ZnO Nanoclusters Research Articles

Charge separation at an organic/inorganic nano-hybrid interface: atomistic simulations of a para-sexiphenyl ZnO system.

A prototypical organic/inorganic interface is considered which is formed by vertical stacking of 20 para-sexiphenyl molecules physisorbed on a ZnO nano-cluster of 3903 atoms. Charge separation kinetics at the interface are investigated for their dependence on ultrafast optical excitation. In order to analyze the spatio-temporal evolution of the Frenkel exciton in the organic part and the formation of charge separated states a first principles parameterized Hamiltonian is introduced and the related time-dependent Schroedinger equation is solved. By determining the interface absorption spectrum the optically addressable states can be uncovered. The work continues our previous studies of J. Phys. Chem. Lett., 2018, 9, 209, but with a changed type of surface passivation. This prevents trapping of electrons close to the surface. Charge separated states are formed by direct optical excitation and also by exciton decay at the interface. Electron migration away from the interface into bulk regions becomes possible. The hole stays close to the interface for all excitation scenarios. Finally, it is demonstrated that energetic disorder is of minor influence.

Atomistic Simulations of Charge Separation at a Nanohybrid Interface: Relevance of Photoinduced Initial State Preparation.

Charge separation kinetics at a nanohybrid interface are investigated in their dependence on ultrafast optical excitation. A prototypical organic/inorganic interface is considered. It is formed by a vertical stacking of 20 para-sexiphenyl molecules physisorbed on a ZnO nanocluster of 3783 atoms. A first principle parametrized Hamiltonian is employed, and the photoinduced subpicosecond evolution of Frenkel-excitons in the organic part is analyzed besides the formation of charge separated states across the interface. The interface absorption spectrum is calculated. Together, the data indicate that the charge separation is based on the direct excitation of the charge separated states but also on the migration of created Frenkel excitons to the interface with subsequent decay. Further, the photoinduced interface dynamics are compared with data resulting from direct set-ups of an initially excited state. Mostly such set-ups lead to substantially different charge separation processes.

Indium-doped and positively charged ZnO nanoclusters: versatile materials for CO detection

Indium-doped ZnO nanoparticle has been recently synthesized using a modified sol-gel technique. This study has revealed that the In-doped ZnO nanoparticles represent a higher sensitivity than the pristine ZnO nanoparticles to the carbon monoxide gas and can detect it at sub-ppm concentrations. Motivated by this study, in the present work using first-principles calculations, we study the effect of In-doping on the sensing properties of a ZnO nanocluster. In our survey, we have explored the sensitivity of pristine as well as In-doped ZnO nanoclusters towards CO detection. In contrast to the pristine form, the In-doped ZnO nanocluster can detect the CO molecule due to significant decrease in the HOMO-LUMO energy gap and thereby in the resistivity. As a secondary objective of the present study, electrical charging of the ZnO nanocluster is proposed as an approach for electrocatalytically switchable CO adsorption. We found that the CO molecule is weekly adsorbed on the neutral ZnO nanocluster. Our results show that the interaction between CO molecule and ZnO nanocluster is dramatically increased by introducing extra positive charges into the nanocluster. Once the charges are removed, the CO molecule spontaneously desorbed from the ZnO absorbent. Therefore, this approach promises both facile reversibility and tunable kinetics without the need of specific catalysts.

ReaxFF Molecular Dynamic Simulations of ZnO Nanocluster and Films in H2 Atmosphere

Reactive molecular dynamics simulations were performed to explore the structural evolution of a ZnO nanocluster and (0001) and (1010) surfaces under a H2 atmosphere at different temperatures. The mechanisms of H2 dissociation and water formation were analyzed. Our simulations reveal that there are two pathways for H2 dissociation and three routes for water formation on the surfaces. The nanocluster is more active for H2 dissociation and water formation than the two surfaces. The gas–solid interactions lead to outward displacement of the substrate O atoms. While the O-terminated surface of the (0001) facet is active for H2 dissociation and water formation, the Zn-terminated one is inactive for the dissociation. Unlike the (0001) surface which is more easily reduced, the (1010) surface is readily hydroxylated. Water formation and desorption results in surface oxygen depletion and Zn aggregation which lead to surface metallization, in accordance with the experimental observations. Our simulations show that...

Rapid and multimodal in vivo bioimaging of cancer cells through in situ biosynthesis of Zn&Fe nanoclusters

Early diagnosis remains highly important for efficient cancer treatment, and hence, there is significant interest in the development of effective imaging strategies. This work reports a new multimodal bioimaging method for accurate and rapid diagnosis of cancer cells by introducing aqueous Fe2+ and Zn2+ ions into cancer cells (i.e., HeLa, U87, and HepG2 cancer cells). We found that the biocompatible metal ions Fe2+ and Zn2+ forced the cancer cells to spontaneously synthesize fluorescent ZnO nanoclusters and magnetic Fe3O4 nanoclusters. These clusters could then be used for multimodal cancer imaging by combining fluorescence imaging with magnetic resonance imaging and computed tomography imaging. Meanwhile, for normal cells (i.e., L02) and tissues, neither fluorescence nor any other obvious difference could be detected between preand post-injection. This multimodal bioimaging strategy based on the in situ biosynthesized Zn&Fe oxide nanoclusters might therefore be useful for early cancer diagnosis and therapy.

PH-Responsive ZnO Nanocluster for Lung Cancer Chemotherapy.

Here, we demonstrated a pH-responsive nanocluster based on ZnO quantum dots (QDs) and investigated its potential in drug delivery with tumor-specific accumulation. The nanoclusters were composed of small single ZnO QDs by cross-linking dicarboxyl-terminated poly(ethylene glycol) (PEG), showing high stability and biocompatibility in physiological fluids. The clustered ZnO QDs were capable of loading a large quantity of doxorubicin (DOX) via complexation and covalent interactions. After cellular uptake, the drug was efficiently released because the carrier was completely dissolved; the metal-drug complex was disassembled in response to decreasing pH in the endosomes within tumor cells. Moreover, the viability of cancer cells was significantly decreased because the ZnO QDs exhibited cytotoxicity postdissolution and preferentially killed cancerous cells compared to normal cells. Furthermore, this pH-responsive PEG-cZnO QDs cluster system may be capable of tumor homing while circulating in the blood via the enhanced permeability and retention (EPR) effect.

Accurate assembly of ZnO nanoclusters at the ends of multi‐walled carbon nanotubes in a microemulsion system

A novel method is reported here to attach ZnO nanoclusters accurately at the end of multi-walled carbon nanotubes (MWCNTs) in a hydrothermal system. The ends of MWCNTs were selectively functionalised using gas-phase functionalisation in ammonia. ZnO/MWCNT heterojunctions are formed accurately at the nanotube-ends through hydrothermal microemulsion synthesis. The reverse micelle of sodium dodecyl sulphate combines with the positive nitrogen-containing basic groups at the MWCNT end and act as the microreactors for ZnO nanoclusters. The sidewalls of the MWCNTs remain intact without any ZnO particles functionalisation. The results represent a versatile approach that is highly scalable and could pave the way toward versatile heterojunction devices fabrication.

Enhanced charge separation through modulation of defect-state in wide band-gap semiconductor for potential photocatalysis application: Ultrafast spectroscopy and computational studies

Abstract Structural defects of wide band gap semiconductors play important role in their functionality. Defect mediated recombination of photoinduced electron-hole pair in the semiconductors for their photocatalytic activities, is detrimental. In the case of ZnO nanostructures, radiative recombination upon band-gap photo-excitation (3.37 eV) originated from different surface defects (mainly Oxygen vacancies at 2.50 eV (V + ) and 2.25 eV (V ++ ) with respect to valance and conduction bands respectively) of crystal lattice, acquires immense interest for both fundamental scientific point of view and for the betterment of their manifold applications. The present work indicates that for transition metal semiconducting oxides, use of anionic attachment like Cl − as surface defect healer proves to be more useful for photocatalytic application than bulk doping using cationic dopant like Mn. ZnO NPs of different sizes (5 nm and 30 nm) are synthesized via precipitation method and allowed to interact with chloride ions in aqueous solution. A variety of electron microscopy and picosecond resolved spectroscopic techniques have been employed to study the role of chloride ions for the enhanced photoinduced charge separation in the aqueous environments. Our first principles density-functional calculations for ZnO nanoclusters with surface oxygen vacancy indicate introduction of trap states within the band gap of the nanoclusters. These states effectively confine the photoinduced electrons and thus essentially reduce the photocatalytic yield with respect to pristine ZnO. However, upon Cl − attachment to the defect states, the energy of the trap states were found to be healed, recovering the efficacy of reactive oxygen species (ROS) generation in the aqueous solution. We have also impregnated Mn 2+ ions to the ZnO lattice using precipitation method in solution phase. The DFT calculation on the Mn 2+ ion impregnated ZnO lattice reveals more defect states compare to that of pristine lattice. The rate of electron recombination is found to be much faster through non-radiative pathway (ground state recovery), leading to a decrease in photocatalytic activity in the case of Mn-doped ZnO. Attachment of Cl − to the Mn-doped ZnO partially recovers the ROS generation, which is consistent with healing of deep trap states. The present work is anticipated to provide a new insight into the surface defect modulation of ZnO in potential photocatalysis application.

Mechanism of phase transition, from vapor to solid: Transient liquid phase is between the two

The mechanism of phase transition, from vapor to solid, is studied by producing non-stoichiometric ZnO and CdS nanoclusters (NCs) by low-energy cluster beam deposition technique, and examining their morphological and compositional evolution over a long span of time. It is concluded that the transition of vapor to solid goes through a transient liquid phase: coagulation of a large number of atomic clusters first forms liquid NCs which then solidify. The nature of the material and the experimental conditions determine crystallinity and shape of the NCs during the solidification process.

Single-Electron Activation of CO2 on Graphene-Supported ZnO Nanoclusters: Effects of Doping in the Support

Use of solar energy to convert the greenhouse gas CO2 into useful chemicals or fuels could not only reduce the accumulation of CO2 in the atmosphere but also provide a solution to sustainable energy development. There has been much interest in understanding the mechanistic role of graphene when added to semiconductor nanostructures to reduce CO2 because of the observation of enhanced photocatalytic activities in recent experiments. In this work, we investigate the adsorption and single-electron activation of CO2 on ZnO nanoclusters with and without a modified graphene support using a first-principles approach, with a special focus on the effect of the support. The formation of the CO2– anion is identified under simulated photoexcitation conditions and is energetically more favorable than for previously studied oxide photocatalysts. The calculated results suggest that single-heteroatom doping in graphene has a significant impact on the catalytic activity of ZnO. The electronic coupling between the support ...

A computational study on the experimentally observed sensitivity of Ga-doped ZnO nanocluster toward CO gas

Metal doped ZnO nanostructures have attracted extensive attention as chemical sensors for toxic gases. An experimental study has previously shown that Ga-doped ZnO nanostructures significantly show a higher electronic response than the undoped sample toward CO gas. Here, the electronic sensitivity of pristine and Ga-doped ZnO nanoclusters to CO gas is explored using density functional theory computations (at B3LYP, PBE, M06-2X, and ωB97XD levels). Our results reproduce and clarify the electrical behavior which has been observed experimentally from the ZnO nanoparticles after the exposure to CO gas. We showed that the calculated change of HOMO-LUMO gap may be a proper index for the change of electrical conductance which is measurable experimentally. It was found that, in contrast to the pristine ZnO nanocluster, the electronic properties of Ga-doped cluster are sharply sensitive to the presence of CO gas which is in good accordance with the results of the experimental study.

Transition metal doped ZnO nanoclusters for carbon monoxide detection: DFT studies.

Metal doped ZnO nanomaterials have attracted considerable attention as a chemical sensor for toxic gases. Here, the electronic sensitivity of pristine and Sc-, Ti-, V-, Cr-, Mn-, and Fe-doped Zn12O12 nanoclusters toward CO gas is investigated using density functional theory calculations. It is found that replacing a Zn atom by a Sc or Ti atom does not change the sensitivity of cluster but doping V and Cr atoms significantly increase the sensitivity. Also, Mn, or Fe doping slightly improves the sensitivity. It is predicted that among all, the Cr-doped ZnO cluster may be the most favorable sensor for CO detection because its electrical conductivity considerably changes after the CO adsorption, thereby, generating an electrical signal. The calculated Gibbs free energy change for the adsorption of CO molecule on the Cr-doped cluster is about -51.2kcal mol(-1) at 298.15K and 1 atm, and the HOMO-LUMO gap of the adsorbent is changed by about 117.8%.

Rare earth functionalization effect in optical response of ZnO nano clusters

The electronic structure of rare earth (RE) doped Zn12O12 clusters – namely, REZn11O12 and RE2Zn10O12 with RE = Nd, Eu and Gd have been investigated within the framework of density functional theory formalism. Doping of a RE atom is found to be energetically favorable in this zinc oxide cluster. We have found that the cage structure of the host cluster Zn12O12 does not change significantly by the substitutional doping of a RE atom on Zn sites. The magnetic coupling between RE ions in the host cluster is found to be ferromagnetic. The static polarizabilities and optical properties of the RE doped Zn12O12 clusters have been studied based on the time dependent density functional theory. With RE doping, the polarizability increases as compared to that of the host cluster. The analysis of the optical absorption spectra indicate that the f electrons in RE doped clusters are significantly more involved in low-energy transitions. For Eu doped clusters give rise to more quenched oscillator strengths as compared to that of Nd and Gd doped zinc oxide clusters. With the increase in number of RE atoms, the red shift is observed in the optical spectrum of the zinc oxide cluster.

Photoelectrochemical water splitting strongly enhanced in fast-grown ZnO nanotree and nanocluster structures.

We demonstrate selective growth of ZnO branched nanostructures: from nanorod clusters (with branches parallel to parent rods) to nanotrees (with branches perpendicular to parent rods). The growth of these structures was realized using a three-step approach: electrodeposition of nanorods (NRs), followed by the sputtering of ZnO seed layers, followed by the growth of branched arms using hydrothermal growth. The density, size and direction of the branches were tailored by tuning the deposition parameters. To our knowledge, this is the first report of control of branch direction. The photoelectrochemical (PEC) performance of the ZnO nanostructures follows the order: nanotrees (NTs) > nanorod clusters (NCs) > parent NRs. The NT structure with the best PEC performance also possesses the shortest fabrication period which had never been reported before. The photocurrent of the NT and NC photoelectrodes is 0.67 and 0.56 mA cm-2 at 1 V vs. Ag/AgCl, respectively, an enhancement of 139% and 100% when compared to the ZnO NR structures. The key reason for the improved performance is shown to be the very large surface-to-volume ratios in the branched nanostructures, which gives rise to enhanced light absorption, improved charge transfer across the nanostructure/electrolyte interfaces to the electrolyte and efficient charge transport within the material.

Co-doped ZnO Thin Films Fabricated by a Nanocluster-Beam Deposition System and the Influence of Flow Rate of Helium Gas on Their Properties

As a pioneer, Co-doped ZnO nanocluster-assembled thin films were fabricated by a nanocluster-beam deposition system and the influence of the flow rate of helium gas on the properties of the films was also investigated. Transmission electron microscopy indicated that Co-doped ZnO nanoclusters maintained a wurtzite structure as that of bulk materials. Also, it is found that the average size of ZnO nanoclusters decreased with the increased flow rate of helium gas. Two photoluminescence (PL) bands at 378 and 510 nm were observed. The Co-doped ZnO nanocluster-assembled thin films exhibited ferromagnetic property at room temperature. As the flow rate of helium gas increased, the corresponding saturation magnetization of the nanocluster-assembled thin films decreased from 11 to 6 μemu.

Anchoring Mechanism of ZnO Nanoparticles on Graphitic Carbon Nanofiber Surfaces through a Modified Co-Precipitation Method to Improve Interfacial Contact and Photocatalytic Performance.

A facile three-step co-precipitation method is developed to synthesize graphitic carbon nanofibers (CNFs) decorated with ZnO nanoparticles (NPs). By interchanging intermediate steps of the reaction processes, two kinds of nanohybrids are fabricated with stark morphological and physicochemical differences. The morphologies differ because of the different chemical environments of the NP/nanocluster formation. The hybrid with larger and non-uniform ZnO nanocluster size is formed in liquid phase and resulted in considerable interfacial defects that deteriorate the charge-transfer properties. The hybrid with smaller and uniform ZnO NPs was formed in a dry solid phase and produced near-defect-free interfaces, leading to efficient charge transfer for superior photocatalytic performance. The results broaden the understanding of the anchoring/bonding mechanism in ZnO/CNF hybrid formation and may facilitate further development of more effective exfoliation strategies for the preparation of high-performance composites/hybrids.

Quantum Chemical Studies Of Nitrogen Substitution On ZnO Nanoclusters Stability

Importance of p-type transparent conducting oxide (TCO) is much needed in the optoelectronics industry. Due to lack of intrinsic p-type TCO, it is necessary to design or tune the properties existing n-type TCO are very essential. This present work describes, n-type ZnO is tuned to p-type by doping of nitrogen on to the nanocluster. The structural stability of ZnxOx-1N for x=(2-5) is optimized using Gaussian 09 program package with a B3LYP/6-31G level basis set. The optimization result shows that when the cluster size increases the stability also increases. The dipole moment depends on the structure of the ZnxOx-1N cluster. These optimized structural geometries are used to calculate the binding energy, HOMO-LUMO energy gap, ionization potential and electron affinity of nanoclusters. The binding energy for ring structures is found to be more than the other two structures. Vibrational analysis is carried out for all the structures and reported. The ring structure is found to be more stable than the linear and 3D structures. The findings of the present work will provide an insight to synthesis, p-type ZnO nanoclusters.

Pressurized polyol synthesis of Al-doped ZnO nanoclusters with high electrical conductivity and low near-infrared transmittance

In this study, a novel pressurized polyol method is proposed to synthesize aluminum-doped ZnO (AZO) nanoclusters without utilizing additional thermal treatment to avoid the merging of nanoclusters. The size of the AZO nanoclusters range from 100 to 150nm with a resistivity of 204Ωcm. The AZO nanoclusters primarily consist of approximately 10-nm nanocrystals that form a spherically clustered morphology. A two-stage growth model has been proposed based on the results of scanning electron microscopy and transmission electron microscopy images, nanocluster sizes, and X-ray diffraction patterns. The primary AZO nanocrystals first nucleate under pressurized conditions and then spontaneously aggregate into larger nanoclusters. Optically, the AZO nanoclusters exhibit a significant decrease in the near-infrared (NIR) transmittance compared to pure ZnO nanoparticles. The NIR blocking efficiency of AZO nanoclusters reached 85%. Moreover, the doping efficiency, resistivity, and NIR transmittance of AZO nanoclusters are influenced by the reaction time in the pressurized polyol solution. On the other hand, the reaction time has no effect on the particle size and crystallinity. An optically transparent coating for the AZO nanoclusters, which consisted of iso-propanol solvent and ultraviolet-curable acrylic binder, was also demonstrated.

Si@C nanosponges application for lithium ions batteries synthesized by templated magnesiothermic route

Carbon coated silicon nanosponges (Si@C NSs) used for anode in lithium ion batteries have been synthesized at a large scale via a templated magnesiothermic route, in which cheap ZnO nanoclusters and non-toxic glucose were employed to serve as sacrificial template and carbon source, respectively. The microstructure of Si@C NSs and its electrochemical performance in lithium ion batteries were investigated in detail. A reversible capacity with little fading after 500 cycles was obtained and the possible reasons for such superiority were discussed. The present results indicate potential application of the as-obtained Si@C NSs in lithium ion batteries.

Ab initio study of vibrational and optical properties of stable ZnmOn(m + n = 2 to 5) nanoclusters

An ab initio study has been performed for the various properties of the most stable configuration out of the various configurations having “m ” number of Zn and “n ” number of O atoms i.e. ZnmOn(m + n = 2 to 5) nanoclusters by employing B3LYP-DFT/6-311G(3df) method. We report here the vibrational frequencies, IR intensities, Rel IR intensities, Raman scattering activities and optical absorption for these nanoclusters. The structure having minimum energy out of all the configurations having similar values of “m ” and “n ” is considered as the most stable. We found that all the different configurations of ZnO4, Zn2O3 and Zn4O complexes are not stable because they possess at least one vibrational frequency which is imaginary. The high vibrational frequencies of each nanocluster arise from the symmetrical and asymmetrical stretching vibrations whereas the lower frequencies belong to the wagging, rocking and the out-of-plane vibrations of Zn and O atoms. Our predicated results for the most intense minimum excitation energies of the ZnO and Zn2O2 nanoclusters exhibit excellent agreement with the available experimental data. All the nanoclusters show strong absorption in the ultraviolet region but some also exhibit weak absorption in the visible region.