N-type ZnO Research Articles

Synthesis of ZnO and CuO co-decorated porous carbon spheres with simultaneous accessibility to small biomelucules

ZnO and CuO co-decorated porous carbon spheres (PCS) were prepared via a simple hydrothermal method, denoted as ZnO-CuO/PCS. An electrochemical sensor based on the ZnO-CuO/PCS was fabricated for simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA). Three well-resolved anodic peaks along with greatly increased peak currents were obtained at the ZnO-CuO/PCS modified glassy carbon electrode (GCE) on the differential pulse voltammograms. The anodic peak separations at the ZnO-CuO/PCS/GCE were 204 mV between AA and DA and 132 mV between DA and UA. Notably, the analytical performances of the ZnO-CuO/PCS outperform those of ZnO/PCS and CuO/PCS, which can be attributed to the synergistic effect of the two metallic oxides, i.e., the formation of p-n heterojunctions between p-type CuO and n-type ZnO allows sensitive transduction into remarkable changes in the electrochemical signals. The ZnO-CuO/PCS/GCE also presented high anti-interference ability, storage stability and repeatability in the determination of AA, DA and UA. Finally, the practical applicability of the prepared sensor was successfully demonstrated by the simultaneous determination of AA, DA and UA in human urine samples.

Functional TiO2 interlayer for all-transparent metal-oxide photovoltaics

Metal oxide has a high energy bandgap and passes the visible light to enlighten the vision to human eyes. However, the strong and harmful UV light is easily captured by large bandgap metal oxide materials. One of the promising applications of metal oxide manipulation would be the transparent solar cells for the transparent window, to guarantee the view of visible light and generate electric power from the invisible UV radiation. Herein, we demonstrate the all-transparent photovoltaics for see-through applications with the functional deployment of TiO2 layer. P-type NiO and n-type ZnO form a heterojunction to establish a photovoltage. TiO2 layer with donor concentration >1019 cm−3 has flat-band potential of 0.4 V vs reversible hydrogen electrode (RHE) and is significantly higher than that of the photoactive ZnO layer, TiO2 layer insertion enables the multifunctions of giving a back surface field and also serving as a carrier selective transport layer. The ultrathin TiO2 embedded ZnO/NiO device has Ag nanowire top electrode and is highly transparent (>50%) in the visible range. This transparent solar cell provides power conversion efficiency of 6.1% and incident photon to charge carrier efficiency of 79.5% under UV light illumination. Mott-Schottky analysis showed the flat band potential to be 0.9 V by using the TiO2 layer insertion to induce the significant higher photovoltage and photocurrent on/off ratio of higher than 5 × 105 and played a vital role in the enhanced performance of ZnO/NiO heterostructure. We demonstrated the enhancement of the minority carrier lifetime for a broadband of light illumination via back surface field formation and proposed the energy band-diagram. We may suggest that this high transparent photovoltaic device can be functionally applied on-demands of power generation windows of electronic devices, vehicles and buildings.

Activated carbon fibers for toxic gas removal based on electrical investigation: Mechanistic study of p-type/n-type junction structures

In this study, we evaluated the potential use of CuO-ZnO combination structures with activated carbon fibers (ACFs) for the adsorption (by ACFs) and electrochemical detection (by CuO-ZnO) by of SO2 gas. The gas adsorptivity was concluded to improve as a result of the synergetic effects of physical adsorption by the micropores and mesopores, the specific surface area developed by chemical activation and the chemical adsorption reaction between SO2 and the transition metals introduced in the CuO-ZnO combination structures. From comparison of the SO2 sensing properties, the CuO-ZnO combination structures with ACFs exhibited the fastest sensing capability. This result can be attributed to the larger specific surface area of the semiconductor, which extended its depletion layer by forming p-type CuO/n-type ZnO junctions. This phenomenon led to good SO2 detection through a decrease in the resistance; thus, the contributions of the sensing responses of p-type CuO and n-type ZnO represent a predominant characteristic of the sensor. These types of mechanisms were proven through various physicochemical and electrical characterization methods, especially through evaluation of the SO2 sensing capability of the CuO-ZnO combination structures with ACFs. The reversible sensing capability indicates that the p-n junction structure changed the electrical properties of the ACFs, leading to an intriguing sensing mechanism.

Air-stable n-type Fe-doped ZnO colloidal nanocrystals.

The synthesis of Al and Fe codoped ZnO colloidal nanocrystals (NCs) using a modified etching-regrowth-doping method is presented. We show that the spectroscopic signatures associated with Fe3+ in ZnO disappear upon introduction of Al3+ donor defects into the ZnO lattice. The presence of Al3+ is confirmed by the appearance of a localized surface plasmon resonance feature indicating excess free carriers in the codoped NCs. These spectral changes suggest that Al3+ doping results in a reduction of Fe3+ dopants to the electron paramagnetic resonance-silent Fe2+ dopants that are stable under ambient conditions. These colloidal NCs provide a potential building block for manipulating magneto-optical properties and plasmon responses in colloidal NCs and higher-order nanostructures.

Construction of Cu doped ZnO nanorods by chemical method for Low temperature detection of NO2 gas

In this report, we have prepared Cu-doped ZnO nanorods by a simplistic wet chemical method with subsequent annealing. The phase formation, optical, surface morphological studies along with the electrical properties of the Cu doped ZnO thin films were characterized by X-ray diffraction (XRD), photoluminescence (PL), field emission scanning electron microscopy (FESEM) and Hall measurements, respectively. The addition of copper (Cu) in to the matrix of ZnO influenced the surface morphology of the product dramatically. The regular hexagonal nanorods-like surface morphology of ZnO becomes a broken rod-like morphology, after addition of copper. Owing to the complexes surface morphology of Cu doped ZnO, the system was used to detect the toxic gases like NO2. The effect of doping on the gas-sensing properties of ZnO was slightly counterintuitive in that they initially increased up to Cu concentrations of 1% before beginning to decline. Meanwhile, the Cu doped ZnO exhibited the gas response of ˜ 71 for 1 ppm of NO2 gas at low operating temperatures. This higher gas sensing response is due to the defect states observed in Cu doped ZnO films. In addition to above mentioned findings, the transformation of n-type ZnO to p-type Cu doped ZnO was also observed. The p-type conductivity is attributed to the holes created in the merging of Cu on Zn sites.

Preparation and Gas Sensing Performance of Hierarchical Porous ZnO-based Materials with Sunflower Rods as a Biological Template

A graded porous structure SnO2/ZnO composite was prepared with sunflower rods as a biological template. The prepared samples were subjected to phase analysis by scanning electron microscopy(SEM), transmission electron microscopy(TEM), X-ray diffractiometry(XRD) and X-ray photoelectron spectroscopy(XPS). ZnO sample was a pure phase of hexagonal wurtzite, and the template had been completely removed. The surface of the sample presented a honeycomb-like structure of a sunflower rod template, which was formed by interconnecting the porous channels, and had a smaller average size and exhibited n-n heterojunction at the n-type ZnO interface. Compared with that of pure ZnO, the response of the hierarchical porous structure SnO2/ZnO composite to 100 mg/L n-butanol reached a maximum of 40.61 at 240 °C, about 2.7 times higher than that of pure ZnO. Its response time and recovery time are 6 and 3 s, respectively, which are also better than those of pure ZnO. SnO2/ZnO composite exhibits good gas selectivity, which is related to the improvement of the structure and the forming of n-n heterojunctions of the material.

Influence of Doping Concentration on the Outputs of a Bent ZnO Nanowire.

In this article, the governing equation on electric potential is established by coupling both the semiconducting characteristics and the piezo-effect property of a ZnO nanowire (ZNW). Carrier redistributions are solved for different doping concentrations while considering the effect of both types of charge carriers. The reason for the semiconducting characteristics of a ZNW to induce electric leakage in energy-harvesting is illustrated in detail and a proper initial carrier concentration, n0 or p0 , is obtained as follows: for a n-type ZnO fiber and for a p-type one under the intrinsic carrier concentration ni = 1.0×1016(1/m3) , which is of significance in both the design and the practical applications of piezotronics and piezo-phototropic devices.

Construction of ZnO/SnO2 Heterostructure on Reduced Graphene Oxide for Enhanced Nitrogen Dioxide Sensitive Performances at Room Temperature.

The employment of n-n homotypic heterogeneous junctions is an efficient method to improve sensitive performance of metal oxide-based gas sensors owing to the generation of charge accumulation regions. Herein, in order to further enhance nitrogen dioxide (NO2) sensing properties of the sensors based on reduced graphene oxide (RGO) at room temperature (RT), n-type ZnO nanoparticles (NPs) decorated n-type SnO2 NPs heterojunctions were successfully constructed on RGO nanosheets (NSs) by combination of the hydrothermal method and the wet-chemical deposition method. The formation of heterostructures between ZnO NPs and SnO2 NPs was confirmed by the nonlinear behavior of current versus voltage (I-V) curve of ZnO/SnO2-RGO. ZnO/SnO2-RGO based sensor displayed remarkably enhanced response (141.0%) for detecting 5 ppm of NO2 at RT, which is almost 4 and 3 times higher than that of SnO2-RGO (34.8%) and ZnO-RGO (43.3%), respectively. Moreover, as far as the ZnO/SnO2-RGO-based sensor is concerned, its response and recovery time (33 and 92 s) are also significantly decreased, compared to SnO2-RGO-based sensor (70 and 39 s) and ZnO-RGO-based sensor (272 and 1297 s). In this work, the improved NO2 sensing properties of the sensors based on RGO not only benefit from the effects of the heterostructures between SnO2 and ZnO, but also derive from the superior electrical characteristics of RGO. In particular, the n-n heterojunctions could offer facile access to effective electronic interaction and improve transfer efficiency of the charges at the interface to adsorbed oxygen. Meanwhile, the n-n heterojunctions can also provide additional reaction center for adsorbing gas.

AZO 버퍼층을 이용한 ZnO/NiO 이종접합 금속 산화물 투명 광전소자 특성 향상

All transparent metal oxide photoelectric device was fabricated with structure of Ag nanowire/NiO/ZnO/AZO/FTO by magnetron sputtering system. In order to achieve p/n junction, p-type NiO was deposited onto the n-type ZnO layer. The AZO (Aluminium-doped zinc oxide) was applied as buffer layer for effective transport and collection of the photo-induced electrons. Under light illumination, electrons and holes are generated in the hetero-junction of p-type NiO and n-type ZnO. The AZO layer between ZnO and NiO layers induces the efficient carrier collection from the ZnO side to the negative electrode, due to the similar structure of AZO to ZnO. In addition, the AZO insertion layer is efficient to suppress the loss of hole carriers, resulting in the low leakage current value. The overall transparency of the Ag nanowire/NiO/ZnO/AZO/FTO device is about 70% for visible light range, and thus which can be applied in the invisible transparent electronics, including photodetectors and solar cells.

MoS2/ZnO-Heterostructures-Based Label-Free, Visible-Light-Excited Photoelectrochemical Sensor for Sensitive and Selective Determination of Synthetic Antioxidant Propyl Gallate

Propyl gallate (PG) as one of the important synthetic antioxidants is widely used in the prevention of oxidative deterioration of oils during processing and storage. Determination of PG has received extensive concern because of its possible toxic effects on human health. Herein, we report a photoelectrochemical (PEC) sensor based on ZnO nanorods and MoS2 flakes with a vertically constructed p-n heterojunction. In this system, the n-type ZnO and p-type MoS2 heterostructures exhibited much better optoelectronic behaviors than their individual materials. Under an open circuit potential (zero potential) and visible light excitation (470 nm), the PEC sensor exhibited extraordinary response for PG determination, as well as excellent anti-inference properties and good reproducibility. The PEC sensor showed a wide linear range from 1.25 × 10-7 to 1.47 × 10-3 mol L-1 with a detection limit as low as 1.2 × 10-8 mol L-1. MoS2/ZnO heterostructure with proper band level between MoS2 and ZnO could make the photogenerated electrons and holes separated more easily, which eventually results in great improvement of sensitivity. On the other hand, formation of a five membered chelating ring structure of Zn(II) with adjacent oxygen atoms of PG played significant roles for selective detection of PG. Moreover, the PEC sensor was successfully used for PG analysis in different samples of edible oils. It demonstrated the ability and reliability of the MoS2/ZnO-based PEC sensor for PG detection in real samples, which is beneficial for food quality monitoring and reducing the risk of overuse of PG in foods.

Improved rectification behaviour in ZnO nanorods homojunction by suppressing Li donor defects using Li-Ni co-doping

Abstract We synthesized p-type Zn0.98Li0.02O and Zn0.96Li0.02Ni0.02O nanorods in conjunction with pristine n-type ZnO nanorods, which are further, used in synthesizing ZnO nanorods based homojunction diodes using a simple two-step solution process. The XRD analysis showed that lattice parameters of the doped samples have no significant changes in comparison to pristine ZnO, substantiating uniform doping. The cross-sectional images of the homojunction substantiate the formation of the p-n junction. The UV-Vis spectroscopy measurement showed the lowering of the optical band gap for Zn0.96Li0.02Ni0.02O nanorods. The presence of higher oxygen vacancies in Zn0.96Li0.02Ni0.02O nanorods is noticed in photoluminescence measurements. The acceptor concentration is ∼1.43 × 1018/cm3 for Zn0.96Li0.02Ni0.02O nanorods and is higher with respect to Zn0.98Li0.02O nanorods. We observed large rectification ratio ∼892 with a lower turn-on voltage ∼3.4 V for Zn0.96Li0.02Ni0.02O based homojunction, relatively better than that of Zn0.98Li0.02O based homojunction.

The s-d exchange model as the underlying mechanism of magnetoresistance in ZnO doped with alkali metals

High field magnetoresistance has been studied in epitaxial n-type ZnO:Na and ZnO:Li thin films in a temperature range between 4 K and 150 K. The resulting negative magnetoresistance can be well fitted using a semiempirical model of Khosla and Fischer based on third order contributions to the s-d exchange Hamiltonian. The parameters obtained from this model were carefully analyzed. One of these parameters is related to a ratio between electron mobilities at zero field (a non-exchange scattering mobility and an exchange or spin dependent one ). From Hall effect measurements was obtained, displaying a weak temperature dependence in accordance with highly n-doped ZnO while the extracted exhibits an anomalous T-dependence. On the other hand, our magnetoresistance data cannot be properly fitted using Kawabata’s expression based on a weak-localization model.

Photovoltaic-pyroelectric effect coupled broadband photodetector in self-powered ZnO/ZnTe core/shell nanorod arrays

Broadband photodetection is widely used in commercial and military fields. However, most semiconductor-based detectors face the constraint of light absorption, needing external bias, comparatively lower responsivity and slower response time that limit their practical applications. Herein, a self-powered core/shell photodetector was fabricated by sputtering a uniform p-type ZnTe layer on n-type ZnO nanorod array. By integrating pyroelectric and photovoltaic effects, the photodetector realizes broadband detection from 325 nm ultraviolet to 1064 nm near infrared under zero bias. The maximum responsivity and detectivity reach 196.24 mA/W and 3.47 × 1012 cm Hz1/2/W for 325 nm laser illumination with power density 2.13 mW/cm2, respectively, which are improved 10-fold relating to the device responded to photovoltaic effect only. While the rise and fall time are drastically reduced from 1.222 ms to 62 μs and 1.563 ms to 109 μs, respectively. In addition, the effects of light wavelength, power density and bias voltage on photocurrent response induced by photovoltaic-pyroelectric effect are systematically characterized and discussed. Our work not only provides a easy yet efficient procedure to construct broadband photodetector, but also broadens the application prospects of core/shell nanorod array based on photovoltaic-pyroelectric effect.

Photo-Electric Response of Polyaniline/ZnO Nanocomposites Based on Inorganic/Organic Heterojunctions

The samples were analyzed and characterized by scanning electronic microscope (SEM) and X-ray diffraction (XRD).The XRD results showed that the diffraction peaks of PANI/ZnO nanocomposites agreed with the pure ZnO. The as-prepared nanocomposites with different amounts of PANI dopants exhibited different sensing properties. When the weight ratio of PANI is 5%, the photoelectric response of nanocomposites to UV light reaches maximum (1938). Compared with ZnO, the response shows 60 times improvement. Additionally, the probable photoelectric response mechanism was also proposed. Firstly, the existence of p-n heterojunctions formed between p-type PANI and n-type ZnO interfaces was confirmed by the IV characteristics. It is believed that the p-n heterojunctions can promote the charge separation and further increase the amount of absorbed negative oxygen ions. Secondly, the highly dispersed stated of PANI could suppress the formation of interfacial recombination centers and accelerate p-type hole transporting. The photocurrent of composites is improved significantly. And we hope this work can help for designing more photoelectric materials with the higher response.

Effect of series and shunt resistance on the photovoltaic properties of solution-processed zinc oxide nanowire based CZTS solar cell in superstrate configuration

Here we report the chemical synthesis of a hetero-junction solar cell in superstrate configuration using nanocrystalline p-type Cu2ZnSnS4 (CZTS) as absorber layer, vertically aligned n-type ZnO nanowire array as window layer and very thin ZnS as buffer layer. CZTS thin films comprised of nano-sized grains with pure kesterite phase have been synthesized by simple and cost-effective sol-gel spin coating technique without any post-deposition sulfurization process. ZnS coated vertically aligned ZnO nanowire arrays have been grown by seed-assisted chemical bath deposition technique followed by post deposition chemical sulfurization of nanowire surface. The fabricated solar cell in superstrate configuration, FTO/ZnONW/ZnS/CZTS/Au, has exhibited power conversion efficiency ∼1.07% with open circuit voltage ∼0.454 V and short circuit current ∼5.652 mAcm−2 when measured in one sun illumination of AM1.5 solar spectrum. The effects of ZnS coating on the photovoltaic properties in association with the series and shunt resistances of the hetero-junction have been studied here.

Effect of the Electronic State of Cu, Ag, and Au on Diesel Soot Abatement: Performance of Cu/ZnO, Ag/ZnO, and Au/ZnO Catalysts.

Noble metals such as Au, Ag, and Cu supported over semiconducting ZnO are well-known heterogeneous oxidation catalysts. All of them have been utilized for the oxidation of diesel soot with varied success. However, Au-supported ZnO is seen to be superior among them. Here, we present a comparative study of all these three catalysts for diesel soot oxidation to explain why Au/ZnO is the best among them, demonstrating the contribution of electronic states of metals in composite catalysts. The electronic states of Cu, Ag, and Au determined by X-ray photoelectron spectroscopy on 1 wt % Cu/ZnO, 1 wt % Ag/ZnO, and 1 wt % Au/ZnO catalysts were correlated with their diesel soot oxidation activities. Although all three catalysts present reasonable diesel soot oxidation activities at relatively low temperature, 1% Cu/ZnO and 1% Ag/ZnO oxidize only about 60% of the deposited diesel soot around 250 °C and 1% Au/ZnO oxidizes 100% of the deposited diesel soot, at a temperature as low as 230 °C. The activity of the catalysts is attributed to the formation of stable M0–Mδ+ bifunctional catalytic sites at the metal–ZnO interface, which enhances the contact efficiency of solid diesel soot on Mδ+ and generates the superoxide species on M0 moieties. The stability of the bifunctional M0–Mδ+ sites is controlled by the electronic interactions between the metal (M) and n-type semiconductor ZnO at their interface. Very high activity of 1% Au/ZnO is attributed to the presence of Au3+ at the catalyst surface, which generates a stronger Coulombic force with diesel soot electrons. We demonstrate a direct relation between the diesel soot oxidation activity of these three metals and their electronic states at the catalyst surface.

MOF-templated synthesis of nano Ag2O/ZnO/CuO heterostructure for photocatalysis

Conversion of metal-organic frameworks to metal oxide nanoparticles is subject of significant research interest for developing high surface area light-active catalysts. Here, for the first time, we demonstrate synthesis of hybrid metal oxide nanocomposites, CuO/ZnO and Ag2O/ZnO/CuO, from bimetallic Zn-Cu metal-organic frameworks using temperature-programmed oxidation method. As-prepared hybrid photocatalysts show very high surface area compared to typical metal oxide nanoparticles with an increase in the surface to bulk Cu ratio due to the synergistic effect between CuO and ZnO. Photocatalytic measurements for degradation of Acid Blue 92 show a pronounced activity increase under visible light irradiation afforded by efficient carrier separation and transfer from coupling a high band gap n-type ZnO with low band gap p-type CuO and Ag2O. Both photocatalysts exhibit excellent stability and reusability for five successive operating cycles.

Determination of Diffusion Coefficient of Copper in ZnO (001) Single Crystals at 1000 °C

Copper from a solid source was diffused into undoped n-type bulk ZnO (001) single crystals at 1000 °C under a nitrogen atmosphere at different diffusion times. The Cu diffusion profiles were obtained by Secondary ion mass spectroscopy (SIMS), and the fitting reveals a diffusion case from a constant concentration source. A value for the diffusion coefficient of 2.42(±0.2) × 10−12 cm2∙s−1 was obtained. Electrical measurements present an increment of carrier concentration with diffusion time, but remains n-type which indicates an increase in the donor levels produced by structural defects in ZnO. Photoluminescence (PL) spectra showed an increment of green emission intensity associated with Cu incorporation.

Piezotronic Effect of a Thin Film With Elastic and Piezoelectric Semiconductor Layers Under a Static Flexural Loading

We theoretically study the electromechanical behaviors of a laminated thin-film piezoelectric semiconductor (PS) composite plate with flexural deformation. The nonlinear equations for drift currents of electrons and holes are linearized for a small carrier concentration perturbation. Following the structural theory systemized by R. D. Mindlin, a system of two-dimensional (2D) equations for the laminated thin-film PS plate, including the lowest order coupled extensional and flexural motion, are presented by expanding the displacement, potential, and the incremental concentration of electrons and holes as power series of the plate thickness. Based on the derived 2D equations, the analytical expressions of the electromechanical fields and distribution of electrons in the thin-film PS plate with an n-type ZnO layer subjected to a static bending are presented. The numerical results show that the electromechanical behaviors and piezotronic effects can be effectively controlled by the external applied force and initial concentration of carriers. The derived 2D equations and numerical results in this paper are helpful for developing piezotronic devices.

Study on material properties of Sn- and Cu-doped ZnO thin films as n- and p-type thermoelectric materials based on wet solution synthesis

Sn-doped n-type and Cu-doped p-type ZnO films were fabricated by depositing solutions derrived by sol–gel route on glass substrates through dip-coating cycles. The X-ray diffractometry demonstrates the typical crystalline structure of hexagonal wurzite of ZnO. The surface morphology was made through scanning electronmicroscopy showing the density and shape of films’ grains of which size was determined between 40 and 55 µm. The results of Hall measurement indicate clearly the material nature of fabricated films at room temperature. The best electrical property of Sn-doped n-type ZnO films is corresponding to Sn content of 2 at.% with carrier concentration of 3.0 × 1018 cm−3. The formation of p-type ZnO thin films was elucidated by the manner of solution synthesis that glucose was added into the solube mixer of zinc acetate and copper(II) nitrate trihydrate and the way of thermal treatment for films in vacuum with pretty good hole concentration of about 1015 cm−3. The electrical transport properties characterizing thermoelectric (TE) behavior of all films were investigated between room temperature to 400 °C (or from 300 to 673 K). The obtained results of electrical conductivity, Seebeck coefficient and power factor revealed that Sn-doped n-type ZnO films have the quality and properties similar to Al-doped ones and Cu-doped ZnO films can be promising p-type materials for TE applications.