Photosensitization of TiO2 nanorods with CdS quantum dots for photovoltaic devices

CdS quantum dots (QDs) coated TiO₂ nanorod arrays have been prepared via a two-step method. TiO₂ nanorod arrays were synthesized by a facile hydrothermal method, and CdS QDs were deposited on the nanorods by a sequential chemical bath deposition (S-CBD) technique. The surface morphology, structure, optical and photoelectrochemical behaviors of the core-shell nanorod array films are considered. A photocurrent of 2.5 mA/cm², an open circuit photovoltage of 1.10 V, and a conversion efficiency of 1.91% were obtained under an illumination of 100 mW/cm², when the CdS QDs deposited on TiO₂ nanorods film for about 7 cycles. The results demonstrate that the composite films are of excellence with respect to photovoltaic conversion.     

CdS quantum dot-sensitized solar cells based on nano-branched TiO2 arrays

Nano-branched rutile TiO₂ nanorod arrays were grown on F:SnO₂ conductive glass (FTO) by a facile, two-step wet chemical synthesis process at low temperature. The length of the nanobranches was tailored by controlling the growth time, after which CdS quantum dots were deposited on the nano-branched TiO₂ arrays using the successive ionic layer adsorption and reaction method to make a photoanode for quantum dot-sensitized solar cells (QDSCs). The photovoltaic properties of the CdS-sensitized nano-branched TiO₂ solar cells were studied systematically. A short-circuit current intensity of approximately 7 mA/cm² and a light-to-electricity conversion efficiency of 0.95% were recorded for cells based on optimized nano-branched TiO₂ arrays, indicating an increase of 138% compared to those based on unbranched TiO₂ nanorod arrays. The improved performance is attributed to a markedly enlarged surface area provided by the nanobranches and better electron conductivity in the one-dimensional, well-aligned TiO₂ nanorod trunks.     

Synthesis and photoelectrochemical response of CdS quantum dot-sensitized TiO2 nanorod array photoelectrodes

A continuous and compact CdS quantum dot-sensitive layer was synthesized on TiO₂ nanorods by successive ionic layer adsorption and reaction (SILAR) and subsequent thermal annealing. The thickness of the CdS quantum dot layer was tuned by SILAR cycles, which was found to be closely related to light absorption and carrier transformation. The CdS quantum dot-sensitized TiO₂ nanorod array photoelectrodes were characterized by scanning electron microscopy, X-ray diffraction, ultraviolet–visible absorption spectroscopy, and photoelectrochemical property measurement. The optimum sample was fabricated by SILAR in 70 cycles and then annealed at 400°C for 1 h in air atmosphere. A TiO₂/CdS core-shell structure was formed with a diameter of 35 nm, which presented an improvement in light harvesting. Finally, a saturated photocurrent of 3.6 mA/cm² was produced under the irradiation of AM1.5G simulated sunlight at 100 mW/cm². In particular, the saturated current density maintained a fixed value of approximately 3 mA/cm² without decadence as time passed under the light conditions, indicating the steady photoelectronic property of the photoanode.     

Capability of coupled CdSe/TiO2 heterogeneous structure for photocatalytic degradation and photoconductivity

Highly ordered TiO₂ nanotube arrays (TiO₂-NTAs), with a uniform tube size on titanium substrate, were obtained by means of reoxidation and annealing. A composite structure, CdSe quantum dots@TiO₂ nanotube arrays (CdSe QDs@TiO₂-NTAs), was fabricated by assembling CdSe quantum dots into TiO₂-NTAs via cyclic voltammetry electrochemical deposition. The X-ray diffractometer (XRD), field-emission scanning electron microscope (SEM), and transmission electron microscope (TEM) were carried out for the determination of the composition and structure of the tubular layers. Optical properties were investigated by ultraviolet-visible spectrophotometer (UV-Vis). Photocurrent response under visible light illumination and photocatalytic activity of samples by degradation of methyl orange were measured. The results demonstrated that the photo absorption of the composite film shifted to the visible region, and the photocurrent intensity was greatly enhanced due to the assembly of CdSe QDs. Especially, photocurrent achieved a maximum of 1.853 μA/cm² after five voltammetry cycles of all samples. After irradiation under ultra violet-visible light for 2 h, the degradation rate of composition to methyl orange (MO) reached 88.20%, demonstrating that the CdSe QDs@TiO₂-NTAs exhibited higher photocatalytic activity.     

CdS Quantum Dot‐Sensitized Vertical TiO2 Nanorod Arrays by a Simple Linker‐Assisted SILAR Method

The homogeneously deposition of quantum dots (QDs) on the mesoporous or one‐dimensional TiO₂ film with a well‐covered layer has become one major challenge in quantum dot‐sensitized solar cell (QDSSC). In this study, one‐dimensional TiO₂ nanorod arrays based photoanode prepared by a simple linker‐assisted SILAR method is studied. The influence of the short organic ligand (TGA) on the homogeneity of QDs on TiO₂ nanorod arrays is discussed and the photoelectrochemical performance of QDSSC is evaluated by UV–vis absorbance spectroscopy, current–voltage performance and electrochemical impedance spectroscopy. The results show that the TGA ligand can act as an efficient bridge between CdS QDs and TiO₂, whereas the content and particle size of the QDs can be easily tuned by controlling the dipping time and SILAR cycle. Owing to the presence of TGA, the charge‐transfer rate at CdS QDs and TiO₂ interfacial region is noticeable enhanced.     

Efficient perovskite solar cells based on low-temperature solution-processed (CH3NH3)PbI3 perovskite/CuInS2 planar heterojunctions

In this work, the solution-processed CH₃NH₃PbI₃ perovskite/copper indium disulfide (CuInS₂) planar heterojunction solar cells with Al₂O₃ as a scaffold were fabricated at a temperature as low as 250°C for the first time, in which the indium tin oxide (ITO)-coated glass instead of the fluorine-doped tin oxide (FTO)-coated glass was used as the light-incidence electrode and the solution-processed CuInS₂ layer was prepared to replace the commonly used TiO₂ layer in previously reported perovskite-based solar cells. The influence of the thickness of the as-prepared CuInS₂ film on the performance of the ITO/CuInS₂(n)/Al₂O₃/(CH₃NH₃)PbI₃/Ag cells was investigated. The ITO/CuInS₂(2)/Al₂O₃/(CH₃NH₃)PbI₃/Ag cell showed the best performance and achieved power conversion efficiency up to 5.30%.     

In situ growth of CuInS2 nanocrystals on nanoporous TiO2 film for constructing inorganic/organic heterojunction solar cells

Inorganic/organic heterojunction solar cells (HSCs) have attracted increasing attention as a cost-effective alternative to conventional solar cells. This work presents an HSC by in situ growth of CuInS₂(CIS) layer as the photoabsorption material on nanoporous TiO₂ film with the use of poly(3-hexylthiophene) (P3HT) as hole-transport material. The in situ growth of CIS nanocrystals has been realized by solvothermally treating nanoporous TiO₂ film in ethanol solution containing InCl₃ · 4H₂O, CuSO₄ · 5H₂O, and thioacetamide with a constant concentration ratio of 1:1:2. InCl₃ concentration plays a significant role in controlling the surface morphology of CIS layer. When InCl₃ concentration is 0.1 M, there is a layer of CIS flower-shaped superstructures on TiO₂ film, and CIS superstructures are in fact composed of ultrathin nanoplates as ‘petals’ with plenty of nanopores. In addition, the nanopores of TiO₂ film are filled by CIS nanocrystals, as confirmed using scanning electron microscopy image and by energy dispersive spectroscopy line scan analysis. Subsequently, HSC with a structure of FTO/TiO₂/CIS/P3HT/PEDOT:PSS/Au has been fabricated, and it yields a power conversion efficiency of 1.4%. Further improvement of the efficiency can be expected by the optimization of the morphology and thickness of CIS layer and the device structure.     

Self-ordered TiO2 quantum dot array prepared via anodic oxidation

The template-based methods belong to low-cost and rapid preparation techniques for various nanostructures like nanowires, nanotubes, and nanodots or even quantum dots [QDs]. The nanostructured surfaces with QDs are very promising in the application as a sensor array, also called ''fluorescence array detector.'' In particular, this new sensing approach is suitable for the detection of various biomolecules (DNA, proteins) in vitro (in clinical diagnostics) as well as for in vivo imaging.The paper deals with the fabrication of TiO₂ planar nanostructures (QDs) by the process of titanium anodic oxidation through an alumina nanoporous template on a silicon substrate. Scanning electron microscopy observation showed that the average diameter of TiO₂ QDs is less than 10 nm. Raman spectroscopic characterization of self-organized titania QDs confirmed the presence of an anatase phase after annealing at 400°C in vacuum. Such heat-treated TiO₂ QDs revealed a broad emission peak in the visible range (characterized by fluorescence spectroscopy).     

Improving the performance of dye-sensitized solar cells with TiO2/graphene/TiO2 sandwich structure

This study investigates the extent to which the TiO₂/graphene/TiO₂ sandwich structure improves the performance of dye-sensitized solar cells (DSSCs) over that of DSSCs with the traditional structure. Studies have demonstrated that the TiO₂/graphene/TiO₂ sandwich structure effectively enhances the open circuit voltage (V_oc), short-circuit current density (J_sc), and photoelectrical conversion efficiency (η) of DSSCs. The enhanced performance of DSSCs with the sandwich structure can be attributed to an increase in electron transport efficiency and in the absorption of light in the visible range. The DSSC with the sandwich structure in this study exhibited a V_oc of 0.6 V, a high J_sc of 11.22 mA cm^-2, a fill factor (FF) of 0.58, and a calculated η of 3.93%, which is 60% higher than that of a DSSC with the traditional structure.     

Controlled fabrication of Sn/TiO2 nanorods for photoelectrochemical water splitting

In this work, we investigate the controlled fabrication of Sn-doped TiO₂ nanorods (Sn/TiO₂ NRs) for photoelectrochemical water splitting. Sn is incorporated into the rutile TiO₂ nanorods with Sn/Ti molar ratios ranging from 0% to 3% by a simple solvothermal synthesis method. The obtained Sn/TiO₂ NRs are single crystalline with a rutile structure. The concentration of Sn in the final nanorods can be well controlled by adjusting the molar ratio of the precursors. Photoelectrochemical experiments are conducted to explore the photocatalytic activity of Sn/TiO₂ NRs with different doping levels. Under the illumination of solar simulator with the light intensity of 100 mW/cm², our measurements reveal that the photocurrent increases with increasing doping level and reaches the maximum value of 1.01 mA/cm² at −0.4 V versus Ag/AgCl, which corresponds to up to about 50% enhancement compared with the pristine TiO₂ NRs. The Mott-Schottky plots indicate that incorporation of Sn into TiO₂ nanorod can significantly increase the charge carrier density, leading to enhanced conductivity of the nanorod. Furthermore, we demonstrate that Sn/TiO₂ NRs can be a promising candidate for photoanode in photoelectrochemical water splitting because of their excellent chemical stability.     

Fe effect on the optical properties of TiO2:Fe2O3 nanostructured composites supported on SiO2 microsphere assemblies

The effect of Fe ion concentration on the morphological, structural, and optical properties of TiO₂ films supported on silica (SiO₂) opals has been studied. TiO₂:Fe₂O₃ films were prepared by the sol-gel method in combination with a vertical dip coating procedure; precursor solutions of Ti and Fe were deposited on a monolayer of SiO₂ opals previously deposited on a glass substrate by the same procedure. After the dip coating process has been carried out, the samples were thermally treated to obtain the TiO₂:Fe₂O₃/SiO₂ composites at the Fe ion concentrations of 1, 3, and 5 wt%. Scanning electron microscopy (SEM) micrographs show the formation of colloidal silica microspheres of about 50 nm diameter autoensembled in a hexagonal close-packed fashion. Although the X-ray diffractograms show no significant effect of Fe ion concentration on the crystal structure of TiO₂, the μ-Raman and reflectance spectra do show that the intensity of a phonon vibration mode and the energy bandgap of TiO₂ decrease as the Fe⁺³ ion concentration increases.     

3-D solar cells by electrochemical-deposited Se layer as extremely-thin absorber and hole conducting layer on nanocrystalline TiO2 electrode

Ag₂S quantum dots were deposited on the surface of TiO₂ nanorod arrays by a two-step photodeposition. The prepared TiO₂ nanorod arrays as well as the Ag₂S deposited electrodes were characterized by X-ray diffraction, scanning electron microscope, and transmission electron microscope, suggesting a large coverage of Ag₂S quantum dots on the ordered TiO₂ nanorod arrays. UV–vis absorption spectra of Ag₂S deposited electrodes show a broad absorption range of the visible light. The quantum dot-sensitized solar cells (QDSSCs) based on these electrodes were fabricated, and the photoelectrochemical properties were examined. A high photocurrent density of 10.25 mA/cm² with a conversion efficiency of 0.98% at AM 1.5 solar light of 100 mW/cm² was obtained with an optimal photodeposition time. The performance of the QDSSC at different incident light intensities was also investigated. The results display a better performance at a lower incident light level with a conversion efficiency of 1.25% at 47 mW/cm².     

Self-compliance RRAM characteristics using a novel W/TaO x /TiN structure

Self-compliance resistive random access memory (RRAM) characteristics using a W/TaO _x /TiN structure are reported for the first time. A high-resolution transmission electron microscope (HRTEM) image shows an amorphous TaO _x layer with a thickness of 7 nm. A thin layer of TiO _x N _y with a thickness of 3 nm is formed at the TaO _x /TiN interface, owing to the oxygen accumulation nature of Ti. This memory device shows 100 consecutive switching cycles with excellent uniformity, 100 randomly picked device-to-device good uniformity, and program/erase endurance of >10³ cycles. It is observed that the 0.6-μm devices show better switching uniformity as compared to the 4-μm devices, which is due to the thinner tungsten (W) electrode as well as higher series resistance. The oxygen-rich TaO _x layer at the W/TaO _x interface also plays an important role in getting self-compliance resistive switching phenomena and non-linear current-voltage (I-V) curve at low resistance state (LRS). Switching mechanism is attributed to the formation and rupture of oxygen vacancy conducting path in the TaO _x switching material. The memory device also exhibits long read endurance of >10⁶ cycles. It is found that after 400,000 cycles, the high resistance state (HRS) is decreased, which may be due to some defects creation (or oxygen moves away) by frequent stress on the switching material. Good data retention of >10⁴ s is also obtained.     

Simple fabrication of N-doped mesoporous TiO2 nanorods with the enhanced visible light photocatalytic activity

N-doped mesoporous TiO₂ nanorods were fabricated by a modified and facile sol–gel approach without any templates. Ammonium nitrate was used as a raw source of N dopants, which could produce a lot of gasses such as N₂, NO₂, and H₂O in the process of heating samples. These gasses were proved to be vitally important to form the special mesoporous structure. The samples were characterized by the powder X-ray diffraction, X-ray photoelectron spectrometer, nitrogen adsorption isotherms, scanning electron microscopy, transmission electron microscopy, and UV-visible absorption spectra. The average length and the cross section diameter of the as-prepared samples were ca. 1.5 μm and ca. 80 nm, respectively. The photocatalytic activity was evaluated by photodegradation of methylene blue (MB) in aqueous solution. The N-doped mesoporous TiO₂ nanorods showed an excellent photocatalytic activity, which may be attributed to the enlarged surface area (106.4 m² g^-1) and the narrowed band gap (2.05 eV). Besides, the rod-like photocatalyst was found to be easy to recycle.     

Enhanced resistive switching memory characteristics and mechanism using a Ti nanolayer at the W/TaOx interface

Enhanced resistive memory characteristics with 10,000 consecutive direct current switching cycles, long read pulse endurance of >10⁵ cycles, and good data retention of >10⁴ s with a good resistance ratio of >10² at 85°C are obtained using a Ti nanolayer to form a W/TiO_x/TaO_x/W structure under a low current operation of 80 μA, while few switching cycles are observed for W/TaO_x/W structure under a higher current compliance >300 μA. The low resistance state decreases with increasing current compliances from 10 to 100 μA, and the device could be operated at a low RESET current of 23 μA. A small device size of 150 × 150 nm² is observed by transmission electron microscopy. The presence of oxygen-deficient TaO_x nanofilament in a W/TiO_x/TaO_x/W structure after switching is investigated by Auger electron spectroscopy. Oxygen ion (negative charge) migration is found to lead to filament formation/rupture, and it is controlled by Ti nanolayer at the W/TaO_x interface. Conducting nanofilament diameter is estimated to be 3 nm by a new method, indicating a high memory density of approximately equal to 100 Tbit/in.².     

Effect of TiO2 nanotubes with TiCl4 treatment on the photoelectrode of dye-sensitized solar cells

In this study, we used the electrochemical anodization to prepare TiO₂ nanotube arrays and applied them on the photoelectrode of dye-sensitized solar cells. In the field emission scanning electron microscopy analysis, the lengths of TiO₂ nanotube arrays prepared by electrochemical anodization can be obtained with approximately 10 to 30 μm. After titanium tetrachloride (TiCl₄) treatment, the walls of TiO₂ nanotubes were coated with TiO₂ nanoparticles. XRD patterns showed that the oxygen-annealed TiO₂ nanotubes have a better anatase phase. The conversion efficiency with different lengths of TiO₂ nanotube photoelectrodes is 3.21%, 4.35%, and 4.34% with 10, 20, and 30 μm, respectively. After TiCl₄ treatment, the efficiency of TiO₂ nanotube photoelectrode for dye-sensitized solar cell can be improved up to 6.58%. In the analysis of electrochemical impedance spectroscopy, the value of R_k (charge transfer resistance related to recombination of electrons) decreases from 26.1 to 17.4 Ω when TiO₂ nanotubes were treated with TiCl₄. These results indicate that TiO₂ nanotubes treated with TiCl₄ can increase the surface area of TiO₂ nanotubes, resulting in the increase of dye adsorption and have great help for the increase of the conversion efficiency of DSSCs.     

Fabrication and NO2 gas sensing performance of TeO2-core/CuO-shell heterostructure nanorod sensors

TeO₂-nanostructured sensors are seldom reported compared to other metal oxide semiconductor materials such as ZnO, In₂O₃, TiO₂, Ga₂O₃, etc. TeO₂/CuO core-shell nanorods were fabricated by thermal evaporation of Te powder followed by sputter deposition of CuO. Scanning electron microscopy and X-ray diffraction showed that each nanorod consisted of a single crystal TeO₂ core and a polycrystalline CuO shell with a thickness of approximately 7 nm. The TeO₂/CuO core-shell one-dimensional (1D) nanostructures exhibited a bamboo leaf-like morphology. The core-shell nanorods were 100 to 300 nm in diameter and up to 30 μm in length. The multiple networked TeO₂/CuO core-shell nanorod sensor showed responses of 142% to 425% to 0.5- to 10-ppm NO₂ at 150°C. These responses were stronger than or comparable to those of many other metal oxide nanostructures, suggesting that TeO₂ is also a promising sensor material. The responses of the core-shell nanorods were 1.2 to 2.1 times higher than those of pristine TeO₂ nanorods over the same NO₂ concentration range. The underlying mechanism for the enhanced NO₂ sensing properties of the core-shell nanorod sensor can be explained by the potential barrier-controlled carrier transport mechanism.

PACS

61.46. + w; 07.07.Df; 73.22.-f     

Efficient PbS/CdS co-sensitized solar cells based on TiO2 nanorod arrays

Narrow bandgap PbS nanoparticles, which may expand the light absorption range to the near-infrared region, were deposited on TiO₂ nanorod arrays by successive ionic layer adsorption and reaction method to make a photoanode for quantum dot-sensitized solar cells (QDSCs). The thicknesses of PbS nanoparticles were optimized to enhance the photovoltaic performance of PbS QDSCs. A uniform CdS layer was directly coated on previously grown PbS-TiO₂ photoanode to protect the PbS from the chemical attack of polysulfide electrolytes. A remarkable short-circuit photocurrent density (approximately 10.4 mA/cm²) for PbS/CdS co-sensitized solar cell was recorded while the photocurrent density of only PbS-sensitized solar cells was lower than 3 mA/cm². The power conversion efficiency of the PbS/CdS co-sensitized solar cell reached 1.3%, which was beyond the arithmetic addition of the efficiencies of single constituents (PbS and CdS). These results indicate that the synergistic combination of PbS with CdS may provide a stable and effective sensitizer for practical solar cell applications.     

Structural and electrical characteristics of high-κ Er2O3 and Er2TiO5 gate dielectrics for a-IGZO thin-film transistors

In this letter, we investigated the structural and electrical characteristics of high-κ Er₂O₃ and Er₂TiO₅ gate dielectrics on the amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT) devices. Compared with the Er₂O₃ dielectric, the a-IGZO TFT device incorporating an Er₂TiO₅ gate dielectric exhibited a low threshold voltage of 0.39 V, a high field-effect mobility of 8.8 cm²/Vs, a small subthreshold swing of 143 mV/decade, and a high I_on/I_off current ratio of 4.23 × 10⁷, presumably because of the reduction in the oxygen vacancies and the formation of the smooth surface roughness as a result of the incorporation of Ti into the Er₂TiO₅ film. Furthermore, the reliability of voltage stress can be improved using an Er₂TiO₅ gate dielectric.     

Hydrothermal growth of TiO2 nanowire membranes sensitized with CdS quantum dots for the enhancement of photocatalytic performance

In this paper, TiO₂ nanowires (NWs) on Ti foils were prepared using a simple hydrothermal approach and annealing treatment. CdS quantum dots (QDs) were assembled onto the crystallized TiO₂ NWs by sequential chemical bath deposition. Ultraviolet-visible absorption spectra showed that CdS adds bands in the visible to the TiO₂ absorption and exhibited a broad absorption band in the visible region, which extended the scope of absorption spectrum and helped improve the photocatalytic degradation efficiency. The results of photocatalytic experiment revealed that CdS-TiO₂ NWs possessed higher photocatalytic activities toward methyl orange than pure TiO₂ nanowires. The degradation efficiency of 96.32% after ten cycles indicated that the as-prepared CdS-TiO₂ composite exhibited excellent long-time recyclable ability and can be reused for the degradation of contaminants.