Synthesis of stable dispersion of ZnO quantum dots in aqueous medium showing visible emission from bluish green to yellow

Aqueous dispersion of 4-8 nm size stable ZnO quantum dots (QDs) exhibiting luminescence in the visible region have been synthesized by a simple solution growth technique at room temperature. Silica has been used as capping agent to control the particle size as well as to achieve uniform dispersion of QDs in aqueous medium. X-ray diffractometer (XRD) analysis reveals formation phase pure ZnO particles having wurzite (hexagonal) structure. Atomic force microscope (AFM) images show that the particles are spherical in shape, having average crystalline sizes ∼4, 5.5 and 8 nm for samples prepared at pH values of 10, 12 and 14, respectively. From the optical absorption studies, the band gap energy of QDs is found to be blue shifted as compared to bulk ZnO (3.36 eV) due to the quantum confinement effect and is consistent with the band gap calculated by using effective-mass approximation model. The photoluminescence (PL) observed in these QDs has been attributed to the presence of defect centers.     

Electronic state of ruthenium deposited onto oxide supports: An XPS study taking into account the final state effects

The electronic state of ruthenium in the supported Ru/EO_x (EO_x = MgO, Al₂O₃ or SiO₂) catalysts prepared by with the use of Ru(OH)Cl₃ or Ru(acac)₃ (acac = acetylacetonate) and reduced with H₂ at 723 K is characterized by X-ray photoelectron spectroscopy (XPS) in the Ru 3d, Cl 2p and O 1s regions. The influence of the final state effects (the differential charging and variation of the relaxation energy) on the binding energy (BE) of Ru 3d_5/2 core level measured for supported Ru nanoparticles is estimated by comparison of the Fermi levels and the modified Auger parameters determined for the Ru/EO_x samples with the corresponding characteristics of the bulk Ru metal. It is found that the negative shift of the Ru 3d_5/2 peak which is observed in the spectrum of ruthenium deposited onto MgO (BE = 279.5-279.7 eV) with respect to that of Ru black (BE = 280.2 eV) or ruthenium supported on γ-Al₂O₃ and SiO₂ (BE = 280.4 eV) is caused not by the transfer of electron density from basic sites of MgO, as considered earlier, but by the differential charging of the supported Ru particles compared with the support surface. Correction for the differential charging value reveals that the initial state energies of ruthenium in the Ru/EO_x systems are almost identical (BE = 280.5 ± 0.1 eV) irrespectively of acid-base properties of the support, the mean size of supported Ru crystallites (within the range of 2-10 nm) and the surface Cl content. The results obtained suggest that the difference in ammonia synthesis activity between the Ru catalysts supported on MgO and on the acidic supports is accounted for by not different electronic state of ruthenium on the surface of these oxides but by some other reasons.     

Effect of N2 ion flux on the photocatalysis of nitrogen-doped titanium oxide films by electron-beam evaporation

Nitrogen-doped titanium oxide (TiO_xN_y) films were prepared with ion-assisted electron-beam evaporation. The nitrogen (N) incorporated in the film is influenced by the N₂ flux modulated by the N₂ flow rate through an ion gun. The TiO_xN_y films have the absorption edge of TiO₂ red-shifted to 500 nm and exhibit visible light-induced photocatalytic properties in the surface hydrophilicity and the degradation of methylene blue. The structures and states of nitrogen in the films are investigated by X-ray diffraction patterns (XRD), and X-ray photoelectron spectroscopy (XPS) and related to their visible light-induced photocatalytic properties. The results indicate that the substitutional N in anatase TiO₂ can induce visible light photocatalysis. The substitutional N is readily doped by the energetic nitrogen ions from the ion gun. The best photocatalytic activity is obtained at the largest N loading about 5.6 at.%, corresponding to the most substitutional N in anatase TiO₂. The film exhibits the degradation of methylene blue with a rate-constant (k) about 0.065 h⁻¹ and retaining 7° water contact angle on the surface under visible light illumination.     

Photocatalytic thin films containing TiO2:N nanopowders obtained by the layer-by-layer self-assembling method

In this work, TiO₂-N powders were synthesized by high-energy ball milling, using commercial titanium dioxide (TiO₂) in the anatase phase and urea to introduce nitrogen into TiO₂ in order to enhance their photocatalytic properties in the visible spectral region. Several samples were prepared by milling a mixture of TiO₂-urea during 2, 4, 8, 12 and 24 h and characterized by spectroscopic and analytical techniques. X-ray diffraction (XRD) results showed the coexistence of anatase and high-pressure srilankite TiO₂ crystalline phases in the samples. Scanning electron microscopy (SEM) revealed that the grain size of the powder samples decreases to 200 nm at 24 h milling time. UV-Vis diffuse reflectance spectroscopic data showed a clear red-shift in the onset of light absorption from 387 to 469 nm as consequence of nitrogen doping in the samples. The photocatalytic activity of the TiO₂-N samples was evaluated by methylene blue degradation under visible light irradiation. It was found that TiO₂-N samples had higher photocatalytic activity than undoped TiO₂ samples, which could be assigned to the effect of introducing N atoms and XPS results confirm it. Using polyethylenimine (PEI), transparent thin films of TiO₂-N nanoparticles were prepared by layer-by-layer self assembly method. UV-visible spectrophotometry was employed in a quantitative manner to monitor the adsorbed mass of TiO₂ and PEI after each dip cycle. The adsorption of both TiO₂ and PEI showed a saturation dip time of 15 min.     

Preparation and characterization of TiO2 photocatalysts co-doped with iron (III) and lanthanum for the degradation of organic pollutants

Titanium dioxide photocatalysts co-doped with iron (III) and lanthanum were prepared by a facile sol-gel method. The structure of catalysts was characterized by X-ray diffraction (XRD), Raman spectroscopy, UV-vis diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy (XPS). The photocatalytic activities of the samples were evaluated by the degradation of methylene blue in aqueous solutions under visible light (λ > 420 nm) and UV light irradiation. Doping with Fe³⁺ results in a lower anatase to rutile (A-R) phase transformation temperature for TiO₂ particles, while doping with La³⁺ inhibits the A-R phase transformation, and co-doping samples indicate that Fe³⁺ partly counteracts the effect of La³⁺ on the A-R transformation property of TiO₂. Fe-TiO₂ has a long tail extending up the absorption edges to 600 nm, whereas La-TiO₂ results in a red shift of the absorption. However, Fe and La have synergistic effect in the absorption of TiO₂. Compared with Fe³⁺ and La³⁺ singly doped TiO₂, the co-doped simple exhibits excellent visible light and UV light activity and the synergistic effect of Fe³⁺ and La³⁺ is responsible for improving the photocatalytic activity.     

Pillaring and photocatalytic properties of mesoporous α-Fe2O3/titanate nanocomposites via an exfoliation and restacking route

Mesoporous α-Fe₂O₃-pillared titanate nanocomposites have been successfully synthesized through an exfoliation−restacking route. Powder X-ray diffraction and N₂ adsorption-desorption isotherms revealed that the α-Fe₂O₃ pillared titanate has an interlayer distance of 3.27 nm, a specific surface area of 66 m²/g and an average pore size of 7.6 nm, suggesting the formation of a mesoporous pillared structure. UV-vis diffuse reflectance spectra show a red shift, indicative of a narrow band gap energy of ∼2.1 eV compared to the host layered titanate, which is essential in creating a visible light photocatalytic activity. The results of degradation of rhodamine B reveal that the present pillared mesoporous composites exhibit better photocatalytic activities than those of the pristine materials under visible irradiation, based on the band gap excitement and the dye-sensitized path, originated from their high surface area, mesoporosity and the electronic coupling between the host and the guest components.     

Preparation and characterization of Cobalt Sulfophthalocyanine/TiO2/fly-ash cenospheres photocatalyst and study on degradation activity under visible light

Cobalt Sulfophthalocyanine (CoSPc) sensitized TiO₂ sol samples were prepared through a Sol-Gel method using Cobalt Sulfophthalocyanine as a sensitizer. Loading and modified floating photocatalyst was prepared by hydrothermal method using fly-ash cenospheres as a carrier. The properties of the samples were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and UV-vis diffuse reflectance spectrum (DRS). Photocatalytic activity was studied by degrading wastewater of methylene blue under visible light. The results indicate that the fly-ash cenospheres are covered by modified TiO₂ film which composed of the anatase, brookite and rutile misch crystal phase. CoSPc/TiO₂/fly-ash cenospheres samples have good catalytic activity under visible light, and have strong absorbency during 600-700 nm. The sensitization of CoSPc can enhance visible light catalytic activity of TiO₂/fly-ash cenospheres. The degradation rate of methylene blue reaches 73.36% in 180 min under the visible light illumination. But too much CoSPc can decrease its catalytic activity.     

Silica-tin nanotubes prepared from rice husk ash by sol-gel method: Characterization and its photocatalytic activity

Silica-tin material has been synthesized by simple sol-gel method using rice husk ash as the source of silica and cetyltrimethylammonium bromide as the surfactant at room temperature. Calcination of the material at 500 °C for 5 h gave nanotubes with external diameter of 2-4 nm and an internal diameter of 1-2 nm. The BET specific surface area was found to be 607 m² g⁻¹. Nitrogen sorption analysis exhibits a type IV isotherm with H3 hysteresis loop. The powder X-ray diffraction pattern showed that the material is amorphous. The photocatalytic activity of the prepared material was studied towards degradation of methylene blue under UV-irradiation. According to the experimental results the silica-tin nanotubes exhibit high photocatalytic activity compared to pure rice husk silica.     

Size-dependent photoluminescent properties of uncapped CdS particles prepared by acoustic wave and microwave method

CdS particles with crystallite size of 5-12 nm have been prepared via acoustic wave stimulated (sonochemical) route and microwave initiated combustion method. X-ray line broadening and transmission electron microscopy (TEM) suggest that sonochemical powders are more amorphous (5-10 nm) compared to microwave-synthesized sulphides (10-15 nm). The photoluminescent (PL) properties of powders with size <10 nm show a clearly blue shifted, resolved emission with full-width at half-maxima (FWHM) ∼100 nm, while powders with size >15 nm show dominant blue to green narrow emission with FWHM ∼60 nm. The mechanistic details of the synthetic route appear to affect the morphology and consequently the PL properties to a significant extent.     

Design and fabrication of a TiO2/nano-silicon composite visible light photocatalyst

Nano-silicon (nc-Si) was utilized as the charges generator to promote the photocatalytic and super-hydrophilic reactivity of TiO₂ film under visible light irradiation. The photocatalytic ability of TiO₂/nc-Si composite photocatalyst was evaluated by a set of experiments to photodecompose 100 ppm methylene blue (MB) in aqueous solution. And the super-hydrophilic property was characterized by measuring the water droplet contacts angle, under visible light irradiation in atmospheric air and at room temperature. Under 100 mW/cm² visible light irradiation, the droplet contact angles were reduced to 0° within 4 h with nc-Si charge generator. Additionally, the rate constant of MB photo-degradation was promoted 6.6 times.     

Photocatalytic performance of ZnO:Fe array films under sunlight irradiation

ZnO:Fe array thin films were prepared by the hydrothermal method using the sol-gel grown film as a seed layer. The samples were characterized by X-ray diffraction (XRD), ultraviolet/visible absorption spectra (UV-vis) and scanning electron microscopy (SEM). The photocatalytic activities of the prepared samples were investigated for the photodegradation of methylene blue (MB) under sunlight irradiation. The results show that the lattice constant a and the cell volume of ZnO:Fe film increase due to the substitution of Fe for Zn. The absorption edge of Fe-doped ZnO displays a red shift with a significant absorption between 600 and 700 nm. The ZnO:Fe array film is composed of disk particles with uniformity and compactness. Doping Fe ions enhances the photodegradation rate of ZnO array film for MB. 1.5% Fe doped ZnO sample exhibits the highest activity under irradiation time of 4 h. Its degradation rate increases about 1.6 times compared to the undoped ZnO.     

Enhanced photocatalytic activity and stability of nano-scaled TiO2 co-doped with N and Fe

Titanium dioxide (TiO₂) nanoparticles co-doped with N and Fe were prepared via modified sol-gel process. The products were characterized by transmission electron microscopy (TEM), N₂ adsorption, X-ray diffraction (XRD), Raman spectroscopy, UV-vis spectroscopy, photoluminescence (PL), and X-ray photoelectron spectroscopy (XPS). It is shown that the prepared TiO₂ particles were less than 10 nm with narrow particle size distribution. The addition of MCM-41 caused the formation of Ti-O-Si bond which fixed the TiO₂ on MCM-41 surface, thus restricted the agglomeration and growth of TiO₂ particles. The photocatalytic performance in the degradation of methylene blue showed that N, Fe co-doped TiO₂ exhibited much higher photocatalytic activity than doped sample with nitrogen or Fe³⁺ alone under both UV and visible light. N, Fe co-doping decreased the loss of doping N during the degradation reaction, thus increased the photocatalytic stability. It was also found that the nitridation time had significant influence on the photocatalytic activity of prepared TiO₂ catalysts.     

Ultrasonic-assisted preparation and characterization of CdS nanoparticles in the presence of a halide-free and low-cost ionic liquid and photocatalytic activity

An ultrasonic-assisted and environmentally favorable method is proposed for preparation of CdS nanoparticles in the presence of a halide-free and low-cost room-temperature ionic liquid (RTIL), 1-ethyl-3-methylimidazolium ethyl sulfate, ([EMIM][EtSO₄]). The prepared samples were characterized by powder X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM) and Fourier transform-infrared (FT-IR). Diffuse reflectance spectra (DRS) of the products reveal a blue shift of 1.45 eV relative to bulk CdS, which can be attributed to the quantum confinement effect of the prepared nanoparticles. Investigation of photocatalytic activity for the CdS nanoparticles towards photodegradation of methylene blue (MB) demonstrates that photodegradation of MB increases with RTIL content of the media using visible and UV irradiations.     

Modification of TiO2 nanotube arrays by solution coating

Packed and sintered titania (TiO₂) nanoparticles are conventionally used as a photoanode for dye-sensitized solar cells. As a result, barriers are created for the electron transport and electron-hole recombination due to the large number of interfacial boundaries and voids in such a mesoporous photoanode. A high aspect ratio of length/diameter of titania nanotube arrays (TNAs) is required to meet the high surface area and the reduction of interfacial boundaries. TNAs prepared by the organic anodic oxidation method in this work possess a vertical nanotube structure that exhibits a high surface area, high degree of crystallinity, geometric pre-orientation and active electro-optical nanograins. Such a synthesized mesoporous structure is typically obtained by 60 V/2 h and 450 °C/30 min post-annealing and has a tube diameter of 110 nm, and a tube length of 15 mm. When the TNAs are modified by a TiO₂ nano-layer (TNL), the photocatalytic efficiency for methylene blue (MB) decomposition is further improved by up to 90%. Photosensitivity shows an obvious improvement when illuminating the UV light for the TNL modified TNAs and the visible light for the CuInS₂ layer modified TNAs.     

Photoelectrochemical property and photocatalytic activity of N-doped TiO2 nanotube arrays

N-doped TiO₂ nanotube arrays (NTN) were prepared by anodization and dip-calcination method. Hydrazine hydrate was used as nitrogen source. The surface morphology of samples was characterized by SEM. It showed that the mean size of inner diameter was 65 nm and wall thickness was 15 nm for NTN. The ordered TiO₂ nanotube arrays on Ti substrate can sustain the impact of doping process and post-heat treatment. The atomic ratio of N/Ti was 8/25, which was calculated by EDX. Photoelectrochemical property of NTN was examined by anodic photocurrent response. Results indicated the photocurrent of NTN was nearly twice as that of non-doped TiO₂ nanotube arrays (TN). Photocatalytic activity of NTN was investigated by degrading dye X-3B under visible light. As a result, 99% of X-3B was decomposed by NTN in 105 min, while that of TN was 59%.     

三氧化钨/氧化银复合材料的水热法合成及其光催化降解性能研究

染料污染是水污染中最严重的问题之一,吸引了很多科学家的关注.人们尝试了很多方法去解决该问题,如化学氧化法、物理吸附法、光催化降解法和生物降解法等.与其他几种方法相比,光催化法有着低能耗、环保以及高效等优势.三氧化钨是常见的半导体材料,具有独特的光学性能,近年来受到了广泛的研究.本文以钨酸钠和硫脲为前驱体,通过水热法制备了三氧化钨/氧化银(WO_3/Ag_2O)复合材料,并用光催化降解亚甲基蓝来分析其光催化性能.通过X射线光电子能谱、X射线衍射、透射电子显微镜、扫描电子显微镜、紫外可见吸收光谱等表征手段对样品的形貌、晶格结构和光催化的性能进行表征.氧化银的带宽为1.2 e V,对可见光很敏感,三氧化钨和氧化银的复合使材料在可见光下的光催化活性显著增强,在可见光下对亚甲基蓝染料的光降解率可以达到98%.实验结果表明,复合材料中的三氧化钨纳米棒为六方相,其平均直径约为200 nm,平均长度约为4μm.而复合材料中的氧化银纳米颗粒为六方相,附着在氧化钨纳米棒的表面,平均晶粒尺寸为20 nm.氧化银的存在为复合材料提供了更多的反应活性位点.相较于单一组分,复合材料在可见光下的光吸收度更高,这说明三氧化钨和氧化银的复合改变了材料的能带结构.研究发现,三氧化钨和氧化银之间形成的异质结构是其优良光催化性能的来源.此外,三氧化钨和氧化银复合材料还具有良好的催化稳定性和化学稳定性.本文结果表明,可以通过给宽带隙的半导体材料复合一些带隙合适的金属氧化物以提升其光催化活性.     

One-step,template-free hydrothermal synthesis of CuO tetrapods

The present investigation reports one-step template-free hydrothermal synthesis of CuO tetrapods (CuO-T) and its characterization. The CuO tetrapods have been prepared in moderate condition without using any surfactant. The prepared sample was characterized by powder X-ray diffraction (PXRD), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), Fourier Transform Infrared (FTIR) spectroscopy, UV–vis–NIR (DRS) spectroscopy and Photoluminescence (PL) spectroscopy. The X-ray diffraction and Fourier transform infrared spectrum analysis confirm clearly the formation of a pure phase high-quality CuO with monoclinic crystal structure. X-ray peak broadening analysis was used to evaluate the average crystallite size (∼30 nm) and lattice strain by the Williamson–Hall (W–H) method. Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) result reveals that the particles are tetrapods in shape with an average length ∼50 nm. Additionally the optical properties were investigated by using UV–vis reflectance spectra with considerable blue-shift in the optical band gap (Eg = 1.45 eV) due to quantum confinement effect. Photoluminescence (PL) spectrum showed both UV as well as visible emission peaks indicating their good optical properties.     

Ag2S nanoparticle encapsulated in mesoporous material nanoparticles and its application for photocatalytic degradation of dye in aqueous solution

Semiconductor loaded mesoporous materials in general possess greater photocatalytic activity than pure semiconductors. Hence, with an attempt to achieve higher photocatalytic activity, Ag₂S/MCM-41 photocatalysts were prepared by ion exchange method and used for the photocatalytic degradation of methylene blue. The materials were characterized by different analytical techniques such as transmission electron microscopy (TEM), X-ray diffraction (XRD), infrared spectroscopy (IR) and BET (Brunauer-Emmert-Teller) experiments. The effect of Ag₂S, MCM-41 support and different wt% of Ag₂S over the support on the photocatalytic degradation and influence of parameters such as Ag₂S loading, catalyst a mount, pH and initial concentration of dye on degradation are evaluated. The degradation reaction follows pseudo-first order kinetics. It was seen that 0.6 g/L of photocatalyst is an optimum value for the dosage of photocatalyst. The degradation efficiency was decreased in dye concentration above 3.2 ppm for dye.     

Preparation of Ru-loaded CdS/Al-HMS nanocomposites and production of hydrogen by photocatalytic degradation of formic acid

The composite of aluminum-substituted mesoporous silica (Al-HMS) molecular sieve coupled with CdS (CdS/Al-HMS) was prepared by template, ion exchange and sulfurization reactions. The result of low angle XRD patterns showed that the low content of 2.5 wt% CdS is incorporated inside Al-HMS channels. The results of diffuse reflectance UV-visible spectra and fluorescence emission spectra exhibited that the absorption edge and photoluminescence peak for CdS/Al-HMS are blue-shifted about 75 nm and 40 nm in comparison to bulk CdS, respectively. The activities of hydrogen production by photocatalytic degradation of formic acid were evaluated under visible light irradiation (λ ≥ 420 nm) and the CdS/Al-HMS loaded 0.07 wt% Ru showed the highest H₂ evolution at a rate of 3.7 mL h⁻¹ with an apparent quantum yield of 1.2% at 420 nm.     

Enhanced photocatalytic degradation of Safranin-O by heterogeneous nanoparticles for environmental applications

Nanostructure titanium dioxide (TiO₂) has been synthesized by hydrolysis of titanium tetrachloride in aqueous solution and Ag-TiO₂ nanoparticles were synthesized by photoreduction method. The resulting materials were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), Fourier-transform infrared (FT-IR) and UV-vis absorption spectroscopy. The experimental results showed that the sizes of the synthesized TiO₂ and Ag-TiO₂ particles are in the range of 1.9-3.2 nm and 2-10 nm, respectively. Moreover, Ag-TiO₂ nanoparticles exhibit enhanced photocatalytic activity on photodegradation of Safranin-O (SO) dye as compared to pure TiO₂. The positive effect of silver on the photocatalytic activity of TiO₂ may be explained by its ability to trap electrons. This process reduces the recombination of light generated electron-hole pairs at TiO₂ surface and therefore enhances the photocatalytic activity of the synthesized TiO₂ nanoparticles. The effects of initial dye and nanoparticle concentrations on the photocatalytic activity have been studied and the results demonstrate that the dye photodegradation follows pseudo-first-order kinetics. The observed maximum degradation efficiency of SO is about 60% for TiO₂ and 96% for Ag-TiO₂.