Abstract
Titanium dioxide (TiO2) has attracted interest for sensory applications due to its high surface area and the high density of active adsorption sites. This review shows the effect of the nanostructure, synthesis technique, operating temperature, target gas, and the impact of incorporating metallic elements on the detection properties of TiO2. The studies showed that the TiO2 gas detection process is closely related to surface reactions. Therefore, sensing properties, such as sensitivity, response time, and recovery, vary with factors that influence the surface reactions, such as chemical elements, morphology, microstructure of the depletion layer, and operating temperature.
Keywords:
titanium dioxide; metallic elements; detection properties
The indispensable monitoring of the rapid expansion of ecological pollution, together with the need for more accurate information from technological advances, has made gas sensors increasingly essential 11 H. Akimoto, Science 302(2003) 1716.)-(44 S. Pandey, K.K. Nanda, ACS Sens. 1(2016) 55.. Several applications, such as industrial manufacturing, aerospace, ocean exploration, environmental protection, medical diagnostics, and bioengineering, have been developing optimizations in sensors taking as main requirements high sensitivity, fast response, good selectivity, low-cost materials, and easy manufacturing 55 J. Bai, B. Zhou, Chem. Rev. 114(2014) 10131.)- (77 F. Yang, J. Zhu, X. Zou, X. Pang, R. Yang, S. Chen, Y. Fang, T. Shao, X. Luo, L. Zhang, Ceram. Int. 44(2018) 1078.. Sensors can be made of various materials, depending on the purposes they serve, so it is essential to evaluate the physical and chemical properties of the compounds involved in the formation of gas sensors to achieve the best results in detecting multiple gases 88 I. Kim, W.-Y. Choi, Int. J. Nanotechnol. 14(2017) 155.. Generally, the types of gas sensors widely used can be classified into: metal oxide gas sensors, gas acoustic wave sensors, gas capacitance sensors, optical gas sensors, and calorimetric gas sensors 99 J.M. Rzaij, A.M. Abass, J. Chem. Rev. 2 (2020) 114.. Semiconductor metal oxide gas sensors are currently one of the most investigated gas sensor groups. They have attracted a lot of attention in gas detection in atmospheric conditions due to their low cost, flexibility in production, simplicity of use, and many application fields for detectable gases 99 J.M. Rzaij, A.M. Abass, J. Chem. Rev. 2 (2020) 114.), (1010 G. Korotcenkov, Mater. Sci. Eng. B 139 (2007) 1..
Through conductive measurements, various metal oxides, such as Cr2O3, Mn2O3, Co3O4, NiO, CuO, SrO, In2O3, WO3, TiO2, V2O3, Fe2O3, GeO2, Nb2O5, MoO3, Ta2O5, La2O3, CeO2, and Nd2O3 are used for the detection of combustible, reducing, or oxidizing gases 1111 E. Kanazawa, G. Sakai, K. Shimanoe, Y. Kanmura, Y. Teraoka, N. Miura, N. Yamazoe, Sens. Actuator B Chem. 77(2001) 72.. However, transition metal oxides perform better than pre-transition metal oxides (MgO, for example), as these oxides are relatively inert due to the large bandgap, while transition metal oxides behave differently because the energy difference between a cationic dn configuration and a dn+1 or dn-1 configuration is often relatively small 1212 V.E. Henrich, P.A. Cox, The surface science of metal oxides, Cambridge Un. Press, New York (1996).. In this way, transition metal oxides are more sensitive than pre-transition metal oxides. The transition metal oxides with electronic configurations d0 (TiO2, V2O5, WO3) and d10 (ZnO, SnO2) stand out for their gas sensor application, as they present less instability of their structures 1313 C. Wang, L. Yin, L. Zhang , D. Xiang, R. Gao, Sensors 10(2010) 2088.. Titanium dioxide (TiO2), in particular, has received attention since 1972, when Fujishima and Honda 1414 A. Fujishima, K. Honda, Nature 238(1972) 37. discovered the photocatalytic division of water on a TiO2 electrode under ultraviolet (UV) light. Studies have grown in the last decades for sensory applications since this semiconductor can present a high surface area (anatase phase), a high density of active adsorption sites, in addition to being non-toxic, biocompatible, free of photocorrosion, and economical 1515 M.R. Al-Mamun, S. Kader, M.S. Islam, M.Z.H. Khan, J. Environ. Chem. Eng. 7(2019) 103248.)- (2020 F. Li, H. Song, W. Yu, Q. Ma, X. Dong, J. Wang, G. Liu, Mater. Lett. 262(2020) 127070.. Therefore, this work reviews the use of TiO2 in gas sensing, highlighting its nanostructured use and application in composite systems.
TiO2 STRUCTURE
TiO2 can be found in three different phase structures, known as anatase, brookite, and rutile 2121 A. Yamakata, J.J.M. Vequizo, J. Photochem. Photobiol. C Photochem. Rev. 40(2019) 234.. The anatase and rutile phases have a tetragonal crystalline structure (Figs. 1a and 1b), and brookite crystallizes in the orthorhombic system (Fig. 1c) 2222 T.R. Esch, I. Gadaczek, T. Bredow, Appl. Surf. Sci. 288(2014) 275., with an energy gap of 3.20 eV for anatase 2323 E. Zhang, Y. Pan, T. Lu, Y. Zhu, W. Dai, Appl. Phys. A 126 (2020) 606., 3.00 eV for rutile 2424 D. Zhang, S. Dong, Prog. Nat. Sci. Mater. Int. 29(2019) 277., and 3.14 eV for brookite 2525 C.G. Ezema, A.C. Nwanya, B.E. Ezema, M. Maaza, P.O. Ukoha, F.I. Ezema, J. Solid State Electrochem. 21(2017) 2655.. Rutile is the stable phase; both anatase and brookite are metastable phases. However, at room temperature, the process of converting anatase to rutile is so slow that it does not happen in practice 99 J.M. Rzaij, A.M. Abass, J. Chem. Rev. 2 (2020) 114.. Temperatures above 600 ºC, depending on pressure, are necessary to promote this phase transformation that involves a solid-state diffusion of atoms in a process of nucleation and crystalline growth 1919 D.A.H. Hanaor, C.C. Sorrell, J. Mater. Sci. 46(2011) 855.. Anatase has lower surface energy than rutile, which favors its production at low temperatures 2626 M. Cargnello, T.R. Gordon, C.B. Murray, Chem. Rev. 114(2014) 9319.. Higher temperatures are required to obtain the rutile phase 2727 Y. Wang, L. Zhang , K. Deng, X. Chen, Z. Zou, J. Phys. Chem. C 111 (2007) 2709.. Understanding the process of transforming anatase into rutile and the variables that influence it is of great relevance since the type of crystalline phase is one of the main critical parameters determining the application of these materials 2828 O. Carp, C.L. Huisman, A. Reller, Prog. Solid State Chem. 32(2004) 33.. Brookite is the least studied polymorph and has the least applicability. There is no agreement in the literature on the relative stability between brookite and anatase, and this is more likely to depend on the initial size of their particles 2929 H. Zhang, J.F. Banfield, J. Phys. Chem. B 104 (2000) 3481.. Literature also relates the thermodynamically stable phase with the crystallite size: anatase is stable for crystallite sizes below 11 nm, brookite between 11 and 35 nm, and rutile for sizes greater than 35 nm 3030 S.-D. Mo, W.Y. Ching, Phys. Rev. B 51 (1995) 13023..
Structures of TiO2: a) anatase; b) rutile; and c) brookite. Orange spheres represent Ti atoms and the blue spheres represent O atoms.
In all three polymorphisms, each titanium atom (Ti4+) coordinates with six oxygen atoms (O2-) to form an octahedron 3131 M. Pelaez, N.T. Nolan, S.C. Pillai, M.K. Seery, P. Falaras, A.G. Kontos, P.S.M. Dunlop, J.W.J. Hamilton, J.A. Byrne, K. O’Shea, M.H. Entezari, D.D. Dionysiou, Appl. Catal. B Environ. 125(2012) 331.. The difference between the three crystalline structures is the deformation of each octahedron and its chains’ organization. In the anatase phase, adjacent octahedral are shared by vertices. In the rutile phase, the edges are shared, and in the brookite phase, the vertices and edges are shared 2828 O. Carp, C.L. Huisman, A. Reller, Prog. Solid State Chem. 32(2004) 33.. As confirmed by the stoichiometric theory for semiconductors, this type of crystal is rich in electrons and belongs to the n-type semiconductor 3232 A. Sclafani, J.M. Herrmann, J. Phys. Chem. 100(1996) 13655.. Within this perspective, TiO2 becomes a promising candidate for gas sensor applications. The TiO2 gas sensor can detect different gases, including oxidizing gases and reducing gases, representing the increase and decrease in resistance, respectively 3333 P. Shankar, J.B.B. Rayappan, Sci. Lett. J. 4(2015) 126.. Generally, the microscopic reactions between these gases and the TiO2 surface can be very different according to the gas type, humidity, and environmental conditions. However, the following two processes can summarize the detection mechanisms: the receptor process and the transducer process (Fig. 2) 3434 A. Oprea, U. Weimar, Anal. Bioanal. Chem. 411(2019) 1761..
The receptor process is related to the relationship between gas molecules and the TiO2 surface 3535 T.L. Thompson, J.T. Yates, Chem. Rev. 106(2006) 4428.. The sensor permits the oxygen to be adsorbed on a superficial level (Fig. 2a) and then oxygen becomes charged negatively and the surface charge layer becomes depleted of electrons 99 J.M. Rzaij, A.M. Abass, J. Chem. Rev. 2 (2020) 114.. When a reducing gas is adsorbed on the oxygen (anionic) of the TiO2 surface, electrons are injected on its surface, which reduces the depletion region and increases the conductivity of the surface. On the other hand, when oxidizing gases are adsorbed on the surface, they gain electrons from the adsorbed oxygen (anionic), which increases the depletion region and decreases conductivity 55 J. Bai, B. Zhou, Chem. Rev. 114(2014) 10131.. It reaches a steady-state level, resulting in a decrease in the work function 3636 S. Ma, M.E. Reish, Z. Zhang, I. Harrison, J.T. Yates , J. Phys. Chem. C 121 (2017) 1263.. This function can also be modified by doping with a foreign receptor 3737 D.E. Williams, Sens. Actuator B Chem. 57(1999) 1.. The transducer process involves electrons transmission of the semiconductor materials and converting electrons into external signals. This process is affected by electron transport modes, including surface-controlled, grain-controlled, and neck-controlled modes, as shown in Fig. 2b. The surface-controlled mode is generally related to compact layer structures 3737 D.E. Williams, Sens. Actuator B Chem. 57(1999) 1.. Gases affect its geometric surface other than the bulk solution; thus, the compact layer’s sensitivity is mainly determined by the thin film thickness 3838 E. Comini, Anal. Chim. Acta 568(2006) 28.. On the contrary, all parts of the porous layer contact gases, which results in more activated sites in the porous layer. Because of this non-dense contact manner, each grain possesses a surface-depleted area 3939 S. Capone, A. Forleo, L. Francioso, R. Rella, P. Siciliano, J. Spadavecchia, D.S. Presicce, A.M. Taurino, J. Optoelectron. Adv. Mater. 5(2003) 1335.. The current has to pass through the intergranular contacts; therefore, the sensitivity of nanostructured TiO2 is affected by the layer thickness and the pore size, and the carrier’s diffusion length 55 J. Bai, B. Zhou, Chem. Rev. 114(2014) 10131.. This resistance change induced by those interactions is considered one of the most critical gas sensors’ characteristics 4040 N. Yamazoe , G. Sakai , K. Shimanoe , Catal. Surv. Asia 7(2003) 63.. The signal reflects the effects of the gas concentration and its diffusivity. Thus, at an equilibrium state in a gas sensor, the gas sensor’s sensitivity is defined as the ratio of the resistance in the air to the resistance after exposure to the analytic gas 4141 S. Ahlers, G. Müller, T. Doll, Sens. Actuator B Chem. 107(2005) 587..
NANOSTRUCTURED TiO2
Significant progress has been made in recent years in the development of nanostructured materials for sensor applications 4242 G. Atanasova, A.O. Dikovska, T. Dilova, B. Georgieva, G.V. Avdeev, P. Stefanov, N.N. Nedyalkov, Appl. Surf. Sci. 470(2019) 861.)- (4646 D. Nunes, A. Pimentel, A. Gonçalves, S. Pereira, R. Branquinho, P. Barquinha, E. Fortunato, R. Martins, Semicond. Sci. Technol. 34(2019) 43001.. In addition to the advantages inherent to the TiO2, its nanostructures due to the large surface-to-volume ratio can significantly improve the sensors’ sensitivity and selectivity compared to traditional materials, as their high surface area promotes an increase in the concentration of active sites for adsorption of oxygen 4747 H.S. Ferreira, M.C. Rangel, Quím. Nova 32(2009) 1860.. Also, the availability of various nanostructures allows them to reach unique chemical, physical, and electronic properties 4848 S.H. Salman, A.A. Shihab, A.-H. Kh. Elttayef, Energy Procedia 157(2019) 283.), (4949 S. Taha, S. Begum, V.N. Narwade, D. Halge, J.W. Dadge, M.P. Mahabole, R.S. Khairnar, K.A. Bogle, AIP Conf. Proc. 2220(2020) 20195.. Furthermore, nanostructured TiO2 can be prepared on a large scale in facile conditions and temperatures, facilitating low-cost manufacturing. Therefore, great interest has been shown in studies of TiO2 with nanostructures, its transduction principle, and system simulation functions for sensor applications. Techniques commonly used to obtain TiO2 nanostructures include sol-gel method 5050 A. Giampiccolo, D.M. Tobaldi, S.G. Leonardi, B.J. Murdoch, M.P. Seabra, M.P. Ansell, G. Neri, R.J. Ball, Appl. Catal. B Environ. 243(2019) 183., electrodeposition 5151 H. Sopha, Y. Norikawa, M. Motola, L. Hromadko, J. Rodriguez-Pereira, J. Cerny, T. Nohira, K. Yasuda, J.M. Macak, Electrochem. Commun. 118(2020) 106788., chemical vapor deposition 5252 A.M. Alotaibi, S. Sathasivam, B.A.D. Williamson, A. Kafizas, C. Sotelo-Vazquez, A. Taylor, D.O. Scanlon, I.P. Parkin, Chem. Mater. 30(2018) 1353., physical vapor deposition 5353 A.K. Vishwakarma, L. Yadava, Adv. Sci. Eng. Med. 10(2018) 723., direct oxidation 5454 M. Daraee, M. Baniadam, A. Rashidi, M. Maghrebi, Chem. Phys. 511(2018) 7., hydrothermal 5555 K.M. Emran, S.M. Ali, H.E. Alanazi, J. Electroanal. Chem. 856(2020) 113661., etc. These methodologies allow the production of TiO2 in the crystalline structure of rutile or anatase, with varied morphology, from nanoparticles (1D) to three-dimensional(3D) nanostructures. Nanotubes (NTs) are one of the most widespread nanostructures for gas sensors. The primary way of obtaining them is by electrochemical oxidation reaction of a metallic titanium substrate 5656 X. Chang, J. van der Zalm, S.S. Thind, A. Chen, J. Electroanal. Chem. 863(2020) 114049.)- (6060 X. Tian, L. Liu, Y. Li, C. Yang, Z. Zhou, Y. Nie, Y. Wang, Sens. Actuator B Chem. 256(2018) 135.. More recently, Bindra and Hazra 6161 P. Bindra, A. Hazra, Sens. Actuator B Chem. 290(2019) 684. used electrochemical anodization with a restricted supply of H2O in the electrolyte to synthesize TiO2 NTs, aiming to select organic vapors (methanol, ethanol, acetone, and 2-propanol). The formation of highly ordered and porous TiO2 NTs was obtained, where diameters and lengths of the tubes were in the range of 110-150 nm and 2.5-2.7 μm, respectively. This type of structure offered a large surface area, which made it possible to adsorb organic vapors with a significant increase in the sensor’s resistive response, even at low concentrations and low temperatures.
TiO2 nanowires (NWs) are another typical one-dimensional structure with high sensitivity for detecting certain gaseous species 6262 N.D. Chinh, N. Van Toan, V. Van Quang, N. Van Duy, N.D. Hoa, N. Van Hieu, Sens. Actuator B Chem. 201 (2014) 7.), (6363 J.-S. Lee, A. Katoch, J.-H. Kim, S.S. Kim , J. Nanosci. Nanotechnol. 16(2016) 11580.. However, wet chemical synthesis methods, such as solid-liquid-vapor (SLV) 6464 G.-H. Lee, Mater. Res. Innov. 20(2016) 421.)- (6666 S.R. Sani, A.M. Ali, R. Jafari, Physica E Low Dimens. Syst. Nanostruct. 43(2011) 1809., thermal oxidation 6767 K. Huo, X. Zhang, L. Hu, X. Sun, J. Fu, P.K. Chu, Appl. Phys . Lett. 93(2008) 13105.), (6868 X. Peng, A. Chen , Appl. Phys. A 80 (2005) 473., hydrothermal 6969 B. Poudel, W.Z. Wang, C. Dames, J.Y. Huang, S. Kunwar, D.Z. Wang, D. Banerjee, G. Chen, Z.F. Ren, Nanotechnology 16(2005) 1935., sol-gel 7070 B. Bhowmik, K. Dutta, N. Banerjee, A. Hazra , P. Bhattacharyya, Nanotechnology 24 (2013) 553.), (7171 M. Epifani, T. Andreu, R. Zamani, J. Arbiol, E. Comini , P. Siciliano , G. Faglia, J.R. Morante, CrystEngComm 14(2012) 3882., pulsed laser deposition 7272 J.A. Losilla, C. Ratanatawanate, K.J. Balkus, J. Exp. Nanosci. 9(2014) 126.), (7373 M.A. Rahman, S. Bazargan, S. Srivastava, X. Wang, M. Abd-Ellah, J.P. Thomas, N.F. Heinig, D. Pradhan, K.T. Leung, Energy Environ. Sci. 8(2015) 3363., electrospinning 7474 A. Nikfarjam, S. Hosseini, N. Salehifar, ACS Appl. Mater. Interfaces 9(2017) 15662., and anodizing method 7575 J. Wang , Z. Lin, J. Phys. Chem. C 113 (2009) 4026., have a high cost as they require more cleaning processes and the transfer of nanostructures on an appropriate substrate. Moreover, TiO2 NWs are typically made in the form of a single nanowire. Despite its good detection characteristics, the practical application of available nanowire sensors has been limited by some severe disadvantages, including the difficulty of manufacture, low reliability, and high cost 7676 D. Zhang , Z. Liu, C. Li, T. Tang, X. Liu, S. Han, B. Lei, C. Zhou, Nano Lett. 4 (2004) 1919.. Lee et al. 6363 J.-S. Lee, A. Katoch, J.-H. Kim, S.S. Kim , J. Nanosci. Nanotechnol. 16(2016) 11580. were the pioneers in obtaining TiO2 nanowires in monocrystalline mesh in the rutile phase with an average diameter of 90 nm using the steam phase growth technique. In this work, the gas detection properties of sensors manufactured from networks of TiO2 nanowires were comparable to that of alternative sensors that use other forms of TiO2 and demonstrated that the networked TiO2 nanowires represent a potential detection platform. In order to reduce the operational cost of synthesis by wet chemistry methods, Arachchige et al. 7777 H.M.M.M. Arachchige, D. Zappa , N. Poli, N. Gunawardhana, N.H. Attanayake, E. Comini , Nanomaterials 10 (2020) 935. synthesized for the first time TiO2 NWs by thermal oxidation directly on the alumina substrate, in which the thin layer of Ti was deposited on the substrate using little oxygen as a reducing gas (dry physical method). The NW of TiO2 obtained had an average diameter of 20-40 nm and several micrometers in length. The gas detection measurements showed selectivity for ethanol and H2 at an ideal temperature of 400 ºC, and detection limits below 50 and 100 ppm for these gases, respectively.
A variation of the nanowire morphology for gas sensors is the nanorod (NR). They differ in length/diameter ratio and stiffness, as NRs have a smaller length/diameter ratio and greater stiffness than NWs. It is observed that TiO2 NRs can be produced in the backbones of the Si NWs matrix through a process of pulsed chemical vapor deposition 7878 J. Shi, Y. Hara, C. Sun, M.A. Anderson, X. Wang , Nano Lett. 11(2011) 3413.. This work also provides evidence that uniform TiO2 NRs can be grown on several surfaces. Wang et al. 7979 H. Wang, Q. Sun, Y. Yao, Y. Li , J. Wang , L. Chen, Ceram. Int. 42 (2016) 8565. reported the influence of temperature on the diameter of the TiO2 NRs (rutile) when obtained by the physical vapor deposition. They showed the excellent response of this nanostructure to O2 at room temperature, with a fast response time of 55 s and a recovery time of 51 s. TiO2 NRs for acetone detection could also be obtained through electrospinning 8080 H. Bian, S. Ma , A. Sun, X. Xu, G. Yang, J. Gao, Z. Zhang , H. Zhu, Superlattices Microstruct. 81(2015) 107.. Bian et al. [80] studied TiO2 NRs in a random network structure composed of a mixture of the anatase and rutile phases. The sensor showed high sensitivity (Rair/Rgas≈20), good selectivity, and reproducibility for acetone with the response and recovery time of 11 and 8 s, respectively, at 500 ºC. Electrospinning was also used for the synthesis of TiO2 nanofibers. Park et al. 8181 J.-A. Park, J. Moon, S.-J. Lee, S.H. Kim, T. Zyung, H.Y. Chu, Thin Solid Films 518(2010) 6642. reported that a random network structure of several layers of titanium dioxide nanofibers can be manufactured by calcination (400, 600, and 800 ºC) of the electrospun hybrid fibers of TiO2/PVP. After calcination at 600 ºC, this structure displayed the highest gas response (Rair/Rgas≈4.3) in a concentration of 25 ppm CO at 200 ºC, compared to those calcined at 400 ºC (Rair/Rgas≈3.1) and 800 ºC (Rair/Rgas≈2.2). The sensor based on TiO2 nanofibers calcined at 600 ºC showed a gas response to CO concentration as low as 1 ppm. This ability of TiO2 nanofibers to detect low concentrations of CO was attributed to the unique geometry and distribution characteristics of the TiO2 nanofiber.
In addition to the one-dimensional nanostructures, there is a great deal of attention from researchers regarding TiO2 nanoparticles. The TiO2 microsphere’s geometry can be controlled by adding different chemicals in the titanium precursors’ aqueous solutions. Navale et al. 8282 S.T. Navale, Z.B. Yang, C. Liu, P.J. Cao, V.B. Patil, N.S. Ramgir, R. S. Mane, F.J. Stadler, Sens. Actuator B Chem. 255 (2018) 1701. obtained TiO2 nanoparticles (NPs) using a titanium glycolate precursor using a simple hydrothermal route, which their chemosensitive activity was considered for various target gases. The gas detection results demonstrated that the TiO2 NP-based sensor has high selectivity for CH3COCH3 and is capable of detecting its concentration up to the limit of ppb level at an operating temperature of 270 ºC. Taha et al. 4949 S. Taha, S. Begum, V.N. Narwade, D. Halge, J.W. Dadge, M.P. Mahabole, R.S. Khairnar, K.A. Bogle, AIP Conf. Proc. 2220(2020) 20195. prepared thick films of titanium dioxide nanoparticles, titanium dioxide nanowires, and titanium dioxide nanotubes to analyze three different TiO2 nanostructures about the detection parameters, such as operating temperature, response/recovery time, and absorption capacity of alcohol vapors, such as methanol, ethanol, and propanol vapors at a low concentration level (10 ppm). The results showed that the nanostructured morphologies exhibit different behaviors concerning sensitivity, operating temperature, and response/recovery time. NPs showed the lowest operating temperature for alcohols than the rest of the nanostructure. NWs have an excellent response to alcohols, but the recovery was better observed in NPs’ nanostructure. Table I lists different types of TiO2 nanostructures and their responses to different types of gases.
TiO2-DOPING OR ASSISTANT METALS
The introduction of some defects in the nanometer scale, such as noble metals, can increase the responses of TiO2 sensors to detect gases. According to the metal, atoms modify the surface sensitization and doping 9595 Y. Luo , C. Zhang, B. Zheng, X. Geng, M. Debliquy, Int. J. Hydrog. Energy 42(2017) 20386.. Noble metals (e.g., Au, Pd, Pt) are often used to improve surface sensitivity and selectivity because their Fermi level is usually lower than TiO29696 F.E. Annanouch, Z. Haddi, M. Ling, F. Di Maggio, S. Vallejos, T. Vilic, Y. Zhu , T. Shujah, P. Umek, C. Bittencourt, C. Blackman, E. Llobet, ACS Appl. Mater. Interfaces 8(2016) 10413.. The detection property rises with an increase in adsorption sites for incoming gas molecules. The selectivity increases due to these metals’ catalytic effect since much of the catalytic reactions’ efficiency occur on the material’s surface. The transition metals (Co and Zr, for example) are more used as doping elements, as they change the structure of TiO2 since they have similarities with the atomic radius of Ti. However, it generates distortions in the crystalline network and functional defects such as oxygen vacancy (Vö) or interstitial titanium (Tiδ+), which significantly increases the number of oxygen adsorption sites around transition metal atoms 9797 X. Yue, S. Jiang, L. Ni, R. Wang, S. Qiu, Z. Zhang , Chem. Phys . Lett. 615(2014) 111..
Bastakoti et al. 9898 B.P. Bastakoti, N.L. Torad, Y. Yamauchi, ACS Appl. Mater. Interfaces 6(2014) 854. found that the introduction of platinum (Pt) on the surface of the mesoporous TiO2 increased the sensor’s sensitivity to acetaldehyde at room temperature, as well as, the sensor exhibited a higher adsorption rate and excellent absorption of acetaldehyde adsorption compared to pure mesoporous TiO2. The use of Pt in TiO2 is also reported as a noble metal that favors the sensitivity of TiO2 for the detection of other gases 9999 M.F. Fellah, Int. J. Hydrog. Energy 44(2019) 27010.), (100100 X. Zhang , J. Tie, Q. Chen, P. Xiao, M. Zhou, IEEE Trans. Dielectr. Electr. Insul. 22(2015) 1559.. Xing et al. 101101 X. Xing, N. Chen, Y. Yang, R. Zhao, Z. Wang, Z. Wang , T. Zou, Y. Wang , Phys. Status Solidi 215(2018) 1800100. synthesized via hydrothermal method sensors of mesoporous TiO2 and Pt-doped mesoporous TiO2. The sensors’ gas detection performance showed favorable selectivity, good gas response value, fast response/recovery time, low detection concentration, and good long-term stability for acetone (300 ºC). TiO2 doped with 0.5% Pt showed the highest gas response value of 29.51 for 200 ppm acetone at 300 ºC, that is, 5.2 times greater than that of a gas sensor based on pure TiO2 (5.67). The authors 101101 X. Xing, N. Chen, Y. Yang, R. Zhao, Z. Wang, Z. Wang , T. Zou, Y. Wang , Phys. Status Solidi 215(2018) 1800100. attributed this better performance of the Pt/TiO2 sensor to the catalytic effect of Pt, which increases the ability to absorb oxygen molecules and accelerate the reaction between adsorbed oxygen species (Ox -) and acetone gas molecules, as well as the fact that Pt atoms introduce an intermediate energy level in the bandgap, making it easier for excited electrons to migrate from the valence band to conduction band. Abe et al. 102102 H. Abe, Y. Kimura, T. Ma, D. Tadaki, A. Hirano-Iwata, M. Niwano, Sens. Actuator B Chem. 321(2020) 128525. also reported that the introduction of Pt on the surface of TiO2 favors the detection of gases (hydrogen, carbon monoxide). They synthesized films of TiO2 nanotubes (NTs) with Pt and by anodic oxidation and atomic layer deposition. When exposed to H2 gas and 1% CO diluted in nitrogen at 300 ºC, the Pt nanoparticles promoted the dissociative adsorption of H2 gas molecules on the surface of the TiO2 NTs, leading to an increase of 7 order of magnitude in the sensor response. Rane et al. 103103 S.S. Rane, S. Arbuj, N. Joshi, R. Ghuge, S.B. Rane, S.W. Gosavi, Sens. Lett. 17(2019) 269. revealed that Pt in TiO2 films, through photochemical reduction, is responsible for decreasing the operating temperature of the gas sensor (≥90 ºC) for different test gases, such as hydrogen, ammonia, and ethanol.
By changing the function of TiO2 by noble metals, they can be categorized into ‘chemical sensitization’ and ‘electronic sensitization’, in which chemical sensitization manifests itself mainly as an overflow effect without changing the resistance of TiO2. In contrast, electronic sensitization can be attributed to the overflow of the target gas in the noble metals on the TiO2 surface and the decrease in the resistance of TiO2 by electron transfer at the interface (95), 104104 D.S. Vlachos, C.A. Papadopoulos, J.N. Avaritsiotis, Sens. Actuator B Chem. 44(1997) 458.. Incorporating Au into the TiO2 surface is an example of chemical sensitization, as Au favors the connection between the molecules of the target gases and the metallic network. Abbasi and Sardroodi 105105 A. Abbasi, J.J. Sardroodi, J. Nanostructure Chem. 7(2017) 121. made Au/TiO2 sensors for the detection of NO2, where they revealed that the atoms that make up the target gas chemically bond to Au. This agrees well with Chomkitichai et al. 106106 W. Chomkitichai, N. Tamaekong, C. Liewhiran, A. Wisitsoraat, S. Sriwichai, S. Phanichphant, Eng. J. 16(2012) 135., who concluded that the H2 gas detection performance increased with the introduction of 0 to 0.75% Au in TiO2. The use of nanostructured noble metals as TiO2’s assistant metals optimizes the sensor responses 107107 N. Mintcheva, P. Srinivasan, J.B.B. Rayappan , A.A. Kuchmizhak, S. Gurbatov, S.A. Kulinich, Appl. Surf. Sci. 507(2020) 145169. . Mintcheva et al. 107107 N. Mintcheva, P. Srinivasan, J.B.B. Rayappan , A.A. Kuchmizhak, S. Gurbatov, S.A. Kulinich, Appl. Surf. Sci. 507(2020) 145169. synthesized nanomaterials prepared via irradiation of TiO2 nanopowders by pulsed laser in milliseconds followed by the deposition of Au nanoparticles and observed that the synthesis method led to the formation of Ti3+ ions and oxygen vacancies on the surface, which appear to be related to nucleation and growth of Au nanoparticles deposited on the Ti support. Thus, laser-irradiated semiconductor nanomaterials improved the sensor’s sensitization and adjusted the selectivity. Nikfarjam et al. 7474 A. Nikfarjam, S. Hosseini, N. Salehifar, ACS Appl. Mater. Interfaces 9(2017) 15662. analyzed the detection of TiO2-aligned nanofibers doped with Au nanoparticles obtained by electrospinning equipped with electrostatic fields for CO detection. Sensor response and recovery times have been improved with the introduction of Au nanoparticles. The addition of Au in pure TiO2 (300 ºC) nanoparticles, when exposed to a concentration of 200 ppb of CO, increased the response from 190% to 597% and decreased the recovery time. The authors 7474 A. Nikfarjam, S. Hosseini, N. Salehifar, ACS Appl. Mater. Interfaces 9(2017) 15662. revealed that Au nanoparticles act as catalysts, form a Schottky barrier between them and TiO2, and reduce the activation energy required for the interaction between CO and O-, which improves the sensor’s response. The same gas detection mechanism was exposed by Zhang et al. 108108 Y. Zhang, D. Li , L. Qin, D. Liu, Y. Liu, F. Liu, H. Song , Y. Wang , G. Lu, Sens. Actuator B Chem. 255(2018) 2240., who synthesized TiO2 hierarchical architectures and Au-loaded TiO2 for the detection of toluene. The results indicated that Au improved the performance of the TiO2 sensor, especially the 5% Au/TiO2 sensor that showed a better response (7.3), short response (4 s) and recovery (5 s) times, excellent repeatability, and stability at 100 ppm toluene at 375 ºC.
Nataraj et al. 109109 J.R. Nataraj, P.Y. Bagali, M. Krishna, M.N. Vijayakumar, Mater. Today Proc. 5(2018) 10670. analyzed the CO detection properties of Ag/TiO2 sensors using an orthogonal matrix of experiments, in which the annealing temperature (200, 250, and 300 ºC), the amount of dopant (0.025, 0.050, and 0.075 atomic ratios), and the concentration of CO gas (1, 3, and 5 ppm). Silver loaded in TiO2 provides a more active site for sorption of gas, as well as Ag also acts as a catalyst to increase the adsorption of gas molecules and accelerate the exchange of electrons between the sensor and the test gas. Wang et al. 110110 Z. Wang , A.A. Haidry, L. Xie, A. Zavabeti, Z. Li, W. Yin, R.L. Fomekong, B. Saruhan, Appl. Surf. Sci. 533(2020) 147383. also showed that an Ag (2 mol%) Ti modified NP has excellent sensory properties, such as sensitivity (SR ~13.9), response and recovery times (11 s), and good long-term stability (30 days) for 100 ppm acetone. The improvement of detection properties was attributed to the electronic sensitization mechanism since the resistance of TiO2 (Rair ~ 465 MΩ) is more significant than Ag/TiO2 (Rair ~133 MΩ). TiO2 gas sensors doped with noble materials usually only show excellent results at elevated temperatures. However, Rahbarpour et al. 111111 S. Rahbarpour, S. Sajed, N. Ghodsi, H. Ghafoorifard, Mater. Res. Express 6(2019) 85905. analyzed the sensitivity versus operating temperature studies in an Ag/TiO2 to detect methanol vapor. The operating temperature of this type of sensor depends on the gas concentration, such that the higher the target gas concentration, the lower the operating temperature. The results were described based on the calculation of the oxygen coverage of the silver surface under different conditions. Şennik et al. 112112 E. Şennik , O. Alev , Z.Z. Öztürk , Sens. Actuator B Chem. 229(2016) 692. studied the improvement of the TiO2 NRs gas detection parameters through the Pd addition process. When Pd nanoparticles were added to the TiO2 NR sensor, there is a ~250 response when exposed to 1000 ppm of hydrogen at 30 ºC; pure TiO2 had no response at these conditions. When both were tested in this concentration of target gas at 200 ºC, the TiO2 Pd/NRs showed a sensitivity 35 times better than the TiO2 NRs sensor. This improvement in temperature decrease for hydrogen detection was justified as the particles of Pd dispersed on the surface form active sites that increase hydrogen absorption since the Pd behaves as a ‘collector of hydrogen’, reducing the working temperature to 30 ºC. Pan et al. 113113 F. Pan, H. Lin, H. Zhai, Z. Miao, Y. Zhang , K. Xu, B. Guan, H. Huang, H. Zhang, Sens. Actuator B Chem. 261(2018) 451. showed that the TiO2 nanofilm sensors with Pd (1 wt%) responded quickly to the change in CO concentration since CO molecules were more easily absorbed and, consequently, activated in the surface of the PdO, and then transferred to the grain surface of the TiO2 film to react with the chemically adsorbed oxygen species. The depletion layer became stronger due to the unoccupied d orbital and the unrelated valence electron of the palladium ion (Pd2+), which made the PdO a strong electron acceptor, removing electrons from TiO2. In this same perspective, Zhang et al. 114114 D. Zhang , C. Jiang, X. Zhou, Talanta 182(2018) 324. synthesized Pd-TiO2/MoS2 ternary nanocomposite films to detect benzene gas at room temperature. They confirmed that Pd has a significant electronic sensitization effect and availability of active sites for benzene gas adsorption. However, they clarified that contents above 4 wt% of Pd cause a decrease in the sensor’s sensitivity.
When transition metals are used to dope TiO2, other mechanisms are predominant. When TiO2 is doped with Co3+, for instance, Co replaces part of the Ti network; therefore, it changes the network parameter and transforms positive and negative charge centers of the octahedron 9797 X. Yue, S. Jiang, L. Ni, R. Wang, S. Qiu, Z. Zhang , Chem. Phys . Lett. 615(2014) 111., which significantly increases the number of oxygen adsorption sites around Co atoms. In this perspective, Fomekong and Saruhan 115115 R.L. Fomekong , B. Saruhan , Front. Mater. 6(2019) 252. reported that replacing Ti4+ with Co3+ creates oxygen vacancies and promotes the transformation of anatase to rutile. Likewise, they said that TiO2 doped with Co reveals a conductive p-type behavior that produces an enhanced NO2 response at 600 ºC under air as a carrier gas. Kumar et al. 116116 M. Kumar, A.K. Gupta, D. Kumar, Ceram. Int. 42(2016) 405. synthesized TiO2 films doped with different Mg concentrations (1, 3, 5, and 7 wt/v% of Mg) using the sol-gel spin coating technique. When deposited on silicon substrates, these films were tested for CO detection (120 to 920 ppm). It was observed that the electron density of the crystalline network increased due to the incorporation of Mg2+ ions, causing a decrease in the resistivity of TiO2 doped with Mg when compared to pure TiO2. As for the detection of CO, the composite film with the highest level of Mg doping (7 wt/v%) obtained the best responses since it presented the lowest sensitivity (0.304 MΩ/ppm) and the shortest response and recovery time (41 and 22 s, respectively).
Chromium (Cr) for having an atomic radius similar to Ti (Cr3+=0.61 Å, Ti4+=0.60 Å) is a non-noble metal suitable for application as a TiO2 dopant for gas detection since Cr3+can replace Ti4+ in the TiO2 network to form additional defects (such as oxygen vacancies and interstitial Ti atoms), which can alter the electronic structure of TiO2 and transform TiO2 into the p-type semiconductor. Based on its p-type conductivity, Cr3+ doped TiO2 exhibits better sensitivity to H2117117 A. Monamary, K. Vijayalakshmi, S.D. Jereil, Physica B Condens. Matter 553(2019) 182.. Xie et al. 118118 L. Xie , Z. Li , L. Sun, B. Dong, Q. Fatima, Z. Wang , Z. Yao, A.A. Haidry, Front. Mater. 6 (2019) 96. also synthesized Cr/TiO2 films. Different Cr concentrations (1, 3, 5, 10 at%) were used and detection of H2 was performed at different temperatures (300 to 500 ºC). With the addition of chromium dopant, the interpretation of p-type behavior was associated with the presence of acceptor states. Generally, chromium dopant affects the electronic structure of TiO2 and forms acceptor levels located in the bandgap. In this work, the TiO2 gas sensor doped with 5 at% of Cr showed the best gas detection performance at 500 ºC. The response value is 152.65, the response/recovery time is fast (142/123 s) when exposed to 1000 ppm of hydrogen and it has good selectivity to hydrogen. The effectiveness in improving gas detection caused by the introduction of Cr in the TiO2 structure can also be seen for other gases, as reported by Sertel et al. 119119 B.C. Sertel, H.I. Efkere, S. Ozcelik, IEEE Sens. J. 20(2020) 13436., which observed that the response of the sensor to propane gas in concentrations of 250, 500, and 1000 ppm at an operating temperature of 300 ºC increases with increasing Cr content. The use of Cr as a TiO2 doping element for CO2 detection was reported by Mardare et al. 120120 D. Mardare, N. Cornei, C. Mita, D. Florea, A. Stancu, V. Tiron, A. Manole, C. Adomnitei, Ceram. Int. 42(2016) 7353., which observed an increase of 9 times in the response for CO2 detection (10000 ppm) when TiO2 was doped with 4 at% Cr at 55 ºC. However, this same research group observed that TiO2 doped with Fe has excellent sensitivity to high concentrations of CO2 at room temperature, although the detection mechanism employed is different due to the diverse nature of the crystalline structure 121121 D. Mardare , C. Adomnitei , D. Florea , D. Luca, A. Yildiz, Physica B Condens. Matter 524(2017) 17.. It is noted that TiO2 films doped with Cr have an organized crystalline structure, in which electrical transport takes place through the grain, while TiO2 films doped with Fe are amorphous. At low concentrations of Fe, it was observed that the change in conductivity in the presence of CO2 occurred through the n-type conductivity. However, with the increase in Fe’s concentration, the increase in conductivity is attributed to the decrease in the charge carriers, which leads to a rise in the level of Fermi in the presence of CO2. Tong et al. 122122 X. Tong, W. Shen, X. Zhang , J.-P. Corriou, H. Xi, J. Alloys Compd. 832(2020) 155015. have also studied the use of Fe as a doping element of TiO2. They analyzed the film performance of Fe-doped TiO2 nanotubes to detect H2S at low working temperatures. The results showed that Fe does not alter the surface morphology of the film since Fe ions are part of the TiO2 structure, favoring the reduction of the Fermi level, which consequently reduces the thermal excitation energy used to increase the free-electron electricity in the TiO2 conduction band, which implied in reducing the sensor’s working temperature. Also, Fe/TiO2 nanotube films showed a sensor response 3.03 times greater than TiO2 nanotubes for 50 ppm H2S at 100 ºC. In 123123 J. Wu , D. Zhang , Y. Cao, Colloid Interface Sci. 529(2018) 556., two mechanisms have been reported to explain the improvement in the detection properties of Fe-TiO2/MoS2 films to ethanol at room temperature when compared to pure TiO2. Firstly, the doping of Fe3+ in TiO2 can produce more oxygen vacancy due to Fe3+ replacing Ti4+, which is useful for improving ethanol detection performance, as an impurity energy level is created by doping with Fe3+, which reduces the TiO2 bandgap. Another possible mechanism for the Fe-TiO2/MoS2 sensor is the p-n heterojunctions generated at the n-type Fe-TiO2 and p-type MoS2 interface, which can promote the reaction between the adsorbed oxygen and the detection film. As the Fermi level of MoS2 is greater than that of Fe-TiO2, electrons flow from MoS2 to Fe-TiO2 and holes flow from Fe-TiO2 to MoS2 until the Fermi level reaches the equilibrium. Therefore, a depletion layer forms at the interface of Fe-TiO2 and MoS2, where electrons accumulate on the side of Fe-TiO2 and holes accumulate on MoS2. In this way, the sensor’s resistance decreases in the air and increases in the ethanol gas due to the modulation of the depletion layer’s width 123123 J. Wu , D. Zhang , Y. Cao, Colloid Interface Sci. 529(2018) 556..
The doping of TiO2 with 0.5 mol% Ni presented another H2 gas detection mechanism 124124 R.L. Fomekong , K. Kelm, B. Saruhan , Sensors 20(2020) 5992.. The authors suggested that the best performance in the sensor’s response (72%) of Ni (0.5 mol%)/TiO2 in detecting 10000 ppm of H2 at 600 ºC was based on the higher formation of n-junction numbers present between the anatase and rutile phases. This behavior was observed only at this concentration of Ni. This behavior can be explained by forming a Schottky barrier at the anatase/rutile junction. They clarify that electrons flow from the anatase to the rutile when an electric field is applied to this composition. The holes flow in the opposite direction, which brings a balance at the Fermi level, and generates electron depletion in the anatase/rutile interface layer. This interaction facilitates more significant oxygen adsorption on the sensor surface due to more active reaction sites. Therefore, when in contact with H2, more electrons are released back into the conduction band, eventually leading to an increased sensor response. Vijayalakshmi and Monamary 125125 K. Vijayalakshmi , A. Monamary, J. Mater. Sci . Mater. Electron. 27(2016) 140., on the other hand, attributed the improvement in the performance of the Ni/TiO2 sensor for the detection of 400 sccm of H2 at room temperature to the behavior of the p-type semiconductor. The acceptor impurity level in p-type Ni/TiO2 films generates trapped hole centers since the presence of Ni as an impurity in the TiO2 network is also known to create insufficient oxygen since it induces an impurity level when Ni/TiO2 film adsorbs a certain amount of hydrogen and the bandgap increases, which increases the sensor resistance. Another doping metal is niobium (Nb). Galstyan et al. 126126 V. Galstyan, E. Comini , C. Baratto, A. Ponzoni, M. Ferroni, N. Poli , E. Bontempi, M. Brisotto, G. Faglia , G. Sberveglieri, Sens. Actuator B Chem. 209(2015) 1091. indicated that the detection performance of TiO2 nanotubes, in terms of response magnitude, ideal operating temperature, and baseline conductivity, can be optimized with the doping of Nb for the detection of some gases, such as H2, CO, acetone, and ethanol. Galstyan et al. 127127 V. Galstyan , A. Ponzoni , I. Kholmanov, M.M. Natile, E. Comini , G. Sberveglieri, Sens. Actuator B Chem. 303 (2020) 127217. reported the analysis of Nb-doped TiO2 nanotubes obtained through anodic oxidation to detect dimethylamine (DMA), which is an essential indicator for checking the degradation of seafood. Detection measurements of TiO2 nanotubes and TiO2/Nb nanotubes were performed at 300 ºC for 5, 10, 25, and 50 ppm DMA. However, the TiO2 nanotubes showed only sensitivity from 10 ppm. TiO2/Nb nanotubes showed a better response than TiO2 NTs, related to sensor response, response time, and recovery time, since the Nb5+ ions, which are present in the cationic sites replacing Ti, act as donor center and promote the gas adsorption process on the material, improving the structure response. However, when Nb was introduced to TiO2 in a structure similar to a Pt/Nb-TiO2/Pt capacitor to detect hydrogen gas, an unexpected result occurred. The response of the sensor with Nb-doped TiO2 film was lower compared to the non-doped Pt/TiO2/Pt sensor, which exhibited the highest response and the shortest response time for 1000 ppm of hydrogen at 100 ºC 128128 Z. Li , Z. Yao , A.A. Haidry , T. Plecenik, B. Grancic, T. Roch, M. Gregor, A. Plecenik, J. Alloys Compd. 806(2019) 1052.. This behavior was attributed to a decrease in surface roughness and the porosity of TiO2 films doped with Nb that limited the gas diffusion rate.
Abbasi and Sardroodi 129129 A. Abbasi , J.J. Sardroodi, Comput. Theor. Chem. 1095(2016) 15. investigated the adsorption behavior of the O3 molecule in N and Zr/N doped TiO2 anatase nanoparticles. Although the O3 molecule is poorly adsorbed in the pure nanoparticle (no doped), it tends to have chemical adsorption in the nanoparticles doped with N. The results suggested that the titanium sites five times coordinated are the preferential site of adsorption of the molecules of O3, when compared with the sites of nitrogen and oxygen. The adsorption of the ozone molecule on the N-doped particle was energetically more favorable than the adsorption on the non-doped particle, indicating the dominant effects of nitrogen doping on the performance of the nanoparticles. Pan et al. 130130 W. Pan, Y. Zhang , D. Zhang, Appl. Surf. Sci. 527(2020) 146781. observed that nanocomposites of TiO2 nanospheres/tungsten diselenide (WSe2) nanofibers produced by a hydrothermal route presented excellent sensory sensing responses (43.8 to 100 ppm) and response/recovery time (2/1 s at 30 ppm) at room temperature. The predominant factor for the improvement in the sensory properties of TiO2 with the introduction of WSe2 is the fact that WSe2 acts as a transmission path for charge transfer owing to its high carrier mobility and small natural bandgap as well as the van der Waals interaction between TiO2 and WSe2, which facilitates the hybridization of the two materials, due to the large specific surface area of WSe2 nanosheets and the uniform adhesion of the TiO2 nanospheres. This nanocomposite provides many active sites for the adsorption of ethanol gas, which is a crucial element to increase the ethanol detection property of the TiO2/WSe2 compound. This contact between the components provides the formation of heterojunction at the interface of p-type WSe2 and n-type TiO2. This interface does not only function as a catalytic center where ethanol molecules are readily adsorbed on it, but it also promotes the electron transfer rate and rapid oxidation of ethanol gas. The TiO2 in this same structure (nanospheres) was used to manufacture TiO2/tin disulfide (SnS2) nanocomposites 131131 D. Zhang , X. Zong, Z. Wu, Y. Zhang, Sens. Actuator B Chem. 266(2018) 52.. The authors showed that the use of TiO2 nanospheres in SnS2 increases the moisture detection properties in high response, low hysteresis, rapid response and recovery times, and good reproducibility. This nanocomposite has more active sites and nanopores than the isolated elements, which can provide high surface exposure for water molecules’ adsorption. Although SnS2 has greater moisture detection capacity than TiO2 due to the different hydrophilic functional groups, the nanomaterial’s synergetic effect gives the SnS2/TiO2 sensor better detection properties, reaching an impedance response of up to 2000 and a sensitivity of 442000 Ω/%RH. Finally, Table II summarizes recent research on several metals used as dopants for TiO2 with the possible application of gas sensors.
CONCLUSIONS
The reports indicated that the variation in the synthesis technique, morphology, operating temperature, and introduction of metallic elements modify the detection mechanism of TiO2 when exposed to the most varied target gases. It is noteworthy that the large surface/volume ratio found in the nanostructures and the use of auxiliary metals positively alters the detection properties in such a way as to increase the sensitivity and reduce the response and recovery times when compared with pure TiO2. These changes can be due to changes in the reaction surface of TiO2 and the creation of adsorption sites in the crystalline structure of titanium dioxide.
ACKNOWLEDGEMENTS
Authors acknowledge Brazilian agencies Coordenação de Aperfeiçoamento de Pessoal e Nível Superior - Brasil (CAPES) - Finance Code 001 (scholarship granted to Rubens Alves Junior) and CNPq (grants Nos. 308822/2018-8 and 420004/2018-1) for support the research.
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Publication Dates
-
Publication in this collection
27 Sept 2021 -
Date of issue
Jul-Sep 2021
History
-
Received
02 Jan 2021 -
Reviewed
24 Feb 2021 -
Accepted
01 Mar 2021