Aligned ZnO Nanorod Arrays Research Articles

Growing nearly vertically aligned ZnO nanorod arrays on porous α-Al2O3 membranes to enhance the separation of MTBE from aqueous solution

The separation of methyl tertiary butyl ether (MTBE) from aqueous solution by pervaporation-based membrane technology has attracted much attention recently. The present work proposed a novel strategy to enhance the MTBE/water separation performance of porous α-Al2O3 membranes. ZnO seeds were planted on the surface of α-Al2O3 membranes by the deposition and decomposition of zinc acetate, followed by growing ZnO nanorod arrays (ZNA) on the seeded α-Al2O3 membranes by hydrothermal reaction in the mixed hydrated zinc nitrate (HZN) and hexamethylenetetramine (HMTA) solution. Finally, the membranes were hydrophobized by grafting fluoroalkylsilane (FAS) onto their surface. The results show that the MTBE/water separation factor has been dramatically promoted from 7.7 for the α-Al2O3 membranes to 55.7 for the ZNA-covered α-Al2O3 membranes. The promotion may be attributed to the nearly vertically aligned ZnO nanorod arrays, which provide much surface area to load more fluorocarbon groups that are amiable to MTBE molecules and supply a superhydrophobic surface. The MTBE in the permeate mixture has been concentrated to as high as 72.9 wt%, in striking contrast to 4.6 wt% in the feed solution. The obtained membranes are highly stable upon long-term operation.

Effects of Reaction Parameters on the Geometry and Crystallinity of Hydrothermally Synthesized ZnO Nanorods for Bio-Fouling Applications

In this present work, we have successfully synthesized vertical aligned ZnO nanorod arrays (ZnO NRs) on conductive stainless-steel substrate. These ZnO NRs were obtained by using a low temperature, low cost, and eco-friendly hydrothermal approach. The effect of the sol-gel deposited ZnO seed layer on the vertical alignment ZnO NRs has been thoroughly studied. Characterizations of the seed layer and the as-obtained nanorods were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM), high-resolution transmission electron microscopy (HR-TEM), and water contact angles (WCA) measurement. SEM analysis shows that these nanorods were grown with a high uniformity and density over the entire substrate. TEM and XRD studies confirmed the hexagonal wurtzite structure of ZnO nanorods. Finally, field immersion tests using the optimized samples were conducted in a coastal sea environment to study biofouling settlement. To the best of our knowledge, there are few articles that investigate the influence of the seed layer deposited on stainless steel on the formation of vertically aligned nanorods for antifouling applications. The novelty of this work is manifested by optimizing the ZnO seed layer to obtain either thick or fine ZnO nanorods (NRs). Moreover, another novelty of this study is the use of the hydrothermal deposition of ZnO NRs on stainless steel for biofouling to replace paints in marine applications.

Growth of highly aligned ZnO nanorod arrays on zinc plates: Morphological and structural characterization

In this work, uniform and highly aligned ZnO nanorod arrays were simply synthesized on zinc plates by hydrothermal method using zinc–ammonia complex solution. Effects of ZnO seed layer formation, Zn(NO3)2 concentration and NH3/Zn(NO3)2 mole ratio (R) in the reaction medium on the morphology and crystal structure of nanorods were investigated in detail. ZnO nanorods grown on zinc surfaces were highly crystalline and had a hexagonal wurtzite structure, as revealed by XRD analysis. In addition, the dominant crystal growth direction was in c-axis, indicating the verticality of the nanorods. SEM images showed that one-dimensional (1D) ZnO nanorods with high verticality and number density were synthesized on seeded plates whereas randomly oriented arrays were grown on non-seeded surfaces. Increase in the Zn2+ concentration changed the top ends of the rods from tapering to hexagonal ends; and also led to an increase in the average size and verticality of the nanorods. The increase in R value (or i.e., amount of NH3) caused the rods to misalign on the surface and decrease in size. The size of well-aligned nanorods can be adjusted from 50 to 260 nm in diameter and 0.11 to 6 m in length by changing the reaction parameters, implying their large potential to be used in photocatalytic and also in electronic applications.

Novel heterojunction superstrate Cu2ZnInS4−x (CZIS) thin film kesterite solar cell with vertical arrays of hexagonal ZnO nanorods window layer

Quaternary Cu2ZnInS4−x (CZIS) thin films have been prepared by a facile and cheap sol-gel spin coating technique. Low-temperature solution-based methods were used to fabricate a heterojunction solar cell in the superstrate architecture with CZIS thin film as the absorber, vertically aligned ZnO nanorod arrays, and CdS as the window and buffer layers respectively. ZnO nanorod arrays were prepared by hydrothermal technique and nanocrystal layer deposition technique were employed for the deposition of CdS-coated ZnO nanorod arrays. CZIS absorber layer was spin coated on the CdS-coated ZnO nanorod arrays and annealed at different temperatures. The vertically aligned ZnO nanorod arrays, and uniformly distributed CdS shell layer were confirmed from morphological studies. The device had a final configuration of Glass/ITO/ZnO NRs/CdS/CZIS/Ag. HRSEM revealed a nanoflake-like morphology and a band gap between 1.5 and 1.77 eV for the CZIS thin films. CZIS superstrate solar cell had a power conversion efficiency of ∼ 0.61%, an open circuit voltage of ∼ 0.8 V, a short circuit current of ∼ 0.95 mA cm−2 and a fill factor of ∼ 61.35%. This method demonstrates a novel, facile and eco-friendly technique for synthesizing nanocrystalline CZIS thin films with promising photo response from the fabricated device indicating a proof of principle that this material can find application in solar cells.

Defect promoted photothermoelectric effect in densely aligned ZnO nanorod arrays for self-powered position-sensitive photodetection

Sustainable light energy from ambient environment has attracted particular attention to meet the ever-growing need of small-scale electronics. The modulation of intercorrelated thermal and electronic transport is one of the crucial aspects for reliable photothermoelectric electronics. Herein, a defect-promoted photothermoelectric effect is demonstrated in densely aligned ZnO nanorod array with rich lattice defects. The defect-rich ZnO device delivers high electrical conductivity and large Seebeck coefficient to enable significant improvement of photothermoelectric energy conversion and self-powered photodetection. The position sensitivity reaches approximately 0.19 mV mm−1, and the temperature gradient induced electric field makes up for the suppression in the photothermoelectric process. The synergism between intrinsic defects and extra temperature field plays an important role in promoting the photothermoelectric properties of dense ZnO nanorod array. This study is interesting for interpreting the thermo-phototronic phenomena as well as demonstrating the possibility of defect engineering and phonon engineering to enable highly efficient light energy scavenging and self-powered photodetection.

Construction of ZnO@NiO heterostructure photoelectrodes for improved photoelectrochemical performance

In this report, a p-n junction has been constructed using ZnO/NiO heterostructured photoelectrode by spin coating NiO layers over vertically aligned ZnO nanorod arrays to demonstrate its potential in water splitting applications. Before investigating their PEC performance, we thoroughly studied the introduction of NiO layers on the structure, morphology and light absorption property of ZnO nanorods. 9 layered NiO coated ZnO nanorods exhibited optimum photocurrent density of 0.251 mA/cm2 at 0.8 V vs. Ag/AgCl which is attributed to its high absorbance and better charge transfer as recorded from UV–Vis and EIS data. Furthermore, we also studied the effect of (cation (Mg) and anion (Cl)) doping in PEC performance of ZnO nanorods on this optimized sample. Cl_ZnO/NiO showed high Jph of 1.282 mA/cm2 at 1.2 V vs. Ag/AgCl under visible light illumination. The reason behind better photoresponse is its enhanced absorption and well-defined p-n heterojunction between Cl_ZnO and NiO which favoured the separation and transfer of the photocarriers. The results displayed in this work provides a suitable approach of building p-n junction for high performance PEC water oxidation.

Well-aligned ZnO nanorod array covered with ruthenium layers for alternative counter electrodes in dye-sensitized solar cells

As an alternative material for conventional counter electrodes (CEs), which are usually made from Pt/fluorine doped tin oxide (FTO), well aligned ZnO nanorod arrays have great potential, their extremely high surface area can significantly increase the number of interfacial reaction sites in contact with the electrolyte. However, in order to use ZnO nanorod arrays as CEs, inferior charge collecting efficiency is a crucial obstacle that must be overcome. Here, patterned ZnO nanorod arrays are synthesized using a hydrothermal method and an Ru metal layer is deposited using a radio frequency sputtering process. We found the optimal conditions for ZnO nanorod growth that enable us to form vertically aligned ZnO nanorod arrays on only the ZnO seed layer. The Ru metal layer, coated onto the ZnO nanorod array serves both the role of the catalyst and the charge collecting electrode, its deposition process is controlled in such a way to maximize electrode performance. The results found in this study reveal the combination of Ru metal and the ZnO nanorod array has great potential as a lower cost alternative to Pt and FTO.

Well-ordered vertically aligned ZnO nanorods arrays for high-performance perovskite solar cells

ZnO as a typical wide bandgap n-type semiconductor has been broadly employed as an electron transport layer in perovskite solar cells. In this work, we proposed a mesoporous scaffold constructed of vertically aligned ZnO nanorods through a low-temperature water bath. By precisely adjusting the reaction time, well-ordered ZnO nanorods were grown on FTO substrate with different lengths. Taking advantage of the highly visible transmittance, enhanced contact area, shortened electron transport route and compact integrated interface, high-performance mesoporous perovskite solar cells (PSCs) were fabricated. The optimum length of ZnO nanorod based PSC yielded a power conversion efficiency of 14.22% under AM 1.5G illumination. This research suggests that ZnO nanorods with appropriate length are an efficient electron transport layer in mesoporous perovskite solar cells.

Dye-sensitized solar cells based on TiO2 nanoparticles-decorated ZnO nanorod arrays for enhanced photovoltaic performance

Herein, we report a successful approach for the vertically aligned ZnO nanorod arrays (ZNAs) deposited on fluorine-doped tin oxide (FTO) substrate by hydrothermal process. TiO2 nanoparticles were grown on the ZNAs by dip-coating. ZNAs and TiO2 nanoparticles (TNPs)-decorated ZNAs (TNPs@ZNAs) were used as a photoanode in dye-sensitized solar cells. The influence of bath temperature for the attainment of uniform decoration of TiO2 nanoparticles onto ZnO nanorod arrays and the photovoltaic performances of the assembled dye-sensitized solar cells were studied. The power conversion efficiency (PCE) of the TNPs@ZNAs arrays prepared at a bath temperature of 120 °C can reach 4.47%, representing better improvement when compared with ZNAs photoanode (3.15%).

Vertically aligned ZnO nanorods arrays grown by chemical bath deposition for ultraviolet photodetectors with high response performance

Well-aligned ZnO nanorods (NRs) arrays were grown on ZnO seed layer/ITO substrate using chemical bath deposition (CBD) technique and then fabricating ultraviolet (UV) photodetectors (PDs). The effect of precursor concentration of sols on the properties of ZnO seed layer was investigated, and indicated that the crystallinity and growth orientation of samples depend significantly on precursor concentration. ZnO NRs deposited on seed layer exhibit hexagonal wurtzite structure and found that changing the concentration not only affects seed layer, but also influence the rods growth, making it possible to improve the aspect ratio and crystallinity of ZnO NRs. Moreover, the performance of PDs was further discussed and suggested that the maximum signal-to-noise ratio (SNR) and sensitivity (S) of PDs are 67 dB and 2348, when the concentration is 0.5 mol/L, also the shortest rise time of 0.993 s and fall time of 2.036 s can be obtained in such preparation conditions, which is related to larger aspect ratio and higher crystal quality of ZnO NRs. The fabricated UV PDs with high performance are helpful to its application in ultraviolet astronomy, fire detection, especially in military fields, and also are promising for large-scale production.

Nitrogen-Implanted ZnO Nanorod Arrays for Visible Light Photocatalytic Degradation of a Pharmaceutical Drug Acetaminophen.

The present study focuses on the effects of nitrogen (N) ion implantation in vertically aligned ZnO nanorod arrays (NRAs) and the photocatalytic degradation of acetaminophen. The X-ray diffraction of these NRAs exhibit a wurtzite structure with a predominant (002) diffraction peak that shifts slightly after N-ion implantation. The field emission scanning electron microscopic images of as-prepared NRAs show a length of ∼4 μm and diameter of ∼150 nm. UV–visible spectroscopy reveals that the band gap of pristine ZnO NRAs decreases from 3.2 to 2.18 eV after N-ion implantation. Under visible irradiation, the N-ion-implanted ZnO catalyst exhibits significant enhancement of the photocatalytic degradation of acetaminophen from 60.0 to 98.46% for 120 min.

Effect of Ethane-1,2-Diamine on Growth of ZnO Nanorods and Cyclohexane Sensing by Current-Voltage Characteristics Investigations

The growth of vertically aligned ZnO Nanorods arrays using Zinc Nitrate hexahydrate and Hexamethylene Tetramine (HMTA), by Chemical Bath Deposition on Silicon Wafer was investigated. The growth is conducted under influence of Ethane-1,2-diamine, the amine based enhancer was evaluated based on three different ratios (1:0.5, 1:1, 1:1.5) of enhancer to the precursor (Zinc Nitrate and HMTA). The effect different ratios of enhancers on morphology aspect ratio and crystallinity of the as grown Nanorods were studied under Scanning electron microscope (SEM) and X-ray powder diffraction (XRD). Electrical Properties such as current–voltage characteristics were investigated, its correlation to the morphology and aspect ratio of the Nanorods in the presence of 100μL-500μL of Aromatic Compound Cyclohexane and at different applied voltages.

Transparent Thin-Film Transistors Based on c-Axis Oriented, Vertically Aligned ZnO Nanorod Arrays via Solution Processing

A bottom-gate, top-contact transparent thin film transistor (TFT) based on c-axis oriented, vertically aligned ZnO nanorod arrays was fabricated on glass substrates via solution processing, in which ZnO nanorod arrays were synthesized on ZnO seed layers through a simple hydrothermal route. This TFT used SiO2 and indium tin oxide as the gate insulator and gate electrode, respectively. The source and drain electrodes were formed by radio frequency sputtered Au through a shadow mask. This ZnO TFT exhibited n-channel enhancement behavior with a field effective mobility of 3.86 cm2 V−1 s−1, a current on-to-off ratio of 65.5 and a threshold voltage of 1 V. Moreover, the ZnO TFT has a high transmittance of 80% in the visible spectrum. Our results demonstrate that hydrothermally grown, vertically aligned ZnO nanorod arrays are very promising for the fabrication of cost effective and high performance transparent thin-film transistors.

Fabrication and Characterization of Vertically Aligned ZnO Nanorod Arrays via Inverted Monolayer Colloidal Crystals Mask

The periodically ordered ZnO nanorod (NR) arrays have been successfully synthesized via a hydrothermal approach on the silicon substrates by templating of the TiO2 ring deriving from the polystyrene (PS) nanosphere monolayer colloidal crystals (MCC). With the inverted MCC mask, sol–gel-derived ZnO seeds could serve as the periodic nucleation positions for the site-specific growth of ZnO NRs. The large-scale patterned arrays of single ZnO NR with good side-orientation can be readily produced. According to the experimental results, the as-integrated ZnO NR arrays showed an excellent crystal quality and optical property, very suitable for optoelectronic applications such as stimulated emitters and ZnO photonic crystal devices.

Effective shell wall thickness of vertically aligned ZnO-ZnS core-shell nanorod arrays on visible photocatalytic and photo sensing properties

Development of hierarchical core-shell semiconductor heterostructures ensue significant advancement in catalytic functional structures with improvised optical functionalities. Shell wall controlled vertically aligned ZnO-ZnS core-shell nanorod (NR) heterostructures were grown on transparent conductive substrates along the c-axis by sulfidation of aligned ZnO nanorod arrays for visible photocatalytic properties. The effects of the sulfidation time on the morphology, crystalline properties, optical property, photocurrent response, and photocatalytic activity of the catalyst arrays were studied under UV and visible light irradiation. The shell wall thickness of these heterostructures influenced in great extent the effective photo responsive charge separation and improved carrier mobility. ZnO-ZnS core-shell heterostructure having the shell wall thickness of 20 nm has exhibited more efficient visible photocatalytic behavior due to effective separation of carriers and improved visible absorption. On further increasing the wall thickness the catalytic efficiency was reduced due to the poor carrier (hole) mobility in the polycrystalline shell grains which induced the higher recombination rate. Stability and reusability of ZnO-ZnS core-shell nanostructures reveals that the ZnS acted as a protective layer over the ZnO NR arrays. In appraisal with ZnO NR arrays, the control over the shell wall thickness of ZnO-ZnS core-shell NR array attributed to the excellent visible photocatalytic activity and improvised absorption of light in visible region at ZnO-ZnS interface and effective separation of photogenerated electron-hole pairs at ZnO-ZnS heterojunctions.

Swift heavy ion induced effects on structural, optical and photo-catalytic properties of Ag irradiated vertically aligned ZnO nanorod arrays

Development of 1D array platforms for environmental remediation is of vitality in the current scenario where water bodies are overborne by organic pollutants. ZnO nanorod arrays (ZNRA) were synthesized using solution growth method and were irradiated by 120 MeV Ag ions at various ion fluences of 1E11, 5E11, 1E12 and 5E12 ions/cm2. The structural, morphological and optical properties were studied using X-ray diffraction (XRD), Field Emission Scanning Microscopy (FESEM), UV and Photoluminescence spectroscopy respectively. While the FESEM analysis shows that the length of ZNR is in the range of ∼4 μm and with a diameter ranging from 100 nm to 350 nm, the XRD results confirmed the formation of wurtzite hexagonal structure (0 0 2) of ZnO. The XRD peak broadened with the increase of Ag ion fluences from 1E11 to 5E12 ions/cm2. The UV absorption spectra show a red shift of the band edge absorption with increase in ion fluences and bandgap decreases from 3.24 eV to 2.94 eV. The photocatalytic activities of Ag irradiated ZNRA were evaluated through the degradation of methylene blue under the visible region of light irradiation. The degradation efficiencies for pristine and 5E12 ions/cm2 Ag irradiated ZNRA were 54.24% and 87.98% respectively for methylene blue after 120 min. These photocatalytic results show that the degradation efficiency increases with the Ag ion irradiations. The possible mechanism behind this enhancement due to the Ag ion irradiation is also discussed.

Surface plasmon-driven photoelectrochemical water splitting of aligned ZnO nanorod arrays decorated with loading-controllable Au nanoparticles

In this paper, we designed a series of well-aligned ZnO nanorod arrays decorated with loading-controllable Au nanoparticles and studied their surface plasmon-driven photoelectrochemical (PEC) water splitting performances. The PEC water splitting ability of Au-ZnO nanorod arrays was evaluated under illumination with λ > 420 nm light. These nanorod arrays show remarkable PEC water splitting performances and achieve the highest photocurrent density of 30 μA cm−2 at 0.8 V versus Ag/AgCl. Furthermore, the PEC performance for heterogenous nanorod arrays can be effectively adjusted by controlling loading amounts of Au nanoparticles. We experimentally demonstrate that the Au-ZnO nanorod arrays show enhanced visible light absorption ability. The superior PEC performance of Au-ZnO nanorod arrays is attributed to the synergistic effects of plasmonic Au nanoparticles, ZnO semiconductor and Schottky barrier built in heterogenous nanorod array. This work provides a facile strategy to manipulate the PEC water splitting activity of Au-ZnO hybrid nanostructures by simply controlling the loading amounts of metallic Au nanoparticles. Furthermore, our research offers a potentially efficient strategy for the design and fabrication of new types of plasmonic-metal/semiconductor hybrid nanostructures with a plasmonic-enhanced PEC water splitting activity under the visible light, which are as valuable photocatalysts for solar-to-chemical/electrical energy conversion.

Preparation of dye-sensitized solar cells using template free TiO2 nanotube arrays for enhanced power conversion

By using the vertically aligned ZnO nanorod arrays (NRAs), TiO2 nanoparticles attached ZnO nanorods (TiO2@ZnO) and TiO2 nanotube arrays (NTAs) were prepared from feasible seed-induced template free sol-gel dip coating method. TiO2 NTAs were prepared by removing the ZnO nanorod cores using wet-chemical etching. By using TiO2 NTAs as working electrode, dye-sensitized solar cell with enhanced power conversion efficiency was obtained. To verify the formation of TiO2 NTAs various characterization techniques such as X-ray diffraction, UV-VIS absorbance spectra, Field Emission Scanning Electron Microscopy (FESEM), Energy-Dispersive Spectra (EDS), and High-Resolution Transmission Electron Microscopy (HRTEM) have been used. X-ray diffraction patterns of ZnO NRAs and TiO2@ZnO NRAs indicate that prepared films possess both the ZnO wurtzite (002) and TiO2 anatase (101) phase. FESEM image clearly shows that vertically aligned ZnO nanorods having the diameter and length of ∼180–240 nm and ∼1.5 μm, respectively, have been formed. The FESEM image also clearly showed that diameter and length of TiO2@ZnO NRAs are ∼250–320 nm and ∼1.5 μm, respectively. By using UV-VIS absorption spectra, the band gap of vertically aligned ZnO NRAs, TiO2@ZnO NRAs, and TiO2 NTAs have been calculated and its values were 3.14, 3.20, and 3.52 eV, respectively. The dye-sensitized solar cell was prepared by using three different working electrodes ZnO NRAs, TiO2@ZnO, and TiO2 NTAs. The power conversion efficiency of dye-sensitized solar cells prepared using ZnO NRAs, TiO2@ZnO, and TiO2 NTAs are 3.53%, 4.04%, and 5.18%, respectively. TiO2 NTAs with 10 s HCl etching exhibited the highest photoelectric conversion efficiency of 5.18% with short-circuit photocurrent density (Jsc) = 13.34 mA/cm2 and open-circuit photovoltage (Voc) = 0.63 V.

Hydrothermal growth of ZnO: a substrate-dependent study on nanostructures formation

ZnO has received lots of attention in recent years because of its unique properties, rendering it a good candidate material for various industrial applications. Amongst the methods used for synthesizing ZnO structures, hydrothermal technique offers a simple, cost-effective and yet environment friendly synthesis route for the materials. To investigate the effect of substrate on the formation of hydrothermally grown ZnO, several types of substrates were used and the structures of the ZnO were systematically studied by field emission scanning electron microscopy (FESEM). It was demonstrated that the type of substrates used had a great influence on the morphologies, density and alignment of the ZnO formed. Pre-coated substrate with Au yielded highly aligned ZnO nanorod arrays compared to bare Si and Al substrates. Possible mechanisms for the variation of morphology obtained are discussed.

A self-powered nano-photodetector based on PFH/ZnO nanorods organic/inorganic heterojunction

Abstract PFH/ZnO nanorods heterojunctions were fabricated by spin-coating p-type Poly (9,9-dihexylfluorene) (PFH) on n-type vertically aligned ZnO nanorod arrays grown by a facile hydrothermal method on indium tin oxide (ITO) transparent conductive glass. A typical p-n junction behavior was observed in the fabricated heterojunction. The current of heterojunction increases and decreases dramatically by switching the illumination on and off at zero bias, showing potential self-powered photodetector applications. The heterojunction were capable of generating negative current when illuminated under an appropriate wavelength. The photoresponse properties of the heterojunction can be tuned by the applied bias. In vacuum, the rectifying behavior disappeared, and show only simple semiconductor behavior. Band structure of the heterojunction was schematic drawn and explain the mechanism of the properties of PFH/ZnO nanorods heterojunctions.