Structural and optical properties of ZnO nanorods by electrochemical growth using multi-walled carbon nanotube-composed seed layers

We reported the enhancement of the structural and optical properties of electrochemically synthesized zinc oxide [ZnO] nanorod arrays [NRAs] using the multi-walled carbon nanotube [MWCNT]-composed seed layers, which were formed by spin-coating the aqueous seed solution containing MWCNTs on the indium tin oxide-coated glass substrate. The MWCNT-composed seed layer served as the efficient nucleation surface as well as the film with better electrical conductivity, thus leading to a more uniform high-density ZnO NRAs with an improved crystal quality during the electrochemical deposition process. For ZnO NRAs grown on the seed layer containing MWCNTs (2 wt.%), the photoluminescence peak intensity of the near-band-edge emission at a wavelength of approximately 375 nm was enhanced by 2.8 times compared with that of the ZnO nanorods grown without the seed layer due to the high crystallinity of ZnO NRAs and the surface plasmon-meditated emission enhancement by MWCNTs. The effect of the MWCNT-composed seed layer on the surface wettability was also investigated.     

ZnO Nanorods as Antireflective Coatings for Industrial‐Scale Single‐Crystalline Silicon Solar Cells

In this work, both planar and textured, industrial scale (156 mm × 156 mm) single‐crystalline silicon (Si) solar cells have been fabricated using zinc oxide (ZnO) nanorods as antireflection coating (ARC). ZnO nanorods were grown in a few minutes via hydrothermal method within a commercially available microwave oven. Relative improvement in excess of 65% in the reflectivity was observed for both planar and textured Si surfaces. Through ZnO nanorods, effective lifetime (τ_eff) measurements were presented to investigate the surface passivation property of such an ARC layer. ZnO nanorods increased the τ_eff from 9 to 71 μs at a carrier injection level of 10¹⁵ cm^?3. Increased carrier lifetime revealed the passivation effect of the ZnO nanorods in addition to their ARC property. 33% and 16% enhancement in the photovoltaic conversion efficiency was obtained in planar and textured single‐crystalline solar cells, respectively. Our results reveal the potential of ZnO nanorods as ARC that can be deposited through simple solution‐based methods and the method investigated herein can be simply adapted to industrial scale fabrication.     

Effect of ZnO seed layers on the solution chemical growth of ZnO nanorod arrays

High density ZnO nanorod arrays were grown on Si substrates coated with ZnO seed layers via aqueous solution route. The ZnO seed layers were deposited on the substrate using DC reactive sputtering and RF magnetron sputtering. It was found that ZnO seed layer with (1 0 3) preferred orientation, prepared using DC reactive sputtering, did not facilitate the formation of ZnO nanorods in the solution grown process. Prior seeding of the surface by ZnO layer with (0 0 2) preferred orientation, deposited using RF magnetron sputtering, leads to nucleation sites on which ZnO nanorod arrays can grow in a highly aligned fashion. ZnO nanorods with well-defined hexagonal facets (0 0 2) were grown almost vertically over the entire substrate. The uniformity and alignment of the nanorod arrays are strongly related to the properties of underneath ZnO seed layers.     

Optimized properties of ZnO nanorod arrays grown on graphene oxide seed layer by combined chemical and electrochemical approach

Novel hybrid architectures based on ZnO nanostructures supported on reduced graphene oxide films are reported by a facile two-step aqueous electrochemical approach. Graphene oxide (GO) obtained by oxidizing graphite with either H₂SO₄:H₃PO₄ mix or H₂SO₄ was subjected to electrochemical reduction (ErGO) by constant potential and sweeping potential methods and further employed as seeding layer for electrodeposition of ZnO nanorods. The results showed the GO reduction rate was markedly influenced by oxygen content while Infrared spectroscopy indicated the sweeping potential was effective in removing the carbonyl and alkoxy groups but also as more defect-inducing reduction approach than constant potential, according to the Raman measurements. The subsequent electroreduction of ErGO during the electrodeposition of ZnO resulted in interfacial interaction between the seed layer and ZnO nanorods. H₂SO₄:H₃PO₄ mix induced a less disrupted graphitic plane in GO which proved beneficial for the growth of higher density ZnO nanorods with improved crystalline quality. By controlling the chemical introduction/electroreduction conditions for oxygen groups in GO materials, the interfacial interaction with ZnO nanorods and their properties could be tailored in order to obtain high performance nanoplatforms.     

Cu-doped ZnO nanorod arrays: the effects of copper precursor and concentration

Cu-doped ZnO nanorods have been grown at 90°C for 90 min onto a quartz substrate pre-coated with a ZnO seed layer using a hydrothermal method. The influence of copper (Cu) precursor and concentration on the structural, morphological, and optical properties of ZnO nanorods was investigated. X-ray diffraction analysis revealed that the nanorods grown are highly crystalline with a hexagonal wurtzite crystal structure grown along the c-axis. The lattice strain is found to be compressive for all samples, where a minimum compressive strain of −0.114% was obtained when 1 at.% Cu was added from Cu(NO₃)₂. Scanning electron microscopy was used to investigate morphologies and the diameters of the grown nanorods. The morphological properties of the Cu-doped ZnO nanorods were influenced significantly by the presence of Cu impurities. Near-band edge (NBE) and a broad blue-green emission bands at around 378 and 545 nm, respectively, were observed in the photoluminescence spectra for all samples. The transmittance characteristics showed a slight increase in the visible range, where the total transmittance increased from approximately 80% for the nanorods doped with Cu(CH₃COO)₂ to approximately 90% for the nanorods that were doped with Cu(NO₃)₂.     

Optimization of an Electron Transport Layer to Enhance the Power Conversion Efficiency of Flexible Inverted Organic Solar Cells

The photovoltaic (PV) performance of flexible inverted organic solar cells (IOSCs) with an active layer consisting of a blend of poly(3-hexylthiophene) and [6, 6]-phenyl C₆₁-butlyric acid methyl ester was investigated by varying the thicknesses of ZnO seed layers and introducing ZnO nanorods (NRs). A ZnO seed layer or ZnO NRs grown on the seed layer were used as an electron transport layer and pathway to optimize PV performance. ZnO seed layers were deposited using spin coating at 3,000 rpm for 30 s onto indium tin oxide (ITO)-coated polyethersulphone (PES) substrates. The ZnO NRs were grown using an aqueous solution method at a low temperature (90°C). The optimized device with ZnO NRs exhibited a threefold increase in PV performance compared with that of a device consisting of a ZnO seed layer without ZnO NRs. Flexible IOSCs fabricated using ZnO NRs with improved PV performance may pave the way for the development of PV devices with larger interface areas for effective exciton dissociation and continuous carrier transport paths.     

One-dimensional ZnO nanostructure growth prepared by thermal evaporation on different substrates: Ultraviolet emission as a function of size and dimensionality

Large-scale uniform one-dimensional ZnO nanostructures were fabricated through thermal evaporation via the vapor solid mechanism on different substrates. The effects of Si (100), Si (111), SiO₂ and sapphire substrates with constant oxygen treatment on the morphology and diameter of ZnO nanostructures were investigated. It is found that the type of substrate has a great effect on the shape and diameter of the synthesized nanowires, nanorods, and nanotubes. It is noticed that the size and dimensionality were the most influential parameters on both structural and optical properties of the grown ZnO nanostructures. X-ray diffraction analysis confirms the stability of the wurtzite crystal structure for all grown ZnO nanostructures and the preferred orientation is substrate dependent. The crystallinity as well as the defects within the crystal lattice of the grown ZnO nanostructures was studied through Raman spectroscopy. The photoluminescence spectra of ZnO nanostructures grown on Si (100), Si (111), SiO₂ and sapphire substrates showed two peaks at a near-band-edge (NBE) emission in the ultraviolet region and a broad deep-level emission (DLE) around the green emission.     

水热合成法制备ZnO纳米棒有序阵列薄膜

采用水热合成法在预先生长的ZnO种子层的玻璃衬底上制备出ZnO纳米棒有序阵列薄膜。通过X射线衍射、扫描电镜、透射电镜和选区电子衍射分析表明：所制备的薄膜由垂直于ZnO种子层的纳米棒组成,呈单晶六角纤锌矿ZnO结构,且沿[001]方向择优生长,纳米棒的平均直径和长度分别为10．0nm和3．3μm。     

Enhanced wettability and photocatalytic activity of seed layer assisted one dimensional ZnO nanorods synthesized by hydrothermal method

The wettability and photocatalytic activity of ZnO nanostructures synthesized by hydrothermal method are reported. XRD, FESEM, XPS, TEM, AFM, Contact angle, UV/Vis and photoluminescence spectroscopy are used to characterize the samples. It is observed that ZnO seeded layer results in the formation of nanorods whereas the absence of seed gives rise to flake like morphology. The XRD indicates that ZnO nanorods have preferred orientation along (002) direction. The formation of ZnO nanorods along (002) direction is due to the existence of nucleation sites resulting from the lattice matching of ZnO seed. Wettability studies show that the ZnO nanorods grown on seeded substrate approaches superhydrophobic state with water contact angle (WCA) of 137.0°. The high contact angle is due to the large surface roughness and low surface energy. The enhanced catalytic performance of ZnO nanorods is attributed to the 1D structure, enhanced roughness, crystallinity and a large number of reactive oxidizing species.     

Designing zinc oxide nanostructures (nanoworms,nanoflowers, nanowalls,and nanorods) by pulsed laser ablation technique for gas-sensing application

In this study, pulsed laser ablation technique, also known as pulsed laser deposition (PLD), is used to design and grow zinc oxide (ZnO) nanostructures (nanoworms, nanowalls, and nanorods) by template/seeding approach for gas-sensing applications. Conventionally, ZnO nanostructures used for gas-sensing have been usually prepared via chemical route, where the 3D/2D nanostructures are chemically synthesized and subsequently plated on an appropriate substrate. However, using pulsed laser ablation technique, the ZnO nanostructures are structurally designed and grown directly on a substrate using a two-step temperature-pressure seeding approach. This approach has been optimized to design various ZnO nanostructures by understanding the effect of substrate temperature in the 300-750°C range under O₂ gas pressure from 10-mTorr to 10 Torr. Using a thin ZnO seed layer as template that is deposited first at substrate temperature of ~300°C at background oxygen pressure of 10 mTorr on Si(100), ZnO nanostructures, such as nanoworms, nanowalls, and nanorods (with secondary flower-like growth) were grown at substrate temperatures and oxygen background pressures of (550°C and 2 Torr), (550°C and 0.5 Torr), and (650°C and 2 Torr), respectively. The morphology and the optical properties of ZnO nanostructures were examined by Scanning Electron Microscope (SEM-EDX), X-ray Diffraction (XRD), and photoluminescence (PL). The PLD-grown ZnO nanostructures are single-crystals and are highly oriented in the c-axis. The vapor-solid (VS) model is proposed to be responsible for the growth of ZnO nanostructures by PLD process. Furthermore, the ZnO nanowall structure is a very promising nanostructure due to its very high surface-to-volume ratio. Although ZnO nanowalls have been grown by other methods for sensor application, to this date, only a very few ZnO nanowalls have been grown by PLD for this purpose. In this regard, ZnO nanowall structures are deposited by PLD on an Al₂O₃ test sensor and assessed for their responses to CO and ethanol gases at 50 ppm, where good responses were observed at 350 and 400°C, respectively. The PLD-grown ZnO nanostructures are very excellent materials for potential applications such as in dye-sensitized solar cells, perovskite solar cells and biological and gas sensors.     

Structural and photoluminescence studies on catalytic growth of silicon/zinc oxide heterostructure nanowires

Silicon/zinc oxide (Si/ZnO) core-shell nanowires (NWs) were prepared on a p-type Si(111) substrate using a two-step growth process. First, indium seed-coated Si NWs (In/Si NWs) were synthesized using a plasma-assisted hot-wire chemical vapor deposition technique. This was then followed by the growth of a ZnO nanostructure shell layer using a vapor transport and condensation method. By varying the ZnO growth time from 0.5 to 2 h, different morphologies of ZnO nanostructures, such as ZnO nanoparticles, ZnO shell layer, and ZnO nanorods were grown on the In/Si NWs. The In seeds were believed to act as centers to attract the ZnO molecule vapors, further inducing the lateral growth of ZnO nanorods from the Si/ZnO core-shell NWs via a vapor-liquid-solid mechanism. The ZnO nanorods had a tendency to grow in the direction of [0001] as indicated by X-ray diffraction and high resolution transmission electron microscopy analyses. We showed that the Si/ZnO core-shell NWs exhibit a broad visible emission ranging from 400 to 750 nm due to the combination of emissions from oxygen vacancies in ZnO and In₂O₃ structures and nanocrystallite Si on the Si NWs. The hierarchical growth of straight ZnO nanorods on the core-shell NWs eventually reduced the defect (green) emission and enhanced the near band edge (ultraviolet) emission of the ZnO.     

Number Density and Diameter Control of Chemical Bath Deposition of ZnO Nanorods on FTO by Forced Hydrolysis of Seed Crystals

ZnO nanorods have been studied extensively due to facile synthesis and useful optoelectronic properties for applications in nanoscale devices. In a common two‐step procedure, an ethanolic Zn²⁺ precursor solution is used to deposit ZnO seed crystals on a substrate, which is then immersed in an aqueous Zn²⁺ precursor solution to grow the nanorods. Here, a forced hydrolysis technique was employed based on additions of water and heat to the seed precursor solution before depositing the seeds on commercial fluorine‐doped tin oxide (FTO)/glass substrates. ZnO nanorods were then grown from these seeds by chemical bath deposition. Analyses showed that the forced hydrolysis resulted in an increase in seed crystallite size and a decrease in the number of seeds deposited. With increasing seed size, the number density of nanorods decreased, while the length and diameter of each rod increased. These findings offer a simple method for exerting control over the number density of ZnO nanorods that is compatible with the rough FTO surface, unlike other methods that require smoother substrates.     

Controllable size and photoluminescence of ZnO nanorod arrays on Si substrate prepared by microwave-assisted hydrothermal method

High-quality ZnO nanorod arrays were grown on silicon substrates by microwave-assisted hydrothermal method. A ZnO seed layer deposited by magnetron sputtering was used for promoting nanorod growth. Process optimization indicates that the size and surface density of nanorods can be controlled individually by varying process parameters including precursor concentration, heating temperature, and heating time. The photoluminescence performance of the nanorods is closely dependent on the mean size of the rods. Reducing rod diameter leads to decreased UV emission and visible emission intensity ratio, which has been attributed to the increased impurities or defects on the rod surface. The present results provide a feasible approach to modify the optical properties of transparent ZnO nanorod arrays.     

Preparation,characterization of 1D ZnO nanorods and their gas sensing properties

The 1D ZnO nanorods (NR's) were grown with Zinc (Zn) ion precursor concentration variation on seed layer glass substrate by the low temperature hydrothermal method and utilized for nitrogen dioxide (NO₂) gas sensing application. Zn ion precursor concentration varied as 0.02, 0.03, 0.04, 0.05 and 0.06 M and thin films were characterized for structural, morphological, optical, electrical, surface defect study and gas sensing properties. All the film showed dominant orientation along the (002) direction, the intensity of the peak vary with the length of the nanorods. SEM cross images confirmed that nanorods had vertical alignment perpendicular to the plane of the substrate surface. The PL intensity of oxygen vacancy related defects for prepared samples was found to be linearly proportional to gas sensing phenomena. This result in good agreement with the theoretical postulation that, oxygen vacancies plays the important role for adsorption sites to NO₂ molecule. The gas sensing performance was studied as a function of operating temperature, Zn ion precursor concentration variation, and gas concentration. The maximum gas response is 113.32–100 ppm NO₂ gas at 150 °C for 0.05 M sample out of all prepared samples. Additionally, ZnO thin film sensor has potential to detect NO₂ as low as 5 ppm.     

Supercritical Carbon Dioxide Treatment of Porous Silicon Increases Biocompatibility with Cardiomyocytes

Porous silicon is of current interest for cardiac tissue engineering applications. While porous silicon is considered to be a biocompatible material, it is important to assess whether post-etching surface treatments can further improve biocompatibility and perhaps modify cellular behavior in desirable ways. In this work, porous silicon was formed by electrochemically etching with hydrofluoric acid, and was then treated with oxygen plasma or supercritical carbon dioxide (scCO₂). These processes yielded porous silicon with a thickness of around 4 μm. The different post-etch treatments gave surfaces that differed greatly in hydrophilicity: oxygen plasma-treated porous silicon had a highly hydrophilic surface, while scCO₂ gave a more hydrophobic surface. The viabilities of H9c2 cardiomyocytes grown on etched surfaces with and without these two post-etch treatments was examined; viability was found to be highest on porous silicon treated with scCO₂. Most significantly, the expression of some key genes in the angiogenesis pathway was strongly elevated in cells grown on the scCO₂-treated porous silicon, compared to cells grown on the untreated or plasma-treated porous silicon. In addition, the expression of several apoptosis genes were suppressed, relative to the untreated or plasma-treated surfaces.     

Well-integrated ZnO nanorod arrays on conductive textiles by electrochemical synthesis and their physical properties

We reported well-integrated zinc oxide (ZnO) nanorod arrays (NRAs) on conductive textiles (CTs) and their structural and optical properties. The integrated ZnO NRAs were synthesized by cathodic electrochemical deposition on the ZnO seed layer-coated CT substrate in ultrasonic bath. The ZnO NRAs were regularly and densely grown as well as vertically aligned on the overall surface of CT substrate, in comparison with the grown ZnO NRAs without ZnO seed layer or ultrasonication. Additionally, their morphologies and sizes can be efficiently controlled by changing the external cathodic voltage between the ZnO seed-coated CT substrate and the counter electrode. At an external cathodic voltage of −2 V, the photoluminescence property of ZnO NRAs was optimized with good crystallinity and high density.     

Fabrication and characterization of silicon wire solar cells having ZnO nanorod antireflection coating on Al-doped ZnO seed layer

In this study, we have fabricated and characterized the silicon [Si] wire solar cells with conformal ZnO nanorod antireflection coating [ARC] grown on a Al-doped ZnO [AZO] seed layer. Vertically aligned Si wire arrays were fabricated by electrochemical etching and, the p-n junction was prepared by spin-on dopant diffusion method. Hydrothermal growth of the ZnO nanorods was followed by AZO film deposition on high aspect ratio Si microwire arrays by atomic layer deposition [ALD]. The introduction of an ALD-deposited AZO film on Si wire arrays not only helps to create the ZnO nanorod arrays, but also has a strong impact on the reduction of surface recombination. The reflectance spectra show that ZnO nanorods were used as an efficient ARC to enhance light absorption by multiple scattering. Also, from the current-voltage results, we found that the combination of the AZO film and ZnO nanorods on Si wire solar cells leads to an increased power conversion efficiency by more than 27% compared to the cells without it.     

Porous silicon pillar and bilayer structure as a nucleation center for the formation of aligned vanadium pentoxide nanorods

Porous silicon single layer (PSM), bilayer (PSB) and pillar (PSP) structures have been evaluated as nucleation centers for vanadium pentoxide (V₂O₅) crystals. Deposition of vanadium precursor over different substrates (drop casting technique), followed by annealing treatment under Ar-H₂ (5% H₂) atmosphere, induced crystallization of vanadium oxide. With respect to c-Si/SiO₂ substrate, V₂O₅ nanorods with relatively large aspect ratio were formed over and within PSP structures. On the other hand, pores in PSM and PSB were found to be filled with relatively smaller crystals. Additionally, PSB provided a nucleation substrate capable to align the nanocrystals in a preferential orientation, while V₂O₅ crystals grown on PSP were found to be randomly aligned around the nanoporous pillar microstructure. Nanorods and nanocrystals were identified as V₂O₅ by temperature-controlled XRD measurements and evidence of their crystalline nature was observed via transmission electron microscopy. A careful analysis of electronic microscopy images allows the identification of the facets composing the ends of the crystals and its corresponding surface free energy has been evaluated employing the Wulff theorem. Such high surface area composite structures have potential applications as cathode material in Lithium-ion batteries.     

Incorporation of carbon nanotube and graphene in ZnO nanorods-based hydrogen gas sensor

In this study, ZnO nanorods (NRs) were grown using solgel/hydrothermal methods on SiO₂, carbon nanotube (CNT)/SiO₂, and graphene/SiO₂ substrates to form hydrogen gas sensing chips. Results indicate that ZnO NRs/CNT/SiO₂ structures exhibited better H₂ sensing performance than the other two types of ZnO NRs-based structures. Furthermore, multiple electrical and material characterizations show that ZnO NRs/CNT/SiO₂ structures had a stronger (002) crystalline phase, with nanorod fusion near the bottom, and more oxygen-related defects. Owing to their small size, simple fabrication, and low cost, the ZnO NRs/CNT/SiO₂based H₂ gas sensors are promising for future industrial H₂ sensing applications.     

SURFACE MODIFICATION OF HYDROTHERMALLY GROWN ZnO NANOSTRUCTURES WITH PROCESS PARAMETERS

ZnO nanorods (NRs) were hydrothermally synthesized by using equimolar zinc nitrate hydrate (Zn(NO₃)₂ [sdot] 6H₂O) and hexamethylenetetramine (C₆H₁₂N₄) solutions. The shape of the nanostructures, obtained by aqueous method, was greatly influenced by the growth temperature and the molar concentrations. NRs grown at higher temperature (90°C) have rounded tips, whereas nanostructures of hexagonal flat-end shape were obtained at 75°C. Hardly any nanostructures were observed by further reducing the temperature to 60°C. In addition, solutions with higher molarity favored the appearance of nanoflowers. Scattered ZnO NRs were observed on silicon substrate, whereas aligned ZnO nanowires (NWs) 50–70 nm in diameter were obtained at 75°C by introducing sputtered ZnO film as a seed layer. High-resolution transmission electron microscopy (HRTEM) confirmed the growth of ZnO nanowires along [001] direction. A band-edge luminescence along with a broad visible spectrum was observed for the ZnO nanowires.