Synthesis and characterization of TiO2 doped polyaniline composites for hydrogen gas sensing

The Polyaniline (PANI) and Titanium dioxide (TiO₂)/PANI composite thin film based chemiresistor type gas sensors for hydrogen (H₂) gas sensing application are presented in this paper. Pure PANI and TiO₂/PANI composites with different wt% of TiO₂ were synthesized by chemical oxidative polymerization of aniline using ammonium persulfate in acidic medium at 0-5 °C. Thin films of PANI and TiO₂/PANI composites were deposited on copper (Cu) interdigited electrodes (IDE) by spin coating method to prepare the chemiresistor sensor. Finally, the response of these chemiresistor sensors for H₂ gas was evaluated by monitoring the change in electrical resistance at room temperature. It was observed that the TiO₂/PANI composite thin film based chemiresistor sensors show a higher response as compared to pure PANI sensor. The structural and optical properties of these composite films have been characterized by X-ray diffraction (XRD) and UV-Visible (UV-Vis) spectroscopy respectively. Morphological and structural properties of these composites have also been characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) respectively.     

Room temperature hydrogen gas sensors of functionalized carbon nanotubes based hybrid nanostructure: Role of Pt sputtered nanoparticles

Fast detection of H₂ gas at room temperature has constantly remained a challenge. The metal-oxide based gas sensors have shown excellent sensing properties for gases like H₂, NO, CO and NH₃. In the present work, the H₂ gas sensing characteristics of multiwalled carbon nanotubes based hybrid sensor (F-MWCNTs/TiO₂/Pt) has been reported. The fabricated sensor shows 3.9% sensitivity for low concentration i.e. 0.05% of H₂ with good repeatability and stability at room temperature. The sensing response of F-MWCNTs/TiO₂/Pt is interrelated to change in their resistance on the introduction of H₂ gas and this phenomenon is required for deep understanding the effect of H₂ adsorption on their electronic conduction. The improvement in sensitivity of F-MWCNTs/TiO₂/Pt as compared to MWCNTs/TiO₂ towards H₂ is because of the catalytic role of dispersed Pt nanoparticles deposited by sputtering.     

Flexible hydrogen gas sensor based on a capacitor-like Pt/TiO2/Pt structure on polyimide foil

Metal oxide semiconductor gas sensors of hydrogen with a typical capacitor-like Pt/TiO₂/Pt electrode arrangement exhibit excellent sensitivity to hydrogen even at room temperature. At the same time, very similar Pt/TiO₂/Pt cells can also be used as memristive elements exhibiting resistive switching between two resistive states, which has been recently exploited to create a gas sensor with built-in memory. Merging of these two functionalities within a single device also opens new possibilities for smart gas sensor arrays. However, so far such sensors have been prepared only on rigid substrates. In this work, a flexible hydrogen gas sensor with such capacitor-like Pt/TiO₂/Pt electrode arrangement fabricated on polyimide foil is presented and characterized in terms of hydrogen gas sensing properties and bending endurance. The sensor exhibits high response (R_air/R_H2) of more than 10⁵ to 10 000 ppm H₂ at 150 °C with minor decline at elevated humidity and is capable of room temperature operation. The lowest detected concentration was 3 ppm at 150 °C and 300 ppm at room temperature in dry conditions. Bending the sensor 10⁵ times over diameter of 10 mm led to slight improvement of the sensing performance.     

Fast response of hydrogen sensor using palladium nanocube-TiO2 nanofiber composites

In this work, we investigated the properties of resistivity type hydrogen (H₂) sensor for monitoring in H₂ gas. The H₂ sensor was made of Pd nanocube (NCs) and TiO₂ nanofiber (NFs) composites. The Pd NCs was synthesized by seed-mediated growth and TiO₂ nanofiber was synthesized via electrospinning method. The two nanomaterials are then converted into nanocomposites by ultrasonication process. Pd NCs-TiO₂ NFs composite was characterized by scanning electron microscope (SEM) and high resolution transmission electron microscope (HRTEM). The H₂ sensing properties including the response/recovery time, the response value and linearity of the synthesized samples were investigated toward to various H₂ concentrations (0.6, 0.8 and 1%). The response of H₂ sensor is S = 40.8% and the response/recovery time are 25/1 s with 0.6% at working temperature of 150 °C. Moreover, the H₂ sensor has excellent cross-selectivity for H₂ compared to ethanol, nitrogen dioxide and isopropyl alcohol.     

Augmented hydrogen adsorption on metal (Mg,Mn) doped α-phase TeO2: A DFT investigation

TeO₂, as a promising gas sensor material, has been extensively studied for its capacity to detect hydrogen with high sensitivity. First-principles calculations were applied to explore the adsorption properties of hydrogen (H₂), carbon dioxide (CO₂), methane (CH₄), and hydrogen sulfide (H₂S) on TeO₂ doped with either Mg or Mn to explore this compound's potential as hydrogen sensors. Hydrogen is more readily adsorbed on pure-TeO₂, Mg–TeO₂ and Mn–TeO₂ than CO₂, CH₄ and H₂S molecules by calculating their adsorption energy and charge transfer; the sequence of adsorption strength is H₂>H₂S > CO₂>CH₄. The hydrogen molecules and pure-TeO₂, Mg–TeO₂ and Mn–TeO₂ form H–O bonds with lengths of 0.98, 0.98 and 0.99 Å, respectively, indicating that chemical adsorption is dominant between them. The adsorption of hydrogen leads to significant changes in the density of states (DOSs) of pure-TeO₂, Mg–TeO₂ and Mn–TeO₂, which may lead to changes in their electrical conductivity. Moreover, the larger diffusion coefficients for hydrogen on the surfaces of pure-TeO₂, Mg–TeO₂ and Mn–TeO₂ relative to other gases indicates that hydrogen diffuses readily in TeO₂-based sensing materials, and the higher gas concentration contributes to improvements in response performance. This finding offers a theoretical basis for experimental explorations of the influence of metal dopants on TeO₂ hydrogen sensing performance.     

A resonant photoacoustic cell for hydrogen gas detection

Hydrogen gas (H₂) detection plays an important role in many fields. With the continuous demand and development of clean energy, it is urgent to study new hydrogen gas sensors for stable and accurate H₂ detection. The purpose of this research is to develop a new H₂ sensor based on the resonant photoacoustic (PA) cell as the sensing element. The sensitivity of the resonant PA cell to the resonant frequency is sufficiently utilized. The optimization of its resonance frequency was investigated minutely for the H₂ sensor. Detection utilizes resonance frequency differences between H₂ and air as a sensing mechanism. The resonance frequency tracking is adopted and implemented by the field-programmable gate array (FPGA) device. The minimum detection limit of about 74 ppm for H₂ has been demonstrated by preliminary experiments. The response time of the sensor is about 5 s. This sensor detects concentrations ranging from 74 ppm to 100% in 1 atm. The preliminary test result shows that the H₂ sensor based on this structure has a larger application perspective.     

Hydrogen sensing properties of a Pd/oxide/InAlAs metamorphic-based transistor

A Pd/oxide/InAlAs metal–oxide–semiconductor (MOS) type metamorphic high electron mobility transistor (MHEMT)-based hydrogen sensor is fabricated and investigated. In comparison with the conventional HEMT-based sensors, the MOS MHEMT-based sensor exhibits significantly high sensitivity to the hydrogen. The found hydrogen sensing response is as high as 300%. Using the thermodynamic analysis to estimate the enthalpy value of hydrogen adsorption, the value for the proposed sensor is much lower than that for the other reported HEMT-based sensors. The MHEMT-based sensors are demonstrated to have a relatively fast response as comparing to other HEMT-based ones. The response time of the device is approximately 10 s under exposure to a 1% H₂/air gas. Consequently, the performance of the studied sensors shows the promise characteristics for practical applications.     

Extending the detection range and response of TiO2 based hydrogen sensors by surface defect engineering

With the increasing usage of hydrogen energy, the requirements for hydrogen detection technology is increasingly crucial. In addition to bringing down the working temperature, further improvement in the response and broadening the detection range of hydrogen sensors in particular are still needed. TiO₂ based sensors show great promise due to their stable physical and chemical properties as well as low cost and easy fabrication, but their detection range and low concentration response requires further improvement for practical applications. Here (002) oriented rutile TiO₂ thin films are prepared by a hydrothermal method followed by annealing in either air, oxygen, vacuum or H₂ and the hydrogen sensing performance are evaluated. Raman results show that TiO₂ thin films annealed in vacuum and hydrogen have more oxygen vacancies, while those annealed in air and oxygen have a more stoichiometric surface. Annealing in an oxygen-rich atmosphere is shown to extend the detection range of the TiO₂ sensors while annealing in anaerobic atmospheres increases their response. At high hydrogen concentrations surface adsorbed O₂⁻ is the dominant factor, while at low concentrations the Schottky barrier between Pt and TiO₂ is key to achieving a high response. Here we show controlling the TiO₂ surface properties is essential for optimizing hydrogen detection over specific concentration ranges. We demonstrate that adjusting the annealing conditions and ambient provides a simple method for tuning the performance of room temperature operating TiO₂ based hydrogen sensors.     

Toward the design of asymmetric photocatalytic membranes for hydrogen production: Preparation of TiO2-based membranes and their properties

Hydrogen production from water using solar light energy is a significant contribution to green renewable energy economy. Separation of water splitting products is essential for this and approached by creating membrane photocatalytic system (MPS) without macroscopic metallic electrodes. The MPS has a layered structure Pt/chemically loaded TiO₂/filtration loaded TiO₂/porous polymer membrane/support. Influence of MPS preparation conditions on its TiO₂ content, permeability, diffuse reflectance spectra, mechanical stability, Pt loading and membrane morphology was investigated. Chemical bath deposition of TiO₂ followed by aging was found to be essential for mechanical stability and high activity in hydrogen production. Loading TiO₂ by filtration alone is ineffective for achieving low permeability. The detected products of ethanol dehydrogenation in gas phase were H₂, CO₂, CH₄ and C₂H₆ and in liquid phase CH₃COOH and CH₃CHO. Optimum mass of TiO₂ and photodeposited Pt were found for high rate of H₂ generation. The highest quantum efficiency of H₂ production was 13%.     

Sensors for anaerobic hydrogen measurement: A comparative study between a resistive PdAu based sensor and a commercial thermal conductivity sensor

Hydrogen sensors able to perform measurements in real time in anaerobic environment such as natural gas (NG) will greatly help the development of power to gas technology. For now, thermal conductivity (TC) gas sensors and Pd thin film based sensors have demonstrated their capability to measure H₂ in air and N₂ but there is still lack of testing in natural gas environment. In this study, the sensing performances (response, hysteresis, response time and selectivity) of two sensors were assessed in three anaerobic environments: N₂, CH₄, and NG. The first one is a homemade resistive sensor based on a PdAu thin film and the second one is a commercial thermal conductivity sensor. While most performances are equivalent for both technologies, only the PdAu sensor is able to detect selectively H₂, without any interfering effect with NG components. Thus, Pd based thin film sensors are promising for H₂ detection in anaerobic environments.     

Recent advances in non-metals-doped TiO2 nanostructured photocatalysts for visible-light driven hydrogen production,CO2 reduction and air purification

The generation of hydrogen and oxygen from the photocatalytic water splitting reaction under visible light is a promisingly renewable and clean source for H₂ fuel. The transition metal oxide semiconductors (e.g. TiO₂, WO_3, ZnO, and ZrO₂) are have been widely used as photocatalysts for the hydrogen generation. Because of safety, low cost, chemical inertness, photostability and other characteristics (bandgap, corrosion resistance, thermal and environmental stability), TiO₂ is considered as a most potential catalyst of the semiconductors being investigated and developed. However, the extensive applications of TiO₂ are hampered by its inability to exploit the solar energy of visible region. Other demerits are lesser absorbance under visible light, and recombination of photogenerated electron-hole pairs. In this review, we focus on the all the possible reactions taking place at the catalyst during photo-induced H₂ from water splitting reaction, which is green and promising technology. Various parameter affecting the photocatalytic water splitting reactions are also studied. Predominantly, this review is focussed on bandgap engineering of TiO₂ such as the upward shift of valence band and downward shift of conduction bands by doping process to extend its light absorption property into the visible region. Furthermore, the recent advances in this direction including various new strategies of synthesis, multiple doping, hetero-junction, functionalization, perspective and future opportunities of non-metals-doped TiO₂-based nanostructured photocatalysts for various photocatalytic applications such as efficient hydrogen production, air purification and CO₂ reduction to valuable chemicals have been discussed.     

Improved low-temperature hydrogen generation from NH3BH3 and TiO2 composites pretreated with water

Ammonia borane (AB, NH₃BH₃) is nontoxic easily transportable solid hydride with high stability in air. In this work we demonstrate that simple mixing of AB with TiO₂ (anatase) allows for hydrogen gas to be generated at temperatures as low as 80 °C. No losses of hydrogen have been observed during preparation of hydride-containing composites. It was shown that the adsorption of water vapor on TiO₂ and the increase of TiO₂ loading considerably accelerated the rate of AB decomposition. The experimentally observed formation of B–O chemical bonds and the elevated heat emission suggest strong interaction of AB with the adsorbed water species on TiO₂ surface. It has been found that this interaction proceeds at a higher rate comparing with binary AB/H₂O systems. The heat being released in the process is thought to contribute to overcoming the activation barrier in the dehydrogenation of ammonia borane to produce hydrogen gas.     

Hydrogen generation from the ball milled composites of sodium and lithium borohydride (NaBH4/LiBH4) and magnesium hydroxide (Mg(OH)2) without and with the nanometric nickel (Ni) additive

The composites of (NaBH₄+2Mg(OH)₂) and (LiBH₄+2Mg(OH)₂) without and with nanometric Ni (n-Ni) added as a potential catalyst were synthesized by high energy ball milling. The ball milled NaBH₄-based composite desorbs hydrogen in one exothermic reaction in contrast to its LiBH₄-based counterpart which dehydrogenates in two reactions: an exothermic and endothermic. The NaBH₄-based composite starts desorbing hydrogen at 240 °C. Its ball milled LiBH₄-based counterpart starts desorbing at 200 °C. The latter initially desorbs hydrogen rapidly but then the rate of desorption suddenly decelerates. The estimated apparent activation energy for the NaBH₄-based composite without and with n-Ni is equal to 152 ± 2.2 and 157 ± 0.9 kJ/mol, respectively. In contrast, the apparent activation energy for the initial rapid dehydrogenation for the LiBH₄-based composite is very low being equal to 47 ± 2 and 38 ± 9 kJ/mol for the composite without and with the n-Ni additive, respectively. XRD phase studies after volumetric isothermal dehydrogenation tests show the presence of NaBO₂ and MgO for the NaBH₄-based composite. For the LiBH₄-based composite phases such as MgO, Li₃BO₃, MgB₂, MgB₆ are the products of the first exothermic reaction which has a theoretical H₂ capacity of 8.1 wt.%. However, for reasons which are not quite clear, the first reaction never goes to full completion even at 300 °C desorbing ∼4.5 wt.% H₂ at this temperature. The products of the second endothermic reaction for the LiBH₄-based composite are MgO, MgB₆, B and LiMgBO₃ and the reaction has a theoretical H₂ capacity of 2.26 wt.%. The effect of the addition of 5 wt.% nanometric Ni on the dehydrogenation behavior of both the NaBH₄-and LiBH₄-based composites is rather negligible. The n-Ni additive may not be the optimal catalyst for these hydride composite systems although more tests are required since only one n-Ni content was examined.     

Effective monitoring and classification of hydrogen and ammonia gases with a bilayer Pt/SnO2 thin film sensor

The monitoring and classification of different gases, such as H₂ and NH₃ using a low-cost resistive semiconductor sensor is preferred in practical applications in hydrogen energy, breath analysis, air pollution monitoring, industrial control, and etc. Herein, porous bi-layer Pt/SnO₂ thin film sensors were fabricated to enhance H₂ and NH₃ sensing performance for effective monitoring and classification. Different Pt film thicknesses of 2, 5, 10, and 20 nm were deposited on 150 nm SnO₂ film-based sensors by sputtering method to optimize the response to H₂ and NH₃ gases. Gas sensing results showed that the fabricated Pt/SnO₂ films significantly improved the sensor response to NH₃ and H₂ compared to pure SnO₂ thin film. The sensors based on 5 and 10 nm Pt catalyst layers presented the highest responses to H₂ and NH₃, respectively. The optimal working temperature for NH₃ was in the range from 250 °C to 350 °C, and that for H₂ gas is less than 200 °C. The response of Pt/SnO₂ sensors to CH₄, CO, H₂S, and liquefied petroleum gas was much lower than that to NH₃ and H₂ supporting the high selectivity. On the basis of sensing results at different working temperatures or Pt thicknesses, we applied a radar plot and linear discriminant analysis methods to distinguish NH₃ and H₂. The results showed that H₂ and NH₃ could be classified without any confusion with different Pt layer thicknesses at a working temperature of 250 °C.     

Hydrogen sensors based on noble metal doped metal-oxide semiconductor: A review

With the exhaustion of traditional energy, increasing attention has been paid on the new energy resources like H₂. Because of its inflammable and explosive properties, it is imperative and challenging to detect ppm-level H₂ during the transport and use process. This paper firstly introduces the working principles and sensing mechanism of the hydrogen sensors based on noble metal doped metal-oxide semiconductors. Then, this paper focuses on the advancement of noble metal doped metal oxide hydrogen sensors, especially the room temperature hydrogen sensors, in the recent years. At the end, we propose that fabricating semiconductors with special morphologies, using two different noble metals for bimetallic doping or composite semiconductors with 2D nanomaterials like graphene/MoS₂ to improve the room temperature sensing properties towards low H₂ concentration should be the emphasis for the future work.     

Characteristics of resistivity-type hydrogen sensing based on palladium-graphene nanocomposites

We describe the characteristics of resistivity-type hydrogen (H₂) sensors made of palladium (Pd)-graphene nanocomposites. The Pd-graphene composite was synthesized by a simple chemical route capable of large production. Synthesis of Pd nanoparticles (PdNPs) of various sizes decorated on graphene flakes were easily controlled by varying the concentration of Pd precursors. Resistivity H₂ sensors were fabricated from these Pd-graphene composites and evaluated with various concentrations of H₂ and interfering gases at different temperatures. Characteristics for sensitivity, selectivity, response time and operating life were studied. The results from testing the Pd-graphene indicated a potential for hydrogen sensing materials at low temperature with good sensitivity and selectivity. Specifically H₂ was measurable with concentrations ranging from 1 to 1000 ppm in laboratory air, with a very low detection limit of 0.2 ppm. The response of the sensors is almost linear. The resistivity of sensors changed approximately 7% in its resistance with 1000 ppm H₂ even at room temperature. The robust mechanical properties of graphene, which supported these PdNPs, exhibit structural stability and durability in H₂ sensors for at least six months. Moreover, the advantages in this work are experimental reproducibility in synthesis Pd-graphene composite and sensor fabrication process.     

High optical response NiO,Pd/NiO and Pd/WO3 hydrogen sensors

In this study, NiO and WO₃ oxide semiconductors were fabricated on glass substrates by RF Magnetron Sputtering technique. Structural and optical characterizations of the semiconductors were performed using XRD, SEM, and optical absorption measurements. NiO and WO₃ thin films were occasionally coated with palladium. In order to investigate the optical response of these semiconductors under hydrogen gas exposure, an optical gas sensor test system was installed and programmed. In both of the coated and uncoated cases, optical absorption changes due to hydrogen gas exposure on the surface were investigated. It was observed that these changes occur between 450 and 850 nm wave lengths range. The absorption in the NiO semiconductor was reduced between these wave lengths, while the absorption was increased in the WO₃ semiconductor. In the uncoated state, only NiO gave an optical response to hydrogen gas. While the palladium coated NiO (Pd/NiO) sensor had the best response and recovery times of respectively 70 s and 206 s for 2% fraction of H₂ gas at 300 °C constant temperature, the Pd/WO₃ sensor gave the best response time of 340 s. Palladium coating resulted in approximately 150% increase in the responses of the NiO sensors at higher H₂ concentration. The lower limit of H₂ sensing of the Pd/NiO sensors at 300 °C was at the H₂ fraction of 0.05%, while for Pd/WO₃ sensors this value was 0.025%.     

Enhancement of hydrogen energy from Casuarina equisetifolia using NiO–Pr2O3/TiO2 and NiO/TiO2 nano composites

Hydrogen is the eco friendly fuel that can supplement the requirements for power generation.Nickel based nano composites attracted much researchers due to its wide applications in various fields viz., solar cells, capacitors, batteries and aerospace engineering. NiO-Pr₂O₃/TiO₂ and NiO/TiO₂ nano composites have high catalytic activity for producer gas conversion by the process of decomposition of tar molecules generated during biomass gasification. In this work, we have investigated the growth of NiO-Pr₂O₃/TiO₂ and NiO/TiO₂ nano composites by the method of deposition precipitation technique. Structural investigation represented that the TiO₂ possess cubic structure with preferential orientation along (111) plane. Morphological features denoted the formation of non agglomerated grains. The sizes of the grains with nanometer scale have been identified by transmission electron microscopy. The surface area and porosity nature of the nano composites has been analyzed using Branauer Elmer Teller and Barrett-Joyner - Halenda techniques, respectively. The estimated value of surface area is found to be 110 and 81 m²g^-1 for NiO-Pr₂O₃/TiO₂ and NiO/TiO₂ nano composites, respectively. The increment in the content of hydrogen present in the producer gas has been investigated using catalytic tar cracking method. The NiO-Pr₂O₃/TiO₂ nano composite exhibited better tar cracking with higher content of hydrogen conversion than NiO/TiO₂.     

Boron nitride nanotubes (BNNTs) decorated Pd-ternary alloy (Pd63·2Ni34·3Co2.5) for H2 sensing

The micro-electro-mechanical system (MEMS)-based field effect transistor (FET) sensor for hydrogen detection was fabricated by modifying the gate electrode with boron nitride nanotubes (BNNTs) decorated Pd-ternary alloy (Pd_63·2Ni_34·3Co_2.5) as a hydrogen sensing layer Electro-thermal properties of the micro-heater embedded under sensor membrane were analyzed by a finite element method (FEM) simulation. The structural and morphological properties of the gate electrode were studied by Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and field emission scanning electron microscopy (FESEM). A variation in gate potential is observed due to the H₂ atmosphere that leads to the variation in the depletion region, therefore, changing the current in the channel (BNNTs decorated Pd-ternary alloy). The BNNTs-decorated Pd ternary alloy displayed high sensing response, fast response and recovery time for H₂ gas, low power consumption, long-term stability, and wide detection range from 1 to 5000 ppm H₂. The drain current of the H₂ FET sensor varied significantly at hydrogen gas exposure and increased with H₂ concentration. As proposed H₂ FET sensor can be utilized to the H₂ leak detection system for safe applications.     

A review on WO3 gasochromic film: Mechanism,preparation and properties

WO₃, which can change from yellow to blue when exposed to H₂, is a potential material for hydrogen sensors. Compared with other kinds of hydrogen sensors, WO₃ optical hydrogen gas sensor has many unique advantages. However, as a versatile material, WO₃ attracts much more attention of researchers in the fields of photocatalysis, electrochromism and resistance based sensors than in the field of gasochromism and the progress in gasochromism during last 10 years is rarely systematically summarized. In this synthetic review, three mechanism models of WO₃ gasochromism are introduced in detail. Then the preparation methods of WO₃ gasochromic film, which can be divided to four aspects, are extensively surveyed and their advantages and disadvantages are discussed in detail. Finally, the WO₃ gasochromic films with excellent performances made from different methods are summarized and the future research perspectives are presented.