Surface Of ZnO Nanocrystals Research Articles

Enhanced visible light photocatalytic and electrochemical performance of Au modified zinc oxide nanocolumns

Research on the treatment of Acid Black 1 as an azo dye is significant because it is toxic and can have adverse effects on both human health and the ecosystem if not properly treated. The purpose of the research was the preparation and application of Au-ZnO nanocolumn (Au-ZnO NCs) photocatalysts to treat the azo dye Acid Black 1 (AB1) in industrial wastewater when it was exposed to light. The structural, electrochemical, and optical properties of prepared photocatalysts were analyzed. Results of structural analyses using SEM and XRD indicated that the surfaces of the ZnO NCs were successfully coated with Au nanoparticles to produce a large effective surface area. Results of electrochemical experiments showed that depositing conductive Au nanoparticles on the surface of ZnO nanocrystals significantly increased the charge transfer rate, implied a longer lifetime for photo-generated electrons, and increased the separation efficiency of Au-ZnO NCs. Results of optical studies showed that the band gap energies of ZnO NCs and Au-ZnO NCs could be measured to be 3.19 eV and 2.96 eV, respectively, which demonstrated that Au nanoparticles narrowed the band gap energy and caused the absorption to extend to the visible range. Results of studies on photocatalytic degradation showed that Au-ZnO NCs, as opposed to ZnO NCs, removed AB1 photocatalytically more quickly. After 20, 30, 45, 50, and 100 min of visible-light illumination, respectively, the total degradation of AB1 was 10, 25, 100, 125, and 300 mg/L. The Au-ZnO NCs' ability to treat 200 ml of a 100 mg/L AB1 solution using a real industrial wastewater sample and deionized water was studied. The results showed that because there are more pollutants in industrial wastewater, the total treatment of the real sample required more time. The results thus showed that AB1 from actual industrial wastewater was successfully photocatalytically treated in an Au-ZnO NCs photocatalyst.

Amino-capped zinc oxide modified tin oxide electron transport layer for efficient perovskite solar cells

Electron transport layer (ETL)/perovskite interface passivation is particularly challenging because of the use of polar solvents (e.g., DMF) for perovskite solution deposition, which usually destroy the bottom as-formed defect passivation layers. Herein, a novel multifunctional composite ETL, NH2[email protected]2, is prepared by mixing amino-capped ZnO (NH2-ZnO) nanocrystals (NCs) with SnO2 nanoparticles. The best-performing PSCs on the basis of NH2[email protected]2 achieve efficiency of 22.52%, which is significantly higher than that of the pristine SnO2 counterpart (18.45%). The enhanced performance of the NH2[email protected]2 ETL can be attributed to higher electron extraction capacity, better energy-level alignment with perovskite material, and more efficient carrier transport in device. Most important, the NH2 groups on the surface of ZnO NCs can effectively passivate the under-coordinated Pb2+ ions from perovskite films, thus reducing charge recombination at ETL/perovskite interface. The results suggest that NH2-ZnO [email protected]2 composite is a promising ETL for improving the performance of PSCs.

Performance enhancement of heterojunction ZnO/PbS quantum dot solar cells by interface engineering

Effective charge transport through heterojunction interface in Quantum dots (QDs) based solar cells is difficult due to the spherical structure of QDs. Therefore tuning of QDs size for uniform and defect free heterojunction interface becomes very important to get optimum device performance. Here we have optimized the performance of ZnO Nanocrystals (NCs)/lead sulfide (PbS) QDs heterojunction solar cell by controlling the surface roughness of ZnO NCs thin films. The synthesized ZnO NCs aggregation was tuned by varying the binary solvent volume ratio of chloroform and methanol (CF:MeOH), which leads to the different surface roughness. The thin film of ZnO NCs having surface roughness comparable to the size of PbS QDs performs better as compared to those devices having different surface roughness. The optimum device fabricated in an ambient atmosphere with ZnO NCs thin film having surface roughness 4.36 nm, and nanocrystal size 3.32 nm as synthesized with (CF:MeOH) volume ratio of (2:1), shows the highest efficiency of 2.94% among other devices in the same structure due to decreased leakage current and improved junction interface contact between active materials. This work reveals that the controllability of ZnO NCs surface roughness and its correlation with the size of QDs can contribute to further improvement of low cost QDs based solar cells.

Adsorption behaviors of gas molecules on the surface of ZnO nanocrystals under UV irradiation

A UV-activated room temperature chemiresistive gas sensor based on ZnO nanocrystals film was fabricated. Its gas sensing properties under various conditions were also investigated in detail. The results shed new insight into the adsorption behaviors of gas molecules on the surface of ZnO nanocrystals under UV irradiation. The chemisorbed oxygen species (O2(ads)−(hv)) induced by UV light govern the adsorption and desorption ways of other gas molecules on the surface of ZnO nanocrystals, which is dependent on the electron affinity of gas molecules. Gas molecules with higher electron affinity than oxygen molecules can be adsorbed on the surface by the competitive adsorption way, extracting electrons from the surface. Gas molecules with lower electron affinity than oxygen molecules are attracted by the adsorbed O2(ads)−(hv) layer, releasing electrons to the surface. These processes can influence the gas sensing properties of the sensor. Our findings will pave the way for the fundamental understanding and design of UV-activated gas sensor in the future.

Emission and Structure-Varying ZnO and Carbon Nanocrystal Composite in Mechanical Processing

Morphology, photoluminescence (PL), and Raman scattering spectra have been investigated for mixtures of ZnO+0.1%C nanocrystals (NCs) at different stages of mechanical processing (MP). The transformation of graphite into graphene monolayers covering the ZnO NC surface is revealed for the first MP stage. The interaction with oxygen has been detected in the second MP stage which leads to the dissolution of oxygen interstitials in the ZnO NCs and to the formation of graphene (graphite) oxides. Increasing the concentration of the oxygen interstitials in ZnO NCs also enhances the intensity stimulation of the orange PL band (2.18eV). Simultaneously, the PL band peaking at 2.82–2.90 eV is detected in the PL spectra of the ZnO+0.1%C NC mixture after MP for 9–90 min. Then, the variation of the ZnO NC shape, agglomeration of ZnO NCs, modification of ZnO defects and decreasing PL intensity have been detected after prolonged MP for 390 min. It is expected that short stages of MP can be useful for ZnO NC surface covering by graphene layers or graphene (graphite) oxides.

A highly sensitive chemical gas detecting device based on N-doped ZnO as a modified nanostructure media: A DFT+NBO analysis

We presented a density functional theory study of the adsorption of O3 and NO2 molecules on ZnO nanoparticles. Various adsorption geometries of O3 and NO2 over the nanoparticles were considered. For both O3 and NO2 adsorption systems, it was found that the adsorption on the N-doped nanoparticle is more favorable in energy than that on the pristine one. Therefore, the N-doped ZnO has a better efficiency to be utilized as O3 and NO2 detection device. For all cases, the binding sites were located on the zinc atoms of the nanoparticle. The charge analysis based on natural bond orbital (NBO) analysis indicates that charge was transferred from the surface to the adsorbed molecule. The projected density of states of the interacting atoms represent the formation of chemical bonds at the interface region. Molecular orbitals of the adsorption systems indicate that the HOMOs were mainly localized on the adsorbed O3 and NO2 molecules, whereas the electronic densities in the LUMOs were dominant at the ZnO nanocrystal surface. By examining the distribution of spin densities, we found that the magnetization was mainly located over the adsorbed molecules. For NO2 adsorbate, we found that the symmetric and asymmetric stretches were shifted to a lower frequency. The bending stretch mode was shifted to the higher frequency. Our DFT results thus provide a theoretical basis for why the adsorption of O3 and NO2 molecules on the N-doped ZnO nanoparticles may increase, giving rise to design and development of innovative and highly efficient sensor devices for O3 and NO2 recognition.

ADSORPTION OF H2 AND O2 GASES ON ZnO WURTZOID NANOCRYSTALS: A DFT STUDY

In the present work, we apply wurtzoids nanocrystals with density functional theory to explain the sensitivity of ZnO nanostructures towards hydrogen and oxygen molecules. Present results of ZnO nanocrystals’ sensing to H2 and O2 molecules show a reduction in the energy gap and hence electrical resistivity of ZnO nanocrystals upon attachments of these molecules in agreement with experiment. The results also show that higher temperatures increase the sensitivity of ZnO wurtzoids towards H2 and O2 molecules with the maximum sensitivity approximately at 390[Formula: see text]C and 417[Formula: see text]C for H2 and O2 molecules, respectively, after which it begins to decline according to calculated Gibbs free energy. These temperatures are comparable with experimentally reported operating temperatures of 325[Formula: see text]C and 350[Formula: see text]C for the two gases, respectively. The main reaction mechanism is the dissociation of H2 or O2 molecules on ZnO nanocrystal surface in which hydrogen and oxygen atoms are attached to neighboring Zn and O surface atoms. The removal of these molecules from the surface is also performed by the formation of H2 and O2 molecules prior to their removal from the ZnO nanocrystal surface. Electronic charge transfers to the adsorbed atoms and molecules confirm and illustrate the mechanism mentioned above.

Chlorine gas reaction with ZnO wurtzoid nanocrystals as a function of temperature: a DFT study.

In the present work, we applied density functional theory and temperature-dependent Gibbs free energy calculations to wurtzoid structures to explain the sensitivity of ZnO nanocrystals towards chlorine molecules. In agreement with experimental findings, our results revealed that chlorine sensing under ambient conditions is feasible. Higher temperatures increased the sensitivity of ZnO nanocrystals towards chlorine gas molecules. Peak calculated sensitivities were in the temperature ranges (167-220°C), (447-578°C) and (952-1159°C), which is in good agreement with experimentally determined temperatures. According to the calculated Gibbs free energy, these three ranges correspond to the van der Waals attachment of Cl2 molecules on Zn-polar sites, van der Waals attachment of Cl2 molecules on O sites, and dissociation of Cl2 molecules on ZnO nanocrystal surfaces, respectively. The removal of chlorine atoms from the surface of ZnO nanocrystals is difficult at low temperatures because of the high electron affinity of chlorine gas atoms, which results in a long recovery time and accumulation of chlorine atoms and molecules on the ZnO surface. Atomic charges and charge transfer are depicted using natural bond orbital analysis to explain the present mechanisms.

In-situ second harmonic generation by cancer cell targeting ZnO nanocrystals to effect photodynamic action in subcellular space.

This paper introduces the concept of in-situ upconversion of deep penetrating near infrared light via second harmonic generation from ZnO nanocrystals delivered into cells to effect photo activated therapies, such as photodynamic therapy, which usually require activation by visible light with limited penetration through biological tissues. We demonstrated this concept by subcellular activation of a photodynamic therapy drug, Chlorin e6, excited within its strong absorption Soret band by the second harmonic (SH) light, generated at 409nm by ZnO nanocrystals, which were targeted to cancer cells and internalized through the folate-receptor mediated endocytosis. By a combination of theoretical modeling and experimental measurements, we show that SH light, generated in-situ by ZnO nanocrystals significantly contributes to activation of photosensitizer, leading to cell death through both apoptotic and necrotic pathways initiated in the cytoplasm. This targeted photodynamic action was studied using label-free Coherent Anti-Stokes Raman Scattering imaging of the treated cells to monitor changes in the distribution of native cellular proteins and lipids. We found that initiation of photodynamic therapy with upconverted light led to global reduction in the intracellular concentration of macromolecules, likely due to suppression of proteins and lipids synthesis, which could be considered as a real-time indicator of cellular damage from photodynamic treatment. In prospective applications this in-situ photon upconversion could be further extended using ZnO nanocrystals surface functionalized with a specific organelle targeting group, provided a powerful approach to identify and consequently maximize a cellular response to phototherapy, selectively initiated in a specific cellular organelle.

Au/ZnO hybrid nanocatalysts impregnated in N-doped graphene for simultaneous determination of ascorbic acid, acetaminophen and dopamine

The formation of nitrogen-doped (N-doped) graphene uses hydrothermal method with urea as reducing agent and nitrogen source. The surface elemental composition of the catalyst was analyzed through XPS, which showed a high content of a total N species (7.12at.%), indicative of the effective N-doping, present in the form of pyridinic N, pyrrolic N and graphitic N groups. Moreover, Au nanoparticles deposited on ZnO nanocrystals surface, forming Au/ZnO hybrid nanocatalysts, undergo a super-hydrophobic to super-hydrophilic conversion. Herein, we present Au/ZnO hybrid nanocatalysts impregnated in N-doped graphene sheets through sonication technique of the Au/ZnO/N-doped graphene hybrid nanostructures. The as-prepared Au/ZnO/N-doped graphene hybrid nanostructure modified glassy carbon electrode (Au/ZnO/N-doped graphene/GCE) was first employed for the simultaneous determination of ascorbic acid (AA), dopamine (DA) and acetaminophen (AC). The oxidation over-potentials of AA, DA and AC decreased dramatically, and their oxidation peak currents increased significantly at Au/ZnO/N-doped graphene/GCE compared to those obtained at the N-doped graphene/GCE and bare CCE. The peak separations between AA and DA, DA and AC, and AC and AA are large up to 195, 198 and 393mV, respectively. The calibration curves for AA, DA and AC were obtained in the range of 30.00–13.00×103, 2.00–0.18×103 and 5.00–3.10×103μM, respectively. The detection limits (S/N=3) were 5.00, 0.40 and 0.80μM for AA, DA and AC, respectively.

Surface and defect modifications in mixture of ZnO and carbon nanocrystals at mechanical processing

The Raman scattering and photoluminescence (PL) spectra, PL temperature dependences and scanning electronic microscopy (SEM) images have been studied in the mixture of ZnO+1.0%C nanocrystals (NCs) before and after intensive mechanical processing (MP). The study reflects the diversity of physical processes occurring at MP: the amorphization of the ZnO NC surface, transforming an amorphous graphite into graphene monolayers covered of ZnO NCs, changing the ZnO NC shape owing to the plastic deformation and crushing the individual ZnO NCs, the partial oxidation of graphene layers with the formation of graphene oxide passivating ZnO NCs, the formation of graphite oxide when temperature increasing at MP, as well as the ZnO defect modification near the surface of ZnO NCs, etc. Additionally the new ‘blue” PL band peaked at 2.82–2.92eV has been revealed in PL spectra after MP, which is assigned to emission of graphite (graphene) oxides. All mentioned processed have been studied and discussed in details.

Surface modification in mixture of ZnO + 3%C nanocrystals stimulated by mechanical processing

The photoluminescence (PL), Raman scattering and SEM images for the mixture of ZnO + 3% C nanocrystals (NCs) have been studied before and after of intensive mechanical processing (MP) with the aim to identify the nature of defects. The study reflects the diversity of physical processes occurring at MP: amorphizating the surface of ZnO NCs, crushing the individual ZnO NCs and carbon nanoparticles, covering the ZnO NC surface by the graphene layers, the oxidation partially of the graphene layers, carbon and ZnO NCs etc. Three stages of MP have been revealed which are accompanied by PL spectrum transformations: i) amorphizating the ZnO NC surface together with the generation of nonradiative recombination centers, ii) passivating the ZnO NC surface by the graphene layer with its oxidation partially and iii) further crushing of ZnO NCs, the oxidation of ZnO NCs and the formation of graphene (graphite) oxides. The new PL band peaked at 2.88 eV has been detected after 9 min of MP. Note that the passivation of the ZnO NC surface by graphene layer can be interesting for future technological applications.

Ion exchange induced removal of Pb(ii) by MOF-derived magnetic inorganic sorbents.

Nanoporous adsorbents of ZnO/ZnFe2O4/C were synthesized by using a metal organic framework (Fe(III)-modified MOF-5) as both the precursor and the self-sacrificing template. The adsorption properties of ZnO/ZnFe2O4/C toward Pb(ii) ions were investigated, including the pH effect, adsorption equilibrium and adsorption kinetics. The adsorption isotherms and kinetics were well described by using the Langmuir isotherm model and pseudo-second-order model, respectively. The MOF-derived inorganic adsorbents exhibited high absorption performance with a maximum adsorption capacity of 344.83 mg g(-1). X-ray powder diffraction and high-resolution X-ray photoelectron spectroscopy suggest that Zn(ii) was substituted by a significant portion of Pb(ii) on the surface of ZnO nanocrystals. Microscopic observations also demonstrate the effect of Pb(ii) ions on ZnO crystals as reflected by the considerably reduced average particle size and defective outer layer. Quantitative measurement of the released Zn(ii) ions and the adsorbed Pb(ii) ions indicated a nearly linear relationship (R(2) = 0.977). Moreover, Pb-containing ZnO/ZnFe2O4/C adsorbents are strongly magnetic allowing their separation from the water environment by an external magnet.

'Clickable' ZnO nanocrystals: the superiority of a novel organometallic approach over the inorganic sol-gel procedure.

We demonstrate for the first time a highly efficient Cu(i)-catalyzed alkyne-azide cycloaddition reaction on the surface of ZnO nanocrystals with retention of their photoluminescence properties. Our comparative studies highlight the superiority of a novel self-supporting organometallic method for the preparation of brightly luminescent and well-passivated ZnO nanocrystals over the traditional sol-gel procedure.

Switch-On fluorescence and photo-induced electron transfer of 3-aminopropyltriethoxysilane to ZnO: Dual applications in sensors and antibacterial activity

Binding interaction studies of 3-aminopropyltriethoxysilane (APTS) with sol–gel synthesized ZnO nanocrystals have been carried out by absorption and fluorescence spectral studies. Astonishing consequences have been observed due to electron transfer from APTS to sol–gel synthesized ZnO. APTS acts as a new host for sol–gel synthesized ZnO nanocrystals that augment its fluorescence due to the electron transfer. APTS adsorbs on the surface of the ZnO nanoparticles. Dispersion studies, SEM and FT-IR confirm the adsorption of APTS on the surface of ZnO nanocrystals. Surface treatment of nanoparticles by the fluorophore APTS and its mechanism are discussed in detail. Bacteriological tests such as disk diffusion have been done in nutrient agar media on solid agar plates using different concentrations of ZnO. f-ZnO nanoparticles show remarkable biocidal activity when compared with n-ZnO in repeated experiments.

Origin of thermal annealing-induced orange emissions from solution-grown ZnO nanocrystals

ZnO nanocrystals were synthesised via solution route and then thermally annealed in air in the temperature range of 60–900°C. The correlation between the hydroxyl groups on the surface of ZnO nanocrystals and the orange photoluminescence (PL) of ZnO nanocrystals was investigated. The crystal structure, surface chemistry and luminescent properties of thermally annealed ZnO nanocrystals were examined with X-ray diffractometer, Fourier transform infrared (FTIR) spectroscope, PL spectrophotometer, transmission electron microscope (TEM) and selected area electron diffraction technique, respectively. It is found that an orange PL band appeared in the PL spectrum at about 600 nm only when the annealing temperature was higher than 600°C. By contrast, the hydroxyl groups were always present on the solution-grown ZnO nanocrystals as the annealing temperature varied in the range of 60–900°C. Our results demonstrate that the orange PL band has no direct correlation with the hydroxyl groups on the surfaces of the ZnO nanocrystals. Thermally induced defects are proposed to be responsible for the orange PL band.

Understanding the binding interaction of imidazole with ZnO nanomaterials and clusters

The order of binding energy values for imidazole adsorbed ZnO clusters through the preferred azomethine nitrogen site is imidazole–Zn4O4 (R) > imidazole–Zn3O3 > imidazole–Zn4O4 (W) > imidazole–Zn2O2.

Donor–acceptor binding interaction of 1-(naphthalene-1-yl)-2,4,5-triphenyl-1H-imidazole with semiconductor nanomaterials

The dynamics of photoinduced electron injection from 1-(naphthalene-1-yl)-2,4,5-triphenyl-1H-imidazole (NTI) to pristine ZnO, Mn-doped TiO2 and BaTiO3 nanoparticles have been studied by absorption, fluorescence and lifetime spectroscopic methods. Both the absorption and fluorescence results suggest the association between the nanoparticles and NTI. The calculated free energy change (ΔGet) confirms the electron injection from NTI to nano semiconductors. The critical energy transfer distance between NTI and the nanoparticles have been deduced. The emission of NTI is enhanced by pristine ZnO and quenched by Mn-doped TiO2 and BaTiO3 nanoparticles which are likely due to change of LUMO and HOMO levels of NTI on its association with nano semiconductors. The strong adsorption of the NTI on the surface of ZnO nanocrystals is likely due to the chemical affinity of the nitrogen atom of the NTI to the zinc ion on the surface of nanocrystals. Electron injection from photoexcited NTI to the CB(S∗→S++e−CB) is likely to be the reason for the fluorescence enhancement.

PbS纳米晶修饰的ZnO/PVP复合电双稳器件阻变机制

An organic /inorganic composite electrical bistable device with the sandwich structure of ITO / PVP /ZnO nanocrystal(NCs) /PbS nanocrystal(NCs) /PVP /Al was fabricated.Compared with the device structure of ITO /PVP /ZnO NPs /PVP /Al,the ON /OFF ratio of target device was improved by 2 orders of magnitude.The carrier transportation and resistance switching mechanism of device were discussed via I-V characteristics and the energy band structures of the devices.The effective capture /release of electrons could be attributed to the optimization of device structure and modification of ZnO NCs surface by PbS NCs layer.

Influence of ZnO nanocrystals surface modification on structure and photovoltaic properties of MEH-PPV/nc-ZnO nanocomposite films

Influence of ZnO nanocrystals surface modification on structure and photovoltaic properties of MEH-PPV/nc-ZnO nanocomposite films