Controllable size and photoluminescence of ZnO nanorod arrays on Si substrate prepared by microwave-assisted hydrothermal method

High-quality ZnO nanorod arrays were grown on silicon substrates by microwave-assisted hydrothermal method. A ZnO seed layer deposited by magnetron sputtering was used for promoting nanorod growth. Process optimization indicates that the size and surface density of nanorods can be controlled individually by varying process parameters including precursor concentration, heating temperature, and heating time. The photoluminescence performance of the nanorods is closely dependent on the mean size of the rods. Reducing rod diameter leads to decreased UV emission and visible emission intensity ratio, which has been attributed to the increased impurities or defects on the rod surface. The present results provide a feasible approach to modify the optical properties of transparent ZnO nanorod arrays.     

Influence of nanocrystalline diamond powder on the growth and photoluminescence of open-ended ZnO nanorods

This paper reports the influence of introducing nanocrystalline diamond powder on the growth and photoluminescence of ZnO nanorods fabricated by hydrothermal technique. The majority of ZnO nanorods show an open-ended feature due to the addition of nano-diamond powder in the reaction solution. It is speculated that the diamond nanocrystallines dropped on the top of the growing ZnO nanorods would suppress the further growth at the localized positions to form the open-ended nanorods. The photoluminescence spectra of the ZnO products show the band-edge-related UV emission of ZnO nanorods at ~ 380 nm, in addition to a broad visible band centered at about 650 nm, which may originate from the emissions related to defects in the open-ended ZnO nanorods. The open-ended ZnO nanorods and/or combined with diamond nanocrystallines would be favorable for potential applications including UV sensors, electron field emitter, and various photoelectric nanodevices.     

Enhanced Fluorescence and Local Vibrational Mode in Near‐White‐Light‐Emitting ZnO:Mg Nanorods System

We report exciton and phonon properties of ZnO:Mg nanorods of different Mg doping concentration. X‐ray diffraction studies (XRD) confirm the growth of wurtzite phase ZnO nanostructures. XRD reveals doping‐induced shift in peaks and formation of secondary phase related to Mg. Optical properties of the prepared nanorods are investigated by using UV‐Visible absorption and photoluminescence spectroscopic techniques. Optical absorption studies show strong free excitonic absorption of ZnO and extra absorption bands related to the defect centers of the secondary phase (MgO) formed after Mg doping. Photoluminescence studies show sharp band in UV region and defects‐related broad band emission in the visible range. Gaussian‐fitted photoluminescence spectra show that the emission is composed of free exciton recombination and its longitudinal optical (LO) phonon replica. In addition, Mg‐related local vibrational mode observed in Raman and FTIR spectra after Mg doping, indicates the incorporation of Mg into the lattice positions of wurtzite ZnO.     

Hydrothermal Synthesis of Vertically Aligned Cesium-Doped ZnO Nanorods for Solar Cell Applications

Single crystalline cesium-doped ZnO nanorods with homogeneous size and shape were hydrothermally grown on Cs–ZnO nucleated glass substrate. The effect of hydrothermal growth duration on the characteristics of Cs-doped ZnO nanorods was examined. The samples were analyzed by X-ray diffraction (XRD), energy dispersive X-ray analysis, scanning electron microscopy, and electrical conductivity, optical transmission and photoluminescence (PL) measurements. XRD analysis showed that Cs-doped ZnO nanorods are wurtzite single crystals and are grown preferentially along the c-axis. Elemental analysis confirmed the presence of 1 at.% of Cs, according to the composition of growth solution. Electrical conductivity of typical samples showed higher values for the 1 at.%-doped sample, which confirmed incorporation of the Cs dopant. The samples were optically transparent and showed two UV and visible PL peaks from which the former peak experienced a red shift and a pronounced increase in intensity with increasing growth time. The fabricated Cs-doped ZnO nanorods are suitable candidates for applications as excitonic solar cells due to their ease of fabrication, morphology control and optical properties.     

Tuning of defects in ZnO nanorod arrays used in bulk heterojunction solar cells

With particular focus on bulk heterojunction solar cells incorporating ZnO nanorods, we study how different annealing environments (air or Zn environment) and temperatures impact on the photoluminescence response. Our work gives new insight into the complex defect landscape in ZnO, and it also shows how the different defect types can be manipulated. We have determined the emission wavelengths for the two main defects which make up the visible band, the oxygen vacancy emission wavelength at approximately 530 nm and the zinc vacancy emission wavelength at approximately 630 nm. The precise nature of the defect landscape in the bulk of the nanorods is found to be unimportant to photovoltaic cell performance although the surface structure is more critical. Annealing of the nanorods is optimum at 300°C as this is a sufficiently high temperature to decompose Zn(OH)₂ formed at the surface of the nanorods during electrodeposition and sufficiently low to prevent ITO degradation.     

Effects of Ni doping on structural,optical and dielectric properties of ZnO

Structural, optical and dielectric properties of Ni doped ZnO samples prepared by the solid state route are presented. X-ray diffraction confirmed the substitution of Ni on Zn sites without changing the hexagonal structure of ZnO. NiO phase appeared for 6% Ni doping. Fourier transform infrared measurements were carried out to study phonon modes in Ni doped ZnO. Significant blueshift with Ni doping was observed in UV–visible studies, strongly supported by photoluminescence spectra that show a high intensity UV emission peak followed by the low intensity green emission band corresponding to oxygen vacancies and defects. The photoluminescence analysis suggest that doping of Ni can affect defects and oxygen vacancies in ZnO and give the possibility of band gap tuning for applications in optoelectronic devices. High values of dielectric constant at low frequency and a strong dielectric anomaly around 320 °C were observed.     

Selective patterning of ZnO nanorods on silicon substrates using nanoimprint lithography

In this research, nanoimprint lithography (NIL) was used for patterning crystalline zinc oxide (ZnO) nanorods on the silicon substrate. To fabricate nano-patterned ZnO nanorods, patterning of an n-octadecyltrichlorosilane (OTS) self-assembled monolayers (SAMs) on SiO₂ substrate was prepared by the polymer mask using NI. The ZnO seed layer was selectively coated only on the hydrophilic SiO₂ surface, not on the hydrophobic OTS SAMs surface. The substrate patterned with the ZnO seed layer was treated with the oxygen plasma to oxidize the silicon surface. It was found that the nucleation and initial growth of the crystalline ZnO were proceeded only on the ZnO seed layer, not on the silicon oxide surface. ZnO photoluminescence spectra showed that ZnO nanorods grown from the seed layer treated with plasma showed lower intensity than those untreated with plasma at 378 nm, but higher intensity at 605 nm. It is indicated that the seed layer treated with plasma produced ZnO nanorods that had a more oxygen vacancy than those grown from seed layer untreated with plasma. Since the oxygen vacancies on ZnO nanorods serve as strong binding sites for absorption of various organic and inorganic molecules. Consequently, a nano-patterning of the crystalline ZnO nanorods grown from the seed layer treated with plasma may give the versatile applications for the electronics devices.     

Color-tunable photoluminescence in Bi3.6Eu0.4Ti3O12/ZnO nanorods composite films

Color-tunable photoluminescence has been realized by constructing Bi_3.6Eu_0.4Ti₃O₁₂/ZnO (BEuT/ZnO) nanorods composite films. The composite films were prepared by a hybrid chemical solution method. Effects of ZnO nanorods on photoluminescence of BEuT were studied. On one hand, the near-band-edge ultraviolet emission of ZnO nanorods can greatly improve the red emission of Eu³⁺ ions in BEuT, and on the other hand, ZnO nanorods can also produce a strong green deep-energy-level luminescence. Our study demonstrates that the color of photoluminescence in the composite films can be tuned from green to yellow to red, through suitable control of ZnO nanorods. The BEuT/ZnO nanorods composite films with the color-tunable photoluminescence may have potential applications for white light emission and other integrated optoelectronic devices.     

Improved photocatalytic performance of Gd and Nd co-doped ZnO nanorods for the degradation of methylene blue

This paper reports the synthesis of pure ZnO, Gd and Nd co-doped ZnO nanorods based nanocomposites via simple hydrothermal method. Subsequently, the prepared photocatalysts were characterized using XRD, SEM/EDX, TEM, UV–visible and PL spectroscopy. The XRD results demonstrate that Gd and Nd ions were incorporated into ZnO lattice in the synthesized ZnO based nanocomposites and showed hexagonal wurtzite structure. The SEM and TEM results show that nanorods having nanoscale diameter and length were successfully synthesized by hydrothermal method. The UV–visible spectroscopy verified that the band gap of ZnO was reduced due to incorporation of Gd and Nd into ZnO photocatalyst. Similarly, Gd and Nd incorporation into ZnO was found effective to reduce the recombination of electrons and holes as confirmed by PL spectroscopy. Moreover, the prepared nanocomposites with various atomic ratios (0.5–2%) were tested for photocatalytic degradation of methylene blue (MB), under visible light irradiation. The highly efficient and optimized 1.5% Nd/ZnO nanocomposite demonstrated enhanced photocatalytic performance for the degradation of methylene blue compared to pure ZnO and other nanocomposites. Furthermore, the recycling results show that the 1.5% Nd/ZnO nanocomposites displayed good stability and long-term durability. These finding suggest that the ZnO based nanocomposite could be efficiently used in various energy and environmental applications.     

Solid-state thermal dewetted silver nanoparticles onto electrochemically grown self-standing vertically aligned ZnO nanorods for three-dimensional plasmonic nanostructures

Three dimensional (3D) plasmonic nanostructures composed of silver nanoparticles decorated ZnO NRs arrays, have been fabricated by a process combining the electrochemical growth of ZnO NRs and further formation of Ag nanoparticles by the solid-state thermal dewetting (SSD) process. The effect of SSD parameters on the morphological, structural and optical properties of the Ag NPs decorated ZnO NRs arrays has been investigated. It is possible to tune the bandgap of the Ag NPs@ZnO nanorods array 3D plasmonic nanostructure by tailoring the Ag nanoparticle sizes, allowing light manipulation at the nanoscale. The silver nanoparticles attached to the ZnO NRs arrays experienced surface plasmonic coupling effect, causing enhancement in the room temperature photoluminescence (PL) UV emission and quenching the corresponding visible light one. An enhancement in the near band edge emission PL intensity of ZnO to the deep level emission PL intensity ratio after Ag NPs decoration of the ZnO nanostructures corresponding to ca. 11 folds has been observed, indicating that the defect emission is obviously suppressed.     

Cu-doped ZnO nanorod arrays: the effects of copper precursor and concentration

Cu-doped ZnO nanorods have been grown at 90°C for 90 min onto a quartz substrate pre-coated with a ZnO seed layer using a hydrothermal method. The influence of copper (Cu) precursor and concentration on the structural, morphological, and optical properties of ZnO nanorods was investigated. X-ray diffraction analysis revealed that the nanorods grown are highly crystalline with a hexagonal wurtzite crystal structure grown along the c-axis. The lattice strain is found to be compressive for all samples, where a minimum compressive strain of −0.114% was obtained when 1 at.% Cu was added from Cu(NO₃)₂. Scanning electron microscopy was used to investigate morphologies and the diameters of the grown nanorods. The morphological properties of the Cu-doped ZnO nanorods were influenced significantly by the presence of Cu impurities. Near-band edge (NBE) and a broad blue-green emission bands at around 378 and 545 nm, respectively, were observed in the photoluminescence spectra for all samples. The transmittance characteristics showed a slight increase in the visible range, where the total transmittance increased from approximately 80% for the nanorods doped with Cu(CH₃COO)₂ to approximately 90% for the nanorods that were doped with Cu(NO₃)₂.     

Preparation of rare earth-doped ZnO hierarchical micro/nanospheres and their enhanced photocatalytic activity under visible light irradiation

Rare earth-doped ZnO hierarchical micro/nanospheres were prepared by a facile chemical precipitation method and characterized by X-ray diffraction, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, UV-visible diffuse reflectance spectroscopy and photoluminescence spectroscopy. The results showed that the as-synthesized products were well-crystalline and accumulated by large amount of interleaving nanosheets. It was also observed that the rare earth doping increased the visible light absorption ability of the catalysts and red shift for rare earth-doped ZnO products appeared when compared to pure ZnO. The photocatalytic studies revealed that all the rare earth-doped ZnO products exhibited excellent photocatalytic degradation of phenol compared with the pure ZnO and commercial TiO₂ under visible light irradiation. Nd-doped ZnO had the highest photocatalytic activity among all of the rare earth-doped ZnO products studied. The optimal Nd content was 2.0 at% under visible light irradiation. The enhanced photocatalytic performance of rare earth-doped ZnO products can be attributed to the increase in the rate of separation of photogenerated electron–hole pairs and hydroxyl radicals generation ability as evidenced by photoluminescence spectra.     

n-ZnO nanorods/p-CuSCN heterojunction light-emitting diodes fabricated by electrochemical method

n-ZnO nanorods/p-CuSCN heterojunction light-emitting diodes (LEDs) have been fabricated using low-temperature electrochemical method. The I-V characteristics of the heterojunction LEDs show obvious rectifying behavior. The typical room-temperature electroluminescence spectra obtained from this heterostructure device under forward bias exhibit a strong visible emission across the spectral region from 350 to 600 nm centered at 530 nm. The intensity of the visible emission rises more quickly than that of the ultraviolet emission with the increasing bias. Photocurrent and Raman spectra reveal that the growth process of CuSCN can induce more surface states and defects in the ZnO nanorods, which confirms the enhancement of visible emission from the ZnO nanorods/p-CuSCN heterostructure. Compared with the ZnO-only LEDs, a p-type CuSCN layer used as a hole-transporting material can balance the electrons and holes injection rates in the heterojunction LEDs. The processing procedure in this work is a low-cost, low-temperature and convenient way to fabricate ZnO-based heterojunction light-emitting diodes.     

Enhanced red photoluminescence in a nanocomposite thin film composed of Bi3.6Eu0.4Ti3O12 and Au-decorated ZnO nanorods

This paper firstly reports on a series of nanocomposite thin films composed of a ferroelectric Bi_3.6Eu_0.4Ti₃O₁₂ (BET) and Au-decorated ZnO nanorods (Au-ZnO), which are prepared by a ultraviolet-induced hybrid chemical solution method. The effects of Au nanoparticles (NPs) and ZnO nanorods on the significantly red photoluminescence enhancement of Eu³⁺ ions are investigated. The results indicate that the larger near band edge (NBE) emission of ZnO by the SPR effect of Au NPs can make an energy transfer from ZnO to Eu³⁺ ions. Simultaneously, the depression of the deep-level emission of ZnO can improve the monochromaticity in the visible range. Furthermore, the dielectric and ferroelectric properties of the thin films are also enhanced. The findings suggest that the nanocomposite thin films of a ferroelectric BET and Au-ZnO could be used as a multifunctional material in the ferroelectric optoelectronic devices with bright light emitting.     

Synthesis of Na-doped ZnO nanowires and their antibacterial properties

Na-doped ZnO nanowires with an average diameter of ~ 40 nm have been fabricated by a thermal decomposition route at temperatures around 400 °C. Their properties have been investigated using X-ray diffraction, field emission scanning electron microscope, Raman spectra, photoluminescence spectra and X-ray photoelectron spectra. Photoluminescence spectra showed that the as-synthesized ZnO samples exhibited strong visible emission with a major peak at 420 nm. Furthermore, intensity of the visible emission at 420 nm enhanced as the increase of Na concentration. The improvement of visible emission in the Na-doped ZnO samples should be a result of the surface defects increased by doping of Na in ZnO. Their antibacterial activities were also evaluated by determination of minimum inhibitory concentration (MIC) against Escherichia coli 25922 in vitro. Antibacterial tests indicated these nanomaterials showed good antibacterial properties after UV illumination for a short time.     

Enhanced wettability and photocatalytic activity of seed layer assisted one dimensional ZnO nanorods synthesized by hydrothermal method

The wettability and photocatalytic activity of ZnO nanostructures synthesized by hydrothermal method are reported. XRD, FESEM, XPS, TEM, AFM, Contact angle, UV/Vis and photoluminescence spectroscopy are used to characterize the samples. It is observed that ZnO seeded layer results in the formation of nanorods whereas the absence of seed gives rise to flake like morphology. The XRD indicates that ZnO nanorods have preferred orientation along (002) direction. The formation of ZnO nanorods along (002) direction is due to the existence of nucleation sites resulting from the lattice matching of ZnO seed. Wettability studies show that the ZnO nanorods grown on seeded substrate approaches superhydrophobic state with water contact angle (WCA) of 137.0°. The high contact angle is due to the large surface roughness and low surface energy. The enhanced catalytic performance of ZnO nanorods is attributed to the 1D structure, enhanced roughness, crystallinity and a large number of reactive oxidizing species.     

Selective Growth of Vertical-aligned ZnO Nanorod Arrays on Si Substrate by Catalyst-free Thermal Evaporation

By thermal evaporation of pure ZnO powders, high-density vertical-aligned ZnO nanorod arrays with diameter ranged in 80–250 nm were successfully synthesized on Si substrates covered with ZnO seed layers. It was revealed that the morphology, orientation, crystal, and optical quality of the ZnO nanorod arrays highly depend on the crystal quality of ZnO seed layers, which was confirmed by the characterizations of field-emission scanning electron microscopy, X-ray diffraction, transmission electron microscopy, and photoluminescence measurements. For ZnO seed layer with wurtzite structure, the ZnO nanorods grew exactly normal to the substrate with perfect wurtzite structure, strong near-band-edge emission, and neglectable deep-level emission. The nanorods synthesized on the polycrystalline ZnO seed layer presented random orientation, wide diameter, and weak deep-level emission. This article provides a C-free and Au-free method for large-scale synthesis of vertical-aligned ZnO nanorod arrays by controlling the crystal quality of the seed layer.     

Synthesis and characterization of ZnO NRs with spray coated GO for enhanced photocatalytic activity

Zinc oxide nanorods, ZnO NRs, were synthesized on a clean glass and coated with graphene oxide (GO) using spray coating method to enhance the photocatalytic activity in wastewater treatment. The ZnO NRs were synthesized using the solution process synthesis that was optimized using Taguchi method. Several synthesis parameters have been optimized and studied to determine the best synthesis parameter to grow ZnO NRs for the photodegradation of organic contaminants. Field emission scanning electron microscopy (FESEM) with EDX, X-ray diffraction (XRD), Raman, ultraviolet visible near-infrared (UV-VIS-NIR), and photoluminescence (PL) spectroscopies were used to investigate the structural and optical properties of the produced nanorods. FESEM images revealed the vertical growth of ZnO NRs as well as layers of GO covering the ZnO NRs' top surface. The Raman study demonstrates the combination peak of GO and ZnO, hence proving the GO layer's successful coating. After the GO coating, decrease in the bandgap of the synthesized photocatalyst was detected by PL and UV–Vis absorption measurements. Under UVC exposure with treatment time of 6 h, the degradation of MB with ZnO NRs/GO photocatalyst reached a degradation percentage of 97.86%, which is greater than the degradation percentage achieved using pristine ZnO NRs, which is 93.28%. The results validated that the coating of GO enhances the photocatalytic activity of the host material, ZnO NRs.     

Sonochemical synthesis of ZnO‐ZnS core‐shell nanorods for enhanced photoelectrochemical water oxidation

The ultrasonic‐assisted synthesis method provides a fast, simple, and large‐scale route for synthesizing desired materials under ambient conditions. In this work, we report on the facile preparation of ZnO‐ZnS core‐shell nanorods on a fluorine‐doped tin oxide (FTO) substrate. The core‐shell nanorods were synthesized by sequential nanoscale reactions involving the preparation of ZnO nanorods and conversion of the ZnO surface into a ZnS shell on the FTO substrate, using an in situ sonochemical method. The ZnO‐ZnS core‐shell nanorods showed improved photocurrents compared with ZnO nanorods for the water oxidation reaction. During the water oxidation reaction, the ZnS shell passivates the surface‐defects of the ZnO, which results in enhanced charge separation in the ZnO nanorods and higher performance.     

Dominant free exciton emission in ZnO nanorods

Nanostructured ZnO is considered to be a promising building block in the design of nanoscale optoelectronic devices. It usually shows dominant donor-bound exciton (DX) emission at low temperatures. In this study, ZnO nanorods with high crystallinity and optical quality were grown by metal-organic chemical vapor deposition on a-plane sapphire (1120) substrates. Dominant free exciton (FX) emission at a low temperature (14 K) was observed by photoluminescence spectroscopy. It was attributed to both the enhancement of the FX emission induced by the high crystalline quality of the nanorods and the suppression of the DX emission induced by hydrogen out-diffusion. The latter reason is believed to be more important from the analysis of the hydrogen distribution in the nanorods through photoluminescence spectroscopy and secondary ion mass spectrometry. A slow cooling process during the deposition is suggested to result in a better optical quality. These results can promote our understanding of the optical properties of ZnO nanostructures.