Hydrogen gas sensing based on polyaniline/anatase titania nanocomposite

Polyaniline (emeraldine)/anatase TiO₂ nanocomposite (PA-NC) was prepared by a chemical oxidative polymerization. The thin films of PA-NC for hydrogen gas sensing application were deposited on Cu-interdigited electrodes by spin coating technique. A study on characteristics of PA-NC thin films was demonstrated by a porous cylindrical morphology. The response and response/recovery time of sensors for hydrogen gas were evaluated by the change of TiO₂ wt% at environmental conditions. Resistance-sensing measurement was exhibited a high sensitivity about 1.63, a good Long-term response, low response time and recovery time about 83 s and 130 s, respectively, at 0.8 vol% hydrogen gas for PA-NC including 25% wt of anatase nanoparticles. The current–voltage characteristics of PA-NC gas sensors before and after hydrogen gas injection showed a nonlinear ohmic current. Moreover, we studied the formation of PA-NCs and their hydrogen gas sensing mechanism based on contact regions in PA-NC.     

Ultra-fast response and highly selectivity hydrogen gas sensor based on Pd/SnO2 nanoparticles

It is still a challenging task to achieve the rapid detection of hydrogen (H₂) with the rapid development of hydrogen energy sector. In this work, the H₂ sensing capabilities of pristine and Pd-modified SnO₂ nanoparticles with the size of ～7 nm were systematically evaluated. The SnO₂ nanoparticles were synthesized via hydrothermal method and Pd modification was performed using impregnation route. Pd modification remarkably upgraded the H₂ sensing performances compared with the pristine SnO₂ gas sensor. The working temperature of SnO₂ decreased from 300 °C to 125 °C after Pd loading. Among the prepared Pd/SnO₂ gas sensors, 0.50 at.% Pd/SnO₂ sensor exhibited the highest response magnitude of 254 toward 500 ppm H₂ and rapid response/recovery time of 1/22 s at 125 °C. The enhanced H₂ sensing capabilities by Pd modification may be related to the catalytic effect and the resistance modulation.     

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.     

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.     

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.     

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.     

The decoration of TiO2/reduced graphene oxide by Pd and Pt nanoparticles for hydrogen gas sensing

Reduced graphene oxide (RGO) was used to improve the hydrogen sensing properties of Pd and Pt-decorated TiO₂ nanoparticles by facile production routes. The TiO₂ nanoparticles were synthesized by sol–gel method and coupled on GO sheets via a photoreduction process. The Pd or Pt nanoparticles were decorated on the TiO₂/RGO hybrid structures by chemical reduction. X-ray photoelectron spectroscopy demonstrated that GO reduction is done by the TiO₂ nanoparticles and Ti–C bonds are formed between the TiO₂ and the RGO sheets as well. Gas sensing was studied with different concentrations of hydrogen ranging from 100 to 10,000 ppm at various temperatures. High sensitivity (92%) and fast response time (less than 20 s) at 500 ppm of hydrogen were observed for the sample with low concentration of Pd (2 wt.%) decorated on the TiO₂/RGO sample at a relatively low temperature (180 °C). The RGO sheets, by playing scaffold role in these hybrid structures, provide new pathways for gas diffusion and preferential channels for electrical current. Based on the proposed mechanisms, Pd/TiO₂/RGO sample indicated better sensing performance compared to the Pt/TiO₂/RGO. Greater rate of spill-over effect and dissociation of hydrogen molecules on Pd are considered as possible causes of the enhanced sensitivity in Pd/TiO₂/RGO.     

Synthesis and characterizations of highly responsive H2S sensor using p-type Co3O4 nanoparticles/nanorods mixed nanostructures

High-quality p-type semiconducting Co₃O₄ with mixed morphology of nanoparticles/nanorods are synthesized using a hydrothermal route for high response and selective hydrogen sulphide (H₂S) sensor application. XRD and Raman studies revealed the crystal structure and molecular bonding for obtained Co₃O_4, respectively. The nanoparticles/nanorods-like structures were confirmed for Co₃O₄ using FESEM and TEM analysis. The EDS and XPS spectra analysis were carried out for elemental composition and chemical atomic states of Co₃O₄. The Co₃O₄ sensor is investigated for gas sensing properties in dynamic conditions. The sensor exhibited the highest selectivity towards H₂S among various hydrogen-contained gases at 225 °C. The sensor revealed a high response of 357% and 44% for 100 and 10 ppm H₂S gas concentrations, respectively. The Co₃O₄ sensor exhibited a systematic dynamic resistance response for 100–10 ppm range H₂S gas. The excellent dynamic resistance repeatability of the sensor was shown towards 25 ppm H₂S gas. The response of Co₃O₄ sensor was investigated at different operating temperatures and H₂S concentrations. The sensor stability and H₂S sensing mechanism for the Co₃O₄ sensor have been reported. Highly uniform and mixed nanostructures of Co₃O₄ can be the potential sensor material for real-time high-performance H₂S sensor application.     

Hydrogen sensing characteristics of Pd/SiO2-nanoparticles (NPs)/AlGaN metal-oxide-semiconductor (MOS) diodes

A Pd/SiO₂-nanoparticles (NPs)/AlGaN metal-oxide-semiconductor (MOS) structure is used to fabricate interesting Schottky diode-type hydrogen sensors. The employment of SiO₂-NPs could effectively increase the specific surface area of Pd catalytic metal and the Schottky barrier height. Good hydrogen sensing performance is obtained. Experimentally, as compared to a conventional Pd/AlGaN MS diode, a significant 34.5-fold improvement on hydrogen sensing response is obtained under an introduced 1% H₂/air gas at 300 K when a 10 wt% concentration of SiO₂-NPs is employed in the studied device. Yet, the increase in SiO₂-NP concentration relatively deteriorates the ability to detect very low hydrogen concentration levels (≦1 ppm H₂/air). In addition, the increase in SiO₂-NP concentration creates a decrease and increase on response and recovery time constants of transient behaviors, respectively.     

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%.     

Understanding on the hydrogen detection of plasma sprayed tin oxide/tungsten oxide (SnO2/WO3) sensor

The plasma spray technique was well proven in producing metal oxide based gas sensors in the last two decades using different powder feedstocks. However, limited research was made to fabricate hydrogen gas sensor from tin oxide layer coated over tungsten oxide layer. This paper attempts to interpret the hydrogen gas sensing performances of plasma sprayed coating derived by depositing tin oxide layer over tungsten oxide (SnO₂/WO₃) layer. Plasma sprayed SnO₂/WO₃ sensor showed maximum response of 90% at 150 °C in contrast to stand-alone WO₃ (89% at 350 °C) and stand-alone SnO₂ (89% at 250 °C). The lower operating temperature of SnO₂/WO₃ sensor without compromising gas response was attributed to the WO₃–SnO₂ hetero-junction. SnO₂/WO₃ sensor showed selective sensing towards hydrogen with respect to carbon monoxide and methane gases. This sensor also possessed repeatable characteristics after 39 days from the initial measurement. In a nut-shell, plasma spayed SnO₂/WO₃ sensor showed stability of base resistance, repeatability after successive response and recovery cycles, selective sensing towards 500 ppm H₂ with significant magnitude of gas response of 90%, response time of 35 s and recovery time of 269 s at a temperature of 150 °C.     

Efficient H2 gas sensor based on 2D SnO2 disks: Experimental and theoretical studies

2D SnO₂ disks with excellent purity and crystallinity were synthesized through a low cost, facile hydrothermal process and were characterized in terms of their morphological, structural, optical and electrochemical properties. The 2D disk-like morphology of synthesized SnO₂ presented the average thickness of ∼1 μm and possessed the typical rutile tetragonal phase for the SnO₂ with preferred growth along (100) plane. As-synthesized SnO₂ disks were used for the fabrication of gas sensors for reducing gases like H₂, CO, and C₃H₈. With the optimized temperature at 400 °C, the as-synthesized SnO₂ electrode expressed the gas responses of 14.7, 9.3 and 8.1 for H₂, CO, and C₃H₈, respectively. Contrary, the reasonable response times of 4 s, 3 s, and 8 s and the recovery times of 331 s, 201 s, and 252 s were recorded for H₂, CO, and C₃H₈ gases, respectively. The DFT studies conducted herein suggest that the adsorbed oxygenated species act as a primary redox mediator for gas sensing reaction between reductive gases like H₂, CO and C₃H_8, and SnO₂ sensor. From DFT analysis, a very low heat of adsorption (≤0.2 eV) estimated which suggested the physisorption of the H₂ molecules on the surface of the sensing material (i.e. SnO₂). In contrast, the deposited oxygen atom forms strong chemical bonds with O_2c and O_3c sites. The oxygen atom bonded to O_2c site control the conductivity of the sensor better than the O_3c sites.     

Selective and tunable H2 adsorption/sensing performance of W-doped graphene under external electric fields: A DFT study

W-doped graphene and its selective gas adsorption/sensing performance are studied through first-principles density functional theory (DFT) calculations. A single W atom is stably anchored into the graphene plane with a high binding energy of ?9.325 eV. The W-doped graphene interacts more strongly with H₂ compared to NH₃, CH₄, CO, SO₂ or H₂S. The H₂ adsorption system also has a higher adsorption energy of ?1.035 eV. Furthermore, the W-doped graphene exhibits the highest sensor response to H₂ with the largest number of transferred charges and the biggest change in the band gap. A negative electric field improves the interaction between the H₂ and the W-doped graphene by increasing the adsorption energy and promoting charge transfer. However, the adsorption of the H₂ is significantly weakened upon the application of a positive electric field; the adsorbed H₂ is easily desorbed from the W-doped graphene with a modulated recovery time as short as ～4.099 s at room temperature (300 K) upon a +0.4 V Å^?1 increase in the electric field. These results reveal that the W-doped graphene has promising selective and tunable H₂ adsorption/sensing performance upon the application of external electric fields.     

Facial fabrication of an inorganic/organic thermoelectric nanocomposite based gas sensor for hydrogen detection with wide range and reliability

A novel method for fabrication of a thermochemical hydrogen (TCH) gas sensor composed of platinum (Pt)-decorated graphene sheets and a thermoelectric (TE) polymer nanocomposite was investigated. The hydrogen sensing characterization for the device included gas response, response time (T₉₀), recovery time (D₁₀), and reliability testing, which were systematically conducted at room temperature with a relative humidity of 55%. Here, the Pt-decorated graphene sheets act as both an effective hydrogen oxidation surface and a heat-transfer TE polymer nanocomposite having low thermal conductivity. This property plays an important role in generating output voltage signal with a temperature difference between the top and bottom surfaces of the nanocomposite. As a result, our TCH gas sensor can detect the range of hydrogen from 100 ppm to percentage level with good linearity. The best response and recovery time revealed for the optimized TCH gas sensor were 23 s and 17 s under 1000 ppm H₂/air, respectively. This type of sensor can provide an important component for fabricating thermoelectric-based gas sensors with favorable gas sensing performance.     

Light-assisted synthesis of Au/TiO2 nanoparticles for H2 production by photocatalytic water splitting

A series of Au/TiO₂ photocatalysts was synthesized via the light assistance through the photo-deposition for H₂ production by photocatalytic water splitting using ethanol as the hole scavenger. Effect of solution pH in the range of 3.2–10.0 on the morphology and photocatalytic activity for H₂ production of the obtained Au/TiO₂ photocatalysts was explored. It was found that all Au/TiO₂ photocatalysts prepared in different solution pH exhibited comparable anatase fraction (～0.84–0.85) and crystallite size of TiO₂ (21–22 nm), but showed different quantity of deposited Au nanoparticles (NPs) and other properties, particularly the particle size of the Au NPs. Among all prepared Au/TiO₂ photocatalysts, the Au/TiO₂ (10.0) photocatalyst exhibited the highest photocatalytic activity for H₂ production, owning to its high metallic state and small size of Au NPs. Via this photocatalyst, the maximum H₂ production of 296 μmol (～360 μmol/g?h) was gained at 240 min using the 30 vol% ethanol as the hole scavenger at the photocatalyst loading of 1.33 g/L under the UV light intensity of 0.24 mW/cm² with the quantum efficiency of 61.2% at 254 nm. The loss of the photocatalytic activity of around 20% was observed after the 5th use.     

Enhanced hydrogen generation using ZrO2-modified coupled ZnO/TiO2 nanocomposites in the absence of noble metal co-catalyst

Mesoporous ZrO₂-modified coupled ZnO/TiO₂ nanocomposites were prepared by a surfactant assisted sol–gel method. The photocatalytic performance of these materials was investigated for H₂ evolution without noble metal co-catalyst using aqueous methanol media under AM1.5 simulated light. The H₂ evolution was compared with coupled ZnO/TiO₂, TiO₂, ZnO and Degussa P25. The ZrO₂-modified nanocomposites exhibited higher H₂ generation, specifically 0.5 wt.% ZrO₂ loading produced 30.78 mmol H₂ g⁻¹ compared to 3.55 mmol H₂ g⁻¹ obtained with coupled ZnO/TiO₂. A multiple absorbance thresholds at 435 nm and 417 nm were observed with 0.5 wt.% ZrO₂ loading, corresponding to 2.85 eV and 2.97 eV band gap energies. The high surface area, large pore volume, uniform crystallite sizes and enhanced light harvesting observed in ZrO₂-modified nanocomposites were contributing factors for effective charge separation and higher H₂ production. The possible mechanism of H₂ generation from aqueous methanol solution over ZrO₂-modified nanocomposite is presented.     

Hydrogen sensing characteristics of a Pd/Nickel oxide/GaN-based Schottky diode

Hydrogen sensing characteristics of a novel metal-oxide-semiconductor (MOS) Schottky diode are thoroughly investigated. The MOS structure consists of a gallium nitride (GaN)-based semiconductor system, a nickel oxide (NiO) layer, and palladium (Pd) catalytic materials. A well-prepared Pd/NiO/GaN-based diode shows several advantages in relation to hydrogen sensing, including a simple structure, high sensing speed, wide flexibility for operation under both forward and reverse applied voltages, and a good sensing response of 8.1 × 10³ under an applied forward voltage of 0.25 V, at 300 K in a 1% H₂/air ambience. Furthermore, under an applied reverse voltage of −2 V and at a high temperature of 573 K, this MOS diode shows a response as high as 1.8 × 10⁴ towards 1% H₂/air mixture gas. The Schottky diode sensor with a novel Pd/NiO/GaN structure demonstrated in this study is a promising candidate for high-performance hydrogen sensing applications.     

Enhancement of the room-temperature hydrogen sensing performance of MoO3 nanoribbons annealed in a reducing gas

Uniform-sized orthorhombic MoO₃ nanoribbons were synthesized by a simple hydrothermal method at 240 °C. The nanoribbons grew along the [001] orientation, with average length, width and thickness of approximately 20 μm, 270 nm and 90 nm, respectively. The obtained nanoribbons were further annealed in a hydrogen atmosphere at different temperatures to modify their surface states. The treatment of the nanoribbons at 300 °C significantly elevated the concentration of non-stoichiometric Mo⁵⁺ to 24.7%, much larger than the original concentration (∼14.8%). A positive relationship was found between the non-stoichiometric Mo⁵⁺, chemisorbed oxygen ion and sensor response. The sensor based on the MoO₃ nanoribbons treated at 300 °C exhibited a faster response time of approximately 10.9 s, and a higher sensor response of 17.3 towards 1000 ppm H₂, compared with the results of original tests (∼21 s and ∼5.7, respectively), indicating the significantly improved gas sensing performance of the treated MoO₃. Meanwhile, the sensor also exhibited excellent repeatability and selectivity toward hydrogen gas. The enhancement of the hydrogen gas sensing performance of treated MoO₃ nanoribbons was attributed to the more effective adjustment of the width of the depletion region on the nanoribbon surface and the height of the potential barrier at the junctions, induced by the interaction between hydrogen molecules and higher-concentration oxygen ions. Our research implied that the gas sensing performance of nanostructured metal oxides could be successfully enhanced through annealing in the reducing gas.     

An excellent room-temperature hydrogen sensor based on titania nanotube-arrays

Hydrogen sensors have been fabricated from highly ordered TiO₂ nanotube arrays through anodization of a Ti substrate in an ethylene glycol solution containing NH₄F. The vertically oriented TiO₂ nanotube arrays containing Pt electrodes exhibit an ability to detect a wide-range of hydrogen concentrations at room temperature. On exposure to 2000 ppm (parts per million) hydrogen, the sensors exhibit seven orders of magnitude change in resistance with a response time of 13 s at room temperature. The TiO₂ nanotube arrays sensor equipped with Pt electrodes exhibited a diode-type current–voltage (I–V) characteristic in air, but nearly ohmic behavior in hydrogen balanced with argon. A significant response to hydrogen was observed without the presence of oxygen in the base atmosphere. The response of two kinds of sensors with either Pt or Pt/Ti electrodes to 500 ppm hydrogen was measured and the results suggested that the excellent hydrogen sensing properties in air resulted primarily from the variation of the Schottky barrier height at the Pt/TiO₂ interface.     

Electrospun Pt/TiO2 hybrid nanofibers for visible-light-driven H2 evolution

One-dimensional (1D) Pt/TiO₂ hybrid nanofibers (HNFs) with different concentrations of Pt were fabricated by a facile two-step synthesis route combining an electrospinning technique and calcination process. X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) results showed that the Pt nanoparticles (NPs) with the size of 5–10 nm were well dispersed in the TiO₂ nanofibers (NFs). Further investigations from the UV–Vis diffuse reflectance (DR) and X-ray photoelectron spectroscopy (XPS) analysis revealed that some Pt ions were incorporated into the TiO₂ lattice as Pt⁴⁺ state, which contributed to the visible light absorption of TiO₂ NFs. Meanwhile, the Pt²⁺ ions existing on the surface of Pt NPs resulted in the formation of Pt–O–Ti bond at Pt NPs/TiO₂ NFs interfaces that might serve as an effective channel for improving the charge transfer. The as-electrospun Pt/TiO₂ HNFs exhibited remarkable activities for photocatalytic H₂ evolution under visible light irradiation in the presence of l-ascorbic acid as the sacrificial agent. In particular, the optimal HNFs containing 1.0 at% Pt showed the H₂ evolution rate of 2.91 μmol h⁻¹ and apparent quantum efficiency of 0.04% at 420 nm by using only 5 mg of photocatalysts. The higher photocatalytic activity could be ascribed to the appropriate amount of Pt ions doping and excellent electron-sink effect of Pt NPs co-catalysts.