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Metal oxide semiconductor hetero- and homojunctions are commonly constructed to improve the performance of hydrogen sensors at room temperature. In this study, a simple two-step hydrothermal method was employed to prepare TiO2 films with homojunctions of rutile and anatase phases (denoted as TiO2-R/A). Then, the microstructure of anatase-phase TiO2 was altered by controlling the amount of hydrochloric acid to realize a more favorable porous structure for charge transport and a larger surface area for contact with H2. The sensor used a Pt interdigital electrode. At an optimal HCl dosage (25 mL), anatase-phase TiO2 uniformly covered rutile-phase TiO2 nanorods, resulting in a greater response to H2 at 2500 ppm compared with that of a rutile TiO2 nanorod sensor by a factor of 1153. The response time was 21 s, mainly because the homojunction formed by the TiO2 rutile and anatase phases increased the synergistic effect of the charge transfer and potential barrier between the two phases, resulting in the formation of more superoxide (O2−) free radicals on the surface. Furthermore, the porous structure increased the surface area for H2 adsorption. The TiO2-R/A-based sensor exhibited high selectivity, long-term stability, and a fast response. This study provides new insights into the design of commercially competitive hydrogen sensors. [ABSTRACT FROM AUTHOR]

Hydrogen sensors with high sensitivity, rapid response and low cost are urgently demanded in today’s industry. In this work, we reported our research on the fabrication process and the relevant sensing mechanisms of a TiO 2 film based hydrogen sensor which showed an excellent performance at room temperature in air. The sensor, prepared by hydrothermal methods with a seed layer being introduced during the growth of the thin films, exhibited the concentration detection limit as low as 1 ppm and the response time as short as 9 s. Structural analysis revealed that the introduction of the seed layer resulted in a significant improvement in the quality of the TiO 2 thin film, which was intimately related to the sensing properties. The dynamical response process was studied and the corresponding sensing mechanisms were analyzed in detail. It was found that, in addition to the generally accepted Schottky barrier mechanism, the adsorption of H 2 molecules at (002) surface of the TiO 2 thin film played a key role in achieving such a low detection limit and a short response time for the sensor. [ABSTRACT FROM AUTHOR]

Abstract: Self-aligned titania thin films of either the rutile or the anatase phase were successfully grown on FTO (F-doped SnO2) substrates with hydrothermal methods. The selection of the targeted phase, either rutile or anatase, in the thin films was controlled by adjusting the content and type of acidic anions such as Cl− and SO4 2−. The growth kinetics depended on the synthesis temperature and the types of alcohol in the solvent. The self-alignment in both the anatase and rutile thin films was attributed to the minimization of surface energies during growth. The hydrogen sensing characteristics of dense films of both rutile and anatase phases were investigated and there was remarkable improvement of sensitivity response over reported data. It was found that rutile films had higher sensitivity while anatase films had faster response. [Copyright &y& Elsevier]

Early detecting of the released gas during charging and discharging of lithium ion battery (LIB) is critical for safety monitoring of the equipment and devices which use LIB as power source. H 2 is one of the typical component of the released gas. The main challenges for detecting H 2 in LIB is the room temperature working, small size of the sensor and hypoxic environment. To address these problems, a room temperature working MEMS hydrogen sensor was developed with Pt-modified Nb-doped TiO 2 nanosheet as sensing material. The sensor has the advantages of a compact size (0.05 cm3), low power consumption (0.1 mW at room temperature), easy integration, and excellent sensing performance. The response achieved 12.3, and the response and recovery time are 31 s/270 s at room temperature to 1000 ppm H 2 (40 °C). The working mechanism of the sensor in the hypoxic environment, which is quite different from the traditional oxygen adsorption mechanism, is proposed according to the characterization of the MEMS sensors. [Display omitted] • The sensor with volume (0.05 cm3) and power consumption (0.1 mW at room temperature) have greatly the potential applications. • MEMS H 2 sensor shows high sensitivity to hydrogen at room temperature (S = 12.3–1000 ppm H 2). • Clarifies the mechanism of sensing mechanism in hypoxic environment. • The sensor shows excellent selectivity and long-term stability for 100 days. [ABSTRACT FROM AUTHOR]

Developing linear ultra-high response hydrogen sensors that operate at room temperature without additional heating is paramount for real applications. WO 3 /Pt-ZnO thin films are fabricated via a two step method. The Pt2+ ions are reduced to metallic Pt0 clusters and co-decorated with WO 3 nanoparticles grown on the well-aligned ZnO nanowalls. The H 2 response is highly linear and strongly dependent on the metallic Pt0 content. The optimized sample shows a response of 13 and 3870 to 1 ppm and 4000 ppm hydrogen, respectively, with excellent gas selectivity and cycling stability. The microstructure and band energy level analyses indicate that the well-aligned porous nanowall structure and the double charge transport channel enable the ultrahigh hydrogen response. • The well aligned porous ZnO nanowall with Pt nanoparticles covered by a WO 3 layer. • The response is 13 at 1 ppm H 2 and the selectivity factor is larger than 40. • Double charge transport channels enable an ultrahigh H 2 response at room temperature. [ABSTRACT FROM AUTHOR]

• Nb doped rutile/anatase TiO 2 heterophase junction thin film was synthesized through one-step hydrothermal self-adjusted Nb doping. • The heterophase junction sensors show remarkably extended detection range and significantly enhanced response for H 2 detecting. • The Schottky junction at Pt/TiO 2 and the heterophase junction at anatase/rutile TiO 2 enhances the sensing response in low H 2 concentration. • The results provides experimental verification of inverse dependence of detectivity on H 2 concentration in high concentration range. Hydrogen sensors combining wide detection range and high response, that can operate at room temperature, are urgently demanded for monitoring different concentrations of hydrogen on a single device. Here, a novel one-step hydrothermal method is reported to construct Nb doped rutile/anatase TiO 2 heterophase junctions for a highly sensitive H 2 sensor. The Nb concentration in the solvent plays an important role for the heterophase juncton self-assemble, and the reaction time determines the surface morphology giving either a rutile TiO 2 nanorod decorated anatase form, or a Nb doped rutile/anatase TiO 2 bi-layer structure. An inverse dependence of the detectivity on H 2 concentration is obtained as theoretically predicted but previously not experimentally verified for the high concentration region at low temperatures. The H 2 concentration detection range is remarkably expanded, extending from 1 ppm to 12000 ppm for nanorod decorated film. The H 2 response at 1 ppm is significantly enhanced to 22.5%, and reaches 98.9% in 8000 ppm H 2 for the bilayer structure. The remarkablely extended detection range is the result of the massive accumulation of reactive pre-absorbed O 2 − on the surface due to the Nb doping, and the significantly enhanced response at room temperature results from the joint contributions of pre-absorbed O 2 − , film compactness and the heterophase junction. Moreover, Nb doped rutile/anatase TiO 2 heterophase juncton films show high stability and humidity resistance for hydrogen sensing. [ABSTRACT FROM AUTHOR]

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. TiO 2 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 TiO 2 thin films are prepared by a hydrothermal method followed by annealing in either air, oxygen, vacuum or H 2 and the hydrogen sensing performance are evaluated. Raman results show that TiO 2 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 TiO 2 sensors while annealing in anaerobic atmospheres increases their response. At high hydrogen concentrations surface adsorbed O 2 − is the dominant factor, while at low concentrations the Schottky barrier between Pt and TiO 2 is key to achieving a high response. Here we show controlling the TiO 2 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 TiO 2 based hydrogen sensors. (002) oriented rutile TiO 2 thin films are prepared by a hydrothermal method followed by annealing in either air, oxygen, vacuum or H 2. Annealing in an oxygen-rich atmosphere is shown to extend the detection range of the TiO 2 based hydrogen sensors while annealing in anaerobic atmospheres increases their response. At high hydrogen concentrations surface adsorbed O 2 − is the dominant factor, while at low concentrations the Schottky barrier between Pt and TiO 2 is key to high response. Image 1 • An annealing method is used to regulate the performance of TiO 2 hydrogen sensors. • The sensor annealed in oxygen shows fast response and excellent stability. • The sensor annealed in hydrogen has a low detection limit with remarkable response. • Surface adsorbed O 2 − plays a dominant role at high hydrogen concentrations. • Schottky barrier determines the high response at low hydrogen concentrations. [ABSTRACT FROM AUTHOR]

Safety is the most important concern for hydrogen production, transportation and storage. A hydrogen sensor based on (004) oriented anatase TiO 2 thin film is fabricated with high density and specific orientation by introducing a seed layer and an annealing process for the TiO 2 thin film. The sensor has a detection limit as low as 1 ppm with a remarkable sensitivity of 17% and fast response and recovery times as short as 5 s and 64 s respectively. Modelling results based on density functional theory indicate that the energy barrier for H 2 dissociation on the (004) surface is about 0.458 eV. Both Ti and O atoms at the TiO 2 (004) surface take part in the H 2 dissociation process. The experimental and theoretical results indicate that surface oxygen has played an important role in hydrogen sensing; lowering the density of surface oxygen vacancies is beneficial in promoting hydrogen sensing. • (004) oriented anatase TiO 2 thin films was synthesized by hydrothermal method. • A hydrogen sensor with response and recovery times of 5 s and 64 s was developed. • The sensor has a low, 1 ppm, detection limit with a remarkable sensitivity of 17%. • Both Ti and O are shown to play key roles in the sensing. • The energy barrier for H 2 dissociation on the (004) surface is 0.458 eV. [ABSTRACT FROM AUTHOR]

Constructing a heterojunction is an effective way to improve the gas sensing properties of metal oxide semiconductors. In this work, a hydrogen sensor containing hierarchical and porous NiO/TiO 2 heterojunction bilayer structure was designed. The top layer is formed by the NiO nanowalls grown along the [220] direction with exposed (111) facets, and the bottom layer is formed by the [002] oriented TiO 2 nanorod arrays. The exposed high energy crystal surfaces of NiO and TiO 2 , together with the adapted surface depletion region, induced by the formation of the heterojunction, enable the hydrogen sensor to have both a wide detection range and a much higher response to hydrogen than single layer materials. With the optimized size of the pores existing between the walls, the hydrogen sensor shows excellent prevention to interference from CO. Both the gas diffusion regulated by the porous structure and the surface interaction between H 2 /CO and O 2 - controlled by the heterojunction barrier contribute to the distinctive detection of H 2 and CO at room temperature (25 ℃ ± 2 ℃). • A (111)NiO/(002)TiO 2 hierarchical heterojunction was designed for hydrogen sensing. • The sensor has wide detection range and high response than single layer materials. • The hydrogen sensor shows excellent performance to prevent interference from CO. • The hierarchical diffusion and the interface adsorption reaction are the key factors. [ABSTRACT FROM AUTHOR]

Development of advanced hydrogen sensors with high sensitivity, fast response speed, and long term stability is essential for safe utilization of hydrogen energy. In this work, a gas sensor with TiO 2 hierarchical trunk/branch structure as the sensing material is prepared by hydrothermal and immersion method. The experimental results show that facet homojunctions are formed by the facet contacts at the trunk/branch interfaces. The optimized sensor works at room temperature with an excellent sensing performance of S = 31.6% when exposed to 1 ppm H 2 concentration and a fast response and recovery time of about 10 s. The sensor also shows long-term stability, high sensing gas selectivity and high humidity resistance. It is found that the facet homojunction in the hierarchical structure of TiO 2 results in a significantly increased density of the charged oxygen ions absorbed on the surface and an enhanced effective barrier height, which are responsible for the much promoted sensing performance. This work provides a new way for designing gas sensors with highly commercial competitiveness. • TiO 2 hierarchical trunk/branch structure are fabricated. • Facet homojunctions are formed at the trunk/branch interfaces. • The hydrogen sensing performance is comprehensively enhanced. [ABSTRACT FROM AUTHOR]