Perforated Metal Oxide–Carbon Nanotube Composite Microspheres with Enhanced Lithium-Ion Storage Properties

Metal oxide–carbon nanotube (CNT) composite microspheres with a novel structure were fabricated using a one-step spray pyrolysis process. Metal oxide–CNT composite microspheres with a uniform distribution of void nanospheres were prepared from a colloidal spray solution containing CNTs, metal salts, and polystyrene (PS) nanobeads. Perforated SnO₂–CNT composite microspheres with a uniform distribution of void nanospheres showed excellent lithium storage properties as anode materials for lithium-ion batteries. Bare SnO₂ microspheres and SnO₂–CNT composite microspheres with perforated and filled structures had a discharge capacity of 450, 1108, and 590 mA h g^–1 for the 250th cycle at a current density of 1.5 A g^–1, and the corresponding capacity retention compared to the second cycle was 41, 98, and 55%, respectively. The synergetic combination of void nanospheres and flexible CNTs improved the electrochemical properties of SnO₂. This effective and innovative strategy could be used for the preparation of perforated metal oxide–CNT composites with complex elemental compositions for many applications

Flexible Room-Temperature NH₃ Sensor for Ultrasensitive, Selective, and Humidity-Independent Gas Detection

Ammonia (NH₃) is an irritant gas with a unique pungent odor; sub-parts per million-level breath ammonia is a medical biomarker for kidney disorders and Helicobacter pylori bacteria-induced stomach infections. The humidity varies in both ambient environment and exhaled breath, and thus humidity dependence of gas-sensing characteristics is a great obstacle for real-time applications. Herein, flexible, humidity-independent, and room-temperature ammonia sensors are fabricated by the thermal evaporation of CuBr on a polyimide substrate and subsequent coating of a nanoscale moisture-blocking CeO₂ overlayer by electron-beam evaporation. CuBr sensors coated with a 100 nm-thick CeO₂ overlayer exhibits an ultrahigh response (resistance ratio) of 68 toward 5 ppm ammonia with excellent gas selectivity, rapid response, reversibility, and humidity-independent sensing characteristics at room temperature. In addition, the sensing performance remains stable after repetitive bending and long-term operation. Moreover, the sensors exhibit significant response to the simulated exhaled breath of patients with H. pylori infection; the simulated breath contains 50 ppb NH₃. The sensors thus show promising potential in detecting sub-parts per million-level NH₃, regardless of humidity fluctuations, which can open up new applications in wearable devices for in situ medical diagnosis and indoor/outdoor environment monitoring

Performance enhancement through post-treatments of CdS-sensitized solar cells fabricated by spray pyrolysis deposition

The CdS-sensitized solar cells (CdS-SSC) were fabricated by spray pyrolysis deposition (SPD) method. The performance of the cells was greatly improved through post-treatments that included thermal oxidation at 500 °C for 30 min in an air atmosphere and subsequent chemical etching by 40 mM aqueous HCl solution at room temperature for 30 min, as compared to as-deposited CdS-SSC. The CdS-SSC in a I−/I3− electrolyte system was resulted in the improvement of Jsc (3.3 → 5.2 mA/cm2), Voc (697 → 758 mV), FF (41.4% → 46.9%), and (0.95% → 1.84%). Similarly, the efficiency of CdS-SSC in a noncorrosive polysulfide electrolyte system was also enhanced by the proposed thermal oxidation and etching process. The increase in the cell efficiency is attributed to the reduced charge recombination among sensitizer themselves through the mitigation of overaggregated CdS sensitizers deposited by SPD

Transformation of ZnO Nanobelts into Single-Crystalline Mn₃O₄ Nanowires

Single-crystalline Mn₃O₄ nanowires were prepared using the vapor-phase transformation of ZnO nanobelts. Mn₃O₄-decorated ZnO nanobelts and ZnO–ZnMn₂O₄ core–shell nanocables (NCs) were also obtained as reaction intermediates. Heteroepitaxial growth of tetragonal spinel Mn₃O₄ (or ZnMn₂O₄) on wurtzite ZnO is a possible reason for the growth of single-crystalline Mn₃O₄ nanowires. Growth interfaces are possibly formed between the wurtzite (101̅0)/(21̅1̅0) and spinel (1̅01)/(4̅11) planes. Various one-dimensional homonanostructures and heteronanostructures consisting of <i>n</i>-ZnO, <i>p</i>-Mn₃O₄, and <i>p</i>-ZnMn₂O₄ can be used to design high-performance gas sensors

Co-Doped Branched ZnO Nanowires for Ultraselective and Sensitive Detection of Xylene

Co-doped branched ZnO nanowires were prepared by multistep vapor-phase reactions for the ultraselective and sensitive detection of <i>p</i>-xylene. Highly crystalline ZnO NWs were transformed into CoO NWs by thermal evaporation of CoCl₂ powder at 700 °C. The Co-doped ZnO branches were grown subsequently by thermal evaporation of Zn metal powder at 500 °C using CoO NWs as catalyst. The response (resistance ratio) of the Co-doped branched ZnO NW network sensor to 5 ppm <i>p</i>-xylene at 400 °C was 19.55, which was significantly higher than those to 5 ppm toluene, C₂H₅OH, and other interference gases. The sensitive and selective detection of <i>p</i>-xylene, particularly distinguishing among benzene, toluene, and xylene with lower cross-responses to C₂H₅OH, can be attributed to the tuned catalytic activity of Co components, which induces preferential dissociation of <i>p</i>-xylene into more active species, as well as the increase of chemiresistive variation due to the abundant formation of Schottky barriers between the branches

Performance enhancement through post-treatments of CdS-sensitized solar cells fabricated by spray pyrolysis deposition

The CdS-sensitized solar cells (CdS-SSC) were fabricated by spray pyrolysis deposition (SPD) method. The performance of the cells was greatly improved through post-treatments that included thermal oxidation at 500 °C for 30 min in an air atmosphere and subsequent chemical etching by 40 mM aqueous HCl solution at room temperature for 30 min, as compared to as-deposited CdS-SSC. The CdS-SSC in a I−/I3− electrolyte system was resulted in the improvement of Jsc (3.3 → 5.2 mA/cm2), Voc (697 → 758 mV), FF (41.4% → 46.9%), and (0.95% → 1.84%). Similarly, the efficiency of CdS-SSC in a noncorrosive polysulfide electrolyte system was also enhanced by the proposed thermal oxidation and etching process. The increase in the cell efficiency is attributed to the reduced charge recombination among sensitizer themselves through the mitigation of overaggregated CdS sensitizers deposited by SPD

Dual Role of Multiroom-Structured Sn-Doped NiO Microspheres for Ultrasensitive and Highly Selective Detection of Xylene

Sn-doped NiO multiroom spheres with unique microreactor morphology were prepared by facile ultrasonic spray pyrolysis of a solution containing tin oxalate, nickel nitrate, and dextrin and subsequent heat treatment. The multiroom structure was formed by phase segregation between the molten metal source and liquidlike dextrin and sequent decomposition of dextrin during spray pyrolysis, which played the dual roles of enhancing gas response and selectivity. The response (resistance ratio) of the Sn-doped NiO multiroom spheres to 1 ppm <i>p</i>-xylene was as high as 65.4 at 300 °C, which was 50.3 and 9.0 times higher than those of pure NiO multiroom spheres and Sn-doped NiO dense spheres, respectively. In addition, the Sn-doped NiO multiroom sensors showed a high selectivity to xylene. The unprecedented high response that enables the sensing of sub-ppm xylene was explained by the high gas accessibility of the multiroom structures and the Sn-doping-induced change in oxygen adsorption as well as the charge carrier concentration, whereas the high xylene selectivity was attributed to the decomposition/re-forming of xylene into smaller or more active species within the unique multiroom structure of Sn-doped NiO microreactors characterized by high catalytic activities. The multiroom oxide spheres can be used as a new and generalized platform to design high-performance gas sensors

Co₃O₄–SnO₂ Hollow Heteronanostructures: Facile Control of Gas Selectivity by Compositional Tuning of Sensing Materials via Galvanic Replacement

Co₃O₄ hollow spheres prepared by ultrasonic spray pyrolysis were converted into Co₃O₄–SnO₂ core–shell hollow spheres by galvanic replacement with subsequent calcination at 450 °C for 2 h for gas sensor applications. Gas selectivity of the obtained spheres can be controlled by varying the amount of SnO₂ shells (14.6, 24.3, and 43.3 at. %) and sensor temperatures. Co₃O₄ sensors possess an ability to selectively detect ethanol at 275 °C. When the amount of SnO₂ shells was increased to 14.6 and 24.3 at. %, highly selective detection of xylene and methylbenzenes (xylene + toluene) was achieved at 275 and 300 °C, respectively. Good selectivity of Co₃O₄ hollow spheres to ethanol can be explained by a catalytic activity of Co₃O₄; whereas high selectivity of Co₃O₄–SnO₂ core–shell hollow spheres to methylbenzenes is attributed to a synergistic effect of catalytic SnO₂ and Co₃O₄ and promotion of gas sensing reactions by a pore-size control of microreactors

Humidity-Independent Oxide Semiconductor Chemiresistors Using Terbium-Doped SnO₂ Yolk–Shell Spheres for Real-Time Breath Analysis

The chemiresistive sensing characteristics of metal oxide gas sensors depend closely on ambient humidity. Herein, we report that gas sensors using Tb-doped SnO2 yolk–shell spheres can be used for reliable acetone detection, regardless of the variations in humidity. Pure SnO2 and Tb-doped SnO2 yolk–shell spheres were prepared via ultrasonic spray pyrolysis and their chemiresistive sensing characteristics were studied. The sensor resistance and gas response of the pure SnO2 yolk–shell spheres significantly changed and deteriorated upon exposure to moisture. In stark contrast, the Tb-doped SnO2 yolk–shell spheres exhibited similar gas responses and sensor resistances in both dry and humid [relative humidity (RH) 80%] atmospheres. In addition, the Tb-doped SnO2 yolk–shell sensors showed a high gas response (resistance ratio) of 1.21 to the sub-ppm-levels (50 ppb) of acetone with low responses to the other interference gases. The effects of Tb oxide and the chemical interactions among the Tb oxide, SnO2, and water vapor on this humidity-independent gas sensing behavior of the Tb-doped SnO2 yolk–shell sensors were investigated. This strategy can provide a new road to achieve highly sensitive, selective, and humidity-independent sensing of acetone, which will facilitate miniaturized and real-time exhaled breath analysis for diagnosing diabetes