Hydrothermal ZnO Research Articles

Tailoring the Surface Properties of ZnO Nanowires by ALD Deposition

AbstractInnovative research on metal‐oxide gas sensors involves nanostructuring and surface modification as key elements to tailor sensitivity and selectivity. This work addresses a ZnO nanowire‐based sensing device obtained by coupling a lithographically prepared substrate with hydrothermal ZnO growth, to align and interconnect the nanowires between two electrical contacts. Furthermore, conformal coating by atomic layer deposition technique allows functionalization of the surface of the nanowires with sub‐monolayers of Al2O3 and TiO2. A detailed analysis is carried out from a morphological and structural point of view with photoluminescence and Raman spectroscopy and electron microscopy. The material characterization results are analyzed in comparison with the functional characterization in gases toward reducing (NO2) and oxidizing (H2S) gases. Unparalleled sensing enhancement with Atomic Layer Deposition functionalization is obtained for NO2 detection. The passivation role of surface states is discussed combining information from experimental techniques with a proposed model.

Engineering of ceramic membranes with superhydrophobic pores for different size water droplets removal from water-in-oil emulsions

Superhydrophobic (SHB) ceramic membrane surfaces have proved their superiority in water-in-oil (W/O) emulsion separation. However, the construction of ceramic membranes with SHB pores remains challenging and the interaction between the SHB pores and water droplets is still unclear. Herein, by growing uniform ZnO nanoclusters (NCs) onto the SiC grains of a symmetric SiC ceramic membrane and then grafting with triethoxy(octyl)silane, a series of ZnO NCs@SiC membranes with SHB pores were prepared. Prior to secondary hydrothermal synthesis ZnO NCs, the small-sized ZnO nanoparticles (NPs) were grown onto the SiC grains via a sol–gel process. All the ZnO NCs@SiC membranes with SHB pores showed improved water removal performance from W/O emulsions compared with that of the pristine membrane. The optimal membrane with SHB pores displayed a pore size of 0.19 μm with a water contact angle of 163.96°. The smaller water droplets in W/O emulsions, the membrane pores are easier to be polluted. When separating a 1000 ppm W/O emulsion with a water droplet size of 500 nm at a transmembrane pressure of 0.5 bar, the water rejection reached 98.90 %, and the steady-state oil flux was 92.99 L·m−2·h−1. Based on the separation experiments, the separation mechanism was revealed. This work provides significant perceptions into constructing SHB pores of ceramic membranes, which might be useful for other possible applications.

Long-lived excitons in thermally annealed hydrothermal ZnO

Applying thermal annealing to hydrothermal ZnO crystals an enhancement of exciton lifetime from 80 ps to 40 ns was achieved boosting PL quantum efficiency of the UV luminescence up to 70 %. The lifetime improvement is related to the reduced density of carrier traps by a few orders of magnitude as revealed by the reduction of the slow decay tail in pump probe decays coupled with weaker defects-related PL. The diffusion coefficient was determined to be 0.5 cm2/s, providing a large exciton diffusion length of 1.4 μm. The UV PL lifetime drop at the lowest exciton densities was explained by capture to traps. Release of holes from acceptor traps provided delayed exciton luminescence with ∼200 μs day time and 390 meV thermal activation energy. Pump-probe decays provided exciton absorption cross-section of 9 × 10−18 cm2 at 1550 nm wavelength and verified the PL decay times of excitons. Amplitudes and decay times of the microsecond slow decay tails have been correlated with the trap densities and their photoluminescence. A surface recombination velocity of 500 cm/s and the bimolecular free carrier recombination coefficient 0.7 × 10−11 cm3/s were calculated. Therefore, the properly annealed hydrothermally grown ZnO can be a viable and integral part of many functional devices as light-emitting diodes and lasers.

Relevance of alcoholic solvents in the growth of ZnO nanoparticles and ZnO hierarchical nanorod structures on their optical and opto-electrical properties

We report on the synthesis of ZnO nanoparticles and ZnO hierarchical nanorod structures using four different alcohols i.e. methanol, isopropanol, ethanol, and aqueous ethanol (70% alcohol, 30% water). The syntheses of the nanoparticles were carried out by non-aqueous and hydrothermal routes. In general, absolute alcohol allows a better control of the synthesis reaction and nanoparticles as small as 5 nm were obtained, confirmed by TEM. XPS analysis elucidated the chemical states that were correlated to the synthesis reaction. For the nanorod growth, these four alcohols were used as seeding solvents, followed by hydrothermal ZnO nanorod growth. Here, the seed layer tailored the nanorod diameters and surface defects, which were studied by SEM and photoluminescence spectroscopy. Subsequently, the ZnO nanorods were electrically characterized and exhibited persistent photoconductivity under UV irradiation of 365 nm. The differences in conductivity in dark and under UV irradiation were attributed to the size of the nanorods, defect states, semiconductor band bending and oxygen adsorption–desorption mechanisms. Parameters such as photoresponse and photosensitivity are also calculated in order to evaluate their applicability in UV sensors. This work demonstrates optimization of the physical, chemical, electrical and optical properties of both ZnO nanostructures via the use of alcoholic solvents.

Significant Enhanced SO2 Resistance of Pt/SiO2 Catalysts by Building the Ultrathin Metal Oxide Shell for Benzene Catalytic Combustion.

A noble metal catalyst shows excellent low-temperature oxidation activity in the catalytic combustion of benzene but has the problem of SO2 poisoning. We all know that SO2 easily competes with the reactant molecules for adsorption of the active site and has electronic effects on the active site to deactivate the catalyst. Therefore, the sulfur resistance of catalysts is the key problem to be solved in the process of catalytic combustion of benzene. Herein, the Pt/SiO2 catalyst with an ordered mesoporous structure was prepared by a one-step hydrothermal method, and MgO, ZnO, and MnOx were, respectively, coated on the surface of Pt/SiO2 as ultrathin shells to improve the sulfur resistance of Pt/SiO2. We observed that the sulfur resistance of the Pt/SiO2 catalyst was significantly improved due to the protective effect of the metal oxide shell. By comparing the three core-shell catalysts, it was found that the Pt/SiO2@MnOx catalyst coated with a MnOx shell had the best performance. The reason was that the MnOx shell not only protected the Pt active site but also had a good electron transfer effect on the core Pt, so it could effectively avoid the rapid adsorption poisoning of SO2 on the active Pt0 site. In addition, it was verified that the excellent redispersion of MnOx species in a SO2 atmosphere could increase the low-temperature oxidation activity of the Pt/SiO2@MnOx catalyst. Meanwhile, in situ DRIFT results also confirmed that the MnOx shell could significantly promote the oxidation of benzene molecules in the SO2 atmosphere.

Investigation on Optically Ultraviolet Response of Low-dimensional ZnO Structures Prepared by Chemical Processes

Chemical processes were used to synthesize ZnO low-dimensional structure including ZnO thin film ZnO nanorod and ZnO nanoparticle. ZnO thin film was prepared by sol-gel spin coating method while ZnO nanorod was prepared by hydrothermal process and ZnO nanoparticle was synthesized via co-precipitation route. The structural, morphological, optical and photoresponse properties were investigated by XRD, FESEM, UV-Visible spectrophotometer, I-V and I-t measurement. Comparison study on photoresponse properties of all structures was conducted under UV light with UV LED wavelength 375 nm. The UV photodetector based on ZnO nanoparticle sample showed superior photoresponse time to another ZnO low-dimensional structures. The results informed crucial factors including active surface area per volume, interaction in low-dimensional structures and specific properties in each structure playing key role on optical ultraviolet response.

Enhanced in-situ oxygen evolution and hydrogen peroxide production by a floatable ZnO-incorporated polyurethane photocatalyst for sulfamethoxazole degradation

A highly efficient rigid floatable photocatalyst was prepared by seeding ZnO in polyurethane (PU) and subsequent hydrothermal ZnO coating (P-ZsZc). Compared to pristine ZnO, granular P-ZsZc had an uplifted valence band (VB) and generated new covalent Zn-O-C bond between PU and ZnO, resulting in a significantly enhanced photocorrosion resistance by boosting the mobility of photoexcited holes. In addition, the VB of P-ZsZc was favorable for the in-situ oxygen evolution reaction (OER) and H2O2 production. As a result, the P-ZsZc achieved ∼ 98% removal of sulfamethoxazole (SMX) within 2 h for 5 consecutive cycles. Moreover, the SMX removal rate and Zn(II) leaching remained at ∼ 0.036 min−1 and 0.17 mg L−1, respectively, even during the fifth cycle for 10 h. Furthermore, scavenger experiments and electron spin resonance (ESR) spin trap analysis revealed that the main photogenerated reactive oxygen species (ROS) was H2O2. Floatable P-ZsZc showed constant SMX removal of ∼ 90% in continuous flow experiments (CFEs) for up to 10 h of operation. This study offers a new approach to prepare floatable rigid photocatalysts capable of in-situ oxygen and H2O2 production to degrade organic pollutants.

Photocatalytic Concrete Developed by Short Seedless Hydrothermal Method for Water Purification

Stormwater runoff management and treatment are significant topics for designing a sustainable city. Therefore, photocatalytic, permeable, and removable concrete is a promising solution to reduce pollution through leaching with permeable and scalable road. The objective of this work was to develop cost-effective and greener photocatalytic concretes that can be easily scaled-up, and to demonstrate their photocatalytic activities. To achieve this, seedless hydrothermal ZnO nanostructures (NSs) in 2 h were employed to functionalize a concrete surface by a soft functionalization process, avoiding overconsumption of energy and chemical products. In this work, two different concretes were studied and used for the degradation of organic dye in water. The results demonstrated the universality of the proposed functionalization process by showing similar gap values, ZnO NSs morphologies, and XRD pattern, compared to the concrete functionalized by the traditional two-step hydrothermal synthesis. The XRD results certified the presence of the ZnO Würtzite phase on the concrete surface. The synthesis feasibility was attributed to the basic pH and O− groups’ presence in concrete. Then, their photocatalytic efficiency was proved for organic dye removal in water. An almost total degradation was recorded after 5 h under artificial solar light, even after several uses, demonstrating a similar efficiency to the photocatalytic concrete functionalized by the traditional two-step synthesis.

Preparation of MOF‐Zn@ZnO composite and its properties in SBR

AbstractThe synthesis and application of metal–organic skeleton materials (MOFs) have attracted much attention in recent years due to their special structures and properties. In order to expand the application field of MOFs materials, this paper designed experiments to explore the application prospect of MOFs materials in the field of rubber. In this study, zinc acetate dihydrate was used as the central metal ion source, terephthalic acid as organic ligand and deionized water as solvent. MOF‐Zn was prepared by hydrothermal method and ZnO nanoparticles were grown on the surface of MOF‐Zn in situ to obtain MOF‐Zn@ZnO. At the same time, the composite material is filled into the SBR by mechanical blending method. The experimental results show that ZnO nanoparticles adsorb on the MOF‐Zn surface and have strong interfacial interaction with MOF‐Zn. Moreover, MOF‐Zn@ZnO was uniformly dispersed in the rubber matrix, which improved the cross‐linking effect of the composite material and promoted the vulcanization of rubber. Compared with ZnOC/SBR composites, the modulus and tensile strength of MOF‐Zn@ZnO/SBR composites at 100%, 200%, and 300% elongation are increased by 33.6%, 38.2%, 30.8%, and 16.8%, respectively. The excellent performance of MOF‐Zn@ZnO/SBR composite is due to the inhibition of agglomeration of ZnO nanoparticles by MOF‐Zn, which can be used as an effective vulcanization activator in rubber. This study provides a simple synthesis route for the preparation of ZnO nanofillers for high performance SBR composites.

Optical properties of graphene/ZnO spheres/Au heterostructure for enhanced van der Waals saturable absorbers

Van der Waals (vdW) heterostructures offer a promising platform for optimizing and controlling physical properties of a variety of materials and devices, which has attracted extensive attentions in ultrafast photonic applications. Here, a hybrid vdW saturable absorber (SA) based on ZnO sphere heterostructures has been fabricated by transferring the low-temperature hydrothermal method ZnO spheres onto gold mirrors and covering with Au nanoparticles and the graphene layer. In comparison with the ZnO sphere SA, the vdW SA exhibits the stronger emission intensity in the ultraviolet-visible range and the higher optical absorption in the near-infrared range, respectively. Meanwhile, by using the balanced twin-detector technology, a modulation depth of 6.6% has been obtained from vdW SA, which is 2.8 times greater than that of the solo ZnO SA due to the harvest of photons and the transfer of photon-generated carriers in the vdW heterostructure. Based on the hybrid vdW SA, a quite stable Q-switched laser and a mode-locked laser operation with a tunable wavelength range from 1531.4 to 1560.4 nm have been achieved. These results demonstrate a effective strategy for enhancing nonlinear optical properties of SAs and pave a way for developing novel types of SAs for the application of ultrafast lasers.

Synthesis of ZnO nanostructures by hydrothermal method deposited on porous silicon for photo-conversion application

The hydro-thermal ZnO nanostructure synthesis is described in this paper. ZnO nanoparticles have been studied with the use of a scanning electron microscope (SEM), and X-ray diffraction (XRD) confirmed the formation of ZnO's hexagonal structure of the wurtzite. The produced nanostructure's average crystallite size was 21 nm. From UV–Vis absorption spectra, the bandgap of ZnO nanostructures was estimated, and the bandgap was 3.0 eV for ZnO. The porous silicon (PS) layers have been made from n-type silicon (100) orientation using the photo-electrochemical etching (PECE) process. Electrical attributes and a photodetector are utilized to characterize the product. The findings indicate that such hydrothermal synthetic method may produce high-quality ZnO nanoparticles. The photo-conversion characteristics of ZnO NPs/PS are improved, resulting in an increase in the responsivity. As a result, ZnO NPs/PS might be employed as a photodetector with a broad wavelength spectral range.

ZnO Nanorods Grown on Heterogenous Ag Seed Layers for Single-Cell Fluorescence Bioassays

Here we demonstrate the controllability of the morphology of hydrothermal ZnO nanorods (ZnO-NRs) grown on heterogenous Ag seed layers. By varying the crystal orientation of silver thin films (Ag), a high density of ZnO-NRs could be obtained. We find that the density of ZnO-NRs strongly relates to the peak intensity ratio between (111) and (200) planes of Ag thin films due to a heteroepitaxy between (0002) ZnO and (111) Ag rather than that of grain boundary nucleation and/or surface nucleation. In addition, the optimized heterostructure of ZnO nanorod/Ag arrays is investigated via a critical concentration for nucleation and used as a fluorescence enhancement substrate (FES). The experimental results have shown that the FES presents an ability to detect a biological sample (PC-3 cell) with a high sensitivity and low detection limit of 1 cell/μL. Our results highlight that understanding an important key to control and design the morphology of heterogeneous hydrothermal ZnO-NR growth is essential to open up the opportunities for fundamental studies and applications in high-performance integrated nanodevices.

Design of SnO2/ZnO@ZIF‐8 Hydrophobic Nanofibers for Improved H2S Gas Sensing

AbstractSnO2/ZnO@ZIF‐8 core‐shell nanofibers were synthesized by electrospinning and hydrothermal etching ZnO component of SnO2/ZnO nanofibers. It is found that the ZIF‐8 coated SnO2/ZnO nanostructures are obtained, and the ZIF‐8 coating can obviously increase the specific surface area of nanofibers. H2S sensing properties of the sensors based on composite nanofibers were investigated. The results show that SnO2/ZnO@ZIF‐8 core‐shell nanostructures can not only improve the response to H2S, but also effectively reduce the interference of ambient humidity on semiconductor gas sensors. The response of the optimal sample to 5 ppm H2S can reach 202.3, which is about 19 times higher than that of pure SnO2 nanofibers. Compared with the response at the ambient humidity of 55 %RH, the response at the ambient humidity of 90 %RH can still maintain 84.7 %. Good selectivity and stability are also observed in our investigations.

Controlled Synthesis of ZnO Nanorods Using Different Seed Layers

Single-crystal, low-cost, low-temperature, hydrothermal synthesis ZnO nanorods were grown on ZnO fine grained random nanocrystalline seed layers prepared by atomic layer deposition (ALD) and benchmarked against spin coating techniques for seed layers. As the growth temperature increased to 90 °C, more nanorods were observed on the samples. Increasing the growth time from 16 h to 24 h resulted in higher nanorod density for the ALD seeded samples, but less nanorods for the spin-coated seeded samples. Our work demonstrates that the final density of ZnO nanorods and their shape and size are primarily influenced by the characteristics of the ZnO seed layer, such as composition, morphology, grain size, impurity content and thickness, as well as the time spent heated. At ∼10 nm the ALD generated random polycrystalline seed layer grains were one order of magnitude smaller compared to the spin coating generated seed layer grains which appear structurally different as ∼100 nm columnar grains. This morphological seed layer difference caused the ZnO nanorods grown on spin coated seed layers to reach only ∼34% of the average nanorod length achieved on ALD seeds with comparable rod diameter and can also account for the diminished nanorod density. The exact mechanism of how the seed layer affects nucleation and subsequent nanorod growth is unknown, but results suggest that there is a significant impact. Future research can be directed to investigating the ability of metal nanoparticles to self-assemble on these nanorods and to further nanoscale catalysis.

Low-temperature large-area fabrication of ZnO nanowires on flexible plastic substrates by solution-processible metal-seeded hydrothermal growth

We have developed the low-temperature conformal ZnO nanowire fabrication on flexible plastic substrates by utilizing the solution-processible metal seed-assisted hydrothermal ZnO crystallization. Structural evolution of ZnO nanowires controlled by major parameters involving growth temperature, growth time, and seed coating condition, has been systematically investigated towards uniform and large-area growth of conformal ZnO nanowires. Direct ZnO nanowire growth on flexible plastics without undergoing the high-temperature seed sintering has been realized by developing the low-temperature Ag-seeded hydrothermal ZnO nanowire growth. The nanoporous Ag layer favorable for ZnO crystal nucleation and continued nanowire growth can be reduced from the Ag ion solution coating at the temperature as low as 130 °C. This tactfully enables the selective hydrothermal growth of ZnO nanowires on the Ag patterns on flexible plastics. Such an all-solution-processible low-temperature fabrication protocol may provide an essential and practical solution to develop many diverse applications including wearable and transparent electronics, sensors, and photocatalytic devices. As one example, we demonstrate that a transparent UV sensor can be devised based on the ZNW growth on the Ag micromesh electrode.

Tuneable Functionalization of Glass Fibre Membranes with ZnO/SnO2 Heterostructures for Photocatalytic Water Treatment: Effect of SnO2 Coverage Rate on the Photocatalytic Degradation of Organics

The construction of a ZnO/SnO2 heterostructure is considered in the literature as an efficient strategy to improve photocatalytic properties of ZnO due to an electron/hole delocalisation process. This study is dedicated to an investigation of the photocatalytic performance of ZnO/SnO2 heterostructures directly synthesized in macroporous glass fibres membranes. Hydrothermal ZnO nanorods have been functionalized with SnO2 using an atomic layer deposition (ALD) process. The coverage rate of SnO2 on ZnO nanorods was precisely tailored by controlling the number of ALD cycles. We highlight here the tight control of the photocatalytic properties of the ZnO/SnO2 structure according to the coverage rate of SnO2 on the ZnO nanorods. We show that the highest degradation of methylene blue is obtained when a 40% coverage rate of SnO2 is reached. Interestingly, we also demonstrate that a higher coverage rate leads to a full passivation of the photocatalyst. In addition, we highlight that 40% coverage rate of SnO2 onto ZnO is sufficient for getting a protective layer, leading to a more stable photocatalyst in reuse.

The I-V characteristics of hydrothermal growth ZnO nanorods

This study aims to analyze the conductivity of ZnO nanorods grown by using the hydrothermal method. We use the I-V characteristic test to determine the conductivity. In this research, the process of making samples of ZnO nanorods was carried out in two stages: preparing the seed layer and then growing ZnO nanorods using the hydrothermal method. After testing the characteristics of using SEM, the resulting crystal structure of ZnO nanorods is hexagonal wurtzite with the direction of growing perpendicular to the substrate. Besides, the resulting I-V characteristics indicate a physical phenomenon that occurs at a temperature variation of ≥ 190°C which means a peak. While the details of phenomena still need to be elaborated. Besides, a gas effect test was also conducted which showed that the sensor was at a maximum working temperature of 190°C with an input voltage of 8 volts and reached a gas sensitivity at 220°C.

Fabrication and UV Sensitivity of a ZnO Decorated NiO Thin Film Field Effect Transistor

In this letter we propose a new field effect transistor (FET) with a ZnO nanorod decorated NiO thin film acting as the source-drain channel. The sensor was fabricated by immersing a 40 nm Ni thin film on a p-type Si/SiO2 wafer in a standard hydrothermal ZnO nanorod growth solution. After this step, vertically aligned ZnO nanorods were grown on the Ni layer. We have argued the existence of a thin layer of NiO on the Ni layer, caused by partial oxidization of Ni in this aqueous solution. The resulting ZnO nanorods form p-n junctions with the NiO layer which creates a depletion layer for the separation of photogenerated electron-hole pairs. Gate voltage is applied using the Si layer and the effect of gate voltage on the depletion layer is discussed. High sensitivity UV detection was performed under positive gate voltages where the depletion layer extends over the entire width of the NiO layer. The proposed sensor, with an effective surface area of 0.6 cm $\times \,\,0.6$ cm, can detect UV intensities as low as $30~\mu \text{W}$ /cm2. The sensor shows good quantum efficiency, reasonable responsivity and a low field mobility of 50%, $0.16~\frac {A}{W}$ and 59.6 cm2/V.s respectively.

Substantial Narrowing on the Width of \u201cConcentration Window\u201d of Hydrothermal ZnO Nanowires via Ammonia Addition

A crystal growth of hydrothermal ZnO nanowires essentially requires a concentration control within so-called “concentration window”, where the anisotropic crystal growth of ZnO nanowires preferentially occurs. Although understanding what exactly determines the width of “concentration window” is important to tailor the anisotropic crystal growth process, the fundamental knowledge as to “concentration window” is still scarce. Here we report the effect of ammonia addition on the width of “concentration window” using conventional hydrothermal ZnO nanowire growth. We found that the ammonia addition substantially narrows the width of “concentration window”. Within the narrow range of zinc complex concentration, we found a significant increase of growth rate (up to 2000 nm/h) of ZnO nanowires. The narrowed “concentration window” and the resultant increased growth rate by the ammonia addition can be understood in terms of synchronized effects of both (1) a reduction of zinc hydroxide complex (precursor) concentration and (2) a fast rate limiting process of ligand exchange between different zinc complexes. Thus, the present knowldege as to “concentration window” will accelerate further tailoring an anisotropic crystal growth of hydrothermal ZnO nanowires.

Investigations on room temperature dual sensitization of ZnO nanostructures towards fish quality biomarkers

We report a facile and controlled synthesis of ZnO nanostructures using hydrothermal and sol-gel synthesis routes. Morphological features such as nanoaggregates and nanospheres were observed for hydrothermal and sol-gel grown ZnO nanostructures respectively. Preferential crystal orientation was observed to be along (1 0 1) plane for both the nanostructures. Excitation dependent defect centers of the nanostructures were observed. Nanospheres showed a selective detection of dimethyl sulphide (C2H6S) at room temperature. On the other hand, ZnO nanoaggregates showed dual gas sensing properties towards ammonia (NH3) and trimethylamine (C3H9N) vapours at room temperature with a maximum sensing response of 1207 and 758 respectively. Trace level detection of below 5 ppm was observed towards NH3 and C3H9N vapour. Sensing responses of these nanostructures in different relative humidity conditions were also studied and reported. The sensors showed good stability over a period of time confirmed the suitability of the sensor for practical applications in fish quality analysis.