Ordered networks of ZnO-nanowire hierarchical urchin-like structures for improved dye-sensitized solar cells

Quasi-1D ZnO nanowires (NWs) ordered as patterned 3D hollow hierarchical urchin-like structures have been prepared on transparent conducting substrates by electrodeposition. The ZnO NWs have been grown on self-assembled ordered polystyrene microspheres with electrical charge densities ranging from 5 to 30 C cm(-2) and organized arrays of mono and multi-urchin layers have been built. These layers have been sensitized by the highly absorbing D149 indoline organic dye. The optical characterizations and dye titrations have shown a significant increase in the light scattering and absorption as well as dye loading for the organized structures compared to randomly vertically aligned ZnO NWs grown under the same conditions. The dye-sensitized solar cells (DSSC) prepared using the sensitized layers have been characterized by current-voltage (J-V) measurements, IPCE and by electrochemical impedance spectroscopy. We show that the best performances are obtained for the 3D urchin monolayer structures. The conversion efficiency is increased by up to 4 times compared to their counterparts made of randomly dispersed vertical ZnO NWs. Impedance spectroscopy results show a very fast charge transfer in the ZnO NWs and urchin monolayers and that the electron lifetime is in the 4-14 ms range.     

Synthesis and thermal stability of ZnO nanowires

ZnO nanowires (NWs) were synthesized on Au-coated Si (100) substrates by vapor transport method. The effect of high temperature annealing on the structural and chemical composition as well as thermal stability was studied. The as-prepared ZnO NWs was nearly stoichiometric and identified as hexagonal ZnO phase. After annealing at 1,473 K, the atomic ratio of O/Zn, the intensity of the diffraction peaks, and the diameter of nanowires were increased. The ZnO NWs were fragmented into nanocrystals and the fragments coalesced with each other after annealing at 1,673 K. The thermal stability of ZnO NWs was studied by thermo-gravimetric (TG) analysis. A sharp increase in the TG curves was observed and can be attributed to the oxidation of some possibly presented Zn atoms. The activation energy of oxidation of Zn interstitial atoms was found to be 484.81 kJ mol^?1. A mass gain peak was observed after annealing at 1,473 K, but it was completely eliminated after annealing at 1,673 K.     

Effects of ZnO nanowire length on surface-assisted laser desorption/ionization of small molecules

The effects of nanowire (NW) length on surface-assisted laser desorption/ionization (SALDI) mass spectrometry (MS) of small molecules were investigated using ZnO NWs of 50 nm diameter with a broad range of lengths ranging from 25 to 1600 nm. Characterization of the ZnO NWs revealed that the length was the only parameter that varied in this study, while other properties of the NWs remained essentially the same as the bulk properties. Experiments on SALDI efficiency exhibited that the SALDI processes on NWs have a certain length window. In the present case of ZnO NWs, the SALDI efficiency was found to be enhanced on the nanowires of 250 nm length, corresponding to an aspect ratio of 5. The roles of NW length in the SALDI processes were discussed from the viewpoint of efficient energy-transfer media as well as physical obstacles screening laser irradiation and preventing the escape of nascent ions from NW surfaces. The existence of the length window may provide valuable insight for tailoring new nanostructures for efficient SALDI of small molecules.     

介电力显微镜探测氧化锌纳米线表面吸附分子对电导率的影响

利用场效应晶体管器件和介电力显微镜来研究氧化锌纳米线表面吸附分子对其电导率的影响. 相比于空气中,ZnO纳米线场效应晶体管器件在氮气中电导率更高,介电力显微镜得的介电信号也是在氮气中更大. 影响ZnO纳米线电导率变化的主要原因是表面吸附分子数量的变化,而并不是电极与材料之间接触性质的变化.     

Characteristics of Ga and Ag-doped ZnO-based nanowires for an ethanol gas sensor prepared by hot-walled pulsed laser deposition

Pure ZnO and Ga (3 % w/w) and Ag (3 % w/w)-doped ZnO nanowires (NWs) have been grown by use of the hot-walled pulse laser deposition technique. The doping characteristics of Ga and Ag in ZnO NWs were analyzed by use of photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS) and the results were compared with those for pure ZnO NWs. We also fabricated gas sensors by use of pure ZnO and Ga and Ag-doped ZnO NWs. Among the NW sensors, the Ag-doped NW sensor was most sensitive. We synthesized the NWs on sapphire substrates under different conditions, for example temperature, time, gas flow, and distance between target and substrate. The diameter and length of NWs were <100 nm and several microns, respectively. To analyze the effect of Ag doping on ZnO NWs, we investigated the near band edge emission by use of low-temperature PL and XPS. Significant changes in resistance and sensitivity were observed. When the sensors were used at 300 °C for detection of 1 ppm ethanol vapor, the sensitivity of the pure ZnO and the Ga and Ag-doped ZnO NW gas sensors was 97, 48, and 203 %, respectively.     

One-step electrodeposition of Ag-decorated ZnO nanowires

A new route for synthesizing Ag-decorated ZnO nanowires (NWs) on conductive glass substrates using a one-step electrodeposition technique is described here. The structural, optical, and photoelectrochemical properties of Ag-decorated ZnO nanowires were studied in detail using techniques such X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, UV-visible spectroscopy, photoluminescence, and photoelectrochemical measurements. Both pure and Ag-decorated ZnO nanowires were found to crystallize in the wurtzite structure, irrespective of their Ag contents. Increasing the Ag content from pure ZnO NWs to 3% Ag ZnO NWs decreases the photoluminescence intensity, shifts the optical band gap to the red, and increases the photocurrent up to threefold. This behavior was attributed to the surface plasmon resonance effect induced by the Ag nanoparticles, which inhibits charge recombination and improves charge transport on the ZnO surface.     

Enhanced UV photosensing properties of ZnO nanowires prepared by electrodeposition and atomic layer deposition

A new process enabling the synthesis of zinc oxide (ZnO) and Al-doped ZnO nanowires (NWs) for photosensing applications is reported. By combining atomic layer deposition (ALD) for the seed layer preparation and electrodeposition for the NW growth, high-quality ZnO nanomaterials were prepared and tested as ultraviolet (UV) sensors. The obtained NWs are grown as arrays perpendicular to the substrate surface and present diameters between 70 and 130 nm depending on the Al doping, as seen from scanning electron microscopy (SEM) studies. Their hexagonal microstructure has been determined using X-ray diffraction and Raman spectroscopy. An excellent performance in UV sensing has been observed for the ZnO NWs with low Al doping, and a maximal photoresponse current of 11.1 mA has been measured. In addition, initial studies on the stability have shown that the NW photoresponse currents are stable, even after ten UV on/off cycles.     

The fabrication of TiO<Subscript>2</Subscript> nanoparticle/ZnO nanowire arrays bi-filmed photoanode and their effect on dye-sensitized solar cells

Porous TiO₂ nanoparticles coated on ZnO nanowire arrays (TiO₂ NP/ZnO NW) as photoanode for dye-sensitized solar cell (DSSC) has been fabricated and investigated to improve the power conversion efficiency. The TiO₂ NP/ZnO NW photoanode consists of single crystalline ZnO NWs synthesized via hydrothermal method and porous TiO₂ NP film covered on the surface of ZnO NW arrays by screen printing technique. The effect of TiO₂ NPs thickness of the bi-filmed photoanode on the cell performance has been investigated, and TiO₂ NP/ZnO NW DSSC with NP thickness of ~5 μm exhibits the best efficiency of 4.68%, higher than 1.16% of ZnO NW DSSC and 3.18% of TiO₂ NPs DSSC, prepared and tested under identical conditions. The efficiency increase is attributed to the enlarged photocurrent, due to the greatly enhanced surface area for dye absorption and light harvesting efficiency resulted from TiO₂ NPs, and improved open-circuit voltage, due to reduced electron recombination by providing direct conduction pathway along ZnO NWs.     

Tailoring the charge carrier dynamics in ZnO nanowires: the role of surface hole/electron traps

Post-fabrication thermal-annealed ZnO nanowires (NWs) in an oxidizing (or a reducing) ambient were investigated using transient photoluminescence and X-ray photoelectron spectroscopy. Our findings reveal an ultrafast hole-transfer process to the surface adsorbed oxygen species (e.g., O(2)(-)) occurring within a few hundred picoseconds (ps) in the air-annealed samples; and an ultrafast electron-transfer process to charged oxygen vacancies (i.e., V(O)(2+)) occurring within tens of ps in the H(2)-annealed samples. Contrary to the common perception that the band edge emission (BE) dynamics are strongly influenced by the carrier trapping to the green emission related defect states (i.e., V(Zn)), these above processes compete effectively with the ZnO BE. Hole trapping by ionized V(Zn), which occurs in an ultrashort sub-ps-to-ps timescale (and hence limits its effective hole capture radius), however, has less influence on the BE dynamics. Importantly, our findings shed new light on the photoinduced charge transfer processes that underpins the novel properties of enhanced photocatalytic activity, photovoltaic performance, and photoconductivity response of ZnO NWs, thereby suggesting a strategy for tailoring the ultrafast carrier dynamics in ZnO NW-based devices.     

Investigation of the growth parameters of hydrothermal ZnO nanowires for scale up applications

Zinc oxide nano-wires (ZnO NWs) are synthesized reproducibly with high yield via a low temperature hydrothermal technique. The influence of the growth duration time, growth temperature, zinc precursor and base concentration of Na₂CO₃ on the morphology of NWs is investigated. The growth products are characterised using scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL). SEM analysis shows that the optimum growth temperature is 140 °C and finds that length and diameter of ZnO NWs have a relationship with growth duration time and base concentrations of Na₂CO₃. In addition, it is reported that a high (～90%) yield of ZnO NWs can be synthesised via using any of three different precursors: zinc chloride, zinc acetate and zinc nitrate. TEM and XRD results indicate the high purity and the single crystalline nature of the ZnO NWs. XPS confirms the absence of sodium contaminants on the surface and indicates a near flat band surface condition. PL shows a large visible band in the yellow part of the spectrum, and a small exciton emission peak, indicating a large defect concentration, which is reduced after annealing in air.     

Enhancing Electron Collection Efficiency and Effective Diffusion Length in Dye‐Sensitized Solar Cells

Intensity‐modulated photocurrent spectroscopy and intensity‐modulated photovoltage spectroscopy are employed to measure the dynamics of electron transport and recombination in the ZnO nanowire (NW) array‐ZnO/layered basic zinc acetate (LBZA) nanoparticle (NP) composite dye‐sensitized solar cells (DSSCs). The roles of the vertical ZnO NWs and insulating LBZA in the electron collection and transport in DSSCs are investigated by comparing the results to those in the TiO₂–NP, horizontal TiO₂–NW and vertical ZnO–NW‐array DSSCs. The electron transport rate and electron lifetime in the ZnO NW/NP composite DSSC are superior to those in the conventional TiO₂–NP cell due to the existence of the vertical ZnO NWs and insulating LBZA. It indicates that the ZnO NW/NP composite anode is able to sustain efficient electron collection over much greater thickness than the TiO₂–NP cell does. Consequently, a larger effective electron diffusion length is available in the ZnO composite DSSC.     

Mesoporous CeO2 nanowires as recycled photocatalysts

Mesoporous CeO2 nanowires(NWs) were synthesized through a facile hydrothermal process by using triblock copolymer F127 as the template.XRD analysis confirmed the cubic phase of the synthesized CeO2 NWs.High-yield one-dimensional NWs with accessible mesopores could be observed from SEM and TEM images,and the surface area of the material was confirmed to be 273 m2 g-1 with pore width distribution of 6.9-13.8 nm.The mesoporous CeO2 NWs could be used as efficient photocatalysts for organic dye degradation under UV light irradiation,which was superior compared with commercial photocatalyst P-25 and commercial CeO2 powders.The NW structure facilitates the recovery of catalyst by sedimentation,leading to the impressive reusability of these mesoporous CeO2 NWs.     

Large-scale Ni-doped ZnO nanowire arrays and electrical and optical properties

Large-scale Ni-doped ZnO nanowire (NW) arrays are grown. The electrical conductivity of a single Ni-doped ZnO NW has been increased for 30 times. The photoluminescence (PL) spectrum of the doped ZnO NWs has a red shift, suggesting possible doping induced band edge bending. The doped NW arrays could be the basis for building integrated nanoscale transistors, sensors, and photodetectors.     

Patterned growth of vertically aligned ZnO nanowire arrays on inorganic substrates at low temperature without catalyst

We report an approach for growing aligned ZnO nanowire arrays with a high degree control over size, orientation, dimensionality, uniformity, and possibly shape. Our method combines e-beam lithography and a low temperature hydrothermal method to achieve patterned and aligned growth of ZnO NWs at <100degreesC on general inorganic substrates, such as Si and GaN, without using catalyst. This approach opens up the possibility of applying ZnO nanowires as sensor arrays, piezoelectric antenna arrays, two-dimensional photonic crystals, IC interconnects, and nanogenerators.     

Enhanced visible-light-driven photocatalytic activities of 0D/1D heterojunction carbon quantum dot modified CdS nanowires

Zero-dimensional carbon dots (0D C-dots) and one-dimensional sulfide cadmium nanowires (1D CdS NWs) were prepared by microwave and solvothermal methods, respectively. A series of heterogeneous photocatalysts that consisted of 1D CdS NWs that were modified with 0D C-dots (C-dots/CdS NWs) were synthesized using chemical deposition methods. The mass fraction of C-dots to CdS NWs in these photocatalysts was varied. The photocatalysts were characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and ultraviolet-visible spectroscopy. Their photocatalytic performance for the spitting of water and the degradation of rhodamine B (RhB) under visible light irradiation were investigated. The photocatalytic performance of the C-dots/CdS NWs was enhanced when compared with that of the pure CdS NWs, with the 0.4% C-dots/CdS NWs exhibiting the highest photocatalytic activity for the splitting of water and the degradation of RhB. The enhanced photocatalytic activity was attributed to a higher carrier density because of the heterojunction between the C-dots and CdS NWs. This heterojunction improved the electronic transmission capacity and promoted efficient separation of photogenerated electrons and holes.     

Environmentally Benign and Efficient Ag2S‐ZnO Nanowires as Photoanodes for Solar Cells: Comparison with CdS‐ZnO Nanowires

In this work, we develop a low‐temperature, facile solution reaction route for the fabrication of quantum‐dot‐sensitized solar cells (QDSSCs) containing Ag₂S‐ZnO nanowires (NWs), simultaneously ensuring low manufacturing costs and environmental safety. For comparison, a CdS‐ZnO NW photoanode was also prepared using the layer‐by‐layer growth method. Ultraviolet photoelectron spectroscopy analysis revealed type‐II band alignments for the band structures of both photoanodes which facilitate electron transfer/collection. Compared to CdS‐ZnO QDSSCs, Ag₂S‐ZnO QDSSCs exhibit a considerably higher short‐circuit current density (J_sc) and a strongly enhanced light‐harvesting efficiency, but lower open‐circuit voltages (V_oc), resulting in almost the same power‐conversion efficiency of 1.2 %. Through this work, we demonstrate Ag₂S as an efficient quantum‐dot‐sensitizing material that has the potential to replace Cd‐based sensitizers for eco‐friendly applications.     

ZnO纳米线负载Pd1单原子催化剂催化若干反应性能的考察

将孤立的Pd原子分散到ZnO纳米线(NWs)上作为单原子催化剂(SACs),并考察了它们在若干反应中的催化性能.Pd1/ZnO SAC对甲醇蒸汽重整制氢反应表现出高的活性、稳定性和CO2选择性.该催化剂体系对CO和H2的氧化也具有高活性,但在富氢物料中CO优先氧化反应中的催化剂性能较差,这主要是由于在ZnO负载的Pd1原子上H2氧化的强竞争反应所致.常压下在Pd1/ZnO SAC上就可发生逆水汽变换反应.该系列催化反应测试结果清楚地表明,选择合适金属与载体对开发分子催化转化用单原子催化剂至关重要.     

CPF/ZnO纳米复合催化材料的制备

通过原位聚合-热转化两步法,利用ZnO纳米微粒和糠醇(F)制备出了具有大共轭结构的高分子(CPF)和ZnO的纳米复合催化材料(CPF/ZnO);用TG-DTA、TEM、XRD、XPS、IR和UV-Vis等技术对其热稳定性、形貌、尺寸、结构及吸光特性等进行了表征,以亚甲基蓝(MB)溶液的催化降解研究了该材料在自然光条件下的催化性能。结果表明,由该方法可以得到平均尺寸约为 50 nm的CPF/ZnO纳米复合催化材料;其中的CPF为具有极性基团和大共轭结构的高分子;ZnO与CPF化学键合在一起;CPF的引入将ZnO的光谱响应拓展到了整个紫外-可见区,从而极大地改善了ZnO在自然光条件下的催化性能。如在460 ℃下处理40 min所得的纳米复合材料,在自然光条件下,10 min即可使MB溶液完全脱色,而在相同条件下,纯纳米ZnO仅能使MB的脱色率为10%左右;该催化材料重复使用3次仍可使MB溶液的脱色率保持在80%以上。     

Photoconductive UV Detectors Based on ZnO Films Prepared by Sol-Gel Method

Highly c-axis oriented ZnO thin films were deposited on single crystal Si (111) substrates by sol-gel method. The photoconductive UV detectors based on ZnO thin films, being a metal-semiconductor-metal (MSM) structure with interdigital (IDT) configuration, were fabricated by using Au as contact metal. The characteristics of dark and photocurrent of the UV detector, the UV photoresponse of the detector were investigated. The linear current-voltage (I-V) characteristics under both forward and reverse bias exhibit ohmic metal-semiconductor contacts. Under illumination using monochromatic light with a wavelength of 365 nm, photo-generated current arrived at 44.89 μ A at a bias of 6 V. The detector exhibits an evident wide-range spectral responsivity and shows a trend similar to that in photoluminescence (PL) spectrum. PL spectrum of detector exhibits two peaks, one is the near band edge emission, and another is the deep-level emission in the visible region.     

复合Ag/ZnO分级结构微球的制备及其光催化性能研究

采用化学沉淀法制备ZnO微球,利用柠檬酸三钠（TCD）避光还原硝酸银在ZnO表面沉积银粒子制备Ag/ZnO复合材料.利用XRD、SEM、TEM、EDS、FTIR、UV-vis DRS、PL、BET等对Ag/ZnO的结构、组分、形貌及光谱性质进行了表征,通过紫外及可见光照降解甲基橙溶液评价样品的光催化性能.结果表明：ZnO纳米微球是由ZnO纳米片相互交错构筑而成的具有丰富孔道的分级结构,Ag纳米粒子均匀沉积在ZnO纳米片上.Ag的沉积显著增加了ZnO的可见光吸收,猝灭了ZnO荧光,提高了ZnO催化活性.