A novel hydrogen-sensitive sensor based on Pd nanorings/TNTs composite structure

Hydrogen sensors with a novel composite structure comprised of Pd nanorings distributed on TiO₂ nanotube arrays were developed and tested. Effect of the TiO₂ nanotube diameter size, Pd nanorings thickness on the sensors' hydrogen response characteristics were investigated. Time dependence of resistance of the Pd nanorings/TNTs composite structure on various hydrogen concentrations was also carried out and demonstrated good room temperature hydrogen sensitive characteristics. Optimized experiments demonstrated that the hydrogen sensor composed of 25 nm-thickness Pd nanorings distributed on the 77 nm-diameter size TiO₂ nanotube showed a fast response time (3.8 s) and high sensitivity (92.05%) at 0.8 vol% H₂. A hydrogen sensitive characteristics model is proposed and the Pd nanorings' important role in the hydrogen sensitive mechanisms is described. The hydrogen sensor's excellent hydrogen sensitive characteristics is ascribed to the Pd nanorings' quick and continual formation and breakage of multiple passages due to absorption and desorption of hydrogen atoms.     

Hydrogen sensor using the Pd film supported on anodic aluminum oxide

Highly sensitive hydrogen gas sensors were fabricated using a microelectromechanical system (MEMS) and anodic aluminum oxides (AAOs) process. MEMS based gas sensor platform was designed with the multi-layer type for Pd film morphology manipulations. The operating temperature of the micro heater was positively correlated with the heater. Hydrogen sensing response of the sensor showed a good positive linearity as the gas sensitivity increased with increasing hydrogen concentration. The hydrogen sensitivity (defined as ratio of sensor resistances in air and after the hydrogen gas injection) was 0.638% at hydrogen concentration of 2000 ppm. The H₂ sensitivity was very dependent on the thickness and morphology of Pd-nanosized film. The gas sensitivity and response properties showed different behaviors when palladium film was deposited on the anodic aluminum oxide (AAO) layer. The hydrogen sensitivity for the Pd on AAO layer was about 0.783% at the hydrogen concentration of 2000 ppm. The sensitivity of the Pd-AAO layer improved with respect to the pure Pd thin film due to nanoporous nature of AAO.     

Effect of pigment concentration and particle size of TiO2 support on performance of chemochromic hydrogen tape sensor

A chemochromic hydrogen tape sensor has been developed to detect hydrogen leaks using titania (TiO₂) supported palladium oxide (PdO) pigments encapsulated within a silicone matrix. This study has been carried out to investigate the effects of pigment (PdO-TiO₂) concentration and particle size of TiO₂ support on detection performance in terms of color contrast of a chemochromic hydrogen tape sensor. The irreversible hydrogen tape sensors were tested with different concentration from 0.2 wt% to 10.0 wt%. Several pigments were synthesized using three different TiO₂ support with particle sizes ranging from 100 nm to 5 μm. The experimental results exhibit that the color of the pigment with 0.2 wt% shows distinctive color change in minimum and the optimal pigment concentration for silicone matrix type irreversible tape sensor is 3.0 wt%. In addition, TEM analysis revealed that the PdO particles become larger and agglomerated as increasing the particle size of TiO₂ support. The pigment with Aldrich TiO₂ particle size ≤5 μm has a good performance than smaller one.     

Effect of PBI-HFA surface treatments on Pd/PBI-HFA composite gas separation membranes

For pure hydrogen separation, palladium was deposited on surface-treated polybenzimidazole (PBI-HFA, 4,4′-(hexafluroisopropylidene)bis(benzoic acid)) via the vacuum electroless plating technique (VELP). Since the hydrophobic characteristics of the polymer surface restrict strong adhesion of Pd on it and cause the peel-off of Pd film, various surface treatments have been employed. To increase the number of Pd anchoring sites on the PBI-HFA surface, mechanical abrasion (polishing) was applied, and to increase the hydrophilicity of the PBI-HFA surface, wet-chemical and O₂ plasma treatment (dry etching) were used. The thickness and effective permeating area of the deposited Pd films on the PBI-HFA membranes were estimated to be in the range of 160–340 nm and 8.3 cm², respectively. Among the tested membranes, membranes with Pd layers deposited on O₂ plasma treated PBI-HFA surfaces had the most uniform microstructure and the least number of defects compared to the other membranes. Gas permeation experiments were performed as a function of temperature and pressure. The gases used in the permeation measurements were H₂, N₂, CO₂, and CO (99.9% purity). A Pd-O₂30 m membrane, fabricated by O₂ plasma surface treatment during 30 min, exhibited superior gas separation performance (H₂ permeability of 275.5 Barrer), and proved to be impermeable to carbon monoxide. Enhancement of H₂ permselectivity of Pd/PBI-HFA composite membrane treated by O₂ plasma shows promising hydrogen separation membrane.     

On-chip growth of patterned ZnO nanorod sensors with PdO decoration for enhancement of hydrogen-sensing performance

In this study, we used a low-temperature hydrothermal technique to fabricate arrays of sensors with ZnO nanorods grown on-chip. The sensors on the glass substrate then were sputter decorated with Pd at thicknesses of 2, 4, and 8 nm and annealed at 650 °C in air for an hour. Scanning electron microscopy, high resolution transmission microscopy, X-ray diffraction, and surface analysis by X-ray photoelectron spectroscopy characterization demonstrated that decoration of homogenous PdO nanoparticles on the surface of ZnO nanorods had been achieved. The sensors were tested against three reducing gases, namely hydrogen, ethanol, and ammonia, at 350, 400, and 450 °C. The ZnO nanorods decorated with PdO particles from the 2 and 4 nm layers showed the highest responses to H₂ at 450 and 350 °C, respectively. These samples also generally exhibited better selectivity for hydrogen than for ethanol and ammonia at the same concentrations and at all tested temperatures. However, the ZnO nanorods decorated with PdO particles from the 8 nm layer showed a reverse sensing behaviour compared with the first two. The sensing mechanism behind these phenomena is discussed in the light of the spillover effect of hydrogen in contact with the PdO particles as well as the negative competition of the PdO thin film formed between the sensor electrodes during sputter decoration, Pd–Zn heterojunction that forms at high temperature and thus influences the conductivity of the ZnO nanorods.     

Cracked palladium films on an elastomeric substrate for use as hydrogen sensors

We have investigated a lithography-free technique for On-Off type hydrogen sensors using a cracked palladium (Pd) film on an elastomeric substrate. Cracks were induced in a sputtered Pd film simply by undergoing hydrogen absorption and desorption processes. Compared to the same thickness of a Pd film on a Si/SiO₂ substrate that relied on the electron scattering mechanism, a cracked Pd film on an elastomeric substrate operated as a reversible On-Off hydrogen sensor based on the crack open-close mechanism when exposed to hydrogen. The thickness of a Pd film on the elastomeric substrate plays a significant role in determining the sensing mode of the cracked Pd film. The cracked Pd film with a thickness of 9–11 nm on the elastomeric substrate showed reversible and perfect On-Off responses under a wide range of hydrogen concentrations with large current variations and a fast response time of less than 1 s.     

Hierarchical spheres assembled from large ultrathin anatase TiO2 nanosheets for photocatalytic hydrogen evolution from water splitting

We report the synthesis of TiO₂ hierarchical spheres (THS) with large specific surface area via a facile one-pot solvothermal method. The as-prepared THS are self-assembled by ultrathin TiO₂ nanosheets with thickness of several nanometers and they show a uniform spherical morphology with an average size of 500–700 nm. However, the as-prepared light yellow THS exhibit inferior photocatalytic activity for hydrogen evolution from water splitting due to the poor crystallization of TiO₂ and the existence of oxygen vacancies. Significantly, a subsequent thermal treatment improves the crystallinity of THS, reduces the oxygen vacancies, and thereby enhances the photocatalytic performance. It demonstrates that the sample annealed at 550 °C (THS550) exhibits the highest photocatalytic activity, about 5 times higher than that of commercial TiO₂ nanoparticles (CTiO₂). Moreover, the THS550 sample loaded with 1 wt% Pt exhibits an hydrogen evolution rate as high as 17.9 mmol h^?1g^?1, and the corresponding apparent quantum efficiency has been determined to be 28.46% under 350 nm light irradiation.     

Feasibility of H2 sensors composed of tungsten oxide nanocluster films

The hydrogen (H₂) sensing properties, including the sensor response, response time and recovery time, of different sensor architectures based on tungsten oxide (WO₃) were investigated to assess the feasibility of using WO₃ in producing practical H₂ sensors. Each of the different sensor architectures consists of 3 layers. The first layer is a 2.5-nm palladium (Pd) layer, which is always deposited onto a highly porous WO₃ nanocluster layer. The third layer is an Au/Ti electrode layer, which may be constructed in the form of interdigitated electrodes or 5 × 5 mm² pad electrodes, which is located either on the top surface of the Pd layer or at the bottom of the WO₃ film. Furthermore, the WO₃ layer was also constructed to be either 11.2 nm or 153 nm thick. The sensor design consisting of a 2.5-nm Pd layer on an 11.2-nm WO₃ layer with interdigitated electrodes at the bottom of the layer was found to exhibit the best overall H₂ sensing properties, with excellent cyclic stability over 600 cycles of operation.     

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.     

Hydrogen gettering of titaniumpalladium/palladium nanocomposite films synthesized by cosputtering and vacuum-annealing

Nano-engineered composite film, prepared by the combination of titanium (Ti) nanoparticles with surrounding layers of palladium (Pd), has been suggested as a high performance hydrogen (H₂) getter. Uniform TiPd film covered by a 35-nm-thick Pd layer was deposited on a silicon wafer via cosputtering and post-vacuum-annealing. As the annealing temperature increased from 200 to 400 °C, amorphous alloy and nano-aggregates were observed, and efficient structural modulation occurred at 400 °C, where dewetting of Pd cover layer from the getter surface was observed. This led to the enhancement of the chemisorption capacity of the 400^oC-annealed sample, two-times higher than that of the 300^oC-annealed sample. Abrupt change in residual gases, which typically come from a bonding process, can be mitigated by minimizing the gas transfer distance through the dewetting of the cover layer; since Ti nanoparticles surrounded by Pd exist independently of each other in the gettering layer, external H₂ gas molecules can be continuously adsorbed onto still-unreacted Ti particles by passing through the dewetted channels in the Pd cover layer. This concept demonstrates a pathway towards a useful synthetic approach for high-performance thin-film getters with high adsorption capacity, fast gettering rate and good device compatibility.     

A comparative study on the hydrogen absorption of thin films at room temperature deposited on non-porous glass substrate and nano-porous anodic aluminum oxide (AAO) template

The influence of different morphology of the thin films deposited on the non-porous glass and nano-porous anodic aluminum oxide (AAO) substrates on the hydrogen absorption at room temperature was studied. A well-known sandwich-like Pd/Mg/Pd film was investigated. It is observed that the film deposited on the porous AAO template demonstrates a better hydrogen absorption in the H₂ pressure range 10-600 mbar with respect to the same film supported by the non-porous glass substrate. Moreover, the layer grown on the AAO, owing to its specific morphology inherited from the nano-porous substrate, has revealed its resistance toward stress accumulation caused by the lattice expansion, showing no buckle-to-crack network formation upon the hydrogen uptake. This interesting feature is expected to improve the cycling properties and structural stability of the system, and may help to investigate better the interaction of the H₂ with metal.     

Room temperature response and enhanced hydrogen sensing in size selected Pd-C core-shell nanoparticles: Role of carbon shell and Pd-C interface

In the present work, the effect of carbon shell around size selected palladium (Pd) nanoparticles on hydrogen (H₂) sensing has been studied by investigating the sensing response of Pd-C core-shell nanoparticles having a fixed core size and different shell thickness. The H₂ sensing response of sensors based on Pd and Pd-C nanoparticles deposited on SiO₂ and graphene substrate has been measured over a temperature range of 25 °C–150 °C. It is observed that Pd-C nanoparticle sensor shows higher sensitivity with increase in shell thickness and faster response/recovery in comparison to that of Pd nanoparticle samples. Pd-C nanoparticles show room temperature H₂ sensitivity in contrast to Pd nanoparticles which respond only at higher temperatures. Role of carbon shell is also understood by investigating H₂ sensing properties of Pd and Pd-C nanoparticles on graphene substrates. These results show that higher catalytic activity and electronic interaction at Pd-C interface, a complete coverage and protection of Pd surface by carbon and presence of structural defects in nanoparticle core are important for room temperature and higher sensing response.     

Hydrogen gas sensor based on mesoporous In2O3 with fast response/recovery and ppb level detection limit

Hydrogen gas sensors were fabricated using mesoporous In₂O₃ synthesized using hydrothermal reaction and calcination processes. Their best performance for the hydrogen detection was found at a working temperature of 260 °C with a high response of 18.0 toward 500 ppm hydrogen, fast response/recovery times (e.g. 1.7 s/1.5 s for 500 ppm hydrogen), and a low detection limit down to 10 ppb. Using air as the carrier gas, the mesoporous In₂O₃ sensors exhibited good reversibility and repeatability towards hydrogen gas. They also showed a good selectivity for hydrogen compared to other commonly investigated gases including NH₃, CO, ethyl alcohol, ethyl acetate, styrene, CH₂Cl₂ and formaldehyde. In addition, the sensors showed good long-term stability. The good sensing performance of these hydrogen sensors is attributed to the formation of mesoporous structures, large specific surface areas and numerous chemisorbed oxygen ions on the surfaces of the mesoporous In₂O₃.     

TiO2/ZnO/TiO2 sandwich multi‐layer films as a hole‐blocking layer for efficient perovskite solar cells

A continuous and compact hole‐blocking layer is crucial to prevent photocurrent recombination at the photoanode/electrode interface of high‐performance mesostructure perovskite‐based solar cells. Novel TiO₂/ZnO/TiO₂ sandwich multi‐layer compact film prepared as hole‐blocking layer for perovskite solar cell. Herein, TiO₂, ZnO, and TiO₂ layers were successfully deposited by spin‐coating onto FTO glass substrate in sequence. The fill factor and power conversion efficiency of the perovskite solar cell are remarkably improved by the employment of a TiO₂/ZnO/TiO₂ sandwich compact layer. Perovskite solar cell based on TiO₂/ZnO/TiO₂ sandwich film has been observed to exhibit maximum incident‐photon‐to‐current conversion efficiency in the visible region (400–780 nm) and reach a power conversion efficiency of 12.8% under AM1.5G illumination. Copyright © 2016 John Wiley & Sons, Ltd.     

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.     

Effect of Pd overlayer and mixed gases on hydrogen permeation of Pd/Nb30Hf35Co35/Pd composite membranes

Effect of Pd overlayer and mixed gases on hydrogen permeation of Pd/Nb₃₀Hf₃₅Co₃₅/Pd composite membranes was investigated. The diameter of Pd particle increases with increasing sputtering power. With this change, the membrane shows a signification reduction in hydrogen permeability/or flux, but its durability and stability increases significantly, which can be mainly attributed to a decrease in hydrogen solubility coefficient. In addition, H₂S impurity in mixed gases can greatly degrade membrane performance, especially the hydrogen permeability, whereas the Ar impurity content has less effect in the temperature range of 523–673 K. Lowering of permeability caused by the change of gas purity can be attributed to a decrease in hydrogen solubility, which is closely related to the stronger adsorption of H₂S molecules to the Pd overlayer of the membrane. Thus, it is concluded that aside from the optimum design for composition of Nb-based hydrogen permeable alloy to improve their permeability, the control of Pd overlayer film on membrane surface and gas purity in the feed gas is important.     

Hydrogen absorption by Ti-implanted Zr-1Nb alloy

This paper describes the hydrogenation behavior of Zr-1Nb alloy Ti-implanted by plasma immersion ion implantation (PIII). Hydrogen sorption kinetics of the Ti-modified alloy was investigated under gas-phase hydrogenation at 400 °C for 1 h. The influence of implantation time on the protective properties of the modified layer was shown. The lowest hydrogen absorption as well as the highest hydrogen trapping efficiency was achieved after PIII for 30 min. The main contribution to the reduction of hydrogen permeation is the formation of an oxide layer consisting of mixed TiO₂ and ZrO₂ on the modified surface of the alloy. X-ray photoelectron spectroscopy (XPS) revealed that PIII titanium oxide exists on the surface in the form of TiO₂, which transforms to mixed Ti₂O₃ and TiO₂ after hydrogenation. The thickness of the modified layer increases with implantation time that improves the efficiency of hydrogen trapping. All the absorbed hydrogen is gradually distributed in the modified layer and no hydrides are formed after hydrogenation in Ti-modified Zr-1Nb for 15 and 30 min.     

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%.     

Nucleate Pool Boiling Heat Transfer of Hydro-Fluorocarbon Refrigerant R134a on TiO2 Nanoparticle Coated Copper Heating Surfaces

ABSTRACT

Pool boiling heat transfer performance of hydro-fluorocarbon refrigerant R-134a on titanium dioxide (TiO₂) nanoparticle coated surface is experimentally studied in the article. The test surfaces, viz, 100 nm, 200 nm and 300 nm thick TiO₂ nanoparticle coated surfaces over 100 nm thin film surface are used in this experimentation. The surfaces are synthesized and fabricated by simple and cost-effective electron beam evaporation method. The test surfaces were characterized by scanning electron microscope and atomic force microscope to uncover the formation of crystalline structure on coated surfaces. These surfaces are utilized in pool boiling test rig using refrigerant R134a at 10°C saturation temperatures. The result indicated that a maximum of 87.5% augmentation in the boiling heat transfer has been achieved by higher thickness of TiO₂ coated surface than the bare copper surface. In addition, the incipience wall superheat is reduced for higher thickness coated surface. The augmentation of heat transfer coefficient might be the reason for increase in micro/nano-porosity, active nucleation site density and surface area of the heating surface. It is observed that with the increase of sub-cooling temperature of liquid, the bubble departure diameter was reduced while the heat transfer coefficient has been increased.     

Unique photoelectrochemical behavior of TiO2 nanorods wrapped with novel titanium Oxy-Nitride (TiOxNy) nanoparticles

In this work, we developed novel titanium oxynitride (TiO_xN_y) nanoparticles with diameter of 25 ± 2 nm and crystalline size of ～15 nm on hydrothermally grown one-dimensional (1D) TiO₂ nanorod (TNR) arrays. Herein, the TiO_xN_y nanoparticles were synthesized by facile nitridation using TiO₂ powder at 100% NH₃ gas atmosphere. Titanium oxynitride composed of potentially energetic metal-nitrogen bonds (TiN), compared to the weaker TiO bond, becomes chemically stable in the alkaline environment, and is considered as a suitable material for photoelectrochemical (PEC) system. The PEC performance of TiO_xN_y decorated TNR (abbreviated as TiO_xN_y @TNR) films was evaluated in 0.1 M KOH solution under solar illumination condition, and achieved the potentially high photocurrent density (J) of 2.1 mA/cm² at 1.23 V versus reversible hydrogen electrode (RHE) (abbreviated as V_RHE) in the TiO_xN_y@TNR arrays, in comparison with the poor photoresponse (0.7 mA/cm² at 1.23 V_RHE) of the pristine TNR arrays. A nearly three-fold enhancement was attained in the TiO_xN_y decorated TNR arrays, attributed to the high visible light absorption and fast carrier separation, due to the hybridization with the visible active TiO_xN_y nanoparticles in the cascading band alignment between the TiO_xN_y and TNR materials. Furthermore, the introduction of TiO_xN_y layer on the TNR surface quite reduces the interfacial resistance in the solid-liquid interface region, and further, the TiO_xN_y layer contributes to the passivation of the surface states (e.g., defect, trap sites etc.) where the charge recombination reaction frequently happens, leading to the improvement of PEC performance.