Hydrothermal synthesis of 3D TiO2 nanostructures using nitric acid: Characterization and evolution mechanism

Various morphologies of TiO₂ nanostructures were synthesized by HNO₃ assisted hydrothermal treatment with respect to the acid molarity (1 M, 3 M, and 8 M), temperature (110, 140, and 180 °C), and time (1, 3, and 6 h). An additional sample was synthesized inside the protonated titanate nanoribbon coated vessel with the acid molarity of 8M at 140 °C for 3 h. The crystal structure and morphology of the nanostructures synthesized were investigated using X-Ray diffractometer, scanning electron microscope, and transmission electron microscope. The results revealed that lower acid concentrations, longer synthesis durations and higher temperatures favored anatase phase formation. Meanwhile, a phase pure 3D lotus structure rutile TiO₂ could be obtained by hydrothermal synthesis at 8M HNO₃ concentration at 140 °C for 3 h using protonated H-titanate nanoribbons. A probable mechanism for the evolution of 3D rutile lotus structure was highlighted.     

Polypyridyl ruthenium complexes containing anchoring nitrile groups as TiO2 sensitizers for application in solar cells

Ruthenium polypyridyl complexes of general formula [Ru(bpy)_3 − x(Mebpy-CN)_x]^2 + (x = 1,2 and 3, bpy = 2,2′-bipyridine, Mebpy-CN = 4-methyl-2,2′-bipyridine-4′-carbonitrile) can be used as visible dyes in novel solar cells formed with a porous TiO₂ film (1 cm²), Pt counter-electrode and iodine/iodide as the redox mediator electrolyte dissolved in a polymeric matrix. These complexes can be anchored over the surface of nanocrystalline TiO₂ through nitrile groups, as evidenced by Raman spectra of the adsorbed species. Irradiated by a solar simulator (67 mW cm^− 2), the cells assembled with the Ru complexes with x = 2 and 3 as TiO₂ sensitizers exhibit almost identical current–potential curves, with short-circuit photocurrents of 1.25 mA cm^− 2, fill factors of 0.5 and overall efficiencies around 0.44%. The Ru complex with x = 1 and a similar Re complex did not perform as well as sensitizers. These data were consistent with results obtained from quantum efficiency curves and impedance spectra. We conclude that complexes with nitrile groups as anchoring entities are promising candidates for designing efficient DSCCs.     

All-solid-state quantum-dot-sensitized solar cells with compact PbS quantum-dot thin films and TiO2 nanorod arrays

To improve the electron injection efficiency from PbS quantum dots to TiO₂ nanorods and prevent the direct contact of spiro-OMeTAD and TiO₂ nanorods, a compact PbS quantum-dot thin film can be successfully obtained on TiO₂ nanorod arrays 360 nm in length by repeated spin coating of Pb(Ac)₂, Na₂S and 1,2-ethanedithiol solution in a step-by-step process. The corresponding solid-state quantum-dot-sensitized solar cells are fabricated using a novel structured FTO/compact PbS quantum-dot thin film sensitized TiO₂ nanorod array/spiro-OMeTAD/Au that achieves a photoelectric conversion efficiency of 3.57% under AM 1.5 G illumination (100 mW cm⁻²), which represents a high value among all-solid-state PbS quantum-dot-sensitized TiO₂ nanorod array solar cells.     

Electrochemically assisted photocatalytic degradation of methyl orange using anodized titanium dioxide nanotubes

The TiO₂ nanotubes have demonstrated potential in the photoelectrocatalytic degradation of methyl orange dye (MO). TiO₂ nanotubes were prepared using anodization of titanium foils in phosphoric acid (PA) and ethylene glycol (EG) by mechanical stirring and ultrasonic method. The TiO₂ nanotubes prepared in EG under ultrasound followed by annealing in nitrogen atmosphere showed higher activity towards dye degradation as compared to the stirring method. Dye degradation shows improved activity under an external bias compared to degradation performed in the absence of an external bias. An increase in the external bias from +0.0 to +0.1 V versus calomel electrode (SCE) is sufficient to improve the degradation rates of MO from 22% to 57% within the first 10 min. At +0.1 V, a complete degradation of 40 μM MO is observed within 30 min. The addition of oxidants such as oxygen and hydrogen peroxide demonstrate improvement in the MO degradation.     

Hydrothermal preparation of Ag-TiO2 nanostructures with exposed {001}/{101} facets for enhancing visible light photocatalytic activity

Nano-composite materials of Ag nanoparticles dispersed TiO₂ nanocubes with exposed {001}/{101} crystal faces were fabricated mainly via a flexible one-step method of hydrothermal treatment with different content of Ag from 1 up to 3 mol%. Prepared photocatalysts were characterized by X-ray diffraction, scanning electron microscope, transmission electron microscopy, UV–vis absorption spectroscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. These analysis was carried out for understanding the contribution of different content of silver for enhancing the photocatalytic activity of TiO₂ nanocubes. Prepared silver nanoparticles had small particle size and grafted to the {101} crystal face of TiO₂ with the role of template control agent and linking agent. The photocatalytic performance of Ag-TiO₂ nanocubes were researched via Rhodamine B dye removal under visible light irradiation ( ≧420 nm). Ag-TiO₂ composite materials with the content of 2 mol% Ag showed the best photocatalytic activity for degradation of Rhodamine B, which was five times more than bare TiO₂ and associated with the localized surface plasmon resonance (LSPR) propelled effect. The mechanism by which silver enhanced the photocatalytic activity of TiO₂ was also demonstrated.     

Synthesis and characterisation of thin-film TiO2 dye-sensitised solar cell

A TiO₂ dye-sensitised solar cell (DSSC) is fabricated and characterised using: X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), electron diffraction X-ray (EDX) analysis, UV–vis spectrometry and a current?voltage (I?V) test. Thicker anatase TiO₂ gives rise to better crystallinity and subsequently leads to better cell efficiency. Mesoporous TiO₂ with a suitable, average pore size results in higher conversion efficiency. Smaller particle sizes lead to higher dye uptake and increase short circuit current density, J_sc. Addition of scattering layer and/or dual TiCl₄ treatment for DSSCs having optimum thickness enhanced their performance. A DSSC having double TiO₂ layers (20 nm+50 nm) with dual TiCl₄ treatment achieved the highest conversion efficiency of 9.78%.     

Synthesis,structure, and properties of dinuclear copper (II) complex with a (H2O)12 cluster

A novel pyrazole complex, namely [Cu₂(4,4′-bpy)₂(2,2′-bpy)(L)₂]·6H₂O has been synthesized under mild condition (4,4′-bpy = 4,4′-bipyridine, 2,2′-bpy = 2,2′-bipyridine). We report the structural evidence in the solid state of discrete water hexamers chair-like conformation. These units were found to act as supramolecular glue in the aggregation of dinuclear copper (II) complex to give a three dimensional network through hydrogen-bonding. The preliminary investigation on the electrochemical property and fluorescence property of the complex are presented.     

A donor–chromophore complex containing the polyazine bridging ligand 2,3-bis(2-pyridyl)pyrazine

The complex [(PTZpbpy)₂Ru(dpp)](PF₆)₂ coupling a phenothiazine derived electron donor to a ruthenium complex with a polyazine bridging ligand dpp (PTZpbpy = 4-methyl-4′-(4-(N-phenothiazinato)pentyl)-2,2′-bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine) has been synthesized and the redox, spectroscopic, and excited state properties elucidated. In the coupled electron donor system, the emission intensity from the Ru(dπ) → dpp(π¹) ³MLCT state is quenched by 94% of that of the analogous system [(bpy)₂Ru(dpp)](PF₆)₂ (bpy = 2,2′-bipyridine) which lacks a coupled electron donor. Coupling a donor–chromophore unit to a bridging ligand will allow the expansion of the molecular architecture to form complex molecular assemblies which contain covalently attached electron donors.     

Preparation of dye sensitized solar cell by using supercritical carbon dioxide drying

Dye-sensitized solar cells (DSSC) derived from TiO₂ aerogel film electrodes were fabricated. TiO₂ aerogels were obtained by using sol–gel method and supercritical carbon dioxide (sc-CO₂) drying. First, TiO₂ wet gels were obtained by sol-gel method. Then, the solvents in the TiO₂ wet gels were replaced by acetone. The TiO₂ aerogels were obtained by using sc-CO₂ drying from the TiO₂ wet gels. The conditions of sc-CO₂ drying were at 313, 323 K and 7.8–15.5 MPa. The electrodes with TiO₂ aerogel films were obtained by deposition of the aerogels on glass substrates. The electrodes with TiO₂ aerogel films and a commercial particle film of various thickness were obtained by repetitive coatings and calcinations. The amount of dye adsorbed on the TiO₂ films with sc-CO₂ drying was higher than that of commercial particle film. The amount of dye adsorbed on the TiO₂ films increased with increasing surface area of the TiO₂ film. DSSCs were assembled by using the TiO₂ aerogel film electrodes and their current–voltage performance was measured. The power performance of DSSC made by supercritical drying was higher than that of commercial particles. The DSSC with the film electrode made at 313 K and 15.5 MPa showed the best power performance (J_sc = 7.30 mA/cm², V_oc = 772 mV, η = 3.28%).     

Microwave dielectric properties and microstructures of Nb2O5-Zn0.95Mg0.05TiO3 + 0.25TiO2 ceramics with Bi2O3 addition

The effects of Bi₂O₃ addition on the microwave dielectric properties and the microstructures of Nb₂O₅-Zn_0.95Mg_0.05TiO₃ + 0.25TiO₂ (Nb-ZMT′) ceramics prepared by conventional solid-state routes have been investigated. The results of X-ray diffraction (XRD) indicate the presence of four crystalline phases, ZnTiO₃, TiO₂, Bi₂Ti₂O₇, and (Bi_1.5Zn_0.5)(Ti_1.5Nb_0.5)O₇ in the sintered ceramics, depending upon the amount of Bi₂O₃ addition. In addition, in order to confirm the existence of (Bi_1.5Zn_0.5)(Ti_1.5Nb_0.5)O₇ phase in the samples, the microstructure of Nb-ZMT′ ceramic with 5 wt.% B₂O₃ addition was analyzed by using a transmission electron micrograph. The dielectric constant of Nb-ZMT′ samples was higher than ZMT′ ceramics. The Nb-ZMT′ ceramic with 5 wt.% Bi₂O₃ addition exhibits the optimum dielectric properties: Q × f = 12,000 GHz, ?_r = 30, and τ_f = ?12 ppm/°C. Unlike the ZMT′ ceramic sintered at 900 °C, the Nb-ZMT′ ceramics show higher Q value and dielectric constant. Moreover, there is no Zn₂TiO₄ existence at 960 °C sintering. To understand the co-sinterability between silver electrodes and the Nb-ZMT′ dielectrics, the multilayer samples are prepared by multilayer thick film processing. The co-sinterability (900 °C) between silver electrode and Nb-ZMT′ dielectric are well compatible, because there are no cracks, delaminations, and deformations in multilayer specimens.     

A homochiral Cu(II) coordination polymer built from helical motif based on two V-shaped ligands

By using a V-shaped N-containing ligand and a V-shaped carboxylate ligand, we have successfully synthesized a 3D novel copper homochiral coordination polymer, namely [Cu(OPY)(1,3-bdc)]_n (complex 1), [1,3-H₂bdc = isophthalic acid, OPY = 4,4′-(oxybis(4,1-phenylene))dipyridine]. The compound has been solvothermally synthesized and characterized by IR, elemental analysis, single crystal X-ray diffraction, XRD, UV–vis, and TG analysis. The complex 1 crystallizes in the chiral space group P6₅ and possesses an unprecedented uninodal 3D 7⁵∙ 8 topology. However, the complex was synthesized by achiral materials. The complex 1 displays a moderate SHG response and its SHG efficiency is about 0.3 times that of urea. In addition, the compound shows high thermal stability.     

A luminescent quadruple stranded dinuclear Eu(III) complex based on 2,8-bis(4′,4′,4′-trifluoro-1′,3′-dioxobutyl)-dibenzothiophene for light-emitting diodes

A luminescent quadruple stranded dinuclear Eu(III) complex Na₂[Eu₂(dbt)₄] was synthesized, where H₂dbt was 2,8-bis(4′,4′,4′-trifluoro-1′,3′-dioxobutyl)-dibenzothiophene. The complex emits the characteristic red luminescence of Eu³⁺ ion due to the ⁵D₀ → ⁷F_J (J = 0–4) transitions under 395 nm-light excitation with a luminescent quantum efficiency of 23%. Bright red light-emitting diodes (LEDs) were fabricated by coating the complex onto 395 nm-emitting InGaN chip. The results indicate that the complex may act as a red component in the fabrication of near UV-based white light-emitting diodes with high color-rendering index.     

Visible light-driven mesoporous Au–TiO2/SiO2 photocatalysts for advanced oxidation process

Au–TiO₂/SiO₂ heterogeneous catalysts with different Au contents were successfully synthesized by a facile hydrothermal process and their photocatalytic activity towards reduction of Rose Bengal (RB), Methyl Blue (MB), Rhodamine B (RhB) and Congo Red (CR) was investigated in the presence of sodium borohydride (NaBH₄) for advanced oxidation process (AOP). The results reveal that 3 wt% Au loaded in TiO₂/SiO₂ can significantly degrade high RB concentration dye (>95%, 0.3 g/L, 12 pH) within 20 min of irradiation time. All catalysis reaction followed the pseudo-first order rate reaction with high correlation coefficient. The effect of loading of Au nanoparticles (1–5 wt%) along with variation in dye concentration (100–500 ppm), pH of solution (2–12), catalysts dosage (0.1–0.5 g/L), and reaction temperature (30–80 °C) were also studied. The present works shows the superior performance of Au–TiO₂/SiO₂ heterogeneous catalysts to be related to the high dispersion of Au nanoparticles in the TiO₂/SiO₂ and to the catalytic effect between gold and TiO₂.     

Formation of one-dimensional metal–water chain containing discrete water octamers

Discrete water octamers containing 1D metal–water chains have been observed in the complex of [Tb₂(bpdc)(Pic)₂(OH₂)₁₂](Pic)₂ · 4H₂O · (CH₃OH)₂ (Pic = picrate, bpdc = 2,2′-bipyridine-3,3′-dicarboxylate) (1). The water clusters are trapped by the cooperative association of coordinate interactions as well as hydrogen bonds. The intermolecular interaction is robust enough to really act as a ‘supramolecular glue’ to assemble the binuclear coordination complex and picratol into three-dimensional arrays.     

Combination of photocatalytic and photoelectro-Fenton/citrate processes for dye degradation using immobilized N-doped TiO2 nanoparticles and a cathode with carbon nanotubes: Central composite design optimization

In this report, commercial TiO₂ nanoparticles were doped with nitrogen by a manual grinding method using urea. The prepared catalyst was characterized by X-ray diffraction (XRD), diffuse reflectance spectra (DRS), and transmission electron microscopy (TEM). N-doped TiO₂ was immobilized on ceramic plates by methyl tri-methoxy silane. Next, multi-walled carbon nanotubes (CNTs) were stabilized on carbon paper to fabricate the cathode. Scanning electron microscopy (SEM) was employed to confirm stabilization of the CNTs. The prepared cathode and immobilized catalyst were utilized for the degradation of C.I. Direct Red 23 (DR23) by the photoelectro-Fenton (PEF) process in the presence of citrate (Cit) combined with a photocatalytic process. The coupled PEF/Cit/N-TiO₂ process could be performed under visible light, not only due to the formation of iron–citrate complexes, but also because of the incorporation of nitrogen to the crystalline structure of TiO₂ and the generation of TiO₂ complexes with electrogenerated H₂O₂. Results demonstrated that the degradation efficiency of DR23 (20 mg/L) using the identical operational conditions, followed a decreasing order of: PEF/Cit/N-TiO₂ > PEF/Cit > PEF > EF > N-TiO₂. Eventually, a model was developed by the central composite design (CCD) method, describing the degradation efficiency as a function of the operational parameters.     

TiO2 nanofibers doped with rare earth elements and their photocatalytic activity

In the present study rare earth doped (Ln³⁺–TiO₂, Ln = La, Ce and Nd) TiO₂ nanofibers were prepared by the sol–gel electrospinning method and characterized by XRD, SEM, EDX, TEM, and UV-DRS. The photocatalytic activity of the samples was evaluated by Rhodamine 6G (R6G) dye degradation under UV light irradiation. XRD analysis showed that all the synthesized pure and doped titania nanofibers contain pure anatase phase at 500 °C but at 700 °C it shows both anatase and rutile phase. XRD result also shows that Ln³⁺-doped titania probably inhibits the phase transformation. The diameter of nanofibers for all samples ranges from 200 to 700 nm. It was also observed that the presence of rare-earth oxides in the host TiO₂ could decrease the band gap and accelerate the separation of photogenerated electron–hole pairs, which eventually led to higher photocatalytic activity. To sum up, our study demonstrates that Ln³⁺-doped TiO₂ samples exhibit higher photocatalytic activity than pure TiO₂ whereas Nd³⁺-doped TiO₂ catalyst showed the highest photocatalytic activity among the rare earth doped samples.     

Photocatalytic decolourisation and degradation of Reactive Orange 4 by TiO2-UV process

The photocatalytic decolourisation and degradation of an azo dye Reactive Orange 4 (RO4) in aqueous solution with TiO₂-P25 (Degussa) as photocatalyst in slurry form has been carried out using UV-A light (365 nm). There is a significant difference in adsorption of dye on TiO₂ surface with the change in the solution pH. The effect of various parameters such as catalyst loading, pH and initial concentration of the dye on decolourisation and degradation have been determined. The dye is decolourised in 80 min and completely degraded in 180 min under optimum conditions. The degradation was strongly enhanced in the presence of electron acceptors such as H₂O₂, (NH₄)₂S₂O₈ and KBrO₃. The photodecolourisation and degradation kinetics are discussed in terms of Langmuir–Hinshelwood kinetic model. The degradation intermediates were analysed by GC–MS technique.     

Synthesis and investigation of TiO2 nanotube arrays prepared by anodization and their photocatalytic activity

TiO₂ nanotube arrays were successfully prepared by anodic oxidation method in the electrolyte of ethylene glycol and deionized water mixed in 9:1 volumetric ration including 0.5 wt.% NH₄F. The microstructure and phase compositions of samples annealing from 0 °C to 800 °C were characterized by field-emission scanning electron microscope (FESEM) and X-ray diffraction (XRD). FESEM showed that the obtained nanotubes with diameter 80–100 nm and length 4.89 μm were highly ordered and perpendicular to Ti substrate. The tubular structure collapsed at 680 °C. The photocatalytic activity of samples annealing at different temperature were calculated by the degradation of a model dye, methyl orange (MO), under UV light illumination. The results indicated the phase composition and the morphology of TiO₂ nanotubes both played an important role in the degradation of MO. In addition, the effects of initial solution pH and dye concentration on degradation of MO had also been investigated. As a result, the optimum values of calcination temperature, initial solution pH and dye concentration were found to be 550 °C, 3, 10 mg/l, respectively. The best photodegradation of MO was 76% under illumination for 3 h.     

Influence of sprayed nanocrystalline Zn-doped TiO2 photoelectrode with the dye extracted from Hibiscus Surattensis as sensitizer in dye-sensitized solar cell

Highly oriented Zn doped TiO₂ thin films (0, 2, 4, 6 and 8 at%) were deposited by spray pyrolysis technique. X-ray diffraction analysis showed a strong orientation along (101) direction for 6 at% Zn with polycrystalline tetragonal anatase phase. Scanning electron microscopy observations revealed uniform distribution of spherical-shaped grains, whereas columnar arrangement of tetragonal-shaped grains with porous nature was revealed from atomic force microscopy. Transmittance spectra indicated a decrease in the energy band gap with increasing doping concentration; i.e. 3.55 up to 3.21 eV, attributed to grain refinement to the nanoscale regime. The optical constants such as refractive index and extinction coefficient as a function of wavelength, were determined; the low extinction coefficient values confirmed the good quality of the thin films. Photoluminescence spectra showed strong emissions at 423 and 437 nm with a weak emission at 505 nm, which confirmed the lesser defect density in 6 at% Zn film. The electrical properties studied by Hall Effect measurements revealed that the 6 at% Zn led to an increase in the carrier concentration, as well as an increase in the mobility with a least resistivity. The efficiency of dye sensitized solar cells, assembled by using natural dye extracted from Hibiscus Surattensis as sensitizer and Zn-doped TiO₂ nanocrystalline thin films as a photoelectrode, was found to be around 1.22%.     

Improved photo-catalytic activity of single-crystalline TiO2 nanowires surrounded by Pt cube nanoparticles

We report improved photo-catalytic properties of single-crystalline TiO₂ nanowires surrounded by Pt cubes for methanol electrooxidation under UV illumination. The TiO₂–Pt consists of single-crystalline TiO₂ nanowires grown along [0 0 1] direction and Pt cube nanoparticles with dominantly exposed {1 0 0} facets confirmed by transmission electron microscopy images and fast Fourier transform patterns. The nanostructure electrode consisting of TiO₂ nanowires and Pt cubes exhibits a remarkably improved performance for methanol electrooxidation in acid solution as compared to that of TiO₂ nanowires.