Electrodeposition of ZnO Nanorod Arrays on Transparent Conducting Substrates–a Review
ZnO nanorods (NRs) are promising components in a wide range of nanoscale devices for future applications in photocatalysis, solar cells, optical devices and biochemical sensing. The nanorods in the form of arrays vertically oriented to the substrate may be obtained by electrochemical deposition but morphology of the film is very sensitive to the synthesis conditions. This article provides a comprehensive review on various electrosynthesis procedures developed to obtain the nanorods of desired structure, diameter, density on the substrate.
We discuss the growth mechanisms and influence of different parameters such as the type and concentration of Zn2+ and OH− precursors, the value of applied potential or current density on the morphology of obtained ZnO films and the role of various structural modifiers on the shape of ZnO nanostructures. We present a brief analysis of the influence of electrosynthesis conditions and postannealing of the samples on optical and electrical properties of ZnO nanowires deposited on the conducting substrate. A short summary of the practical applications of ZnO nanorods is also provided.
- ZnO nanorods;
- structure modifiers;
- electrical properties, ZnO nanorods applications
ZnO is a wide bandgap semiconductor with a direct bandgap of about 3,37 eV and relatively high exciton binding energy (60 meV) at room temperature. ZnO has attracted increasing interest due to its unique ability to form a variety of nanostructures such as nanowires, nanoribbons, nanobelts, nanocombs, nanospheres, nanofibers, nano-tetrapods. A special attention is focused on the ZnO in the form of nanorods (NRs) and nanorod arrays vertically arranged with respect to the substrate due to their unique properties. The ordered ZnO nanostructures are expected to enhance performance of various technologically important devices such as short-wavelength lasers  and , electroluminescent devices , ,  and , sensors , , ,  and , photocatalytic systems , , ,  and  and third generation of solar cells , , , , ,  and . In the latter case, the ZnO NRs are usually deposited on a transparent conducting oxide (TCO) substrate i.e. glass plate covered either with a thin layer of indium-tin oxide (ITO) or F-doped SnO2 (FTO) and the length, diameter and density of nanorods are important parameters influencing the solar cell efficiency.
Over past decades, many methods have been developed for preparation of ZnO of various morphologies and size, including sol–gel processes , , ,  and , chemical vapor deposition , , , ,  and , hydrothermal , , ,  and  and electrochemical methods , , , ,  and . Electrodeposition provides several advantages in comparison with other methods, like low cost, large-scale deposition and possibility of morphology control of resultant films, from rough deposits to well aligned nanorods arrays. The electrochemical synthesis from aqueous solution containing Zn2+ ions and source of OH-, consisted in electroreduction of hydroxide precursor, followed by precipitation of Zn(OH)2 and dehydration to ZnO, was firstly reported in 1996 independently by Izaki et al.  and by Peulon and Lincot . A numerous papers published up to date have shown that the shape and the properties of obtained films may be varied by the change of parameters such as the type and concentrations of Zn and O precursors, temperature, type of a substrate and method of its pretreatment as well as by manipulation with electrochemical parameters (current density or electrodeposition potential). A few studies have been also performed in non-aqueous solutions, such as propylene carbonate , dimethylsulfoxide  and ionic liquids ,  and .
The aim of this review is to sum up the recent advances in electrochemical deposition of ZnO nanorods on the transparent conducting substrates from aqueous solutions. In the following sections we discuss the stability diagrams of ZnO, processes involved in the nucleation and growth of ZnO on the electrodes and analyze correlations between the electrosynthesis conditions and morphology of the nanostructures, their density on the substrate, diameter and length as well as the crystal structure, optical and electrical properties.
2. Basic information on stability of Zn(OH)2 and ZnO–thermodynamic data
A fundamental reaction in electrodeposition of ZnO is based on reduction of an oxygen precursor to OH-, followed by reaction with Zn2+ ions, according to the scheme:
The zinc hydroxide formed in this reversible reaction is transformed at elevated temperature into zinc oxide:
Among several different forms of Zn(OH)2 denoted by α-β-, γ-, δ- and ɛ-Zn(OH)2,  and  the latter one is the most stable and this form will be considered further in the text.
One of the crucial aspects which should be considered in ZnO deposition is limited pH stability of Zn(OH)2 and ZnO, as it is illustrated in the stability diagrams presented in Fig. 1. The solid lines represent solubility of the solid phases as a function of pH, i.e. indicate the total concentration of the Zn(II) soluble species, while the dashed lines denote the thermodynamic equilibria between soluble species and the solid phases . It means, that in aqueous solution at the temperature of 25 °C the ZnO and Zn(OH)2 are formed at concentrations of Zn (II) above 10−5.9 and 10−5.6, respectively. In the solution containing for example 10−4 M Zn (II), both Zn(OH)2 and ZnO are thermodynamically stable over the pH ranging from 8 to 13, whereas outside this range Zn(II) is present in the solution in the form of Zn2+ ions or soluble complex Zn(OH)+ at pH < 8 or Zn(OH)42− at pH > 13.
The influence of temperature on the solubility of ZnO and Zn(OH)2 in the solution containing Cl- ions has been reported by Lincot et al. . As it is illustrated in solubility-pH diagrams presented in Fig. 2, the increase of temperature leads to the shift of the solubility curves towards lower pH and the shift is stronger for zinc oxide than for zinc hydroxide. The difference between the V-shape of solubility curves presented in Fig. 2 and U-shape reported by this group in the previous paper  (similar to that presented in Fig. 1) has been explained as a result of the use of another set of data.
The studies of distribution of Zn(II) in different forms in aqueous solutions at pH > 6.7 at several temperatures from the range between 12.5 °C and 75 °C have been carried out by Reichle et al. . The solubility data of zinc hydroxide obtained from atomic absorption spectrophotometric measurements were used by them to evaluate the solubility products of Zn(OH)2 at different pH and various temperatures and the equilibrium constants of different hydroxide complexes of zinc. The results presented in ref.  allow calculating the fractions of Zn(II) existing in the solution in the form of Zn2+(aq), Zn(OH)+(aq), Zn(OH)2(aq), Zn(OH)3-(aq), and Zn(OH)42-(aq). These data were used by us to construct the plots of distribution of different forms of Zn(II) as a function of pH at different temperatures, presented in Fig. 3.
The obtained results are consistent with solubility-pH diagrams presented in Fig. 2 and demonstrate the shift of the curves towards lower pH values with the increase of temperature. Namely, the Zn(OH)2 is the major soluble species in the pH range from 9 to 11.2 at the temperature of 25 °C and from pH 7.8 to 10.5 at 75 °C.
The dependence of temperature on the formation of Zn(OH)2 and ZnO films during electrochemical deposition will be discussed in more detail further in the text.
The data presented in Fig. 3 may be useful in estimation of isoelectric point (IP) of ZnO, i.e. the pH value corresponding to zero net surface charge of ZnO. The establishment of a surface charge on the metal oxide in water is a result of two effects: amphoteric dissociation of surface M-OH groups and/or adsorption of metal hydroxo complexes formed as the hydrolysis products of dissolved species  and . Both phenomena may explain qualitatively the pH dependence of the surface charge. The isoelectric point of ZnO nanoparticles reported in the literature ranges from 8.7 to 10.3 ( and references therein) and its high value is very useful for immobilization of enzymes by electrostatic interactions (see more details in Section 8).
3. Crystallographic structure of ZnO nanorods
The ZnO crystal in hexagonal wurtzite-type structure, which is thermodynamically stable at ambient conditions, may be considered as a number of alternating planes composed of four-fold coordinated O2− and Zn2+ ions stacked alternately along the c-axis ,  and , as shown in Fig. 4a. It is characteristic that hexagonal ZnO has polar and nonpolar planes. The top, Zn-terminated (0001) plane is positively charged and exhibits high surface energy. Therefore, the anions from the solution (OH-, Zn(OH)3-, Zn(OH)42−) may be adsorbed on this polar phase, resulting in fast growth of along  direction. The second polar plane is negatively charged, O-terminated basal plane. Beside these two primary polar planes in the wurtzite crystal structure one can distinguish many other secondary non-polar planes, for example or side planes with 3-fold coordinated atoms , as well as other non-polar surfaces, as pyramidal plane  (see Figs 4b and 4c).
According to the theoretical models concerning the ZnO crystal growth under hydrothermal conditions presented in the literature  and , the fastest growth rate is in direction , which leads to the increase of the surface area of the or facets, which is consistent with observed growth habit. According to the model proposed Li et al. , the crystal growth rates along different directions decrease are in the order: . However, in practice, the presence of selected crystal face is strongly dependent on the growth conditions, such as pH, presence of ligands and concentration of Zn2+ and . In turn, in the case of electrochemical growth the main parameters influencing the shape of ZnO nanocrystals and its orientation with respect to the substrate are: the type and concentrations of ZnO precursors, electric parameters of deposition (potential or current density), pretreatment of the substrate as well as the presence of different organic or inorganic additives, such as ethylenediamine (EDA), hexamethylenetetramine (HMT) or Cl-, considered as structure modifiers. By the change of these parameters, ZnO may be electrodeposited in the form of compact film, hexagonal particles, hexagonal plates, hexagonal rods, flower-like rod bundles and hexagonal nanowire arrays.
The preferred crystallographic orientation of the deposit in nano/microscale may be determined by analysis of its XRD pattern, as it has been illustrated by Xu et al.  (see Fig. 5).
The crystal growth of hexagonal nanowire arrays along  direction with their c-axis perpendicularly oriented to the substrate results in highly intensive peak at 2θ=34.4o corresponding to (0002) plane (Fig. 5e). The relative intensities of two other important diffraction peaks: at 31.7o and at 36.3o with respect to the (0002) signal depend strongly on the crystal shape of ZnO and its orientation on the substrate. For example, formation of vertically arranged ZnO plates results in high relative intensity of the and peaks. The other signals at 47.5°, 56.6° and 62.8° corresponding respectively to the crystal planes , and are of lower intensities or are absent in the spectrum of nanorods and they are less important in determination of the ZnO crystal shape from XRD spectra.
A quantitative analysis of the preferred orientation may be done by determination of a texture coefficient for considered orientation, TC(hkl), by means the equation :
where I is the measured relative intensity, Io is relative intensity of corresponding plane given in the JCPDS data and N is the number of diffraction peaks. The texture coefficient approaches unity for a randomly distributed powder sample, while it is larger than unity if the (hkl) plane is preferentially oriented.
4. General rules for electrodeposition of ZnO
4.1. Pretreatment of the substrate–seeding process
The first step in ZnO synthesis is nucleation occurring when the solution in the electrode vicinity achieves supersaturation. This process is promoted by the presence of underlying thin ZnO film, so called seed layer, previously deposited on the substrate and playing a role of nucleation centers. Heterogeneous nucleation taking place on the substrate in most cases has lower free energy barrier of activation than homogeneous nucleation and therefore, it occurs at lower supersaturation level than that for the process in the bulk of the solution .
In electrochemical deposition the seeding stage may be omitted but in general it enhances formation of the ZnO nanorods , , , ,  and , improves alignment of ZnO nanowires and allows controlling their diameter and density. All these effects will be discussed in more detail in the next sections.
The seed layer may be formed by chemical methods, for example by thermal decomposition of zinc acetate deposited on the substrate  and , sol-gel method  as well as by electrochemical deposition , ,  and . The advantage of the latter method is quantitative control of seeding by the electrodeposition charge. The seed layer should be thin and uniform to provide a homogenous postdeposition of vertically aligned ZnO nanorods.
The electrochemical seeding is usually performed in the solution containing soluble zinc salt of concentration ranging from 0.005 M to 0.5 M, at constant potential or constant current density. In the former case, the value of the potential applied should not be very far from that commonly required for the electrodeposition of ZnO, which is usually in the range -1.0 V ÷ −1.2 V vs. saturated calomel electrode (SCE). It was found that the increase of the pretreatment potential to -1.4 V resulted in the increase of the number of ZnO crystallites and in effect the ZnO nanorods grown on a such layer formed a very dense film  and . The similar effect may be obtained by application of high current density in the galvanostatic pretreatment. According to the results presented by Wei et al. , the increase of the cathodic current density from j = −1.3 mA cm−2 to -3 mA cm−2 resulted in the increase of nucleation centers density. This may lead to decrease of the size of postdeposited ZnO nanorods, higher compactness of the film up to the coalescence of the grains at some critical current density.
A comparison of two different methods of seeding, spray pyrolysis and galvanostatic deposition at the current density −0.13 mA cm−2 was reported by Elias et al. . The ZnO nanowires deposited on both types of buffer layers revealed higher density (up to a factor of 6) in comparison to that formed on naked TCO substrate. It was also found that the morphology of the seed layer influenced on the diameter of nanowires, which varied from 45 nm for arrays deposited on the substrate seeded electrochemically up to 160 nm for those grown on the substrate pretreated by spraying. On the other hand, in the latter case a dispersion of ZnO nanowire diameters was very large.
Another important parameter which also influences the morphology of the seed layer electrodeposited on the substrate is temperature of the process. Lincot et al.  on the base of chronoamperometric results obtained on a rotating FTO electrode under potentiostatic conditions (shown in Fig. 6a), have proposed a general mechanism for thin film deposition, illustrated in Fig. 6b.
According to this mechanism, the polarization of FTO in the solution of ZnCl2 saturated with molecular oxygen, at the temperatures below 34 °C leads to a slow electrode passivation with a thin layer of Zn(OH)2. This results in a monotonic decrease of the current density (curves 1,2 in Fig. 6a). Transformation of Zn(OH)2 into ZnO takes place at higher temperatures due to slow dehydration of hydroxide layer initially formed on the electrode. This transition process occurs after some induction period which lengths decreases with increasing temperature . The XRD studies  of the film electrodeposited from the solution of Zn(NO3)2 at low temperature (25 °C) indicated however, that amorphous zinc hydroxide deposited in the form of uniform layer on ITO electrode already contains the zones of ZnO (of wurzite structure), which act as the nucleation centers in transformation of Zn(OH)2 to ZnO.
Dehydration of Zn(OH)2 at higher temperatures leads to formation of polycrystalline islands (see the scheme in Fig. 6b and SEM picture D in Fig. 7). The process seems to occur by curling the Zn(OH)2 fibrils into grains which then play the role of the growing centers (Fig. 7 B–E). It was found that the full coverage with ZnO is achieved at the deposition charge density above 100 mC cm−2 but at the same time a transparency of the sample strongly decreases . In contrast, annealing of the sample seeded at room temperature leads to transformation of zinc hydroxide to oxide without changes in the film coverage but with the increase of transparency and therefore, this seeding procedure may be very useful in preparation of ZnO rod arrays for solar cell application. A compact and transparent buffer layer predeposited on TCO substrate prevents a direct contact of electrolyte or solid hole transporting material and TCO and hinders the charge carrier recombination (see Section 8.4).
The influence of the bath temperature and concentration of KCl (0.1 M or 1 M) added to the solution of 5 mM ZnCl2, on morphology, composition and transparency of thin layers deposited at the charge density of 800 mC cm−2, have been studied by Ivanova et al. . It was found that at higher concentration of KCl the thin layers are amorphous regardless of the bath temperature and probably contain hydrochlorides (Zn(OH)xCly). Lowering the KCl concentration to 0.1 M leads to various film morphology, dependent of the deposition temperature.
4.2. Electrochemical deposition of ZnO nanorod arrays
In electrochemical deposition of ZnO, the interfacial concentration of OH− ions may be adjusted by reduction of different precursors, such as molecular oxygen (O2) , ,  and  nitrate ions (NO3−) ,  and  or hydrogen peroxide (H2O2) , , ,  and . However, according to the literature, deposition of ZnO from the solution of H2O2 precursor leads to formation of more or less porous films but not nanorods vertically orientated to the substrate. Therefore, in this review we focus only on the results obtained in the presence of O2 and NO3− precursors.
4.2.1. Molecular oxygen precursor
The presence of molecular oxygen in the solution may be provided by bubbling of oxygen or O2/Ar mixture of different volumetric ratio to control O2 concentration in the solution from zero to saturation value, which in aqueous solution is about 10−3 M at room temperature and 8 × 10−4 M at 80 °C .
Electroreduction of oxygen to hydroxide ions is a four-electron reaction:
O2 +2H2O +4e → 4OH-.
Although the standard potential of this process, derived from thermodynamic data is 0.40 V vs. SHE, at 25 °C , the reaction takes place on ITO electrode at the potentials more negative than -0.65 V vs. SCE (i.e. -0.41 V vs. SHE), in the solution of 0.1 M KCl electrolyte .
In the presence of Cl− ions, the reaction (4) may be accompanied by a parallel 2-electron process leading to H2O2 byproduct:
O2 +2H2O + 2e → H2O2 + 2OH-
A combined measurements of the current intensity and interfacial pH performed by Peulon and Lincot  in aqueous solution of 0.1 M KCl without Zn2+ ions indicated that electroreduction of oxygen upon scanning of the electrode potential from -0.6 V to -0.9 V vs. SCE leads to the increase of pH in the vicinity of the electrode from the initial value of 6 to about 10. In the presence of ZnCl2, the OH− ions generated at the electrode are consumed in precipitation of ZnO and therefore, the interfacial pH remains nearly constant (6.5 ± 0.2) within the same polarization range. Thermodynamic calculations have indicated that in the solutions containing ZnCl2 at pH ranging from 0 to 6.5 and temperature above 50 °C, the predominant soluble species is ZnCl+. Thus, the electrosynthesis of ZnO may occur via two alternative schemes:
Zn2+ + 2 OH− → ZnO + H2O
or ZnCl+ + 2OH− → ZnO + H2O + Cl−
The studies performed by many groups, in a wide range of ZnCl2 concentrations (from 5 × 10−5 M to 0.02 M), at different polarization potentials have shown that the morphology of the deposit may be changed from a compact film to nanowires, depending on both chemical and electrochemical parameters.
The mechanism of electrodeposition of ZnO nanowires on the FTO substrate seeded with a thin buffer layer of nanocrystalline ZnO has been discussed by Elias et al.  and Tena-Zaera et al. . According to their reasoning, the precipitation of ZnO is kinetically fast and therefore, the aspect ratio of growing nanowires (the ratio of their length to the width) depends on relative concentration of OH− and Zn2+ ions at the substrate. When the electrode reaction is the only source of hydroxide ions, the [OH−]/[Zn2+] ratio is mainly determined by the relative rates of OH− generation and diffusion of Zn2+ to the electrode. From this point of view it is important to know that diffusion coefficient of Zn2+ (6.2–7.4 × 10−6 cm2 s−1 in aqueous solution of ZnCl2) is much lower than that of molecular oxygen, 3.4 × 10−5 cm2 s−1 at 75 °C and 2.1 × 10−5 cm2 s−1 at 25 °C . If production of hydroxide ions is much faster than the transport of Zn2+ ions, the majority of Zn2+ ions arriving at the electrode are consumed in reaction with OH− ions adsorbed on nanowire tips, hindering the lateral growth of nanowires and promoting the growth along longitudinal axis . However, under these conditions the deposition efficiency, defined as the ratio of OH− ions consumed in reaction with Zn2+ and the total amount of OH− generated in electroreduction of O2, is very low (below 17%) and may be improved by the increase of ZnCl2 concentration or by decrease of the current density . It has been also found that significant increase of the deposition efficiency (above 40%) may be achieved by the increase of KCl concentration to the value of 3.4 M. This effect may be explained by modification of the oxygen reduction mechanism, from four-electron reaction (4) to two-electron process (5) at high Cl− concentration, leading to decrease of the current density. Some effect may be also ascribed to the lowered solubility of O2 in the chloride-containing solution.
Since the oxygen reduction rate is dependent on the potential or current density applied to the electrode and the rate of Zn2+ ion transport to the electrode may be controlled by ZnCl2 concentration in the solution (namely, concentration gradient of Zn2+ across the diffusion layer), the growth process is a complex interplay of electrical and chemical parameters. However, there are some threshold values for deposition of ZnO in the form of nanorods. The lowest concentration limit of ZnCl2 below which ZnO is not deposited on the electrode is 5 × 10−5 M, whereas at concentrations above 5 × 10−3 M the close-packed ZnO nanorods of large diameter (>200 nm) undergo to coalescence leading to formation of a compact layer on the electrode .
On the other hand, it was found that the films obtained at the concentration of Zn2+ higher than 5 × 10−2 M in the presence of KCl may exhibit the sheet-like morphology  and  and this transformation in the growth habit was explained by adsorption and directional effect of Cl− ions. This effect will be discussed in more detail in Section 5.1.
For the reasons stated above, the studies of the influence of ZnCl2 concentration on morphology and the size of ZnO nanorods are usually performed in the concentration range from 5 × 10−5 M to 5 × 10−3 M, as illustrated in Fig. 8.
According to the data presented by Elias et al. , the nanowire dimensions are the most sensitive to concentration at [ZnCl2] < 5 × 10−4 M (see Fig. 8e). At the lowest concentration (5 × 10−5 M) the mean diameter of electrodeposited nanorods (∼25 nm) matched fairly well with the size of the crystallites of the nanocrystalline ZnO buffer layer which are considered as the nucleation sites. Moreover, the nanorod diameter, in contrast to the length, is only slightly dependent on the charge density passed during deposition, i.e. on the deposition time.
The nanorods of smaller diameter (18–20 nm) were obtained by Guo et al.  under similar deposition conditions, i.e. in aqueous solutions of ZnCl2 in the presence of 0.1 M KCl supporting electrolyte, at deposition potential of −1.0 V vs. SCE and temperature of 70 °C. Since the only difference in the procedures applied by these two groups was the method of seeding of the substrate, namely a spin coating of colloidal zinc acetate followed by annealing , or electrodeposition , this indicates that the pretreatment of the substrate is of crucial importance for the width of the nanorods. A strong influence of the morphology, thickness and grain size of the seed layer on the nanorods diameter has been also confirmed by Ivanova et al. .
Several attempts were also done to prepare the film of well aligned ZnO nanorods from ZnCl2 solution on non-seeded substrate , ,  and . Although the nanorods were formed, their density in the film was 6 times lower than density of nanorods grown on the substrate seeded electrochemically (1 × 109 cm−2 and 6 × 109 cm−2, respectively) . According to Li et al. , the initial 2 min of applied bias are the most important for determining the diameter of ZnO nanowires because this initial time corresponds to formation of nuclei for further growth. However, since the nucleation and growth of individual nanowires tends to work in a cooperative fashion, it results in proximate nanowires coalescing into a pseudo-film on non-seeded substrate even at ZnCl2 concentration of 2.8 × 10−3 M.
The mechanism of ZnO electrodeposition on non-seeded FTO substrate in the chloride medium has been discussed by Belghiti et al. . They considered two different regimes, depending on the pH in the vicinity of the electrode. In the classical deposition conditions, at pH close to neutral, the Zn(II) is present in the form of Zn2+ ions and zinc complexes, ZnCl+ and Zn(OH)+, which may be adsorbed on the negatively polarized electrode. These ions rapidly react with generated OH-, giving rise to the ZnO growth both in lateral and vertical directions. This situation usually occurs in the initial deposition period. If zinc ions are in excess in the solution, this growth regime occurs during long time period, leading to coalescence of the growing crystals and formation of a continuous, dense film. If after the initial period the pH of the solution increases above 9 (OH− ions are generated at the electrode in excess), the hydroxide complexes Zn(OH)3− and Zn(OH)42− become predominant soluble species. These ions are not attracted by the negatively charged electrode and in effect the lateral growth is completely quenched. However, the growth of nanorods is not fully stopped but is continued along c-axis due to higher reactivity of the (0001) plane of ZnO  (see Section 3).
4.2.2. Nitrate precursor
The nitrate precursor of hydroxide ions is used in the form of Zn(NO3)2 salt, well soluble in water. It seems to be advantageous that it can act as the precursor of both reagents, Zn2+ and OH− ions. Cox and Brajter have shown  that the reduction of nitrate is catalyzed by the presence Zr(IV) and La(III) ions adsorbed on the electrode surface. According to Izaki  and Yoshida et al. , the same effect is responsible for the shift of the reduction potential of NO3− ions towards less negative values and the mechanism of ZnO electrodeposition from the solution of Zn(NO3)2 has been proposed as follows:
Since Zn2+ plays the role both of the catalyst and reagent, the reaction occurs more likely in one step and it may be written in the form:
However, Zn2+ ions adsorbed on the electrode may also mediate the charge transfer in the reduction of nitrate to ammonia, according to the scheme:
but it is not clear whether this process may have a negative effect on morphology of electrodeposited ZnO.
Another effect which should be also considered is aging of the stock solution of Zn(NO3)2 which causes that electrosynthesis of ZnO from the solution stored for a few days is failed.
Since ZnO electrodeposition is usually performed in aerobic conditions, the second precursor of OH− is molecular oxygen dissolved in the deposition bath.
The two-step procedure of formation of ZnO films revealing wurzite structure with (0001) preferred orientation, from 0.1 M Zn(NO3)2 precursor has been developed by Izaki . In the first step performed at -1.2 V the substrate was covered with granular ZnO particles, whereas the film grown in the second step at the potential of -0.7 V was composed of the aggregates of hexagonal columnar grains. Under such conditions a high concentration of Zn2+ at the electrode provides immediate consumption of generated OH− and the ZnO is deposited in the form of the film with a smooth surface. The kinetic studies performed by Yoshida et al.  with the use of rotating disc electrode in the solution of 0.1 M Zn(NO3)2 have shown the rate constant of NO3− reduction is very low (5.98 × 10−10 cm2 s−1) and therefore, this reaction is a rate determining step in electrodeposition of ZnO. Thus, in order to obtain ZnO in the form of well separated nanorods, one should decrease the Zn(NO3)2 concentration to the value below 5 × 10−3 M or manipulate with the concentration ratio of Zn2+ to OH− ions at the electrode by addition of NaNO3 electrolyte or/and by the increase of deposition potential to speed up the rate of the charge transfer at the electrode/solution interface  and .
The results of systematic studies on the influence of different parameters on the growth of ZnO nanorods from the solution of Zn(NO3)2 have been reported recently by Tena-Zaera et al. . Similarly to the process carried out in the presence of O2, two different regimes of electrodeposition, controlled either by Zn2+ diffusion or OH− generation were distinguished. If diffusion of Zn2+ is significantly slower than generation rate of OH− ions, the growth of nanowires takes place mainly along longitudinal axis (Fig. 9a). When the rates of these two processes are of the same order, the growth occurs both along longitudinal and transversal axis leading to nanorods of larger width (Fig. 9b).
Since the diffusion rate of Zn2+ ions depends on the concentration gradient across the diffusion layer, the diameter of ZnO nanorods may be controlled by the change of Zn(NO3)2 concentration in the bulk of solution, at constant and high concentration of NaNO3, as it is illustrated in Fig. 10. The observed tendency of the increase of nanorod's diameter is similar to that reported for ZnO deposition from the solution containing ZnCl2 and O2, discussed earlier in the text (Section 4.2.1) (see also Fig. 8).
It is worth noting that the results reported above were obtained on the FTO substrate seeded with ZnO thin layer. Electrodeposition of ZnO in the form of nanorods from the solution of low Zn(NO3)2 concentration (10−4 M) is also possible on the bare substrate (without predeposited seed layer) but the nanorod density in a such film is very low  (see Fig. 11 a,b).
The studies on the influence of Zn(NO3)2 concentration on the average nanostructure diameter and surface coverage ratio performed by Sun et al.  have demonstrated that at [Zn(NO3)2] < 1 mM only 30% of the whole non-seeded substrate was covered with ZnO nanowires ( Fig. 12). The increase of Zn(NO3)2 concentration led to the improvement of the substrate coverage with nanorods and to the increase of their diameter but on the other hand, a dispersion of nanorods diameters also increased (Fig. 11c).
A strong influence of Zn(NO3)2 concentration on the quality of nanorods grown on non-seeded ITO substrate is also confirmed by the results presented by Lin et al. . A film of vertically oriented rods with the lengths of about 4 μm and diameter ∼450 nm was obtained after 4 h deposition from the solution of 5 mM Zn(NO3)2 at constant current density of -0.9 mA cm−2 and temperature of 70 °C (see Figs 13 a,b). However, the increase of zinc nitrate concentration above 10−2 M resulted in the growth of the rods of greater diameter, randomly oriented to the surface with tendency to coalescence (Fig. 13c).
Lin et al.  have also investigated the influence of deposition current density on the nanorod diameter and length, at constant concentration of Zn(NO3)2 (0.005 M). It was found that the increase of the current density from −0.9 mA cm−2 to −2 mA cm−2, and in consequence the increase of concentration of OH− generated at the electrode, resulted in the increase of both dimensions in such way that the aspect ratio of nanorods also increased from about 4.6 to 8.4.
The ZnO nanorods vertically arranged on non-seeded substrate were also successfully obtained from the solution of 0.005 M Zn(NO3)2 by potentiostatic deposition at the potentials from the range between -0.6 V and −1.1 V . However, the rods were of submicron width and the diameter distribution was rather large. Moreover, the size of nanorods was very sensitive to deposition potential. Namely, the thinnest nanorods, of average diameter 0.24 μm and aspect ratio 5.4, were obtained at the deposition potential -1.0 V but the increase of the potential to −1.1 V resulted in significant thickening of nanorods to 0.73 μm and their coalescence. A broad distribution of the NR diameters was also observed after decrease of Zn(NO3)2 concentration to 10−4 M (at the deposition potential −1.0 V vs. SCE). The influence of concentration of the second precursor, NO3-, on diameter of ZnO nanorods grown under potentiostatic conditions on non-seeded ITO electrode has been demonstrated by Xue et al. . It was found that one can control the average diameter of the nanorods by addition an extra amount of NaNO3 to the deposition bath, keeping other parameters of the process constant ([Zn2+] = 0.005 M and deposition time 1800 s at the potential of −1.0 V), as it is presented in Fig. 14. It is worth noting that morphology, size and density of the ZnO NRs obtained under these conditions were similar to those of the nanorods formed on the seeded substrate  (for comparison see Fig. 10).
As it has been discussed above, irrespectively of the type of hydroxide precursor, the small changes in interfacial concentration of Zn2+ and OH− ions lead to strong changes of quality, aspect ratio and arrangement of electrodeposited nanorods with respect to the surface, making difficult the precise control of growth process. Therefore, intensive studies have been undertaken towards improvement of the quality of ZnO nanostructures and modification of their shape by addition of organic or inorganic compounds to the deposition bath.
5. Modification of the ZnO structure by different solution additives
5.1. The influence of KCl
Since ZnCl2 salt is used in electrodeposition of ZnO from the solutions containing oxygen precursor, the chloride ion is a natural component of the deposition bath. Another source of Cl− ions is KCl or NaCl very often added to the deposition bath to provide a good conductivity of the solution at low concentration of Zn2+. It has been found however, that Cl− ion due to strong adsorption on ZnO nanocrystals acts as a capping agent blocking certain faces and enhancing the growth of other facets  and . The scheme of the process, in which the preferable adsorption of Cl− onto positively charged, Zn-terminated (0001) plane improves the growth along direction leading to plate-shaped ZnO, is illustrated in Fig. 15.
It should be stressed however, that the results referred above were obtained on non-seeded ITO substrate, in the solution of relatively high Zn2+concentration (0.05 M) which is likely too high to form the ZnO nanorods.
The studies reported by Taena-Zera et al.  and  carried out on the seeded ITO at concentration of ZnCl2 two orders lower (5 × 10−4 M) have demonstrated that the arrays of vertically oriented ZnO nanorods were formed even in the solution saturated with KCl (3.4 M) (see Section 4.2.1). As it has been already discussed, a high concentration of chlorides results in slowing down the rate of generation OH− ions due to the change of the mechanism of O2 reduction from 4-electron process to 2-electron reaction with formation of H2O2 intermediate species at high [Cl−]. In effect, the concentration ratio [Zn2+]/[OH−] at the electrode becomes suitable for efficient growth of ZnO and formation of longer nanowires . However, at [KCl] >1 M a significant increase of the nanowire thickness (from 80 nm up to 300 nm) was also observed which was explained by preferential adsorption of Cl− on the (0001) plane of ZnO which stabilizes of the (0001) face and favors the lateral growth of nanowires.
Cui et al.  have studied the growth of ZnO nanowires from the solution containing 2.5 mM Zn(NO3)2 in the presence of equimolar concentration of hexamethylenetetramine (2.5 mM) and different concentration of NH4Cl (from the range 0-50 mM). Their results confirm that the increase of Cl− concentration suppresses the growth along c-axis and promotes the lateral growth of nanowires and therefore, the growth rate decreased from 2.5 μm h-1 in the absence of NH4Cl, to 1.3 μm h-1 in the presence of 50 mM NH4Cl. The presence of the peak at 2.63 keV in EDX spectrum and the increase of its high with increasing amount of NH4Cl in the deposition bath led them to the conclusion that chlorine is incorporated into ZnO nanowires. The increase of ammonium chloride resulted also in the shift of the diffraction peaks in the XRD spectrum to lower angles which was explained by lattice expansion of ZnO nanowires due to substitution of O2− (of the size 1.4 Å) by larger Cl− ions (1.81 Å).
5.2. The influence of hexamethylenetetramine (HMT)
One of the most popular organic compounds added to the deposition bath to support the growth of ZnO nanorods of required morphology is methenamine, also known as hexamethylenetetramine (HMT) or hexamine. This non-ionic, cyclic tertiary amine, highly soluble in water has been widely used in hydrothermal growth of ZnO rod arrays , ,  and . However, the mechanism which governs the deposition process in the presence of HMT is still not clear. It is known that in acidic aqueous solution at elevated temperature HMT hydrolyses to formaldehyde and ammonia, which undergoes to acid-base equilibrium, according to the schemes:
Thus, HMT acts as an additional source of OH-, promoting the nucleation and growth of ZnO. Besides, it is supposed that HMT plays a role of buffer because the rate of its hydrolysis decreases with increasing pH and vice versa . Different approach has been presented by Sugunan et al.  who proposed that HMT, being a nonpolar chelating agent is preferentially attached to the nonpolar and facets of the ZnO crystal, thereby cutting off the access of Zn2+ ions to their surfaces, leaving only the polar (0001) face for epitaxial growth along  direction. However, it seems that this mechanism may be taken into account at higher pH when hydrolysis of HTM is strongly inhibited.
The shaping properties of HTM has been also studied with respect to ZnO nanowires obtained by electrodeposition. It has been demonstrated that addition of equimolar concentration of HTM to the solution of 0.03 M Zn(NO3)2 led to significant improvement of crystallinity and alignment along c-axis of the ZnO nanorods deposited on FTO substrate seeded with a thin layer of TiO2 (Fig. 16).
The electrodeposition of aligned compact ZnO arrays on the glass slides covered with thin Ag film from the solution containing equimolar concentrations (0.03 M) of Zn(NO3)2 and HMT was studied by Chandler et al. . Although the layer of Ag provided a good electrical conductivity of the substrate, a distribution of diameters of electrodeposited ZnO nanorods was very large, from 50 nm up to 500 nm.
The HMT–assisted procedure has been also applied for electrodeposition of large-scale and dense ZnO nanorod arrays on a smooth ITO without any seeding .
A comparison of morphology and XRD patterns of ZnO films electrodeposited on a bare substrate from the solution of 0.01 M Zn(NO3)2 of pH 5.5, in the presence of HMT and without it, presented in Fig. 17, indicate that addition of equimolar amount of HTM improves orientation of ZnO nanorods with respect to the substrate, prevents from their coalescence and provides uniform distribution of nanorods diameters (200–300 nm).
The influence of excessive amount of HTM on the morphology and optical properties of ZnO grown under potentiostatic regime in the solution of 6.3 mM Zn(NO3)2 has been studied by Cui . It was found that the increase of HTM concentration above 25 mM up to 43 mM resulted in the formation of corn-like shape ZnO nanorods with a rough surface, in contrast to smooth surface obtained at equimolar concentration of Zn2+ and HTM. This effect was interpreted as a consequence of microscopic variation of hexamine attachment to the nonpolar facets of nanorods, leading to inhomogeneous growth of ZnO.
5.3. The influence of ethylenediamine (EDA)
EDA is a strong bidentate chelating agent, which complexes zinc ions with high stability constant (higher than those of ammonia and OH−). At elevated temperature zinc-EDA complex decomposes and released EDA molecules undergo to hydrolysis. This results in the formation of , bearing two positive charges:
The influence of EDA on morphology of ZnO nano/microstructures electrodeposited on non-seeded substrate from the solution of 0.05 M Zn(NO3)2 has been reported by and F. Xu et al.  and L. Xu et al. . As it is illustrated in Fig. 18a, addition of EDA to the deposition bath allows formation of hexagonal taper-like nanostructures even at relatively high concentration of Zn(NO3)2. The tapers of the length 2 μm are randomly oriented to the substrate and their diameter decrease gradually from 100–500 nm at the base to several nm at the top.
It was also found that in the presence of 0.06 M KCl in the electrodeposition bath containing 0.05 M Zn(NO3)2 and 0.01 M EDA, the obtained ZnO film consisted of tightly arranged hexagonal nanorods of diameter 100–300 nm (instead of tapers), vertically aligned on the ITO substrate (Fig. 18 b). In addition, it was possible to diminish the aspect ratio of nanorods by the increase of KCl concentration or decrease of EDA concentration . This structure-modifying effect was explained by adsorption of neutral EDA molecules on the lateral () face and negatively charged Cl− ions on the polar (0001) plane. In the absence of Cl− ions a strong coordination ability of EDA to zinc limits the growth along the side direction and enhances the crystal growth along  direction. As a result, the (0001) face contracts and even disappears to form a hexagonal taper shape. In the presence of both capping agents, a simultaneous adsorption of EDA on lateral plane and Cl− on (0001) face, results in the decrease of the contracting rate of the (0001) plane and in effect the ZnO grows in the form of hexagonal nanorods.
The change of morphology from the taper-like ZnO nanostructures to nanorods was also observed by F. Xu et al.  upon increase of concentration of EDA in the electrodeposition bath of 0.05 M Zn(NO3)2 (in the absence of Cl−). In contrast to the results discussed above, the taper-like structures obtained at low EDA concentration (0.005 M) were arranged upside-down, as shown in Fig. 19a.
This special arrangement of the tapers on the substrate was explained by adsorption of positively charged product of EDA hydrolysis on the negatively charged cathode in the initial stage of the process. A gradual increase of EDA concentration to 0.03 M resulted in the change of ZnO morphology from taper via short-fat and finally to long-thin rods (see the scheme in Fig. 19 d). It has been postulated that at longer time scale the tapers transform into the short-fat ZnO rods. The formation of longer and thinner rods at higher concentration of EDA was explained by total coverage of lateral planes of ZnO crystal by adsorbed EDA molecules which enhances the growth in  direction .
Preferential adsorption of EDA molecules and OH− anions on different crystal faces (on the lateral and (0001) plains, respectively) was also used to achieve a selective dissolution of the ZnO nanorods . According to the postulated mechanism, the adsorbed EDA molecule provides the electrons to Zn atoms, changing the charge distribution between Zn and O atoms, and strengthening the Zn-O bond. As a result, the ZnO dissolution rate by OH− ions at the edge of the (0001) surface becomes much slower than in the center leading to formation of nanotubes. The quality of nanotubes may be additionally improved by decrease of etching rate by application of a small positive potential to the nanotubes.
6. Optical properties of ZnO nanorods
Practical applications of ZnO nanorods in optoelectronic devices, such as short-wavelength lasers, light-emitting diodes and hybrid solar cells, require the nanocrystals of high quality. Therefore, the preparation procedures including synthesis of nanostructures and their later treatment (such as annealing) should be optimized to minimize the structural defects which may influence the device performance. One of the methods applied to investigate the optical properties of ZnO is fluorescence spectrophotometry.
The photoluminescence (PL) spectrum of ZnO at room temperature usually shows a near band-edge UV emission peak centered at 3.26 eV (380 nm) and at least one broad band extending between 400 nm and 750 nm, i.e. in the whole visible region. The broadness of the band results from overlapping of several visible emissions localized in green (centered around 2.4 eV), yellow (about 2.2 eV) and red or orange (around 2.07 eV) ranges  and .
At low temperature of 4 K, the UV near band edge peak of electrodeposited ZnO is shifted to higher energy and splits into two bands, one centered at 3.366 eV attributed to band-to-band transitions and the second one, at 3.328 eV, ascribed to recombination process including surface states . The contribution of the latter peak in the PL spectrum increased with the decrease of the nanorod diameter due to rising surface-to-volume ratio. The improvement of the PL spectrum (sharpening of exciton-related transition peaks) was achieved by thermal annealing of the sample for 60 min at 500 °C leading to decrease of donor density by a factor of 20 (from about 8 × 1019 cm−3 to 4 × 1018 cm−3). In the presence of high donor densities, the overlapping of the wave functions of neighboring donors results in formation of a band of donor states  and then a recombination processes takes place between this band and the valence band.
The UV emission peak is expected to be a dominant signal in the PL spectrum of ZnO nanorods of good crystallinity, free of structural defects. Some shift of its position in the spectrum as well as the change of the shape may be ascribed to the presence of ZnO doping  and  or/and quantum confinement effect (blue-shift) . The broad visible band, known also as deep-level emission (DLE) peak is usually ascribed to intrinsic “native” defects in ZnO, such as zinc or oxygen vacancies and interstitials or antisite oxygen  and . However, according to the literature, Zn antisites, O antisites and O interstitials have high formation energies and under normal conditions they are probably not present in large concentration . Thus, the oxygen and Zn vacancies as well as Zn interstitials seem to be the predominant ionic defects and the growth environment controls their concentration in ZnO. These defects introduce the levels in the band gap of the semiconductor, responsible for transitions between different charge states. Although the exact assignment of the visible emission peaks is still a matter of debate, it is generally accepted that the green luminescence band centered around 510 nm (2.4–2.5 eV) results from recombination of electrons in singly ionized oxygen vacancy with photogenerated holes in the valence band  and/or electron transition from conduction band to zinc vacancies level (Vzn) situated 0.9 eV above the valance band .
The relative intensity of UV exciton emission to visible DLE band (IUV/Ivis or Iexc/IDLE) is usually the way of quality evaluation of different ZnO samples . It should be stressed however, that this ratio may be also dependent on excitation efficiency of the photons with different energies and excitation area ,  and  and therefore, the spectra used for comparison of different samples should be obtained under the same excitation conditions.
A systematic study on the influence of the main growth parameters of ZnO electrodeposition (temperature, Zn2+ concentration, overpotential) on the optical parameters of ZnO nanorods has been reported by Pauporte et al. . The deposition was performed on non-seeded FTO, from the solution of Zn2+ at low concentration (0.2 and 5 mM), saturated with molecular oxygen as the source of OH−. It has been demonstrated that the increase of deposition temperature from 70 °C to 88 °C resulted in significant increase of UV exciton emission and quenching the visible band (Fig. 20 a). This means that electrodeposition temperature is a key parameter in the growth of high quality ZnO nanorods containing low concentration of structural defects.
It was found  that the excitonic band also increased and became more narrow when ZnCl2 precursor was replaced by Zn(ClO4)2, which indicates that the presence of chlorides enhances the formation of defects in the ZnO structure. In contrast, Cui et al.  have observed an increase of relative intensity of the UV and visible peaks (IUV/Ivis) with the increase of amount of ammonium chloride in deposition bath (Fig. 20 b), which was interpreted by reduction of the oxygen vacancies in ZnO nanowires. Tena-Zaera et al.  analyzed the evolution of the lattice parameters at increasing KCl concentration and concluded that the formation of the structural defects is a result of the change of local composition of the electrolyte around the nanowires.
According to the results of Cao et al. , the intensity ratio Iexc/IDLE is also dependent on the deposition current density which controls the growth rate along the c-axis (see Section 4). The most favorable ratio (about 15), obtained for the noncompact and well-faceted ZnO nanorod array films deposited at the current density of −1.5 mA/cm2, was similar to that of the film prepared by plasma-assisted MBE method on sapphire substrate . Very strong and sharp UV emission peak and a weak green emission signal were also obtained for well-aligned nanoneedle arrays galvanostatically deposited on the silicon substrate coated with an oriented gold film (Au/Si) .
The increase of the intensity ratio Iexc/IDLE was also observed by Cui et al.  upon the increase of ZnO deposition potential of and this effect was ascribed to improvement of crystalline quality and reduction of defect density in the nanowires. The concentration of the defects, controlled by the change of hexamine in the electrodeposition bath, was found to play an important role in the exciton emission from ZnO nanorods. A decrease of the exciton emission at high concentration of the defects in ZnO grown at low concentration of HTM, has been ascribed to the exciton scattering by the defects .
Another important parameter enhancing the relative intensities of UV and visible emission peaks is seeding of the substrate with a thin ZnO film and it was found that the intensity ratio was dependent on the seeding current density . This confirms that the nucleation centers formed on the substrate during seeding play important role in improvement of the structural properties of the ZnO nanowires.
The improvement of the crystallographic structure of ZnO nanorods may be also achieved by annealing of the sample. Tena-Zaera et al.  have explained this effect by reduction of defects density due to out-diffusion of the Zn interstitials at high temperature. The enhancement of the PL spectrum may also result from transformation of remaining traces of amorphous Zn(OH)2 into ZnO, especially in the case of the samples electrodeposited at relatively low temperature . The removal of Zn(OH)2 traces in effect of annealing at 380 °C leads also to the increase of transmittance of the sample in the visible wavelength range as well as to the shift of absorption edge towards lower energies due to decrease of the band gap energy to the value typical of ZnO (3.2–3.4 eV).
The influence of doping of ZnO nanorods by metal ions (Cu, Co, Al) via electrodeposition on electroluminescence spectra have been studied by Seipel et al. . Incorporation of metal ions, confirmed by SIMS analysis, resulted in a shift of a broad defect luminescence peak, from 600 nm for undoped ZnO nanowires towards longer wavelengths. Although the ionic radius of the host ion is larger than the ionic radii of the dopants, no clear trend toward decreasing lattice constants was observed.
A vertical arrangement of nanorods with respect to the substrate and formation of dense nanorod arrays result in a strong light scattering which is important feature in terms of application of ZnO in the solar cells. Tena Zaera et al.  have analyzed a relationship between dimension of ZnO NRs and light scattering with the goal to optimize the optical configuration of the nanorod arrays and attain efficient solar light absorption. It was found that for small and constant NR diameter the maximum of total ZnO reflectance (maximum of scattering) increased from 8.6% to 20.4% with increasing nanorod length (from 0.5 to 2.0 μm, respectively). The most pronounced enhancement was observed at the wavelength of 400 nm. On the contrary, for a constant NRs length (1.5 μm), the increase of their diameter (from 105 nm to 330 nm) gave rise to considerable redshift of the reflectance maximum without the change of its value. This means that the control of the nanorod dimensions allows monitoring of the light scattering over a wide wavelength range. These results are important in terms of the increase of the solar light harvesting in nanostructural solar cells. This aspect will be discussed further in Section 8.
7. Electrical properties of electrodeposited ZnO nanorods
Determination of electric properties of ZnO nanorod arrays, such as charge carrier density and flatband potential, is a challenging task due to particular morphology of the arrays. Mora-Sero et al.  have adapted electrochemical impedance spectroscopy to derive information on electronic properties of the entire nanowire surface. A cylindrical Mott-Schottky model, taking into account geometry of nanowires, has been developed by them to determine the carrier density in ZnO nanorods electrodeposited on FTO substrate. According to this model, each nanowire is described as a cylinder of radius R with axial symmetry and donor density ND. A circular depletion layer is formed from the surface towards the center of nanowire, as illustrated in Fig. 21a and the potential difference (Vsc) across the space-charge region was defined as :
It was also found  that the dependence of the total capacitance of the nanowire array on the radius of the neutral region (x) may be expressed by the equation:
The impedance spectra obtained at different potentials were fitted to a simple equivalent circuit with a resistance (corresponding to a sum of resistances of the solution, ohmic contacts, etc.) in series with the loop containing parallel resistance (corresponding to charge transfer resistance) and capacitance. The extracted capacitance was then used in Mott-Schottky analysis.
The influence of the chloride concentration (KCl) ranging from 0.05 M to 3.4 M in the deposition bath (containing also 5 × 10−5 M ZnCl2 and saturated with O2) and annealing of resultant samples on the donor density in ZnO nanowires have been studied by Tena-Zaera et al.  and . A linear Mott-Schottky plots (1/C2 in a function of applied potential), typically observed for flat samples , were achieved for as-grown nanorods. The donor density in these samples was found to increase from 5 × 1018 cm−3 to 2.4 × 1020 cm−3 when the KCl concentration in the deposition bath was changed from 0.05 M to 3.4 M, respectively (Fig. 21 b). These values are significantly higher than those obtained for ZnO nanowires formed by vapor phase techniques (1016–1018 cm−3)  and . Higher donor density in the nanorods deposited at higher concentration of KCl was ascribed to the decrease of the formation energy of ZnO intrinsic defects (oxygen vacancies and Zn interstitials) in reach [Zn2+] conditions . The latter one is a consequence of the increase of [Zn2+]/[OH−] ratio because of low generation rate of OH− at high chloride concentration (see Sections 4.2.1 and 5.1). It was also found that a thermal annealing of electrodeposited ZnO nanowires at 450 °C (for 1 hour) results in the curved Mott-Schottky plots and leads to the lowering of donor density to 1017–1018 cm−3  and . The decrease of donor density upon annealing was explained by the out-diffusion of the Zn interstitials.
The spatially resolved insight in the free-carrier distribution in ZnO nanorods electrodeposited from the solution containing 3.4 M KCl was presented by Stiegler et al. . As-grown and annealed samples were studied by means of scattering-type scanning near-field optical microscopy at infrared frequency (infrared s-SNOM). In this technique a metalized tip is illuminated by a focused IR laser beam and back scattered radiation is recorded simultaneously with topography, yielding nanoscale resolved images . Due to interactions between the tip and the sample, the light scattered by the tip is modified in both its amplitude and its phase, depending on the local dielectric properties of the sample. The optical resolution of s-SNOM is only limited by the tip's radius of curvature, usually 10-20 nm. From the nanoscale-resolved IR amplitude and phase images, one can conclude on the local structural properties, material composition  and  and the free-carrier concentration in doped semiconductor nanowires . The results presented by Steigler et al.  revealed a large amount of free carriers (leading to high conductivity) due to the point defects in the shell region of as-deposited ZnO nanorods. Lower conductivity in the core of nanowires was explained by trapping or scattering of free-carriers at the grain boundaries. It was also found that annealing results in the reduction of free carriers concentration in the shell without structural changes in this region. Moreover, the annealing improved crystallinity of the nanowire core.
8. Applications of ZnO nanorods
The unique properties of ZnO nanostructures and development of various cheap methods of their synthesis have triggered a great interest in practical industrial and medical applications of ZnO nanostructures. Over previous several years there have been published a number of reviews on application of ZnO in different morphological forms (nanorods, nanotubes, nanobelts, nanosheets, nanostars, etc.) , , , ,  and . In this section we focus only on selected applications of the nanorods and discuss the aspects related to their properties discussed in the previous sections. Although the nanorods in the references quoted below were mainly formed by hydrothermal methods, the considerations could be likely extended to ZnO nanorods obtained by electrochemical deposition.
ZnO in the form of nanorods is attractive material for gas sensors with great possibility for overcoming the fundamental limitations owing to their ultrahigh surface-to-volume ratio and electronic parameters strongly influenced by surface processes. ZnO nanorods demonstrate high sensitivity even at room temperature, whereas a thin-film gas sensors often need to be operated at elevated temperatures . Therefore, application of the ZnO nanorod (or nanowire) arrays for sensing various gases, including H2, NO2, O2, CO, and NH3, have been reported extensively over the previous years , , ,  and . The sensing process is related to crystallographic structure of ZnO and/or oxygen vacancies in the ZnO nanorods. The adsorption of gas molecules on Zn-terminated (0001) polar surface and/or on the structural defects influence the electrical behavior of nanorods. Namely, the adsorption of electron accepting molecules, such as O2 results in withdrawing of electrons, leading to formation of O2− and decrease of conductivity of ZnO . On the other hand, when ZnO is exposed to the target gas of reducing molecules, these molecules could react with oxygen ions adsorbed on the surface and release the trapped electrons back to the conduction band, resulting in the increase of conductivity . The real problem is gas selectivity and sufficient sensitivity. These parameters may be improved by deposition of Pd clusters on ZnO nanorods , manipulation of the nanorod diameter or modulation by the voltage.
A metal–semiconductor junction (Schottky diode), consisted of ZnO nanorods electrodeposited on FTO substrate with a silver contact formed on the top of nanorods, has been used as a room temperature sensor for relative humidity . The sensing process in this device is based on the adsorption of water molecules on the nanorod surface and decrease of concentration of majority carriers in the conduction band of semiconductor, according to the relation associated to the work-function of Schottky barrier. The response time was around 5 s and the sensitivity showed a logarithmic relation in the range between 20% and 75% of relative humidity.
ZnO nanorods with high isoelectric point, ranging from 8.7 to 10.3, are attractive substrate for loading of enzymes of low isoelectric point, such as cholesterol oxidase (IP = 4.7) , glucose oxidase (IP = 4.2)  and  and uricase (IP = 4.3) , through electrostatic interactions. Important advantage is also excellent biological compatibility of ZnO helping enzyme to retain its bioactivity. Besides, it provides a direct electron transfer between enzyme's active site and the electrode and therefore, it can be employed for developing implantable biosensors. A glassy carbon electrode modified with Sb-doped ZnO nanowires has been used for determination of L-cysteine by it electrochemical oxidation . Such electrode exhibited a high reproducible sensitivity, a low limit of detection, favorable stability and high resistance to interference. ZnO-based biosensors have been developed in various configurations, depending on the type of answer, such as field effect transistors (FET), ion-sensitive field effect transistors (ISFET), optical devices, piezoelectric devices and electrochemical transducers. Recent advances in application of ZnO nanostructures in biosensor application have been summarized in the review papers  and .
8.3. Light emitting diodes
ZnO is promising material for short-wavelength luminescent and lasing devices due to its characteristic direct and wide bandgap and large excitation binding energy of 60 meV. The p-n heterojunction electroluminescent device based on ZnO nanowires showed a high current density and strong luminescence even at a reverse bias voltage of 3 V . The electroluminescent diode, consisting of a p-type conducting polymer and electrodeposited ZnO nanorods, embedded in an insulating polystyrene layer and sandwiched between transparent SnO2 films, has been constructed by Könenkamp et al.  and . As-grown nanorods showed a broad electroluminescence band over the visible spectrum but annealing at moderate temperature of 300 °C increased the emission and strongly raised the excitonic contribution at 390 nm.
Light emitting diode with tunable emission wavelength, based on Cd-alloyed ZnO (n-Zn1-xCdxO) electrodeposited on (0001) oriented p-GaN has been developed by Pauporte et al.  and . The increase of the Cd content in the deposition bath resulted in the redshift of the room temperature UV emission peak, with a width at half-maximum of 10 nm, from 397 nm for pure ZnO(NRs)/p-GaN to 417 nm for the LED of Zn0.92Cd0.08O(NRs)/p-GaN structure. The effect was ascribed to expansion of the lattice parameters with increasing Cd content.
8.4. Solar cells
Applications of ZnO nanorods, obtained by different synthetic methods, in the dye- and semiconductor sensitized solar cells have been reported in several review papers , ,  and . In this Section we only refer to the main problems connected with relatively low efficiency of the ZnO-based solar cells and focus on the last achievements in the field.
The design and performance of dye-sensitized solar cells (DSSCs) based on ZnO nanowires grown on FTO by chemical vapor deposition and hydrothermal method have been reported for the first time by Baxter et al.  and . However, the described solar cells, sensitized with the ruthenium dye N719, revealed very low power efficiency (0.5%). Although the device performance increased with increasing nanowire length, the efficiency still remained much lower than that of the cells based on mesoporous TiO2. This was explained by low light harvesting efficiency resulting from a small surface area of the nanowires as compared to that of mesoporous films.
The studies with dense and long ZnO NRs synthesized by hydrothermal method, performed by Law et al.  have shown that the longest nanorods reaching 20–25 μm with a diameter ranging from 130 to 200 nm have the surface area only one-fifth of the nanoparticle film.
Since even a very dense array of long nanowires is not capable to permit a large amount of adsorbed dye and, in consequence, the solar light harvesting is not sufficient, an interesting option is application of hierarchical ZnO nanostructures ,  and  instead of non-branched nanowires. An attractive approach also consists in combining nanowires and nanoparticles. It was found that specific surface area of hierarchical structures of ZnO covered with nanoparticles increased by more than one order of magnitude compared to uncoated nanowires and larger amount of dye was accommodated ,  and .
The experiments performed with a dye of high extinction coefficient has recently pointed out that the main limitation for high efficiency of the cells may also arise from high recombination rate due to high surface states concentration in ZnO nanorods obtained in wet synthesis . Coating the ZnO nanorods with ZnO nanocrystalline layer (ZnO core-shell) significantly slowed down the recombination dynamics and led to enhancement of photovoltage (of more than 250 mV) with respect to the device based on bare nanorods .
However, so far, the dye-sensitized solar cells utilized ZnO nanowires have relatively low power conversion efficiency (below 3%) in comparison with the systems based on mesoporous TiO2 (efficiency up to 12%) . Among the reasons of limited performance in ZnO-based DSSCs is instability of ZnO in acidic dye due to dissolution of ZnO surface, resulting in the formation of Zn2+/dye agglomerates, retarding the electron injection from the dye to ZnO .
To overcome these problems, the dye sensitizer may be replaced by semiconductor quantum dots (QDs) or extremely thin absorber film (in ETA cells). Semiconductor QDs (CdS, CdSe, CdTe, PbS, PbSe) offer several significant advantages over dyes, such as higher extinction coefficient, easily tuned band gap from infrared to ultraviolet by the control of QD size and composition, multiple exciton generation owing to utilization hot electrons in QDs . Most of the papers in this area concern the solar cells based on mesoporous TiO2 but recently many attempts have been also done towards ZnO NR-based systems. Despite of very promising features, the QD semiconductor-sensitized solar cells (QD-SSSCs) reveal rather low power conversion efficiency and the main limiting factor is probably recombination of the charge carriers due to slow electron transfer from semiconductor to the electron acceptor (ZnO or TiO2). According the Chen et al. , a suppression of the charge carrier recombination at the semiconductor surface may be achieved by incorporation of graphene thin film into the ZnO NR/CdSe solar cell. This enhanced the electron transport and improved efficiency almost twice with respect to the device without graphene layer, whereas fill factor, 62%, was one of the highest reported so far for the QD-SSSCs based on ZnO nanorods.
Levy-Clément and Elias have recently showed that the increase of the solar cell efficiency may be also achieved by the light scattering over a large wavelength range, close to the CdSe absorber bandgap, leading to significant enhancement in the effective light absorption . This was realized by the change of ZnO nanorods diameter at constant NR length, as it has been reported in ref  (see discussion in Section 6)
Another way of performance improvement of the QDs solar cells is broadening the absorption spectral region by co-sensitization of the nanorods by two semiconductors of different size or type, for example CdS/CdSe , ,  and  or CdS/CdTe . Seol et al.  have demonstrated that the CdS interlayer between ZnO nanorods (of the lengths 10 μm and diameter 100 nm) and CdSe QDs deposited on the top, passivates the ZnO suppressing the charge carrier recombination and in effect, the power conversion efficiency of the cell increased to 4.15%.
ZnO nanowire arrays have been also applied as electron transport building blocks in organic-inorganic hybrid solar cells, in which a conducting polymer (for example poly(3-hexylthiophene)) and (6,6)-phenyl C61 butyric acid methyl ester (PCBM) play the role of electron donor and electron acceptor, respectively. Improvement of efficiency of the cells with configuration ITO/ZnO/P3HT:PCBM/Ag from about 3% to 3.9% was achieved by incorporation of vanadium oxide which acts as an electron-blocking layer between P3HT:PCBM and the Ag electrode, enhancing the electron collection via ZnO nanorods .
Further increase of efficiency of the hybrid ZnO-based solar cells (to 4.1%) has been achieved by covering of ZnO nanorods with ZnO nanoparticles, leading to enlargement of interfacial surface area (as in the case of DSSCs) and improvement of the charge lifetime .
The recent studies on the improvement of efficiency and long-term stability of hybrid solar cells based on TiO2 and ZnO are focused on utilization of organic-inorganic perovskites, of the type CH3NH3PbX3 (X = Cl, Br, I), as the light harvesting materials  and . The lead perovskite is a direct band semiconductor, have a large absorption coefficient and a high carrier mobility. The use of CH3NH3PbI3 absorber in combination with ZnO nanorod arrays and spiro-MeOTAD as a hole transporting material has been reported recently  and . It was shown that by employing the electrodeposited ZnO compact layer as a hole blocking layer preventing the recombination of electrons in FTO with the holes in spiro-MeOTAD, the power conversion efficiency of perovskite ZnO-based solar cells may be increased even up to 8.9% . The unique nature of these very promising organometal halide light absorbers is still not clear and therefore, further systematic investigations are expected.
In this review we summarized the achievements on electrochemical synthesis of ZnO nanorod arrays on the transparent conducting electrodes. We have attempted to highlight the factors crucial for formation of uniform films of nanorods, vertically oriented to the substrate of desired length, diameter, density and optoelectronic properties.
We have discussed the mechanisms of the ZnO deposition in the solutions containing two different precursors of hydroxide ions (molecular oxygen and nitrate ions) and advantageous or disadvantageous aspects of the use of each of them. The third potential precursor, hydrogen peroxide, is not suitable because it does not lead to formation of ZnO nanorods.
Since transformation of Zn(OH)2 into ZnO is achieved via dehydration of hydroxide, the electrodeposition should be performed at the temperature above 65 °C. Further increase of deposition temperature even up to 88 °C is recommended because of significant reduction of structural defects in ZnO, supported by the increase of relative intensity of UV and visible emission peaks (IUV/Ivis) in the photoluminescence spectrum of ZnO nanorods.
Irrespectively of the chosen OH− precursor (O2 or NO3−), one of the main parameters which decides on the form of ZnO deposit (compact film or nanorods) is concentration of Zn2+ ions. In general, formation of nanorods is possible at 5 × 10−5 M ≤ [Zn2+] < 5 × 10−3 M. The upper range is not so strict because the quality of nanorods depends also on the additives present in deposition bath, such as Cl− ions, hexamethylenetetramine and ethylenediamine. However, at concentrations of Zn2+ above 5 × 10−3 M the ZnO nanorods have tendency to coalescence.
A diameter of nanorods grown on the electrode may be controlled by the concentration ratio of Zn2+ and OH− ions in the vicinity of the electrode. Namely, a decrease of [Zn2+]/[OH−] ratio results in the formation of ZnO nanowires of greater diameter. This may be achieved by:
- manipulation of the electrical parameters, i.e. potential and current density applied to the electrode, which control the rate of OH− electrogeneration,
- addition of extra amount of NaNO3 supporting electrolyte, if electrodeposition is performed from nitrate precursor,
- addition of KCl into the deposition bath because of preferential adsorption of Cl− ions onto positively Zn-terminated (0001) plane, which favors the lateral growth of nanorods.
However, addition of high amount of Cl− ions, slows down the rate of electrogeneration of OH− from oxygen precursor due to the change of mechanism of O2 reduction from four-electron to two-electron reaction, leading in consequence to the formation of longer nanowires.
The increase of the nanowire length may be obtained by addition of equimolar amount of hexamethylenetetramine (HMT) to the solution of Zn2+ as a result of attachment of this chelating agent to nonpolar facets of ZnO crystal and its preferential growth along c-axis. However, an excessive amount of HTM results in the formation ZnO nanorods with corn-like surface due to microscopic variation of HTM attachment to ZnO.
The change of local composition of the electrolyte around growing nanowires, for example due to the presence of high concentration of additives (Cl− or HTM), is responsible for formation of structural defects. Annealing of the samples at 450 °C improves crystallinity of the nanorods and leads to decrease of structural defects density.
The improvement of morphology and density of ZnO nanorods on the electrode may be achieved by seeding of the substrate before the NRs deposition. It consists in formation of the thin ZnO layer by chemical or electrochemical methods. The grains of the seed layer act as nucleation centers and their size influences on the diameter of post-deposited nanorods. The deposition of ZnO nanorods is also possible on non-seeded substrate, but then the parameters, such as orientation, density and diameter of nanorods are much more sensitive to electrodeposition conditions than in the case of the growth on the seeded substrate.
ZnO nanorods are attractive candidates for applications in various devices like solar cells, sensors, biosensors or fluorescent devices. Nowadays the studies in this direction are very advanced and it is expected that in the next few years the ZnO nanorod arrays will be widely used in different branches of industry.