Decrease in Operation Temperature of Zinc Oxide Nanomarkers

A lot of publications and patents about semiconductor gas
sensors are published today in several scientific journals, materials
of many Meetings, etc. Confine ourselves only of citations of
four books published in the field...

sensors devices is a control the electronic and structural properties of the materials in order to modify the sensing properties. There are various techniques for modifying these properties. It is known that ruthenium in tin oxide matrix acts as an oxidative catalyst for hydrocarbons to achieve a considerable degree of sensitivity and selectivity. Therefore, an interesting approach is a modification of metal oxide surfaces using noble metal catalysts like Pt, Ru and Pd (see for example, [1,6]).
Another way to improve of characteristics of sensors is the appropriate doping of metal oxides with different metal impurities.
Aluminum-doped ZnO thin films were manufactured in [16,17] on glass-ceramic substrates by the high-frequency magnetron sputtering method. The obtained ZnO<Al> thin films have nanosize grains (~20-30nm). The glass ceramic/ZnO<Al>/Pt structure showed sufficient sensitivity to hydrogen at the heating of the working body already up to 400 0 C. The investigations have shown that this structure has fast response and recovery time periods.
We have shown that in contrast to many other metal-oxide materials, using our aluminum-doped ZnO films, it is possible to realize hydrogen sensors with remarkable low OT of pre-heating of the working body. The structure obtained satisfies the basic requirements to gas sensors. Analysis of the literature in [16] shown that ZnO nanorods sensors for detection of ethanol and hydrogen have OT=350 0 C [18], ZnO nanowire sensors for detection of ethanol 300 0 C [19], ZnO thin film sensors for detection of methane 150-350 0 C [20] and ethanol [21], Fe 2 O 3 -ZnO sensors for detection of NH3 350 0 C [22], ZnO sensors for detection of LPG 400 0 C [23], ZnFe2O4 sensors for detection of ethanol and acetone 300 0 C [24].
The other two investigations of aluminium doped ZnO films were reported in [25,26]. Pre-heating of the working body NOx sensors reported in [27] was 100-300 0 C. Thin-film hydrogen peroxide vapor sensors made from lantana-doped ZnO were manufactured using the high-frequency magnetron sputtering method [28]. Sensors made from La-doped doped ZnO exhibit a sufficient response to 10ppm of hydrogen peroxide vapors at the 220 0 C operating temperature. It was established that the dependencies of the response on hydrogen peroxide vapor concentration have a linear character for prepared structures at the 150 0 C operating temperature and can be used for determination of hydrogen peroxide vapor concentration. The response of such detectors of hydrogen peroxide vapors was dramatically increased after this pre-heating temperature of work body [27]. The indium was added to increase the ZnO resistive response and reduce the operating temperature of the 3D ZnO sensor. The highest sensitivity and selectivity toward ethanol gas at 250 0 C was observed for 5 at.% In doped ZnO. The response was about three times higher than that of the pure ZnO at 285 0 C, which was probably connected with the unique 3D ordered macroporous morphology of the ZnO: In, increased surface area and higher electron carrier concentration [28,29]. Mn /ZnO/Au/ZnO, Ag/Al-ZnO sensors of acetone and formaldehyde at OT=t240-275 0 C were reported in [30][31][32][33]. Porous ZnO acetone sensor with polymer colloids and OT=300 0 C was proposed in [34].
ZnO nanofibers, nanoplates, nanoflowers have been successfully synthesized by simple electrospinning hydrothermal routes and other methods [4]. One dimensional (1D) nanomaterials including nanowires, nanofibers, nanorods, and nanotubes have attracted great attention for sensing applications due to their unique morphology and the high surface-to-volume ratio [35]. Significant enhancement of hydrogen-sensing properties of ZnO nanofibers through NiO loading and ZnO nanowires decorated with WO 3 nanoparticles [36]. Sensor based on 1D fluorine-doped zinc oxide (1D-FZO) was reported in [37]. A sensitive H 2 S gas sensor made of ZnO QDs of less than 4nm in diameter was fabricated in [38].
The average QD' grain size was below twice the Debye length.
High sensitivity (Rair/Rgas) of 75 and 567 at room temperature and 90°C, correspondingly, was realized. Microwave-assisted hydrolysis preparation of highly crystalline ZnO nanorod array was proposed in [39] for the manufacture of room temperature photoluminescence-based CO gas sensor. Nanotubes have attracted great attention as promising nanostructures for fabricating highly sensitive and selective gas sensors due to their vast surface area. In fact, the meso-and nanosized pores formed on various nanotubes surfaces during synthesis can significantly enhance the gas sensing performance by facilitating the penetration of targeting gas into the deepest parts of the sensing device. It was shown in Yerevan State University [6,16,40] that the functionalization of multi-walled carbon nanotube (MWCNT) / SnO 2 thick-film structures with Ru leads to a considerable increase in response signal to methanol, ethanol, acetone, toluene vapors as well as to isobutane gas.  for the marker with Zn0.9853La0.0147O films deposited on Multi-Sensor-Platform c.
[27].   For over-addition of MWCNTs, the increasing number of electrons in the grain boundary reduce the resistance and decrease the sensitivity of the metal oxide + MWCNT sensors. Similar behavior is also reported for NO 2 by Sharma et al. [43]. The response toward acetone vapor was jumped to 72% after addition of 0.25% MWCNT also. Note that nanonsensors to hydrogen dioxide made from SnO 2 / MWCNT were reported also in [44][45][46][47][48][49][50][51] and ZnO/CNTin [49]. ZnO nanoflowers acetone and ZnO hollow microspheres ethanol sensors with high OT reported in [50][51][52]   Typical TEM image of ZnO QDs obtained through a wet synthesis method based on alkaline activated hydrolysis and condensation of zinc acetate solutions [62].

c.
SEM image of ZnO nano-sheets formed through a simple mixed hydrothermal synthesis method [64].

e.
ZnO ultraporous film made by flame spray pyrolysis [66].   ed, which are applied in improving NO 2 gas sensing performance [82][83][84][85][86][87]. Among many metal oxides, n-type Zn 2 SnO 4 has received considerable attention because of its high electron mobility, high electrical conductivity and wide band gap (3.6eV), which make it suitable for gas sensor applications [88,89]. When Zn 2 SnO 4 is coupled with RGO, its electrons can transfer from Zn 2 SnO 4 to RGO. As a result, the effective electronic contact between Zn 2 SnO 4 and RGO helps to improve the gas sensing performance. It was reported that  [53,54], respectively.
In contrast to that, the sensors presented in [74] not only show high sensitivity to lower concentrations of NO 2 (2.86 for 150ppb in 50% RH) but are also able to retain this quality in a high humidity atmosphere (80%). NO 2 and O 3 sensing mechanisms were discussed in details in [74].

Sensitivity of Sensors Under Illumination
The PbS QDs-decorated ZnO NRs with a hierarchical mesoporous structure were investigated in [94]. The nanocomposites showed near-infrared (NIR) adsorption derived from the PbS QDs with a narrow bandgap. Compared to the pristine ZnO nanorods, the ZnO/PbS nanocomposites-based sensor with the optimal loading amount of PbS exhibited higher response and quicker response/ recovery rates to ppm level of NO 2 at room temperature under NIR illumination. Moreover, the sensor exhibited full reversibility, low detection limit, good selectivity, and long-term stability to NO 2 . In the last few years, it has been shown that UV enhances the gassensing properties of metal oxides at room temperature [95][96][97][98][99][100].
UV illumination reduces response and recovery times and increases sensitivity. By using ZnO as the sensor material, it is possible to measure ppb-level NO 2 concentrations [101,102]. A transducer by which UV-supported metal oxide sensors can be miniaturized consuming only little energy has reported [103].

Some Information about Theoretical Investigations
To date, many of the mechanisms of the gas adsorption process and the nature of active centers responsible for the adsorption of gases on the surface of metal oxide materials are far from fully understood. These processes are very complex and their theoretical analysis requires ab initio calculations of the adsorption energies, equilibrium geometry, and charge transfer between the surface and the adsorbent, based on the density functional theory (DFT) [109][110][111][112]. We performed calculations, in particular, based on the DFT implemented in the Quantum Espresso package [110][111][112].
We use the generalized gradient approximation (GGA) in the form suggested by Purdue, Burke, and Ernzerhof [113] to describe the exchange-correlation functional. We have so far investigated the interaction of hydrogen peroxide with the stoichiometric surface analysis method [114]. Since defects, impurities, quantum-size effects, and gas molecules in the environment can have a significant effect on adsorption processes [115,116], additional research is needed to find out how all this can affect the process. An analysis of the latest book on quantum dots [117] shows no progress in studies of adsorption phenomena at the QD interface metal oxide -gas.