Thickness dependent wavelength selective NIR-UV-visible photoresponse in ZnO nanowires and silicon heterojunction

In this paper, we report ZnO nanowires (NWs) and silicon-based type-II PN heterojunction for UV–Visible–Infrared self-powered photodetection. The as-grown ZnO NWs were highly crystalline and aligned along the c-axis in the [002] direction revealed in the HRTEM and XRD measurements. The Hall measurements revealed the n-type behavior for ZnO and p-type for p-Si with carrier concentrations of 4.09 × 10¹⁶ cm^?3 and 1.38 × 10¹⁷ cm^?3, respectively. The depletion widths were estimated to be ～35 nm and ～120 nm, respectively for p-Si and n-ZnO NWs. The Ag/n-ZnO NWs/p-Si/Ag PN heterojunction showed large photoresponse, even at zero bias, under the illumination of commercially available UV–Visible–NIR LEDs, thus acting as a self-powered photodetector. It was interesting to observe that the photoresponse was dependent on the growth time and hence the thickness of ZnO NWs thin film. A maximum zero bias responsivity of ～0.1 A/W at green (515 nm) was observed and was large for the junction with thicker ZnO NWs film (5 h growth), compared with thinner (3 h growth) device under IR (950 nm) LED illumination, however, it was observed otherwise for UV (395 nm) LED. This suggests that tuning the thickness of the ZnO NWs thin film results in the wavelength selective photoresponse, consequently, paving the way towards UV blind IR-visible photodetector based on ZnO NWs. The transient short circuit current (I_sc vs t) and open circuit voltage (V_oc vs t) properties showed fast and large responses under periodic illumination of all LEDs (UV–Vis–NIR). The response was observed to depend on the intensity of light and the maximum V_oc comes out to be ～102 mV and I_sc ～5.58 μA, under the illumination of a red laser diode.     

用热氧化法在微米丝上制备CuO纳米线

采用电镀和热氧化相结合的方法,在直径为30 ?m的丝上成功制备了直径为50～80 nm、长度在几个至十几个微米的CuO纳米线,并研究了温度和热氧化时间对其生长情况的影响。实验结果表明,随着热氧化时间延长,纳米线长度增加,保温4 h后均匀性也得到较好保证。热氧化温度在450～500 ℃内CuO纳米线可以较好生长。在500 ℃下保温4 h后,自然冷却至室温,沿着微米丝表面垂直生长成均一排列的CuO纳米线阵列。     

Fabrication of vertical GaN/InGaN heterostructure nanowires using Ni-Au bi-metal catalysts

We have fabricated the vertically aligned coaxial or longitudinal heterostructure GaN/InGaN nanowires. The GaN nanowires are first vertically grown by vapor–liquid-solid mechanism using Au/Ni bi-metal catalysts. The GaN nanowires are single crystal grown in the [0001] direction, with a length and diameter of 1 to 10 μm and 100 nm, respectively. The vertical GaN/InGaN coaxial heterostructure nanowires (COHN) are then fabricated by the subsequent deposition of 2 nm of In_xGa_1-xN shell on the surface of GaN nanowires. The vertical GaN/InGaN longitudinal heterostructure nanowires (LOHN) are also fabricated by subsequent growth of an InGaN layer on the vertically aligned GaN nanowires using the catalyst. The photoluminescence from the COHN and LOHN indicates that the optical properties of GaN nanowires can be tuned by the formation of a coaxial or longitudinal InGaN layer. Our study demonstrates that the bi-metal catalysts are useful for growing vertical as well as heterostructure GaN nanowires. These vertically aligned GaN/InGaN heterostructure nanowires may be useful for the development of high-performance optoelectronic devices.     

Simple and low-cost synthesis of Al-doped ZnO/CuO composite nanowires for highly efficient hydration level sensing

In this paper, we report on our investigation of the impact of Al doping on the primary physical properties of SILAR fabricated CuO/ZnO composite nanowires. Our characterization on surface and structural analysis showed that we were able to lightly dope the nanocomposites effectively using the SILAR method, which is cost-effective and repeatable. Conductivity of the Al-doped films changed significantly which resulted in greater sensing response for hydration. We utilized artificial sweat solutions to characterize the sensing response of the films and we demonstrated that sensing response almost doubled with Al doping. Transient response of sweat application also showed that response times were less than 10 s for 2% Al-doping. We conclude that Al doping on CuO/ZnO nanocomposites is an excellent candidate to be utilized for hydration sensing through sweat.     

Ultrafast hole carrier relaxation dynamics in p-type CuO nanowires

Ultrafast hole carrier relaxation dynamics in CuO nanowires have been investigated using transient absorption spectroscopy. Following femtosecond pulse excitation in a non-collinear pump-probe configuration, a combination of non-degenerate transmission and reflection measurements reveal initial ultrafast state filling dynamics independent of the probing photon energy. This behavior is attributed to the occupation of states by photo-generated carriers in the intrinsic hole region of the p-type CuO nanowires located near the top of the valence band. Intensity measurements indicate an upper fluence threshold of 40 μJ/cm² where carrier relaxation is mainly governed by the hole dynamics. The fast relaxation of the photo-generated carriers was determined to follow a double exponential decay with time constants of 0.4 ps and 2.1 ps. Furthermore, time-correlated single photon counting measurements provide evidence of three exponential relaxation channels on the nanosecond timescale.     

Synthesis and electrical property of metal/ZnO coaxial nanocables

ABSTRACT: Ag/ZnO and Cu/ZnO coaxial nanocables were fabricated using AgNO3 or copper foil as source materials by the vapor-liquid-solid process. The coaxial nanocables consist of a crystalline metallic Ag or Cu core and a semiconductor ZnO shell. The evolution of the Ag/ZnO products having different morphologies was investigated by stopping the heating at different temperatures. The diameters of the Ag/ZnO nanocables and the Ag cores could be modulated by changing Ag ratio in the source. The electrical characteristics of the Ag/ZnO contact and the influence of annealing reveal a Schottky diode behavior for a single Ag/ZnO nanocable device. The nanocables with uniform shape and controlled size are expected to provide a new choice in various applications of biological detection, nanothermometer, and photocatalysis.     

纳米CuO/ZnO去除H_2S反应条件及机理研究

以纳米CuO/ZnO作为去除H2S的活性组分,考察了反应温度、空速、氧分压等反应条件和掺铜量对纳米CuO/ZnO脱硫性能的影响,并分析了CuO对脱硫剂脱硫机理的影响。结果表明:脱硫剂TZ2(Cu:Zn物质的量比为1:18.40)有较高的脱硫性能;空速越低脱硫性能越高;反应温度由25℃升高到120℃,脱硫性能先下降后升高,120℃的脱硫性能略高于25℃;氧分压为10%时,有最佳的脱硫性能;CuO的加入增大了脱硫产物向多硫化物及单质硫的转化。     

Highly sensitive characteristic of surface enhanced Raman scattering for CuO/Au core/shell nanowires substrate

In this paper, we report a facile process to fabricate CuO/Au core/shell nanowires, where CuO core and Au shell were prepared by thermal oxidation and sputtering, respectively. The as-prepared CuO/Au nanowires are highly sensitive surface-enhanced-Raman-scattering (SERS) substrates, which can detect methylene blue down to a very low concentration of 10^?13 M. The major advantages of SERS substrates based on CuO/Au core/shell nanowires compared with others SERS substrates are the high sensitivity, uniformity, and purity due to the absence of any organic surfactants in the synthesis process.     

Extended photoresponse and multi-band luminescence of ZnO/ZnSe core/shell nanorods

Enhanced resistive switching phenomena of IrO _x /GdO _x /W cross-point memory devices have been observed as compared to the via-hole devices. The as-deposited Gd₂O₃ films with a thickness of approximately 15 nm show polycrystalline that is observed using high-resolution transmission electron microscope. Via-hole memory device shows bipolar resistive switching phenomena with a large formation voltage of -6.4 V and high operation current of >1 mA, while the cross-point memory device shows also bipolar resistive switching with low-voltage format of +2 V and self-compliance operation current of <300 μA. Switching mechanism is based on the formation and rupture of conducting filament at the IrO _x /GdO _x interface, owing to oxygen ion migration. The oxygen-rich GdO _x layer formation at the IrO _x /GdO _x interface will also help control the resistive switching characteristics. This cross-point memory device has also Repeatable 100 DC switching cycles, narrow distribution of LRS/HRS, excellent pulse endurance of >10,000 in every cycle, and good data retention of >10⁴ s. This memory device has great potential for future nanoscale high-density non-volatile memory applications.     

Li-doped Cu2O/ZnO heterojunction for flexible and semi-transparent piezoelectric nanogenerators

We have investigated the characteristics of p-type Li-doped Cu₂O (LCO) films grown by radio frequency magnetron sputtering to use as p-n heterojunction for flexible and semi-transparent piezoelectric nanogenerators (PENGs). Electrical, optical, morphological properties of the LCO films were examined as a function of Ar/O₂ flow ratio as well as work function. The LCO films grown at Ar/O₂ ratio of 20/4 sccm film showed a p-type behavior with resistivity of 2.12 Ω-cm, mobility of 0.364 cm²/V-s, and carrier concentration of 8.07×10¹⁹ cm^-3. To overcome the piezoelectric potential screening effect of conventional ZnO-based PENGs, the p-type LCO layer was employed. Due to the enhanced piezoelectric potential coupled with the reduced total capacitance, the PENG with a p-LCO/n-ZnO heterojunction demonstrates the much higher output power up to ~52 μW than PENG only with ZnO layer (7 μW). The improved output power of PENGs indicates that sputtering of the p-type LCO layer on the n-type ZnO is the effective method to overcome the limit of the ZnO-based PENGs.     

Synthesizing vertical porous ZnO nanowires arrays on Si/ITO substrate for enhanced photocatalysis

In this work, porous ZnO nanowires arrays (NWAs) which are perpendicular to the substrate were synthesized using facile and simple hydrothermal process approaches. The surface of ZnO nanowires (NWs) was increased by chemical electroless etching many macropores on them, which results in better photocatalytic activity, which is 1.73 times in terms of the rate constant for methyl orange (MO) degradation for porous ZnO NWAs compared with ZnO NWAs. The absorption of the vertical porous ZnO NWAs was considerably enhanced in the visible region. The optical band-gap became slightly narrower from 3.24 eV (ZnO NWAs) to 3.22 eV (porous ZnO NWAs). Thus, the vertical porous ZnO NWAs can be employed for various applications, such as gas sensor, photocatalyst, and solar cell.     

Rapid synthesis and photocatalytic activity of ZnO nanowires obtained through microwave-assisted thermal decomposition

This work reports a new method for large scale production of ZnO nanowires (ZnO-NWs) by microwave assisted thermal decomposition (MATD). This method is simple, economical and reproducible; in addition, the production of material exceeds 95% without using preferential growth precursors. The reaction occurs in only 3 min with minimal energy expenditure. ZnO-NWs produced at 1200 W had diameters ranging between 20 and 70 nm and lengths that varied between 1 and 15 µm, were totally crystalline and showed preferential growth in the [001] direction. Using XRD and HRTEM, it was determined that ZnO presented a hexagonal, wurtzite structure. The relationship between graphite and ZnO, the power of the reaction system and the reactor humidity each played an important role in this method and need to be properly controlled to optimize the production of nanowires. The reactor used allowed for the separation of the reaction products; for that reason, the ZnO-NWs produced had a high degree of purity, as determined by XRD. The photocatalytic activity of ZnO-NWs in methyl orange (MO) decreased as the pH increased; this behavior is common in ZnO because basic pH increased the interaction between the MO and ZnO. The synthesis process, growth mechanisms and photocatalytic activity of ZnO-NWs are discussed in this work.     

Characterization and evaluation of ZnO/CuO catalyst in the degradation of methylene blue using solar radiation

Catalysts based on the combination of zinc oxide and copper oxide were synthesized at a 80:20 mass ratio by the Pechini method and calcined at 500, 600 and 700 °C for 1 h. These catalysts were characterized by XRD, SEM, FT-IR, BET, UV-Vis, TGA and XRF. They were subsequently tested for the removal of methylene blue dye by means of heterogeneous catalysis combined with solar radiation through a RCCD experimental design, analyzing the concentrations of H₂O₂ and methylene blue, as well as radiation exposure time and pH. The average crystallite size obtained was of 26.21, 28.21 and 35.91 nm for the respective calcined samples. The XRF was effective in determining the elements present in the catalyst, consisting of 75% zinc oxide and 25% copper oxide. The values of surface area were of 7.54, 7.19 and 3.92 m²/g, respectively. The experimental design showed that the catalyst calcined at 500 °C exhibited the highest removal efficiency (93%) of methylene blue with a dye concentration of 20 mg/L. Despite the need to carry out new studies to optimize the process, results suggest that the application of solar photocatalysis in the treatment of methylene blue with ZnO/CuO is a feasible alternative.     

Electrochemical performance of polycrystalline CuO nanowires as anode material for Li ion batteries

CuO nanowires were prepared by wet-chemical method through the reaction of CuSO₄, KOH and ammonia in aqueous solution and subsequent aging process. The XRD, SEM, TEM, HRTEM, CV and galvanostatic method are used to characterize the structure, morphology and electrochemical performance of the as-prepared CuO nanowires. The CuO nanowires are polycrystalline microstructure, which facilitates the electrochemical storing Li. Therefore, the polycrystalline CuO nanowires exhibit a good electrochemical performance as Li ion batteries anode. The CuO nanowires showed a high reversible capacity of 720 mAh/g. The capacity keeps up 650 mAh/g over 100 cycles.     

氧化铜/钒酸铋异质结薄膜的制备及其光电性能研究

采用电化学沉积法在FTO玻璃上制备了具有纳米多孔网状结构的氧化铜/钒酸铋薄膜。利用X射线衍射（XRD）、拉曼光谱（Raman）、扫描电子显微镜（SEM）、能谱分析（EDS）对薄膜做成分及结构分析,采用线性伏安扫描（LSV）、频率阻抗测试（EIS）对薄膜做光电性能测试。氧化铜的掺入能够提高钒酸铋薄膜的光电性能,在1.23 Vvs.RHE时,40 mmol/L 氧化铜/钒酸铋薄膜的光电流密度为1.39 mA/cm²,比纯钒酸铋薄膜的光电流密度（0.7 mA/cm²）增大了1倍左右。结果表明,纳米多孔的网状结构,提高了薄膜对光的利用效率,同时也增加了薄膜和电解液的接触面积。氧化铜和钒酸铋复合形成异质结后,抑制了光生电子-空穴对的复合,从而提高了氧化铜/钒酸铋薄膜的光电流密度。     

Low-temperature and highly sensitivity H2S gas sensor based on ZnO/CuO composite derived from bimetal metal-organic frameworks

The bimetallic metal-organic frameworks (MOF) Zn/Cu-BTC were prepared by a facile solvothermal method in one step and used as a self-sacrificed template to obtain the ZnO/CuO composites. The composites with different Cu/Zn molar ratios were characterized by XRD, FESEM, and XPS. The ZnO/CuO composite exhibited an octahedral structure, and a p-n heterojunction may be formed between p-type CuO and n-type ZnO. To prove its functional characteristics, the ZnO/CuO composite was used as a sensing material to test its gas sensitivity. The effect of Cu/Zn molar ratios was examined, and the results showed that the optimized ZnO/CuO (1: 0.33) composite based gas sensor exhibited reasonable selectivity to 10 ppm H₂S, operated at 40 °C. The sensitivities were improved by 17.1 times and 327.8 times compared with the pristine CuO and ZnO based gas sensors, respectively. Moreover, the detection limit to H₂S of such sensors could be reduced as low as 300 ppb. The sensing mechanism has been thoroughly studied and such ZnO/CuO composite is an ideal candidate for highly sensitive detection for H₂S with low power consumption in the real application.     

Preparation and electrochemical performances of ZnO nanowires as anode materials for Ni/Zn secondary battery

ZnO nanowires were synthesized by a hydrothermal route without any substrate or template. Structure analyses through XRD, SEM, TEM and HRTEM indicated that ZnO nanowires had high purity and perfect crystallinity, and grew along [0 0 0 1]. The diameter was 50-80 nm, the length was about several micrometers and length-diameter ratio was more than 100. As electrode materials of Ni/Zn batteries, ZnO nanowires showed the obviously improved cycle stability, average discharge capacity of 609 mAh g⁻¹, higher discharge voltage/lower charge voltage. Slow rate cyclic voltammetry showed that electrochemical activity of ZnO nanowires was superior to that of the conventional ZnO. The improvements of electrochemical performance were ascribed to the unique nanowire structure. During the charging/discharging cycles, nanowires were broke, grew in diameter, and changed into nanorods. Nanowires lying parallel to the anodes could suppress the growth of dendrite clusters perpendicular to the anodes.     

ZnO/SBA-16不同焙烧温度对CuO/ZnO/SBA-16催化CO活性的影响

通过浸渍法将氧化锌负载与载体SBA-16上,制得不同焙烧温度的ZnO/SBA-16,再通过沉淀沉积法将氧化铜负载与ZnO/SBA-16上,制备出高分散的催化剂10%CuO/6%ZnO/SBA-16并考察了其对CO氧化的活性,通过XRD、TG-DTA、H2-TPR表征及活性测试,发现300℃、400℃、500℃预先焙烧过的ZnO/SBA-16制得的催化剂中CuO分散度更好,活性更高。     

Application of impregnation combustion method for fabrication of nanostructure CuO/ZnO composite oxide: XRD,FESEM, DRS and FTIR study

Nanostructured CuO/ZnO composite oxide was prepared by a novel impregnation combustion method using copper nitrate and zinc oxide tetrapod. The X-ray diffraction patterns revealed that CuO/ZnO composite oxide was formed. The effects of different impregnation combustion parameters on the properties of composite were studied by field-emission scanning electron microscope (FESEM), powder X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR) and UV–vis diffuse reflectance spectrum (DRS). The synthesis of ZnO–CuO nanocomposites through impregnation of a zinc oxide tetrapod with copper nitrate aqueous sodium carbonate solutions is reported. During thermal treatment the samples evolve toward the formation of nanocrystalline ZnO particles (zincite phase) dispersed onto tenorite, CuO annealed at 450 °C. XRD patterns of the precursors calcined at 450 °C showed the formation of the zincite–tenorite phases. Field emission scanning electron microscopy (FESEM) exhibited loosely agglomerated hexagonal particles with uniform morphology having a size around 50 nm.     

新型CuO/ZnO催化剂的制备及其在水煤气变换反应中的应用

首次采用沉积沉淀法,以Cu（OH）₂为前驱体制备不同CuO负载质量分数的CuO/ZnO水煤气变换（WGS）催化剂,并运用XRD、N₂物理吸附和TPR等方法对催化剂进行结构表征。结果表明,活性组分CuO在载体ZnO表面的分散程度、颗粒大小及CuO和ZnO之间相互作用对催化剂的活性均有影响。CuO的适宜负载质量分数为20％,所得CuO/ZnO催化剂样品WGS反应活性较好,在350 ℃,CO转化率可达95.5％。