Aligned TiO2 Nanorod Arrays Research Articles

Fabrication of Bi2MoO6 Nanosheets/TiO2 Nanorod Arrays Heterostructures for Enhanced Photocatalytic Performance under Visible-Light Irradiation.

Bi2MoO6/TiO2 heterostructures (HSs) were synthesized in the present study by growing Bi2MoO6 nanosheets on vertically aligned TiO2 nanorod arrays using a two-step solvothermal method. Their morphology and structure were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. Excellent visible-light absorption was observed by UV–Vis absorption spectroscopy, which was attributed to the presence of the Bi2MoO6 nanosheets with a narrow-band-gap. The specific surface area and pore volume of the photocatalysts were significantly increased due to the hierarchical structure composed of Bi2MoO6 nanosheets and TiO2 nanorods. The photoluminescence and photoelectrochemical characterizations showed improved separation and collection efficiency of the Bi2MoO6/TiO2 HSs towards the interface charge carrier. The photocatalytic analysis of the Bi2MoO6/TiO2 HSs demonstrated a significantly better methylene blue (MB) degradation efficiency of 95% within 3 h than pristine TiO2 nanorod arrays under visible-light irradiation. After three photocatalytic cycles, the degradation rate remained at ~90%. The improved performance of the Bi2MoO6/TiO2 HSs was attributed to the synergy among the extended absorption of visible light; the large, specific surface area of the hierarchical structure; and the enhanced separation efficiency of the photogenerated electron-hole pairs. Finally, we also established the Bi2MoO6/TiO2 HSs band structure and described the photocatalytic dye degradation mechanism. The related electrochemical analysis and free-radical trapping experiments indicated that h+, ·O2− and ·OH have significant effects on the degradation process.

Aligned TiO2 nanorod arrays decorated with closely interconnected Au/Ag nanoparticles: Near-infrared SERS active sensor for monitoring of antibiotic molecules in water

Near-infrared (NIR) excited surface-enhanced Raman scattering (SERS) spectroscopy is expected to have more appealing multiple-features than that of traditional ultraviolet (UV) or visible light excitation. Herein, we demonstrate an interesting and highly active hybrid semiconductor-based NIR-SERS sensor via well-aligned TiO2 nanorod arrays (NRAs) decorated with bimetallic Au/Ag nanoparticles (NPs). The obtained Au/Ag NPs-TiO2 NRAs with a stronger absorption capacity in 400–1300 nm region exhibit a remarkable higher NIR-SERS activity than that of monometallic Au or Ag loaded on TiO2 NRAs. Then, the established NIR-SERS sensor can provide ultrasensitive 785 nm laser excited-SERS detection of antibiotic ciprofloxacin and chloramphenicol in real-world water samples with detection limits as low as 10–9 M and 10–8 M, respectively. Interestingly, superior to the present bare metallic nanosubstrates, the Au/Ag NPs-TiO2 NRAs with unique heterostructures can also be served as reusable NIR-SERS sensor due to its excellent photocatalytic degradation ability, and ~84.2% residual SERS activity can be maintained after 5 recycling tests. Therefore, it is believed that the established multifunctional NIR-SERS sensor holds greater potential for ultrasensitive monitoring of diverse biomolecules in many specific applications.

Enhanced solar photoelectrochemical conversion efficiency of the hydrothermally-deposited TiO2 nanorod arrays: Effects of the light trapping and optimum charge transfer

The vertically aligned TiO2 nanorod arrays (NRA) with manipulated aspect ratio were hydrothermally synthesized by changing the amount of the titanium (Ti) precursor in the initial growth solution. FE-SEM images show the optimum morphology, density and aspect ratio of the well-aligned TB-1.2 NRs on the surface of the FTO substrate. The UV-vis-absorption measurements revealed that a sample prepared at TB-1.2 can provide an increased light trapping effect. PEC analyses demonstrated that the TiO2 nanorods deposited at TB-1.2 of Titanium butoxide show a relatively high PEC conversion efficiency (3.5 times) compared with the TB-0.8 prepared TiO2 at a 1.0 V versus RHE. The higher PEC performance is believed to be the result of an enhancement of the optimum aspect ratio, light trapping, an efficient charge separation, and the high carrier transport in the vertically aligned TiO2 NRs. Further, the PEC based organic dye degradation experiments showed 77% and 94% removal of Orange II and methylene blue respectively. Additionally, 109 μmol h−1 cm−2 hydrogen generations were attributed using optimized vertically aligned TiO2 NRA’s. Thus, the appropriate morphology manipulated the TiO2 NRAs are useful for solar conversion applications.

Fabrication of Ag2S quantum dots decorated TiO2 nanorod arrays for organic/inorganic hybrid solar cells

Core/shell shaped nanorod arrays with type-II band alignment heterojunction is a promising nanobuilding block for efficient charge transfer at the interface. Here in this work, we have fabricated Ag2S quantum dots decoration on surface of vertically aligned TiO2 nanorod arrays. A modified two-step solution process is used to obtain uniform Ag2S quantum dots shell that promotes exciton dissociation at the organic/inorganic hybrid interface. Because of the enhanced charge transfer, the photovoltaic conversion efficiency shows a drastic improvement of up to 73% for the solar cell adopting TiO2/Ag2S core/shell nanorod arrays as the electron acceptor.

Constructing aligned single-crystalline TiO2 nanorod array photoelectrode for PbS quantum dot-sensitized solar cell with high fill factor

Low bandgap PbS quantum dots (QDs) have attracted special interests serving as promising sensitizers for developing high performance near-infrared responsive photovoltaics. However, the commonly studied successive ionic layer adsorption and reaction (SILAR) processed PbS QD-sensitized solar cells (QDSCs) constructed with mesoporous TiO2 nanoparticle-photoelectrodes generally experience poor fill factor (FF) values lower than 0.50, arising from severe interfacial charge recombination because of the structural disorder of nanoparticles. This work demonstrates the construction of aligned TiO2 nanorod arrays (NRAs), vertically grown on substrates, for SILAR processed PbS QDSCs with high FF. The microscopic, structural, and optical characterizations reveal a rutile phase of single-crystalline structure in nature for TiO2 NRAs. The as-designed solar cell affords the advantages of panchromatic absorption for adequate light harvesting, reasonable band structure for efficient electron injection, and one dimensional (1D) oriented architecture for radial directional charge transport. A solar cell based on a 1.9 μm TiO2 NRA photoelectrode yields a considerable power conversion efficiency of 1.15% under 1 sun, coupled with a significantly enhanced FF of 0.51 in contrast to a conventional TiO2 nanoparticle-based device (FF = 0.38). The high FF is attributed to the outstanding electron transport behavior from QD sensitizers to conducting substrates via a shortest pathway (i.e., through radial direction of NRAs), thereby suppressing charge recombinations in solar devices. Therefore, the architecture of aligned 1D TiO2 NRAs is of great prospect to construct high performance PbS QDSCs. The performance improvement is promisingly expected through fully understanding of the radial directional charge transport mechanism and further optimization of NRA photoelectrodes and QD assemblies.

Enhanced photoelectrochemical and photocatalytic behaviors of MFe2O4 (M = Ni, Co, Zn and Sr) modified TiO2 nanorod arrays.

Modified TiO2 nanomaterials are considered to be promising in energy conversion and ferrites modification may be one of the most efficient modifications. In this research, various ferrites, incorporated with various cations (MFe2O4, M = Ni, Co, Zn, and Sr), are utilized to modify the well aligned TiO2 nanorod arrays (NRAs), which is synthesized by hydrothermal method. It is found that all MFe2O4/TiO2 NRAs show obvious red shift into the visible light region compared with the TiO2 NRAs. In particular, NiFe2O4 modification is demonstrated to be the best way to enhance the photoelectrochemical and photocatalytic activity of TiO2 NRAs. Furthermore, the separation and transfer of charge carriers after MFe2O4 modification are clarified by electrochemical impedance spectroscopy measurements. Finally, the underlying mechanism accounting for the enhanced photocatalytic activity of MFe2O4/TiO2 NRAs is proposed. Through comparison among different transition metals modified TiO2 with the same synthesis process and under the same evaluating condition, this work may provide new insight in designing modified TiO2 nanomaterials as visible light active photocatalysts.

Photoelectrochemical determination of Pb2+ ions by using TiO2 nanorod arrays grown on FTO substrates via a facile two-stage hydrothermal route

Aligned TiO2 nanorod arrays (NRAs) were fabricated on TiCl4 pretreated fluorine doped tin oxide (SnO2:F, FTO) substrates by a facile hydrothermal route. The effects of TiCl4 concentration (20, 40 and 60 mM) and post-growth treatments were investigated on the PEC performance of TiO2 NRAs. The optimal PEC performance, owning a low Pb2+ detection limit of ~2 nM, was achieved in the TiO2 NRAs which was pretreated with 20 mM TiCl4 and sintered after growth. It implied that the modification of FTO substrates with a low TiCl4 concentration was critical for achieving the ideal morphology and performance of TiO2 NRAs which reached a balance of light harvesting, carrier transport and ion diffusion. Additionally, it was also worth mentioning that the TiCl4 post-treatment wasn’t beneficial to improve the PEC performance of the compact TiO2 NRAs.

Fabrication of Morphology Controllable Vertically Aligned TiO<sub>2</sub> Nanorod Arrays for Dye-Sensitized Solar Cells

In this work, vertically aligned TiO2 nanorod arrays (NR) are synthesized directly on FTO coated glass substrate by hydrothermal method. The samples are characterized by XRD, SEM, TEM and the results indicate that the 1-D nanorods are of single crystal rutile structure with growth direction along the [001] direction. The morphology (diameter, thickness and density) of the nanorods can be adjusted by changing the precursor amounts. The possible growth mechanism of TiO2 nanorods on FTO substrate has also been briefly discussed in this work. For dye-sensitized solar cells (DSSCs) fabricated of different thickness nanorods, a power conversion efficiency (PCE) of 1.74% has been achieved by using ~3μm nanorod arrays under simulated AM 1.5 illumination (100 mW cm-2). It is expected that the 1-D nanorods can be composited with other nanomaterial of different structures and morphologies to enhance the efficiency of DSSCs.

Conductivity improvement of CuCrO2 nanoparticles by Zn doping and their application in solid-state dye-sensitized solar cells

Undoped and Zn-doped CuCrO2 nanoparticles were synthesized by sol–gel method as promising wide band gap p-type semiconductor materials for solid-state dye-sensitized solar cells (DSSCs). We studied the influence of Zn dopant concentration on structural, electrical and optical properties of CuCrO2 nanoparticles. The X-ray diffraction data indicated that the delafossite-to-spinel ferrite phase transition occurs by increasing the amount of Zn doping. The average nanoparticle size was determined about 40nm. A minimum value of electrical resistivity of 5.7Ωcm was obtained for doping concentration of 5%. Having optimized the Zn-doped CuCrO2 nanoparticles, solid-state DSSCs were fabricated using undoped and Zn-doped CuCrO2 (5%) as solid electrolytes. As the photoanode layer, the vertically aligned TiO2 nanorod arrays were grown on FTO glass using a hydrothermal method. Compared with undoped CuCrO2, the Zn-doped nanoparticles exhibited an improvement in photovoltaic properties. The overall efficiency enhancement of 39% was obtained for the dopant concentration of 5%. The improved power conversion efficiency is attributed to the lowered electrical resistivity and enlarged work function of Zn-doped CuCrO2 nanoparticles.

Controlled growth of vertically aligned TiO2 nanorod arrays using the improved hydrothermal method and their application to dye-sensitized solar cells

Well-aligned rutile TiO2 nanorod arrays were synthesized directly on FTO substrate by hydrothermal method. By controlling the growth parameters such as precursor concentration, acidity, growth temperature and growth time, we obtained various morphologies of nanorods. Optimization of the growth recipe leads to well-aligned vertical arrays of TiO2 nanorods with appropriate length to width ratio. Field-emission scanning electron microscopy (FESEM) and x-ray diffraction (XRD) are used to characterize TiO2 nanorods. Dye-sensitized solar cell (DSSC) assembled with the optimized nanorod arrays achieves an overall photoelectric conversion efficiency of 1.86%. Electrochemical impedance spectroscopy (EIS) measurement was applied to compare the electron transport and recombination properties of DSSCs using TiO2 nanorods with different length.

Enhancing efficiency of CdS/TiO2 nanorod arrays solar cell through improving the hydrophilicity of TiO2 nanorod surface

CdS nanoparticles with well-defined crystallinity were assembled on vertically aligned TiO2 nanorod arrays (TiO2 NRAs) to form CdS/TiO2 NTAs heterostructures by cyclic voltammetry electrochemical deposition. The morphology and structure of CdS/TiO2 NTAs heterostructure were investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results show that the amount of CdS nuclei on TiO2 surface and the bond between CdS nuclei and TiO2 nanorod were improved by increasing hydrophilicity of TiO2 nanorod surface, and well crystalline CdS nanocrystals were deposited on TiO2 nanorod (001) surface with good bonding between CdS nanoparticle and TiO2 nanorod. With increased hydrophilicity of TiO2 nanorod surface, the Jsc, Voc and τe of CdS/TiO2 NRAs heterostructure were increased. Especially, PEC (2.29%) was increased by near three times. This is because that CdS nanoparicles were uniformly dispersed on TiO2 NRAs and had good bond with TiO2 nanorods.

Synthesis and application of TiO2 single-crystal nanorod arrays grown by multicycle hydrothermal for dye-sensitized solar cells

TiO2 is a wide band gap semiconductor with important applications in photovoltaic cells. Vertically aligned TiO2 nanorod arrays (NRs) are grown on the fluorine-doped tin oxide (FTO) substrates by a multicycle hydrothermal synthesis process. The samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and selected-area electron diffraction (SAED). It is found that dye-sensitized solar cells (DSSCs) assembled by the as-prepared TiO2 single-crystal NRs exhibit different trends under the condition of different nucleation and growth concentrations. Optimum cell performance is obtained with high nucleation concentration and low growth cycle concentration. The efficiency enhancement is mainly attributed to the improved specific surface area of the nanorod.

Changes of Voc in Hybrid Solar Cells by TiO2 Nanoarray with Different Crystallinity of Shell

Polymer-inorganic hybrid solar cells with pure vertically aligned TiO2 nanorod arrays (TiO2-NRA) normally suffer from a rather low (<0.4 V) open-circuit voltage (Voc). Recently, it has been demonstrated that the Voc in polymer/TiO2-NRA solar cells can be improved to 0.7 V by formation of homostructured core-shell structures with single-crystalline TiO2 nanorod as core and the polycrystalline TiO2 quantum dots aggregation layer as shell. However, the Voc was not achieved high enough, and effects of different crystallinity of shell on device performance have not been investigated. In this paper, we obtained three types of homostructured TiO2 core-shell nanoarrays with different crystallinity of TiO2 shell (amorphous, amorphous&crystalline, and polycrystalline) through controlling shell reaction time. The changes of device performance (in particular, the Voc) in solar cells with these arrays are studied. Results show that the device Voc gradually decreases when crystallinity of shell changes from amorphous to crystalline. Moreover, the reason for a larger Voc by amorphous shell is analyzed. More dipoles could be produced at α-TiOx shell/polymer interface than crystalline TiO2 shell/polymer interface, which results in a further up-shifting conduction band edge in TiO2 nanorod core toward the local vacuum level of the polymer, along with a further enhanced Voc.

Growth of Aligned Single-Crystalline TiO2 Nanorod Arrays on Transparent Conducting Oxide Coated Glass

In this work, we report a simple hydrothermal method for growth of aligned TiO2 nanorod arrays directly on SnO2:F (FTO) coated glass substrates without any seed layer. The rectangular nanorod is single crystalline with a tetragonal rutile structure. The diameter, length and density of the nanorods are determined by the synthesis temperature. When synthesis temperature increases from 110 to 190 °C, the diameter increases from 53 to 249 nm, and the length increases from to 324 to 1904 nm. However, the highest density is obtained at 150 °C due to nuclei coalescence at the high synthesis temperature. The activation energy for formation of TiO2 nanorod arrays is 102 kJ/mol obtained by using an Arrhenius equation.

Synthesis and characterization of TiO2/CdS core–shell nanorod arrays and their photoelectrochemical property

TiO2/CdS core–shell nanorod arrays have been fabricated via a two-step method. Vertically aligned TiO2 nanorod arrays (NRs) were synthesized by a facile hydrothermal method, and followed by depositing CdS nanoparticles on TiO2 NRs by spin-coating successive ion layer adsorption and reaction (spin-SILAR) method. The surface morphology, structure, optical and photoelectrochemical behaviors of the core–shell NRs films are considered. The UV–vis absorption spectrum results suggested that the absorption peak of the TiO2/CdS core–shell NRs shifts from the ultraviolet region to the visible region in comparison to that of the pure TiO2 NRs. The obviously enhanced photoelectrochemical (PEC) performances of the heterojunction NRs were found under illumination of the simulated sunlight in comparison with that of the TiO2 NRs. The enhanced PEC performance and formation mechanism of TiO2/CdS core–shell NRs were discussed in detail.

Branched TiO2 Nanorods Covered with TiO2 Nanosheets for Harvesting Solar Energies in Dye-Sensitized Solar Cells

A hierarchical structure of branched TiO2 nanorods covered with TiO2 nanosheets was designed and constructed for harvesting solar energies in dye-sensitized solar cells. Vertical aligned TiO2 nanorod arrays were synthesized on fluorine-doped SnO2 (FTO) by using the electrochemically deposited ZnO nanorod arrays as a template and liquid phase deposition (LPD) method. The resultant TiO2 nanorod was used as a seed and the rutile TiO2 nanorods were firstly synthesized on the surface of anatase TiO2 nanorod by using hydrothermal method. In photocurrent-voltage measurements, 3.56% efficiency was achieved for 1.5um thick nanosheets coated TiO2 nanobranch (NB), which is about 239% higher than the original TiO2 nanorod arrays.

Building three-dimensional Pt catalysts on TiO2 nanorod arrays for effective ethanol electrooxidation

Pt nanoparticles on TiO2 nanorod arrays have been prepared and used for the electrooxidation of ethanol in acidic and alkaline media. The vertically aligned TiO2 nanorod arrays provide a three-dimensional frame for supporting Pt nanoparticles. FESEM and TEM show that the Pt nanoparticles about 3nm in diameter are evenly distributed on the surface of TiO2 nanorods. Cyclic voltammetry and chronoamperometry are performed to investigate electrochemical properties of Pt/TiO2/Ti, Pt/C, and Pt/Ti electrocatalysts. The Pt/TiO2/Ti electrocatalyst shows better catalytic activity and higher catalytic stability compared with other electrocatalysts.

Vertically Aligned TiO2 Nanorod Arrays as a Steady Light Sensor

Vertically aligned nanorods have been prepared directly on Ti foil through a one-step environmentally friendly and template-free solution method. X-ray diffraction, field-emission-scanning electron microscopy, and energy-dispersive X-ray spectroscopy have been employed to characterize the synthesized samples. The control of morphology and size of the nanorods can be achieved by varying the solution concentration, hydrothermal temperature, and synthesis time. The influence of nanorod size on the conductivity has been investigated by electrochemical impedance spectroscopy, and the results indicate that the electrical conductivity of thin nanorod arrays is better than that of thick nanorod arrays. Meanwhile, the electrode based on the thin nanorod arrays has shown a highly sensitive and steady photocurrent response under simulated sunlight and UV illumination, which indicates that the nanorod arrays could be a candidate for use in optoelectronic sensor.

The effect of Ag nanoparticle loading on the photocatalytic activity of TiO2 nanorod arrays

Ag nanoparticles with different loadings were systematically coated onto uniformly aligned TiO2 nanorod arrays. The enhanced photocatalytic decay rates were found to increase linearly with the Ag nanoparticle load until a maximum photodecay rate was reached at 0.25 wt.% Ag loading, and then decrease almost linearly with further increasing Ag loading. A simple reaction model based on the Ag nanoparticle coverage and charge trapping is proposed, and the predicted results agree very well with the experimental results quantitatively. The effective lifetime of photogenerated holes due to the trapping of the Ag nanoparticles on the surface of TiO2 nanorods was estimated to be approximately three times that of pure TiO2 nanorods.

Photoelectrochemical Water Splitting Using Dense and Aligned TiO2 Nanorod Arrays

Dense and aligned TiO2 nanorod arrays are fabricated using oblique-angle deposition on indium tin oxide (ITO) conducting substrates. The TiO2 nanorods are measured to be 800-1100 nm in length and 45-400 nm in width with an anatase crystal phase. Coverage of the ITO is extremely high with 25 x 10(6) mm(-2) of the TiO2 nanorods. The first use of these dense TiO2 nanorod arrays as working electrodes in photoelectrochemical (PEC) cells used for the generation of hydrogen by water splitting is demonstrated. A number of experimental techniques including UV/Vis absorption spectroscopy, X-ray diffraction, high-resolution scanning electron microscopy, energy-dispersive X-ray spectroscopy, and photoelectrochemistry are used to characterize their structural, optical, and electronic properties. Both UV/Vis and incident-photon-to-current-efficiency measurements show their photoresponse in the visible is limited but with a marked increase around approximately 400 nm. Mott-Schottky measurements give a flat-band potential (V(FB)) of +0.20 V, a carrier density of 4.5 x 10(17) cm(-3), and a space-charge layer of 99 nm. Overall water splitting is observed with an applied overpotential at 1.0 V (versus Ag/AgCl) with a photo-to-hydrogen efficiency of 0.1%. The results suggest that these dense and aligned one-dimensional TiO2 nanostructures are promising for hydrogen generation from water splitting based on PEC cells.