Synthesis of defect-rich, (001) faceted-ZnO nanorod on a FTO substrate as efficient photocatalysts for dehydrogenation of isopropanol to acetone

Highly oriented ZnO nanorod was successfully synthesised on Ag nanoseed coated FTO substrate via a microwave hydrolysis approach. It was found that the morphology and the optical properties of the ZnO nanorod are strongly influenced by the power of the microwave irradiation used during the growth process. The aspect ratio of the nanorods changed from high to low with the increasing of microwave power. It was also found that the optical band gap of the ZnO nanorod red shifted with the increasing of the microwave power, reflecting an excellent tune ability of the optical properties of ZnO nanorods. The photocatalytic activity of these unique nanorod was evaluated by a dehydrogenation process of isopropanol to acetone in the presence of ZnO nanorod. It was found that the ZnO nanorod exhibited an excellent catalytic performance by showing an ability to accelerate the production of 0.031 mol L⁻¹ of acetone within only 35 min or 0.9 mmol L⁻¹ min⁻¹ from isopropyl alcohol dehydrogenation. It was almost no conversion from isopropyl alcohol when ZnO nanorods was absence during the reaction. In this report, a detailed mechanism of ZnO nanorod formation and the relationship between morphology and optical energy band gap are described.     

Utilization of surface plasmon resonance of Au/Pt nanoparticles for highly photosensitive ZnO nanorods network based plasmon field effect transistor

Hydrothermally processed highly photosensitive ZnO nanorods based plasmon field effect transistors (PFETs) have been demonstrated utilizing the surface plasmon resonance coupling of Au and Pt nanoparticles at Au/Pt and ZnO interface. A significantly enhanced photocurrent was observed due to the plasmonic effect of the metal nanoparticles (NPs). The Pt coated PFETs showed I_on/I_off ratio more than 3 × 10⁴ under the dark condition, with field-effect mobility of 26 cm² V⁻¹ s⁻¹ and threshold voltage of −2.7 V. Moreover, under the illumination of UV light (λ = 350 nm) the PFET revealed photocurrent gain of 10⁵ under off-state (−5 V) of operation. Additionally, the electrical performance of PFETs was investigated in detail on the basis of charge transfer at metal/ZnO interface. The ZnO nanorods growth temperature was preserved at 110 °C which allowed a low temperature, economical and simple method to develop highly photosensitive ZnO nanorods network based PFETs for large scale production.     

Optical and physical characteristics of In-doped ZnO nanorods

In this paper the effect of indium dopants on structure, optical, electrical and mechanical properties of ZnO nanorods are studied. The average surface potentials and the surface currents of ZnO:In nanorods were 0.25–0.84 mV and 2.2–200 MΩ-cm, respectively. The turn-on threshold field for the vertical ZnO nanorods was around 2–16 V μm⁻¹. Emission current densities of 3.3–911.4 mA cm⁻² were obtained for an electrical field of 60–160 V μm⁻¹. The photoluminescence (PL) spectrum measured at 15–300 K showed that the intensity of the peak at 2.06 eV increased with decreasing temperature, while the peak at 2.06 eV further red shifted and the peak at 3.39 eV blue shifted.     

ITO-free inverted organic solar cells fabricated with transparent and low resistance ZnO/Ag/ZnO multilayer electrode

Inverted organic solar cells (OSCs) based on poly (3-hexylthiophene) (P3HT):[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) bulk heterojunctions (BHJ) were fabricated with optimized ZnO/Ag/ZnO multilayer and conventional indium–tin oxide (ITO) cathode electrodes and their performance was compared. The ZnO/Ag/ZnO multilayer films showed sheet resistances in the range 3.6–3.9 Ω/sq, while ITO exhibited 14.2 Ω/sq. On the one hand, the carrier concentration gradually decreased from 1.74 × 10²² to 4.33 × 10²¹ cm⁻³ as the ZnO thickness increased from 8 to 80 nm, respectively. The transmittance of the ZnO(40 nm)/Ag(19 nm)/ZnO(40 nm) films was ∼95% at 550 nm, which is comparable to that of ITO (∼96%). The multilayer films were smooth with a root mean square (RMS) roughness of 0.81 nm. OSCs fabricated with the ZnO(40 nm)/Ag(19 nm)/ZnO(40 nm) film showed a power conversion efficiency (2.63%) comparable to that of OSCs with a conventional ITO cathode (2.71%).     

Fabrication and characteristics of photodiode based on ZnO nanowire arrays and spray-coated regioregular P3HT layers

A photodiode based on well-aligned ZnO nanowire arrays (ZNAs) and spray-coated regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) layers hybrid hetero junctions was fabricated, and its electrical characteristics in dark and under illumination with a solar simulator were investigated. Current–voltage (I–V) data of ITO/ZNAs/P3HT/Ag device in dark and under illumination showed typical diode characteristics. A rectification ratio (RR) of 22.7 at 1.7 V and a low turn-on voltage of 0.4 V in dark were obtained. Also, the photodiode with high photo-response in the order of 0.31 A/W at -2 V using 80 mW/cm² illumination power was observed. Upon increasing illumination power from 40 to 100 mW/cm², the RR value for the photodiode continuously was improved with a highest value of 12.5.     

The effect of Ag layer thickness on the properties of WO3/Ag/MoO3 multilayer films as anode in organic light emitting diodes

Transparent conductive WO₃/Ag/MoO₃ (WAM) multilayer electrodes were fabricated by thermal evaporation and the effects of Ag layer thickness on the optoelectronic and structural properties of multilayer electrode as anode in organic light emitting diodes (OLEDs) were investigated using different analytical methods. For Ag layers with thickness varying between 5 and 20 nm, the best WAM performances, high optical transmittance (81.7%, at around 550 nm), and low electrical sheet resistance (9.75 Ω/cm²) were obtained for 15 nm thickness. Also, the WAM structure with 15 nm of Ag layer thickness has a very smooth surface with an RMS roughness of 0.37 nm, which is suitable for use as transparent conductive anode in OLEDs. The current density?voltage?luminance (J?V?L) characteristics measurement shows that the current density of WAM/PEDOT:PSS/TPD/Alq₃/LiF/Al organic diode increases with the increase in thickness of Ag and WO₃/Ag (15 nm)/MoO₃ device exhibits a higher luminance intensity at lower voltage than ITO/PEDOT:PSS/TPD/Alq₃/LiF/Al control device. Furthermore, this device shows the highest power efficiency (0.31 lm/W) and current efficiency (1.2 cd/A) at the current density of 20 mA/cm², which is improved 58% and 41% compared with those of the ITO-based device, respectively. The lifetime of the WO₃/Ag (15 nm)/MoO₃ device was measured to be 50 h at an initial luminance of 50 cd/m², which is five times longer than 10 h for ITO-based device.     

Current transport mechanism and photovoltaic properties of photoelectrochemical cells of ITO/TiO2/PVC–LiClO4/graphite

This paper deals with the current transport mechanism of solid state photoelectrochemical cells of ITO/TiO₂/PVC–LiClO₄/graphite as well as the physical properties of a component of a device affecting its performance. The principle of operation and a schematic energy level diagram for the materials used in the photoelectrochemical cells are presented. The device makes use of ITO films, TiO₂ films, PVC–LiClO₄ and graphite films as photoanode, photovoltaic material, solid electrolyte and counter electrode, respectively. The device shows rectification. The J_sc and V_oc obtained at 100 mW cm⁻² were 0.95 μAcm⁻² and 180 mV, respectively.     

Controlled repeated chemical growth of ZnO films for dye-sensitized solar cells

The controlled growth of ZnO nanorods perpendicular to substrate surface i.e. c-axis by a repeated chemical deposition method for efficient dye-sensitized solar cell application is described. X-ray diffraction study shows the wurtzite structure of ZnO with high crystallinity. Intensity and newly evolved peaks of the ZnO are found to be thickness dependent. Dye loving flower-like globular architecture of ZnO is observed after 8 μm thickness. Dye-sensitized solar cell studies show the solar-to-electrical conversion efficiency of 2.21% for 11 μm ZnO electrode when illuminated with 80 mW/cm².     

Effect of GZO thickness and annealing temperature on the structural,electrical and optical properties of GZO/Ag/GZO sandwich films

The GZO/Ag/GZO sandwich films were deposited on glass substrates by RF magnetron sputtering of Ga-doped ZnO (GZO) and ion-beam sputtering of Ag at room temperature. The effect of GZO thickness and annealing temperature on the structural, electrical and optical properties of these sandwich films was investigated. The microstructures of the films were studied by X-ray diffraction (XRD). X-ray diffraction measurements indicate that the GZO layers in the sandwich films are polycrystalline with the ZnO hexagonal structure and have a preferred orientation with the c-axis perpendicular to the substrates. For the sandwich film with upper and under GZO thickness of 40 and 30 nm, respectively, it owns the maximum figure of merit of 5.3 × 10⁻² Ω⁻¹ with a resistivity of 5.6 × 10⁻⁵ Ω cm and an average transmittance of 90.7%. The electrical property of the sandwich films is improved by post annealing in vacuum. Comparing with the as-deposited sandwich film, the film annealed in vacuum has a remarkable 42.8% decrease in resistivity. The sandwich film annealed at the temperature of 350 °C in vacuum shows a sheet resistance of 5 Ω/sq and a transmittance of 92.7%, and the figure of merit achieved is 9.3 × 10⁻² Ω⁻¹.     

Experimental investigation on the structural,dielectric, ferroelectric and piezoelectric properties of La doped ZnO nanoparticles and their application in dye-sensitized solar cells

Pure and lanthanum (La) doped ZnO nanorods were synthesized via co-precipitation method. The structure and morphology of as grown ZnO and La-ZnO nanoparticles were studied using transmission electron microscopy (TEM) and powder X-ray diffraction (XRD) methods. The values of remnant polarization and coercive field were found to be 0.027 μC/cm² and 1.33 kV/cm, respectively, for as grown La-ZnO nanostructures. High Curie temperature (276 °C) for doped ZnO was observed in dielectric study. Piezoelectric coefficient at room temperature was found to be 101.30 pm/V. I-V characteristics were studied for both pure and doped ZnO nanoparticles. Photo-anodes of dye-sensitized solar cells (DSSCs) were made using ZnO and La-ZnO nanorods. The conversion efficiency and short circuit current density of La-ZnO nanorods based DSSC were 0.36% and 1.31 mA/cm², respectively, which were found to be largely enhanced when compared with that of pure ZnO based DSSC (0.20% and 0.94 mA/cm²).     

Plasmon enhanced CdS-quantum dot sensitized solar cell using ZnO nanorods array deposited with Ag nanoparticles as photoanode

CdS-quantum dot sensitized solar cell using ZnO nanorods (ZnO NRs) array deposited with Ag nanoparticles (Ag NPs) as photoanode was fabricated. Light absorption effect of Ag NPs on improvement of the cell performance was investigated. Performance improvement of metal nanoparticles (MNPs) was controlled by the structure design and architecture. Different decorations and densities of Ag NPs were utilized on the photoanode. Results showed that using 5% Ag NPs in the photoanode results in the increased efficiency, fill factor, and circuit current density from 0.28% to 0.60%, 0.22 to 0.29, and 2.18 mA/cm² to 3.25 mA/cm², respectively. Also, incident photon-to-current efficiencies (IPCE) results showed that cell performance improvement is related to enhanced absorption in the photoanode, which is because of the surface plasmonic resonance and light scattering of Ag NPs in the photoanode. Measurements of electrochemical impedance spectroscopy revealed that hole transfer kinetics increases with introduction of Ag NPs into photoanode. Also, it is shown that chemical capacitance increases with introduction of Ag NPs. Such increase can be attributed to the surface palsmonic resonance of Ag NPs which leads to absorption of more light in the photoanode and generation of more photoelectron in the photoanode.     

Electrical characteristics and efficiency of organic light-emitting diodes depending on hole-injection layer

In a device structure of ITO/hole-injection layer/N,N′-biphenyl-N,N′-bis-(1-naphenyl)-[1,1′-biphthyl]4,4′-diamine(NPB)/tris(8-hydroxyquinoline)aluminum(Alq₃)/Al, we investigated the effect of the hole-injection layer on the electrical characteristics and external quantum efficiency of organic light-emitting diodes. Thermal evaporation was performed to make a thickness of NPB layer with a rate of 0.5–1.0 Å/s at a base pressure of 5 × 10⁻⁶ Torr. We measured current–voltage characteristics and external quantum efficiency with a thickness variation of the hole-injection layer. CuPc and PVK buffer layers improve the performance of the device in several aspects, such as good mechanical junction, reducing the operating voltage, and energy band adjustment. Compared with devices without a hole-injection layer, we found that the optimal thickness of NPB was 20 nm in the device structure of ITO/NPB/Alq₃/Al. By using a CuPc or PVK buffer layer, the external quantum efficiencies of the devices were improved by 28.9% and 51.3%, respectively.     

AZO/Ag/AZO multilayer films prepared by DC magnetron sputtering for dye-sensitized solar cell application

Ag films were deposited on Al-doped ZnO (AZO) films and coated with AZO to fabricate AZO/Ag/AZO multilayer films by DC magnetron sputtering on glass substrates without heating of glass substrates. The best multilayer films have low sheet resistance of 19.8 Ω/Sq and average transmittance values of 61% in visible region. It was found that the highest figure of merit (F_TC) is 6.9 × 10⁻⁴ Ω⁻¹. For the dye-sensitized solar cell (DSSC) application, the multilayer films were used as transparent conductive electrode (multilayer films/ZnO + Eosin-Y/LiI + I₂/Pt/FTO). The best DSSC based on the multilayer films showed that open circuit voltage (V_oc) of 0.47 V, short circuit current density (J_sc) of 2.24 mA/cm², fill factor (FF) of 0.58 and incident photon-to-current conversion efficiency (η) of 0.61%. It was shown that the AZO/Ag/AZO multilayer films have potential for application in DSSC.     

Near-white emitting QD-LED based on hydrophilic CdS nanocrystals

In this work we report fabrication of a nanocrystal (NC)-based hybrid organic–inorganic LED with structure of ITO/PEDOT:PSS/PVK/CdS-NCs/(Al or Mg:Ag). The hydrophilic CdS NCs were synthesized using a novel aqueous thermochemical method at 80 °C and sizes (around 2 nm) were controlled by thioglycolic acid (TGA) as the capping agent. The favorite feature of these NCs is their relatively high emission intensity and broad, near-white emission. The hydrophilic CdS NCs were successfully spin coated using Triton X-100 as the wetting agent. The fabricated LEDs demonstrated a turn on voltage about 7 V for Al metallic contact. The electroluminescence was a broad spectrum at 540 and 170 nm width, which was about 50 nm red shifted compared to photoluminescence spectra. The CIE color coordinates of the LED at (0.33, 0.43) demonstrated a near white light LED with an emission on green–yellow boundary of white. Annealing of the device up to 190 °C had a positive effect on the performance, possibly due to better contacts between layers. Replacing Al contacts with Mg:Ag reduced the turn-on voltage to 6 V and changed CIE color coordinate to (0.32, 0.41). The EL peak was also shifted to 525 nm, with a brightness of 15 Cd/m² at working voltage of 15 V. The current efficiency and external quantum efficiency of device were 0.08 Cd/A and 0.03% at current densities higher than 10 mA/cm².     

Studies on structural and magnetic properties of Co-doped pyramidal ZnO nanorods synthesized by solution growth technique

Large-area arrays of highly oriented Co-doped ZnO nanorods with pyramidal hexagonal structure are grown on silica substrates by wet chemical decomposition of zinc–amino complex in an aqueous medium. In case of undoped ZnO with an equi-molar ratio of Zn²⁺/hexamethylenetetramine (HMT), highly crystalline nanorods were obtained, whereas for Co-doped ZnO, good quality nanorods were formed at a higher Zn²⁺/HMT molar ratio of 4:1. Scanning electron microscope (SEM) studies show the growth of hexagonal-shaped nanorods in a direction nearly perpendicular to the substrate surface with a tip size of ～50 nm and aspect ratio around 10. The XRD studies show the formation of hexagonal phase pure ZnO with c-axis preferred orientation. The doping of Co ions in ZnO nanorods was confirmed by observation of absorption bands at 658, 617 and 566 nm in the UV–vis spectra of the samples. The optical studies also suggest Co ions to be present both in +2 and +3 oxidation states. From the photoluminescence studies, a defect-related emission is observed in an undoped sample of ZnO at 567 nm. This emission is significantly quenched in Co-doped ZnO samples. Further, the Co-doped nanorods have been found to show ferromagnetic behavior at room temperature from vibrating sample magnetometer (VSM) studies.     

Effect of growth time of hydrothermally grown cobalt hydroxide carbonate on its supercapacitive performance

We present the time-dependent synthesis of cobalt hydroxide carbonate nanorods by hydrothermal method with a systematic increase of different parameters such as specific surface area and specific capacitance as a function of different synthesis time. Morphological characterization of the cobalt hydroxide carbonate nanorods were carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that variation of the time of reaction plays a crucial role in the transformation of samples’ morphology. Cobalt hydroxide carbonate nanorods synthesized with 12 h reaction time, which is the reaction just before the materials transforms into cobalt oxide under the same synthesis conditions exhibited the highest specific capacitance of 466 F g⁻¹ at a current density of 1 A g⁻¹ in 6 M KOH electrolyte and also showed excellent stability with ∼99% capacitance retention after 2000 cycles at a current density of 10 A g⁻¹. Based on the above results, the cobalt hydroxide carbonate nanorods show a considerable potential as electrodes materials for supercapacitor applications.     

Ohmic contact and space-charge-limited current in molybdenum oxide modified devices

The effect of indium-tin oxide (ITO) surface treatment on hole injection of devices with molybdenum oxide (MoO₃) as a buffer layer on ITO was studied. The Ohmic contact is formed at the metal/organic interface due to high work function of MoO₃. Hence, the current is due to space charge limited when ITO is positively biased. The hole mobility of N, N′-bis-(1-napthyl)-N, N′-diphenyl-1, 1′biphenyl-4, 4′-diamine (NPB) at various thicknesses (100–400 nm) has been estimated by using space-charge-limited current measurements. The hole mobility of NPB, 1.09×10⁻⁵ cm²/V s at 100 nm is smaller than the value of 1.52×10⁻⁴ cm²/V s at 400 nm at 0.8 MV/cm, which is caused by the interfacial trap states restricted by the surface interaction. The mobility is hardly changed with NPB thickness for the effect of interfacial trap states on mobility which can be negligible when the thickness is more than 300 nm.     

RGB tricolor produced by white-based top-emitting organic light-emitting diodes with microcavity structure

RGB pixels by microcavity top-emitting organic light-emitting diode (TOLED) is beneficial to both minimizing the loss of light and improving the color purity and the efficiency. Based on the multi-emitting layers, white organic light-emitting diodes (OLEDs) and microcavity TOLEDs were prepared. TOLEDs were fabricated using Ag/ITO as the reflector and adjusting layer, Al/Ag as semi-transparent cathode, Alq:DCJTB/TBADN:TBPe/Alq:C545 as white light emitting layer. By adjusting the thickness of ITO, optical length of cavity and the color of the device have been changed. So we get RGB tricolor devices. The peak wavelengths are 476 nm, 539 nm, 601 nm, Commission Internationale d’Eclairage (CIE) coordinates are (0.133, 0.201), (0.335, 0.567), (0.513, 0.360), FWHM are 32 nm, 50 nm, 73 nm for blue, green and red, respectively.     

High quality nitrogen-doped zinc oxide thin films grown on ITO by sol–gel method

Highly transparent N-doped ZnO thin films were deposited on ITO coated corning glass substrate by sol–gel method. Ammonium nitrate was used as a dopant source of N with varying the doping concentration 0, 0.5, 1.0, 2.0 and 3.0 at%. The DSC analysis of prepared NZO sols is observed a phase transition at 150 °C. X-ray diffraction pattern showed the preferred (002) peak of ZnO, which was deteriorated with increased N concentrations. The transmittance of NZO thin films was observed to be ~88%. The bandgap of NZO thin films increased from 3.28 to 3.70 eV with increased N concentration from 0 to 3 at%. The maximum carrier concentration 8.36×10¹⁷ cm⁻³ and minimum resistivity 1.64 Ω cm was observed for 3 at% N doped ZnO thin films deposited on glass substrate. These highly transparent ZnO thin films can be used as a window layer in solar cells and optoelectronic devices.     

Application of ZnO nanorods loaded on activated carbon for ultrasonic assisted dyes removal: Experimental design and derivative spectrophotometry method

A method based on application of ZnO nanorods loaded on activated carbon (ZnO-NRs-AC) for adsorption of Bromocresol Green (BCG) and Eosin Y (EY) accelerated by ultrasound was described. The present material was synthesized under ultrasound assisted wet-chemical method and subsequently was characterized by FE-SEM, TEM, BET and XRD analysis. The extent of contribution of conventional variables like pH (2.0–10.0), BCG concentration (4–20 mg L⁻¹), EY concentration (3–23 mg L⁻¹), adsorbent dosage (0.01–0.03 g), sonication time (1–5 min) and centrifuge time (2–6 min) as main and interaction part were investigated by central composite design under response surface methodology. Analysis of variance (ANOVA) was adapted to experimental data and guide the best operational conditions mass by set at 6.0, 9 mg L⁻¹, 10 mg L⁻¹, 0.02 g, 4 and 4 min for pH, BCG concentration, EY concentration, adsorbent dosage, sonication and centrifuge time, respectively. At these specified conditions dye adsorption efficiency was higher than 99.5%. The suitability and well prediction of optimum point was tested by conducting five experiments and respective results revel that RSD% was lower than 3% and high quality of fitting was confirmed by t-test. The experimental data were best fitted in Langmuir isotherm equation and the removal followed pseudo second order kinetics. The experimentally obtained maximum adsorption capacities were estimated as 57.80 and 61.73 mg g⁻¹ of ZnO-NRs-AC for BCG and EY respectively from binary dye solutions. The mechanism of removal was explained by boundary layer diffusion via intraparticle diffusion.