Highly selective GaN-nanowire/TiO2-nanocluster hybrid sensors for detection of benzene and related environment pollutants

Nanowire-nanocluster hybrid chemical sensors were realized by functionalizing gallium nitride (GaN) nanowires (NWs) with titanium dioxide (TiO(2)) nanoclusters for selectively sensing benzene and other related aromatic compounds. Hybrid sensor devices were developed by fabricating two-terminal devices using individual GaN NWs followed by the deposition of TiO(2) nanoclusters using RF magnetron sputtering. The sensor fabrication process employed standard microfabrication techniques. X-ray diffraction and high-resolution analytical transmission electron microscopy using energy-dispersive x-ray and electron energy-loss spectroscopies confirmed the presence of the anatase phase in TiO(2) clusters after post-deposition anneal at 700?°C. A change of current was observed for these hybrid sensors when exposed to the vapors of aromatic compounds (benzene, toluene, ethylbenzene, xylene and chlorobenzene mixed with air) under UV excitation, while they had no response to non-aromatic organic compounds such as methanol, ethanol, isopropanol, chloroform, acetone and 1,3-hexadiene. The sensitivity range for the noted aromatic compounds except chlorobenzene were from 1% down to 50 parts per billion (ppb) at room temperature. By combining the enhanced catalytic properties of the TiO(2) nanoclusters with the sensitive transduction capability of the nanowires, an ultra-sensitive and selective chemical sensing architecture is demonstrated. We have proposed a mechanism that could qualitatively explain the observed sensing behavior.     

Platinum-nanoparticle-modified TiO2 nanowires with enhanced photocatalytic property

Highly crystalline Pt nanoparticles with an average diameter of 5 nm were homogeneously modified on the surfaces of TiO(2) nanowires (Pt-TiO(2) NWs) by a simple hydrothermal and chemical reduction route. Photodegradation of methylene blue (MB) in the presence of Pt-TiO(2) NWs indicates that the photocatalytic activity of TiO(2) NWs can be greatly enhanced by Pt nanoparticle modification. The physical chemistry process and photocatalytic mechanism for Pt-TiO(2) NWs hybrids degrading MB were investigated and analyzed. The Pt attached on TiO(2) nanowires induces formation of a Schottky barrier between TiO(2) and Pt naonoparticles, leading to a fast transport of photogenerated electrons to Pt particles. Furthermore, Pt incoporation on TiO(2) surface can accelerate the transfer of electrons to dissolved oxygen molecules. Besides enhancing the electron-hole separation and charge transfer to dissolved oxygen, Pt may also serve as an effective catalyst in the oxidation of MB. However, a high Pt loading value does not mean a high photocatalytic activity. Higher content loaded Pt nanoparticles can absorb more incident photons which do not contribute to the photocatalytic efficiency. The highest photocatalytic activity for the Pt-TiO(2) nanohybrids on MB can be obtained at 1 at % Pt loading.     

Growth and characterization of V-shaped IrO(2) nanowedges via metal-organic vapor deposition

We report in detail the synthesis and characterization of V-shaped IrO(2) nanowedges (NWs) with an angle of 110° between the two arms. The NWs were grown on top of rutile (R) phase TiO(2) nanorods (NRs) sitting on a sapphire (SA)(100) substrate via metal-organic chemical vapor deposition (MOCVD) by using (C(6)H(7))(C(8)H(12))Ir and titanium-tetraisopropoxide (TTIP, Ti[OCH(CH(3))(2)](4)) as the source reagents. The surface morphology, structural, and spectroscopic properties of the as-deposited nanocrystals (NCs) were characterized by field emission scanning electron microscopy (FESEM), x-ray diffraction (XRD), micro-Raman spectroscopy, transmission electron microscopy (TEM), and selected-area electron diffractometry (SAED). The FESEM images and XRD patterns indicated growth of V-shaped IrO(2)(101) NWs on top of R-TiO(2) NRs. The Raman spectrum showed the nanosize induced redshift and peak broadening of the IrO(2) and rutile phase of TiO(2) signatures with respect to that of the bulk counterparts. TEM and SAED characterizations of IrO(2) NCs showed that the nanowedges were crystalline IrO(2) with a twin plane of (101) and twin direction of [Formula: see text] at the V-junction. The probable mechanisms for the formation of well-aligned IrO(2) NWs are discussed.     

Carbon nanotube-TiO(2) hybrid films for detecting traces of O(2)

Hybrid titania films have been prepared using an adapted sol-gel method for obtaining well-dispersed hydrogen plasma-treated multiwall carbon nanotubes in either pure titania or Nb-doped titania. The drop-coating method has been used to fabricate resistive oxygen sensors based on titania or on titania and carbon nanotube hybrids. Morphology and composition studies have revealed that the dispersion of low amounts of carbon nanotubes within the titania matrix does not significantly alter its crystallization behaviour. The gas sensitivity studies performed on the different samples have shown that the hybrid layers based on titania and carbon nanotubes possess an unprecedented responsiveness towards oxygen (i.e.?more than four times higher than that shown by optimized Nb-doped TiO(2) films). Furthermore, hybrid sensors containing carbon nanotubes respond at significantly lower operating temperatures than their non-hybrid counterparts. These new hybrid sensors show a strong potential for monitoring traces of oxygen (i.e.?≤10?ppm) in a flow of CO(2), which is of interest for the beverage industry.     

Photoelectrochemical detection of pentachlorophenol with a multiple hybrid CdSe(x)Te(1-x)/TiO2 nanotube structure-based label-free immunosensor

Driven by the urgent demand of detecting trace amounts of pentachlorophenol (PCP) in contaminative water, a label-free immunosensor with ultra sensitivity and high selectivity was constructed based on a hybrid CdSe(x)Te(1-x) (0 ≤ x ≤ 1) nanocrystal (NCs)-modified TiO(2) nanotube (NT) arrays for the first time. The CdSe(x)Te(1-x) NCs were photoelectrodeposited on inner and outer space of the TiO(2) NTs, leading to high photoelectrical conversion efficiency in the visible region. PCP antibodies are covalently conjugated on the TiO(2) NTs due to the large surface area and good biocompatibility. Since the photocurrent is highly dependent on the TiO(2) surface properties, the specific interaction between PCP and the antibodies results in a sensitive change in the photocurrent, with a limit of detection (LOD) of 1 pM. High sensor-to-sensor reproducibility is achieved. The sensor was applied for the direct analysis of river water samples.     

Interface architecture determined electrocatalytic activity of Pt on vertically oriented TiO(2) nanotubes

The surface atomic structure and chemical state of Pt is consequential in a variety of surface-intensive devices. Herein we present the direct interrelationship between the growth scheme of Pt films, the resulting atomic and electronic structure of Pt species, and the consequent activity for methanol electro-oxidation in Pt/TiO(2) nanotube hybrid electrodes. X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) measurements were performed to relate the observed electrocatalytic activity to the oxidation state and the atomic structure of the deposited Pt species. The atomic structure as well as the oxidation state of the deposited Pt was found to depend on the pretreatment of the TiO(2) nanotube surfaces with electrodeposited Cu. Pt growth through Cu replacement increases Pt dispersion, and a separation of surface Pt atoms beyond a threshold distance from the TiO(2) substrate renders them metallic, rather than cationic. The increased dispersion and the metallic character of Pt results in strongly enhanced electrocatalytic activity toward methanol oxidation. This study points to a general phenomenon whereby the growth scheme and the substrate-to-surface-Pt distance dictates the chemical state of the surface Pt atoms, and thereby, the performance of Pt-based surface-intensive devices.     

Silver nanoclusters in BSA template: a selective sensor for hydrogen peroxide

Fluorescent silver nanoclusters (Ag NCs) have been bio-labeled to bovine serum albumin (BSA) protein by wet chemistry. The conformational changes of protein were studied using circular dichroism spectroscopy. The spectroscopic and chemical properties of these BSA-Ag NCs are explored to study their chemical sensing behavior. These NCs exhibit an intense fluorescence peak at 650 nm. Fluorescence behavior of Ag NCs has been explored for sensitive and selective detection of hydrogen peroxide.     

The effect of Au and Pt nanoclusters on the structural and hydrogen sensing properties of SnO2 thin films

Au and Pt nanoparticle modified SnO₂ thin films were prepared by the sol-gel method on glass substrates targeting sensing applications. Structural and morphological properties of these films were studied using X-ray Diffraction and Scanning Electron Microscopy. It was proved that the films crystallized in tetragonal rutile SnO₂ crystalline structure. Scanning Electron Microscopy observations showed that the metallic clusters' dimensions and geometry depend on the kind of the metal (Au or Pt) while SnO₂ films surface remains almost the same: nanostructured granular very smooth. Optical properties of the films were studied using UV-visible spectroscopy. The modified SnO₂ films were tested as hydrogen sensors. The response of SnO₂, SnO₂-Au and SnO₂-Pt thin films against hydrogen was investigated at different operating temperatures and for different gas concentrations. The addition of metal nanoparticles was found to decrease the detection limit and the operating temperature (from 180 °C to 85 °C), while increasing the sensing response signal.     

薄壳层核壳型Ni/Pt纳米粒子的制备及电催化性能

通过胶体-化学镀法制备不同厚度薄壳层核壳型Ni/Pt纳米粒子, 采用HRTEM、EDS、XPS和XRD手段对粒子的形貌、晶型和组成进行物理表征. 采用动电位、循环伏安法对其氧电还原和甲醇电氧化活性进行测试. 实验结果表明, 核壳结构Ni/Pt纳米颗粒基本为球形, 其中Ni₁-Pt_0.067平均直径为7 nm左右, 壳层厚度约1 nm. 与Pt/C相比, 核壳型Ni/Pt纳米粒子对氧电还原和甲醇电氧化的催化活性显著提高. 在所制备的不同壳层厚度催化剂中, Ni₁-Pt_0.067/C在0.5 mol/L H₂SO₄中氧电还原的最大峰电流密度可达到143.06 mA/mg, 是相同反应条件Pt/C峰电流密度的1.4倍; 而Ni₁-Pt_0.067/C在0.5 mol/L H₂SO₄+1.0 mol/L CH₃OH溶液中甲醇电氧化峰电流密度可达538.3 mA/mg, 是Pt/C峰电流密度的5.2倍. 若以1 mg贵金属Pt为基准, Ni₁-Pt_0.067/C的比质量活性相对Pt/C的提高了30倍.     

Pt/Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:Eu<Superscript>3+</Superscript> composite nanotubes: Enhanced photoluminescence and application in dye-sensitized solar cells

Y(OH)3:Eu3+ nanotubes were synthesized using a facile hydrothermal method,and then,Pt particles were grown on the surface of the nanotubes using a combination of vacuum extraction and annealing.The resulting Pt/Y2O3∶Eu3+ composite nanotubes not only exhibited enhanced red luminescence under 255-or 468-nm excitation but could also be used to improve the efficiency of dyesensitized solar cells,resulting in an efficiency of 8.33％,which represents a significant enhancement of 11.96％ compared with a solar cell without the composite nanotubes.Electrochemical impedance spectroscopy results indicated that the interfacial resistance of the TiO2-dye|I3-/I-electrolyte interface of the TiO2-Pt/Y2O3:Eu3+ composite cell was much smaller than that of a pure TiO2 cell.In addition,the TiO2-Pt/Y2O3:Eu3+ composite cell exhibited a shorter electron transport time and longer electron recombination time than the pure TiO2 cell.     

Response mechanism of SiC-based MOS field-effect gas sensors

SiC-based field-effect gas sensors with catalytic platinum electrodes (Pt-MOSiC) have been subjected to a series of gas response measurements. Structural analysis of the Pt electrodes revealed that the gas sensitivity and the selectivity of Pt-MOSiC gas sensors depend on two major parameters: the porosity and the catalytic activity of the Pt electrodes. Pt-MOSiC gas sensors with thick, dense Pt electrodes only exhibit hydrogen sensitivity, whereas Pt-MOSiCs with thin porous gates exhibit a broad range of gas sensitivities similar to resistive metal-oxide gas sensors. A model is put forward that explains the nonhydrogen gas response of Pt-MOSiC devices and of conventional polycrystalline metal-oxide materials on a common basis.     

A new route to size and population control of silver clusters on colloidal TiO₂ nanocrystals

Formation of hybrid Ag-TiO(2) nanocrystals (NCs) in which Ag clusters are uniformly deposited on individual TiO(2) NC surface has been achieved by using hydrophobic surfactant-capped TiO(2) NCs in combination with a photodeposition technique. The population of Ag clusters on the individual TiO(2) NC surface can be controlled by the degree of hydrophobicity (e.g., the number of vacant sites) on the TiO(2) NC surface while their size may be altered simply by varying irradiation time. A reversible change in color of the resulting hybrid Ag-TiO(2) NCs is induced by alternating UV light and visible-light illumination; however, the size and population of Ag clusters on TiO(2) NCs are almost unchanged. Furthermore, these materials also exhibit much higher photocatalytic performance as compared to that of Ag supported on commercial TiO(2)-P25.     

The study of Pt Schottky contact on porous GaN for hydrogen sensing

This article reports the studies of Pt Schottky contact on porous n-type GaN for hydrogen sensing. A simpler and improved electroless etching method has been developed to generate porous GaN in which high uniformity of the porous area could be achieved. Hydrogen sensor was subsequently fabricated by depositing Pt Schottky contacts onto the porous GaN sample. For comparative study, a standard hydrogen sensor was also prepared by depositing Pt Schottky contacts on the as-grown sample. Hydrogen detection was carried out at room temperature and 100 °C. This Pt/porous GaN sensor exhibited a significant change of current upon exposure to 2% H₂ in N₂ gas as compared to the standard Pt/GaN sensor. Morphological studies by scanning electron microscopy revealed that Pt contact deposited on porous GaN having a very rough surface morphology with pores distributed all over the contact layer. Therefore, the increase of current could be attributed to the unique microstructure at porous Pt/porous GaN interface which allowed higher accumulation of hydrogen and eventually led to stronger effect of the H-induced dipole layer.     

Growth of titanium dioxide films by cluster supersonic beams for VOC sensing applications

We developed and tested gas sensing devices based on TiO/sub 2/ nano-crystalline films produced at room temperature by the novel growth method of cluster beam deposition. The devices show a very good response to ethanol, methanol, and propanol and an overall performance comparable or better to the best devices reported in literature, based on films grown with other techniques. The major advantage of our growth method is that there is no need of the thermal annealing or doping processes usually required to improve sensitivity and reliability of gas sensing devices based on nanostructured thin film. The sensors dynamic response gives a maximum sensitivity for ethanol at about 250/spl deg/C. As the morphological (AFM) and structural (XRD) characterizations of the films show, the high performance of our sensors could only be achieved because their nano-crystalline structure was well controlled by the properties of the cluster precursors in the supersonic beam. We envisage possible further developments in terms of sensitivity and selectivity of gas sensing devices based on films grown by cluster beam deposition.     

AlGaN/GaN-based diodes and gateless HEMTs for gas and chemical sensing

The characteristics of Pt/GaN Schottky diodes and Sc/sub 2/O/sub 3//AlGaN/GaN metal-oxide semiconductor (MOS) diodes as hydrogen and ethylene gas sensors and of gateless AlGaN/GaN high-electron mobility transistors (HEMTs) as polar liquid sensors are reported. At 25/spl deg/C, a change in forward current of /spl sim/6 mA at a bias of 2 V was obtained in the MOS diodes in response to a change in ambient from pure N/sub 2/ to 10% H/sub 2// 90% N/sub 2/. This is approximately double the change in forward current obtained in Pt/GaN Schottky diodes measured under the same conditions. The mechanism appears to be formation of a dipole layer at the oxide/GaN interface that screens some of the piezo-induced channel charge. The MOS-diode response time is limited by the mass transport of gas into the test chamber and not by the diffusion of atomic hydrogen through the metal/oxide stack, even at 25/spl deg/C. Gateless AlGaN/GaN HEMT structures exhibit large changes in source-drain current upon exposing the gate region to various polar liquids, including block co-polymer solutions. The polar nature of some of these polymer chains lead to a change of surface charges in gate region on the HEMT, producing a change in surface potential at the semiconductor/liquid interface. The nitride sensors appear to be promising for a wide range of chemicals, combustion gases and liquids.     

GaN-UV photodetector integrated with asymmetric metal semiconductor metal structure for enhanced responsivity

Fabrication of very thin GaN ultraviolet photodetectors on Si (111) substrate integrated with asymmetric (Pt–Ag, Pt–Cr) metal–semiconductor–metal (MSM) structure have been illustrated. Designed GaN photodetection device displays significant enhancement in responsivity for asymmetric (Pt–Ag) MSM structure (280 mA/W) in comparison to symmetric (Pt–Pt) MSM structure (126 mA/W) at 10 V bias. The fabricated asymmetric and symmetric devices also exhibit fast response time in the range of 30–59 ms. The enhancement in responsivity using asymmetric MSM structure ascribed to large difference in work function which lead to change in Schottky barrier height of the metal semiconductor junction. Additionally, power dependent photoresponse analysis of GaN asymmetric (Pt–Ag) ultraviolet photodetector was showing a responsivity of 116 mA/W at low optical power of 1 mW. Such GaN asymmetric MSM ultraviolet photodetectors having high responsivity can extensively be used for low power, high speed ultraviolet photo detection applications.     

(Ba(x)La(1-x)(2))In(2)O(5+x) (0.4 < or = x < or = 0.6) electrolyte-supported mixed-potential CO sensors

The dense (Ba(x)La(1-x)(2))In(2)O(5+x) electrolytes with different compositions (x = 0.4, 0.5, 0.6) were synthesized by Pechini method. The obtained sintered (Ba(x)La(1-x)(2))In(2)O(5+x) electrolytes showed a high relative density of approximately 98%, and the major phase of three electrolyte compositions was indexed as a cubic phase. The CO sensing properties of as-fabricated planar-type (Ba(x)La(1-x)(2))In(2)O(5+x)-based sensors coupled with ITO and Pt as the sensing electrode and reference electrode, respectively, were investigated. The effects of factors such as gas flow rate, chemical compositions, and density of the electrolytes on the sensing performance were investigated. The sensors showed good sensitivity to different concentrations of CO from approximately 100 to approximately 500 ppm and excellent selectivity over low concentrations of methane (<500 ppm). Linear relationships between emf of the sensors and CO gas concentrations from approximately 100 to approximately 400 ppm were observed. However, the sensors indicated more sluggish response compared with the sensors coupled with a corresponding porous electrolyte. The probable reason has been discussed. The long-term stability of the sensor for the detection of CO was also investigated, which indicated a reasonably stable sensor signal after an initial decline during the incubation period.     

Pt/TiO2-NTs复合材料的制备及其甲醇电氧化机理研究

采用水热法合成锐钛矿型TiO2纳米管(TiO2-NTs),并以其为载体制备了Pt/TiO2-NTs复合材料。用TEM、XRD对复合材料的形貌和结构进行了表征,TEM测试表明Pt纳米粒子以簇状形式均匀地分散在TiO2纳米管表面。运用循环伏安法研究了Pt/TiO2-NTs复合材料在不同条件下对硫酸中甲醇的电催化活性,并讨论了甲醇的电氧化机理。结果表明,Pt/TiO2-NTs复合材料具有出色的电催化活性。因此,TiO2-NTs被认为是非常有潜力的燃料电池贵金属催化剂载体材料。     

Full surface embedding of gold clusters on silicon nanowires for efficient capture and photothermal therapy of circulating tumor cells

We report on rapid thermal chemical vapor deposition growth of silicon nanowires (Si NWs) that contain a high density of gold nanoclusters (Au NCs) with a uniform coverage over the entire length of the nanowire sidewalls. The Au NC-coated Si NWs with an antibody-coated surface obtain the unique capability to capture breast cancer cells at twice the highest efficiency currently achievable (~88% at 40 min cell incubation time) from a nanostructured substrate. We also found that irradiation of breast cancer cells captured on Au NC-coated Si NWs with a near-infrared light resulted in a high mortality rate of these cancer cells, raising a fine prospect for simultaneous capture and plasmonic photothermal therapy for circulating tumor cells.     

Controlled dielectrophoretic nanowire self-assembly using atomic layer deposition and suspended microfabricated electrodes

Effects of design and materials on the dielectrophoretic self-assembly of individual gallium nitride nanowires (GaN NWs) onto microfabricated electrodes have been experimentally investigated. The use of TiO(2) surface coating generated by atomic layer deposition (ALD) improves dielectrophoretic assembly yield of individual GaN nanowires on microfabricated structures by as much as 67%. With a titanium dioxide coating, individual nanowires were placed across suspended electrode pairs in 46% of tests (147 out of 320 total), versus 28% of tests (88 out of 320 total tests) that used uncoated GaN NWs. An additional result from these tests was that suspending the electrodes 2.75 μm above the substrate corresponded with up to 15.8% improvement in overall assembly yield over that of electrodes fabricated directly on the substrate.