ZnO Nanodots Research Articles

Altering defect population during the solvothermal growth of ZnO nanorods for photocatalytic applications

High abundance and low toxicity make zinc oxide very attractive for broad application in the field of photocatalysis for remediation of toxic compounds from water. Its performance could be elegantly improved by inducing native defects into ZnO semiconductor nanocrystals. However, this typically requires post-processing at high temperatures, during which many of the benefits of the original batch of monodisperse small grains are lost due to agglomeration and/or crystal growth. Within this research we show how variations in defect population are inherent to recrystallization process and that they can be tuned to some extent by choosing optimal pathways. The defects were studied by Raman, photoluminescence and EPR spectroscopy, while the surface chemistry of the ZnO nanorods, solvothermally recrystallized from ZnO nanodots, was evaluated by X-ray photoelectron spectroscopy. The produced nanorods with different dimensions were also utilized to show how effective the samples are for degradation of a model pollutant, namely caffeine. Unexpectedly, we found that the surface chemistry of our samples is not the main factor in photocatalytic efficiency. Instead, we show that the differences in photocatalytic activity are mainly determined by the defect population within the bulk formed during the recrystallization process under altered solvothermal conditions.

Exploring Enhanced Hydrolytic Dehydrogenation of Ammonia Borane with Porous Graphene-Supported Platinum Catalysts.

Graphene is a good support for immobilizing catalysts, due to its large theoretical specific surface area and high electric conductivity. Solid chemical converted graphene, in a form with multiple layers, decreases the practical specific surface area. Building pores in graphene can increase specific surface area and provide anchor sites for catalysts. In this study, we have prepared porous graphene (PG) via the process of equilibrium precipitation followed by carbothermal reduction of ZnO. During the equilibrium precipitation process, hydrolyzed N,N-dimethylformamide sluggishly generates hydroxyl groups which transform Zn2+ into amorphous ZnO nanodots anchored on reduced graphene oxide. After carbothermal reduction of zinc oxide, micropores are formed in PG. When the Zn2+ feeding amount is 0.12 mmol, the average size of the Pt nanoparticles on PG in the catalyst is 7.25 nm. The resulting Pt/PG exhibited the highest turnover frequency of 511.6 min-1 for ammonia borane hydrolysis, which is 2.43 times that for Pt on graphene without the addition of Zn2+. Therefore, PG treated via equilibrium precipitation and subsequent carbothermal reduction can serve as an effective support for the catalytic hydrolysis of ammonia borane.

Insights into the Electronic, Optical, and Anti-Corrosion Properties of Two-Dimensional ZnO: First-Principles Study

The electronic, optical, and anticorrosion properties of planer ZnO crystal and quantum dots are explored using density functional theory calculations. The calculations for the finite ZnO quantum dots were performed in Gaussian 16 using the B3LYP/6-31g level of theory. The periodic calculations were carried out using VASP with the plane wave basis set and the PBE functional. The subsequent band structure calculations were performed using the hybrid B3LYP functional that shows accurate results and is also consistent with the finite calculations. The considered ZnO nanodots have planer hexagonal shapes with zigzag and armchair terminations. The binding energy calculations show that both structures are stable with negligible deformation at the edges. The ZnO nanodots are semiconductors with a moderate energy gap that decreases when increasing the size, making them potential materials for anticorrosion applications. The values of the electronic energy gaps of ZnO nanodots are confirmed by their UV-Vis spectra, with a wide optical energy gap for the small structures. Additionally, the calculated positive fraction of transferred electrons implies that electron transfer occurs from the inhibitor (ZnO) to the metal surface to passivate their vacant d-orbitals, and eventually prevent corrosion. The best anti-corrosion performance was observed in the periodic ZnO crystal with a suitable energy gap, electronegativity, and fraction of electron transfer. The effects of size and periodicity on the electronic and anticorrosion properties are also here investigated. The findings show that the anticorrosion properties were significantly enhanced by increasing the size of the quantum dot. Periodic ZnO crystals with an appropriate energy gap, electronegativity, and fraction of electron transfer exhibited the optimum anticorrosion performance. Thus, the preferable energy gap in addition to the most promising anticorrosion parameters imply that the monolayer ZnO is a potential candidate for coating and corrosion inhibitors.

Cd-substitution effect on photoexcitation properties of ZnO nanodots surrounded by carbon moiety

The geometrical structure and photoexcitation properties of Zn27- n Cd n O27C42 complexes are investigated by density functional theory (DFT) and time-dependent DFT calculations at the PBE0/6-31G*/SDD level of theory. The cohesive energy and frequency analysis indicate that the hybrid materials are energetically stable. In presence of Cd substituting atoms, the energy gap of the ZnO nanodots surrounded by carbon moiety is shown to decrease, as compared to Cd-free complex. In-depth excited state analysis including charge density difference (CDD) mapping and absorption spectrum decomposition is performed to reveal the nature of the dominant excited states and to comprehend the Cd-to-Zn substitution effect on the photoexcitation properties of Zn27- n Cd n O27C42. A principal possibility to enhance the intramolecular charge transfer through incorporation of certain number of Cd atoms into the ZnO nanodots is shown. Such Cd-induced modifications in optical properties of semi-spherical Zn27- n Cd n O27C42 complexes could potentially enable use of this hybrid material in optoelectronic and photocatalytic applications.

Combining ZnO and Organosilica Nanodots as a Thick Cathode Interlayer for Highly Efficient and Stable Inverted Polymer Solar Cells

Low-work-function metal oxides as cathode interlayers are widely used in polymer organic solar cells (PSCs), but the surface defect and intrinsic photocatalysis issues severely affect the high efficiency, thickness insensitivity, and stability of PSCs. In this work, we used organosilica nanodots (OSiNDs) to modify ZnO as cathode interlayers via the self-assembly method. The ZnO/OSiNDs bilayer can acquire a suitable work function and a high conductivity of 5.87 × 10–4 S m–1. Through systematic studies, there is stable surface coordination interaction of Zn–N bonding between ZnO and OSiNDs. In i-PSCs, using D18:Y6 as the active layer, the ZnO/OSiNDs-based device achieves the best PCE of 17.87%. More importantly, due to the high conductivity, the PCE for the device based on a 68 nm thick ZnO/OSiNDs interlayer is still high up to 16.53%, while the PCE for the device based on a 66 nm thick ZnO interlayer is only 13.18%. For photostability, the PCE of the device based on the ZnO/OSiNDs interlayer maintains 95% of its original value after continuous AM 1.5G illumination (contains UV light) at 100 mW/cm2 for 600 min, while that of the ZnO-based device only maintains 72% of the original value. This work suggests that ZnO/OSiNDs can be utilized as a cathode interlayer to fabricate highly efficient and stable PSC over a wide range of thicknesses.

Self-assembly of ZnO nanodots on glass substrates by a facile sol-gel method

In this work, we report the self-assembly of ZnO nanodots on the glass substrates by a sol-gel method. Firstly, the precursor solution consisted of zinc acetate and the solvent of ethylene glycol or propylene glycol is employed to fabricate precursor thin film by spin coating. Secondly, the as-grown precursor thin film is annealed at a low temperature to form the self-assembled ZnO nanodots. The effects of solvent type, precursor solution concentration and annealing time on the morphologies of the self-assembled ZnO nanodots were investigated. In addition, the self-assembly evolution of the ZnO nanodots is proposed in order to helping people to understand the formation of the nanodots by mean of sol-gel method.

Improving thermoelectric performance by constructing a SnTe/ZnO core-shell structure.

SnTe is becoming a new research focus as an intermediate temperature thermoelectric material for its environment-friendly property. Herein, the SnTe/ZnO core–shell structure prepared by a facile hydrothermal method is firstly constructed to enhance the thermoelectric performance. The characterization results demonstrate that ZnO nanosheets are coated on the surface of SnTe particles by in situ synthesis and converted into ZnO nano-dots by spark plasma sintering. The energy barriers built by the SnTe/ZnO core–shell structure improve the Seebeck coefficient effectively. Additionally, the increased density of interfaces induced by ZnO can effectively scatter low/medium frequency phonons, reducing the lattice thermal conductivity in the low/medium temperature region. Further, the point defects caused by Cu2Te-alloying strengthen the scattering of high frequency phonons. The lattice thermal conductivity reaches 0.48 W m−1 K−1, which is close to the amorphous limit of pristine SnTe. As a result, a peak ZT value of 0.94 is achieved at 823 K for SnTe(Cu2Te)0.06–1.5% ZnO, benefiting from the synergistic optimization of thermal and electrical properties. This provides a new idea for exploring an optimization strategy of thermoelectric performance.

Lithiophilic sites dependency of lithium deposition in Li metal host anodes

Lithiophilic hosts hold exciting prospects to tackle the challenges of the dendrite growth and volume expansion of Li metal anode. However, insight into the structure of lithiophilic sites on impacting Li deposition which determines the reliability of electrode yet remains elusive. Here, based on nonlinear phase-field simulations, we show that the Li-ion flux in the electrolyte and current density on the electrode surface are closely related to the dispersity of lithiophilic sites. Taking ZnO as an example, we also experimentally demonstrate that host with densely and homogenously distributed ZnO nanodots allows uniform Li nucleation and even Li deposition compared to its counterparts with separately dispersed and randomly coalesced morphologies. Consequently, symmetric cell with the well-designed anode delivers a long-term lifespan of 1300 h with a low overpotential of only 11 mV. Further coupling with a sulfurized polyacrylonitrile cathode, the full cell displays a steady capacity of 1120 mA h g−1 for 800 cycles at 1 C and a superb rate capacity of 702 mA h g−1 at 5 C. These findings correlate the structure of lithiophilic sites with the mesostructure evolution of Li during deposition, and would provide useful guidance for the host designs toward high-performance rechargeable Li batteries.

Photoresponsivity Enhancement of AlGaN/GaN Heterojunction Phototransistor with ZnO Nanodot Coating Layer

The effects of a ZnO nanodot coating layer on the photoresponsivity characteristics of an AlGaN/GaN heterojunction phototransistor was investigated. The ZnO nanodot layer was introduced on the detection surface of the phototransistor using a simple spin-coating process. The ZnO nanodot layer not only reduced the dark current but also significantly increased the photoresponsivity. At the wavelength of 300 nm, the photoresponsivity increased from 1.02 × 10<SUP>6</SUP> to 1.76 × 10<SUP>6</SUP> A/W.

Potent and durable antibacterial activity of ZnO-dotted nanohybrids hydrothermally derived from ZnAl-layered double hydroxides

The search for effective alternatives to traditional antibiotics to avoid antibiotic resistant bacteria is growing worldwide. ZnO nanoparticles are found to effectively inhibit growth and proliferation of bacteria, and ZnO-based layered double hydroxides (ZnO-based LDHs) have been intensively investigated for this purpose. However, the nanocomposites are made in a multi-step preparation process with severe agglomeration and limited bactericidal ability. In this research, ZnO-dotted nanohybrids using Zn3Al-LDHs as precursors (ZnO-dotted LDHs or ZnO/LDHs) were synthesized under facile hydrothermal conditions. An understanding of the transformation of the LDH precursors to the ZnO/LDHs was conducted with TEM/HRTEM/XRD/FTIR. ZnO/LDHs can be transformed from ZnAl-LDHs, with more ZnO nanodots generated upon heating at 150 and 200 °C for 2 h (Zn3Al-150, Zn3Al-200). Zn3Al-200 nanohybrids showed potent antibacterial activity towards Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) at 100–300 μg/mL for 4 days. Antibacterial activity of Zn3Al-200 may be attributed to the synergistic effects (ROS, leached Zn2+ and physical interaction). This research thus suggests a potential economic approach to prepare ZnO/LDH nanocomposites for avoiding the antibiotic resistant bacteria in environmental engineering or clinic fields.

In situ growth of ZnO nanodots on carbon hierarchical hollow spheres as high-performance electrodes for lithium-ion batteries

Metal-organic frameworks (MOFs) is an ideal class of precursors or templates to build up transition metal based composites with unique architectures and high electrochemical performance for energy storage applications. In this work, a hybrid hollow nanoarchitecture made of ultrasmall zinc oxide nanodots (less than 5 nm in diameter) and carbon with 3D ordered hollow structure have been synthesized through a simple surfactant-assisted hydrothermal method followed by thermal annealing treatment. This 3D ordered hollow structure not only offers good electronic transportation routes and ionic conductive channels, but also effectively relieves the stress and mitigates the volume variation during lithiation/delithiation process as reflected by the high cycle stability and excellent rate capability as anode materials for lithium ion batteries. Benefitting from the rational design of 3D hollow structure and the synergistic effect between the ultrasmall ZnO nanodots and carbon framework, the ZnO/C hollow structures exhibit decent electrochemical performance of lithium storage with excellent cycling stability (a reversible capacity of 919 mA h/g over 100 cycles at 100 mA/g) and enhanced rate capability (567 mA h/g at high current density of 2 A/g). In addition, it also can deliver a capacity of 741 mA h/g at 500 mA/g after 120 cycles. These results uncover a facile route for the material design for energy application.

Nano-engineered ZnO/CeO2 dots@CNFs for fuel cell application

Well-dispersed ZnO(x)CeO2(1−x) nanodots@carbon nanofibers as anode catalysts for the electrooxidation of methanol were synthesized by an easy-controlled template-free method. Their structure and morphology were characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM), field-emission scanning electron microscopy (FESEM) equipped with rapid EDX (energy dispersive analysis of X-ray). The appealed characterization techniques specified that the obtained material is carbon nanofibers decorated by ZnO and CeO2 nanodots. The electrochemical oxidation of methanol on ZnO(x)CeO2(1−x) nanodots@CNFs modified glassy carbon electrode in alkaline solutions was systematically evaluated by cyclic voltammetry (CV) method. A detailed investigation is made for the electrocatalytic oxidation of methanol by varying methanol concentration. The corresponding current densities of ZnO(60%)CeO2(40%) nanodots@CNFs and ZnO(40%)CeO2(60%) nanodots@CNFs were 5.3 and 16.3mA/cm2, respectively. Moreover, negative onset potential (−50mV vs. Ag/AgCl) was observed when ZnO(40%)CeO2(60%) nanodots@CNFs were utilized, which is a superior value among the reported non-precious electrocatalysts. These results suggested cheap and effective nanomaterials as non-precious catalyst for DMFCs application and pave the way to further improve the performance in energy and environmental applications.

ZnO and ZnS nanodots deposited by spray methods: A versatile tool for nucleation control in electrochemical ZnO nanorod array growth

ZnO and ZnS nanodots deposited by simple, non-vacuum ultrasonic spray methods, spray pyrolysis and ion layer gas reaction, are used to control the nucleation and thereby the morphology of electrochemically grown ZnO nanorod arrays. The ZnO nanodot density and subsequently the density of electrochemically deposited ZnO nanorods is increased by increasing the aerosol flow and decreasing the substrate temperature during ZnO nanodot deposition. By increasing the substrate temperature during ZnO nanodot deposition, moreover, the nanorod tilt with respect to the substrate (normal) can be decreased. To tune the density of electrochemically deposited ZnO nanorods to lower values compared to growth on the bare substrate, ZnS nanodots are used to block energetically favorable nucleation sites for electrochemical ZnO nanorod growth and thereby reduce the ZnO nanorod density.

Highly efficient ultrathin-film amorphous silicon solar cells on top of imprinted periodic nanodot arrays

The addressing of the light absorption and conversion efficiency is critical to the ultrathin-film hydrogenated amorphous silicon (a-Si:H) solar cells. We systematically investigate ultrathin a-Si:H solar cells with a 100 nm absorber on top of imprinted hexagonal nanodot arrays. Experimental evidences are demonstrated for not only notable silver nanodot arrays but also lower-cost ITO and Al:ZnO nanodot arrays. The measured external quantum efficiency is explained by the simulation results. The Jsc values are 12.1, 13.0, and 14.3 mA/cm2 and efficiencies are 6.6%, 7.5%, and 8.3% for ITO, Al:ZnO, and silver nanodot arrays, respectively. Simulated optical absorption distribution shows high light trapping within amorphous silicon layer.

Dynamic morphology instability in epitaxial ZnO/AZO (aluminum-doped ZnO) core–shell nanowires

Misfit strain relaxation-induced morphology instability is usually observed in epitaxial heterostructures at high temperatures. In this paper, we report that this morphology instability can occur even at room temperature in epitaxial ZnO/AZO (Al-doped ZnO) core–shell nanowires (NWs). As a result, densely distributed ZnO nanodots (NDs) were self-assembled on the NWs. The growth of NDs was slowed down during aging owing to the gradually reduced misfit strain. The final size and shape of the NDs were highly depended on the shell thickness and the doping ratio. It was proved that the morphology stability could be improved by surface passivation, thinning the shell thickness, or lowering the doping ratio. The results may provide instructive suggestions for the reliable design in strain and surface engineering of nanomaterials.

Arrays of nanorods composed of ZnO nanodots exhibiting enhanced UV emission and stability.

A novel one-step coating and assembly approach for fabricating well-defined ZnO nanodot/SiO₂ nanorod arrays by hydrolysis-recrystallization growth from 1-D ZnO nanorods is described. The resultant composite nanorod arrays exhibit much enhanced UV emission efficiencies and excellent stability, and thus offer particular promise for application in UV emission devices operating in harsh environments.

Research on the Control of ZnO Nanostructures Morphology via a Reverse Micellar Route

ZnO nanostructures with controllable morphology were synthesized by confining the reaction of Zn(NO3)2 and MEA in Reverse Micellar system composed with CCl4-AOT-Water. It was found that the ratio of MEA/ Zn(NO3)2 is decisive on the morphology of ZnO nanostructures. As the lower ratio of MEA/ Zn(NO3)2 is lower than 2:1, ZnO nanodots with a size about 5 nm were generated; the increase in the ratio of MEA/ Zn(NO3)2 leads to the formation of ZnO nanorods.

Control on the Photoluminescence of ZnO Nanostructures Synthesized by a Reverse Micellar Route

ZnO nanostructures with controlable properties were synthesized by a reverse micellar system. The morphology and photoluminescence of ZnO nanostructures can be controlled by changing the the ratio of reaction Zn(NO3)2 and MEA. As the ratio of MEA/ Zn(NO3)2 is low, ZnO nanodots were generated with strong blue emission; as the ratio of MEA/ Zn(NO3)2 is increased, the formation of ZnO nanorods with green emission were synthesized. Since the photoluminescence (PL) properties could be adjusted by varying the nanostructures, these ZnO materials have high potential on different applications, for instance, as fluorescence probes.

The effects of magnetic field on pulsed laser deposition of Mg-doped ZnO thin films

Mg-doped ZnO thin films were deposited on Si surfaces by laser ablation of a Zn:Mg target to shed light on the effects of magnetic field in pulsed laser deposition. Both crystalline and photoluminescence properties of the Mg-doped ZnO film were dramatically enhanced by performing laser ablation under transverse magnetic field at room temperature. The ablation wavelength turned out to be crucial to bring about optimum effects of the magnetic field on the laser-induced plasma plume. Besides, cylindrically phase-separated polystyrene-b-poly(methylmethacrylate) employed as a template facilitated fabrication of bandgap-engineered ZnO nanodot arrays.

Epitaxial growth of thin films and nanodots of ZnO on Si(111) by pulsed laser deposition

Epitaxial (0001) ZnO thin films were grown on (111) Si substrates buffered with intermediate epitaxial (111) yttria-stabilized zirconia (YSZ) layers by pulsed laser deposition (PLD). X-ray diffraction and transmission electron microscopy characterizations revealed that the YSZ buffer layers enabled the epitaxial growth of structurally high quality ZnO films and an atomically sharp ZnO/YSZ interface, proving to be an effective epitaxial template. The epitaxial orientation relationships were revealed as follows: (0001) ZnO||(111) YSZ||(111) Si and [1¯21¯0] ZnO||[1¯10] YSZ||[1¯10] Si. Room temperature photoluminescence spectrum of the ZnO films showed the excitonic ultraviolet emission with few green emissions relevant to oxygen vacancies in the film. Furthermore, we fabricated ZnO nanostructures on the same (111) YSZ||(111) Si substrates by simply manipulating PLD conditions for the epitaxial film growth. The size control of the ZnO nanodots was realized by varying the number of laser pulses. A blueshift behavior induced by quantum confinement was observed, as the nanodot size decreases.