n-ZnO Nanorods Research Articles

Comprehensive investigations into the synergy of S-scheme heterojunction between nitrogen-doped ZnO nano-rods and g-C3N4 nanosheets for improved photocatalytic degradation

In this work, N-ZnO nanorods decorated with 2D g-C3N4 prepared by hydrothermal synthesis are studied for their synergistic effect in regard to enhanced photocatalytic activity. The synthesized material is characterized with XRD, FESEM, EDX, TEM and HR-TEM to perform the structural, morphological and elemental analysis along with XPS and FTIR for tracking sample purity and to study the elemental composition on the surface of the material; UV–Visible spectroscopy and PL spectroscopy for investigating the optical properties, bandgap calculation and to inquire about the defects in the structure. N-ZnO/g-C3N4 shows remarkable photocatalytic MB degradation ability upon solar irradiation which is 2.8 and 1.2 times exceeds than pristine ZnO and g-C3N4, attributed to the s-scheme mechanism for charge migration. The construction of s-scheme heterojunction among N-ZnO and g-C3N4 generates an internal electric field that saliently promotes charge separation at the interface. Furthermore, post-stability test for 8 cycles with XRD and FTIR analysis to verify the material integrity is presented. The scavenger experiments revealed the involved active species. This research does seem to pique interest in the creation of cutting-edge photocatalysts that utilize the solar spectrum to remove harmful organic dyes from water.

Electroluminescence enhancement of ZnO nanorods array determined by Au-nanoparticles position under external electric field

In this work, n-ZnO nanorod arrays (NRA)/MgZnO/p-GaN heterojunction light-emitting diodes (LEDs) were successfully prepared. Au nanoparticles (NPs) were introduced in a simple and stable method to improve ultraviolet electroluminescence efficiency of the ZnO NRA. Although Au NPs are commonly seen as a factor to form localized surface plasmons resonance coupling effect in the visible region, ultraviolet electroluminescence enhancement of ZnO NRA was observed in this work. Importantly, the enhancement performance can be dominated by the position of Au NPs relative to ZnO nanorods driven by the external electric field. Au NPs located at the top of ZnO nanorods promise the additional exciton relaxation channels. As a result, the ZnO ultraviolet near-band-edge (NBE) emission efficiency is improved by 4 times in the Au NPs modified devices. This enhancement process provides a new approach to design efficient UV ZnO-based LEDs.

Photodetection Performance of Hydrothermally Grown n-ZnO Nanorods at High Illumination Intensity

Photodetection Performance of Hydrothermally Grown n-ZnO Nanorods at High Illumination Intensity

Effect of copper and silver co-doping on growth behaviour and photoelectric properties of n-ZnO nanorods/p-GaN heterojunction light-emitting diodes

ABSTRACT In order to adjust the band gap of n-ZnO nanorods (NRs)/p-GaN heterojunction and improve its conductivity, a Cu and Ag co-doping ZnO nanorod array has been synthesised on a p-GaN/Al2O3 substrate by a hydrothermal method at low temperature, in addition, investigated the co-doping effect on the morphology, microstructure and electrical/optical properties. Compared with other samples, the average diameter of the co-doped ZnO NRs is larger, and there are fewer NRs per unit area (density). According to X-ray diffraction analysis, the synthesised ZnO NRs had a preferential [001] growth direction and exhibited a hexagonal wurtzite structure. Compared with undoped ZnO NRs, the UV emission peaks of all doped ZnO NRs had a slight red-shift, which may be related to the combined effect of surface resonance and band gap renormalisation effect. Ag+Cu co-doping can also reduce the band gap of the ZnO NRs. The I-V characteristics curve indicates that Ag+Cu co-doping can greatly improve the electrical properties of the heterojunction. This study demonstrates the possibility of ZnO NRs by Ag+Cu co-doping for potential applications in ultraviolet light-emitting diode.

Photo-Electrochemical Performance of Interpenetration-Structured Heterojunction by Overlapping CuO and ZnO Nanorod Arrays

A facile design of interpenetration-structured p–n heterojunction device was fabricated by overlapping p-CuO and n-ZnO nanorod arrays directly. The design showed weak photovoltaic performance. However, the p-n heterojunction system shows the significantly enhanced photocurrent and photovoltage with the assistance of filling electrolyte (I3 −/I−). The charge carrier dynamics indicates that the reversible redox couple (I3 −/I−) electrolyte can promote electron transfer from CuO to ZnO better, i.e. better charge separation efficiency and photocurrent.

Negative Differential Resistance of n-ZnO Nanorods/p-degenerated Diamond Heterojunction at High Temperatures.

In the present study, an n-ZnO nanorods (NRs)/p-degenerated diamond tunneling diode was investigated with regards to its temperature-dependent negative differential resistance (NDR) properties and carrier tunneling injection behaviors. The fabricated heterojunction demonstrated NDR phenomena at 20 and 80°C. However, these effects disappeared followed by the occurrence of rectification characteristics at 120°C. At higher temperatures, the forward current was increased, and the turn-on voltage and peak-to-valley current ratio (PVCR) were reduced. In addition, the underlying mechanisms of carrier tunneling conduction at different temperature and bias voltages were analyzed through schematic energy band diagrams and semiconductor theoretical models. High-temperature NDR properties of the n-ZnO NRs/p-degenerated diamond heterojunction can extend the applications of resistive switching and resonant tunneling diodes, especially in high-temperature, and high-power environments.

Ultraviolet electroluminescence from flowers-like n-ZnO nanorods/p-GaN light-emitting diode fabricated by modified chemical bath deposition

In this work, an ultraviolet (UV) light-emitting diode (LED) based on flowers-like n-ZnO nanorods (NFs)/p-GaN was fabricated using direct heat substrate-modified chemical bath deposition (DHS-MCBD) method. Initially, the ZnO nanoparticles were grown on the top of a p-type GaN substrate as a seed layer which facilitated the growth of high-density of ZnO nanorods in various directions. The current-voltage (I–V) characteristics of the fabricated LED device showed a rectifying behavior of ZnO NFs/p-GaN heterojunction whereas the breakdown mechanism for the device was generated by the tunneling effect. The luminescence properties of the device were studied at room temperature (RT) by using electroluminescence (EL) technique. The back side EL spectrum of the device displayed a sharp UV emission peaks at 367 and 378 nm for GaN and ZnO respectively. On the other hand, the EL spectrum taken from the front side was composed of UV emission, near blue emission along with a broadband green emission. The EL emission for the device was also observed with the naked eye under normal light from both of its sides. The light emission mechanism and the waveguiding effect of the ZnO NRs have been discussed in this work.

Fabrication and Characterization of Light Emitting Diode Based on n-ZnO Nanorods Grown Via a Low-Temperature

In this work, we report the fabrication of a near- ultraviolet (UV) light emitting (LED) device based on the growth of n-ZnO nanorod (NRs) arrays on the p-GaN layer/sapphire substrate heterostructure using the low-cost hydrothermal technique. Morphological, structural and optical properties of the as-fabricated sample are described. The room temperature current-voltage (I–V) measurements of the fabricated LED device confirmed a rectifying diode behaviour. The device presents near UV color under reverse bias. The luminescence properties of were investigated from both sides of the fabricated LED device at room temperature by electroluminescence (EL). EL spectrum of color emitting LED composed of intense peaks centered at 367 nm, 379 nm and a broad band around green emission. EL emission for the device has seen with the naked eye under normal light.

Ultraviolet photodetectors using hollow p-CuO nanospheres/n-ZnO nanorods with a pn junction structure

We report on ultraviolet (UV) photodetectors with a pn junction structure consisting of hollow p-CuO (h-CuO) nanospheres and n-ZnO nanorods (NRs). To form the pn junction structure, thermal annealing was conducted using a transferred monolayer of Cu-ion-incorporated polymer spheres onto the n-ZnO NRs/n-Si substrate. Device performance was evaluated by comparing the effects of h-CuO nanosphere coverage changed by sphere shrinkage during thermal annealing of Cu-ion-incorporated polymer spheres. Three samples were prepared by varying the transfer times of h-CuO on ZnO NRs: 0 times (Reference), 1 time (CZ-I), and 2 times (CZ-II). The CZ-II-based UV detector shows a fast rising time of 1.8 s and a falling time of 0.26 s, which are faster rising by 2.2 and 1.3 times and faster falling by 3.1 and 32.6 times than those of the CZ-I and Reference UV detectors, respectively, under illumination with UV light at 254 nm. Moreover, the On/Off current ratio of the CZ-II UV detector is 4.58, which is about 3.3 times and 3.5 times higher than that of the CZ-I and Reference devices, respectively. The higher h-CuO coverage on the ZnO NRs that form the pn junction structure can effectively separate the electron and hole and suppress recombination by mutual transfer of photo-generated electrons and holes in the heterojunction.

Fast-Response and Self-Powered Cu2O/ZnO Nanorods Heterojunction UV-Visible (570 nm) Photodetectors

A fast, self-powered photodetector (PD) capable of UV to visible (570 nm) sensing based on the p-Cu2O/n-ZnO nanorods (NRs) heterostructure is reported. The PD shows a spectral response range from 380 nm to 570 nm, which is suitable for ambient light sensing applications. The PD exhibits good rectifying characteristics with a low leakage current and a low turn-on voltage of ∼0.3 V. The responsivities of the self-powered PD are 0.24 A W−1 and 0.06 A W−1 at 380 and 450 nm, respectively. The rise/recovery time of the PD are 0.02/0.03 s and 0.01/0.02 s for UV (370 nm) and visible light (450 nm), respectively. Upon UV light illumination, the long, persistent photocurrent (or recovery time) caused by ZnO NRs can be eliminated using the self-powered mode.

N-ZnO nanorod arrays/p-GaN light-emitting diodes with graphene transparent electrode

Few-layer graphene sheets are applied as transparent electrode and current spreading layer for n-ZnO nanorod arrays/p-GaN light-emitting diodes (LEDs). The device with graphene electrode exhibits improved electroluminescence (EL) performance in EL intensity and reliability compared to the n-ZnO nanorod arrays/p-GaN LED with conventional indium-tin-oxide (ITO) electrode. The results prove that, with its superior electrical, optical and thermal properties, graphene can be an ideal candidate for transparent electrodes in nanorod arrays-based LEDs.

UV enhanced white-light response based on p-Si/n-ZnO nanorod heterojunction photosensor

Two-regime photosensor has been fabricated from p-n junction between p-Si and n-ZnO nanorods. The junction is obtained by simply grown vertically-aligned ZnO nanorods on silicon substrate via hydrothermal method. As expected, the device response to visible and UV spectrum due to the band gap of silicon and ZnO, respectively. However, it is found that photoresponse to visible light can be enhanced under UV activation. Detailed mechanism behind the phenomena is proposed along with the photoluminescence results of defect states in ZnO nanorods. The trapped electrons from UV stimulation could be excited to conduction band by white light and contributed to the increase of the photocurrent of the device under white-light irradiation. This low-temperature solution growth ZnO nanorod-based UV–vis photosensor also presents reproducible results and fast recovery time.

Sb-doped p-ZnO quantum dots: Templates for ZnO nanorods homojunction white light-emitting diodes by low-temperature solution process

Zinc oxide (ZnO) is an attractive metal oxide material in optoelectronics because of its visible light-emitting property caused by various deep-level defects. To exploit this property to its full potential in optoelectronic device applications, it is necessary to synthesize quantum-dot sized ZnO and to obtain p-type ZnO for achieving a p-n homojunction. In this study, for the first time, Sb-doped p-ZnO quantum dots (QDs) were successfully synthesized with a doping concentration of 0.5, 1, 2, and 5 at.% by a facile solution process at a low temperature of 30 °C. The obtained Sb-doped p-ZnO QDs exhibited strong and broad deep-level emission due to various deep-level defects. The p-type conductivity of the Sb-doped p-ZnO QDs was confirmed by applying QDs to a channel layer in TFTs. By using the obtained Sb-doped p-ZnO QDs as a p-type layer and growing n-ZnO nanorods (NRs) hydrothermally on top of the p-ZnO QD layer, ZnO-based homojunction white light-emitting diodes (WLEDs) were successfully fabricated. The fabricated WLEDs exhibited naked-eye observable white light emitting over the entire visible spectral region.

Resistive switching in individual ZnO nanorods: delineating the ionic current by photo-stimulation

Resistive switching in nanostructured metal oxide semiconductors has been broadly understood to originate from the dynamics of its native point defects. Experimental results of switching observed in individual n-ZnO nanorods grown on a p-type polymer is presented along with an empirical model describing the underlying defect dynamics necessary to observe bi-polar switching. Selective photo excitation of electrons into the defect states delineates the incidence and role of an ionic current in the switching behavior. The understanding further extends to the observance of a negative differential resistance regime that is often coincident in such systems. The analysis not only unifies the underlying physics of the two phenomena but also offers further confidence in the proposed mechanism. We conclude by demonstrating that the effective memresistance of such devices is a strong function of the operating bias and identify parameters that optimize switching performance.

Mechanism of the photocatalytic activity of p-Si(100)/n-ZnO nanorods heterojunction

A heterojunction of p-Si(100)/n-ZnO nanorods was fabricated by a simple hydrothermal method. The photocatalytic activity of this heterojunction was examined by degradation of Rhodamine B (RhB) under UV light irradiation. The results indicated that the p-Si(100)/n-ZnO nanorods heterojunction exhibits higher photocatalytic activity compared to that of a glass/n-ZnO nanorods. The inner electric field created by the space charge region of heterojunction will oppose the recombination of photogenerated electrons and holes. Furthermore, this heterojunction serves as a convenient recyclable and effective photocatalyst. The photodecomposition rate of RhB after 5 cycles is negligible change in an experiment using this heterojunction.

Nano-engineering of p–n CuFeO2-ZnO heterojunction photoanode with improved light absorption and charge collection for photoelectrochemical water oxidation

The effective utilization of abundant visible solar light for photoelectrochemical water splitting is a green approach for energy harvesting, to reduce the enormous rise of carbon content in the atmosphere. Here, a novel efficient design strategy for p–n type nano-heterojunction photoanodes is demonstrated, with the goal of improving water splitting efficiency by growing low band gap p-CuFeO2 nanolayers on n-ZnO nanorods by an easy and scalable electrochemical route. The photoconversion efficiency of p–n CuFeO2/ZnO photoanodes is found to be ∼450% higher than that of pristine ZnO nanorod electrodes under visible solar light illumination (λ > 420 nm, intensity 10 mW cm−2). The p–n CuFeO2/ZnO nano-engineering not only boosts the visible light absorption but also resolves limitations regarding effective charge carrier separation and transportation due to interfacial band alignment. This photoanode also shows remarkably enhanced stability, where the formation of p–n nano-heterojunction enhances the easy migration of holes to the electrode/electrolyte interface, and of electrons to the counter electrode (Pt) for hydrogen generation. Therefore, this work demonstrates that p–n nano-engineering is a potential strategy to design light-harvesting electrodes for water splitting and clean energy generation.

Facile Construction of Dual p-n Junctions in CdS/Cu2O/ZnO Photoanode with Enhanced Charge Carrier Separation and Transfer Ability.

With the gradually increasing demand for solving the environmental pollution problem and energy crisis, efficient photocatalysts with superior charge carrier separation and transfer ability have attracted extensive research attention. Herein, n-type CdS-decorated p-Cu2O/n-ZnO nanorod arrays (CdS/Cu2O/ZnO NRAs), integrating the merits of both highly ordered structure and synergistic effect derived from dual p–n junctions, were successfully fabricated and further applied to photoelectrocatalysis. In this ternary nanocomposite, fast generation, separation, and transfer of charge carriers were achieved in the Cu2O/ZnO and Cu2O/CdS dual p–n junction regions due to their built-in electric field and appropriate band structures. Moreover, both highly ordered ZnO NRAs and compact CdS shell play the role of an electron collector and a transport channel that efficiently consumes the photoinduced electrons in the conduction band of Cu2O, which considerably reduces the recombination rate of charge carriers. As expected, the perfect cooperation of the three participators leads to the highest photoconversion efficiency of 2.61% at −0.275 V (versus saturated calomel electrode) and an incident photon-to-current conversion efficiency of 14.51% at 380 nm as well as the photoelectrocatalytic degradation ability of the optimized 30 min CdS/Cu2O/ZnO NRAs photoanode as compared to that of the Cu2O/ZnO and ZnO NRAs photoanodes. It is believed that the induced synergistic effect between dual p–n junctions and ZnO NRAs caused the superior performances of the CdS/Cu2O/ZnO NRAs photoanode, and this ternary material with a unique structure may present a new way of thinking for potential applications in the photoelectrochemistry field.

Selective Oxidizing Gas Sensing and Dominant Sensing Mechanism of n-CaO-Decorated n-ZnO Nanorod Sensors

In this work, we investigated the NO2 and CO sensing properties of n-CaO-decorated n-ZnO nanorods and the dominant sensing mechanism in n-n heterostructured one-dimensional (1D) nanostructured multinetworked chemiresistive gas sensors utilizing the nanorods. The CaO-decorated n-ZnO nanorods showed stronger response to NO2 than most other ZnO-based nanostructures, including the pristine ZnO nanorods. Many researchers have attributed the enhanced sensing performance of heterostructured sensors to the modulation of the conduction channel width or surface depletion layer width. However, the modulation of the conduction channel width is not the true cause of the enhanced sensing performance of n-n heterostructured 1D gas sensors, because the radial modulation of the conduction channel width is not intensified in these sensors. In this work, we demonstrate that the enhanced performance of the n-CaO-decorated n-ZnO nanorod sensor is mainly due to a combination of the enhanced modulation of the potential barrier height at the n-n heterojunctions, the larger surface-area-to-volume ratio and the increased surface defect density of the decorated ZnO nanorods, not the enhanced modulation of the conduction channel width.

Characterization and device applications of p-type ZnO films prepared by thermal oxidation of sputter-deposited zinc oxynitride

Formation of nitrogen-doped p-type ZnO (p-ZnO) and related homojunction devices has been reported extensively using a variety of techniques with the exception of thermal oxidation of ZnON that is a simple and less explored method. We converted sputter-deposited ZnON films to p-ZnO at relatively low oxidation temperatures (400–500 °C) as compared with other reports. Chemical bonding states of nitrogen in p-ZnO films were examined by XPS analysis and the mechanism of p-type doping was discussed. Films were characterized by XRD, SEM, photocurrent spectroscopy, and photoluminescence (PL) measurement. Films were composed from nano-size crystallites with (100) preferential orientation. A shallow acceptor level with a 70–90 meV binding energy and a 1.72-eV deep level were measured and identified. PL thermal quenching of the films had activation energy of 34 meV. Schottky barrier (SB) and homojunction diodes, with two different types of n-ZnO, were fabricated on films and their parameters were compared with the reported data. The SB and both types of homojunction diodes had high rectification factors (104-105) and good device parameters. The conductance of homojunction diode based on sputter-deposited n-ZnO showed thermal activation energies of 0.08–0.12 eV, 0.16–0.20 eV, and 0.40–0.50 eV. The homojunction diode based on n-ZnO nano-rods exhibited strong electroluminescence in the visible region from which two defect-related transition energies of 2.21 eV and 2.66 eV were obtained. In conclusion, good quality p-ZnO films and related devices can be prepared by thermal oxidation of ZnON films.

Seed layer synthesis effect on the concentration of interface defects and emission spectra of ZnO nanorods/p‐GaN light‐emitting diode

As the low‐temperature aqueous chemical synthesis (LT‐ACS), holds great promises for the synthesis of one‐dimensional (1D) ZnO nanostructure‐based light‐emitting diodes (LEDs) and hence require parameter tuning for optimal performance. N‐ZnO nanorods (NRs)/p‐GaN heterojunction LEDs have been synthesized by the LT‐ACS using ZnO nanoparticle (NPs) seed layers prepared with different precursor solutions. The effect of these seed layers on the interface defect properties and emission intensity of the as‐synthesized n‐Zn/p‐GaN heterojunction LEDs has been demonstrated by spatially resolved cathodoluminescence (CL) and electroluminescence (EL) measurements, respectively. A significant reduction of the interface defects in the n‐ZnO NRs/p‐GaN heterostructure synthesized from a seed layer prepared from zinc acetate (ZnAc) with a mixture of potassium hydroxide (KOH) and hexamethylenetetramine (HMTA) (donated as ZKH seed) compared with those prepared from ZnAc and KOH (donated as ZK seed) is observed as revealed by spatially resolved CL. Consequently, the LEDs based on n‐ZnO NRs/p‐GaN prepared from ZKH seed show an improvement in the yellow emission (∼578 nm) compared to that based on the ZK seed as deduced from the electroluminescence measurements. The improvement in the yellow EL emission on the ZKH LED probably attributed to the low presence of the non‐radiative defect as deduced by light‐output current (L–I) characteristics analysis.