by 1D SnO 2 nanobelts using a custom evaporation-condensation technique [95]. Pristine SiC, SnO 2, and SiC/SnO 2 composite materials were characterized at RT in the presence of ammonia (NH 3 ) and NO 2 both with and without UV photoactivation. Not surprisingly, both SiC and SnO 2 materials benefit from the presence of the external irradiation, enhancing significantly the response of both gases compared to the dark conditions ( Figure 12). The UV activation also increases the response of SiC/SnO 2 composite materials, which was otherwise negligible. Reproduced with permission [95].
We recently investigated the performances of a SnO2/rGO composite material, fabricated by drop-casting of RGO on SnO2 nanowires [96]. The sensing performances of the composite material were evaluated in the presence of NO2 and CO. Although the overall performances of the composite for the detection of CO were modest, the composite material outperforms pristine SnO2 at a temperature close to RT, and the benefits of UV irradiation are evident. However, in the presence of NO2, the advantages are substantial. Figure 13 reports the response of pristine SnO2 and SnO2/rGO composite towards NO2 at various operating temperatures, in dark and under the UV light irradiation. As for CO, the performances of the composite material outclass the ones of pristine SnO2, and the presence of UV light enhances the response. Nevertheless, the combination of light irradiation and high operating temperatures is not favorable. Since, in this case, the response of the sensor decreases compared to the RT conditions. We recently investigated the performances of a SnO 2 /rGO composite material, fabricated by drop-casting of RGO on SnO 2 nanowires [96]. The sensing performances of the composite material were evaluated in the presence of NO 2 and CO. Although the overall performances of the composite for the detection of CO were modest, the composite material outperforms pristine SnO 2 at a temperature close to RT, and the benefits of UV irradiation are evident. However, in the presence of NO 2, the advantages are substantial. Figure 13 reports the response of pristine SnO 2 and SnO 2 /rGO composite towards NO 2 at various operating temperatures, in dark and under the UV light irradiation. As for CO, the performances of the composite material outclass the ones of pristine SnO 2 , and the presence of UV light enhances the response. Nevertheless, the combination of light irradiation and high operating temperatures is not favorable. Since, in this case, the response of the sensor decreases compared to the RT conditions. Similar investigations were performed with the sensing element that was just a single metal oxide nanowire [97]. In particular, SnO2 and ZnO single nanowire devices were fabricated by electron-beam lithography (EBL) technique and tested towards NH3, CO and NO2 in a humid environment at RT. In the case of NO2, the UV Irradiation of the single SnO2 nanowire helps it to recover the baseline conductance after the target gas is switched off. Quite interestingly, the effect of UV light at two different wavelengths (254 and 365 nm) on the single ZnO nanowire is almost the same, as expected, being the ZnO band gap about 3.3 eV, which is lower than the incident UV photons energies. This does not hold for the single SnO2 nanowire, which exhibits a band gap of about 3.6 eV. In this case, the energy of the 365 nm illumination is not enough to excite electrons from the valence band to the conduction one.
The enhancement of the sensor response due to the UV photoactivation was recorded also in organic/inorganic hybrid devices, such as BC-MOS (bacterial cellulose-metal oxide semiconductor) [28]. In particular, we have fabricated a hybrid device by depositing a sensitive ZnO thin film on porous bacterial cellulose produced by acetic acid bacteria from the Acetobacteraceae family. This novel sensing device was tested towards NO2, acetone, ethanol, and H2. As expected, the response of the device in the presence of these chemical species at RT and without UV irradiation was negligible. Instead, the device exhibits a significant change of the electrical conductance, in particular toward NO2 and acetone under UV irradiation condition.
Up to now, we discussed the effect of UV light (over bandgap) on the electronic and sensing performances of metal oxides. What is happening if we illuminate a metal oxide with visible light, with an energy lower than the material bandgap? We investigated the wavelength dependency of the sensors' response of pristine and Ag-decorated SnO2 nanowires to H2 [98]. For pristine SnO2, we cannot observe any enhancement due to the illumination with 350-650 nm light. However, in the case of Ag decoration, the exposition of the sample to green light (≈ 500 nm) led to a weak increase in its response (Figure 14a). Analyzing the UV-vis spectra of both materials, we observed two plasmonic peaks appearing at around 500 and 350 nm (Figure 14b). The former is responsible for the enhancement of response in the presence of H2. Our obtained results led to the hypothesis that the enhanced gas response under UV−vis light is the effect of plasmonic hot electrons populating the surface of Ag nanoparticles. Similar investigations were performed with the sensing element that was just a single metal oxide nanowire [97]. In particular, SnO 2 and ZnO single nanowire devices were fabricated by electron-beam lithography (EBL) technique and tested towards NH 3 , CO and NO 2 in a humid environment at RT. In the case of NO 2 , the UV Irradiation of the single SnO 2 nanowire helps it to recover the baseline conductance after the target gas is switched off. Quite interestingly, the effect of UV light at two different wavelengths (254 and 365 nm) on the single ZnO nanowire is almost the same, as expected, being the ZnO band gap about 3.3 eV, which is lower than the incident UV photons energies. This does not hold for the single SnO 2 nanowire, which exhibits a band gap of about 3.6 eV. In this case, the energy of the 365 nm illumination is not enough to excite electrons from the valence band to the conduction one.
The enhancement of the sensor response due to the UV photoactivation was recorded also in organic/inorganic hybrid devices, such as BC-MOS (bacterial cellulose-metal oxide semiconductor) [28]. In particular, we have fabricated a hybrid device by depositing a sensitive ZnO thin film on porous bacterial cellulose produced by acetic acid bacteria from the Acetobacteraceae family. This novel sensing device was tested towards NO 2 , acetone, ethanol, and H 2 . As expected, the response of the device in the presence of these chemical species at RT and without UV irradiation was negligible. Instead, the device exhibits a significant change of the electrical conductance, in particular toward NO 2 and acetone under UV irradiation condition.
Up to now, we discussed the effect of UV light (over bandgap) on the electronic and sensing performances of metal oxides. What is happening if we illuminate a metal oxide with visible light, with an energy lower than the material bandgap? We investigated the wavelength dependency of the sensors' response of pristine and Ag-decorated SnO 2 nanowires to H 2 [98]. For pristine SnO 2 , we cannot observe any enhancement due to the illumination with 350-650 nm light. However, in the case of Ag decoration, the exposition of the sample to green light (≈ 500 nm) led to a weak increase in its response (Figure 14a). Analyzing the UV-vis spectra of both materials, we observed two plasmonic peaks appearing at around 500 and 350 nm (Figure 14b). The former is responsible for the enhancement of response in the presence of H 2 . Our obtained results led to the hypothesis that the enhanced gas response under UV−vis light is the effect of plasmonic hot electrons populating the surface of Ag nanoparticles.
The approaches presented in Sections 2 and 3 have been successfully applied in the manufacturing of electronic nose (EN) systems. The EN is an instrument based on different sensors, where the detection of gaseous and volatile compounds can be carried out through the Principal Component Analysis (PCA). The concept of the EN device allows developing a system with enhanced sensitivity and selectivity. We reported a detailed description of the studies of EN systems in our previous papers [44,[99][100][101][102]. The approaches presented in sections 2 and 3 have been successfully applied in the manufacturing of electronic nose (EN) systems. The EN is an instrument based on different sensors, where the detection of gaseous and volatile compounds can be carried out through the Principal Component Analysis (PCA). The concept of the EN device allows developing a system with enhanced sensitivity and selectivity. We reported a detailed description of the studies of EN systems in our previous papers [44,[99][100][101][102].

Conclusions
In this review, we presented an overview of activities on the synthesis and application of metal oxide nanomaterials for the fabrication of gas sensing systems performed at the SENSOR lab in Brescia (Italy). In particular, we focused on the development of the synthesis procedures for the preparation of pure, doped and functionalized 1D metal oxide nanomaterials. The methods for the fabrication of heterojunction were reported as well. The sensing performances of the materials were discussed considering their improvement at relatively low operating temperatures, which is favorable for the development of low power consumption gas sensors. Moreover, the achievements in the applications of energy-efficient and biocompatible substrates were presented. In this regard, the simple production technique of BC substrates and its application in gas sensors open new perspectives for their integration in next-generation biocompatible and flexible functional devices.
The progress in the vapor phase and electrochemical growth techniques allow the fabrication of doped and functionalized 1D metal oxides. The variation of the type and concentration of the dopant and catalyst material enhanced the response and the selectivity of metal oxides towards specific gaseous compounds at relatively low operating temperatures. In the meantime, the preparation of heterostructures by the coupling of different synthesis methods is a promising way to fabricate highperformance chemical sensors. Especially, the changes in the charge transfer mechanism of composite structures due to the formation of a depletion layer at the interface of two materials with different electrical conductivity can essentially improve the response and selectivity of sensors at low working temperatures. Moreover, the investigations showed that the preparation procedures of heterostructures should be performed properly, considering the variations in their composition and morphology for each specific gaseous compound.

Conclusions
In this review, we presented an overview of activities on the synthesis and application of metal oxide nanomaterials for the fabrication of gas sensing systems performed at the SENSOR lab in Brescia (Italy). In particular, we focused on the development of the synthesis procedures for the preparation of pure, doped and functionalized 1D metal oxide nanomaterials. The methods for the fabrication of heterojunction were reported as well. The sensing performances of the materials were discussed considering their improvement at relatively low operating temperatures, which is favorable for the development of low power consumption gas sensors. Moreover, the achievements in the applications of energy-efficient and biocompatible substrates were presented. In this regard, the simple production technique of BC substrates and its application in gas sensors open new perspectives for their integration in next-generation biocompatible and flexible functional devices.
The progress in the vapor phase and electrochemical growth techniques allow the fabrication of doped and functionalized 1D metal oxides. The variation of the type and concentration of the dopant and catalyst material enhanced the response and the selectivity of metal oxides towards specific gaseous compounds at relatively low operating temperatures. In the meantime, the preparation of heterostructures by the coupling of different synthesis methods is a promising way to fabricate high-performance chemical sensors. Especially, the changes in the charge transfer mechanism of composite structures due to the formation of a depletion layer at the interface of two materials with different electrical conductivity can essentially improve the response and selectivity of sensors at low working temperatures. Moreover, the investigations showed that the preparation procedures of heterostructures should be performed properly, considering the variations in their composition and morphology for each specific gaseous compound.
Finally, the studies carried out on the operation of metal oxide gas sensors through their surface photoactivation by UV and visible light were reported. This strategy was employed to fabricate energy-efficient gas sensors working at RT, without the need for a heating element. The kinetics of gas sensors was discussed in terms of the power of UV light and the operating temperature of the structure. The kinetics become faster when increasing the power of UV light and the operating temperature. However, the sensor performance is not always improved by increasing the power of the light source due to the prevalence of the photodesorption effect.