Recovery properties of hydrogen gas sensor with Pd/titanate and Pt/titanate nanotubes photo-catalyst by UV radiation from catalytic poisoning of H2S

Recovery properties after H₂S catalytic poisoning of catalytic-type gas sensor with photo-catalysts and UV radiation have been examined. Each sensing material of the sensor consists of Pd, Pt supported on γ-Al₂O₃ and Pd/titanate, Pt/titanate nanotubes or TiO₂ particles. Pd/titanate and Pt/titanate nanotubes photo-catalyst were synthesized by hydrothermal synthesis method. All the sensors were deactivated after 500 ppm H₂S exposure for 20 h. The sensors with Pd/titanate or Pt/titanate nanotubes showed regenerated voltage response under UV radiation. However the sensor with TiO₂ particles showed negligible regenerated voltage response. Regenerated voltage response with Pd/titanate or Pt/titanate nanotubes may stem from location of Pd or Pt catalyst on the titanate nanotube photo-catalyst.     

Effect of multi-walled carbon nanotubes loaded with Ag nanoparticles on the photocatalytic degradation of rhodamine B under visible light irradiation

Multi-walled carbon nanotubes loaded with Ag nanoparticles (Ag/MWNTs) were prepared by two methods (direct photoreduction and thermal decomposition). The photocatalytic activity of Ag/MWNTs for the degradation of rhodamine B (RhB) under visible light irradiation was investigated in detail. The adsorption and photocatalytic activity tests indicated that the MWNTs served as both an adsorbent and a visible light photocatalyst. The photocatalytic activity of MWNTs was remarkably enhanced when the Ag nanoparticles were loaded on the surface of MWNTs. Moreover, the visible light photocatalytic activity of Ag/MWNTs depended on the synthetic route. On the basis of the experimental results, a possible visible light photocatalytic degradation mechanism was discussed.     

Ag/TiO2纳米材料的制备和光致变色性能研究

 采用光催化还原法在不同温度热处理的TiO2薄膜表面沉积Ag纳米颗粒,制备了Ag/TiO2纳米薄膜材料。通过UV-Vis吸收光谱表征对比了不同温度热处理的TiO2对Ag粒子光催化沉积的影响,发现500℃退火处理TiO2薄膜较利于Ag纳米粒子的光催化沉积;在650 nm红色激光照射下,500℃退火处理的Ag/TiO2样品具有明显的光致变色现象,对此变色过程中涉及的机理进行了讨论,且发现随着Ag纳米颗粒光催化沉积时间的增长,Ag/TiO2薄膜光致变色的响应速率提高,但Ag纳米颗粒过多会抑制Ag/TiO2薄膜的变色响应速率。     

Photoinduced transformations in a thin-film waveguide AgCl-Ag composition under a green (532 nm) laser beam

A linearly polarized (E ₀) laser beam (λ = 532 nm) causes photoinduced transformations in an AgCl-Ag composition consisting of a thin waveguide AgCl film on glass covered by a layer of Ag nanoparticles. Before the illumination the sample exhibits an absorption band due to localized plasmons in nanoparticles. The illumination excites plasmons and leads to scattering of waveguide TE₀ modes. The interference of modes with the incident light beam leads to the development of a periodic structure, the lines of which are formed by Ag particles and directed along E ₀. The measured structure period coincides with the result of calculation based on solving the dispersion equation for the TE₀-mode. Measurement of absorption in the E ‖ E ₀ polarization reveals dichroism and a spectral hole (at λ ≈ nm). It is shown that the structures formed remain on the substrate after removing AgCl in a fixing agent. The dichroism value and dispersion change after fixing. The character of dichroism prior to fixing is recovered after depositing an AgCl layer (with the parameters retained) on the fixed film.     

Effects of NiTiO3 nanoparticles supported by mesoporous MCM-41 on photoreduction of methylene blue under UV and visible light irradiation

In this work, we report the synthesis of nickel titanate nanoparticles loaded on nanomesoporous MCM-41 nanoparticles to determine the effect of MCM-41 nanoparticles on the photocatalytic activities of nickel titanate (NiTiO₃) nanoparticles by using simple solid-state dispersion (SSD) method. Fourier transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and UV–Vis diffuse reflectance spectra (DRS) analysis were used to characterize the size and morphology of the obtained nanocomposite. The photocatalytic activity (PA) of the as-prepared NiTiO₃ loaded on MCM-41 was evaluated by degradation of the methylene blue under irradiation of UV and visible light. The results showed that NiTiO₃ loaded on nanosize MCM-41 has higher photocatalytic activity than that of NiTiO₃ nanoparticles.     

Structural evolution of sol-gel SiO2 heated glasses containing silver particles

SiO₂ based glasses added with nanometric-silver particles have been prepared by the traditional sol-gel process, using the tetraethyl-orthosilicate alkoxide. The Ag particles were prepared using a new method described in this article, the method uses a cementation reaction between Ag ions and an iron electrode. The size of the particles, measured in the dried glass, was in the range of 100-200 nm. The observed structural changes depend on the annealing conditions, such as the annealing temperature, the amount of silver particles and the type of acid (HCl or HNO₃) used to catalyze the hydrolysis/condensation reactions during the sol-gel process. Samples prepared using both acids crystallized into the cristobalite phase after thermal annealing at 800 °C. The amount of SiO₂ crystallized depends on the amount of Ag present in the glass. Samples prepared from solutions catalyzed with HCl acid show the formation of nanometric Ag particles after thermal annealing at 500 °C, these small particles are not observed after similar treatments when HNO₃ is used as the catalytic acid. HCl and Ag₂O form AgCl which is reduced by residual carbon to form Ag ultrafine particles. The diffusion of the reduced Ag spices, which form these particles, is facilitated by the opened structure of the glasses added with Ag, as indicated by IR measurements.     

Construction of Ag/AgCl nanostructures from Ag nanoparticles as high-performance visible-light photocatalysts

A combined strategy of in situ oxidation and assembly is developed to prepare Ag/AgCl nanospheres and nanocubes from Ag nanoparticles under room temperature. It is a new facile way to fabricate Ag/AgCl with small sizes and defined morphologies. Ag/AgCl nanospheres with an average size of 80 nm were achieved without any surfactants, while Ag/AgCl nanocubes with a mean edge length of 150 nm were obtained by introduction of N-dodecyl-N,N-dimethyl-2-ammonio-acetate. The possible formation mechanism involves the self-assembly of AgCl nanoparticles, Ostwald ripening and photoreduction of Ag⁺ into Ag⁰ by the room light. The as-prepared Ag/AgCl nanospheres and nanocubes exhibit excellent photocatalytic activity and stability toward degradation of organic pollutants under visible-light irradiation. It is demonstrated that Ag/AgCl nanocubes display enhanced photocatalytic activity in comparison with Ag/AgCl nanospheres due to the more efficient charge transfer. This work may pave an avenue to construct various functional materials via the assembly strategy using nanoparticles as versatile building blocks.

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Graphical abstract A combined strategy of in situ oxidation and assembly was developed to construct Ag/AgCl nanospheres and nanocubes from Ag nanoparticles, which exhibited highly photocatalytic activity and good stability for degrading methyl orange under visible light irradiation.

     

Photoresponsive conductance switching of multi-walled carbon nanotubes bearing covalently linked spironaphthoxazine

Multi-walled carbon nanotubes functionalized with a photochromic spironaphthoxazines (MWCNTs-SPO) were synthesized by the reaction between the hydroxyl group of SPO and the carbonyl chloride groups on the surface of MWCNTs. The functional spirooxazine photochromic groups on the surface of MWCNTs were identified by the X-ray photoelectron spectroscopy. As a novel derivative of MWCNTs, photoresponsive conductance switching of the MWCNTs-SPO was demonstrated under a 365 nm UV light irradiation. A simple photoinduced resistance measurement following the cyclic irradiation of UV light shows a very reversible response. The effect of the electron injection in the SPO is rationalized in terms of the HOMO–LUMO energy levels of both spiro and merocyanine forms.     

Ag负载WO3/TiO2光催化剂的合成及其催化性能

以钛酸丁酯、无水乙醇、钨酸铵为原料,采用溶胶-凝胶法合成了WO3/TiO2复合光催化剂;采用光还原技术制备了Ag负载WO3/TiO2光催化剂,借助X射线粉末衍射(XRD)和UV-Vis光谱等技术对样品的组成和光吸收性能进行了表征,并以罗丹明B为模型污染物考察样品的光催化活性。XRD分析表明,所得粉体均为锐钛矿型纳米TiO2,且与WO3复合后,纳米TiO2特征衍射峰宽化,强度降低;UV-Vis光谱分析表明,载银使得催化剂在400—700nm的可见光区域对光响应,且在紫外光区吸收显著增强,对光具有更高的利用率;以罗丹明B为降解物的光催化实验表明,WO3复合对纳米TiO2光催化活性有显著的影响,而载Ag后其光催化活性进一步提高,将该光催化剂用于炼油厂废水的处理,效果较好。     

Ag/TiO2 sol prepared by a sol–gel method and its photocatalytic activity

Ag/TiO₂ sol with narrow particle size distribution was synthesized using TiCl₄ as the starting material. TiCl₄ was converted to Ti(OH)₄ gel. The Ag/TiO₂ sol was prepared by a process where H₂O₂ was added and then heated at 90–97 °C. After condensation reaction and crystallization, a transparent sol with suspended Ag/TiO₂ was formed. Ag/TiO₂ was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, contact angle analysis, and X-ray photoelectron spectroscopy. The photocatalytic properties of Ag/TiO₂ film were evaluated by degradation of methylene blue in aqueous solution under UV light irradiation. The suspended Ag/TiO₂ particles were rhombus primary particles with the major axis ca. 40 nm and the minor axis ca. 10 nm. Ag nanoparticles were well dispersed on TiO₂ and the particle size was only 1–2 nm. Ag could restrain the recombination of photo-generated electrons and holes effectively. Transparent thin films could be obtained through dip-coating glass substrate in the sol. The thin film had strong hydrophilicity after being illuminated by UV light. Ag/TiO₂ film showed a significant increase in photocatalytic activity compared to the TiO₂ film. The high amount of surface hydroxyls on Ag/TiO₂ film also played an important role in its photocatalytic activity.     

Unusual photo-induced adsorption-desorption behavior of propylene on Ag/TiO2 nanotube under visible light irradiation

Titanium dioxide (TiO₂) nanotube with a large amount of single-electron-trapped-oxygen-vacancies (coded as T2) was obtained by annealing nanotube H₂Ti₂O₄(OH)₂ (coded as T1) at 400 °C in air. Silver nanoparticles with a diameter of about 30-50 nm were loaded onto the surface of T2 via deposition associated with photochemical reduction under ultraviolet irradiation. The resulting Ag/TiO₂ nanotube (coded as T3) was characterized by means of transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and ultraviolet-visible light diffusion reflectance spectrometry. It was found that C₃H₆ experienced unusual photo-induced adsorption-desorption on T3 under visible light irradiation. Namely, C₃H₆ was initially desorbed from T3 and then adsorbed on T3 under visible light irradiation. On the contrary, C₃H₆ was initially adsorbed on T3 in the dark, followed by desorption. The reason might lie in that two kinds of active sites exist on the surface of T3, corresponding to quite different rates of adsorption and desorption. It was found that oxygen vacancies in association with deposited silver particles, were responsible for the alternative adsorption-desorption of C₃H₆ on T3.     

Thermo- and photostimulated processes of polarization and depolarization in CdI<Subscript>2</Subscript>: Ag

Currents of thermogradient polarization and depolarization of an electret state that arises in a photochromic crystal CdI₂: Ag during one-side cooling of the sample in the dark from 325 to 90 K in the presence of a temperature gradient directed along the crystallographic axis C ₆ have been found. At 90 K, the crystal polarized in the thermogradient electromotive force field is characterized by the photosensitivity in the near-edge, impurity, and infrared spectral regions. It is revealed that the electret state in the CdI₂: Ag crystal is also formed at room temperature during photolysis under irradiation of the samples by integrated light from a xenon lamp. Models of photosensitive centers formed upon doping of the CdI₂ crystal from the melt by the Ag⁺ impurity and during the occurrence of thermo- and photostimulated chemical reactions are proposed. The mechanism of the photochromic effect, including the change in the charge state of silver impurity ions, is considered.     

Microwave-assisted synthesis of silver nanocrystals in benzyl alcohol and their subsequent in situ chemical transformation into Ag–AgCl nanohybrids for plasmonic photocatalysis

In heterostructured metal–semiconductor system, plasmonic metal nanostructures can cooperate with semiconductors to enhance the solar light harness and energy conversion. In this study, we report on the plasmonically photocatalytic system constructed by in situ growth of semiconductor on metal nanostructures with high performance and stability. A facile and rapid microwave-assisted route was first explored to synthesize Ag nanocrystals, and subsequently converting them to Ag–AgCl nanohybrids was realized by in situ chemical transformation strategy at room temperature. These Ag–AgCl nanohybrids were characterized by X-ray diffraction, scanning electronic microscopy, and UV–vis absorption spectroscopy. The resulting Ag–AgCl nanohybrids showed remarkable photocatalytic activity and excellent durability for the degradation of organic pollutants under visible light irradiation. This finding provides a new way to improve photocatalytic efficiency through controllable chemical transformation.     

Photoelectrochemical properties of transparent multilayer films via electrostatic alternating assembly of titanate nanotubes and methylpyridinium porphyrin cobalt

Transparent multilayer films of titanate nanotubes (TNTs) and meso-tetrakis (N-methylpyridinium-4-yl) porphyrin cobalt (CoTMPyP) (TNTs/CoTMPyP)_n were fabricated by the layer-by-layer self-assembly based on TNTs and CoTMPyP in the aqueous solution. The CoTMPyP molecules as an organic “spacer” and a sensitizer were confirmed to play important roles in the performance improvement of TNTs by comparing with the TNTs multilayer films fabricated with poly(diallyldimethylammonium chloride) (PDDA) as the interlayer. The optical absorption and photocurrent output of the resulting (TNTs/CoTMPyP)_n films increased consecutively with the number of the TNTs/CoTMPyP bilayer. The optical and thermal stabilities of the (TNTs/CoTMPyP)₁₂ films were evaluated under UV light irradiation and heat treatment, respectively. The results showed that the UV light irradiation and heat treatment had obvious influences on the photoelectrochemical performance of the resulting films.     

Growth of one-dimensional Ag/Si/SiOx capsule nanostructures by self-assembled SiOx template

One-dimensional Ag/Si/SiO_x capsule nanostructures have been synthesized by thermal evaporation of the mixture of SiO and Ag₂O. Products were analyzed by using SEM, TEM, HREM and element map. Two kinds of morphologies were observed. Inside the amorphous SiO_x shell, Ag nanowires interspersed by short segments of Si were formed when Ag content was higher than Si. Ag and Si contacted well and nanosize MS (metal-semiconductor) structures were obtained. One-dimensional periodic nanostructures that Ag particles embedded in the nanowire were synthesized when Si content was higher than Ag. SiO_x nanotubes were also observed. Structure analysis shows that Ag/Si/SiO_x nanostructures are grown by a self-assembled SiO_x template mechanism. And the growth of SiO_x nanotubes is tightly related to the adding of Ag. PACS 81.07.Bc; 81.10.Bk; 61.14.-x     

Preparation, characterization of Au (or Pt)-loaded titania nanotubes and their photocatalytic activities for degradation of methyl orange

TiO₂ nanotubes were prepared by hydrothermal method and Au (or Pt) was loaded on TiO₂ nanotubes by photodeposition method. The photocatalysts were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and N₂ adsorption technique, respectively. The photocatalytic properties of the samples were also investigated. The results show that TiO₂ nanotubes with uniform diameter were prepared, and they have specific surface areas over 400 m²/g. The specific surface areas of TiO₂ nanotubes decrease with the increasing of calcining temperature, and crystalline phase of TiO₂ in the wall of nanotubes was transformed from anatase into rutile phase in calcination process. The photocatalytic activities of TiO₂ nanotubes are higher than that of nanosized TiO₂, and the photocatalytic activities of TiO₂ nanotubes were enhanced after loading Au (or Pt). After irradiation for 40 min under a 300 W of middle-pressure mercury lamp (MPML), the degradation rate of methyl orange solution using the Au/TiNT-500 (or Pt/TiNT-500) as a catalyst can reach 96.1% (or 95.1%). On the other hand, Au-loaded sample has evident adsorption peak in visible range, indicating that Au-loaded TiO₂ nanotubes are hopeful to become visible light photocatalyst.     

The synthesis and kinetic growth of anisotropic silver particles loaded on TiO2 surface by photoelectrochemical reduction method

Silver nanorods with average diameters of 120-230 nm and aspect ratio of 1.7-5.0 were deposited on the surface of TiO₂ films by photoelectrochemical reduction of Ag⁺ to Ag under UV light. The composite films prepared on soda-lime glass substrates were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results show that the TiO₂ film after UV irradiation in AgNO₃ solution is composed of anatase phase TiO₂ and metallic silver with face centered cubic structure. Other compounds cannot be found in the final films. The maximum deposition content of silver particles on the surface of TiO₂ film was obtained with the AgNO₃ concentration of 0.1 M. The kinetic growth rates of silver particles can be controlled by photocatalytic activity of TiO₂ films. The studies suggest that the growth rates of silver particles increase with the enhancement of photocatalytic activity of TiO₂ films. The maximum growth rate of silver particles loaded on TiO₂ films can be up to 0.353 nm min⁻¹ among samples 1#, 2# and 3#, while the corresponding apparent rate constant of TiO₂ is 1.751 × 10⁻³ min⁻¹.     

Synthesis of Cr2O3/TNTs nanocomposite and its photocatalytic hydrogen generation under visible light irradiation

A novel Cr₂O₃/TNTs nanocomposite was prepared by loaded suitable amount of amorphous Cr₂O₃ on titanate nanotubes (TNTs) via hydrothermal reaction and impregnation process. XRD, SEM and TEM results demonstrated that the amorphous Cr₂O₃ nanoparticles were homogeneously dispersed on the surface of TNTs. The diffuse reflectance UV–visible absorption spectra exhibited that the spectral response of TNTs was extended to visible light region by coupled with Cr₂O₃. The 2.5Cr₂O₃/TNTs nanocomposite showed the highest activity of hydrogen generation by photocatalytic water-splitting under visible light irradiation (λ > 400 nm). The high activity of H₂ evolution for Cr₂O₃/TNTs nanocomposites was associated with the donor level in the forbidden band of TNTs semiconductor provided by dopant Cr³⁺ and a probably photocatalytic mechanism was proposed.     

Study of the growth of CeO2 nanoparticles onto titanate nanotubes

We report the study of the growth of CeO₂ nanoparticles on the external walls and Ce⁴⁺ intercalation within the titanate nanotubes. The materials were fully characterized by multiple techniques, such as: Raman spectroscopy, infrared spectroscopy (FTIR), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The ion exchange processes in the titanate nanotubes were carried out using different concentrations of Ce⁴⁺ in aqueous solution. Our results indicate that the growth of CeO₂ nanoparticles grown mediated by the hydrolysis in the colloidal species of Ce and the attachment onto the titanate nanotubes happened and get it strongly anchored to the titanate nanotube surface by a simple electrostatic interaction between the nanoparticles and titanate nanotubes, which can explain the small size and even distribution of nanoparticles on titanate supports. It was demonstrated that it is possible to control the amount and size of CeO₂ nanoparticles onto the nanotube surface, the species of the Ce ions intercalated between the layers of titanate nanotubes, and the materials could be tuned for using in specific catalysis in according with the amount of CeO₂ nanoparticles, their oxygen vacancies/defects and the types of Ce species (Ce⁴⁺ or Ce³⁺) present into the nanotubes.     

Implantation of periodic structures formed by silver particles into quartz glass

Quartz glass samples with a thin photosensitive AgCl-Ag film, prepared by successive evaporation of AgCl (h _AgCl ≈ 35 nm) and Ag (h _Ag ≈ 8 nm) in vacuum have been investigated. A periodic structure with a period of d = 375 nm formed by Ag particles was obtained in films using a p-polarized laser beam (λ = 532 nm, P = 25 mW) at an angle of incidence of ϕ = 20°. After removing AgCl in a fixing agent, the periodic structure remained on the glass surface. Subsequent irradiation by a CO₂ laser (λ = 10.6 μm, P ≈ 20 W) led to the implantation of this structure into the glass with the conservation of its period d and partial conservation of the related dichroism. The fact of implantation is confirmed by the high mechanical and chemical stability of the structure obtained. A possible implantation mechanism, taking into account the thermionic emission, Ag ionic transport, and the presence of free voids in the quartz glass and defects in its structural network, is discussed.