Resistive-type hydrogen gas sensor based on TiO2: A review

Resistive-type hydrogen gas sensor based on TiO2: A review

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Review Article

Resistive-type hydrogen gas sensor based on TiO2: A review Zhong Li a,b, ZhengJun Yao a,*, Azhar Ali Haidry a,**, Tomas Plecenik b, LiJuan Xie a, LinChao Sun a, Qawareer Fatima a a

College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, 211100 Nanjing, China b Faculty of Mathematics, Physics and Informatics, Comenius University, 84248 Bratislava, Slovak Republic

article info

abstract

Article history:

Owing to its high energy density and environmentally friendly nature, hydrogen has

Received 11 July 2018

already been regarded as the ultimate energy of the 21st century and gained significant

Received in revised form

attention from the worldwide researchers. Meanwhile, there are increasing concerns about

6 September 2018

its safe use, storage and transport as, despite being colorless and odorless, after certain

Accepted 10 September 2018

concentration level it becomes flammable and explosive in air. Therefore, it is imperative

Available online xxx

to develop H2 sensors for real-time monitoring of the H2 leakage for an early warning. This paper firstly introduces the general hydrogen gas sensing mechanism of TiO2-based

Keywords:

hydrogen sensors. Then we summarize and comment on the current hydrogen gas sensor

Hydrogen

based on various TiO2 materials, which include pristine TiO2, metal-assisted TiO2, organic-

Gas sensor

TiO2 composites, carbon-TiO2 composites, MOX-TiO2 composites and novel sensor concept

TiO2

with effective top-bottom electrode configuration. Finally, we briefly discuss the obstacles

Semiconductor

that TiO2-based H2 sensors have to overcome in the progress of the systematically practical application, possible solutions, and future research perspectives that can be focused in this area. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characteristics of TiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General hydrogen gas sensing mechanism based on TiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrogen gas sensor based on TiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pristine TiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metal assisted or doped TiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TiO2 e polymer composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Yao), [email protected] (A.A. Haidry). https://doi.org/10.1016/j.ijhydene.2018.09.051 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Li Z, et al., Resistive-type hydrogen gas sensor based on TiO2: A review, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.051

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TiO2 e carbon composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mixed MOX - TiO2 composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tuning the transducer function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary, outlook, and concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Introduction At atmospheric pressure and temperature, H2 is a colorless, odorless and tasteless diatomic gas. Despite these unusual properties, it is also a highly combustible and explosive gas that can easily be ignited and present an almost invisible pale blue flame yielding an explosion when its volume concentration reaches 4e75% in the air. Furthermore, being the smallest known gas with low density (0.0899 g/L), which is even smaller than air (1/14 of air), H2 is non-toxic but can lead to an extreme decrease of oxygen concentration in human body, a condition called asphyxia, if replaces with the oxygen in air. More importantly, hydrogen is not only the basic raw material in chemical, metallurgy and many other related fields but also a renewable and green energy carrier with high energy density (120e142 MJ/kg). Nowadays, hydrogen is regarded as the ultimate energy of the 21st century and tightly associated with the evolution of world energy system due to its great prospect for replacing traditional fossil energy and reducing the NOx/COx harmful-emissions as well [1e3]. In this context, scientists have carried out significant research to promote its development; these research topics cover almost all aspects related to the production, storage, transportation, utilization, and safety of hydrogen [4e6]. Despite being clean energy, its flammable and explosive nature should be critically taking into consideration in every step, specifically when its concentration exceeds 4% in air [7,8]. Thus, for its safe use, the real-time detection of hydrogen leakage is the most important fundamental to take full advantage of hydrogen energy. Therefore, it is imperative to develop an effective and convenient method to monitor its even low concentrations, parts per billion (ppb), at room temperature [9,10]. On the other hand, according to the citation report from the Web of Science Core Collection, we can conclude that there is a continuous growth in hydrogen sensor SCI publications in recent 10 years, which reaches to ~600 in 2017 (Fig. 1). This report strongly proves that hydrogen sensors are getting increasing attention in the field of gas sensor. Based on the current sensing mechanism, available hydrogen gas sensing technologies can be mainly categorized into optical, electrochemical and chemiresistor types [11,12]. Optical methods generally exhibit high accuracy toward target gases but also need sophisticated instruments or configurations that increase their sizes as well as costs [13]. Electrochemical sensing is a low-cost approach but usually suffers from cross-sensitivity against other reducing gases and longterm stability issue [14]. Chemiresistor type hydrogen sensors are economical devices consisting of a transducing platforms and acceptor materials and have been most widely

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accepted due to its high application potential and long-term stability [15]. The transducing platforms are generally composed of highly conductive metal electrodes (platinum, palladium and their alloys), while acceptor materials are basically sensing elements based on various applied materials. Among these sensing materials, metal oxide (MOX) semiconductors (TiO2, SnO2, ZnO, etc.) are the most commonly used due to their cost-effective fabrication and high sensitivity towards hydrogen gas [16,17]. Hereinto, TiO2, a wide bandgap n-type semiconductor with unique electrical, optical, catalytic and gas sensing properties, possesses the advantages of being nontoxic, low-cost and highly stable material in the harsh environments [18,19]. Thus, TiO2 is considered as the most potential candidate for future smart hydrogen gas sensor. However, despite there are several reports and reviews [20e22] on various application of TiO2, the authors believe that there is a lack of comprehensive review on TiO2-based hydrogen sensors. Therefore, to fulfill this gap for the broad and interdisciplinary readership, the present review summarizes the research progress of hydrogen gas sensor based on TiO2. This review provides a comprehensive overview of current TiO2 fabrication techniques, its main characteristics, and surface reaction mechanisms as a hydrogen gas sensing material. More specifically, we focus on the most significant studies and progress in the fabrication of various TiO2 coatings or powders by sputtering, sol-gel, electrochemical methods, and hydrothermal methods. In addition, effective modified strategies (such as doping, surface modification,

Fig. 1 e The annual number of SCI publications on hydrogen sensing in recent 10 years (2008e2017).

Please cite this article in press as: Li Z, et al., Resistive-type hydrogen gas sensor based on TiO2: A review, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.051

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nano-fabrication, composition and Pt/TiO2/Pt sandwich structure) that aim to enhance the hydrogen sensing performances are also discussed. Furthermore, a brief summary of the hydrogen gas sensing performance of various TiO2 composites is also provided. Finally, some future research perspectives and possibilities in this area are proposed and discussed.

Characteristics of TiO2 In nature, the bulk form of TiO2 exists as three basic crystal structures, generally known as rutile, anatase (both are tetragonal), and brookite (orthorhombic). Hereinto, anatase and brookite are the well-known metastable phases and converts irreversibly to rutile phase at the temperatures range of 400e800  C depending on various conditions [23]. As a gas sensing materials, most widely used forms of TiO2 are tetragonal anatase (I41/amd) and rutile (P42/mnm); their unit cells are shown in Fig. 2. The lattice parameters of anatase are a ¼ b ¼ 0.376 nm, c ¼ 0.948 nm, while of that rutile are a ¼ b ¼ 0.459 nm, c ¼ 0.296 nm. In both forms, each titanium atom is connected with six oxygen atoms and forms a TiO6 octahedron. Through both the anatase and rutile phases belong to orthogonal crystal system, the distorted degree of each TiO6 octahedron present obvious difference. The symmetry of anatase phase is poor as it possesses severe distortion, while rutile phase has near-perfect orthogonal crystal system with small distortion. Such differences in the structure are sufficient to result in different band structure, electronic and optical properties, and thus further influence their performance in many applications including gas sensing. Band structure of TiO2 is one of the most important fundamental properties due to their significant role in electronic and optical properties that embodied in charge carrier mobility, redox potential and light absorbance and so on. Currently, most studies are focused on anatase because of its

Fig. 2 e The unit cell of tetragonal anatase and rutile TiO2 [30].

higher reactivity and simpler structure as compared to the rutile TiO2. Most accepted value of bandgap for bulk anatase and rutile TiO2 are 3.2 eV and 3.0 eV [24], respectively and such bandgap are believed to be related with the existence of the Ti 3d and O 2p states in the conduction band (CB) and valence band (VB) [25]. In addition, it should be noted that doping with metal or nonmetal elements can affect the position of CB and VB or form a new energy level in the bandgap thus changing the band structure of TiO2 [26e28]. The absorbance of TiO2 ultimately depends on its crystal phase [29], to be more specific, the values of light absorption edge (refractive index) of anatase and rutile TiO2 are 384 nm (~2.48) and 410 nm (~2.6), respectively.

General hydrogen gas sensing mechanism based on TiO2 For the structural functionality, basically, a chemiresistor gas sensor consists of sensing element and transducer element. The sensing element deals with the gas adsorption/desorption and subsequent resistance changes upon the sensing materials, whereas the transducer element recognizes these changes and output sensible electrical signal with the help of metallic electrodes and electronics [31,32]. It has already been proved that both elements contribute to the sensing properties explicitly [33e36]. There are two generally accepted and well established sensing mechanisms for metal oxide (TiO2) based gas sensors; (i) oxygen-vacancy mechanism and (ii) ionosorption mechanism [37]. The former is based on the surface reduction/ oxidation of the TiO2 and the simultaneous change of surface oxygen vacancies under reducing and oxidizing gas exposure [38]. In this model, H2 initially reacts with TiO2 surface Eq. (1). Here, the lattice oxygen from the surface of TiO2 is removed by the H2 and producing an oxygen vacancy. Subsequently, oxygen vacancy ionizes positively (Eq. (2)) and the free electron enters into the CB thus decrease the electrical resistance. When H2 is closed, oxygen fills the vacancy and takes electrons from the CB to increase the electrical resistance (Eq. (3)). However, this model is rather complex as many aspects are not reported fully, reader is referred to Ref [37] for further details. H2ðgasÞ þ Oxo 4H2 OðgasÞ þ Vxo

(1)

 Vxo 4V** O þe

(2)

x  2V** O þ O2ðgasÞ þ 2e 42Oo

(3)

Ionosorption mechanism, undisputedly, is the most widely accepted model, in which the H2 reaction takes place at the surface of TiO2 that follows the H2 adsorption, charge transfer and H2O desorption. This model is widely discussed in detail and varies significantly depending on various parameters, such as sensing experimental conditions, nature of the sensing material, transducer specifications, and response measurement methods, i.e., resistive, capacitive, or impedance based. Till date, it is believed that sensor response is the most critically controlled by the nature of sensing material

Please cite this article in press as: Li Z, et al., Resistive-type hydrogen gas sensor based on TiO2: A review, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.051

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(e.g., morphology, thickness, and crystallite size). For instance, for the same experimental conditions and measurement methods, the sensing mechanism of compact layer is different than porous layer; for the former case gas reaction is limited to surface only, whereas gas can penetrate for later case [39]. Here, due the complexity of the topic, we only mention the general H2 sensing mechanism for layers having grain boundaries, the sensing mechanism illustration is shown in Fig. 3; the reader however is referred to review papers [39,40] on conduction mechanism for further details. In this model, typical gas sensing starts with the oxygen adsorption process (Eqs. (4)e(6), depending on operating  temperature the reaction results in ionized oxygen (O 2 , O and 2 O ) on the surface. Oxygen is physically adsorbed (physisorption) in its molecular form ðO 2 Þ at temperature below 100  C and chemically adsorbed (chemisorption) in ionic forms (O , O2 ) at temperatures greater than 100  C. Here, O2 is the main adsorption species over 350  C, while O and O2 are competitively adsorbed at 100e300  C) [41,42]. These oxygen species capture free electrons from the conduction band simultaneously which result in reduced density of free electrons ½e  and increased resistance [43,44]. Once hydrogen is exposed, reactions between the ionized oxygen and hydrogen will be triggered, e.g., the reaction between molecular form ðO 2 Þ and hydrogen (Eq. (7)), such reaction releases the captured electrons e and hence decreases the resistance of TiO2. O2 ðgasÞ þ e 4O 2 ðadsÞ

(4)

O2 ðgasÞ þ 2e 42O ðadsÞ

(5)

O2 ðgasÞ þ 4e 42O2 ðadsÞ

(6)

 2H2 ðgÞ þ O 2 ðadsÞ/2H2 OðgÞ þ e

(7)

As the form and number of adsorbed oxygen given by adsorption and desorption kinetics on the sensor surface changes with varying temperature in general [32], this results in different sensing characteristics at different operating temperatures. It should be further expounded that the even ionosorption model still lacks of convictive

spectroscopic proofs to determine the exact nature of surface reaction [45].

Hydrogen gas sensor based on TiO2 Pristine TiO2 Bulk metal oxides, like TiO2, are generally regarded as n-type semiconductors because their surface features oxygen deficiency that results oxygen vacancies acting as electron donors. Compared to the dominant TiO2 based hydrogen sensing materials, pristine TiO2 are not widely investigated due to its low sensitivity and usually only sensitive at temperature above 150  C [46]. However, several methods are adapted to address these issues including controlled TiO2 synthesis or with the help of photo-assistance. For example, O. Krsko [4] et al. presented a flexible highly sensitive hydrogen gas sensor based on TiO2 thin film prepared by reactive sputtering on polyimide foil. The sensors exhibit high response, Ra/Rg~1.7 and ~104 for low (30 ppm) and high (10000 ppm) concentration of H2, even at room temperature under dry conditions. The authors ascribed such high sensitivity to the very low grain size (~10 nm) that is comparable with the width of the depletion layer. For comparison, the authors also found sensor response enhancement (~106 for 10000 ppm) with low detection limit of 6.8 ppm when the temperature was elevated to 150  C under dry conditions, which however decreased significantly (~1336 for 10000 ppm) with low detection limit of 300 ppm under 32 %RH. This strongly demonstrates the influence of humidity and temperature on sensor performance; the former reduces, while improves the sensor response. Moreover, the TiO2 films exhibited good adhesion to the polyimide foil showing no significant damage or decrease in sensitivity values after bending the sensor 1000 times over diameter of 10 mm; this showed its high potential to be integrated into smart sensors with wearable electronics. According to Xia and co-workers [48], self-aligned [002] TiO2 thin films prepared by the hydrothermal methods possess remarkable hydrogen sensing functionality (the limit of detection as low as 1 ppm with the

Fig. 3 e The illustration of general hydrogen sensing mechanism (Ionosorption model) of TiO2 based sensors for layers with grain boundaries. Please cite this article in press as: Li Z, et al., Resistive-type hydrogen gas sensor based on TiO2: A review, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.051

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response time as short as 9 s) at room temperature. The authors attributed such highly sensitive performance not only to the introduction of TiO2 seed layer deposited by magnetron sputtering but also the improved quality of the well orientation aligned TiO2 thin films that featured with dense (002) surface and effective elimination of defects. This interpretation can be further understood that high-energy (002) surface for rutile TiO2 is significantly more effective in adsorbing hydrogen atoms through dissociative hydrogen molecules, which has been proved by Z. Wang [49] using first-principle calculations in the framework of density functional theory. In generally, TiO2 is well-known oxygen deficient nonstoichiometric metal oxide and its n-type conductivity was mostly reported in previous literature [50,51], but in some case Ti vacancy may also be present and dominant in undoped TiO2 thus be responsible for the p-type behaviour [52,53]. A. Hazra [54] et al. employed sol-gel methods to elaborate prepare nanocrystalline TiO2 thin film that showed p-type behaviour in the temperature range 27e225  C. As-prepared p-type TiO2 chemiresistor sensor showed a consistent and reproducible response for 1% H2 in air at 100  C and a response time of 1.3 s was recorded. The authors indicated that hydrogen is exposed to the surface of p-type TiO2 and reacts with the pre-adsorbed oxygen, and then free electrons are released and partially annihilate the holes thereby reducing the conductivity. Moreover, recently it has already been revealed that in pristine TiO2 can also exhibit p-type conductivity depending on crystal facet direction [55], gas concentration, operating temperature, and applied voltage [56]. Synthesis of low dimensional TiO2 also leads in improved sensitivity, for instance Paulose et al. [57] reported remarkable hydrogen sensing properties of highly ordered micron-length TiO2 nanotube arrays prepared by anodic oxidation of titanium foil in electrolyte containing fluorine ion. High resistance change of 8.7 orders of magnitude toward 1000 ppm H2 is attributed to the high surface area of the nanotube architecture accompanied with highly active surface sites on the nanoscale walls and the well-ordered geometry for hydrogen-sensitive tube-to-tube electrical connections. Aforementioned research strongly demonstrates that well-designed pristine TiO2 can be prepared as a high quality hydrogen gas sensor working at room temperature. Light radiation is another effective method to increase the sensitivity and decrease the operating temperature of gas sensor [58,59]. For example, X.Y. Peng [60] et al. successful prepared a UV irradiation-assisted room temperature hydrogen gas sensor based on anatase TiO2 film, which exhibited a compact structure composed of uniform TiO2 microspheres. However, it should be pointed out that the detailed influence mechanism of the light radiation to the gas sensing is still ambiguous.

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assisted modification effect can be divided into surfacesensitization and doping respectively [10]. The schematic diagram of these two effects is shown in Fig. 4. In general, for surface sensitization, noble metals are widely used because their Fermi level is generally lower than that of TiO2 [10,66]. Specifically, depending on whether the work function of TiO2 is changed by the noble metals, such “promotion effect” [67] from the noble metals can be further categorized as “chemical sensitization” and “electronic sensitization”. Take an example of hydrogen gas reacting with Pt-promoted TiO2. Chemical sensitization mainly manifest as the spill-over effect, which is very common in the surface science. Hydrogen molecules can be dissociated into hydrogen atom upon metal Pt clusters then spill over to the surface of TiO2 thus resulting in the acceleration of reaction [31,68]. In this case, Pt clusters just bring down the reaction activation energy or increase the rate of reaction but don't change the resistance of the TiO2. Electronic sensitization can be ascribed to the interface electronic interaction between Pt clusters and TiO2. In this case, due to the difference in work function and electron affinity, there is a depleted space charge region near the PteTiO2 interface as well as the band bending. Electronic sensitization is controlled spill-over effect of H2 over Pt at the surface of TiO2 and decreases the resistance of TiO2 by electron transfer at the interface [31,69]. Doping is another situation that metal atoms enter the framework of TiO2. As for the dopant, transition metals like Co, Cr and Ni are most used because both metals are efficient catalyst for oxidation process and can be integrated into the framework of TiO2 easily due to the near similar values of the ionic radii of A), Cr3þ (0.61  A), Ni2þ (0.69  A) and Ti4þ (0.60  A). When Co2þ (0.65  this situation happens, the promotion effect is different. Taking Co doped TiO2 as an example, Co2þ replace some of lattice Ti4þ thus change the lattice parameter and shifts the positive and negative charge centers of the octahedron. In addition, Co atoms can also create acceptor levels and can

Metal assisted or doped TiO2 As the metal additives are usually able to improve the photocatalytic properties of TiO2 [27,28], some noble metals like Au, Pd, Pt and transition metals like Co, Cr [61e65] are also well known for their enhancement of TiO2 to hydrogen sensing. Depending on whether the metal atoms just modify the active surface or the bulk framework of TiO2, the metal-

Fig. 4 e The schematic diagram showing the surfacesensitization effect (Chemical sensitization and Electronic sensitization) and bulk doping of TiO2, here M is labelled for metal.

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enter into the octahedral anatase TiO6 structure [70]. Above processes result in the generation of active defects like oxygen dþ vacancies V** O or interstitial titanium Ti , which to some extent greatly enhance the number of oxygen adsorption sites around the Co atoms [71,72]. In other words, TiO2 doped by some specific metal ions will cause lattice distortion and generate much more oxygen defects, which present a high local electric field and promote the dissociation of hydrogen [73,74]. There is a high possibility that such promotion effect increases the reaction rate between hydrogen and adsorbed oxygen thus reduces the response time. Furthermore, as compared to the pristine TiO2, more oxygen species could be adsorbed around the dopant atoms thus increase the baseline resistance of the sensor as well as the sensor's sensitivity. In brief, with the help of metal additives, the gas sensor performance of TiO2-based sensors can be optimized by choosing an appropriate type and/or level of doping. For example, Erdem Sennik [75] et al. conducted research to improve the gas sensitivity of TiO2 nanorods (NRs) via Pd loading process. The authors firstly synthesized the TiO2 NRs on surface of FTO by hydrothermal method and then decorated the surface of TiO2 NRs with Pd nanoparticles aided by heat treatment. The results revealed that there is no response of the TiO2 NRs sensor against hydrogen at 30  C, while the PdeTiO2 NRs sensor showed a response up to ~250 for 1000 ppm hydrogen. The TiO2 and PdeTiO2 NRs sensors were tested for further hydrogen sensing performances at 200  C. Results indicated that PdeTiO2 NRs sensor showed an enhanced response to 1000 ppm hydrogen gas and 35 times better sensitivity than TiO2 NRs sensor. The authors suggested that Pd particles dispersed on the TiO2 surface can enhance gas sensitivity by forming highly activated sites, which enables to efficiently increase the hydrogen absorption even at lower temperature. So the authors concluded that Pd particles behave like a “hydrogen collector” thus reducing the working temperature to 30  C. W.P. Chen [76] et al. presented a porous PteTiO2 nanocomposite material fabricated from TiO2 (P25, 80% anatase and 20% rutile) and Pt (particle sizes of ~10 nm) via pressing and sintering according to the weight ratio of Pt/ TiO2 ¼ 5/95. This porous PteTiO2 nanocomposite exhibited extraordinary hydrogen sensitivities (~6000 for 1000 ppm H2 balanced in N2 with fast response/recovery times of 10/20 s) even at room temperature. In contrast, the sintered pellets of pure P25 showed no detectable response to hydrogen at room temperature. Such excellent hydrogen sensing can be attributed to two aspects; (i) the molecular hydrogen can easily penetrate into the bulk of the porous PteTiO2 nanocomposite thus increasing hydrogen reactivity and (ii) Pt aggregates in the nanocomposite are effective in dissociating the molecular hydrogen and migrate hydrogen atoms into the surfaces of the TiO2 nanoparticles by spill-over process [77]. S.T. Ren [78] et al. reported an ultra-sensitive sensor to 5 ppm hydrogen gas at room temperature. In this work, TiO2 nanowires were firstly synthesized by hydrothermal treat of the Ti foil at 220  C in NaOH aqueous solution for 24 h. Then such TiO2 nanowires were decorated with PdAu alloyed nanoparticles (NPs) via the one-step photochemical deposition method. The effect of PdAu alloyed NPs is estimated by comparing the sensing performance of [email protected] with different Pd/Au ratios and contrast samples (TiO2, [email protected] and [email protected]). Results

indicated that Pd and PdAu alloyed NPs make TiO2 nanowire films highly sensitive to ppm-level H2 at room temperature while the sensor based on pristine TiO2 and [email protected] showed no detectable response to 1250 ppm H2. Meanwhile, the [email protected] film exhibits a tremendous response to 5e1250 ppm H2 while the response of [email protected] is further enhanced under identical conditions. Especially in the low concentration of 5 ppm, [email protected] exhibits a higher (175 times) sensitivity and a faster (2 times) response than [email protected] under identical conditions. Considering that other parameters remain unchanged, the authors indicated that the alloy structure of PdAu NPs contribute to the better performances of [email protected] The non-uniform charge distribution in PdAu NPs and the size decrease of alloyed NPs as well as TiO2 substrates are the plausible reasons for response enhancement. Though above TiO2 modified with the noble metals possess high hydrogen gas sensing performances, there are still increasing demand to find some replaceable metals to keep the high performance but reduce the raw material cost at the same time. In this context, Li [64] et al. reported an evaporation-induced self-assembly (EISA) method that able to prepare the Co-doped anatase TiO2 with ordered mesoporous structure. The authors prepared three types of representative sensors with a different molar ratio of Co/Ti (0, 3.0, and 5.0%). XRD patterns of Co-doped TiO2 and typical mesoporous structure of 3%CoeTiO2 were shown in Fig. 5(a) and (b) respectively. As shown in Fig. 5(c), the sensor based on pristine TiO2 was selected to prove the good reproducibility, while systematic property performances of all sensors towards various concentration of H2 under the dry condition and room temperature are shown in Fig. 5(deg). Furthermore, the effect of doping level toward ordered mesoporous structure integrity and relative humidity (RH) on sensing characteristics were also explored. Irrespective to the experimental conditions, the results showed that the sensor based on 3%CoeTiO2 present the highest response ( 4:1  103 ) and shortest reaction time (66 s) towards 1000 ppm hydrogen. In this case, the authors ascribed such high performance of 3%CoeTiO2 based sensor to the doping effect and structural integrity as compared to the pristine TiO2 with ordered mesoporous and 5%CoeTiO2 that has destroyed mesoporous structure. In general, the synergistic of structure effect and doping effect should be responsible for the high performance of 3%CoeTiO2 based sensor because the mesoporous structure can provide both high surface area and direct pathway for hydrogen gas adsorption and diffusion (structure effect) while doping means the enhancement of defects and active sites for hydrogen gas adsorption and reaction (doping effect). B. Lyson-Sypien [65] et al. used the flame spray synthesis method to prepare the nanocrystalline undoped and Cr-doped TiO2 powders (Cr:TiO2 ¼ 0.1, 0.2, 0.5, 1, 5 and 10 at.%) for hydrogen detection. The gas sensing tests were carried out at the concentration range from 50 to 3000 ppm and the temperatures varied from 200 to 400  C. Fig. 6(a) demonstrated that the sensor response clearly improves with the decrease of operating temperature and the high operating temperature for the undoped TiO2 (300e400  C) can be reduced to 210e250  C by adding Cr element. At the specific doping level (~5 at. %), an interesting reversal of the sensor response from that of n-type to that of p-type semiconductor is seen (as

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Fig. 5 e (a) XRD patterns of the samples. (b) HRTEM image of the 3%CoeTiO2 with typical mesoporous structure. (c) Pristine TiO2 based sensor's dynamic response to 1000 and 500 ppm hydrogen gas at room temperature. (d) TiO2; (e) 3%CoeTiO2 and (f) 5%CoeTiO2 based sensors' room temperature dynamic response curves when exposed to 100e1000 ppm H2 and (g) their corresponding response time at each concentration [64].

Fig. 6 e Gas sensor response S as a function of temperature at the hydrogen concentration of (a) 200 ppm and (b) 1100 ppm [65].

shown in Fig. 6). The authors believed that the effect can be interpreted by the acceptor-type substitutional defects Cr'Ti built into TiO2 lattice. Z.H. Li [79] et al. fabricated the hydrogen gas sensor based on the Ni-doped TiO2 nanotubes fabricated by the electrochemical anodization of NiTi alloy and annealing treatment at 525  C. Such nanotubes present a length of ~220 nm and diameter of ~30 nm. According to the XPS measurement results, the contents of Ti, O and Ni elements were 62.97 at%, 24.12 at%, and 7.11 at%, respectively. Gas sensing test revealed that such sensor possesses good sensitivity for wide-range (50 ppme2%) hydrogen detection and present temperaturedependent sensing varied from 25 to 200  C. To be more specific, the nanotube sensor showed a higher response and quicker response/recovery rate at elevated temperature. To explain the performance improvement of doping and explore

its influence to the energy band structure, authors employed the first-principles calculations to do the simulation study. Consequence of the simulation revealed that the bandgap value decreased from 2.40 eV to 0.26 eV after the doping and presented metallic characteristic due to the concentration of Ni is high enough. Therefore, Ni doping enabled better conductivity and improved the sensing performances at room temperature. In addition, the bandgap further decreased to 0.088 eV after the H2 adsorption, which means Ni doping make the process of hydrogen adsorption much easier. This can be used to explain the enhanced hydrogen sensing behaviour of Ni-doped TiO2. The incorporation Ni2þ inside TiO2 lattice could create defect states in the bandgap thus producing an impurity level. Such impurity level provides a migration pathway to the hydrogen to overcome the activation barrier thus lead to a higher sensitivity.

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TiO2 e polymer composites In the last few years, organic conducting polymers such as polyaniline (PANI) and polypyrrole (PPy) are also established to be a promising candidate for hydrogen gas sensor [80,81]. As compared to metal oxides sensing materials, their selectivity is generally higher but also with the typical drawbacks including incomplete recovery, low response as well as poor stability. Thus, designing the organic-inorganic composites consisting of metal oxides and conducting polymers is a promising method to solve the aforementioned problems. As both materials can be connected by either weak interactions (van der Waals forces and hydrogen bonding) or strong interactions (covalent bonding and ionic covalent bonding) [82,83], the organic-inorganic composite system can present improved mechanical, physical and chemical characteristic via the synergic interaction between them [84,85]. Previous studies also proved that such composite materials are highly suitable for hydrogen sensing applications. For example, S. Nasirian and H.M. Moghaddam [86e88] systematically studied the hydrogen gas sensing features of PANI/TiO2 nanocomposite prepared by an in-situ self-assembly chemical oxidative polymerization method. The response and reaction/

recovery time of sensors for hydrogen gas were evaluated at room temperature under 45e55% RH by changing the phases species of TiO2 (anatase and rutile) and their mass fraction. The obtained thin film sensors consist of PANI/TiO2 nanocomposite with 15, 25 and 40 wt % of anatase (A) or rutile (R) phases were named as PA15, PA25, PA40, PR15, PR25 and PR40, respectively. Fig. 7(aec) showed the dynamic resistance change and response of the sensors exposure to 0.8 vol % H2. It was observed that the PANI/TiO2 nanocomposite system exhibited significant sensing performance enhancement toward hydrogen gas in comparison to pure PANI. Considering the correlation between response and the reaction/recovery time, the PR40 and PA25 thin films sensor are more suitable candidates for practical hydrogen gas sensor. Fig. 7(d and e) shows the dynamic response of the PR40 and PA25 thin films sensor exposed to different concentration of hydrogen gas, meanwhile, their response value and response/recovery time are shown in Fig. 7(f) and (g). The authors ascribed the enhanced hydrogen gas sensing mechanism to the two types of interface regions in the nanocomposite system (the interface among TiO2 grains and the interface between n-type TiO2 grains and p-type PANI) and the correlation of energy levels between these two phases. Unfortunately, the authors did not

Fig. 7 e The dynamic resistance change of the sensors based on (a) PA15, PA25, PA40; (b) PR15, PR25, PR40 and (c) their response versus time plot at the hydrogen gas concentration of 0.8 vol %. Dynamic response of the (d) PR40 and (e) PA25 thin films exposed to different concentration of hydrogen gas. (f) The response value of PA25 and PR40 and (g) their reaction/ recovery time at a different concentration of hydrogen gas [86e88]. Please cite this article in press as: Li Z, et al., Resistive-type hydrogen gas sensor based on TiO2: A review, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.051

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investigate the reasons behind excellent sensing performance of PR40 and PA25 thoroughly. Further, the discussion about the best performance as compared with other nanocomposite and why the optimum mass fraction for hydrogen gas sensing changes along with the phases of TiO2 (anatase and rutile) is found to be incomplete. Based on the fact that the p-n junctions created at the interface of PANI matrix and TiO2 is a suitable strategy for the enhancement of hydrogen gas sensing efficiency at room temperature, it seems that combines PANI with TiO2 and SnO2 nanoparticle simultaneously could obtain better hydrogen gas sensing as compared to PANI/TiO2 nanocomposite because the work function, electronic conductivity and charge transport of TiO2 and SnO2 nanoparticles are different [89,90]. In this context and on the basis of results from PANI/TiO2 nanocomposite, S. Nasirian and H.M. Moghaddam [91] further studied the hydrogen gas sensing features of PANI/TiO2:SnO2 nanocomposite fabricated by the similar methods as reported in Refs. [86e88]. Results showed that PAS3 (30 wt % anatase phase) sensor possess the highest response among this series sensors and PAS2 (20 wt % anatase phase) sensor has the lowest response time at identical conditions. In addition, an increase of TiO2 and SnO2 components wt. % result a change to the surface morphology of nanocomposites thus caused a higher response and lower recovery time. As compared to using rutile TiO2, this rise is higher when the anatase TiO2 is employed. Above scientific research from S. Nasirian and H.M. Moghaddam [86e88,91] demonstrated that not only the morphology of the sensing surface has a significant effect to the sensor performance, but also the change of metal oxide type, mass fraction, phase species and the type/number of contacts in the organic-inorganic composite system are important factors to alter them. Besides polyaniline, polypyrrole (PPy) [92] is another promising organic polymer suitable for hydrogen gas sensing. For instance, Y.J. Zou and co-workers [93] attempted to examine the gas sensing performance of TiO2 combined with metal (Pd nanoparticles) and non-metal elements (PPy) simultaneously. The authors fabricated a core-shell composite by using PPy nanofibers as the core and TiO2 as the shell. Then Pd-doped [email protected] core-shell composites were obtained by further doping Pd nanoparticles upon the [email protected] composite via chemical reduction. Hydrogen sensing tests at room temperature revealed that the sensor based on [email protected] exhibited a response of 8.1% toward 1 vol % of hydrogen gas, which is much larger than the sensors based on only [email protected] and PPy nanofibers. The authors ascribed the enhancement to the large specific surface area of the mesoporous [email protected] composite and the catalytic effect of the Pd nanoparticles. Both factors can improve the interactions between the hydrogen molecules and the sensing surface. In addition, the sensors based on [email protected] also exhibited excellent reproducibility, stability and selectivity that make them potential candidate for practical hydrogen sensor applications. Above works suggest a pathway to optimize the selectivity of sensor based on TiO2 and also proved that composite is a useful method to overcome the shortcomings of metal oxides/ organic conducting polymer based sensing materials.

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TiO2 e carbon composites Carbon-based materials, such as 1D carbon nanotubes (CNTs) and 2D graphene [94e97], have attracted increasing attention as the sensing materials. Although their inherent structure can offer high sensitivity as well as low detection limits towards target gases at room temperature, there are still some challenges including poor selectivity, reversibility and recovery [98]. Fortunately, both experimental results and theoretical researches [99e102] demonstrated that combine CNTs or graphene with inorganic nanoparticles could be a potential solution to above challenges. In this part, we discuss recent advances in hydrogen gas sensors based on carbon-TiO2 composites, specific focus on CNTs-TiO2 and graphene-TiO2. It is well known that surface Ti atom has a coordinative affinity for the oxygen from the carboxylic group of CNTs. Thus, nanocomposite TiO2/CNTs may induce particular charge transfer and enhance the sensitivity due to their excellent electronic property. Following this idea, S. Trocino [103] employed PteTiO2/MWCNTs composites for hydrogen detection at low temperature. Firstly, in order to form enough attachment sites for Ti and Pt, they functionalized the MWCNTs by heat treatment in 15 M nitric acid. Thereafter the TiO2/MWCNTs composites with specific C/Ti molar ratio were prepared by the sol-gel method, a nominal 2%wt Pt were loaded by the wet impregnation to get the PteTiO2/MWCNTs composites. To fabricate the final sensor, a thick film of the composites was painted onto the substrate with interdigital electrodes. Hydrogen sensing tests were carried out at 150  C in the concentration between 0.5 and 3%. Results present that PteTiO2/MWCNTs based sensor are superior to either of its constituent components and the response comparison of TiO2/MWCNTs and PteTiO2/MWCNTs indicated that Pt works like a catalytic additive to promote the reaction. This can be further confirmed by the comparison of pure MWCNTs (insensitive to H2) and Pt-MWCNTs (sensitive to H2). The authors attributed such sensing performance to three factors: (i) enhancement of the specific surface area; (ii) effective electron transfer between constituent components; and (iii) catalytic effect of Pt. Unfortunately, this research mainly focused on the influence of specific Pt contents but pass-over the influence of different Pt content or molar ratio of each constituent. L. De Luca [104] et al. prepared TiO2/MWCNTs nanocomposite with different C/Ti molar ratio (0.3e17.0) by the sol-gel method, then Pt/TiO2/MWCNTs composites with a nominal content of 2 wt % Pt nanoparticles were prepared by wetness impregnation of the TiO2/MWCNTs samples. Hydrogen gas sensing tests indicated that these sensors are capable of monitoring hydrogen gas especially at high concentration levels (up to 100%, balance in nitrogen even at room temperature). Fig. 8(a and b) shown the dynamic resistance change and response of the PtCT3.6 sensor (containing 2.0 wt % Pt nanoparticles, 34.3 wt % MWCNTs and 63.7 wt % TiO2) towards various concentration of hydrogen gas. In order to the comparison, the Pt/TiO2/MWCNTs sensors with various MWCNT contents (0e98 wt %) towards 70% hydrogen gas were also evaluated at the identical conditions. As shown in Fig. 8(c), the results demonstrated that the best performance of Pt/TiO2/MWCNTs series sensor is the sensor based on

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Fig. 8 e (a) Dynamic resistance change and (b) response of the PtCT3.6 sensor toward various concentrations of hydrogen gas. (c) Pt/TiO2/MWCNTs sensors with various MWCNT contents (0e98 wt %) towards 70% hydrogen gas at 50  C. (d) The schematic of the hydrogen gas sensing mechanism for Pt/TiO2/MWCNTs sensors [104].

PtCT3.6 and the content of MWCNT is a significant factor that has great influence on the sensor response. Combine the experimental results and the spill-over effect (as mentioned in metal-assisted TiO2), the hydrogen gas sensing mechanism of Pt/TiO2/MWCNTs sensors can be simply explained as followed: hydrogen molecules initial chemisorbed on the surface of Pt nanoparticles, then hydrogen atoms spill out of the Pt surface and diffusing onto the TiO2/MWCNTs surface layer by the spill-over mechanism (Fig. 8(d)). Meanwhile, electrons transfer from TiO2 nanoparticles to the underlying MWCNTs that causing the recombination of an electron with hole thus resulting in the increase of resistance. Here, MWCNTs not only provide faster hydrogen diffusion channel but also a preferential pathway to the current flow that result in a decrease of the response/recovery time. However, it should be pointed out that the most popular carbon-based gas sensing materials have changed from CNTs to graphene. Compared to the 1D structure of CNTs, atomically thick graphene possesses more flexible 2D planar surface that makes graphene better candidate to composite with TiO2 nanoparticles thus form the p-n junctions with excellent charge transfer [105,106]. Based on that, A. Esfandiar [107] et al. employed reduced graphene oxide (RGO) to improve the hydrogen gas sensing performance of the noble metal (Pd, Pt) decorated TiO2 nanoparticles (as shown in Fig. 9(a and b), Pd/TiO2/RGO composite) by a facile fabrication route. The TiO2 nanoparticles were firstly prepared by sol-gel method and then coupled with GO sheets by photo-reduction process, in which GO was reduced by the TiO2 and formed TieC bonds

between the RGO sheets and TiO2. Finally, the Pd or Pt nanoparticles were decorated on the TiO2/RGO composite by chemical reduction to obtain the Pd/TiO2/RGO or Pt/TiO2/RGO nanocomposites. As shown in Fig. 9(c), gas sensing tests were conducted at the hydrogen gas concentrations ranging from 100 to 10000 ppm at 180  C. As comparison, the sensor based on the composites without noble metal nanoparticles and/or RGO were also tested at the identical conditions, their sensitivity is shown in Fig. 9(d). It is obvious that addition of Pd/Pt nanoparticles along with RGO sheets could enhance sensing performances of the TiO2 based sensors considerably. The mechanism of Pd and Pt/TiO2 sensors has already been discussed above; here we discuss the role of RGO in the enhancement of hydrogen sensing. It is well-know that RGO features with the high surface area and porosity as well as faster charge transport. Moreover, RGO acted as a template for the nucleation of TiO2 nanoparticles thus reducing agglomeration during the hydrolysis. So, such sensor based on TiO2/ RGO has more surface area and adsorption/desorption sites that results faster reaction/recovery time and higher sensitivity as compared with the samples without RGO. In addition, the TiO2/RGO decorated by Pd nanoparticles possesses better performance than that decorated by Pt nanoparticles. Taking the tests at 180  C towards 500 ppm hydrogen gas as an example, Pd/TiO2/RGO samples obtained 92% sensitivity with 18 s reaction time, while Pt/TiO2/RGO samples obtained 45% sensitivity with 54 s reaction time. The authors suggested that metallic Pd possesses higher hydrogen dissolution and a lower

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Fig. 9 e (a) TEM image of the TiO2/RGO. (b) TEM image of Pd decorated TiO2/RGO. (c) Dynamic responses of sensors based on Pd and Pt decorated TiO2/RGO toward various concentrations of H2 at 180  C. (d) The sensitivity of TiO2 based sensors toward various concentration of H2 [107].

fraction of oxidized state relative to metallic form thus leading higher efficient of electronic sensitizations in the Pd/TiO2/RGO and exhibiting better sensitivity as well as faster responses to hydrogen gas.

Mixed MOX - TiO2 composites Combining two or more metal oxides to form composites is another typical method used for the enhancement of gas sensing performance, which can be attributed to the physical, chemical and gas sensing properties of the nanocomposite that differ substantially from the features of its constituents. In addition, inherent characteristics of the MOX composites can be changed and tailored for detection of specific gases thus improve their selectivity. So, it is very appealing that the enhancement of hydrogen gas sensing can be easily realized through functionalization of TiO2 based materials with other metal oxides at an optimum composition. For example, TiO2eSnO2 composites is a promising candidate because both TiO2 and SnO2 (rutile and cassiterite phases) belong to the tetragonal structure P 4(2)/mnm that ensure the formation of composites as well as the decreases electron scattering between contacting crystallites [108,109]. Moreover, their work function, electronic conductivity, and

charge transport also present essential differences. Based on above characteristics, the composite attracted much interest from researchers likes D. Shaposhnik [110]. The authors compared two kinds of hydrogen gas sensor based on SnO2e TiO2 composites that synthesized by co-precipitation and mechanical mixing method respectively. For each method, SnO2/TiO2 weight ratios were fixed at both 9:1 and 7:3. In order to contrast the effect of the admixture, pure SnO2 and TiO2 powders were also used to fabricate the sensor by an identical process. Sensor performances were studied in a wide range of working temperature (300e500  C) and humidity conditions towards 1e500 ppm hydrogen gas. Results revealed that co-precipitated SnO2/TiO2 composite manifested better thermal stability and higher sensitivity as compared to the mechanically mixed SnO2eTiO2 composite and pure SnO2 or TiO2. The authors ascribed the better performance on the basis of several factors: higher surface area, a higher amount of surface hydroxyls and lower carrier concentration. In addition, it was also found that even 80% RH only has the negligible effect to the sensitivity of coprecipitated SnO2eTiO2 composite, which means much better selectivity to hydrogen gas under humid conditions. Unfortunately, the authors didn't explore the reason why the addition of 30% TiO2 enhances the sensitivity more as

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compared to the addition of 10%, whatever the synthesized method is employed. M. Radecka [111,112] et al. systematically studied the hydrogen gas sensing of the TiO2eSnO2 nanocomposite (SEM images are shown in Fig. 10(aee)) prepared by mechanical mixing of their commercial nano-powders ranging from 100 mol. % TiO2 to 100 mol. % SnO2. The gas sensing tablets were prepared by pressing specific composition nanopowders at 450  C under 25 MPa and then performed gas sensing measurements within the temperature ranging from 225  C to 400  C toward 500 ppm hydrogen gas. Obtained results are shown in Fig. 10(f), which indicated that sensor response of the pure TiO2 and SnO2 can be significantly sensitized by addition a small amount of 5 mol. % SnO2 and 2 mol. % TiO2 respectively. The authors believed the sensitization effect to the modification of electronic structure at grain contact that offers better percolation electron transfer paths between the grains. Last but most important, the highest sensitivity was obtained with the composition of 50 mol. % TiO2/50 mol. % SnO2. In this context, the authors further pre-treat this optimal composition samples at 500  C in Ar þ 7% H2 atmosphere before gas sensing tests. Results revealed that kinetics of the sensor responses were improved and the recovery time is significantly reduced. However, unfortunately, this conditioning effect by 500  C annealing under Ar þ 7% H2 atmosphere was not clearly explained. I. Kosc [113] et al. sputtered two types TiO2/NiO composite films for hydrogen detection. Sample-1 was consisted of 100 nm TiO2 thin film supported by 10 nm NiO coating on the top, while for sample-2 the NiO coating was at the bottom. Both samples underwent rapid thermal annealing at 600  C in a mixture of argon and hydrogen (5% of H2). XRD patterns revealed that both samples possess the TiO2 anatase phase

while NiO additives on top of sample-1 are reduced to pure Ni and sample-2 presence of rhombohedral NiO. As shown in Fig. 11(a), sample-1 consisted of TiO2 with Ni clusters on the top revealed typical n-type response to various concentrations of H2. While the dynamic responses of sample-2 (Fig. 11(b)) to different concentration H2 are much more complex because inversion of conductivity response type was observed. It is interesting to note that for each operating temperatures, there exists a corresponding critical break point of H2 concentration (as shown in Fig. 11(c)), at which the conductivity inverses. The authors found the phenomenon is strongly dependent on the quantity of the surface oxygen bonds, the response type inverses when the number of electrons arisen out of broken oxygen bonds exceeds the limit of the holes in the range of Debye length. So, the critical break point of H2 concentration was the special equilibrium point of electrons and holes inside of the TiO2/NiO composite film.

Tuning the transducer function Generally, gas sensor response is controlled by its components function, i.e., receptor function and transducer function. Receptor functions deals with the gas reaction and subsequent changes in the sensing element, whereas the transducer function recognizes these changes and output sensible electrical signal [114] with the help of metallic electrodes and electronics. Both these function are indispensable and contribute to the sensing properties equally and explicitly. However, according to our best knowledge, global researchers paid much attention to study the sensing properties by modifying the receptor function and subsequent sensing mechanism but near completely ignore the important

Fig. 10 e SEM images of (a) SnO2, (b) 2 mol. % TiO2/98 mol. % SnO2 composite, (c) 20 mol. % TiO2/80 mol. % SnO2 composite, (d) 50 mol. % TiO2/50 mol. % SnO2 composite and (e) TiO2. (f) TiO2eSnO2 composites sensor response defined as R0 =R vs. temperature T at 500 ppm H2 [111,112]. Please cite this article in press as: Li Z, et al., Resistive-type hydrogen gas sensor based on TiO2: A review, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.051

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Fig. 11 e (a) Response characteristics of TiO2 thin film with Ni clusters on the top annealed at 600  C. (b) Response characteristics of TiO2 thin film with NiO thin film at the bottom annealed at 600  C. Critical concentration is visible at 1000 ppm. (c) The critical concentration vs. different working temperatures [113].

contribution of the transducer function to the overall sensor response. In many cases, we may speculate a strong possibility that the sensor performance is controlled by transducer function where metal electrodes (like Pt and Au) also play critical role as catalyst activators. In addition, the metal/MOX interface can lead to the formation of Schottky barrier resulting in high resistance of the sensor. In this case, one cannot assure that the sensor signal is only govern by receptor function, which is inherent to intrinsic or modified (with doping or surface catalyst) metal oxide surface with new morphology and phase. On the other hand, electrode transducer function is related to more technological factors, such as electrode structure, material and new electrode design. It is also proven and understood by some previous reports that electrode-semiconductor interface, various electrode geometries and specifications (Fig. 12) play very important role along with electrode material and its geometry [33,115]. Nearly four decades back, L. A. Harris [116] early in 1980 discovered layered TiO2 structure (TiO2/Ti/TiO2/Pt) for hydrogen detection, the device structure however seems to be very complex with low resistivity (~350 U) that can measure even 900 ppm H2 at room temperature. Unfortunately, it was not until 2013 that more specific studies began to emerge. For example, M. Cerchez and M. Strungaru [117,118] anodized Ti foil to produce a nonporous TiO2 layer and then sputtered a top platinum layer to prepare a sandwiched Pt/TiO2/Ti hydrogen sensor. In N2 atmosphere, the sensors present a

detection threshold as low as to 15 ppm with an approximately response time of 120 s at room temperature. Interestingly, with this novel Pt/TiO2/Ti structure, it was found that sensitivity to atmospheric hydrogen exposure and electroforming can coexist and are interdependent. The basic drawback was the high applied voltage ~10 V in this case, which causes the strong electroforming. The simplified Pt/ TiO2/Pt structure, named as top-bottom electrode geometry (as shown in Fig. 12(a, b), was developed by A. A. Haidry et al. [119] that shows very high response (~107) to 10000 ppm H2 at room temperature with very low applied voltage 0.1 V. Later, more particular theoretical explanation was reported by the authors [35]. Schematic diagram of the sensor structure and the sketch of conduction band profile together with the sensor profile are shown in Fig. 13(a, c, d) and the typical AFM topography is shown in Fig. 13(b). The authors fixed the width of the bottom Pt electrode around 100 mm and decreased the width of the top Pt electrode from 1 mm down to 80 nm. The sensors with 80 nm top Pt stripe exhibited increased response (~107) by three orders of magnitude, as shown in Fig. 13(e)) even at room temperature. The authors ascribed such performances enhancement to an extraordinary non-ohmic effect, an immediate decrease of the electrical resistance with the decrease to 80 nm. Specifically, such non-ohmic effect is caused by two nanoscale-related effects: (i) the hydrogen diffusion controlled spatially inhomogeneous resistivity of the TiO2 layer and (ii) onset of the hot electron temperature

Fig. 12 e (a) and (b) shows the top-bottom electrode configuration, and (c) shows the classical interdigitated transducer electrode geometry. Please cite this article in press as: Li Z, et al., Resistive-type hydrogen gas sensor based on TiO2: A review, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.051

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Fig. 13 e (a) Schematic diagram of the sensor structure. (b) Typical AFM morphology of the TiO2 film surface. (c) Cross profile of the sensor structure. (d) The sensor profile together with the sketch of conduction band profile. (e) Sensor resistance change along with the width of top electrode for 0 and 10,000 ppm of H2 at different temperature and their corresponding response at room temperature [35].

instability when the small grains suffer from high electric field. The selectivity, stability and the influences of humidity on sensing was later take into consideration in detail by A.A. Haidry [36]. The authors fabricated a room temperature hydrogen gas sensor based on (Pt/TiO2/Pt structure, as shown in Fig. 14(a)) that yielding response over 6 orders of magnitude

with short reaction times (Tres ¼ 19 s/ Trec ¼ 118 s) when exposed to 10,000 ppm H2. Under 50% relative humidity and 25e100  C condition, an enhanced and temperatureindependent sensor performances toward 10,000 ppm H2 was observed with response/recovery times reduce to 5/9 s respectively (Fig. 14(c)). In addition, a reasonable H2 selectivity was observed against CO and NO2, which present great

Fig. 14 e (a) Schematics of the Al2O3/Pt/TiO2/Pt sensor structure. (b) Illustration of hydrogen sensing mechanism with Pt/ TiO2/Pt structure. (c) Dynamic resistance change of sensor (annealed at 800  C, T800) toward 10,000 ppm hydrogen in dry air or 50% RH at different temperature under TEþ and BEþ. Typical magnified of sensor response/recovery part. The selectivity of the sensor to H2, against CO and NO2 [36]. Please cite this article in press as: Li Z, et al., Resistive-type hydrogen gas sensor based on TiO2: A review, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.09.051

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advantage and high potential for practical applications. More interesting in this research is that the response is highly dependent on the bias polarity of Pt/TiO2/Pt layered structure, i.e. TEþ or BEþ (as shown in Fig. 14(c) Dry air) and this report confirm the existence of both electronic and ionic conductivity within TiO2 sensing layer and their significant alteration under hydrogen exposure. A. A. Haidry et al. found that the hydrogen sensing mechanism with such Pt/TiO2/Pt structure is based on memristor principle as shown in Fig. 14(b) [114]. Above understanding lay the foundation for the design and fabrication for innovative sensors with stable, selective and robust H2 detection.

Summary, outlook, and concluding remarks For convenient comparison, the fundamental parameters of all mentioned and surveyed hydrogen gas sensors based on various TiO2 are summarized and listed in Table 1. It is obvious that most research focuses on developing higher performance hydrogen gas sensors with enhanced response, improved selectivity, good stability and fast response/recovery rate. In addition, due to the explosive characteristics of hydrogen, it is very appealing to fabricate a sensor that can

work at room temperature as this could reduce the risk of any calamity. Meanwhile, working at room temperature also means low power consumption, simplified fabrication, and hence reduced operating cost. As a focal material, TiO2 possesses unique attributes in physical, chemical, photocatalytic, electronic and optical fields due to its nontoxic, environmentally friendly nature, highly reactive, excellent biocompatibility and stability in the harsh-environment. However, as a hydrogen gas sensing material to be systematically practical applied, there are still some obstacles to overcome. Firstly, selectivity is the most urgent challenge for TiO2 based hydrogen gas sensors because reductive molecules (like CO, CH4, and NH3) usually cause some cross response and not easy to discriminate. Secondly, such highly reactive surfaces of TiO2 usually suffer from surface poisoning thus require frequent calibration. Thirdly, TiO2 based hydrogen gas sensor always adhere a good humidity sensing, which has significant influence on sensor response. In addition, relative humidity in different environment usually has a great difference, which results in the drift of the resistance as well as response of TiO2. In order to avoided or partly improve above shortcomings, except loading noble metals, doping with transition metals, fabricating composites with other gas sensing materials, most recommended solution are taking full advantage of

Table 1 e Hydrogen sensing properties of various TiO2 based sensors. Materials TiO2 [47] TiO2 [54] TiO2 [54] * TiO2 [57] * TiO2 [60] * PdeTiO2 [75] PteTiO2 [76] * PdAueTiO2 [78] Ni-doped TiO2 [79] CoeTiO2 [64] CreTiO2 [65] PANI/TiO2 [86] PANI/TiO2 [87] PANI/TiO2:SnO2 [91] PdeTiO2/PPy [93] PteTiO2/MWCNTs [103] * PteTiO2/MWCNTs [104] * PdeTiO2/RGO [107] PteTiO2/RGO [107] SnO2/TiO2 [110] SnO2/TiO2 [111] SnO2/TiO2 [112] TiO2/NiO [113] TiO2 [35] TiO2 [36] PdeTiO2 [121] TiO2/ITO [122] Pt/TiO2/MoS2 [123] SnO2/TiO2 [124] MWCNTs/TiO2/Pt [125] * TiO2 [126] *

Structure

Definition of sensitivity

Sensitivity

Concentration (ppm)

Temperature  ( C)

Reaction time (s)

Thin film Thin film Thin film Nanotube Film Nanorods Pellet Nanowire Nanotube Mesoporous Nano-powder Nanofiber/Nanoparticle Nanofiber/Nanoparticle Nanofiber/Nanocomposite Core-Shell/Nanofiber Particle/Nanotube Particle/Nanotube Nanoparticle/Sheet Nanoparticle/Sheet Nanocomposite Nanocomposite Nanocomposite Thin film Pt/TiO2/Pt Pt/TiO2/Pt Nanorings/Nanotube Thin film Nanocomposite Nanotube Nanotube/Nanoparticle Nanotube

Ra =Rg ðRa  Rg Þ=Ra  100% ðIa  Ig Þ=Ia  100% Gg =Ga Ra =Rg ðIg  Ia Þ=Ia Ra =Rg ðIg  Ia Þ=Ia  100% ðRa  Rg Þ=Rg  100% Ra =Rg ðRa  Rg Þ=Ra Ra =Rg Ra =Rg Ra =Rg Ra  Rg =Rg  100% ðRg  Ra Þ=Ra  100 ðRa  Rg Þ=Ra  100% ðRa  Rg Þ=Ra ðRa  Rg Þ=Ra ðRa  Rg Þ=Rg ðRa  Rg Þ=Ra Ra =Rg Ra =Rg

104 4% 55% 108.7 4.856 250 6000 350% 25% 4082 1.5 1.63 1.54 6.18 8.1% 11.5 6.6% 92 46 11.5 0.93 93 7 107 106 92.05% 57 0.749 1410 3.9% 28

1000 1 10000 1000 20000 1000 1000 5 20000 1000 3000 8000 8000 8000 10000 5000 50000 500 500 20 200 500 10000 10000 10000 8000 800 2000 300 500 1000

RT RT 150 RT 400 30 RT RT 25 RT 250 RT RT RT RT 150 100 180 180 400 275 325 200 RT 100 RT RT 100 250 RT 150

e 9 2 e 18 e 10 21 80 66 e 83 152 245 200 e e 18 54 12 1.3 e e e 19 3.8 e 150 e e e

Ra =Rg Ra =Rg ðRa  Rg Þ=Ra ðRa  Rg Þ=Rg ðRg  Ra Þ=Ra Ra =Rg ðRg  Ra Þ=Ra  100% ðIg  Ia Þ=Ia

Here R, G, and I represent the resistance, conductance, and current of the sensors; the subscript a and g represent their respective values in air and gas; the references with * and - refer to the “N2 background gas” and “data not available” respectively.

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nanotechnology to optimize the properties and micro/nanostructure. For example, TiO2 with special morphologies or nanostructure, use of ZIF-8 or other metal-organic framework materials as a gas molecular filter membrane, which has been proved the feasibility in [email protected] based high-performance hydrogen gas sensor [9,120]. In addition, through the firstprinciple calculations to guide the selection of doped element or doped level and selective exposure the dominant crystal surface of TiO2 is also an effective method to improve the performance as well as shorten the development cycle.

[11]

[12]

[13] [14] [15]

Acknowledgment This work was supported by “Priority Academic Program Development of Jiangsu Higher Education Institutions” (PAPD), “Six Talent Peaks Project of Jiangsu Province” (YPC16005-PT), Natural Science Foundation of Jiangsu Province project (BK20170795), “Postgraduate Research and Practice Innovation Program of Jiangsu Province” (KYCX_0255, KYCX18_0281) and the Slovak Research and Development Agency under contract no. APVV-16-0266.

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