Photochemical properties of CeO2-coated ZnO nanorods

Well-aligned ZnO nanorods were deposited by a mild hydrothermal process and coated with nanosized CeO2 particles (approximately 5 nm) by an oxidative-soak-coating method at 45 degrees C. The low growth temperature proved useful in avoiding interfacial reaction between the two phases. Correlation of photoluminescence results indicated that the defects responsible for the deep level emission (DLE) from ZnO were largely located at the surface. The CeO2 coating reduced the DLE but also the photocatalytic activity as surficial hydroxyl groups were involved in the nucleation of the CeO2 phase and thus not available for absorption of the methylene blue species for degradation. Still, CeO2 coated ZnO nanorods retained their photocatalytic ability and could be useful as bifunctional catalyst to treat multiple contaminants simultaneously.     

Chemical synthesis of CdS onto TiO2 nanorods for quantum dot sensitized solar cells

A quantum dot sensitized solar cell (QDSSC) is fabricated using hydrothermally grown TiO₂ nanorods and successive ionic layer adsorption and reaction (SILAR) deposited CdS. Surface morphology of the TiO₂ films coated with different SILAR cycles of CdS is examined by Scanning Electron Microscopy which revealed aggregated CdS QDs coverage grow on increasing onto the TiO₂ nanorods with respect to cycle number. Under AM 1.5G illumination, we found the TiO₂/CdS QDSSC photoelectrode shows a power conversion efficiency of 1.75%, in an aqueous polysulfide electrolyte with short-circuit photocurrent density of 4.04 mA/cm² which is higher than that of a bare TiO₂ nanorods array.     

Enhancing the performance of dye-sensitized solar cells by ZnO nanorods/ZnO nanoparticles composite photoanode

In this paper, ZnO nanorods (NRs) were prepared by a two-step solution phase reaction. A composite photoanode architecture is fabricated by adding 0–0.20 at.% ZnO NRs into ZnO nanoparticles (NPs). The scanning electron microscopy image shows that the average diameter and length of the ZnO NRs are about 50 nm and 2–5 µm, respectively, and the ZnO NRs are uniformly embedded into the ZnO NPs photoanode. The UV–vis spectrum analysis reveals that the amount of dye adsorption of the composite photoanode decreases with increasing ZnO NRs content. Meanwhile, the influence of ZnO NRs contents on the dye-sensitized solar cells (DSSCs) performance is systematically investigated. The photocurrent density–voltage (J–V) characteristics reveal that the device performance of DSSCs can be significantly enhanced by the composite photoanode. Typically, the DSSC with 0.15 at.% ZnO NRs obtains the optimal energy conversion efficiency of 3.8%, which is 28.4% higher than that of the pristine ZnO DSSCs. The electrochemical impedance spectroscopy (EIS) analysis shows that ZnO NRs can provide a direct pathway for accelerating electron transport, extending the electron lifetime, suppressing electron recombination and improving electron collection efficiency. These results indicate that the incorporation of ZnO NRs in the photoanode is an effective way to improve the performance of DSSCs.     

Sensitization of ZnO nanorods with CdSe quantum dots

We have optimized the low-temperature growth of aligned arrays of zinc oxide nanorods of controlled length and diameter on conductive substrates. Varying the solution concentration and growth time, we were able to tune the nanorod diameter and length in the ranges 40–600 nm and 0.5–15 μm, respectively. The grown zinc oxide nanorods were photosensitized with CdSe quantum dots (QDs) in an oleic shell, which was replaced by pyridine. We studied the optical and transport properties of the ZnO nanorod arrays, with and without CdSe QDs on their surface. The current-voltage characteristics of the ZnO nanorod arrays with CdSe QDs are significantly influenced by illumination with light at a wavelength under the absorption band of the QDs, which points to effective interaction between the QDs and ZnO matrix.     

Electrospun TiO2-MWCNTs nanofibers as photoanode in dye-sensitized solar cell (DSSC)

Electrospinning process was used to fabricate hybrid TiO₂ nanofibrous membrane containing multi-walled carbon nanotubes (MWCNTs). The MWCNTs treated with plasma modification as established in our previous studies are dispersed in the mixture of titanium (IV) isopropoxide and poly(methyl methacrylate) in N,N-dimethylformamide prior to electrospinning. Diameter of the calcined TiO₂-MWCNTs nanofibers (NFs) ranged from 100 to 200 nm, and transmission electron microscopy shows that the MWCNTs are both embedded and lying externally on the NFs. Photoanodes for dye-sensitized solar cells are prepared by first conglutinating the nanofibrous membranes onto conducting glass substrate under methanol vapor treatment followed by calcination and dye sensitization. The NFs exhibit improved conducting behavior (from 10^?8 to 10^?6 S/m) with small quantity (0.5–1.5 wt%) of MWCNTs. An optimum addition of 1.0 wt% MWCNTs into the TiO₂ nanofibrous membrane improves the overall solar conversion efficiency by 47 % with significant increase in the short-circuit photocurrent. Electrochemical impedance spectroscopy and intensity-modulated photocurrent/photovoltage spectroscopy analyses reveal that the enhanced electron transport with smaller resistance is responsible for the improved cell performance. The results suggest that the conducting properties of the MWCNTs are crucial for faster transport of photogenerated electrons, hence retarding charge recombination that could result in poor conversion efficiency.     

Chemical bath deposition of ZnO nanorods for dye sensitized solar cell applications

ZnO nanorods using various molar concentrations have been synthesized through the chemical bath deposition method. X-ray diffraction result shows that the ZnO nanorods are of hexagonal structure. The morphology of the ZnO nanorods has been examined by scanning electron microscopy. The ZnO nanorods have diameters ranging from 100 to 200 nm and length of 1–3 μm. Dye-sensitized solar cells have been assembled by using ZnO nanorod film photoelectrode sensitized using natural dye extracted from lantana camara as sensitizer. The ZnO nanorods have been used as electrode material to fabricate dye sensitized solar cells which exhibited an efficiency of 0.71 %, the maximum efficiency was obtained for films deposited for 0.07 M concentration.     

ZnO hollow quantum dot: a promising deep-UV light emitter

We establish an analytic model to illustrate the energy bandgap of ZnO hollow quantum dots (HQDs) with negative curvature surface from the perspective of nanothermodynamics. It was found that the bandgap of ZnO HQDs shows a pronounced blue-shift as comparable to those of bulk counterpart and free nanocrystals. Furthermore, the photoelectric properties of ZnO HQDs can be effectively modulated by three independent dimensions, including the outer surface, the inner surface and the shell thickness. Strikingly, the emission wavelength of ZnO HQDs can be extended into the deep-ultraviolet (DUV) region, which suggests this kind of nanostructure could be expected to be applicable for the new-generation, compact, and environmentally friendly alternative DUV light emitter.     

Evaluation of all-inorganic CdSe quantum dot thin films for optoelectronic applications

Exchanging the original organic ligands of colloidal CdSe quantum dots (QDs) with inorganic metal chalcogenide SnS(4) ligands resulted in absorption peak redshifts and complete photoluminescence (PL) quenching in QD solids. The SnS(4)-capped QDs, meanwhile, were able to retain strong excitonic absorption. After the ligand exchange, the ITO/QDs/Al structure showed much higher electrical conductivity and reduced space-charge limited current. Its photocurrent spectral response increased by over two orders of magnitude and closely resembled the absorption spectrum of the QDs. However, it was found that mild thermal treatment above 200 °C transformed the SnS(4)-capped QD film into to a more conductive assembly, degrading its absorption and photocurrent generation. These results suggest that the inorganic ligands considerably enhanced the inter-dot electronic coupling in QD solids, leading to facile charge separation and transport. Our study thus demonstrates the potential applicability of colloidal QDs with metal chalcogenide ligands processed at low temperatures for efficient photodetection and solar energy conversion.     

Performance of Eu2O3 coated ZnO nanoparticles-based DSSC

To provide an inherent energy barrier between the electrode and electrolyte interface, the surface of the ZnO nanoparticles has been modified by Eu₂O₃ layer. The synthesis of ZnO, Eu₂O₃ coated ZnO nanoparticles have been carried out by chemical precipitation method and solvothermal treatment. The synthesized samples were characterized by XRD and the diffraction plane (222) of Eu₂O₃ detected, demonstrating the existence of Eu₂O₃ on the surface of ZnO, which is further verified using energy dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy. The strong quenching in photoluminescence emission, in case of Eu₂O₃/ZnO nanoparticles, has been attributed to the decrease in recombination rate of photo-generated electron–hole pairs. Compared to ZnO electrodes, Eu₂O₃ coated ZnO electrodes adsorbed more dye. The photoelectrochemical properties of the Eu₂O₃/ZnO electrodes have been found to improve and the energy conversion efficiency increase from 0.44 to 1.45 % under the illumination of simulated light of 100 mW/cm².     

Quantum dot size dependent J-V characteristics in heterojunction ZnO/PbS quantum dot solar cells

The current-voltage (J-V) characteristics of ZnO/PbS quantum dot (QD) solar cells show a QD size-dependent behavior resulting from a Schottky junction that forms at the back metal electrode opposing the desirable diode formed between the ZnO and PbS QD layers. We study a QD size-dependent roll-over effect that refers to the saturation of photocurrent in forward bias and crossover effect which occurs when the light and dark J-V curves intersect. We model the J-V characteristics with a main diode formed between the n-type ZnO nanocrystal (NC) layer and p-type PbS QD layer in series with a leaky Schottky-diode formed between PbS QD layer and metal contact. We show how the characteristics of the two diodes depend on QD size, metal work function, and PbS QD layer thickness, and we discuss how the presence of the back diode complicates finding an optimal layer thickness. Finally, we present Kelvin probe measurements to determine the Fermi level of the QD layers and discuss band alignment, Fermi-level pinning, and the V(oc) within these devices.     

Array of ZnO nanoparticle-sensitized ZnO nanorods for UV photodetection

Vertically aligned ZnO nanorod (NR) arrays have been successfully synthesized on ITO-glass substrate by hydrothermal growth. Chemical bath deposition method was used to deposit ZnO nanoparticles (NPs) onto the ZnO NRs. These structures were applied in fabricating ZnO NPs sensitized ultraviolet (UV) photodetectors (PDs). Incorporation of ZnO NPs onto ZnO NRs results in distinct improvement of optical properties of ZnO NRs, i.e., significant enhancement of emission as well as effective suppression of defects emission in ZnO. Furthermore, there is a noticeable blue-shift in absorption spectra compared with that of ZnO NRs structure. I–V characteristics show that the sensitized structure improved photocurrent almost twice that of unsensitized ZnO NRs. Consequently, these findings may open new opportunities for the integration of different ZnO nanostructures for application in UV region particularly fabrication of UV PDs.     

Structural and antireflective properties of ZnO nanorods synthesized using the sputtered ZnO seed layer for solar cell applications

We report the structural and antireflective properties of ZnO nanorod arrays (NRAs) on silicon (Si) substrate by wet chemical growth using the sputtered ZnO seed layer for solar cell applications. The size, height, shape, and number of ZnO nanorods depend strongly on the ZnO seed layer thickness as well as the molar zinc nitrate concentration. Clearly, the ZnO nanorods are of wurzite crystal structure from the X-ray diffraction analysis. To achieve the low reflectance over a wide wavelength range, the ZnO seed layer thickness, molar concentration, and growth time are optimized. It is found that the specular reflection spectrum of ZnO NRAs is closely related to the ZnO seed layer thickness. The solar weighted reflectance, Rw, of ZnO NRAs as antireflection coatings for Si solar cells is estimated under AM1.5 g illumination. For ZnO NRAs with 50 nm ZnO seed layer in 10 mM aqueous solution for 12 hours, the low specular reflectance (i.e., <7%) is obtained at wavelengths of 300-1200 nm, indicating a low Rw of 3.86%.     

Modification of ZnO nanorods through Au nanoparticles surface coating for dye-sensitized solar cells applications

Dye-sensitized solar cells (DSSC) based on ZnO nanorods were fabricated and modified through the addition of Au nanoparticles. The as-synthesized ZnO nanorods were well-dispersed and of high crystallinity quality leading to a high cell efficiency of 5.2%. On the other hand, thick layer of Au nanoparticles aggregation may have led to distortion of plasmonic effect. Also, the addition of Au nanoparticles have effectively decreased the surface area of ZnO nanorods with direct contact to the dye molecules, resulting in a lower amount of adhered dye molecules to convert sunlight. The electrons generated by the photo-absorption through thick aggregated Au nanoparticles layer may have a lower injection rate to ZnO nanorods as compared to those absorbed by the dye.     

Detecting single quantum dot motion with nanometer resolution for applications in cell biology

Quantum dots (QDs), semiconductor particles of nanometer dimension, have emerged as excellent fluorescent analogs in tracer experiments with single molecule sensitivity for bioassays. Cell imaging greatly benefits from the remarkable optical and physical properties of these inorganic nanocrystals: QDs are much brighter and exhibit a higher resistance to photobleaching than traditional fluorophores, and their narrow emission spectrum and flexible surface chemistry make them particularly suitable for multiplex imaging. Here, we have demonstrated the achievement of a nanometer spatial resolution on the position of a single QD in a simple optomechanical instrument using a high-sensitivity low-noise detector, an intensified CCD camera. Furthermore, nanometer variations in the amplitude of a QD's sinusoidal oscillations could be quantitatively distinguished after fast Fourier transform (FFT) based data processing. As confirmed by experiments where QDs were attached to the surface of bovine aortic endothelial cells, this method can be exploited in biology to assess molecular and subcellular contributions to responses such as motility, intracellular trafficking, and mechanotransduction, with high resolution and minimal disturbance to cells     

Synthesis of 1-D ZnO nanorods and polypyrrole/1-D ZnO nanocomposites for photocatalysis and gas sensor applications

1-D ZnO nanorods and PPy/1-D ZnO nanocomposites were prepared by the surfactant-assisted precipitation and in situ polymerization method, respectively. The synthesized nanorods and nanocomposites were characterized by UV–Vis spectrophotometer, Fourier transform-infrared spectroscopy (FTIR), X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM), which gave the evidence of 1-D ZnO nanorods, polymerization of pyrrole monomer and strong interaction between PPy and 1-D ZnO nanorods, respectively. Photocatalytic activity of 1-D ZnO nanorods was conducted by 3³ level full-factorial design to evaluate the effect of three independent process variables viz., dye concentration (crystal violet), catalyst concentration (1-D ZnO nanorods) and the reaction time on the preferred response: photodegradation efficiency (%). The PPy/1-D ZnO nanocomposites were used for the sensing of NH₃, LPG, CO₂ and H₂S gases, respectively, at room temperature. It was observed that PPy/1-D ZnO nanocomposites with different 1-D ZnO nanorod weight ratios (15 and 25%) had better selectivity and sensitivity towards NH₃ at room temperature.     

Synthesis of K-doped p-type ZnO nanorods along (100) for ferroelectric and dielectric applications

Urchin-like p-type ZnO nanorods were grown along preferred (100) direction by low temperature solution technique and subjected to morphological, structural, Hall conductivity, dielectric and ferroelectric characterization. Hall voltage, bulk carrier density (hole) and mobility were found to be 0.058 V, 2.36 × 10¹⁹ cm⁻³ and 0.025 cm²/V s, respectively. In the temperature variation of the dielectric constant a phase transition at 343 K was observed at various frequencies. The piezoelectric charge coefficient (d₃₃) was found to be 1.60 pC/N. In the ferroelectric hysteresis loop studies, ZnO exhibited remnant polarization and coercive field at 0.083 µC/cm² and 3.86 kV/cm, respectively.