ZnO Nanorods Grown Electrochemically on Different Metal Oxide Underlays

In this study we present results on electrochemically grown ZnO nanorods on different metal oxide underlays, such as ZnO seed layers with different morphologies, ZnS and TiO2 compact thin films produced by spray pyrolysis on transparent conductive oxide (TCO) substrates. Also in this work we present results on ZnO nanorods directly deposited on some chosen TCO substrates. The relationship between nanorod formation and substrate properties were studied. All ZnO nanorod layers were grown electrochemically using ZnCl2 aqueous solutions (c=0.2 mmol/L) at the bath temperature of 80 °C during one hour. The structural properties and morphology of metal oxide underlays and ZnO nanorods grown on them were studied by scanning electron microscopy (SEM), x-ray diffraction spectroscopy (XRD). Depending on the substrate morphology, ZnO rods with different dimension, orientation, shape and density were obtained. For instance, larger rods (d~200 nm, l~700 nm) were obtained on substrates, such as ITO/glass, FTO/glass and ZnO:In/ITO/glass. Smaller rods (d~60 nm, l~350 nm) were obtained on smooth, uniform and fine-grained underlays, such as ZnS and TiO2.


Introduction
Zinc oxide (ZnO) is an n-type II-VI semiconductor with a direct band gap (3.37 eV) and large exciton binding energy of 60 meV [1][2]. ZnO has attracted research interest in recent years due to its potential applications in various nanodevices such as lasers and light emitting diodes [3], gas sensors [1,4], field emission devices [1,4] and solar cells [4,5]. For hybrid organic/inorganic solar cells with "absorber layer/ZnO nanorod/blocking layer/TCO (transparent conductive oxide)" structure, it is highly important to synthesize ZnO nanorod/blocking layer/TCO structures. In order to increase the solar cell performance, it is desired to obtain high aspect ratio (l/d, where l -length and d -diameter) conductive ZnO nanorods. Among various deposition techniques, electrodeposition is a promising approach for growing ZnO nanorods, because it is simple, low-cost, low temperature and easily scalable to large-area deposition [6][7].
Morphology of the substrate has a particular significance in nanorods growth. A seed layer (usually ZnO thin film) is required prior to the electrodeposition of the ZnO nanorods to contribute the rods growth or improve their vertical alignment [1,8,9]. For some applications (emitting diodes, solar cells), it is required to grow ZnO nanorod directly on a transparent conductive oxide [10][11]. In this paper, we present a study on the growth of ZnO nanorods by electrodeposition on various substrates: 1) commercially available indium tin oxide (ITO) and fluorine doped tin oxide (FTO) coated glass substrates, 2) set of ZnO seed layers with various morphologies prepared by spray pyrolysis onto ITO/glass substrates and 3) ZnS and TiO2 thin films deposited by spray pyrolysis on ITO/glass substrates. Herein we study relationship between the initial morphology of the substrate and final morphology of the ZnO nanorod layer. The morphology and structural properties of the substrates and ZnO nanorods deposited on them, respectively, are studied by means of high resolution scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques.

Synthesis details
Prior the deposition, ITO and FTO covered glass substrates were washed thoroughly with soap, ethanol and sulphuric acid. The samples were rinsed with deionised water after each cleaning step. On some chosen ITO/glass and FTO/glass substrates, the ZnO nanorods were deposited directly onto the substrate. Commercially available ITO/glass substrates have been chosen for the deposition of metal oxide underlays such as ZnO seed layers, ZnS and TiO2 thin films prepared by using spray pyrolysis method as described in details earlier [12][13]. Electrodeposition of ZnO nanorods was carried out potentiostatically in a three-electrode glass cell, where the TCO/glass was used as a working electrode, while a silver/silver chloride/3M KCl (Ag/AgCl/KCl) and a platinum (Pt) wire were used as reference and counter electrodes, respectively. Solution of 0.2 mmol ZnCl2 (Sigma-Aldrich), and 0.1 M KCl in amount of 50 ml was utilized as a supporting electrolyte. The growth temperature was kept at 80 C using a temperature controlled circulating bath. Deposition time was fixed to 1 hour. Electrochemical deposition was done under -1.0 V potential, vs the reference electrode using the Radiometer Analytical potentiostat PGP201.

Characterization
The morphology of the substrates was studied by the high resolution scanning electron microscope Zeiss EVO-MA15 at the operating voltage of 10 kV. The crystal structure of the nanorods was characterized by using X-ray diffraction on a Rigaku Ultima IV diffractometer using CuKα radiation (λ=1.5406Å, 40 kV at 40 mA). Crystals density was evaluated from the SEM surface images, from area of 6.2 µm×4.7 µm.

Deposition of ZnO rods on TCO electrodes
In this study two different types of conducting electrodes were used: commercially available ITO and FTO coated glass substrates. The SEM images of TCO electrodes are shown in figure 1a (from ITO/glass) and in figure 1c (from FTO/glass). Corresponding morphologies of the electrodeposited ZnO layers directly grown on ITO and FTO electrodes are presented in figures 1b and 1d, respectively. As can be seen from the figures, the final shape of the ZnO nanorods differs depending on the substrate morphology. In case of ITO/glass, the rods are thinner (d~170 nm, l~700 nm) and show higher density (~7.4×10 8 cryst./cm 2 ) than those grown on FTO/glass (d~300 nm, l~600 nm, density ~6.5×10 8 cryst./cm 2 ). Such difference can be explained by the difference in the morphology of the substrate. Higher density of the grains on the FTO electrode ( Figure 1c) provoke large amount of nuclei that can further coalescence and promote the lateral growth of the crystal. Similar behaviour has   [14][15]. The resistivity of the substrates might be also a reason of such difference in final shape of the ZnO crystals. In our case, the ITO has lower resistivity (ρ~15-20 Ω•cm) than resistivity of the FTO susbtrate (ρ~150 Ω•cm). It was reported earlier by Kim et al., that conductive substrates greatly affect on the structural and optical properties of ZnO nanorods [16].

Deposition of ZnO nanorods on ZnO seed layers obtained by spray pyrolysis
In previous studies on ZnO thin films by spray pyrolysis [17][18][19], it was shown that temperature, concentration of the precursor solution (zinc acetate), dopant type and amount are the most important technological parameters that influence the morphology of the film. Here we prepared ZnO seed layers by spray pyrolysis on ITO/glass substrates using different technological parameters mentioned above. The morphology of ZnO seed layers are shown in figures 2a, 2c, 2e and 2g. The SEM images of ZnO nanostructures grown on those seed layers are presented in figures 2b, 2d, 2f and 2h, respectively. ). It could be that differences in seed layer thicknesses (SL-2 is deposited from more concentrated solution than SL-1 and thus SL-2 layer is probably thicker [15,22]) or surface morphologies (compare figures 2a and 2c) are responsible for such kind behavior. XRD study shows that the ZnO rods grow preferably along the (002) plane parallel to the substrate independent of the orientation of corresponding ZnO seed layer.

Deposition of ZnO nanorods on ZnS and TiO2 underlays grown by spray on ITO/glass substrates
For some purposes (dye-synthesized solar cell etc.) it is important to have a less conductive thin barrier layer between TCO and ZnO nanorods, therefore it is important to find out technological parameters in order to grow nanorods on such types of the substrate. Here we prepared ZnS and TiO2 thin films by spray pyrolysis using knowledge of the earlier studies on ZnS [23] and TiO2 films deposition by spray [24]. Both ZnS and TiO2 films have smooth morphologies according to SEM images (figures 4a and 4c). Both substrates resulted in very uniform and dense coverage of wellshaped hexagonal ZnO nanorods. The dimension of the rods grown onto ZnS are slightly larger (d~150 nm, l~450 nm), than those grown on TiO2, (d~60 nm and l~370 nm). The difference in rods dimensions could be explained by different surface roughness on ZnS and TiO2 underlays.   Herein we presented a study on the growth of ZnO nanorods by electrodeposition on various substrates: commercially available ITO, FTO substrates on glass, metal oxide underlays such as ZnO seed layers, ZnS and TiO2 thin films prepared by using spray pyrolysis. The underlays with highly structured surface composed of grains with different sizes provoke the growth of ZnO crystals with different sizes and shape. ZnO rods were grown on smooth, flat, uniform and fine-grained substrates, such as ITO, FTO, ZnS and TiO2 thin films, ZnO seed layer obtained at 320 °C and c=0.05 mol/L and indium-doped ZnO seed layer. The conductivity of the substrates might also affect the shape of resultant ZnO crystals. Electrically more conductive substrates-such as ITO, FTO and indium-doped ZnO seed layer provoke the growth of ZnO rods with larger dimensions. Thus, dimensions, shape and density of the rods depends strongly on the properties of the substrate or seed layer used, the orientation of the seed layer has minor effect.