One-dimensional ZnO nanostructures: Solution growth and functional properties

TL;DR: A comprehensive overview of the progress that has been made within the context of 1D ZnO nanostructures synthesized via wet chemical methods can be found in this paper, where the synthetic methodologies and corresponding growth mechanisms, different structures, doping and alloying, position-controlled growth on substrates, and finally, their functional properties as catalysts, hydrophobic surfaces, sensors, and in nanoelectronic, optical, optoelectronics, and energy harvesting devices.

Abstract: One-dimensional (1D) ZnO nanostructures have been studied intensively and extensively over the last decade not only for their remarkable chemical and physical properties, but also for their current and future diverse technological applications. This article gives a comprehensive overview of the progress that has been made within the context of 1D ZnO nanostructures synthesized via wet chemical methods. We will cover the synthetic methodologies and corresponding growth mechanisms, different structures, doping and alloying, position-controlled growth on substrates, and finally, their functional properties as catalysts, hydrophobic surfaces, sensors, and in nanoelectronic, optical, optoelectronic, and energy harvesting devices.

Summary

1. Introduction

• It has been demonstrated to have enormous applications in electronic, optoelectronic, electrochemical, and electromechanical devices [3–8], such as ultraviolet (UV) lasers [9, 10], light-emitting diodes [11], field emission devices [12–14], high performance nanosensors [15–17], solar cells [18–21], piezoelectric nanogenerators [22–24], and nanopiezotronics [25–27].
• Also, the experimental cost is usually very high, so they have been less widely adopted.
• Here in this review, the authors focus on the 1D ZnO nanostructures that have been grown by wet chemical methods, although evaluation of ZnO nanostructures is provided in the vast Ref. [1, 5, 6, 51–53].
• The authors cover the following five main aspects.
• First, the authors will go over the basic synthetic methodologies and growth mechanisms that have been adopted in the literature.

2. Basic synthetic methodologies and growth mechanisms

• ZnO is an amphoteric oxide with an isoelectric point value of about 9.5 [54].
• Due to the 3d10 electron configuration, it is colorless and has zero crystal field stabilization energy.
• Chemical reactions in aqueous systems are usually considered to be in a reversible equilibrium, and the driving force is the minimization of the free energy of the entire reaction system, which is the intrinsic nature of wet chemical methods [56].
• So when a ZnO nucleus is newly formed, owing to the high energy of the polar surfaces, the incoming precursor molecules tend to favorably adsorb on the polar surfaces.
• After adsorption of one layer of precursor molecules, the polar surface transforms into another polar surface with inverted polarity.

2.1 Growth in general alkaline solutions

• An alkaline solution is essential for the formation of ZnO nanostructures because normally divalent metal ions do not hydrolyze in acidic environments [28, 57, 58].
• Therefore growth of ZnO does not necessarily require the solvent to be H2O [65].
• ZnO nanowires with various aspect ratios can be prepared by simply adjusting OH− concentration and reaction time [68].
• The alkaline solution could also be weak bases, such as NH3·H2O and other amine compounds [74].
• At the growth temperature (typically 70– 95 °C), this promoted only heterogeneous growth on the seeded substrate and suppressed the homogeneous nucleation in the bulk solution.

2.2 Growth mediated by hexamethylenetetramine (HMTA) aqueous solution

• Probably the most commonly used chemical agents in the existing literature for the hydrothermal synthesis of ZnO nanowires are Zn(NO3)2 and HMTA [83, 84].
• First, it produces a basic environment that is necessary for the formation of Zn(OH)2.
• All five reactions (6) to (10) are actually in equilibrium and can be controlled by adjusting the reaction parameters, such as precursor concentration, growth temperature and growth time, pushing the reaction equilibrium forwards or backwards.
• Growth time and temperature control the ZnO nanowire morphology and aspect ratio [50, 91].

2.3 Seeded growth on general substrates

• The vertical alignment of the nanowire arrays is usually poor due to the polycrystalline nature of the seed [83, 84].
• There is a competition between homogeneous nucleation and heterogeneous nucleation in solution, and heterogeneous nucleation generally has a lower activation energy barrier than homogeneous nucleation.
• Therefore, there will be growth of ZnO nanowires wherever there are ZnO seeds, and as a result the density of nanowires is typically quite high [84, 94–96].
• The thickness of the seed layer could be small enough that the seeds no longer form a continuous thin film, but form separated islands.

2.4 Electrodeposition

• Electrochemical deposition is a very powerful technique for achieving uniform and large area synthesis of ZnO nanostructures [115], because it exerts a strong external driving force to make the reactions take place, even if they are non-spontaneous.
• It has also been suggested that when using Zn(NO3)2 as the precursor, reduction of NO3 − at the cathode could also provide a possible source of OH− [122], as indicated by equation (12).
• Other than being produced in situ, OH− could also be added to the solution beforehand in the form of alkali precursors [122].
• It has also been pointed out, however, that high KCl concentrations (> 1 mol/L) also favored the radial growth of ZnO nanowires [125].
• It was also found that varying the counter anions could greatly tune the diameter (65–110 nm) and length (1.0–3.4 μm) of the nanowires.

2.5 Templated growth

• ZnO nanowires can be grown by electrodeposition methods in combination with templates, such as anodic aluminum oxide (AAO), polycarbonate membranes, nano-channel glass, and porous films self-organized from diblock copolymers.
• Zn coordination species diffuse towards the cathode and into the pores of the template.
• These two ions react and result in the growth of nanowires inside the pores of the template.
• As an alternative, polycarbonate templates have been shown to be able to produce free standing ZnO nanowire arrays.
• The key issue for semiconductor nanowires fabricated by this technique is the crystalline quality, which in most cases is not perfect.

2.6 Epitaxial growth

• Just as for seeded growth, epitaxial growth is also considered to involve a heterogeneous nucleation and growth process.
• Because of the small interfacial lattice mismatch, dangling bonds can be mostly satisfied and are less critical than for general interfaces.
• The energy benefits from satisfying the interfacial dangling bonds provide the driving force for the epitaxial growth.
• Different substrates have different isoelectric points— the pH where most sites on the substrate are neutral and the numbers of negative and positive sites are equivalent.

2.6.1 Au coated general substrates

• While the formation of well-aligned ZnO nanowires on a pristine Si substrate is difficult because of a large mismatch (~40%) between ZnO and Si, it is appealing to take advantage of the relatively small lattice mismatch between ZnO and other materials, such as Au [10, 137, 138], Pd [139], and Cu [140].
• The ZnO nanowire arrays were c-axis oriented, and had in-plane alignment, which was probed by X-ray pole analysis [142].
• X-ray diffraction studies showed the as-deposited polycrystalline Au thin films were <111> oriented normal to the substrate, even though they had random in-plane orientations [82].
• The nanowire morphology was very sensitive to the growth temperature.
• This was probably due to the electrostatic interaction between the ions in the solution and the polar surfaces, and as a result higher Miller index surfaces became preferred [144].

2.6.2 n-GaN/p-GaN

• Due to a small lattice mismatch, almost perfectly vertically aligned ZnO nanowire arrays can be grown on GaN (n-type [48] and p-type [145–149]), AlN, SiC, Al2O3, and MgAl2O4 substrates [150], either by hydrothermal decomposition [151] or electrodeposition.
• Nevertheless, the nanowires grown on both substrates have uniform length and width, and are well distributed on the substrates with one nanowire growing on one spot.
• The epitaxial relationship between the as-grown ZnO nanowires and the GaN substrate is evidenced by X-ray diffraction (XRD) [136, 153].
• Figure 10(c) shows a six-fold rotational symmetry in the azimuthal scan.
• Furthermore, the small full width at half maximum of the diffraction peaks also showed a good crystalline quality [153].

2.7 Capping agent-assisted growth

• Capping agents can be included in the solution to modify the growth habits of the ZnO nanostructures [154].
• Also after the growth, less precipitate was expected to form in the bulk solution in the presence of PEI coordination than without PEI.
• Thus, longer nanowire arrays could be produced through prolonging the growth time without refreshing the growth solution, because the Zn2+ depleted during the growth would be replenished through the decomposition of PEI-Zn2+ complexes [156].
• Citrate ions are characterized by three negative charges under the normal growth environment.

3.1 Belts

• This configuration has normally been observed using gas phase synthesis approaches [14, 31, 164–166], and is not favorable using aqueous approaches because—as discussed previously—wet chemical methods are usually considered to be under thermodynamic equilibrium, and the driving force is the minimization of the free energy of the entire reaction system [56].
• This eventually led to the formation of unique 1D rectangular cross-sectional ZnO nanobelts.
• The subsequent Ostwald ripening process smoothened out the resulting nanobelts when the precursor concentration was lowered [73].
• Porous ZnO nanobelts, shown in Fig. 12(d), were prepared from the thermal decomposition of synthetic bilayered basic zinc acetate nanobelts obtained by a simple synthetic route under mild conditions.

3.2 Tubes/rings

• Tubular structures are of particular interest for many potential applications, such as in high efficiency solar cells due to the high internal surface area relative to nanowires, and in novel bimolecular or gas sensors due to the well-defined adsorption microcavities [170].
• The nanotube could also be grown on large scale on seeded general substrates [180], with a yield approaching 100% [181].
• By a simple solvothermal method, the atypically shaped coordination polymer particles were formed by the cooperation of two different organic ligands, namely N,N’-phenylenebisdicarboxylic acid (A) and 1,4-benzenedicarboxylic acid (B).

3.4 Hierarchical structures

• It is attractive to fabricate complex three-dimensional nanostructures with controlled morphology and orientation.
• These nucleated clusters perform as seeds to initiate the following vertical growth of the secondary branches on the side surfaces of the primary nanowires.
• Besides microspheres, layered ZnO nanowire arrays have been formed by wet chemical methods.
• Zincowoodwardite belongs to a family of layered compounds with positively charged layers of Zn2+ and Al3+ with hydroxide anions, and interlayer charge-balancing anions SO42−.
• The reaction was conducted through a temperature-dependent multistep process.

3.5 Heterostructures

• Each single component material has its own functions and also limitations.
• Heterostructures of nanomaterials are a preliminary step towards this application.

3.5.1 ZnO compound semiconductors

• XRD results showed that the CdTe shell had a zinc-blende structure, and the crystallinity could be further increased by annealing [213].
• Shi et al. reported the capping of ZnO nanowires with doped SnO2, which could act as the top electrode of the ZnO nanowires [215].
• Plank et al. demonstrated a low temperature wet chemical method to coat a MgO shell layer onto ZnO nanowires that did not require a subsequent high temperature annealing [216].
• The as-coated ZnS shell had a cubic structure and, as can be seen from Fig. 22(e), the substantial surface roughness of the ZnS indicated it was polycrystalline.

3.5.2 ZnO–metals

• Semiconductor–metal heterostructures exhibit many interesting chemical, optical, and electronic properties that have found various applications in catalysis, biomedicine, photonics, and optoelectronics [233].
• This growth mechanism was suggested to result from two factors.
• In addition to Ag, Au nanoparticles have also been decorated onto ZnO nanowires by wet chemical methods.
• In their experiment, the gold colloidal solution was made by laser ablation of a gold target in water, which produced Au nanoparticles with fresh surfaces [242].
• The epitaxial relationship followed [0002] Zn/[0100] ZnO, and there were defects at the interface to accommodate the lattice mismatch [244].

3.5.3 ZnO–carbon nanotubes

• In their method, a thin film of ZnO seed layer was precoated on vertically aligned carbon nanotubes by radio frequency (r.f.) sputtering or ALD [246].
• After sputtering, the carbon nanotubes preserved their vertical alignment.
• The growth solution was made up of saturated Zn(OH)42− formed by dissolving ZnO in NaOH aqueous solution.
• The as-grown high density of ZnO nanowires on the carbon nanotube arrays Nano Res 24 showed greater surface to volume ratio than the ZnO nanowires grown on flat substrates.
• Moreover, the thin ZnO seed layer provided a continuous pathway for carrier transport for electronic applications [245, 247].

4 Rational doping and alloying

• Doping and alloying are the primary techniques to control the physical properties of semiconductor nano- materials, such as electrical conductivity, conductivity type, band gap, and ferromagnetism [86].
• Cui et al. used ammonium chloride to alter the growth properties of ZnO nanowires by electrochemical deposition [119].
• First was the reduced number of oxygen-related defects during ZnO growth.
• There have been many efforts using both vapor phase [252, 253] and solution phase growth approaches [254].
• The difference in the conductivity type were attributed to several factors, such as the dependency of native defect concentrations, the different concentrations of zinc vacancies, and the different incorporation of compensating donor defects, like hydrogen and indium atoms.

4.3 Transition metal doping

• Transition metal doped dilute magnetic semiconductors are of particular research interest for potential applications in spintronic devices and visible light photocatalysis.
• A few studies have been reported of the synthesis and characterization of ZnO nanowires doped with different transition metal ions, like Co, Ni, Mn, Cu, Fe, and Ag [86, 257, 258].
• Annealing of the doped nanowires would help rearrange the dopants and enhance the magnetization.
• But when the annealing temperature was too high, it led to precipitation and clustering of the dopant atoms [260].
• The dopant ions were in a uniform environment which Nano Res 27 did not induce a large degree of disorder in the nanowires.

4.4 Alloying

• Band gap engineering of ZnO by alloying ZnO (Eg = 3.37 eV) with MgO (Eg = 7.7 eV) to form Zn1-xMgxO alloys is an attractive approach for electronic and optoelectronic applications.
• By assuming the percentage of Mg in the as-grown Zn1–xMgxO nanowires was proportional to the percentage of Mg in the precursor [264], the results showed that changing the amount of Mg(NO3)2 relative to Zn(NO3)2 readily changed the value of x in the Zn1–xMgxO alloy.
• Shimpi et al. reported a two-step method for fabrication of uniform and large scale ZnO:MgO nanowire arrays without post-annealing.

5. Patterned growth

• Here the authors will discuss various strategies for defining the spatial distribution of ZnO nanowires on a substrate, including photolithography, electron beam lithography, interference lithography, nanosphere lithography, nanoimprint lithography, micro-contact printing, and inkjet printing.
• Finally the authors will also discuss the feasibility of pattern transfer.

5.1 Photolithography

• In a typical process, first the substrate is spin-coated with a layer of Nano Res 28 photosensitive material (e.g., photoresist), and then is exposed to UV light under a patterned photomask.
• In contrast, for a negative tone photoresist, UV photons provide energy to overcome the energy barrier of forming new chemical bonds crosslinking the side chains of the small monomer molecules producing larger molecules that have a lower solubility in the developer than the original monomer molecules.
• Photolithography could also be realized without a photoresist by using photosurface functionalization [270, 271].
• As shown in Fig. 29(a), the polycarbonate surface could be oxidized and grafted with carboxylic acid groups [272] or sulfate anion groups [273] where it was exposed to UV light in air, which was confirmed by fluorescence microscopy imaging [271].
• As shown in Fig. 29(b), wellorganized ZnO nanowire arrays were grown on the unexposed polymer surfaces [271].

5.2 Electron beam lithography

• Since photolithography has a bottleneck arising from the diffraction limit of UV light, a combination of electron beam lithography and a wet chemical growth method has been developed [48].
• The opening size patterned by electron beam lithography was around 100 nm, in which there were a number of seed grains exposed [48].
• The width of the as-grown nanowires was found to be almost three times the size of the patterned openings.
• In addition, the electron beam lithography system may not be stable, especially over large areas.
• Also, because of the anisotropic growth habits of the ZnO nanowires, the strip shape openings had to be along the [0001] direction of the substrate to grow individually separated nanowires.

5.3 Interference lithography

• Laser ablation was originally used to generate patterned ZnO seeds for the growth of ZnO nanowire arrays [288].
• The laser is primarily used as a heating source to melt the precursor materials, which are later ejected through a mask.
• In addition to being a heating source, it was also found that a femtosecond laser was able to generate patterns on a single crystal ZnO wafer by virtue of the second harmonic generation excited in the ZnO wafer [289].
• Other than that, based on laser interference, two-dimensional (2D) [290] or 3D [291] periodic structures could be generated by so-called laser interference lithography.

5.4 Nanosphere lithography

• Sub-micrometer sized spheres, such as polystyrene and silica spheres, will self-assemble on a water surface into a hexagonally close-packed structure driven by the water surface tension.
• A honeycomb pattern is formed [295], called nanosphere lithography.
• These spheres are commercially available with well-controlled monodisperse sizes.
• The aspect ratio of the nanowires was fairly small.
• Also, there were many defects in the self-assembled spheres.

5.5 Nanoimprint lithography

• Nanoimprint lithography is an embossing technique, which describes the transfer of a sub-micron scale pattern from the mold to the target substrate or device [298].
• Basically, it employs a relatively hard mold template that is used to carve into a relatively soft surface.
• The pattern in the mold is typically written by electron beam lithography.
• An anti-stick coating is often applied on the mold to prevent any residues that will introduce defects into the pattern.
• A mold is then embossed intimately into the seed layer.

5.6 Micro-contact printing

• With a similar methodology, micro-contact printing (μCP) utilizes a mold inked with functional molecules [301].
• When the mold is in contact with a substrate, the functional molecules are transferred and selfassemble on the substrate following the patterns of the mold, as shown in the Fig. 34(a).
• The self-assembled functional molecules can inhibit or promote the growth of ZnO nanowires locally.
• The authors also found that the COO––HMTA–H+ complexes played the major role in suppressing the nucleation of ZnO.
• Also, restricting regions of nucleation resulted in an increase in nucleation density in the unrestricted regions [303].

5.7 Inkjet printing

• For the growth of ZnO nanowire arrays, a sol–gel derived zinc acetate precursor ejected from the nozzle can be dropped on a substrate that was heated to decompose the zinc acetate into ZnO seed crystals [306].
• The typical resolution of an inkjet printed pattern is on the order of micrometers, and depends on many factors, such as the droplet size ejected from the nozzle, the accuracy of the droplet landing on the substrate, the wettability and spreading of the droplet on the substrate, and possibly convolution/interactions between the neighboring droplets.
• Of these factors, the size of the droplets is currently the bottleneck to narrowing down the feature size of this technique, and much effort is nowadays devoted to this topic [307], such as controlling the viscosity, evaporation rate, and surface tension of the precursor ink.
• Sekitani et al. have recently demonstrated an inkjet technology with micrometer line resolution [308].
• In any case, inkjet printing provides a direct writing technique that is simple, versatile, and inexpensive and can potentially be scaled up.

5.8 Pattern transfer

• The as-grown ZnO nanowire arrays can be transferred intact onto flexible substrates [56, 149].
• In a typical pattern transfer process, a thin conformal layer of polymer was coated on the nanowires.
• Then a straightforward peeling off or delamination was applied to the as-coated polymer layers.
• Therefore, the patterned ZnO nanowire arrays can be transferred onto the as-coated polymer substrate, as shown in Fig. 35, which was later used to grow second generation of ZnO nanowire arrays, replicating the original pattern [193].
• This technique has great potential for future flexible and foldable electronic applications [309].

6.1 Catalytic properties

• ZnO has received great attention as a photocatalyst for the degradation and mineralization of environmental Nano Res 34 pollutants due to its large band gap and low fabrication cost [310].
• Zhou and Wong showed that ZnO nanowires had an even higher catalytic activity than nanoparticles and bulk forms due to their high purity and crystallinity [129].
• In addition, with a larger band gap energy, the photogenerated electron and hole pairs will be less likely to recombine, which in turn enhances the charge transfer efficiency between the catalyst and the pollutants [310].
• The intensity of the characteristic absorption peak at 464 nm for methyl orange decreases with increasing exposure time, and the peak completely fades away after about 80 min; as shown in the inset images of Fig. 36(a), the sample is also completely decolorized.
• Therefore ZnO is only suitable for photocatalytic applications in neutral environments [314].

6.2 Hydrophobic properties

• Wettability of a solid surface is of critical importance for many industrial applications [326].
• The superhydrophobicity can be restored by putting the nanowires in dark for 7 days.
• This process can be repeated several times without obvious deterioration as shown in Fig. 37(b).
• An as-grown ZnO nanowire array with appropriate density is a rather porous structure and exposes mostly the low energy non-polar side surfaces [101], which greatly enhances the hydrophobic behavior.
• In Badre’s study, they utilized a ferrocene silane molecule, N(3-trimethoxysilyl)propylferrocenecarboxamide, which has an anchoring group to ZnO nanowire surfaces on one end, and a redox change group on the other end.

6.3 Field emission properties

• Field emission finds applications in photoelectric panel display, X-ray sources, and microwave devices [332].
• Hung et al. reported ZnO nanotip array-based field emitters [334].
• The turn-on field and the threshold field are defined as the macroscopic fields required to produce a current density of 10 μA/cm2 and 10 mA/cm2, respectively [112].
• Figure 38(b) shows a plot of field emission current density as a function of the applied field [304].
• The β values could be derived from fitting the experimental curve to F–N theory according to β = 6.83 × 109 dΦ3/2/k, where d is the gap distance between the two electrodes and k is the slope of the F–N curve.

6.4 Photonic crystals

• Periodically aligned vertical ZnO nanowire arrays give rise to a periodic modulation of dielectric constants for photons traveling inside, resulting in a refractive index contrast; this is called a photonic crystal [340].
• Defects in the photonic crystal can also introduce localized states in the photonic band gap, allowing the propagation of photons with frequencies at the localized states.
• In their calculation, the periodic array of well-aligned nanowires can be treated as a 2D photonic crystal provided that the nanowire height is much larger than the nanowire width.
• Figure 39(d) shows a six-fold optical diffraction pattern of white light passing through a hexagonally patterned ZnO nanowire array.
• Dispersion along the radial directions is clearly seen [343, 344].

6.5.2 Electrically driven light emission

• Light emission driven by electricity is more attractive than that driven by optical pumping, in the sense that the device is capable of being integrated with chip circuits.
• At present, electrically driven light emission from wet chemically grown ZnO nanowires is limited to LEDs.
• Electrically pumped lasers are difficult to realize due to the low crystal quality of the ZnO nanowires which leads to a low internal quantum efficiency and thus a high threshold voltage.
• The device usually fails before reaching the lasing threshold voltage due to Joule heating at high bias voltages.

6.6 Electrochromic displays

• Electrochromic devices undergo color change on charge injection or extraction driven by an external voltage [423].
• There are generally four categories of electrochromic devices [424].
• Therefore the switching speed of the device was very fast, and the nanowire itself provided a direct charge transport path.
• The nanowires decorated with viologen molecules were used as the working electrode, which was sealed together with a reference electrode since the redox reaction of the viologen molecules is sensitive to oxygen.
• When under negative bias, the viologen molecules were reduced and become deep blue within about 170 ms, and the color was retained for over 1 h after the bias was removed.

6.7 Field effect transistors

• Field effect transistors (FETs) based on a single ZnO nanowire [426] or ZnO nanowire thin films [427] have been fabricated for use in optically transparent mechanically flexible electronics [428, 429].
• The fact that ZnO nanowires can grow in situ between the source and drain electrodes is one of the merits of low temperature wet chemical methods [283, 430, 431].
• Therefore the transport properties are not determined by the intrinsic work function difference, but rather by the interfacial defect states [432].
• The output characteristics and averaged transconductance gm are shown in Fig. 52(c), which shows an n-type channel with an on/off current ratio of 104–105.
• The maximum value of gm was about 100 nS.

6.8 Sensors

• The authors noted that the ZnO nanowires were thermodynamically stable over the tested pH range at room temperature [440].
• As shown in Fig. 53(b), the two electrodes were in direct contact with the cell cytoplasm, and the intracellular pH value was read to be 6.81.

6.9 Energy harvesting devices

• ZnO nanowire arrays are playing a very active role in energy harvesting technologies nowadays.
• Preliminary proof-of-concept devices include solar cells, piezoelectric nanogenerators, and water splitting devices, as shown in the following sections.

6.9.1 Solar cells

• Silicon-based conventional solar cells have low defect densities and high carrier mobilities, and currently dominate the solar energy industry due to the relatively high energy conversion efficiency and well-developed fabrication technologies [443].
• Extensive studies of other inorganic and organic materials have been carried out with the aim of further reducing the cost per unit energy output.
• ZnO nanowire arrays are good candidates in particular for solar cell applications for three straightforward reasons: they have low reflectivity that enhances the light absorption, relatively high surface to volume ratio that enables interfacial charge separation, and fast electron transport along the crystalline nanowires that improves the charge collection efficiency.
• ZnO nanowire arrays have been implemented for both conventional p–n junction solar cells and excitonic solar cells (including organic, hybrid organic–inorganic, dye-sensitized, and nanoparticlesensitized solar cells), as discussed individually in the following sections.

6.9.2 Piezoelectric nanogenerators

• The search for sustainable micro/nano-power sources for driving wireless and mobile electronics is an emerging field in today’s energy research, and could offer a fundamental solution to the energy needs of nanodevices/nanosystems [495–498].
• A piezoelectric nanogenerator that converts mechanical energy into electricity was first demonstrated using ZnO nanowire arrays [22].
• A worldwide effort has been launched in this regard, forming a new research field in nanotechnology and energy science [520].

6.9.3 Water splitting

• Hydrogen has one of the highest energy density values.
• In particular, ZnO nanowire arrays have been used for this purpose because of their large surface to volume ratio, appropriate direct band gap and flat band potential, low series resistance, and high electron transfer efficiency when compared with TiO2.
• It is believed that CdTe has a more favorable conduction band energy than CdSe and thus can inject electrons into ZnO more efficiently [558].
• Non-ideal factors with PEC devices include the photodecomposition and dissolution of the anode itself—that is, oxidation of the anode by holes in the depletion region if the photopotential is higher than the redox potential of the anode material.
• When the piezopotential was greater than the standard redox potential of H2O (1.23 eV), then H2O can be split into H2 and O2 in the molar ratio of 2:1.

7. Concluding remarks

• Before reaching a conclusion, the authors would like to share their perspectives on some critical issues in the field and possible solutions to potentially address these issues.
• The high defect level is due to the nature of solution growth at low temperature in a rather complex environment composed of many kinds of ions and molecules.
• To improve the crystal quality, it is of critical importance to start from an appropriate precursor, choose well-controlled reaction parameters, and possibly introduce appropriate capping agents and/or even control some seemingly trivial species, such as the counter ions in the Zn2+ salt precursor and the dissolved oxygen concentration.
• The second is to minimize the surface conductivity for piezoelectric applications.
• All of these advantages combine to make 1D ZnO nanostructures unique building blocks for fabricating diverse novel devices.

Figures

Figure 58 (a) Surface photovoltage spectra and (b) transient photovoltage spectra of ZnO nanowire arrays before and after different numbers of chemical bath deposition (CBD) cycles with CdS quantum dots [472]. Reproduced with permission

Figure 5 Scanning electron microscope (SEM) images of ZnO nanowire arrays grown on a ZnO seeded (a) flat rigid substrate [84], and (b) etched Si wafer (inset is an enlarged view) [82]. (c) Photograph of a four-inch flexible TPU substrate [94], and (d) SEM image of ZnO nanowire arrays with a uniform length on the TPU substrate [94]. (e) SEM image of a looped Kevlar fiber with ZnO nanowire arrays grown on top, showing the flexibility and strong binding of the nanowires [96], and (f) an enlarged local part of (e), showing a uniform distribution at the bending area [96]. (g) SEM image of ZnO nanowire arrays grown on a polystyrene sphere [107]. (h) Cross-section SEM image of ultrathin ZnO nanofibers grown on a Zn metal substrate [108]. (i) Highresolution transmission electron microscopy (HRTEM) image of a single ZnO nanofiber. Inset is the corresponding fast Fourier transform pattern [108]. Reproduced with permission

Figure 10 SEM images of ZnO nanowires on a n-type GaN wafer: (a) top view and (b) oblique view [82]. (c) φ-scan profiles of the ZnO nanowires/GaN/c-sapphire structure with the family of planes of ZnO nanowires on the top, and the GaN film at the bottom [153]. Reproduced with permission

Figure 9 A digital photograph of the reaction container showing the substrate floating on the solution surface by surface tension

Figure 62 (a) Wurtzite ZnO showing the alternating polar surfaces along the [0001] direction. (b) Tetrahedral coordination between Zn (filled circles) and oxygen (open circles). (c) Distortion of the tetrahedral unit under compressive strain, showing the displacement of the positive charge center from the negative charge center [26]. Reproduced with permission

Figure 45 (a) Electroluminescence spectra as a function of the forward bias voltage. Inset shows how by Gaussian deconvolution analysis the blue/near-UV emission can be decomposed into three distinct bands that correspond to three different optoelectronic processes. (b) All of the four emission bands evolve as a function of the bias voltage. Inset schematics show the band diagram of the n-ZnO/p-GaN heterojunction under no or small bias voltage, where the three emission bands making up the blue/near UV band are indicated in different colors [371]. Reproduced with permission

Figure 46 (a) Blue-shift of peak positions as a function of the bias voltage for (i) ZnO near-band-edge emission, (ii) GaN nearband-edge emission, (iii) interfacial emission, and (iv) defect emission. (b) The integrated emission band intensities as a function of the bias voltage. When the bias voltage is below 5.5 V, the defect emission can hardly be distinguished from the noise level [371]. Reproduced with permission

Figure 39 (a) The lowest three photonic bands for TM modes in a triangular lattice of ZnO nanowires when the array pitch a is 240 nm and the nanowire width 2R is 120 nm, where R is the nanowire radius. There is a photonic band gap in the range 2.0–2.4 eV. The inset denotes the first Brillouin zone of a 2D triangular lattice [340]. (b) The dependence of the band gap on R/a when a = 240 nm. The hatched area denotes the band gap. Note that when R/a = 0.21, the photonic band gap center is located at around 2.35 eV, which is the spectral maximum frequency of the green luminescence band of ZnO nanowires due to oxygen vacancies or zinc interstitials [340]. (c) The measured transmission spectra of the ZnO nanowire hexagonal array in the Γ–M direction for both TM and TE polarizations. The array has a photonic lattice constant of 250 nm and a height of about 500 nm [341, 342]. (d) Diffraction pattern using a white light source from a hexagonal array of ZnO nanowires [343, 344]. Reproduced with permission

Figure 27 Magnetization hysteresis loop of Ni-doped ZnO nanowires with the applied magnetic field perpendicular and parallel to the nanowire c axis [120]. Reproduced with permission

Figure 26 Capacitance-voltage measurements for ZnO nanowires grown with different seed layers. (a) Schematic device structure. (b) I–V curves. (c) C–V curves for the seed and rods [254]. Reproduced with permission

Figure 14 SEM images illustrating the formation of ZnO nanotubes at different etching stages: (a) 0 min, (b) 5 min, (c) 10 min, (d) 15 min, (e) 60 min, and (f) 120 min [186]. Reproduced with permission

Figure 49 External quantum efficiencies of two ZnO nanowire/ GaN thin film heterostructural LEDs as a function of the bias voltage/injection current [371]. Reproduced with permission

Figure 50 Spatial distribution of the electric field radiated from a point source at the center of a 2D triangular ZnO nanowire photonic crystal, when the frequency of the photon is (a) inside and (b) outside the photonic band gap [340]. Reproduced with permission

Figure 51 (a) Schematic structure of the electrochromic device, and (b) photographs of the electrochromic device (i) before and (ii) after applying an external voltage, and (iii) at the open circuit state [422]. Reproduced with permission

Figure 63 (a) Experimental setup and working procedure of the nanogenerator based on AFM [22]. (b) The AFM topography image and (c) the corresponding voltage output profile obtained over a 40 μm × 40 μm area of vertically aligned ZnO nanowires [114]. Reproduced with permission

Figure 31 (a) From a crystal structure point of view, wurtzite structure ZnO has a six-fold symmetry at its basal surfaces and a two-fold symmetry at its side surfaces. On a six-fold symmetry epitaxial substrate, ZnO will grow vertically on the substrate. On a two-fold symmetry substrate, it will grow horizontally on the substrate. (b) A 60° tilt view of thick (top row) and thin (bottom row) ZnO nanowire arrays growing out of 2 μm by 400 nm and 2 μm by 200 nm openings, respectively [280]. (c) A 30° tilted view of the horizontal nanowire-array-based five-segment monolithic superstructures [56]. Reproduced with permission

Figure 67 (a) Schematic structure of a vertically integrated alternating current nanogenerator, and (b) the nanogenerator under uniaxial strain. (c) Enhancing the output voltage of the nanogenerator by integrating them in series. Inset in the left panel is the an enlarged view of a single pulse [432]. Reproduced with permission

Figure 66 (a) Schematic design of a laterally integrated nanowire array nanogenerator, in which gold and chromium are used to create Schottky and ohmic contacts at the two ends of the lateral nanowires, respectively. (b) Low-magnification optical image of the laterally integrated nanogenerator. Inset in (b) is a demonstration of the nanogenerator flexibility. (c) Open circuit output voltage measured for a laterally integrated nanogenerator composed of 700 rows of nanowire arrays. The inset in (c) is the output voltage for one cycle of mechanical deformation. The laterally integrated nanogenerator was periodically deformed at a straining rate of 2.13% s–1 to a maximum strain of 0.19% [432]. Reproduced with permission

Figure 29 (a) Schematic flow chart of the use of UV oxidation to pattern a polycarbonate surface by photolithography, and (b) a SEM image of the as-grown ZnO nanowire arrays on the patterned polyester filament surface [271]. Reproduced with permission

Figure 30 SEM images of the ZnO nanowire arrays on a seeded Si wafer grown at (a) 70 °C and (b) 95 °C. (c) Top view and (d) 60° tilt view of the ZnO nanowire arrays on a GaN substrate grown at 95 °C. The inset in (c) is the enlarged view with a pitch of 1 μm. (e) Top view and (f) 60° tilt view of a 200 μm × 200 μm patterned ZnO nanowire array [48]. Reproduced with permission

Figure 40 (a) Room temperature photoluminescence spectrum of ZnO nanowires grown on Au-coated ITO substrates at room temperature [352]. (b) Photoluminescence spectrum of ZnO nanowires and ZnO thin film at 4.2 K [338]. Reproduced with permission

Full text

Nano Res
1
One-Dimensional ZnO Nanostructures: Solution Growth and
Functional Properties
Sheng Xu and Zhong Lin Wang (

)
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, USA
Received: 8 May 2011 / Revised: 14 June 2011 / Accepted: 15 June 2011
© Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011
ABSTRACT
One-dimensional (1D) ZnO nanostructures have been studied intensively and extensively over the last decade
not only for their remarkable chemical and physical properties, but also for their current and future diverse
technological applications. This article gives a comprehensive overview of the progress that has been made
within the context of 1D ZnO nanostructures synthesized via wet chemical methods. We will cover the synthetic
methodologies and corresponding growth mechanisms, different structures, doping and alloying, position-
controlled growth on substrates, and finally, their functional properties as catalysts, hydrophobic surfaces, sensors,
and in nanoelectronic, optical, optoelectronic, and energy harvesting devices.
KEYWORDS
ZnO, one dimensional nanostructures, solution growth, semiconductive, optical, piezoelectric, novel devices
1. Introduction
ZnO is a semiconducting and piezoelectric material
with a direct wide band gap of 3.37 eV and a large
exciton binding energy of 60 meV at room temperature
[1, 2]. It has been demonstrated to have enormous
applications in electronic, optoelectronic, electroche-
mical, and electromechanical devices [3–8], such as
ultraviolet (UV) lasers [9, 10], light-emitting diodes
[11], field emission devices [12–14], high performance
nanosensors [15–17], solar cells [18–21], piezoelectric
nanogenerators [22–24], and nanopiezotronics [25–27].
One-dimensional (1D) ZnO nanostructures have been
synthesized by a wide range of techniques, such as wet
chemical methods [28–30], physical vapor deposition
[31–33], metal–organic chemical vapor deposition
(MOCVD) [34–36], molecular beam epitaxy (MBE)
[37], pulsed laser deposition [38, 39], sputtering [40],
flux methods [41], eletrospinning [42–44], and even
top-down approaches by etching [45]. Among those
techniques, physical vapor deposition and flux methods
usually require high temperature, and easily incorporate
catalysts or impurities into the ZnO nanostructures.
Therefore, they are less likely to be able to integrate
with flexible organic substrates for future foldable
and portable electronics. MOCVD and MBE can give
high quality ZnO nanowire arrays, but are usually
limited by the poor sample uniformity, low product
yield, and choices of substrate. Also, the experimental
cost is usually very high, so they have been less widely
adopted. Pulsed laser deposition, sputtering and top
down approaches have less controllability and repeata-
bility compared with other techniques. Electrospinning
gives polycrystalline fibers. Comparatively speaking,
wet chemical methods are attractive for several reasons:
they are low cost, less hazardous, and thus capable of
Nano Res. 2010, 3(9): 676–684 ISSN 1998-012
4
DOI 10.1007/s12274-011-0160-7 CN 11-5974/O
4
Review Article
Address correspondence to zhong.wang@mse.gatech.edu

Nano Res
2
easy scaling up [46, 47]; growth occurs at a relatively
low temperature, compatible with flexible organic sub-
strates; there is no need for the use of metal catalysts,
and thus it can be integrated with well-developed silicon
technologies [48]; in addition, there are a variety of
parameters that can tuned to effectively control the
morphologies and properties of the final products [49,
50]. Wet chemical methods have been demonstrated as
a very powerful and versatile technique for growing
1D ZnO nanostructures.
Here in this review, we focus on the 1D ZnO nano-
structures that have been grown by wet chemical
methods, although evaluation of ZnO nanostructures
is provided in the vast Ref. [1, 5, 6, 51–53]. We cover the
following five main aspects. First, we will go over the
basic synthetic methodologies and growth mechanisms
that have been adopted in the literature. Second, we
will display the various kinds of novel nanostructures
of ZnO that have been achieved by wet chemical
methods. Third, we will summarize ways to manipulate
the conductivity of the ZnO nanostructures by doping,
such as n-type, p-type, and transition metal doping,
and the ways of engineering the ZnO band gap by
alloying with other metal oxides. Fourth, we will show
the various techniques that have been implemented to
control the spatial distribution of ZnO nanostructures
on a substrate, namely patterned growth. Finally we
will illustrate the functional properties of 1D ZnO
nanostructures and the diverse innovative applications
where 1D ZnO nanostructures play an important role.
2. Basic synthetic methodologies and growth
mechanisms
ZnO is an amphoteric oxide with an isoelectric point
value of about 9.5 [54]. Generally speaking, ZnO is
expected to crystallize by the hydrolysis of Zn salts in
a basic solution that can be formed using strong or
weak alkalis. Zn
2+
is known to coordinate in tetrahedral
complexes. Due to the 3d
10
electron configuration, it is
colorless and has zero crystal field stabilization energy.
Depending on the given pH and temperature [55], Zn
2+
is able to exist in a series of intermediates, and ZnO can
be formed by the dehydration of these intermediates.
Chemical reactions in aqueous systems are usually
considered to be in a reversible equilibrium, and the
driving force is the minimization of the free energy of
the entire reaction system, which is the intrinsic nature
of wet chemical methods [56]. Wurtzite structured
ZnO grown along the c axis has high energy polar
surfaces such as ± (0001) surfaces with alternating
Zn
2+
-terminated and O
2–
-terminated surfaces [28]. So
when a ZnO nucleus is newly formed, owing to the high
energy of the polar surfaces, the incoming precursor
molecules tend to favorably adsorb on the polar
surfaces. However, after adsorption of one layer of
precursor molecules, the polar surface transforms into
another polar surface with inverted polarity. For
instance, a Zn
2+
-terminated surface changes into an
O
2–
-terminated surface, or vice versa. Such a process
is repeated over time, leading to a fast growth along
the ± [0001] directions, exposing the non-polar
{1100}
and
{2110} surfaces to the solution. This is essentially
how a 1D nanostructure is formed.
2.1 Growth in general alkaline solutions
An alkaline solution is essential for the formation of
ZnO nanostructures because normally divalent metal
ions do not hydrolyze in acidic environments [28, 57,
58]. The commonly used alkali compounds are KOH
and NaOH. Generally speaking, the solubility of ZnO
in an alkali solution increases with the alkali con-
centration and temperature. Supersaturation allows a
growth zone to be attained [58]. KOH is thought to
be preferable to NaOH, because K
+
has a larger ion
radius and thus a lower probability of incorporation
into the ZnO lattice [58, 59]. Furthermore, it has been
suggested that Na
+
is attracted by the OH
−
around the
nanocrystal and forms a virtual capping layer, thus,
inhibiting the nanocrystal growth [60].
Zn
2+
+ 2OH
−
←
→ Zn(OH)
2
(1)
Zn(OH)
2
+ 2OH
−
←→ [Zn(OH)
4
]
2–
(2)
[Zn(OH)
4
]
2–
←
→ ZnO
2
2
−
+ 2H
2
O (3)
ZnO
2
2–
+ H
2
O
←
→ ZnO + 2OH
−
(4)
ZnO + OH
−
←
→ ZnOOH
−
(5)
The main reactions involved in the growth are
illustrated in the above equations [61, 62]. For the

Nano Res
3
equation (2), the product is not necessarily Zn(OH)
4
2–
,
but could also be in the form of Zn(OH)
+
, Zn(OH)
2
,
or Zn(OH)
3
−
, depending on the parameters, such as
the concentration of Zn
2+
and the pH value, as shown
in Fig. 1(a). And all of these intermediate forms are
actually in equilibrium, with the major forms being
different under different reaction conditions. The
growth process could be described as follows [63]. At
the very beginning, the Zn
2+
and OH
−
ions coordinate
Figure 1 (a) Phase stability diagrams for the ZnO(s)–H
2
O system
at 25
°
C as a function of precursor concentration and pH, where
the dashed lines denote the thermodynamic equilibrium between
the Zn
2+
soluble species and the corresponding solid phases [64].
(b) Aggregation and nucleation of domains of the wurtzite structured
ZnO, where the characteristic six membered rings in the aggregate
center are highlighted in blue. The two staggered six-rings form a
center of stability and give rise to further ordering in favor of the
wurtzite structure [63]. Reproduced with permission
with each other, and then they undergo dehydration
by proton transfer, forming Zn
2+
···O
2–
···Zn
2+
bonds, and
leading to an agglomerate of the form of [Zn
x
(OH)
y
]
(2x–y)+
,
which has an octahedral geometry. The H
2
O molecules
formed by dehydration migrate into the solution. These
aggregates usually contain fewer than 50 ions, and the
formation of O
2–
ions implies dramatic changes within
the aggregate. After the aggregates reach around 150
ions, wurtzite type (tetrahedral coordination) ZnO
domains are then nucleated in the central region of
the aggregates (shown in Fig. 1(b)). The core comprises
Zn
2+
and O
2–
ions only, while the aggregate surface
still mainly consists of Zn
2+
and OH
−
ions. Aggregates
of over 200 ions exhibit a nanometer-sized core of the
wurtzite structured ZnO which grows as a result of
further association and dehydration of Zn
2+
and OH
−
ions [63].
In the above equations, the O
2–
in ZnO comes from
the base, not from the solvent H
2
O. Therefore growth
of ZnO does not necessarily require the solvent to be
H
2
O [65]. It could be organic solvents, such as methanol
[66], ethanol [67], and butanol [68], or even ionic
liquids [69, 70]. Under alkali conditions, the reactions
could take place at room temperature by adjusting the
ratio of Zn
2+
and OH
−
, giving rise to ZnO nanowires
with diameter even below 10 nm. ZnO nanowires with
various aspect ratios can be prepared by simply
adjusting OH
−
concentration and reaction time [68].
The growth of polar inorganic nanocrystals is sen-
sitive to the reaction solvents, and their morphologies
could be tuned and controlled by the crystal–solvent
interfacial interactions [66]. In such cases, the mor-
phology of ZnO is largely directed by the polarity
and saturated vapor pressure of the solvents [65]. As
shown in Figs. 2(a)–2(c), the aspect ratio of ZnO
nanowires, which is dictated by the relative growth
rates of polar and nonpolar surfaces, can be readily
tuned by varying the polarity of the solvents. Highly
polar solvent molecules have stronger interactions
with the polar surfaces of ZnO, and thus hinder the
precursor molecules from adsorbing and settling down
onto the polar surfaces. The aspect ratio of the ZnO
nanostructures increases on going from the more polar
solvent methanol to the less polar solvent 1-butanol.
All the as-grown ZnO nanowires showed two well-
faceted basal planes along the
± c axis as shown in
Fig. 2(d) [67].

Nano Res
4
Figure 2 Transmission electron microscopy (TEM) images of
ZnO nanowires synthesized in solvents having different polarities:
(a) in methanol [66], (b) in ethanol [66], and (c) in 1-butanol [68].
Even though the reaction temperature and the growth time are
different, we can still see the effect of the solvent polarity on the
nanowire aspect ratio. Insets in (a) and (b) are selected area electron
diffraction patterns. (d) Schematic illustration of growing +c ends
of ZnO with two common interplanar angles [67]. Reproduced
with permission
When the solvent contained nonpolar hexane,
ultrathin ZnO nanowires of diameters of 2 nm could
be synthesized from a simple acetate precursor, as
shown in Fig. 3(a) [71]. These ultrathin nanowires
also self-assembled into uniform stacks of nanowires
aligned parallel to each other with respect to the long
axis [71]. Near-UV absorption and photoluminescence
measurements were able to determine that quantum
confinement effects were present in these ultrathin
nanowires, with an excitonic ground state of about
3.55 eV [71]. The ultrathin nanowires were possibly
grown by oriented coalescence of quantum dots, as
shown in Fig. 3(b). Pacholski et al.
suggested that
oriented attachment of preformed quasi-spherical ZnO
nanoparticles should be a major reaction path during
the formation of single crystalline nanowires [72, 73].
The bottlenecks between the attached adjacent nano-
particles were later filled up and the nanowire surfaces
were thus, smoothened by Ostwald ripening [72].
The alkaline solution could also be
weak bases,
such as
NH
3
·H
2
O and other amine compounds [74].
For examples, growth kinetics of ZnO nanowires in
NH
3
·H
2
O has been well studied in the Ref. [75]. Besides
providing a basic environment, NH
3
·H
2
O is also able
to mediate heterogeneous nucleation of ZnO nano-
wires [75–78]. Experiments have shown that due to
depletion of Zn
2+
ions the growth of the ZnO nanowires
normally slowed down with time and eventually
arrived at growth-dissolution equilibrium for longer
reaction times. This limitation can be overcome by
adding additional Zn nitrate solution [79], or by
replenishing the growth solution [77, 78, 80]. Under
the mediation of NH
3
·H
2
O, however, Zn
2+
could be
stabilized through the reversible reaction shown in
equation (8) below, thus, leading to a relatively low
level of supersaturation being maintained in the
solution. At the growth temperature (typically 70–
95
°
C), this promoted only heterogeneous growth on
the seeded substrate and suppressed the homogeneous
nucleation in the bulk solution. That is also the reason
that why after growth the bulk solution and reaction
container usually remained clear without any preci-
pitation. As the reaction proceeded, Zn
2+
was gradually
Figure 3 (a) TEM image of self-assembled ZnO nanowires with
diameters of about 2 nm (inset: higher resolution image showing
the oriented stacking; nanowires are dark contrast) [71]. (b) TEM
image of the ultrathin nanowire formed by orientational aggregation
of several quantum dots [72]. Reproduced with permission

Nano Res
5
consumed and the zinc–ammonia complex gradually
decomposed, thus, maintaining a stable level of Zn
2+
in the solution. Therefore, all the reaction nutrient
only contributed to the heterogeneous growth of ZnO
nanowires on the seeded substrate, so the growth could
last for a long time without replenishing the solution.
Equations (1) to (5) only describe a simplified version
of the reaction processes. The actual scenario could be
much more complicated than what has been discussed
above. For example, oxygen molecules have not been
considered at all, but in reality, the dissolved O
2
concentration in the solution plays a significant role
in the final crystal quality of the ZnO nanowires. There
is experimental evidence showing that, if the growth
solution was added with extra H
2
O
2
that decomposed
into H
2
O and O
2
, high quality ZnO nanowires with
sharp top surfaces were grown [81]; if the solution was
prepared with boiled de-ionized water to eliminate the
dissolved O
2
, ZnO nanowires with very ragged surfaces
were formed [82].
2.2 Growth mediated by hexamethylenetetramine
(HMTA) aqueous solution
Probably the most commonly used chemical agents in
the existing literature for the hydrothermal synthesis
of ZnO nanowires are Zn(NO
3
)
2
and HMTA [83, 84].
In this case, Zn(NO
3
)
2
provides Zn
2+
ions required for
building up ZnO nanowires. H
2
O molecules in the
solution, unlike for the case of alkali-mediated growth,
provide O
2–
ions.
HMTA is a nonionic cyclic tertiary amine, as shown
in Fig. 4. Even though the exact function of HMTA
during the ZnO nanowire growth is still unclear, it
has been suggested that it acts as a bidentate Lewis
base that coordinates and bridges two Zn
2+
ions [85].
So besides the inherent fast growth along direction
of the polar surfaces of wurtzite ZnO, attachment of
HMTA to the nonpolar side facets also facilitates the
Figure 4 Molecular structure of HMTA
anisotropic growth in the [0001] direction [86]. HMTA
also acts as a weak base and pH buffer [49]. As shown
in Fig. 4, HMTA is a rigid molecule, and it readily
hydrolyzes in water and gradually produces HCHO
and NH
3
, releasing the strain energy that is associated
with its molecular structure, as shown in equations (6)
and (7). This is critical in the synthesis process. If the
HMTA simply hydrolyzed very quickly and produced
a large amount of OH
–
in a short period of time, the
Zn
2+
ions in solution would precipitate out quickly
owing to the high pH environment, and this eventually
would result in fast consumption of the nutrient and
prohibit the oriented growth of ZnO nanowires [87].
From reactions (8) and (9), NH
3
—the product of the
decomposition of HMTA—plays two essential roles.
First, it produces a basic environment that is necessary
for the formation of Zn(OH)
2
. Second, it coordinates
with Zn
2+
and thus stabilizes the aqueous Zn
2+
. Zn(OH)
2
dehydrates into ZnO when heated in an oven [84], in
a microwave [88], under ultrasonication [89], or even
under sunlight [90]. All five reactions (6) to (10) are
actually in equilibrium and can be controlled by
adjusting the reaction parameters, such as precursor
concentration, growth temperature and growth time,
pushing the reaction equilibrium forwards or back-
wards. In general, precursor concentration determines
the nanowire density. Growth time and temperature
control the ZnO nanowire morphology and aspect
ratio [50, 91]. As we can also see from equation (6),
seven moles of reactants produce ten moles of products,
so there is an increase in entropy during reaction,
which means increasing the reaction temperature will
push the equilibrium forwards. The rate of HMTA
hydrolysis decreases with increasing pH and vice
versa [49]. Note that the above five reactions proceed
extremely slowly at room temperature. For example,
when the precursor concentration is below 10 mmol/L,
the reaction solution remains transparent and clear for
months at room temperature [82]. The reactions take
place very fast if using microwaves as the heating
source, and the average growth rate of the nanowires
can be as high as 100 nm·min
–1
[88].
HMTA + 6H
2
O
←
→ 4NH
3
+ 6HCHO (6)
NH
3
+ H
2
O
←
→ NH
4
+
+ OH
−
(7)

Frequently asked questions

Q1. What are the main techniques to control the physical properties of semiconductor nano-materials?

Doping and alloying are the primary techniques to control the physical properties of semiconductor nano-materials, such as electrical conductivity, conductivity type, band gap, and ferromagnetism [86].4.1 n-type dopingZnO nanowires are intrinsically n-type due to the inevitable point defects, such as oxygen vacancies and zinc interstitials. 

Q2. What are the contributions mentioned in the paper "One-dimensional zno nanostructures: solution growth and functional properties" ?

One-dimensional ( 1D ) ZnO nanostructures have been studied intensively and extensively over the last decade not only for their remarkable chemical and physical properties, but also for their current and future diverse technological applications. This article gives a comprehensive overview of the progress that has been made within the context of 1D ZnO nanostructures synthesized via wet chemical methods. 

Q3. What can be done to separate the inorganic substrate and the polymer coating layers?

Rapid heating/cooling can also be used to separate the inorganic substrate and the polymer coating layers due to their different thermal expansion coefficients. 

Q4. Why did the ZnO nanoplates show enhanced photocatalytic properties?

Due to the high roughness factor and/or the large areas ofexposed polar basal planes, the ZnO nanoplates showed enhanced photocatalytic properties for decomposition of volatile organic compounds in comparison with common 1D ZnO nanowire arrays [161]. 

Q5. Why do flat thin film-based LEDs suffer from low light extraction efficiency?

Due to the high refractive index of the active materials, flat thin film-based LEDs suffer from low light extraction efficiency limited by the total internal reflection [403]. 

Q6. Why are dangling bonds less critical than for general interfaces?

Because of the small interfacial lattice mismatch, dangling bonds can be mostly satisfied and are less critical than for general interfaces. 

Q7. What are the characteristics of the ZnO nanostructures grown by wet chemical methods?

ZnO nanostructures grown by wet chemical methods are usually of low crystal quality with abundant defects, such as point defects and voids, which can sometimes be seen from their very rough surfaces. 

Q8. Why did the strip shape openings have to be along the direction of the substrate?

because of the anisotropic growth habits of the ZnO nanowires, the strip shape openings had to be along the [0001] direction of the substrate to grow individually separated nanowires. 

Q9. What is the main reaction path for the formation of nanowires?

Pacholski et al. suggested that oriented attachment of preformed quasi-spherical ZnO nanoparticles should be a major reaction path during the formation of single crystalline nanowires [72, 73]. 

Q10. What was the advantage of growing nanowires on top of a substrate?

the nanowires were directly grown on top of the substrate, which ensured an intimate contact for electron transport between the nanowires and the substrate. 

Q11. What are the other approaches to hinder the recombination of the photogenerated electron and?

There are also other approaches to hinder the recombination of the photogenerated electron and hole pairs, such as using CdS nanoparticle–ZnO nanowire heterostructure arrays [321], and SnO2 and ZnO composites to improve the charge separation efficiency [322]. 

Q12. What was done to ensure a direct charge transport between the nanowire and the viologen?

Unattached viologen molecules were rinsed away, and only those that were physically/chemically adsorbed on the nanowire surface were left to ensure a direct charge transport between the nanowire and the viologen molecule. 

Q13. What are the potential applications of the nonlinear optical properties of semiconductor nanowires?

Nonlinear optical properties of semiconductor nanowires have potential applications in frequency converters and logic/routing elements in optoelectronic nanocircuits [366]. 

Q14. What was the main parameter in the electrodeposition of ZnO nanowires?

In any case, the ratio between the OH− generation rate at the cathode and the Zn2+ diffusion rate to the cathode was proposed to be the major parameter in the electrodeposition of ZnO nanowires [123]. 

Q15. How many layers of nanogenerators could be used to increase the output voltage?

The output voltage could be greatly enhanced by linearly integrating a number of layers of vertically integrated alternating nanogenerators [432, 554]. 

Q16. Why is it difficult to remove the Al2O3 membrane in the presence of ZnO?

because both ZnO and Al2O3 are amphoteric oxides, it is technically difficult to selectively remove the Al2O3 membrane in the presence of ZnO nanowires. 

Q17. How can the aspect ratio of ZnO nanowires be tuned?

As shown in Figs. 2(a)–2(c), the aspect ratio of ZnO nanowires, which is dictated by the relative growth rates of polar and nonpolar surfaces, can be readily tuned by varying the polarity of the solvents. 

Q18. Why did ZnO nanowires have an even higher catalytic activity than bulk materials?

Zhou and Wong showed that ZnO nanowires had an even higher catalytic activity than nanoparticles and bulk forms due to their high purity and crystallinity [129]. 

Q19. How did He et al. decorate ZnO nanowires with Au?

In addition to wet chemical syntheses, He et al. have described a novel approach to decorate ZnO nanowires with Au nanoparticles by electrophoretic deposition [241]. 

Q20. How can the polycarbonate template be used to synthesize ZnO nanostructures?

As shown in Fig. 7, Zhou et al. demonstrated a simple polycarbonate template method to synthesize 1D oxide nanostructures [129], among which, the diameters of the ZnO nanowires could be tuned from 60 to 260 nm, with lengths in the ~μm range, by reliably and reproducibly controlling the template pore channel dimensions [129].