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.     

Improving hydrogen production via water splitting over Pt/TiO2/activated carbon nanocomposite

Mesoporous TiO₂/AC, Pt/TiO₂ and Pt/TiO₂/AC (AC = activated carbon) nanocomposites were synthesized by functionalizing the activated carbon using acid treatment and sol–gel method. Photochemical deposition method was used for Pt loading. The nano-photocatalysts were characterized using XRD, SEM, DRS, BET, FTIR, XPS, CHN and ICP methods. The hydrogen production, under UV light irradiation in an aqueous suspension containing methanol has been studied. The effect of Pt, methanol and activated carbon were investigated. The results show that the activated carbon and Pt together improve the hydrogen production via water splitting. Also methanol acts as a good hole scavenger. Mesoporous Pt/TiO₂/AC nanocomposite is the most efficient photocatalyst for hydrogen production compared to TiO₂/AC, Pt/TiO₂ and the commercial photocatalyst P25 under the same photoreaction conditions. Using Pt/TiO₂/AC, the rate of hydrogen production is 7490 μmol (h g catal.)⁻¹ that is about 75 times higher than that of the P25 photocatalyst.     

Hydrogen production by photocatalytic membranes fabricated by supersonic cluster beam deposition on glass fiber filters

Photoactive membranes coated with TiO₂ and Pt/TiO₂ nanostructured thin films were produced by one-step deposition of gas phase nanoparticles on glass fiber filters. Pt/TiO₂ nanoparticles (0–1.5 wt.% Pt content) were produced by flame spray pyrolysis, starting from liquid solutions of the Ti and Pt precursors, and then expanded in a supersonic beam to be deposited on the filters. The nanostructured coatings were composed of crystalline nanoparticles (mainly anatase phase), without any need of post-deposition annealing. The so obtained photocatalytic membranes were tested in hydrogen production by photo-steam reforming of ethanol in an expressly set-up diffusive photoreactor. The reaction rate was found to increase with increasing the Pt content in the photoactive material, up to 1.5 wt.% Pt. The use of these membranes allowed a significant increase of the hydrogen production rate compared to that obtained with the same photoactive Pt/TiO₂ films deposited on a quartz substrate.     

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.     

Kinetic study of oxygen reduction reaction and PEM fuel cell performance of Pt/TiO2-C electrocatalyst

The electrochemical activity and thermal stability of the Pt/TiO₂-C were evaluated in the oxygen reduction reaction (ORR) in acid medium at different temperatures. The platinum was selectively deposited onto the TiO₂ (E_bg = 2.3 eV) by the photo-irradiation of platinum precursor (Pt⁴⁺→Pt⁰). The Pt/TiO₂-C electrocatalyst prepared was characterized by XRD, TEM/EDS, cyclic and lineal voltammetry techniques. TEM images indicated that platinum nanoparticles (<5 nm) were deposited in agglomerates form around the oxide sites. EDS and XRD results confirm the composition and crystalline structure of Pt/TiO₂-C. The thermal stability and electrochemical activity of the Pt/TiO₂-C for ORR at different temperatures (298–343 K) is higher than Pt/C commercial sample (Pt-Etek). A more favorable apparent enthalpy of activation for Pt/TiO₂-C was greatly influenced by addition of oxide in the catalyst compare to Pt-Etek. Single H₂/O₂ fuel cell performance results of Pt/TiO₂-C show an improvement of the power density with the increase of the temperature.     

Influence of Pt deposition on water-splitting hydrogen generation by highly-ordered TiO2 nanotube arrays

Highly-ordered TiO₂ nanotube arrays (TNTAs) were fabricated on Ti sheets by electrochemical anodization. Uniform Pt nanoparticles with an average diameter of 3 nm could be successfully located on the TiO₂ nanotubes on only one side (Pt/TNTAs) or both sides of the Ti sheet (Pt/TNTAs/Pt). Pt/TNTAs, the single-sided Pt deposited TNTAs, could be directly used to split water without a counter electrode. The hydrogen evolution rate can reach 120 μmol h⁻¹ cm⁻² in a mixed solution of 0.5 M Na₂SO₄ and 0.5 M ethylene glycol without any applied bias, which is six times of that by the pure TNTAs. In comparison to the traditional three electrode system, this single-sided Pt deposited TNTAs is a much more simple and efficient water splitting system. Meanwhile, the photoelectrical conversion mechanism has been investigated in detail.     

A hydrothermal route for constructing reduced graphene oxide/TiO2 nanocomposites: Enhanced photocatalytic activity for hydrogen evolution

A series of reduced graphene oxide/TiO₂ (RGO/TiO₂) nanowire microsphere composites were synthesized with a facile one-step hydrothermal method using TiCl₃ and graphene oxide (GO) as the starting materials, during which the formation of TiO₂ and the reduction of GO occur simultaneously. The obtained nanocomposites were characterized with X-ray diffraction, field emission scanning electron microscope, transmission electron microscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy, respectively. UV–vis absorption spectra showed that the absorption edges of TiO₂ were extended into visible light region with the addition of RGO. The photocatalytic activities of the samples with and without Pt as cocatalysts were evaluated by hydrogen evolution from water photo-splitting under UV–vis light illumination. Enhanced photocatalytic properties were observed for the as-prepared RGO/TiO₂ nanocomposites. The amount of hydrogen evolution from the optimized photocatalyst reached to 43.8 μmol h⁻¹, which was about 1.6 times as high as that of bare TiO₂. The results shown here indicate a convenient and applicable approach to further exploitation of high activity materials for photocatalytic water splitting applications.     

Fabrication of highly dispersed palladium/graphene oxide nanocomposites and their catalytic properties for efficient hydrogenation of p-nitrophenol and hydrogen generation

In this report, graphene oxide (GO) nanosheets decorated with ultrafine Pd nanoparticles (Pd NPs) have been successfully fabricated through a reaction between [Pd₂(μ-CO)₂Cl₄]²⁻ and water in the presence of GO nanosheets without any surfactant or other reductant. The as-synthesized small Pd NPs with average diameter of about 4.4 nm were well-dispersed on the surface of GO nanosheets. The Pd/GO nanocomposites show remarkable catalytic activity toward the hydrogenation of p-nitrophenol at room temperature. The kinetic apparent rate constant (k_app) could reach about 34.3 × 10⁻³ s⁻¹. Furthermore, the as-prepared Pd/GO nanocomposites could also be used as an efficient and stable catalyst for hydrogen production from hydrolytic dehydrogenation of ammonia borane (AB). The catalytic activity is much higher than the conventional Pd/C catalysts.     

A experimental study on the dehydrogenation performance of dodecahydro-N-ethylcarbazole on M/TiO2 catalysts

Liquid organic hydrogen carrier (LOHC) is considered as a promising candidate for large-scale hydrogen storage. In this work, we found that Pt/TiO₂ catalysts exhibited better catalytic activity and selectivity compared to Pd/TiO₂ and commercial Pd/Al₂O₃ catalysts in the dehydrogenation of dodecahydro-N-ethylcarbazole (12H-NECZ) at 453 K. The catalytic activity of the noble metal catalysts followed the trend of Pt/TiO₂ > Pd/TiO₂ > Rh/TiO₂ > Au/TiO₂ > Ru/TiO₂. Compared with the commercial Pd/Al₂O₃, Pt/TiO₂ greatly improved the selectivity and conversion rate, the reaction time was also shortened. In addition, kinetics calculation was carried out to obtain fundamental reaction parameters. It was found that the third step of 4H-NECZ dehydrogenation to NECZ was the rate-limiting step of the entire dehydrogenation reaction for all catalysts.     

Pd–WO3/reduced graphene oxide hierarchical nanostructures as efficient hydrogen gas sensors

Pd–WO₃ nanostructures were incorporated on graphene oxide (GO) and partially reduced graphene oxide (PRGO) sheets using a controlled hydrothermal process to fabricate effective hydrogen gas sensors. Pd–WO₃ nanostructures showed ribbon-like morphologies and Pd–WO₃/GO presented an irregular nanostructured form, while Pd–WO₃/PRGO exhibited a hierarchical nanostructure with a high surface area. Gas sensing properties of thin films of these materials were studied for different hydrogen concentrations (from 20 to 10,000 ppm) at various temperatures (from room temperature to 250 °C). Although adding GO in the Pd–WO₃, after hydrothermal process could increase the film conductivity, gas sensitivity was reduced to half, due to lower surface area of the irregular morphology in comparison with the ribbon-like morphology. The Pd–WO₃/PRGO films showed an optimum sensitivity (∼10 folds better than the sensitivity of Pd–WO₃/GO), and a fast response and recovery time (<1 min) at low temperature of 100 °C. Moreover, the Pd–WO₃/PRGO-based gas sensor was sensitive to 20 ppm concentration of hydrogen gas at room temperature. The results confirmed the effect of residual oxygen-containing functional groups of PRGO on the growth and morphology of Pd–WO₃ as well as gas sensing properties of metal oxide/graphene based hybrid nanostructures.     

Comparison study of electrocatalytic activity of reduced graphene oxide supported Pt–Cu bimetallic or Pt nanoparticles for the electrooxidation of methanol and ethanol

Pt–Cu bimetallic nanoparticles supported on reduced graphene oxide (Pt–Cu/RGO) were synthesized through the simple one-step reduction of H₂PtCl₆ and CuSO₄ in the presence of graphene oxide (GO) at room-temperature. The Pt–Cu/RGO was characterized with UV–vis spectrophotometer, X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy and its catalytic behavior for the direct oxidation of methanol was investigated. Compared to Pt/RGO and Pt/C catalysts, Pt–Cu/RGO hybrids exhibited markedly superior catalytic activity for the electrocatalytic oxidation of methanol and ethanol. This improved catalytic activity can be attributed to the dendritic structure of the Pt–Cu bimetallic nanoparticles.     

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.     

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.     

Efficient photocatalytic hydrogen generation by Pt modified TiO2 nanotubes fabricated by rapid breakdown anodization

Photo-assisted hydrogen generation studies of platinum loaded titanium (IV) oxide nanotubes suspended in ethanol–water mixture were carried out at room temperature. The TiO₂ nanotubes synthesized by rapid breakdown anodization technique were loaded with Pt nanoparticles by chemical reduction of aqueous chloroplatinic acid solution using sodium borohydride. The chemisorption (active) surface area of the synthesized nanocomposites for hydrogen was measured by pulse chemisorption method using temperature programmed desorption reduction oxidation equipment and found to decrease with increase in platinum loading in the range 1–10 wt%. The platinum supported nanotube composites were characterized for phase and morphology by XRD, TEM and SEM. The hydrogen generated by the photocatalytic reduction of water from water–ethanol mixture at different wavelengths of incident light, using the Pt-TiO₂ nanocomposite photocatalyst, was determined by using a proton exchange membrane based hydrogen meter. The highest hydrogen generation efficiency was observed at 1–2.5 wt% of Pt loading. The maximum photocatalytic hydrogen generation of 0.03 mol/h/g of Pt-TiO₂ was observed with a 64 W UV light source (λ = 254 nm). The photoluminescence property of the Pt loaded TiO₂ has been correlated with the hydrogen generation efficiency and the reaction mechanism briefly discussed.     

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.     

Synthesis of reduced graphene oxide–TiO2 nanoparticle composite systems and its application in hydrogen production

The utilization of solar energy for the conversion of water to hydrogen and oxygen has been considered to be an efficient strategy to solve crisis of energy and environment. Here, we report the synthesis of reduced graphene oxide–TiO₂ nanoparticle composite system through the photocatalytic reduction of graphite oxide using TiO₂ nanoparticles. Photoelectrochemical characterizations and hydrogen evolution measurements of these nanocomposites reveal that the presence of graphene enhances the photocurrent density and hydrogen generation rate. The optimum photocurrent density and hydrogen generation rate has been found to be 3.4 mA cm⁻² and 127.5 μmole cm⁻²h⁻¹ in 0.5 M Na₂SO₄ electrolyte solution under 1.5AM solar irradiance of white light with illumination intensity of 100 mW cm⁻². In graphene–TiO₂ nanocomposite, photogenerated electrons in TiO₂ are scavenged by graphene sheets and percolate to counter electrode to reduce H⁺ to molecular hydrogen thus increasing the performance of water-splitting reaction.     

Photoelectrochemical hydrogen production from water/methanol decomposition using Ag/TiO2 nanocomposite thin films

Though less frequently studied for solar-hydrogen production, films are more convenient to use than powders and can be easily recycled. Anatase TiO₂ films decorated with Ag nanoparticles are synthesized by a rapid, simple, and inexpensive method. They are used to cleave water to produce H₂ under UV light in the presence of methanol as a hole scavenger. A simple and sensitive method is established here to monitor the time course of hydrogen production for ultralow amounts of TiO₂. The average hydrogen production rate of Ag/TiO₂ anatase films is 147.9 ± 35.5 μmol/h/g. Without silver, it decreases dramatically to 4.65 ± 0.39 μmol/h/g for anatase TiO₂ films and to 0.46 ± 0.66 μmol/h/g for amorphous TiO₂ films fabricated at room temperature. Our method can be used as a high through-put screening process in search of high efficiency heterogeneous photocatalysts for solar-hydrogen production from water-splitting.     

Electrochemical hydrogen storage of expanded graphite decorated with TiO2 nanoparticles

The electrochemical hydrogen storage of expanded graphite (EG) decorated with TiO₂ nanoparticles (NPs) calcined at different temperatures has been investigated with the galvanostatic charge and discharge method. The TiO₂ NPs are deposited on and between the graphene-like nanosheets of EG by a sol-gel method. The morphology, structure, composition, and specific surface area of the samples were characterized. The electrochemical measurement reveals that the EG decorated with TiO₂ NPs calcined at 500 °C has a discharge capacity of 373.5 mAh/g which is 20 times higher than that of pure EG and quite appealing for the battery applications. The mechanism of enhancement of the electrochemical activity for the TiO₂-decorated EG could be attributed to the preferable redox ability and photocatalytic property of TiO₂ NPs.     

Facile synthesis of well dispersed Pd nanoparticles on reduced graphene oxide for electrocatalytic oxidation of formic acid

In present study, we report a facile synthesis of crystalline, small size Pd nanoparticles (NPs) on reduced graphene oxide (RGO) abbreviated as Pd/RGO for electrocatalytic oxidation of formic acid (FA). Here, first graphene oxide (GO) was reduced by the green method using l-ascorbic acid and citric acid and further Pd NPs were decorated on RGO by a facile method without using any reducing agents. The reduction of GO to RGO and synthesis of Pd NPs was confirmed by the X-ray diffraction (XRD) and X-ray photoelectrons (XPS) techniques. Surface morphology of Pd/RGO nanocomposite was evaluated by the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The electrocatalytic behavior of Pd/RGO nanocomposite was tested by using of cyclic voltammetric (CV) technique for electro-oxidation of FA in mixed solution of 0.5 M HCOOH + 0.5 H₂SO₄ at RT. Results shows that the higher electrocatalytic activity of Pd/RGO nanocomposite compare to Pd NPs.     

Photo-induced reforming of alcohols with improved hydrogen apparent quantum yield on TiO2 nanotubes loaded with ultra-small Pt nanoparticles

Photo-induced reforming of methanol, ethanol, glycerol and phenol at room temperature for hydrogen production was investigated with the use of ultra-small Pt nanoparticles (NPs) loaded on TiO₂ nanotubes (NTs). The Pt NPs with diameters between 1.1 and 1.3 nm were deposited on TiO₂ NTs by DC-magnetron sputtering (DC-MS) technique. The photocatalytic hydrogen rate achieved an optimum value for a loading of about 1 wt% of Pt. Apparent quantum yield for hydrogen generation was measured for methanol and ethanol water solutions reaching a maximum of 16% under irradiation with a wavelength of 313 nm in methanol/water solution (1/8 v/v). Pt NPs loaded on TiO₂ NTs represented also a true water splitting catalyst under UV irradiation and pure distilled water. DC-MS method appears to be a technologically simple, ecologically benign and potentially low-cost process for production of an efficient photocatalyst loaded with ultra-small NPs with precise size control.