Surface Of ZnO Nanorods Research Articles

Fabrication of Bi2MoO6/ZnO hierarchical heterostructures with enhanced visible-light photocatalytic activity

The construction of heterostructures is regarded as an excellent strategy to achieve efficient charge separation and improved photocatalytic activity. Herein, a series of Bi2MoO6/ZnO hierarchical heterostructured photocatalysts were synthesized by a solvothermal method. The morphology of Bi2MoO6 grown on the surface of ZnO nanorods could be controlled by adjusting the experimental conditions. The synthesized samples were characterized by various analytical techniques and their photocatalytic performance was evaluated by photocatalytic reduction of Cr(VI) under visible-light irradiation. Compared with those of pure Bi2MoO6 and ZnO, the Bi2MoO6/ZnO composites showed higher photocatalytic activity towards the reduction of Cr(VI). The enhanced photocatalytic activity was mainly attributed to the formation of a heterojunction between Bi2MoO6 and ZnO, which effectively facilitated the separation and transfer of electrons and holes. In addition, the Bi2MoO6/ZnO photocatalysts maintained good stability after three cycles of Cr(VI) photoreduction. A possible photocatalytic mechanism of the as-synthesized composites was proposed.

Ag2S/ZnO Nanorods Composite Photoelectrode Prepared by Hydrothermal Method: Influence of Growth Temperature

Silver sulfide (Ag2S) attached to ZnO nanorods film has been successfully prepared by a two-step hydrothermal process. The prepared photoelectrodes were characterized by Field Emission Scanning Eletron Microscopy (FESEM), Energy Dispersive Spectroscopy (EDS), X-ray diffractometry (XRD), UV-vis spectrophotometry (UV–vis) and photoelectrochemical measurements (PEC). The results revealed that Ag2S deposited onto the surface of ZnO nanorods was under the influence of growth temperature. The absorption in the visible region and efficient transfer of photoelectron between Ag2S/ZnO/ITO photoelectrode and the redox species in the electrolyte were significantly enhanced under the illumination intensity of 100 mW cm−2. The band gap values of the five different growth temperatures ranging from 50 °C to 90 °C were found to decrease from 2.93 eV to 1.57 eV. Besides, the photoelectrochemical measurements verified that Ag2S/ZnO nanocomposite grown at 60 °C exhibited the highest photocurrent density of 3.97 mA/cm2 at 1.0 V. The findings suggested that Ag2S/ZnO nanocomposite is an efficient alternative for photoelectrode applications.

Tailoring the active surface sites of ZnO nanorods on the glass substrate for photocatalytic activity enhancement

The urgent need for inexpensive, efficient, ease in use and reusable photocatalyst has attracted the attention of many researchers. Here, the photocatalyst ZnO nanorods were synthesized with a simple, low cost and rapid ultrasonic spray pyrolysis method and then grown with a hydrothermal method for 2, 4, 6 and 8 h. The ZnO nanorods grown for 6 h shows the highest photocatalytic efficiency although do not have the largest surface area. This is probably due to its surface is dominated by (002) facets as indicated by the highest texture coefficient (TC) value. The (002) polar facets consisting of a positive Zn-terminated (002) facets and a negative O-terminated (00-2) facets may play an important role in more efficient UV absorption and photogenerated charges separation. Moreover, the highest crystallite size as shown in XRD result provide pathways for electrons and holes for redox reactions on the surface of ZnO nanorods. The synthesized ZnO nanorods also contain high concentration of oxygen interstitial as a source of holes that react with the OH− ions that easily adsorbed on (002) facets.

Preparation of MoS2@ZnO heterostructure with enhanced photocatalytic activity

In order to effectively solve environmental problems induced by organic pollutants, the MoS2@ZnO heterostructure is successfully fabricated via a two-step hydrothermal method, in which the MoS2 nanosheets grew on the surface of ZnO nanorods. The synthetic heterostructure is demonstrated to possess excellent photocatalytic activity in the degradation of methylene blue (MB) under ultraviolet (UV) and visible light irradiation. The corresponding photodegradation rate constant of MoS2@ZnO heterostructure reaches up to 0.02075 min−1 and 0.00916 min−1, which are higher than that of pure ZnO (0.00514 min−1 and 0.00048 min−1) under the same reaction conditions. Obviously, compared with pure ZnO, the photocatalytic activity of the MoS2@ZnO heterostructure has been significantly improved. This could be attributed to the increased specific surface area of photocatalyst and the formation of heterostructure between ZnO and MoS2 after loading the MoS2 on the surface of ZnO nanorods. Which help to provide more reaction sites for the adsorption of pollutant and improve the separation efficiency of photogenerated electron-hole pairs.

Periodic ZnO-Elevated Gold Dimer Nanostructures for Surface-Enhanced Raman Scattering Applications

Electron beam lithography-defined periodic ZnO-elevated gold dimer nanostructures for surface-enhanced Raman spectroscopy with strong electromagnetic field enhancements were fabricated in the present work. The ZnO nanorods (NRs) were grown on the patterned substrate using a hydrothermal process, and Au dimers were deposited on the top surface of ZnO NRs, forming suspended gold dimers and providing plasmonic hot-spots with nanocavity effect. Effects of the dimer radius, gap size, and ZnO NR height on the dark-field scattering spectra and surface-enhanced Raman scattering (SERS) were investigated experimentally and theoretically. The SERS response was enhanced with increasing dimer radius and decreasing gap size. The SERS enhancement factor (EF) due to the localized surface plasmon resonance was calculated from simulation results. The calculated EF as a function of ZnO NR height showed the periodic trend of electric field intensity with the periodicity of about half of the exciting laser wavelength, and was...

Migration Energy Barriers for the Surface and Bulk of Self-Assembly ZnO Nanorods

Post-annealing treatment is a necessary process to create/eliminate/repair defects in self–assembly (SA) metal oxide by providing enough thermal energy to the O atoms to overcome the migration energy barrier in ZnO. The height of migration energy barrier is dependent on the depth from the surface, which is hard to be estimated by theoretical calculations, as well as the optical analyses. SA ZnO nanorods (ZNRs) have high surface-to-volume ratio to provide complete picture between the optical and surface properties obtained by photoluminescence (PL) and ultraviolet/X-ray photoemission spectroscopy (UPS/XPS), which is used to investigate the evolution of structure and chemical states of the surface layers to reveal mutual agreement on all observations in PL, XPS, and UPS. We demonstrate variation of the surface structure of SA-ZNRs by scanning over a range of annealing temperatures and time to regulate the structure variation of SA-ZNRs, and their optical analyses agrees well with PL, XPS and UPS, which indicates the dependence of migration energy barriers on the depth from the surface of ZNR. The results reveal the well ZNRs formed at 570 °C and the further oxidation process and the formation of hydroperoxide on the Zn-rich surface of ZNRs at 640 °C.

New Insights into Mechanism of Surface Reactions of ZnO Nanorods During Electrons Beam Irradiation.

This study provides new insight into mechanisms of ionic reactions on the surface of ZnO nanorod networks, which could result in enhanced performance in optical or molecular sensors. The current- voltage characteristics of ZnO nanorod network devices exhibit typical nonlinear behavior in air, which implies the formation of a Schottky barrier when metals are used as contacts. The conductance of the device increased significantly in vacuum, which can be explained by the desorption of hydroxyl groups at very low pressure. While physisorbed water or oxygen-related ions can detach from the ZnO surface during evacuation, exposure to high energy in the electron beam is believed to detach the chemisorbed anions of O- and O-2 from the surface of ZnO nanorods, which releases more electrons into the channel. The increase in available electrons enhances the conductance of the ZnO nanorods. Slow initialization of the conductance under ambient conditions indicates that the ionic re-adsorption is inactive under these conditions. Thus, the electron irradiation process can be used to reset the surface ionic molecules on metal oxide nano-structures by tuning the surface potential prior to the passivation process.

Sensing Enhancement on ZnO Nanorod Gas Sensors with Novel Nanorod/Nanotube Structures

Among various Metal Oxide Semiconductor (MOS) materials for in gas sensor applications, ZnO has been widely used in gas detection due to its wide band gape of 3.37 eV, large exciton binding energy (~60 meV), good electrical conductivity, and high mechanical stability. Moreover, owning to the high surface-to-volume ratio, ZnO gas sensors based on nanorod structure are extensively applied into gas specie detection and gas concentration detection. Therefore, a large number of methods and techniques have been applied to enhance the sensing performance of ZnO nanorod gas sensors. In this work, the ZnO nanorods with coaxial nanotubes gas sensors were fabricated to detect the concentration of ethanol vapor under the different temperatures and concentration conditions. In addition, Al doped ZnO (AZO) thin films were deposited on the surface of ZnO nanorods by Atomic Layer Deposition (ALD). The additional oxygen vacancies deficiency from the Zn-O and Al-O bands in the Zn-Al-O interface could improve the sensing performance of prepared ZnO gas sensors. In this study, ZnO nanorods were synthesized by hydrothermal method with ZnO seed layers deposited on Si wafer by ALD, as shown in Figure 1(a). The solutions used for ZnO nanorods hydrothermal growth were prepared with zinc nitrate hexahydrate (Zn(NO3)6H2O) and hexamethylenetetramine ((CH2)6N4) dissolved in DI water. To synthesize ZnO nanotubes around nanorods, one additional Al2O3 sacrificial layer was deposited on the surface of ZnO nanorods by ALD with TMA (Al2(CH3)6) and DI water as precursors. Then ZnO thin films were deposited on ZnO nanorods with Al2O3 sacrificial layer wrapped around, as shown in Figure 1(b). To eliminate the sacrificial layer, Precision Ion Polishing System (PIPS) was used to remove the top cover of ZnO nanorods. Figure 1(c) and Figure 1(d) show the Al2O3 sacrificial layer was exposed for Sodium hydroxide (NaOH) etching. After the sacrificial layer was removed by NaOH, ZnO nanorod/nanotube gas sensors were synthesized, as shown in Figure 1(e). To further enhance the sensing performance of ZnO nanorod/nanotube to the ethanol, AZO thin films were deposited on the surface by ALD as shown in Figure 1(f). A testing system of gas sensors was developed with a sealed reaction chamber and control system based on CompactRio from National instrument to investigate the sensing performance of ZnO nanorod/nanotube to ethanol vapor. The sensing response of ZnO nanorods to ethanol vapor was recorded and analyzed from the resistance change of ZnO nanorods. The sensing performances of ZnO nanorods, ZnO nanorod/nanotube, and ZnO nanorod/nanotube after coated with AZO thin films were measured and analyzed under various volumes of ethanol vapor at different temperatures. Base on the results, the sensing performance of ZnO nanorod gas sensor was enhanced by increase surface-to-volume ratio and oxygen vacancies. Figure 1. FESEM results of (a) ZnO Nanorods, (b) ZnO Nanorods with Al2O3 Coating and ZnO thin films, (c) and (d) Al2O3 sacrificial layer exposed after PIPS, (e) ZnO Nanorod/Nanotube after NaOH Etching, (f) ZnO Nanorod/Nanotube with AZO Coating. Figure 1

Enhanced band edge luminescence of ZnO nanorods after surface passivation with ZnS

We report on the passivation of surface defects of ZnO nanorods by surface layer deposition. ZnO nanorods and ZnSZnO hybrid nanostructures are grown on FTO coated glass substrate by chemical bath deposition method. XRD spectrum of ZnO nanorods shows the preferential growth along the c-axis. SEM analysis confirms the nearly aligned growth of the ZnO nanorods with a hexagonal shape. XPS measurements were performed to confirm the deposition of the surface layer and surface stoichiometry. Room temperature photoluminescence of ZnO nanorods showed two emission bands, viz. the band edge emission and the blue-green emission, with the latter being associated with the defect states arising from the surface of ZnO nanorods. The band edge emission is significantly increased as compared to blue-green emission after ZnS surface layer deposition on ZnO nanorods. The quenching of blue-green emission is explained in terms of reduced surface defects after ZnS deposition. Density functional theory (DFT) calculations are used to understand the mechanisms of defect passivation in ZnSZnO nanostructures and we show that the S atoms prefer the O site as compared with the Zn and interstitial sites.

A novel ZnO-based inorganic/organic bilayer with low resistance for Li metal protection

Although Li metal has a great potential for application as anode of the next generation batteries, the existing problems of the lithium dendrite growth and volume expansion need to be solved to pave the way for the real application of Li metal anode. Here, a double-layer structure consisting of poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP)/ZnO composite membrane and ZnO buffer layer was fabricated by solid-state method and spin coating to protect the Li metal. Compared to the bare Li electrode, the protected Li electrode displays the advantages of stable voltage profiles and long lifespan. Interestingly, the ZnO buffer layer between Li metal and PVDF-HFP/ZnO composite membrane can significantly reduce the battery resistance because of the formation of Li2O on the ZnO nanorod surface, which can guide Li+ flux to deposit uniformly onto Li metal. Results show that the double-layer structure on the Li metal surface can inhibit dendrite growth and volume expansion. The analyses in this paper may contribute to a better understanding of the reaction mechanism between ZnO and Li+.

Direct formation of transfer-free graphene as current spreading layers on n-ZnO nanorods/p-GaN light-emitting diodes

Transfer-free graphene has been directly grown on ZnO nanorods (NRs)/p-GaN LEDs by a feasible approach involving rapid thermal annealing of amorphous carbon and nickel layers. Compared with conventional ZnO NRs/p-GaN LEDs without a current spreading layer, the current–voltage characteristic and electroluminescence performance are enhanced, mainly attributed to more efficient current spreading by the transfer-free graphene that conformally covers the entire surface of ZnO NRs, leading to better carrier injection. It is hence expected that our scheme will be of great importance for nanostructured LED arrays with large-scale fabrication.

Effect of Sn/Zn ratio on structure and photoluminescence properties of SnO2@ZnO composites

ABSTRACTThe SnO2@ZnO composites, with Sn/Zn molar ratio (Sn/Zn–ratio) from 4:1 to 8:1, were successfully prepared via two–step hydrothermal method. The microstructure and morphology of the as–prepared Sn/Zn–ratio–dependence SnO2@ZnO composites were investigated. Furthermore, a reasonable growth mechanism of SnO2@ZnO composites was proposed and the results indicated that the dispersity of SnO2 nanowires on the ZnO nanorods surface was obviously influenced by the Sn/Zn–ratio. The photoluminescence (PL) properties of the Sn/Zn–ratio–dependence SnO2@ZnO composites were investigated. The results showed the opposite intensity change of ultraviolet (UV) and visible emission while the Sn/Zn–ratio increased. In addition, the possible emission mechanisms were also discussed that the UV emission is attributed to free–exciton recombination at the near–band edge and the potential difference between SnO2 and ZnO result in the visible emission.

Novel Route for Fabrication of ZnO nanorods-Au Nanoparticles Hybrids Directly Supported on Substrate and their Application as Gas Sensors

The aim of this study was to develop a novel, simple and fast route to prepare ZnO nanorods (NRs) -Au nanoparticles (NPs) hybrids directly supported on a substrate to be used in gas sensor devices. The ZnO NRs were promptly grown on interdigitated Au electrodes Al2O3 substrates by chemical bath deposition at a low temperature. After that, Au NPs were deposited by sputtering. Results obtained by XRD, SEM, EDX and TEM showed the perpendicularly aligned growth of the ZnO NRs with a hexagonal base on the substrate and the Au NPs homogeneously covered the ZnO NRs surface. The ZnO NRs-Au NPs hybrids-based sensor exhibited an improved sensor response for H2 and O2 gases compared to the ZnO NRs at 300 oC. Due to the ability to prepare homogeneous hybrids with high surface directly supported on the substrate; the developed route might provide a convenient approach to preparing gas sensor devices.

A facile route to ionic liquids-functionalized ZnO nanorods for the fluorometric sensing of thiabendazole drug

In this report, we introduce a simple and cost-effective hydrothermal method for the functionalization of ZnO nanorods with three different ionic liquids (ILs (1–3)) to check their sensing response with various drugs including thiabendazole, phenylbutazone, chloramphenicol, neomycin, and theophylline through fluorescence spectroscopy. The rod like-microstructure and hexagonal wurtzite structure of synthesized ZnO nanorods were examined with SEM-EDS and XRD techniques. On the other hand, ionic liquids-functionalized ZnO nanorods (ILs (1–3)/ZnO) exhibited various self-organized (layered-sheet~IL1, distorted nanorods~IL2, peanut beaded surface~IL3/ZnO) structures due to the intercalation of ILs on the surface of ZnO nanorods. Compared to ZnO nanorods, ILs (1–3)/ZnO nanorods showed a blue shift in UV-absorption band which resulted to the variation in optical bandgap values. Ionic liquids-functionalized nanorods samples (ILs (1–3)/ZnO) have been selected for their recognition behaviour with available drug molecules using fluorescence spectroscopic technique. Interestingly, with the addition of thiabendazole drug molecules to two samples (ILs (1,3)/ZnO), a significant response corresponding to the peaks at ~350nm and ~357nm was observed, which led to the development of fluorescent sensors with a detection limit of 12 and 304nM, respectively.

Enhanced photocatalytic activity and synthesis of ZnO nanorods/MoS2 composites

A stable and recyclable organic degradation catalyst based on MoS2 functionalized ZnO nanorods was introduced. ZnO nanorods were synthesized on the glass substrates (2 cm*2 cm) by sol-gel method and hydrothermal method and functionalized with MoS2 via an argon flow annealing method. The structure and morphology of the as-prepared samples were characterized by XRD, SEM and TEM. Results showed that a small amount of MoS2 was successfully wrapped on the surfaces of ZnO nanorods. XPS analyses showed the existence of Zn-S between ZnO and MoS2, indicating that the MoS2 was combined with ZnO through chemical bonds and formed the ZnO/MoS2 heterostructure. PL results revealed that ZnO/MoS2 had lower fluorescence spectra indicating an electron transport channel between ZnO and MoS2 which separated electrons and holes. Photocatalytic experiment showed that ZnO/MoS2 composites showed a better photodegradation performance of Rhodamine B (RhB) after functionalized with MoS2 under the UV light irradiation which could be attributed to the separation and transfer of photogenerated electrons and holes between ZnO and MoS2. Meanwhile, the high active adsorption sites on the edges of MoS2 also accelerated the degradation process. Furthermore, the scavengers were used to investigate the major active species and results indicated that h+ was the major reactive species for the degradation.

ZnO/TiO2 core–shell heterojunction for CdS and PbS quantum dot-cosensitized solar cells

ZnO nanorods (NRs) with regular morphology were prepared through hydrothermal method, and the TiO2 shell was assembled onto the surface of ZnO NRs by spin coating to the ZnO/TiO2 core–shell heterojunction. CdS and PbS quantum dots (QDs) were used to cosensitize the ZnO/TiO2 nanostructure by direct adsorption (DA) and successive ionic layer adsorption and reaction, respectively. SEM, TEM, and HRTEM images show that the samples possessed a rough surface and four lattice fringes indicating the successful synthesis of the ZnO/TiO2/CdS/PbS composite structure. The ZnO/TiO2(10T)/CdS/PbS sample showed a high absorption intensity at a broad range of wavelength to visible light region. The ZnO/TiO2(10T)/CdS/PbS photoelectrode with QDSSCs showed the highest IPCE of 36.04% and photoelectric efficiency (η) of 1.59%; these values increased by approximately 550% and 150% compared with those of unsensitized ZnO (0.29%) and ZnO/TiO2(10T) (1.04%) and about 146% and 120% compared with those of ZnO/TiO2(10T)/CdS and ZnO/TiO2(10T)/PbS, respectively. The fill factor was 0.36, and the photocurrent density (Jsc) and open circuit voltage (Voc) reached the maximum values of 9.73 mA cm−2 and 0.46 V, respectively.

Morphology-Preserving Sensitization of ZnO Nanorod Surfaces via Click-Chemistry.

Films of ZnO nanorods grown by chemical vapor deposition were functionalized with a chromophore in a stepwise process that preserves the surface morphology. In the first step, the ZnO nanorods were functionalized by exposure to prop-2-ynoic acid (propiolic acid) in vacuum, which did bind through the COOH group leading to a ZnO surface functionalized with ethyne moieties (ethyne/ZnO). In the second step, 9-(4-azidophenyl)-2,5-di-tert-butylperylene (DTBPe-Ph-N3) was reacted with the ethyne/ZnO surface via copper-catalyzed azide-alkyne click reaction (CuAAC) in solution to form the DTBPe-functionalized surface (DTBPe/ZnO). The ZnO morphology was preserved after each step, as demonstrated by scanning electron microscopy (SEM). Each step was probed by X-ray photoelectron spectroscopy (XPS), and transient absorption spectroscopy (TA) of the resulting DTBPe/ZnO surface shows interfacial electron transfer following visible light excitation. As expected, attempts to bind the reference compound 1-(4-(8,11-ditert-butylperylen-3-yl)-phenyl)-1H-1,2,3-triazole-4-carboxylic acid (DTBPe-Ph-Tz-COOH) directly from solution lead to etched surfaces (confirmed by SEM) and undefined binding modes (confirmed by TA).

A novel signal-on photoelectrochemical sensing platform based on biosynthesis of CdS quantum dots sensitizing ZnO nanorod arrays

New approach for fabrication of photoelectrochemical sensing platform has been demonstrated for sulfate-reducing bacteria (SRB) detection based on biosynthesis of CdS quantum dots (QDs) sensitizing ZnO nanorod arrays. ZnO nanorod arrays were synthesized on the FTO substrate via galvanic-cell-based approach and then silanized by (3-Aminopropyl)triethoxysilane (APTES) with amino groups to introduce Cd2+. By taking advantage of the characteristic bacterial metabolite (H2S) reaction with Cd2+, CdS QDs can be biosynthesized on the surface of ZnO nanorod and significant enhancement of photocurrent due to the formation of CdS/ZnO heterostructures. The fabricated PEC sensing platform showed detection sensitivity of 32 cfu/mL for SRB. The current work can provide new strategies to construct advanced PEC sensing platforms for detections of physiologic messenger molecules (H2S).

Improved Gas Sensing Performance of ALD AZO 3-D Coated ZnO Nanorods

This paper reports an enhancement on the sensing performance of ZnO nanorod ethanol sensors with a new approach by utilizing nested coatings of Aluminum doped ZnO (AZO) thin films by Atomic Layer Deposition (ALD) technology. ZnO nanorods were grown by the hydrothermal method with the ZnO seed layer synthesized on Silicon wafers by ALD. To enhance the sensing performance of ZnO nanorod ethanol sensors, multiple coated AZO thin film 3-D coatings were deposited on the surface of the intrinsic ZnO nanorods by ALD. To investigate the sensing performance of the ZnO nanorods sensor for the detection of ethanol vapor, a gas sensor testing system was designed and built with a sealed reaction chamber and a temperature controller. The demonstrated sensing performance results include the sensing response comparison between ZnO nanorods before and after ALD coatings with AZO films at different temperatures and with various concentrations of input ethanol vapor. The response times and recovery times of ZnO nanorods before and after ALD coatings with AZO thin films were analyzed to investigate the sensing enhancement. The sensing response improvement peaks at 25°C room temperature with approximately 200% enhancement. However, the sensing response improvement decreases as a function of increasing operating temperature.

Surface modification of ZnO nanorods with CdS quantum dots for application in inverted organic solar cells: effect of deposition duration

Incorporating cadmium sulfide quantum dots (CdS QDs) onto ZnO nanorod (ZNRs) has been investigated to be an efficient approach to enhance the photovoltaic performance of the inverted organic solar cell (IOSC) devices based on ZNRs/poly (3-hexylthiophene) (P3HT). To synthesize CdS/ZNRs, different durations of deposition per cycle from 1 to 9 min were used to deposit CdS via SILAR technique onto ZNRs surface grown via hydrothermal method at low temperature on FTO substrate. In typical procedures, P3HT as donor polymer were spun-coating onto CdS/ZNRs to fabricate IOSC devices, followed by Ag deposition as anode by magnetron sputtering technique. Incorporation of CdS QDs has modified the morphological, structural, and optical properties of ZNRs. Incorporation of CdS QDs onto ZNRs also led to higher open circuit voltage (Voc) and short circuit current density (Jsc) of optimum ZNRs/CdS QDs devices due to the increased interfacial area between ZNRs and P3HT for more efficient exciton dissociation, reduced interfacial charge carrier recombination as a result of lower number of oxygen defects which act as electron traps in ZnO and prolonged carrier recombination lifetime. Therefore, the ZNRs/CdS QDs/P3HT device exhibited threefold higher PCE (0.55%) at 5 min in comparison to pristine ZNR constructed device (0.16%). Overall, our study highlights the potential of ZNRs/CdS QDs to be excellent electron acceptors for high efficiency hybrid optoelectronic devices.