Influence of growth time and annealing on rutile TiO2 single-crystal nanorod arrays synthesized by hydrothermal method in dye-sensitized solar cells

Vertically aligned TiO₂ nanorod (NR) arrays have been grown on the fluorine doped tin oxide (FTO) substrates by hydrothermal methods and the structures were employed to fabricate the dye-sensitized solar cells (DSSCs). The charge transport properties were investigated by the current to voltage curves and the electrochemical impedance spectroscopy. It was found that DSSCs containing the as-prepared and 500 °C annealed TiO₂ NRs exhibit different trends with the growth time (t). The DSSCs assembled with the un-sintered NR arrays showed the highest power conversion efficiency (η) of 1.87%. When DSSCs were assembled with the sintered NR arrays, nearly 400% enhanced efficiency were obtained, and the values (η) showed a positive correlation with t. This behavior may be attributed to the improved adhesion and electric contact between TiO₂ and FTO, as well as the reduced number of recombination sites.     

Fabrication of vertically aligned CuInSe2 nanorod arrays by template-assisted mechanical approach

A facile, low cost, and vacuum free method for fabricating vertically aligned copper indium diselenide (CuInSe₂) nanorod (NR) arrays from pre-synthesized CuInSe₂ nanoparticles (NPs) by mechanical approach using porous anodic aluminum oxide (AAO) as a template was demonstrated. This approach utilizes a rubbing technique to fill CuInSe₂ NPs suspension into AAO template. X-ray diffraction and Raman spectroscopy study is employed to confirm the phase of CuInSe₂ NPs before and after the formation of NRs. The polycrystallinity and composition of NRs are confirmed by using transmission electron microscopy. Optical studies of CuInSe₂ NPs film reveal a reflectance of ∼9.8% while a significant reduction of the reflectance to ∼1.2% is observed after the formation of CuInSe₂ NR arrays. The observed low reflectance behavior is attributed to a concept of gradual refractive index with vertical array structures. From differential reflectance spectra of CuInSe₂ NRs, a band gap of ∼1.01 eV was observed, which is identical to its bulk value.     

Improved photodegradation activity of TiO2 via decoration with SnS2 nanoparticles

The particles of TiO₂ modified with various amounts of SnS₂ nanoparticles (TiO₂/SnS₂) were synthesized via the hydrothermal method by reacting SnCl₄·5H₂O with thioacetamide in 5% (vol.) acetic acid aqueous solution in the presence of TiO₂. The obtained products were characterized by using X-ray diffraction, X-ray photoelectron spectroscopy, UV–Vis diffuse reflection spectra, scanning and transmission electron microscopy. The photodegradation activities of TiO₂/SnS₂ composites have been investigated by using methyl orange as target in water under the light irradiation of 250–400, 360–600 and 400–600 nm. It was found that the photodegradation activity of TiO₂/SnS₂ composites depended on the mass ratio of SnS₂ and the wavelength of the irradiating light. The composites containing 33% SnS₂ exhibited the maximum activity under the light irradiation of 250–400 and 360–600 nm. However, the more SnS₂ in the composites, the higher activity appeared under the irradiation of 400–600 nm light. All the results reveal that the composites possess much better activity than the pristine TiO_2.     

Enhancing efficiency of dye-sensitized solar cells by combining use of TiO2 nanotubes and nanoparticles

Titanium dioxide nanotubes (TiNTs) were fabricated from commercial P25 TiO₂ powders via alkali hydrothermal transformation. Dye-sensitized solar cells (DSCs) were constructed by application of TiNTs and P25 nanoparticles with various weight percentages. The influence of the TiNT concentration on the performance of DSCs was investigated systematically. The electrochemical impedance spectroscopy (EIS) technique was employed to quantify the recombination resistance, electron lifetime and time constant in DSCs both under illumination and in the dark. The DSC based on TiNT/P25 hybrids showed a better photovoltaic performance than the cell purely made of TiO₂ nanoparticles. The open-voltage (V_oc), fill factor (FF) and efficiency (η) continuously increased with the TiO₂ nanotube concentration from 0 to 50 wt%, which was correlated with the suppression of the electron recombination as found out from EIS studies. Respectable photovoltaic performance of ca. 7.41% under the light intensity of 100 mW cm⁻² (AM 1.5G) was achieved for DSCs using 90 wt% TiO₂ nanotubes incorporated in TiO₂ electrodes.     

Hybrid nanocomposite of Fe nanoparticles on SiO2 nanowires by sublimation route

A simple route was developed to synthesize the hybrid nanocomposite with Fe nanoparticles (NPs) dispersed on the surface of SiO₂ nanowires (NWs), where SiO₂ NWs with the diameter of 20-40 nm were produced by heating single-crystal silicon wafer, and Fe NPs in the size range of 3-20 nm were generated by heating Fe powders. The nucleation and growth of Fe NPs follows the solid-vapor-solid (S-V-S) mechanism, namely, Fe powders firstly sublime and then Fe atoms deposit on SiO₂ NWs to form Fe NPs.     

TiO2 coated SnO2 nanosheet films for dye-sensitized solar cells

TiO₂-coated SnO₂ nanosheet (TiO₂-SnO₂ NS) films about 300 nm in thickness were fabricated on fluorine-doped tin oxide glass by a two-step process with facile solution-grown approach and subsequent hydrolysis of TiCl₄ aqueous solution. The as-prepared TiO₂-SnO₂ NSs were characterized by scanning electron microscopy and X-ray diffraction. The performances of the dye-sensitized solar cells (DSCs) with TiO₂-SnO₂ NSs were analyzed by current-voltage measurements and electrochemical impedance spectroscopy. Experimental results show that the introduction of TiO₂-SnO₂ NSs can provide an efficient electron transition channel along the SnO₂ nanosheets, increase the short current density, and finally improve the conversion efficiency for the DSCs from 4.52 to 5.71%.     

Impact of reaction temperature,stirring and cosolvent on the solvothermal synthesis of anatase TiO2 and TiO2/titanate hybrid nanostructures: Elucidating the growth mechanism

One-dimensional anatase TiO₂ and hybrid TiO₂/titanate nanostructures are synthesized by a simple low temperature solvothermal route followed by the Na⁺/H⁺ ion-exchange and final calcination process. We investigated the impact of reaction temperature, stirring conditions and cosolvent on the morphologies of the as-prepared nanostructures. Nanotubes and nanorods are formed in alkaline solution, while nanorods/nanowires and nanoporous nanoribbons are formed in alkaline water–ethanol and alkaline water–ethylene glycol mixed solvents, respectively. X-ray diffraction, Raman scattering and high-resolution transmission electron microscopy studies are employed to identify the structure and phase composition. The formation of different morphologies of the as-synthesized nanostructures is investigated by field emission scanning electron microscopy and transmission electron microscopy. The growth mechanism and reaction process of the as-prepared nanostructures are explained based on the experimental observations. The photoluminescence, optical absorption and the tuning of band gap of the prepared samples are also studied. This work will be valuable for understanding the growth mechanism of various nanostructured TiO₂ and to explore the commercial applications of nanoporous nanoribbons of TiO₂.     

Controllable synthesis of uniformly distributed hollow rutile TiO2 hierarchical microspheres and their improved photocatalysis

We reported the controllable synthesis of uniformly distributed hierarchical hollow microspheres composed of rutile TiO₂ nanorods as building blocks, prepared by a hydrothermal method without employing any templates/substrates or surfactants. The homogenous hollow microspheres were obtained by optimizing the experimental conditions including hydrothermal temperatures and tetrabutyl titanate (TBOT) concentrations. A detailed formation mechanism was also proposed. The samples were analyzed by Brunauer–Emmett–Teller (BET) specific surface area analysis and ultraviolet–visible (UV–Vis) diffuse reflectance spectra (DRS). The photocatalytic results for methylene orange (MO) degradation showed that the hollow hierarchical microspheres exhibited the best photocatalytic activity among the as-synthesized products. Further optimum photocatalytic conditions were determined to study the degradation rate, decolorization and TOC (total organic carbon) removal efficiencies, and reaction kinetics in detail. Under optimum conditions, the contrastive photocatalytic experiments indicated that the photocatalytic activity was enhanced markedly when assembling the single-crystal rutile TiO₂ nanorods into hollow hierarchical microstructures.     

Double-layered TiO2 cavity/nanoparticle photoelectrodes for efficient dye-sensitized solar cells

TiO₂ nanoparticles (NPs) in the size of ~25 nm, namely P25, are very common material as the electron collecting layer in dye-sensitized solar cells (DSSCs). However, the light-scattering improvement of TiO₂ NP photoelectrodes is still a challenge. Here, we built TiO₂ cavities on the top of the TiO₂ NP layer by using carbonaceous microspheres as the template, forming the TiO₂ cavity/nanoparticle (C/NP) photoelectrode for the application in DSSCs. The cavity amount in the TiO₂ C/NP photoelectrode was controlled by adjusting the weight ratio of carbonaceous microspheres. SEM results confirm the successful formation of the double-layered TiO₂ C/NP electrode. J‒V tests show that the optimized TiO₂ C/NP electrode prepared with 25 wt.% carbonaceous microspheres contributes to remarkable improvement of the short-circuit current density (J_sc) and the power conversion efficiency (PCE). The best photovoltaic performance solar cell with the PCE of 9.08% is achieved with the optimized TiO₂ C/NP photoelectrode, which is over 98% higher than that of the TiO₂ NP photoelectrode. Further investigations of UV-vis DRS, IPCE, OCVD, and EIS demonstrate that the competition between light scattering effect and charges recombination in this TiO₂ C/NP photoelectrode is responsible for the PCE enhancement.     

The effect of Pt nanoparticles loading on H2 sensing properties of flame-spray-made SnO2 sensing films

SnO₂ nanoparticles loaded with 0.2–2 wt% Pt have successfully been synthesized in a single step by flame spray pyrolysis (FSP) and investigated for gas sensing towards hydrogen (H₂). According to characterization results by X-ray diffraction, nitrogen adsorption, scanning/high resolution-transmission electron microscopy and analyses based on Hume-Rothery rules using atomic radii, crystal structure, electronegativities, and valency/oxidation states of Pt and Sn, it is conclusive that Pt is not solute in SnO₂ crystal but forms nanoparticles loaded on SnO₂ surface. H₂ gas sensing was studied at 200–10,000 ppm and 150–350 °C in dry air. It was found that H₂ response was enhanced by more than one order of magnitude with a small Pt loading concentration of 0.2 wt% but further increase of Pt loading amount resulted in deteriorated H₂-sensing performance. The optimal SnO₂ sensing film (0.2 wt% Pt-loaded SnO₂, 20 μm in thickness) showed an optimum H₂ response of ∼150.2 at 10,000 ppm and very short response time in a few seconds at a low optimal operating temperature of 200 °C. In addition, the response tended to increase linearly and the response times decreased drastically with increasing H₂ concentration. Moreover, the selectivity against carbon monoxide (CO) and acetylene (C₂H₂) gases was also found to be considerably improved with the small amount of Pt loading. The H₂ response dependence on Pt concentration can be explained based on the spillover mechanism, which is highly effective only when Pt catalyst is well-dispersed at the low Pt loading concentration of 0.2 wt%.     

Characterization of Cu3SbS4 microflowers produced by a cyclic microwave radiation

Copper antimony sulfide (Cu₃SbS₄) crystals were produced from mixtures of different molar ratios of CuCl, SbCl₃ and thiourea in 40 and 60 ml ethylene glycol (EG) by a 300 W cyclic microwave radiation (CMR) for different lengths of time. In the present research, tetragonal Cu₃SbS₄ microflowers, characterized by X-ray and electron diffraction including electron microscopy and Raman analyses, were successfully produced in the 40 ml solution containing 2:2:4 molar ratio Cu:Sb:S for 40 cycles. Their UV-visible absorption was studied to determine the energy gap (E_g). A formation mechanism was also proposed to relate with the experimental results.     

Preparation of pure rutile and anatase TiO2 nanopowders using RF thermal plasma

We synthesized pure rutile and anatase nanopowders using in-flight oxidation of titanium nitride powder in an r.f. thermal plasma reactor. The mechanism of the in-flight oxidation of TiN powder was elucidated, and the effects of oxygen and hydrogen input concentrations on phase composition and morphology were investigated. The characteristics of the synthesized powder were evaluated using X-ray diffraction, optical microscopy, scanning electron microscopy and Raman scattering spectroscopy. The phase composition depended mainly on the ambient oxygen concentration. At high oxygen concentrations, the anatase content increased to over 93%. Introducing hydrogen promoted the formation of oxygen-deficient rutile nanopowder, and pure rutile nanopowder averaging 50 nm in diameter was produced with excess hydrogen. The synthesized powder had good photocatalytic activity in the decomposition of aqueous methylene blue under visible light.     

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).     

Fabrication and photocatalytic activity of porous TiO2 nanowire microspheres by surfactant-mediated spray drying process

A novel, porous TiO₂ nanowire microsphere with a diameter of 3-8 μm was successfully fabricated via spray drying of TiO₂ nanowire suspension with the assistance of surfactant (F127). The products were characterized by FESEM, XRD and N₂ adsorption-desorption analysis and results revealed that the resulting TiO₂ nanowire microspheres possessed a hierarchically macro/mesoporous structure, as well as a high BET surface area of 38.2 m²/g. Systematic studies showed that the presence of surfactant in the suspension feed for spray drying was critical in the formation of porous microspheres. The structure of the fabricated microspheres depends on the nanowire concentration in the feed. The TiO₂ nanowire microspheres exhibited significant photocatalytic degradation of Methylene blue (MB) as compared to commercial TiO₂ nanoparticles (P25). It was also revealed that the microspheres have excellent stability on photocatalytic activity and mechanical strength, which are both crucial factors when considering reuse of these photocatalysts.     

Electrospun polymeric nanofibrous composites containing TiO2 short nanofibers

In this study, polymeric nanofibrous composites containing anatase TiO₂ short nanofibers (TiO₂-SNF) were successfully produced via electrospinning. The fabrication of the nanofibrous composite structure includes two steps. First, anatase TiO₂ nanofibers were obtained by calcination of electrospun PVP/TiO₂ nanofibers and then crushed into short nanofibers ranging from few microns in length. Second, these TiO₂-SNF were dispersed into polymer solutions and then electrospun into nanofibrous composites. We obtained nanofibers containing TiO₂-SNF from different polymer types including PMMA, PAN, PET and PC. The SEM and TEM imaging indicated that some of the TiO₂-SNF were fully covered by the polymeric matrix whereas some TiO₂-SNF were partially covered and/or stick on the surface of the fibers. The photocatalytic activity of nanofibrous composites containing TiO₂-SNF was evaluated by monitoring the photocatalytic decomposition of a model dye (rhodamine-6G) under UV irradiation.     

Structure induced optical properties of anodized TiO2 nanotubes

TiO₂ nanotubes were synthesized by means of anodization and investigated for their structure dependent optical properties. The anodization was conducted at operating voltages between 5 and 30 V for 3 h in a neutral, organic electrolyte consisting of 0.3 wt% NH₄F + 2 wt% H₂O + ethylene glycol and the resulting nanotubes were annealed at 450 °C for 2 h in air at atmospheric pressure. It is shown that an increase in the applied anodization voltage yielded an increase in the wall thickness, diameter and length of the nanotubes and that these varying morphologies have a direct influence on the crystallite size of the nanotubes during annealing. Photoluminescence spectra indicated that the optical bandgap of the TiO₂ nanotube film decreased with the increase in the anodization voltage, whereas supplementary Raman spectra showed a decrease in the confinement of the optical phonon modes as the crystallite sizes increased, in coherence with the phonon confinement model. These results present significant insights into the size-dependent properties of these novel nanostructured forms of TiO₂ and play an important role in their implementation in photovoltaic devices, such as the dye-sensitized solar cell.     

Cathodoluminescence of TiO2 nanotubes prepared by low-temperature anodization of Ti foils

We show that anodization of Ti sheets in an ethylene glycol and HF containing electrolyte at temperatures under 0 °C results in the formation of a self-arranged ordered porous structure at the top surface of the sample. This perforated surface structure initiates the growth of an ordered array of titania nanotubes. The inner diameter of nanotubes can be modified in a controlled fashion in the range from 10 nm to more than 250 nm through the change of the electrolyte temperature from −20 °C to + 50 °C. The spectral distribution of cathodoluminescence from a cluster of nanotubes clearly demonstrates the formation of resonator modes which are separated from each other by around 200 meV.     

Ultrasonic spray pyrolysis of surface modified TiO2 nanoparticles with dopamine

Spherical, submicronic TiO₂ powder particles were prepared in the low temperature process of ultrasonic spray pyrolysis (150 °C) by using as a precursor aqueous colloidal solutions consisting of surface modified 45 Å TiO₂ nanoparticles with dopamine. Detailed structural and morphological characterization of colored submicronic TiO₂ spheres was performed by X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), laser particle size analysis and FTIR techniques. Also, optical characterization of both dopamine-modified TiO₂ precursor nanoparticles and submicronic TiO₂ powder particles was performed using absorption and diffuse reflectance spectroscopy, respectively. A significant decrease of the effective band gap (1.9 eV) in dopamine-modified TiO₂ nanoparticles compared to the band gap of bulk material (3.2 eV) was preserved after formation of submicronic TiO₂ powder particles in the process of ultrasonic spray pyrolysis under mild experimental conditions. Due to the nanostructured nature, surface-modified assemblage of TiO₂ nanoparticles preserved unique ability to absorb light through charge transfer complex by photoexcitation of the ligand-to-TiO₂ band, conventionally associated with extremely small TiO₂ nanoparticles (d < 20 nm) whose surface Ti atoms, owing to the large curvature, have penta-coordinate geometry.     

Charge recombination reduction in dye-sensitized solar cells by means of an electron beam-deposited TiO2 buffer layer between conductive glass and photoelectrode

A thin anatase titanium dioxide compact film was deposited by electron beam evaporation as buffer layer between the conductive transparent electrode and the porous TiO₂-based photoelectrode in dye-sensitized solar cells. The effect of such a buffer layer on the back transfer reaction of electrons to tri-iodide ions in liquid electrolyte-based cells has been studied by means of both electrochemical impedance spectroscopy and open circuit photovoltage decay analysis. The influence of the thickness has been also investigated and an increment in overall quantum conversion efficiency η as high as + 31% with respect to the standard cell - fabricated onto an uncoated conductive glass - has been revealed in the case of a 120 nm thick buffer layer.     

Synthesis and photoluminescence properties of aligned Zn2GeO4 coated ZnO nanorods and Ge doped ZnO nanocombs

Aligned Zn₂GeO₄ coated ZnO nanorods and Ge doped ZnO nanocombs were synthesized on a silicon substrate by a simple thermal evaporation method. The structure and morphology of the as-synthesized nanostructure were characterized using scanning electron microscopy and transmission electron microscopy. The growth of aligned Zn₂GeO₄ coated ZnO nanorods and Ge doped ZnO nanocombs follows a vapor-solid (VS) process. Photoluminescence properties were also investigated at room temperature. The photoluminescence spectrum reveals the nanostructures have a sharp ultraviolet luminescence peak centered at 382 nm and a broad green luminescence peak centered at about 494 nm.