Next Article in Journal
Temperature Sensors Based on Organic Field-Effect Transistors
Next Article in Special Issue
Magneto-Electronic Hydrogen Gas Sensors: A Critical Review
Previous Article in Journal
Improving the Detection Accuracy of an Ag/Au Bimetallic Surface Plasmon Resonance Biosensor Based on Graphene
Previous Article in Special Issue
Macrocycle-Functionalized RGO for Gas Sensors for BTX Detection Using a Double Transduction Mode
 
 
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Adsorption Kinetics of NO2 Gas on Pt/Cr-TiO2/Pt-Based Sensors

1
College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, China
2
Key Laboratory of Materials Preparation and Protection for Harsh Environment, Ministry of Industry and Information Technology, Nanjing 211100, China
3
Department of Physics, University of Okara, 2 KM Multan Road Renala Khurd by-Pass, Okara 56300, Pakistan
4
German Aerospace Center (DLR), Institute of Materials Research, Linder Hoehe, 51147 Cologne, Germany
*
Author to whom correspondence should be addressed.
Submission received: 1 November 2021 / Revised: 15 December 2021 / Accepted: 19 December 2021 / Published: 27 December 2021

Abstract

:
Metal oxides are excellent candidates for the detection of various gases; however, the issues such as the limited operating temperature and selectivity are the most important ones requiring the comprehensive understanding of gas adsorption kinetics on the sensing layer surfaces. To this context, the present study focuses mainly on the fabrication of a Pt/Cr-TiO2/Pt type sensor structure that is highly suitable in reducing the operating temperature (from 400 to 200 °C), extending the lower limit NO2 gas concentration (below 10 ppm) with fast response (37 s) and recovery (24 s) times. This illustrates that the sensor performance is not only solely dependent on the nature of sensing material, but also, it is significantly enhanced by using such a new kind of electrode geometry. Moreover, Cr doping into TiO2 culminates in altering the sensor response from n- to p-type and thus contributes to sensor performance enhancement by detecting low NO2 concentrations selectively at reduced operating temperatures. In addition, the NO2 surface adsorption kinetics are studied by fitting the obtained sensor response curves with Elovich, inter-particle diffusion, and pseudo first-order and pseudo second-order adsorption models. It is found that a pseudo first-order reaction model describes the best NO2 adsorption kinetics toward 7–170 ppm NO2 gas at 200 °C. Finally, the sensing mechanism is discussed on the basis of the obtained results.

1. Introduction

The detection of oxides of nitrogen (NO and NO2) is highly demanding, as the reducing and oxidizing nature of these gases may yield sensor signals in the opposite directions. In the course of the previous two decades, the outdoor concentration of NO2 gas has been increased particularly due to environmental pollution caused by increased transport and industrial human activities; therefore, its consequences affect human health. Some people with bronchitis/asthma are particularly sensitive to NO2, which causes lung irritations and thus leads to breathing difficulties [1,2,3,4]. Furthermore, NO2 is a highly perilous gas for the human and environment as well because of its threshold limit value (TLV), which is about 3 and 5 ppm for 8 h and 15 min (this is estimated for the time-weighted average) [5]. In fact, even the inhalation of low concentrations of NO2 by humans can cause severe damage to the respiratory tract, thus leading to lung cancer. Therefore, the allowed short-time exposure of NO2 gas concentration has been reduced to 1 ppm by the Occupational Safety and Health Administration (US). However, the conventional TLV (as of 2018) is still much lower (≈0.2 ppm). Hence, researchers around the globe are striving to discover new possible solutions for the detection of NO2 with high accuracy, sensitivity, and selectivity to avoid any kind of expected danger to humans and the environment [6].
Semiconducting metal oxides (MOx) offer sensing ability in a large variety. However, some may suffer from high cross-sensitivity to many environmental gases [7,8,9,10]. In this regard, it is found that TiO2 is one of the potential candidates for NO2 sensing and is preferred over other MOx materials due to its low-cost fabrication and chemical stability under harsh and humid environment [11,12,13]. Yet, the high operating temperature (200–600 °C) of the TiO2-based sensors has been depreciating their widespread use. The cost and complexity of conventional sensors increase due to the high power consumption, as they require a heater (generally resistive) at the sensing material. Therefore, there is a need for NO2 gas sensors with improved sensing properties at relatively lower operating temperatures to reduce the power consumption and consequently the cost of sensor device. Recent studies showed that TiO2 with low-dimensional morphology (for example, nanowires and nanotubes) offer excellent opportunity for reducing the operating temperature [14,15,16,17]. However, these nanostructured material-based gas sensors with finer circuitry still have trouble with long-term stability issues. Ultimately, from the commercial point of view, the advanced manufacturing procedures (e.g., use of electron beam lithography and focused ion beam) results in high-cost gas sensor devices.
In general, the response of a gas sensor consists of two functions, receptor and traducer. The receptor (sensing material) enables the identification of gas concentrations by their corresponding interactions. In return, this prompts the relative change in the electrical transport property of the sensing material. Then, the metallic electrodes of the transducer part transfer this change into an electrical output signal. If the transducer function is not properly designed, then the sensor may produce an improper output signal. Importantly, the recent progress in gas sensor is primarily focused on the improvement of receptor function (i.e., the sensing material), while the pre-eminent role of transducer function has been ignored and rarely investigated. Contemporary studies on the effect of transducer function (the role of electrode material, its fabrication methods, its geometry and gap size) have shown that the sensor response can be significantly improved by tuning the transducer properties [18,19,20,21,22]. Recently, our group has proved that a novel sandwiched type configuration (Pt/TiO2/Pt) with top–bottom electrodes can produce fast and highly sensitive response toward NO2 gas and hence reduce the optimal operating temperature from 400 to 200 °C [23].
The present study reports the implementation of sandwiched-type sensor configuration (Pt/TiO2/Pt) for NO2-oxidizing gas. The effects of TiO2 doping with chromium (Cr-TiO2) are investigated on the basis of crystal structure and layer morphology but also when sandwiched between two Pt electrodes after heat treatment. Ultimately, the NO2 adsorption mechanism of pristine TiO2 and Cr-TiO2 with such configuration is interpreted based on the data obtained.

2. Materials and Methods

The sensors were fabricated in three steps (as shown in Figure 1) via the sputtering deposition technique. Firstly, 300 μm wide and 200 nm thick bottom Pt electrodes (BE) were patterned on alumina substrates via sputter coater (BALTEC, Hallbergmoos, Germany). On the patterned BEs, 2 µm thick TiO2 or Cr-TiO2 sensing layers with a columnar structure were deposited by reactive magnetron sputtering from Ti targets (99.95% purity) or Ti and Cr targets (99.99% purity), respectively. The sputtering equipment Z400 (from SVS, Gilching, Germany) allows reactive sputtering from multiple metallic or ceramic sources (targets) under a mixture of 23 sccm (standard cubic centimeter) inert argon (99.999% purity) and 06 sccm reactive oxygen (99.995% purity) flow. Subsequently, as the final step, the top Pt-electrodes (TEs) were deposited as a cross-bar pattern to the bottom Pt electrodes. The width of both the top and bottom electrodes is kept fixed in order to only elucidate the effect of Cr doping on the sensing mechanism.
During the deposition of sensing layers, several other substrates were placed inside the chamber for basic characterization: for instance, sapphire discs of 13 mm diameter (for XRD) and 20 × 20 mm silicon (Si) substrates (for EDX analysis). Thus, the obtained analyses results were free of any interference of Pt from platinum circuitry and Al from Al2O3 substrates. Finally, the sensing structures Pt/TiO2/Pt were post-deposition annealed for 3 h under static air conditions at 800 °C (ramping/heating rate of 6.6 °C per min) in a furnace from HERAEUS Instruments (Hanau, Germany).
The crystal structural analysis was performed in standard Bragg–Brentano geometry by using an X-ray diffractometer XRD-D5000 from SIEMENS AG (Munich, Germany), with a CuKα radiation (λCuKα = 0.15418 nm) and the graphite curved monochromator. For the quantitative analysis, the measured θ/2θ spectra from the 23° to 87° 2θ range were compared with the standard anatase (PDF 21-1272) and rutile (PDF 21-1276) phases. The morphology of the samples (both surface and cross-sections) was investigated by a field effect scanning electron microscope FE-SEM ULTRA 55 equipped with a built-in energy dispersive spectrometer (EDS) having an X-Ray Fluorescence Analyzer MESA 500 from ZEISS AG (Jena, Germany).
The gas-sensing tests of the Pt/TiO2/Pt-based sensors were performed in a sensor and catalyst characterization unit SESAM (German Aerospace Center, Cologne, Germany), which is fully computer-controlled with custom-made LabVIEW software. The SESAM test unit is comprised of MFC-647b type mass flow controllers (MKS Instruments GmbH, Munich, Germany). The unit has the capability of directing eight various kinds of gases simultaneously via eight-channel flow into the gas mixing chamber made of Quartz-glass recipient, which is permanently placed inside a CARBOLITE tube furnace. The setup enables measuring the high resistance ≈1011 Ω of the sensing layer and signal noise is negligible due the usage of a triaxial cable and sensitive resistance measuring source meter Keithley 2635A (Keithley Instruments GmbH, Germering, Germany). The sensors are contacted with thin Pt wires using silver paste solder. During the gas sensing tests, the gas flow rate was kept constant at 400 mL/min, and a constant voltage of 1 V (DC bias) was applied. All instruments included in this unit are controlled with the LabVIEW program.

3. Results and Discussion

3.1. Characterization

In the present case, the coatings were ex situ annealed at 800 °C; contradicting with previous reports [24,25], it can be seen that only anatase phases are present in undoped TiO2 coatings, whereas in Cr-TiO2 coatings, both anatase (PDF 21-1272) and rutile (PDF 21-1276) phases are present in Figure 2, which is in good agreement with [26,27]. From the XRD diffraction patterns, it is notable that the following anatase (101), (004), (200), (101) and rutile (110), (101), (211) and (220) peaks are present. For anatase phases, the peaks are found at 2θ values of 25.41°, 37.76°, 48.10°, and 53.88°, whereas the peaks corresponding to the rutile phase are observed at 2θ values of 27.44°, 36.06°, and 41.18°. It is very interesting to note that the peak of A (004) is comparatively higher than that of the A (101) phase. For the powder diffraction patterns, the A (101) peak of TiO2 is higher than the other peaks, which contradicts the above XRD diffractograms, as shown in Figure 2. This scenario happens due to the corundum substrate C [0006], where the crystallites assimilate preferentially following the substrate crystals orientations. From the previous reports, one can anticipate an anatase to rutile transformation for undoped TiO2 at annealing temperature higher than 800 °C. However, for Cr-TiO2 coatings, this phase transformation already exists at 800 °C, which is in good agreement with the previous reports [28,29,30]. A slight shift in the reflections of TiO2 in the above X-ray diffraction patterns has been observed (Figure 2), which indicates Cr integration into the TiO2 matrix. Furthermore, no significant changes in the values of lattice parameters are anticipated, which is most probably due to the minor variation in the ionic radii of Cr (0.61 ) and Ti (0.60 ). The weight fraction of anatase phase WA(%) was estimated with the help of Spurr–Mayer’s equation for A(101), A(004), and R(110) peaks. The detected weight fraction of anatase is 100 wt % in undoped TiO2, while for Cr-doped TiO2, the respective wt % of anatase phase A(101):R(110) and A(004):R(110) WA(%) is ≈64 wt % and ≈23, respectively (the estimation error with this method is 10%). Thus, one can infer from the quantitative analysis that Cr significantly promotes the phase transformation from anatase to rutile. In addition, metallic Cr and its secondary phases are absent, which affirm that Cr segregates into TiO2 instead of forming metallic or secondary oxides (e.g., Cr2O3).
From the microstructure and compositional analysis of the sensor structure in Figure 3a–c, it is observed that both electrodes show different morphology but are highly stable and do not vanish or crack even after annealing at 800 °C [31]. The Pt layer crystallized with the average faceted grain size of ≈150–200 nm for bottom electrodes and ≈100–150 nm for top electrodes. There are pores (average estimated pore size is ≈350–400 nm) in the top electrodes, which promotes the gas diffusion to the sensing layer surface whereas the pores size in bottom electrode is relatively smaller (≈200–300 nm), and its accumulation is highly in compliance with the substrate even without using the supportive layer for adhesion purpose [32].
A typical micro columnar structure of the sensing layer is observed with excellent adhesion [33,34]. From Figure 3d–i, it can be seen that these vertical columnar structures are around 2 μm in height. After annealing at 800 °C, the grains (amorphous and finely dense in as-deposited form) on the top surface become centered, and these columnar structures transform to crystalline structures; see Figure 3d,g. There is a formation of occasional nanostructured grains existing as pumps or nodules (average size of ≈70–80 nm) grown on the surface. It can be seen in the SEM images that the average bottom diameters of these columns show an irregular trend. For instance, undoped coatings show an average value of ≈400–520 nm, while for Cr-TiO2 coatings, the estimated average diameter of the columns is smaller (≈300–430 nm). From Figure 3g–i, it is observable that there are a significant number of voids and cracks, which increased in the case of Cr-doped TiO2 films as compared to pristine TiO2 layer Figure 3d–f. The voids size varies from 100 to 300 nm for both coatings. However, the widening of these cracks and voids after annealing at 800 °C is more significant in Cr-TiO2 [35].
From the elemental compositional studies with EDX, it was found that the Cr contents (relative to Ti) in the doped coatings are about 0.5 wt % (Pt/Ti0.95Cr0.5O2/Pt). The EDX analysis confirms the transformation of non-stoichiometric TiO2−x to near stoichiometric TiO2 after annealing at 800 °C. From Figure 4, it can be seen that there is a diffusion of Pt top and bottom electrodes into the depth of several nanometers into the sensing layer, which is caused by thermally induced Pt atoms into the sensing layer during the annealing process at 800 °C. Furthermore, it was affirmed by point analysis that the interdiffusion rate is higher at the bottom Pt/TiO2 interface than at the top interface.
From the GDOES analysis shown in Figure 5, it is noticeable from the variation in quantitative elemental distribution profile that both the as-coated samples consist of a highly non-stoichiometric TiO2−x layer as the O 130 has a decreasing trend, while Ti 399 has an increasing trend; see Figure 5a,b. The figure also shows that the bottom part of the sensing layer is rich in metallic Ti 399. Stoichiometric films are formed after annealing at 800 °C with both concentrations, O 130 and Ti 399, has become constant. It is critical to note that the Ti:O ratio in the region near the bottom Pt/TiO2 interface is still non-stoichiometric, and it is rich in metallic Ti, which is in good agreement with the EDX analysis. However, this Ti:O ratio is about ≈0.80:2.10; thus, one can conclude that the gradient of O 130 concentration holds even after 800 °C heat treatment (Figure 5c,d). This is also affirmed by the Pt 266 peaks in the annealed samples, which showed extended inclination into TiO2 from the bottom electrode; such observations were also mentioned in previous reports [36].

3.2. Sensor Performance

The comparative sensing performance of the pristine TiO2 sensing layer and Ti0.95Cr0.5O2 (we simply mention it with Cr-TiO2) are shown in Figure 6. It is found that undoped TiO2 exhibited a typical n-type sensor response (SRn = RNO2/Rair), while there was an inverse p-type response (SRp = Rair/RNO2) for the Cr-doped TiO2 based sensor when exposed to a certain concentration of NO2 gas; here, Rair and RNO2 indicate saturated resistances in the air and NO2 gas, respectively. A low baseline resistance in air was observed for the Pt/Cr-TiO2/Pt sensor (≈3.5 × 105 ohm) compared to that of the undoped TiO2 sensor (≈2.6 × 106 ohm) at 400 °C shown in Figure 6. The inimitable sensing responses of both prepared sensors toward 50–300 ppm of NO2 at 400 °C is illustrated in Figure 7a. Furthermore, it is evident from Figure 7a that the Cr-doped TiO2 sensor showed a superior response toward NO2. For example, the Pt/TiO2/Pt-based sensor showed about SRn ≈ 1.58, while the Pt/Cr-TiO2/Pt based sensor showed about SRp ≈ 33.24 toward 50 ppm NO2 at 400 °C. In both cases, the sensor response increased with the NO2 gas concentration. However, a significant drift in the baseline resistance is found for Pt/TiO2/Pt-based sensor due to incomplete recovery, but then, Pt/Cr-TiO2/Pt-based sensors showed excellent response with full recovery after the NO2 gas is off. In addition, for the sensors based on the Pt/Cr-TiO2/Pt structure, the sensor response values increased as the operating temperature decreased from 400 to 200 °C, compared with Pt/TiO2/Pt-based sensors that showed the best response at 400 °C while observing the decreasing trend with an operating temperature decrease in the same range. One can infer that Cr incorporation in the Ti-O matrix can significantly decrease the operating temperature of the sensor and thus require less power consumption when designing a gas sensor for real-life applications.
Based on the aforementioned superior performance, the further analysis was done on Pt/Cr-TiO2/Pt-based sensors. It is well-known that the response of the gas sensor is strongly dependent on the gas concentration and operating temperature due to the adsorption/desorption processes of various gases on the surface of the sensing layer. Figure 5b depicts the sensing analysis of Pt/Cr-TiO2/Pt sensors at various temperatures (100 °C, 200 °C, 300 °C, and 400 °C) toward increasing NO2 concentration. For the reader’s clarity, it is to be noted that in the current study, the actual gas concentration used is 25–400 ppm NO2, while the curves in the Figure 7b are extrapolated to show the trend of sensor response with gas concentration. The gas-sensing measurements of the Pt/Cr-TiO2/Pt sensor in Figure 7b show that the maximum response was obtained at 200 °C. Thus, the dynamic sensing responses of the Pt/Cr-TiO2/Pt sensor toward low concentrations of NO2 gas were measured at 200 °C. Figure 7c depicts the real-time variation in baseline resistance before and after exposure to 7, 12, 18, 25, 50, 75, and 125 ppm NO2. The gas sensing measurements showed that the sensor could detect NO2 gas at a concentration as low as 7 ppm with reproducible signals at 200 °C. The Ti0.95Cr0.5O2 sensor shows the recoverable and reproducible response at 7 ppm NO2 gas, which indicates that the sensor could detect a low concentration of the NO2 gas, thus leading to the detection of a broad range of NO2 concentration (7–400 ppm). To further elaborate, at 200 °C, the response of Pt/Cr-TiO2/Pt based sensors is about ≈212 and 545 toward 50 and 100 ppm, which is higher than the response mentioned in the reports on the TiO2-based sensors [37,38,39,40]. Furthermore, the Pt/Cr-TiO2/Pt sensors showed remarkable sensing characteristics such as response and recovery times (estimated for 90% resistance change). For instance, at 200 °C, the response and recovery times toward 100 ppm are 37 s and 24 s respectively, which indicates the fast response and recovery of the sensors.

3.3. Adsorption Kinetics

It is well-established that TiO2 offers multifunctional properties [41], and while designing a gas sensor, it is of utmost importance to get scientific insights about the gas adsorption kinetics onto the surface of the sensing layer. In our previous report, we have shown that room temperature hydrogen adsorption onto an ordered mesoporous TiO2 surface is mainly governed by the pseudo second-order rate equations. However, in the present case, we chose the NO2 adsorption fitting at 200 °C, which is the optimum temperature to obtain the highest response, and found that the pseudo first-order model fits well with the obtained dynamic responses. In fact, the obtained adsorption regressions were fitted with the Elovich model (EM), inter-particle diffusion model (IPD), Lagergren’s pseudo first order (or simply pseudo first order—PFO), and Ho’s pseudo second-order (or simply pseudo second-order—PSO) models.
For the Elovich model, in the adsorption regression expression (Equation (1)), we consider that the surface coverage of NO2 onto the sensor layer is θ t   (at time t), the adsorption and desorption are represented by a and b respectively.
d θ t d t = a   e b θ t
Under the condition that θ t = 0 at t = 0, the integration of the Equation (1) results in Equation (2).
θ t = 1 b ln ( t + t 0 ) + 1 b ln ( t 0 )
If the condition t >> t0 is valid, then one may write:
θ t = 1 b ln ( a b ) + 1 b ln ( t ) .
After fitting the obtained data with the Elovich model, we found an irregular trend in the R2 values for each fitting curve. The R2 values range from 0.33 to 0.66, which means that the adsorption does not follow Equation (3) of the Elovich model; the fitting of these curves can be seen in Figure 8a.
θ t = k i d t 0.5 + C i
As demonstrated in Figure 8b, next, the response data were fitted with the inter-particle diffusion (IPD) model, which is generally governed with molecular or ionic transport at the grain boundaries of the polycrystalline sensing layer, and its fitting is done by using Equation (4). Here, θ t is the surface coverage of NO2 gas adsorbed on the sensing layer, Ci is the inter-particle boundary width and directly linked with the thickness of the sensing layer, and kid is the inter-particle diffusion constant. Since our sensing layers are polycrystalline in nature but not purely a porous layer, thus, the IPD fitting values of R2 (0.49–0.88) are also not in accordance with the linearity of the model fitting; see Figure 8b.
d θ t d t k 2   ( θ e θ t ) n
The above equation (Equation (5)) represents the pseudo nth order rate regression. For n = 1 and n = 2, we can fit the pseudo first-order Figure 8c and pseudo second-order Figure 8d adsorption kinetics. In Equation (5), k 2 is the first-order reaction rate constant, θ t is the surface coverage at equilibrium, and θ e is the surface coverage at equilibrium, respectively. The integration of Equation (5) yields the following relationship:
ln ( θ e θ t ) = ln ( θ e ) k 2 t .
In addition, for PSO, the above equation (Equation (5)) can be solved as:
t θ t = 1 k 3 θ e 2 + 1 θ e t .
Then, the obtained sensor dynamic responses are further fitted with PFO and PSO, as shown in Figure 8c,d. The fitting with PFO is more accurate than PSO as the R2 values of PFO fall in the range of 0.93–0.97, while for PSO, the R2 values fall in the range of 0.58–0.89. The R2 values obtained after the fitting of the data with various models are shown in Table 1 and plotted in Figure 9a. Thus, as given in Figure 9b, we further proceed with the fitting of obtained data with the PFO model, and the estimated reaction parameters are shown in Table 1. Here, the surface coverage at equilibrium θ e corresponds to the sensor response SR and reaction constant corresponds to the resistance of the sensor; see Figure 9c. Thus, we can conclude that the PFO model has the highest appositeness to describe the NO2 adsorption on the surface of the Pt/Cr-TiO2/Pt sensor at 200 °C. The values of rate constants k are seven orders of magnitude lower than the saturated sensor resistance in various gas concentrations; however, the surface coverage values (9.03–3866.09) are well pertinent with the experimentally obtained sensor response shown in Figure 9c. Furthermore, the data presented for PFO in Table 1 illustrate that the surface coverage is linearly proportional to the amount of gas adsorption (sensor response) that is also depicted from Figure 8, where its linear fitting is observed with increasing NO2 gas concentrations. Here, the reaction rate is also dependent on the number of adsorption sites and the rate constant k. The reaction constant k is determined, which decreases linearly with gas concentration. This corresponds to the decrease in the sensor’s resistance, indicating that the adsorption reaction in Cr-TiO2 is a pseudo first-order reaction, which is more favorable as compared to the pseudo second-order adsorption reaction, as shown in Figure 8a.

3.4. Sensing Mechanism

The sensing mechanism for NO2 gas onto the TiO2 sensing layer is based on three stages: firstly, the adsorption of gas, secondly, the transfer of charge due to the adsorbed gas, and lastly, desorption of the gas. The surface of the metal oxide (adsorbate) has a natural ability to collect and gather species based on chemicals (adsorbent). As the adsorbate is accessible for air, the oxygen percentage in its molecule (lower than 150 °C) or available ionic phase (higher than 150 °C) can be easily adsorbed, giving a depletion layer by the captured electrons. As shown in Figure 10, the sensing mechanism for NO2 depends on occurrence of the reactions between adsorbate and NO2 gas; this yields an increase or decrease in the resistance value of the sensing layer due to the charge transfer function, as shown by the following Equation (8).
NO 2   ( gas ) + e NO 2   ( ads )  
If the adsorbate is n-type, the increase in resistance is caused by the NO2 surface reaction, this is firstly led by the oxygen vacancies VO that are doubly ionized, and secondly, the encounter between O/O2− and NO2 gas adsorption, as the adsorption of NO2 is more robust, it dominates the O/O2− adsorption. On the other side, the decrease in resistance in the p-type adsorbate is accomplished by an inversion layer which is created by trivalent acceptor-type impurity (e.g., Cr3+). As the chromium Cr3+ is doped with the titanium dioxide, it changes the electronic structure of TiO2 by developing an acceptor level just below the conduction band shown in Figure 10, thus causing the Fermi level to lower down. This variation from n-type to p-type conductivity happens after a certain concentration of Cr3+ doping, as in this scenario, the defects found in the acceptor have outnumbered the donor defects, as shown in the Equation (9) below.
Cr 2 O 3 + 1 2 O 2 2 Cr Ti + 2 h + 4 O o x
According to the literature, this p-type conductivity is attained by at least 5 at % dopant concentration of Cr in TiO2 nanopowders. In our case, 2 at % of Cr dopant has already yielded in p-type sensor behavior; also, there are no chromium oxides peaks detected in the XRD patterns. According to XRD analysis, 64 ± 10% of the anatase phases is found in the undoped TiO2 that has a marginally higher optical bandgap of 3.2 eV than that of the rutile phase after Cr doping, giving the value of 3 eV in Cr-TiO2 films. The better sensitivity of NO2 in Cr-TiO2 cannot be linked with only this point. This bandgap difference of 0.2 eV can alter the height of the activation energy barrier initiated between the grains (eVS). In our case, the bandgap values for undoped TiO2 (0.36 eV) and Cr-TiO2 (0.35) are not much varied.

4. Conclusions

In this work, we demonstrate a highly sensitive Pt/Cr-TiO2/Pt sensor design suitable for NO2 detection with the reduced operating temperatures (200 °C) and lower limit of the NO2 gas concentrations (7 ppm). The XRD, SEM, EDX, and GDOES analyses confirmed that the dominant anatase A (004) crystal structure was preferentially oriented following the crystal orientation of the substrate, which was a morphology of columnar structured TiO2 matrix with a column diameter of ≈400–520 nm for undoped TiO2 and a smaller column diameter ≈300– 430 nm for Cr-TiO2 coatings, and the interdiffusion of Pt/TiO2 specifically more at the bottom interface. The sensor response at 200 °C is relatively high ≈1.08 × 103 toward 300 ppm NO2 with a response (recovery) time of ≈37 s (≈24 s). The adsorption kinetics fitting showed that pseudo first-order reaction regressions are linearly fitted (R2 value above 0.97) with the obtained sensor responses. The obtained reaction rate (32.60–1.09) and surface coverage (9.03–3866.09) showed comparative trends observed for the sensor resistance and sensor response; however, the rate constant value is 107 orders lower than that of the sensor resistance. The sensing mechanism is explained on the basis of surface adsorption kinetics and reduced grain size due to Cr segregation into the Ti-O matrix.

Author Contributions

Conceptualization, methodology and resources, data curation, formal analysis, investigation and writing—original draft, A.A.H.; Formal analysis and writing—original draft, Q.F.; Investigation and writing—review and editing, A.M.; Investigation and writing—review and editing, A.S.; Formal analysis and validation, Y.J.; Writing—review and editing, Project administration and supervision, B.S.; All authors equally contributed to this work. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, grant number 51850410506 and the Central University Basic Scientific Research Business Expenses Special Funds, grant number NG2020002. This work has also been supported by DAAD-DLR Fellowship Program under the fellowship number 165.

Institutional Review Board Statement

The study was conducted according to the guidelines of the Declaration of German Aerospace Center (DLR), German Academic Exchange Services (DAAD) and Nanjing University of Aeronautics and Astronautics (NUAA). The study was funded and approved by the DLR-DAAD fellowship grants and National Natural Science Foundation of China.

Informed Consent Statement

Not applicable as this study does not include any human and/or animal participants under observation.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to DLR/DAAD/NUAA confidentiality and privacy.

Conflicts of Interest

The authors declare no competing financial interest.

References

  1. Myasoedova, T.N.; Plugotarenko, N.K.; Moiseeva, T.A. Copper-Containing Films Obtained by the Simple Citrate Sol–Gel Route for NO2 Detection: Adsorption and Kinetic Study. Chemosensors 2020, 8, 79. [Google Scholar] [CrossRef]
  2. Ueda, T.; Boehme, I.; Hyodo, T.; Shimizu, Y.; Weimar, U.; Barsan, N. Enhanced NO2-Sensing Properties of Au-Loaded Porous In2O3 Gas Sensors at Low Operating Temperatures. Chemosensors 2020, 8, 72. [Google Scholar] [CrossRef]
  3. Levy, J.I. Impact of residential nitrogen dioxide exposure on personal exposure: An international study. J. Air Waste Manag. Assoc. 1998, 48, 553–560. [Google Scholar] [CrossRef]
  4. Tomeček, D.; Piliai, L.; Hruška, M.; Fitl, P.; Gadenne, V.; Vorokhta, M.; Matolínová, I.; Vrňata, M. Study of Photoregeneration of Zinc Phthalocyanine Chemiresistor after Exposure to Nitrogen Dioxide. Chemosensors 2021, 9, 237. [Google Scholar] [CrossRef]
  5. Hjiri, M. Highly sensitive NO2 gas sensor based on hematite nanoparticles synthesized by sol–gel technique. J. Mater. Sci. Mater. Electron. 2020, 31, 5025–5031. [Google Scholar] [CrossRef]
  6. Kaur, N.; Zappa, D.; Comini, E. Shelf Life Study of NiO Nanowire Sensors for NO2 Detection. Electron. Mater. Lett. 2019, 15, 743–749. [Google Scholar] [CrossRef]
  7. Teoh, L.G.; Hung, I.M.; Shieh, J.; Lai, W.H.; Hon, M.H. High sensitivity semiconductor NO2 gas sensor based on mesoporous WO3 thin film. Electrochem. Solid State Lett. 2003, 6, G108. [Google Scholar] [CrossRef] [Green Version]
  8. Barsan, N.; Koziej, D.; Weimar, U.; Chemical, A.B. Metal oxide-based gas sensor research: How to? Sens. Actuators B Chem. 2007, 121, 18–35. [Google Scholar] [CrossRef]
  9. Sholehah, A.; Faroz, D.F.; Huda, N.; Utari, L.; Septiani, N.L.W.; Yuliarto, B. Synthesis of ZnO flakes on flexible substrate and its application on ethylene sensing at room temperature. Chemosensors 2020, 8, 2. [Google Scholar] [CrossRef] [Green Version]
  10. Galstyan, V.; Comini, E.; Ponzoni, A.; Sberveglieri, V.; Sberveglieri, G. ZnO quasi-1D nanostructures: Synthesis, modeling, and properties for applications in conductometric chemical sensors. Chemosensors 2016, 4, 6. [Google Scholar] [CrossRef]
  11. Saruhan, B.; Yüce, A.; Gönüllü, Y.; Kelm, K. Effect of Al doping on NO2 gas sensing of TiO2 at elevated temperatures. Sens. Actuators B Chem. 2013, 187, 586–597. [Google Scholar] [CrossRef]
  12. Haidry, A.A.; Durina, P.; Tomasek, M.; Gregus, J.; Schlosser, P.; Mikula, M.; Truhly, M.; Roch, T.; Plecenik, T.; Pidik, A. Effect of post-deposition annealing treatment on the structural, optical and gas sensing properties of TiO2 thin films. Key Eng. Mater. 2012, 510–511, 467–474. [Google Scholar] [CrossRef]
  13. Gönüllü, Y.; Haidry, A.A.; Saruhan, B. Nanotubular Cr-doped TiO2 for use as high-temperature NO2 gas sensor. Sens. Actuators B Chem. 2015, 217, 78–87. [Google Scholar] [CrossRef]
  14. Zhang, D.; Fan, Y.; Li, G.; Du, W.; Li, R.; Liu, Y.; Cheng, Z.; Xu, J. Biomimetic synthesis of zeolitic imidazolate frameworks and their application in high performance acetone gas sensors. Sens. Actuators B Chem. 2020, 302, 127187. [Google Scholar] [CrossRef]
  15. Yang, X.; Hao, X.; Liu, T.; Liu, F.; Wang, B.; Ma, C.; Liang, X.; Yang, C.; Zhu, H.; Zheng, J.; et al. CeO2-based mixed potential type acetone sensor using La1−xSrxCoO3 sensing electrode. Sens. Actuators B Chem. 2018, 269, 118–126. [Google Scholar] [CrossRef]
  16. Abdelghani, R.; Hassan, H.S.; Morsi, I.; Kashyout, A. Nano-architecture of highly sensitive SnO2–based gas sensors for acetone and ammonia using molecular imprinting technique. Sens. Actuators B Chem. 2019, 297, 126668. [Google Scholar] [CrossRef]
  17. Amiri, V.; Roshan, H.; Mirzaei, A.; Neri, G.; Ayesh, A.I. Nanostructured metal oxide-based acetone gas sensors: A review. Sensors 2020, 20, 3096. [Google Scholar] [CrossRef] [PubMed]
  18. Shaalan, N.; Yamazaki, T.; Kikuta, T. Effect of micro-electrode geometry on NO2 gas-sensing characteristics of one-dimensional tin dioxide nanostructure microsensors. Sens. Actuators B Chem. 2011, 156, 784–790. [Google Scholar] [CrossRef]
  19. Tamaki, J.; Miyaji, A.; Makinodan, J.; Ogura, S.; Konishi, S. Effect of micro-gap electrode on detection of dilute NO2 using WO3 thin film microsensors. Sens. Actuators B Chem. 2005, 108, 202–206. [Google Scholar] [CrossRef]
  20. Li, Z.; Yao, Z.; Haidry, A.A.; Plecenik, T.; Grancic, B.; Roch, T.; Gregor, M.; Plecenik, A. The effect of Nb doping on hydrogen gas sensing properties of capacitor-like Pt/Nb-TiO2/Pt hydrogen gas sensors. J. Alloys Compd. 2019, 806, 1052–1059. [Google Scholar] [CrossRef]
  21. Capone, S.; Siciliano, P.; Quaranta, F.; Rella, R.; Epifani, M.; Vasanelli, L. Moisture influence and geometry effect of Au and Pt electrodes on CO sensing response of SnO2 microsensors based on sol–gel thin film. Sens. Actuators B Chem. 2001, 77, 503–511. [Google Scholar] [CrossRef]
  22. Haidry, A.A.; Xie, L.; Wang, Z.; Zavabeti, A.; Li, Z.; Plecenik, T.; Gregor, M.; Roch, T.; Plecenik, A. Remarkable improvement in hydrogen sensing characteristics with Pt/TiO2 interface control. ACS Sens. 2019, 4, 2997–3006. [Google Scholar] [CrossRef] [PubMed]
  23. Haidry, A.A.; Cetin, C.; Kelm, K.; Saruhan, B. Sensing mechanism of low temperature NO2 sensing with top–bottom electrode (TBE) geometry. Sens. Actuators B Chem. 2016, 236, 874–884. [Google Scholar] [CrossRef]
  24. Bakri, A.; Sahdan, M.Z.; Adriyanto, F.; Raship, N.; Said, N.; Abdullah, S.; Rahim, M. Effect of annealing temperature of titanium dioxide thin films on structural and electrical properties. AIP Conf. Proc. 2017, 1788, 030030. [Google Scholar] [CrossRef] [Green Version]
  25. Zhang, Q.; Li, C. High Temperature Stable Anatase Phase Titanium Dioxide Films Synthesized by Mist Chemical Vapor Deposition. Nanomaterials 2020, 10, 911. [Google Scholar] [CrossRef]
  26. Sertel, B.C.; Efkere, H.I.; Ozcelik, S. Gas Sensing Properties of Cr Doped TiO2 Films Against Propane. IEEE Sens. 2020, 20, 13436–13443. [Google Scholar] [CrossRef]
  27. Su, T.-Y.; Chen, Y.-Z.; Wang, Y.-C.; Tang, S.-Y.; Shih, Y.-C.; Cheng, F.; Wang, Z.M.; Lin, H.-N.; Chueh, Y.-L. Highly sensitive, selective and stable NO2 gas sensors with a ppb-level detection limit on 2D-platinum diselenide films. J. Mater. Chem. C 2020, 8, 4851–4858. [Google Scholar] [CrossRef]
  28. Lee, S.P. Electrodes for semiconductor gas sensors. Sensors 2017, 17, 683. [Google Scholar] [CrossRef]
  29. Li, Z.; Xing, L.; Zhang, Z. Photocatalytic Properties of Columnar Nanostructured Films Fabricated by Sputtering Ti and Subsequent Annealing. Adv. Mater. Sci. Eng. 2012, 2012, 413638. [Google Scholar] [CrossRef] [Green Version]
  30. Spurr, R.A.; Myers, H. Quantitative analysis of anatase-rutile mixtures with an X-ray diffractometer. Anal. Chem. 1957, 29, 760–762. [Google Scholar] [CrossRef]
  31. Lyson-Sypien, B.; Czapla, A.; Lubecka, M.; Gwizdz, P.; Schneider, K.; Zakrzewska, K.; Michalow, K.; Graule, T.; Reszka, A.; Rekas, M.; et al. Nanopowders of chromium doped TiO2 for gas sensors. Sens. Actuators B Chem. 2012, 175, 163–172. [Google Scholar] [CrossRef]
  32. Suhak, Y.; Roshchupkin, D.; Redkin, B.; Kabir, A.; Jerliu, B.; Ganschow, S.; Fritze, H. Correlation of Electrical Properties and Acoustic Loss in Single Crystalline Lithium Niobate-Tantalate Solid Solutions at Elevated Temperatures. Crystals 2021, 11, 398. [Google Scholar] [CrossRef]
  33. Perini, N.; Prado, A.; Sad, C.; Castro, E.; Freitas, M. Electrochemical impedance spectroscopy for in situ petroleum analysis and water-in-oil emulsion characterization. Fuel 2012, 91, 224–228. [Google Scholar] [CrossRef] [Green Version]
  34. Dobromir, M.; Konrad-Soare, C.T.; Stoian, G.; Semchenko, A.; Kovalenko, D.; Luca, D. Surface Wettability of ZnO-Loaded TiO2 Nanotube Array Layers. Nanomaterials 2020, 10, 1901. [Google Scholar] [CrossRef] [PubMed]
  35. Smecca, E.; Sanzaro, S.; Galati, C.; Renna, L.; Gervasi, L.; Santangelo, A.; Condorelli, G.G.; Grosso, D.; Bottein, T.; Mannino, G. Porous Gig-Lox TiO2 Doped with N2 at Room Temperature for P-Type Response to Ethanol. Chemosensors 2019, 7, 12. [Google Scholar] [CrossRef] [Green Version]
  36. Saruhan, B.; Haidry, A.A.; Yüce, A.; Ciftyürek, E.; Mondragón Rodríguez, G.C. A Double Layer Sensing Electrode “BaTi(1−X)RhxO3/Al-Doped TiO2” for NO2 Detection above 600 °C. Chemosensors 2016, 4, 8. [Google Scholar] [CrossRef] [Green Version]
  37. Ruiz, A.M.; Sakai, G.; Cornet, A.; Shimanoe, K.; Morante, J.R.; Yamazoe, N. Cr-doped TiO2 gas sensor for exhaust NO2 monitoring. Sens. Actuators B Chem. 2003, 93, 509–518. [Google Scholar] [CrossRef]
  38. Sivachandiran, L.; Thévenet, F.; Gravejat, P.; Rousseau, A. Investigation of NO and NO2 adsorption mechanisms on TiO2 at room temperature. Appl. Catal. B Environ. 2013, 142, 196–204. [Google Scholar] [CrossRef]
  39. Hope, G.A.; Bard, A.J. Platinum/titanium dioxide (rutile) interface. Formation of ohmic and rectifying junctions. J. Phys. Chem. 1983, 87, 1979–1984. [Google Scholar] [CrossRef]
  40. Zhu, Z.; Lin, S.-J.; Wu, C.-H.; Wu, R.-J.J.S.; Physical, A.A. Synthesis of TiO2 nanowires for rapid NO2 detection. Sens. Actuators A Phys. 2018, 272, 288–294. [Google Scholar] [CrossRef]
  41. Mokrushin, A.S.; Simonenko, E.P.; Simonenko, N.P.; Bukunov, K.A.; Gorobtsov, P.Y.; Sevastyanov, V.G.; Kuznetsov, N.T. Gas-sensing properties of nanostructured TiO2–xZrO2 thin films obtained by the sol–gel method. J. Sol-Gel Sci. Technol. 2019, 92, 415–426. [Google Scholar] [CrossRef]
Figure 1. The schematics showing the steps to fabricate Pt/TiO2/Pt-based sensors. The figure also includes the real photograph of one of the real sensors (photos at the right hand-side) fabricated in our lab with the procedure mentioned in the experimental part and optical images of the Pt/TiO2/Pt based cross-bars in the insets.
Figure 1. The schematics showing the steps to fabricate Pt/TiO2/Pt-based sensors. The figure also includes the real photograph of one of the real sensors (photos at the right hand-side) fabricated in our lab with the procedure mentioned in the experimental part and optical images of the Pt/TiO2/Pt based cross-bars in the insets.
Chemosensors 10 00011 g001
Figure 2. The X-ray diffractograms of the pristine TiO2-sensing layer and Ti0.95Cr0.5O2-sensing layer deposited on corundum [0006] substrate and annealed at 800 °C.
Figure 2. The X-ray diffractograms of the pristine TiO2-sensing layer and Ti0.95Cr0.5O2-sensing layer deposited on corundum [0006] substrate and annealed at 800 °C.
Chemosensors 10 00011 g002
Figure 3. The scanning electron microscope (SEM) analysis of both the coatings. The cross-sectional micrograph of the Pt/TiO2/Pt structure (a) along with surface topography of Pt-TE (b) and Pt-BE (c). The surface topographical and cross-sectional micrographs of the pristine TiO2 sensing layer (df) and Ti0.95Cr0.5O2 (gi) sensing layer deposited on corundum [0006] substrate and annealed at 800 °C. The color mapping of the various elements (Pt, Ti, O and Cr), showing the Cr incorporation into the TiO2 sensing layer.
Figure 3. The scanning electron microscope (SEM) analysis of both the coatings. The cross-sectional micrograph of the Pt/TiO2/Pt structure (a) along with surface topography of Pt-TE (b) and Pt-BE (c). The surface topographical and cross-sectional micrographs of the pristine TiO2 sensing layer (df) and Ti0.95Cr0.5O2 (gi) sensing layer deposited on corundum [0006] substrate and annealed at 800 °C. The color mapping of the various elements (Pt, Ti, O and Cr), showing the Cr incorporation into the TiO2 sensing layer.
Chemosensors 10 00011 g003
Figure 4. The color mapping of the various elements (Pt, Ti, O, and Cr), showing the Cr incorporation into the TiO2 sensing layer.
Figure 4. The color mapping of the various elements (Pt, Ti, O, and Cr), showing the Cr incorporation into the TiO2 sensing layer.
Chemosensors 10 00011 g004
Figure 5. The glow discharge optical emission spectroscopic plots showing the quantitative analysis of evolution of composition of the various elements (Pt, Ti, O, and Cr) in the pristine TiO2 sensing layer (a,b) and Ti0.95Cr0.5O2 (c,d) both as-deposited and annealed at 800 °C. The color elemental composition shows the defected sensing layer in the as-deposited state and stoichiometric state at 800 °C annealing with evidence of Cr incorporation into the TiO2 sensing layer.
Figure 5. The glow discharge optical emission spectroscopic plots showing the quantitative analysis of evolution of composition of the various elements (Pt, Ti, O, and Cr) in the pristine TiO2 sensing layer (a,b) and Ti0.95Cr0.5O2 (c,d) both as-deposited and annealed at 800 °C. The color elemental composition shows the defected sensing layer in the as-deposited state and stoichiometric state at 800 °C annealing with evidence of Cr incorporation into the TiO2 sensing layer.
Chemosensors 10 00011 g005
Figure 6. The dynamic responses of Pt/TiO2/Pt and Pt/Cr-TiO2/Pt sensors measured at 400 °C toward different concentrations (50, 100, 200, 300 ppm) of NO2 gas.
Figure 6. The dynamic responses of Pt/TiO2/Pt and Pt/Cr-TiO2/Pt sensors measured at 400 °C toward different concentrations (50, 100, 200, 300 ppm) of NO2 gas.
Chemosensors 10 00011 g006
Figure 7. The comparative sensing performance of the Pt/TiO2/Pt and Pt/Cr-TiO2/Pt (a) sensors measured at 400 °C and annealed at 800 °C. Further sensing analysis of Pt/Cr-TiO2/Pt sensors at various temperatures 100 °C, 200 °C, 300 °C, and 400 °C (b), the dynamic responses of the Pt/Cr-TiO2/Pt sensor measured at 200 °C toward low concentrations (7, 12, 18, 25, 50, 75, and 125 ppm) of NO2 gas (c), the selectivity of the Pt/Cr-TiO2/Pt sensor toward NO2 again H2, and CO reducing exhaust gases (d).
Figure 7. The comparative sensing performance of the Pt/TiO2/Pt and Pt/Cr-TiO2/Pt (a) sensors measured at 400 °C and annealed at 800 °C. Further sensing analysis of Pt/Cr-TiO2/Pt sensors at various temperatures 100 °C, 200 °C, 300 °C, and 400 °C (b), the dynamic responses of the Pt/Cr-TiO2/Pt sensor measured at 200 °C toward low concentrations (7, 12, 18, 25, 50, 75, and 125 ppm) of NO2 gas (c), the selectivity of the Pt/Cr-TiO2/Pt sensor toward NO2 again H2, and CO reducing exhaust gases (d).
Chemosensors 10 00011 g007
Figure 8. The response and regression analysis of NO2 adsorption (7, 12, 18, 25, 50, and 75 ppm) on the Pt/Cr-TiO2/Pt surface. The original data are shown as color symbols, and their fitting is shown with various color dashed lines. The data were fitted by using the Elovich model (a), inter-particle diffusion model (b), pseudo first-order model also known as Lagergren’s first-order model (c) and pseudo second-order model also known as Ho’s second-order model (d).
Figure 8. The response and regression analysis of NO2 adsorption (7, 12, 18, 25, 50, and 75 ppm) on the Pt/Cr-TiO2/Pt surface. The original data are shown as color symbols, and their fitting is shown with various color dashed lines. The data were fitted by using the Elovich model (a), inter-particle diffusion model (b), pseudo first-order model also known as Lagergren’s first-order model (c) and pseudo second-order model also known as Ho’s second-order model (d).
Chemosensors 10 00011 g008
Figure 9. The comparative plot showing the R2 values for the Elovich, IPD, PFO, and PSO models (a), the plot showing the linear fitting of the data by using a pseudo first-order model also known as Lagergren’s first-order model (b), and the estimated values of its reaction rate constant k (the squared symbols in red color), the values are normalized to 107 to make the plot clear to the reader, and the adsorption capacity θe (circled symbol in blue color) values are shown in (c).
Figure 9. The comparative plot showing the R2 values for the Elovich, IPD, PFO, and PSO models (a), the plot showing the linear fitting of the data by using a pseudo first-order model also known as Lagergren’s first-order model (b), and the estimated values of its reaction rate constant k (the squared symbols in red color), the values are normalized to 107 to make the plot clear to the reader, and the adsorption capacity θe (circled symbol in blue color) values are shown in (c).
Chemosensors 10 00011 g009
Figure 10. The sensing mechanism of NO2 adsorption onto pristine (n-type) and Cr-doped TiO2 (p-type) sensing layers.
Figure 10. The sensing mechanism of NO2 adsorption onto pristine (n-type) and Cr-doped TiO2 (p-type) sensing layers.
Chemosensors 10 00011 g010
Table 1. The R2 values obtained with the help of the Elovich model, IPD, PFO, and PSO model. The table also includes the adsorption constants (reaction rate constant and surface coverage) obtained via the PFO model.
Table 1. The R2 values obtained with the help of the Elovich model, IPD, PFO, and PSO model. The table also includes the adsorption constants (reaction rate constant and surface coverage) obtained via the PFO model.
NO2 Concentration (ppm)R2 ValuePFO
Elovich ModelIPD ModelPFO ModelPSO ModelReaction Constant Surface Coverage
70.416880.496110.931430.585173.26 × 1089.03404
120.333810.641630.93510.676511.41 × 10848.91089
180.431380.665210.968410.693881.05 × 10893.6908
250.662310.888950.974020.89184.93 × 107678.57839
500.410890.856770.94720.859972.6 × 1071450.98803
700.393290.877060.941010.880941.09 × 1073866.0941
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Share and Cite

MDPI and ACS Style

Haidry, A.A.; Fatima, Q.; Mehmood, A.; Shahzad, A.; Ji, Y.; Saruhan, B. Adsorption Kinetics of NO2 Gas on Pt/Cr-TiO2/Pt-Based Sensors. Chemosensors 2022, 10, 11. https://0-doi-org.brum.beds.ac.uk/10.3390/chemosensors10010011

AMA Style

Haidry AA, Fatima Q, Mehmood A, Shahzad A, Ji Y, Saruhan B. Adsorption Kinetics of NO2 Gas on Pt/Cr-TiO2/Pt-Based Sensors. Chemosensors. 2022; 10(1):11. https://0-doi-org.brum.beds.ac.uk/10.3390/chemosensors10010011

Chicago/Turabian Style

Haidry, Azhar Ali, Qawareer Fatima, Ahmar Mehmood, Asim Shahzad, Yinwen Ji, and Bilge Saruhan. 2022. "Adsorption Kinetics of NO2 Gas on Pt/Cr-TiO2/Pt-Based Sensors" Chemosensors 10, no. 1: 11. https://0-doi-org.brum.beds.ac.uk/10.3390/chemosensors10010011

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop