Sol-gel synthesis of silver/titanium dioxide (Ag/TiO2) core-shell nanowires for photocatalytic applications

Silver/titanium dioxide (Ag/TiO₂) core-shell nanowires were synthesized by direct coating of TiO₂ shells on the surface of silver nanowires (AgNWs) through a simple sol-gel process. TEM image and EDX elemental analysis had confirmed the presence of TiO₂ coating on the surface of AgNWs. The thickness of titanium dioxide coating was about 10 nm. These Ag/TiO₂ core-shell nanowires showed good photocatalytic activities in the decomposition of methylene blue as a model organic dye in aqueous solution under UV light irradiation. Ag/TiO₂ core-shell nanowires are potentially useful in photocatalytic applications.     

A green and facile route to synthesize calcium silicate nanowires

Large-scale single crystalline calcium silicate nanowires have been synthesized via a simple and facile hydrothermal route using nanoscale SiO₂ and CaO powders as the starting materials. Xonotlite [Ca₆(Si₆O₁₇)(OH)₂] nanowires were first achieved after hydrothermal treatment at 220 °C for 12 h. After being calcinated at 800 °C for 1 h, the Ca₆(Si₆O₁₇)(OH)₂ nanowires are completely transformed into β-CaSiO₃ nanowires. The β-CaSiO₃ nanowires have a diameter of 30–150 nm and a length of tens of micrometers. The hydrothermal conditions and the size of the raw materials play important roles on the size of the nanowires. A possible growth mechanism of the nanowires is also proposed.     

Sol–gel-hydrothermal synthesis and sintering of K0.5Bi0.5TiO3 nanowires

The sol–gel-hydrothermal processing of K_0.5Bi_0.5TiO₃ (KBT) nanowires as well as their sintering behavior at 1000–1100 °C were investigated. The morphological analyses indicated that sol–gel-hydrothermal route led to the formation of KBT nanowires with diameters of 4 nm and lengths of 100 nm at low processing temperature of 200 °C with KOH concentration of 6 M. It is believed that the gel precursor and hydrothermal environment play an important role in the formation of the nanowires. The KBT ceramics with a relative density of more than 95% can be successfully fabricated from the high quality KBT nanowires even by a conventional sintering process. The KBT ceramics sintered at 1050 °C showed typical characteristics of relaxor ferroelectrics, and the dielectric properties were better than that prepared by all other methods reported previously.     

Quantification of silver ion release,in vitro cytotoxicity and antibacterial properties of nanostuctured Ag doped TiO2 coatings on stainless steel deposited by RF magnetron sputtering

Surface treatments on biomaterials using several methods have greatly reduced the in vivo bacterial attachment, surface colonization and formation of biofilm. In this study, the effect of silver (Ag) ion release against in vitro antibacterial activity and cytotoxicity of 1–4wt% Ag doped titania (TiO₂) thin film coatings were evaluated. These coatings were deposited for 1–6 h onto stainless steel substrate (SS) using (radio frequency) RF magnetron sputtering technique. The coatings predominantly in the crystalline anatase phase were configured using X-ray Diffraction (XRD). Scanning electron microscopy (SEM) observation showed the presence of Ag–TiO₂ nanoparticles of less than 100 nm in all the coated surfaces confirming the formation of nanostructured coatings. An initial rapid release, followed by a sustained lower release of Ag ion concentration was measured between 0.45 and 122 ppb when all the coated substrates immersed in Phosphate Buffered Saline (PBS) for 1–10 days. The obtained concentration was less than the maximum toxic concentration for human cells; yet achieved antibacterial concentration, sufficient to kill or inhibit the growth of bacteria. In vitro cytotoxicity results have indicated that 1–4 wt% of Ag doped TiO₂ coatings had no adverse effect on mouse fibroblast proliferation, confirming its cytocompatibility. The antibacterial assessment was performed on 1 and 2 wt% Ag–TiO₂ coatings using Staphylococcus aureus (S. aureus) whereby significant antibacterial activity was observed in 2 wt% Ag–TiO₂ coatings.     

TiOx/Ag/TiOx multilayer for application as a transparent conductive electrode and heat mirror

Amorphous TiO_x films and Ag layer were deposited by electron-beam evaporation on soda-lime glass at room temperature. The details regarding the structure, surface morphology, and optical properties of the as-prepared TiO_x films were examined by X-ray diffraction, scanning electron microscopy, and ultra-violet (UV) -visible-near-infrared (NIR) spectrometry. The TiO_x films exhibit amorphous phase with an optical band gap of 3.35 eV. The polygrains oriented along the (111) and (200) directions in the Ag films were adopted to supply carriers into the TiO_x film and lower the sheet resistance of the stacked layer. The multilayer exhibited a sufficiently large Ag thickness (>15 nm), low resistance, high UV transmittance, visible transmittance, and high NIR reflection. Dependence of Ag thickness, TiO_x bottom-layer, and TiO_x overlayer on the optical and electrical properties of TiO_x/Ag/TiO_x were explored. A figure of merit (FOM) was used to find an optimal structure for a multilayer with superior conductivity and visible transparency. An FOM of 9.8 × 10^?2 (Ω^?1) at the visible wavelength of 550 nm for a TiO_x/Ag/TiO_x stacked layer with an 18-nm-thick Ag and a 20-nm-thick TiO_x was achieved. The TiO_x/Ag/TiO_x sample annealed at 500 °C 10 min also shows a good thermal stability.     

Photocatalytic properties of porous TiO2/Ag thin films

In this study, nanocrystalline TiO₂/Ag composite thin films were prepared by a sol-gel spin-coating technique. By introducing polystyrene (PS) spheres into the precursor solution, porous TiO₂/Ag thin films were prepared after calcination at a temperature of 500 °C for 4 h. Three different sizes (50, 200, and 400 nm) of PS spheres were used to prepare porous TiO₂ films. The as-prepared TiO₂ and TiO₂/Ag thin films were characterized by X-ray diffractometry (XRD) and by scanning electron microscopy to reveal structural and morphological differences. In addition, the photocatalytic properties of these films were investigated by degrading methylene blue under UV irradiation.When PS spheres of different sizes were introduced after calcination, the as-prepared TiO₂ films exhibited different porous structures. XRD results showed that all TiO₂/Ag films exhibited a major anatase phase. The photodegradation of porous TiO₂ thin films prepared with 200 nm PS spheres and doped with 1 mol% Ag exhibited the best photocatalytic efficiency where ∼ 100% methylene blue was decomposed within 8 h under UV exposure.     

Enhancement of thermal stability of TiO2 nanowires embedded in anodic aluminum oxide template

Titania (TiO₂) nanowires with diameters of 20, 50, and 80 nm were successfully synthesized via the template-assistant method. The TiO₂ nanowires embedded in anodic aluminum oxide template have extremely high crystallization and anatase-to-rutile phase transition temperatures than that of the free-state TiO₂ powders, and the thermal stability of embedded TiO₂ nanowires depends on the diameter of the templates. The growth and nucleation activation energy of rutile in 20 nm nanowires are determined to be _boxclose E_{\rm{g}}  = 2.8 ± 0.2 eV and E_n E_{\rm{n}}  = 2.7 ± 0.2 eV, respectively, much higher than that of the free-state TiO₂ powders with E_g E_{\rm{g}}  = 1.6 ± 0.2 eV and E_n E_{\rm{n}}  = 1.9 ± 0.2 eV. The pressure induced by the difference of thermal expansion coefficient between the TiO₂ and aluminum oxide acts as an effective barrier that prevents phase transition, resulting in the enhancement of the TiO₂ structural stability.     

Morphological and phase dependence of nanotitania materials generated under extreme pH conditions for large scale production of TiO<Subscript>2</Subscript> nanowires (basic) and nanosquares or nanrods (acidic)

The effect that the phase of the starting nanoseed titania (TiO₂), the pH of the solvent solution, and the processing methodology employed have on the properties of the resultant TiO₂ nanomaterials were explored. This led to the development of a new process to produce large-scale, phase pure, thin nanowires of TiO₂ at high pH and nanosquares at low pH. Anatase, rutile, and Degussa P25^TM TiO₂ nanoparticle starting materials (or nanoseeds) were processed in strongly basic (10 M KOH) and strongly acidic (conc. HX, where X = Cl, Br, I) solutions using solvothermal (SOLVO) and solution precipitation (SPPT) methodologies. Under basic SOLVO conditions, the nanoseeds were converted to H₂Ti₂O₅·H₂O nanowires. The SPPT basic conditions also produced the same phased nanowires for the rutile and anatase nanoseeds, while the Degussa nanomaterial yielded mixed phased [anatase:rutile (9:1)] nanowires. The SPPT method was found to produce substantially thinner nanowires in comparison to the SOLVO route, with comparable surface areas but the strong basic media led to etching of the glassware yielding HK₃Ti₄O₄(SiO₄)₃·4H₂O nanorods. Hybridization of these two processing routes led to the use of Nalgene^TM bottle as the reaction flask termed the hybrid (HYBR) route, yielding even thinner H₂Ti₂O₅·H₂O nanowires on a large-scale. Switching to a concentrated halide acid (HX, where X = Cl, Br, I) system, SOLVO, SPPT, and HYBR routes were investigated. The resultant TEM images revealed that the rutile starting material yielded short rods, whereas the anatase seeds formed square or faceted materials.     

Co-catalysis effect of different morphological facets of as prepared Ag nanostructures for the photocatalytic oxidation reaction by Ag–TiO2 aqueous slurry

This paper highlights the comparative co-catalytic efficiency of different shapes of prepared Ag nanoparticles of size much larger as well as smaller than titania for the Ag–TiO₂ photocatalysis. Quantum sized Ag nanospheres (4^_8 nm), nanorod (length 70–75 nm and width 30–38 nm), polygonal nanosphere (80–120 nm) and truncated triangles (side length 70–140 nm) are prepared by solvothermal process. The co-catalytic activities of these Ag nanostructures were investigated by mixing them with TiO₂ for the photocatalytic degradation of aqueous salicylic (0.5 mM) and benzoic acid (0.5 mM) under UV light (125 W-Hg arc, 10.4 mW cm⁻²) irradiation. The Ag co-catalysis effect imparted to TiO₂ follows as polygonal nanosphere > nanorod > truncated triangle > small nanosphere due to the formation of many Ag–TiO₂ interfaces by a single large-sized Ag nanoparticle than smaller one. As the surface coverage of Ag particles by TiO₂ decreases, the Ag–TiO₂ photoactivity is decreased accordingly. The efficient adsorption of salicylic acid to TiO₂ surface through –COOH and –OH groups render its higher photodegradation rate (1.8–2.7 × 10⁻² μmol min⁻¹) than benzoic acid (1.5–2.5 × 10⁻² μmol min⁻¹) having one chelating –COOH group. Zeta potential and conductance measurement of photoreaction mixture were carried out to investigate the ionic interaction-adsorption of reactant substrates over Ag–TiO₂ surface.     

TiO2/Si nanowires hybrid system for efficient photocatalytic degradation of organic dye

Herein, titanium dioxide (TiO₂)-coated vertically aligned silicon nanowires (SiNWs/TiO₂) were fabricated and evaluated for photocatalytic degradation of organic dyes. Aligned SiNWs arrays were prepared by facile metal-assisted chemical-etching process with varying the etching time that was followed by TiO₂ nanoparticles coating using sputtering technique. The TiO₂ film crystallized in pure anatase phase with an average crystalline size of 50 nm, as was elucidated with X-ray diffraction studies. SEM analysis showed nanowires with varying lengths from 2.5 to 13.5 µm and confirmed the homogenous surface decoration with TiO₂. The homogeneous distribution of TiO₂ nanoparticles on nanowires was co-evidenced with Energy-Dispersive X-ray spectroscopy (EDX) and Raman spectra analysis. The developed SiNWs/TiO₂ was exploited for photocatalytic degradation of methylene blue; the role of hydrogen peroxide was also elucidated. The highest photocatalytic efficiency of 96% was achieved for SiNWs/TiO₂ with optimum nanowire length of 3.5 μm. The developed photocatalyst was found to be almost stable even after 190 days (~?5 months) and could be used as reusable and easily removable photocatalysts. The current study highlighted the SiNWs/TiO₂/H₂O₂ system as excellent candidate for water remediation applications.

     

Synthesis of titanate nanostructures using amorphous precursor material and their adsorption/photocatalytic properties

This article reports on a new and swift hydrothermal chemical route to prepare titanate nanostructures (TNS) avoiding the use of crystalline TiO₂ as starting material. The synthesis approach uses a commercial solution of TiCl₃ as titanium source to prepare an amorphous precursor, circumventing the use of hazardous chemical compounds. The influence of the reaction temperature and dwell autoclave time on the structure and morphology of the synthesised materials was studied. Homogeneous titanate nanotubes with a high length/diameter aspect ratio were synthesised at 160 °C and 24 h. A band gap of 3.06 ± 0.03 eV was determined for the TNS samples prepared in these experimental conditions. This value is red shifted by 0.14 eV compared to the band gap value usually reported for the TiO₂ anatase. Moreover, such samples show better adsorption capacity and photocatalytic performance on the dye rhodamine 6G (R6G) photodegradation process than TiO₂ nanoparticles. A 98% reduction of the R6G concentration was achieved after 45 min of irradiation of a 10 ppm dye aqueous solution and 1 g L⁻¹ of TNS catalyst.     

Synthesis and tuning of ordering and crystallinity of mesoporous titanium dioxide film

Synthesis of well-organized and highly crystalline mesoporous titania (TiO₂) film is demonstrated using triblock copolymer (Pluronic P123) as a structure directing template, through the evaporation induced self-assembly (EISA) process. The issue of thermal and structural stability of a mesoporous TiO₂ film was addressed via optimization of annealing temperature and time. An anatase phase, high crystallinity TiO₂ film with ordered pores was obtained at 430 °C after annealing for 15 min. The synthesized film was crack free with TiO₂ nanoparticle size of 10–15 nm, quasi-hexagonal pore diameter in the range of 8–10 nm and film thickness of ~ 150 nm.     

NH4Cl-assisted low temperature synthesis of anatase TiO2 nanostructures from Ti powder

A simple, low temperature and low cost method, which was based on heating the mixture of Ti and NH₄Cl powders in air at 300 °C, has been developed for the controlled synthesis of anatase TiO₂ nanostructures including irregular nanoparticle aggregates, curved nanowires built up by the oriented attachment of nanoparticles, and nanoplates constructed with nanoparticles. The characterization results from X-ray diffraction and Raman spectra indicated that the as-obtained products were anatase TiO₂. Field emission scanning electron microscope images revealed that the products obtained for 3, 10 and 16 h comprised, in turn, irregular nanoparticle aggregates (8-55 nm), curved nanowires built up by the oriented attachment of nanoparticles (~ 9 nm), and nanoplates constructed with nanoparticles (~ 8 nm).     

Titanium and iron oxides produced by sol–gel processing of [FeCl{Ti2(OPri)9}]: structural,spectroscopic and morphological features

The first structurally characterised titanium and iron isopropoxide, [FeCl{Ti₂(OPrⁱ)₉}] (1), has been used as a single-source precursor for TiO₂/Fe₂TiO₅ composites prepared by the sol–gel route. Two distinct hydrolysis and condensation conditions were employed, followed by drying and thermal treatment up to 1000 °C. Product composition and oxide phase transitions were characterised by powder X-ray diffractometry and Raman, electron paramagnetic resonance, Mössbauer and Fourier-transformed infrared spectroscopies. A mixture of nanometric-size TiO₂ (anatase, 3.6–5.8 nm) and amorphous iron(III) oxide was obtained up to 500 °C, while TiO₂ (rutile), α-Fe₂O₃ (hematite) and Fe₂TiO₅ (pseudobrookite) were found at 700 °C. At 1000 °C, only rutile and pseudobrookite were observed. These results suggest that 1 behaves as a type III single-source precursor. Powders calcined at 1000 °C were analysed for surface morphology, microstructure and elemental composition by scanning electron microscopy/energy dispersive X-ray spectroscopy. Results suggest no phase segregation on a sub-micrometer level. Different morphologies were observed for the materials produced by the N₂ route, and this could relate to early crystal growth in an oxygen-deficient environment.     

Fabrication of Ag and Cu nanowires by a solid-state ionic method and investigation of their third-order nonlinear optical properties

Ag and Cu nanowires were separately fabricated in a direct current electric field using a solid-state ionic method, and characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Their optical nonlinearities induced by 8 ns laser pulses from a frequency-doubled, Nd:YAG laser at 532 nm, were investigated using the Z-scan technique. Experimental results indicate the metal nanowires have obvious positive refractive nonlinearities and reverse saturated absorption behaviors. The self-focusing behaviors of Ag and Cu nanowires can be attributed to Kerr-induced self-focusing of laser radiation, the nonlinear refractive indexes of Ag and Cu nanowires are n₂ = 1.7 × 10^− 11 esu and n₂ = 2.4 × 10^− 11 esu respectively, and the two-photon process of Ag and the one-photon process of Cu are responsible for the difference between Ag and Cu nanowires suspended in de-ionized water in nanosecond nonlinear absorptions.     

Low temperature growth of SnO2 nanowires by electron beam evaporation and their application in UV light detection

For the first time, high quality tin oxide (SnO₂) nanowires have been synthesized at a low substrate temperature of 450 °C via vapor–liquid–solid mechanism using an electron beam evaporation technique. The grown nanowires have shown length of 2–4 μm and diameter of 20–60 nm. High resolution transmission electron microscope studies on the grown nanowires have shown the single crystalline nature of the SnO₂ nanowires. We investigated the effect of growth temperature and oxygen partial pressure on SnO₂ nanowires growth. Variation of substrate temperature at a constant oxygen partial pressure of 4 × 10⁻⁴ mbar suggested that a temperature equal to or greater than 450 °C was the best condition for phase pure SnO₂ nanowires growth. The SnO₂ nanowires grown on a SiO₂ substrate were subjected to UV photo detection. The responsivity and quantum efficiency of SnO₂ NWs photo detector (at 10V applied bias) was 12 A/W and 45, respectively, for 12 μW/cm² UV lamp (330 nm) intensity on the photo detector..     

Quantification of silver ion release, in vitro cytotoxicity and antibacterial properties of nanostuctured Ag doped TiO2 coatings on stainless steel deposited by RF magnetron sputtering

Surface treatments on biomaterials using several methods have greatly reduced the in vivo bacterial attachment, surface colonization and formation of biofilm. In this study, the effect of silver (Ag) ion release against in vitro antibacterial activity and cytotoxicity of 1-4wt% Ag doped titania (TiO₂) thin film coatings were evaluated. These coatings were deposited for 1-6 h onto stainless steel substrate (SS) using (radio frequency) RF magnetron sputtering technique. The coatings predominantly in the crystalline anatase phase were configured using X-ray Diffraction (XRD). Scanning electron microscopy (SEM) observation showed the presence of Ag-TiO₂ nanoparticles of less than 100 nm in all the coated surfaces confirming the formation of nanostructured coatings. An initial rapid release, followed by a sustained lower release of Ag ion concentration was measured between 0.45 and 122 ppb when all the coated substrates immersed in Phosphate Buffered Saline (PBS) for 1-10 days. The obtained concentration was less than the maximum toxic concentration for human cells; yet achieved antibacterial concentration, sufficient to kill or inhibit the growth of bacteria. In vitro cytotoxicity results have indicated that 1-4 wt% of Ag doped TiO₂ coatings had no adverse effect on mouse fibroblast proliferation, confirming its cytocompatibility. The antibacterial assessment was performed on 1 and 2 wt% Ag-TiO₂ coatings using Staphylococcus aureus (S. aureus) whereby significant antibacterial activity was observed in 2 wt% Ag-TiO₂ coatings.     

Low-temperature synthesis of Li2TiO3 tritium breeder ceramic pebbles by water controlled-release solvothermal process

In this work, the water-controlled release solvothermal process (WCRSP) method was selected to synthesize nanostructured β-Li₂TiO₃ tritium breeder ceramic pebbles using anhydrous lithium acetate (LiOAc) as lithium source and tetrabutyl titanate (TBOT) as titanium source. The effects of the reaction system, temperature, and amount of acetic acid on the crystal structure and morphology of Li₂TiO₃ power were systematically investigated by using XRD, SEM, and TEM techniques, respectively. The urea or acetone in the reaction system affects the composition of Li₂TiO₃ in the water-controlled release solvothermal process at 550 °C. Initially, the nanostructured single-phase Li₂TiO₃ powders with good dispersion and an average particle size of 35 nm were successfully synthesized at 550 °C when the acetic acid content was 15 vol%. Moreover, the phase transition temperature for the monoclinic phase β-Li₂TiO₃ was as low as 450 °C. The morphology of Li₂TiO₃ powder presents two different shapes, one is micro-spherical, the other is spherical nanoparticles in different content acetic acid. The results indicate that using ethanol/acetic acid mixture as a reaction system is favorable for obtaining nanostructured Li₂TiO₃ powder. Finally, Li₂TiO₃ ceramic pebbles with grain size of 1.4 μm were successfully fabricated at 950 °C for 2 h at room temperature, and the crush load and relative density (theoretical density, T.D) of the pebbles reach 53 N and 85.2%, respectively.     

Sintering and electromagnetic properties of BaFe12O19 ferrite prepared by co-precipitation method

BaFe₁₂O₁₉ (BaM) was synthesized through the co-precipitation route. Pure phase BaM was formed after calcination of precipitated powder at 900 °C. BaM was sintered at three different temperatures; 1100, 1200, and 1300 °C to study the sintering kinetics by varying the sintering time from 1 to 4 h. Apparent porosity decreased, and bulk density increased with increasing sintering temperature and period. A bulk density of about 4.6 g/cm³ was achieved after sintering at 1300 °C/4 h. The rate-controlling mechanism of BaM densification was the diffusion of oxygen, and the activation energy for the sintering process was 274 kJ/mol. The grain size of BaM increased with rising sintering temperatures. Permittivity increased from about 11 to 17 and the permeability increased from about 10 to 16 with the increase in sintering temperature from 1100 to 1300 °C. Saturation magnetization was also enhanced to about 69 emu/g after sintering at 1300 °C/4 h. Therefore, BaM ferrite synthesized through the co-precipitation route can be effectively used for high-frequency applications after sintering at 1300 °C.