ZnO nanowires (or nanorods) have been widely studied due to their unique material properties and remarkable performance in electronics, optics, and photonics. Recently, photocatalytic applications of ZnO nanowires are of increased interest in environmental protection applications. This paper presents a review of the current research of ZnO nanowires (or nanorods) with special focus on photocatalysis. We have reviewed the semiconducting photocatalysts and discussed a variety of synthesis methods of ZnO nanowires and their corresponding effectiveness in photocatalysis. We have also presented the characterization of ZnO nanowires from the literature and from our own measurements. Finally, a wide range of uses of ZnO nanowires in various applications is highlighted in this paper.
Nanomaterials have attracted tremendous interest due to their noticeable performance in electronics, optics, and photonics. Nanomaterials are typically classified into three groups: 0-dimensional, 1-dimensional, and 2-dimensional. 0-dimensional nanostructures, referred to as quantum dots or nanoparticles with an aspect ratio near unity, have been extensively used in biological applications [
ZnO is a semiconductor material with a direct wide band gap energy (3.37 eV) and a large exciton binding energy (60 meV) at room temperature [
ZnO structure: (a) the wurtzite structure model; (b) the wurtzite unit cell (from
This paper reviews recent research in ZnO nanowires (or nanorods) with an emphasis on ZnO nanowires used in photocatalysis. In the following sections we have reviewed different semiconductor photocatalysts, compared their properties, and discussed a variety of synthesis methods of ZnO nanowires. We have also presented the characterization of ZnO nanowires from both the literature and our own measurements. Finally, a wide range of ZnO nanowires in various applications is highlighted in this paper.
Photocatalysis is a promising process for environmental protection because it is able to oxidize low concentrations of organic pollutants into benign products [
A schematic of the principle of photocatalysis (reproduced with permission from [
There are a number of semiconductors that could be used as photocatalysts, such as TiO2, ZnO, and WO3, Fe2O3. The band gap energy plays a significant role in the photocatalytic process. Figure
Band edge positions of common semiconductor photocatalysts (data from [
Since the contaminant molecules need to be adsorbed on the photocatalytic surface before the reactions take place, the surface area plays a significant role in the photocatalytic activity. Although nanoparticles offer a large surface area, they have mostly been used in water suspensions, which limit their practical use due to difficulties in their separation and recovery. Moreover, additional equipment is needed for catalyst nanoparticle separation. Photocatalyst supported on a steady substrate can eliminate this issue. One-dimensional nanostructures, such as nanowires grown on a substrate, offer enhanced photocatalytic efficiency due to their extremely large surface-to-volume ratio as compared to a catalyst deposition on a flat surface [
Comparison of different ZnO nanostructures used in photocatalytic applications.
Nanoparticles | Nanowires | Nanothin film | |||
Advantages | Disadvantages | Advantages | Disadvantages | Advantages | Disadvantages |
Could be suspended in a solution | Particle aggregation in a solution leads to a reduced surface area | Growth could be well aligned on most substrates | Growth conditions are more restricted | Coated on certain substrates | Lower performance because of small surface area |
High performance because of larger surface areas | Posttreatment for catalyst removal is required | Offer larger surface area compared to nanothin film | Lower surface area compared to nanoparticles | Posttreatment for catalyst removal is not required | |
Difficult to recover all the catalyst | Posttreatment for catalyst removal is not required | ||||
Lower crystallinity and more defects |
ZnO nanowires can be either grown independently or grown on certain substrates. However, a vertical aligned growth on a substrate has more advantages in photocatalytic applications. The anisotropy of the ZnO crystal structure assists the growth of nanowires. The most common polar surface is the basal plane (0 0 1) with one end of the basal polar plane terminating in partially positive Zn lattice points and the other end terminating in partially negative oxygen lattice points. The anisotropic growth of the nanowires takes place along the
The synthesis methods of ZnO nanowires could mainly be classified as vapor phase and solution phase synthesis.
Vapor phase synthesis is probably the most extensively explored approach in the formation of 1D nanostructures [
Compared to other vapor phase techniques, VLS method is a simpler and cheaper process, and is advantageous for growing ZnO on large wafers [
Catalyst-free metal-organic chemical vapor deposition (MOCVD) is another important synthesis method for ZnO nanowires [
Physical vapor deposition (PVD) technique has also been used to fabricate ZnO nanowires. The advantages of PVD technique are the following: (1) composition of products can be controlled, (2) there is no pollution such as drain water, discharge gas, and waste slag, and (3) simple process of making samples [
Solution phase synthesis has many advantages when compared to vapor phase synthesis, such as low cost, low temperature, scalability, and ease of handling. Generally, solution phase reactions occur at relatively low temperatures (<200°C) compared to vapor phase synthesis methods. Thus, solution synthesis methods allow for a greater choice of substrates including inorganic and organic substrates. Due to the many advantages, solution phase synthesis methods have attracted increasing interest. In solution phase synthesis, the growth process could be carried out in either an aqueous or organic solution or a mixture of the two [
Generally, solution phase synthesis is carried out in an aqueous solution, and the process is then referred to as the hydrothermal growth method [ A thin layer of ZnO nanoparticles is seeded on a certain substrate. The seeding layer promotes nucleation for the growth of nanowires due to the lowering of the thermodynamic barrier [ An alkaline reagent (such as NaOH or hexamethylenetetramine) and Zn2+ salt (Zn(NO3)2, ZnCl2, etc.) mixture aqueous solution is used as a precursor (or growth solution). The ZnO seeded substrate is kept in the growth solution at a certain temperature and a certain period of time. The resultant substrate and growth layer is washed and dried.
When hexamethylenetetramine ((CH2)6N4, or HTMA) and Zn(NO3)2 are chosen as precursor, the chemical reactions can be summarized in the following equations [
Decomposition reaction:
Hydroxyl supply reaction:
Supersaturation reaction:
ZnO nanowire growth reaction:
One of the key parameters for the growth of ZnO nanowires is controlling the supersaturation of the reactants. It is believed that high supersaturation levels favor nucleation and low supersaturation levels favor crystal growth [
Typical preseeding methods include thermal decomposition of zinc acetate, spin coating of ZnO nanoparticles, sputter deposition, and physical vapor deposition. In order to seed ZnO particles on the substrate, ZnO seeds must be annealed at certain temperature to improve ZnO particle adhesion to the substrate and nanowire vertical growth alignment. Greene et al. [
The texture, thickness, and crystal size of ZnO seed layers also affect the quality of ZnO nanowire growth [
The diameter, density, and length of ZnO nanorods corresponding to the thickness of the seed layer (reproduced with permission from [
Thickness of the seed layer (nm) | Diameter of ZnO nanorods (nm) | Density of ZnO nanorods ( | Length of ZnO nanorods (nm) |
---|---|---|---|
20 | 30 | 213 | 1052 |
40 | 36 | 209 | 1007 |
160 | 51 | 184 | 998 |
320 | 72 | 169 | 967 |
Without a ZnO seeding layer, ZnO nanowires could be grown on an Au/substrate by introducing a suitable content of ammonium hydroxide into the precursor solution [
There are some alkaline reagents that have been used to supply OH- during the reaction process such as NaOH, hexamethylenetetramine (HMTA), Na2CO3, ammonia, and ethylenediamine. When NaOH, KOH, or Na2CO3 is chosen, the synthesis process usually is carried out at elevated temperatures (>100°C) and pressures in a Teflon-sealed stainless autoclave [
To ascertain the relationship between the precursor concentration and the ZnO nanowire growth, Wang et al. [
Average diameters, lengths, and aspect ratios of ZnO nanorods prepared from various precursor concentrations with a Zn(NO3)2/C6H12N4 ratio of 5 (reproduced with permission from [
Average diameters, lengths, and aspect ratios of ZnO nanorods prepared in a 0.04 M precursor solution with various Zn(NO3)2/C6H12N4 ratios (reproduced with permission from [
Yuan et al. [
Average diameter of ZnO nanowires changes with growth time (data from [
Growth time (hr) | Average diameter (nm) |
---|---|
0.5 | 35 |
1.0 | 37 |
1.5 | 55 |
2.0 | 90 |
2.5 | 100 |
3 | 100 |
Baruah and Dutta [
One major advantage of the hydrothermal synthesis method is that almost any substrate can be used for the growth of vertical ZnO nanowires by using ZnO seeding layer. In this way, ZnO nanowires can grow on flat surface regardless of the substrate (polymer, glass, semiconductor, metal, and more) by only controlling the growth conditions. ZnO nanowires can also be grown on organic substrates. ZnO nanowires have been successfully grown on polydimethylsiloxane (PDMS) [
Sugunan et al. [
The aspect ratio of ZnO nanowires could be affected by the addition of additives. Zhou et al. [
Other factors affecting ZnO growth include the heating source, the Zn2+ source, the external electric field, and mechanical stirring. The use of microwave heating instead of conventional heating has recently received great interest [
Other solution phase synthesis methods include the microemulsion and ethanol base methods. Lim et al. [
Doping is the primary method of controlling semiconductor properties such as the band gap, electrical conductivity, and ferromagnetism. Many metals and nonmetals have been successfully used to dope ZnO nanowires by various synthesis methods. ZnO nanowire metal doping includes Ni [
Li et al. [
Under general conditions, ZnO is single crystalline and exhibits a hexagonal wurtzite structure. The structure of ZnO nanowires could be revealed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Figure
XRD of ZnO nanowires on a silicon substrate growth by the hydrothermal synthesis method (from [
SEM image of the ZnO nanorods array on glass substrate by hydrothermal method (from [
Further structural characterizations can be carried out by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). Figure
(a) TEM of ZnO nanowires on an ITO substrate showing a general view. (b) HRTEM image showing individual nanowires with
The ZnO nanowires obtained at different reaction times and examined by Raman spectra are presented in Figure
Raman spectra of the ZnO structures obtained by the hydrothermal method at 130°C for (a) 30 min, (b) 60 min, (c) 120 min, and (d) 180 min (reproduced with permission from [
Figure
UV-Visible absorption spectra of ZnO nanoparticle, ZnO and ZnO/Fe nanowires (from [
Figure
FTIR spectrum of ZnO nanorods prepared at 200°C for 20 h using NaOH (reproduced with permission from [
The X-ray photoelectron spectroscopy (XPS) result Zn(2p) of the ZnO nanorods is shown in Figure
Zn(2p)XPS spectrum of synthesized ZnO nanorods on a ZnO thin film (reproduced with permission from [
Figure
Photocurrent response to UV light at different DC biases (reproduced with permission from [
In addition to the above characterizations, ZnO nanowires also exhibit many other unique chemical and physical properties for many applications such as large surface areas, piezoelectric, piezotronic, and optical.
ZnO nanowires can be used for a number of applications in different fields due to the unique electrical, optical, and mechanical properties.
ZnO is one of the earliest discovered and most widely used oxide gas sensing materials. ZnO functions as a gas sensitive material due to its electrical conductivity that can be dramatically affected by the adsorption or desorption of gas molecules on its surface [
Response curve of a ZnO gas sensor exposed to 10 ppb NO2 gas at 250°C (reproduced with permission from [
Recently, ZnO nanostructures have attracted interest in biosensor applications due to many advantages, including nontoxicity, biosafety, bio-compatibility, high electron-transfer rates, and combination with immobilized enzymes [
(a) Typical steady-state response of the biosensor by the successive addition of specific concentrations of glucose to air-saturated and stirred 0.01 M pH 7.4 PBS solution at −100 mV. (b) Effect of interfering species on the biosensor response (reproduced with permission from [
UV detection is another promising optical application of ZnO nanowires [
Room temperature of ZnO-nanowire-based UV lasing has been recently demonstrated [
Room temperature PL spectra of ZnO nanorods (
The output power of GaN LEDs with ZnO nanotip arrays can be enhanced by up to 50% times [
EL spectra of ZnO nanowires/p-GaN/ZnO nanowires heterostructure at a DC current of 20 mA. The inset is a photograph of the emission of blue light by the heterojunction LED under dc bias (reproduced with permission from [
Dye-sensitized solar cells (DSSCs), which are based on oxide semiconductors and organic dyes or metal organic complex dyes, are one of the most promising candidate systems to achieve efficient solar-energy because they are flexible, inexpensive, and easier to manufacture than silicon solar cells [
Characteristics and parameters of the “Nanoforest” DSSC Solar Cells shown in Figure
Symbol | Backbone NW length [ | Branching times | Configuration | Efficiency [%] | FF | ||
---|---|---|---|---|---|---|---|
LG1 | 7 | 0 | 0.45 | 1.52 | 0.636 | 0.480 | |
LG2 | 13 | 0.71 | 2.37 | 0.64 | 0.486 | ||
LG3 | 18 | 0.85 | 2.87 | 0.645 | 0.484 | ||
BG1 | 7 | 1 | 2.22 | 7.43 | o.681 | 0.522 | |
BG2 | 13 | 2.51 | 8.44 | 0.683 | 0.531 | ||
BG3 | 2 | 2.63 | 8.78 | 0.680 | 0.530 |
(a) Schematic structure of a solar cell and (b)
Lupan et al. [
Due to the noncentrosymmetric symmetry of the wurtzite structure, ZnO exhibits a strong piezoelectric behavior. Recently, nanogenerators (NGs) have been developed by utilizing the piezoelectric effect of ZnO nanowires for scavenging tiny and irregular mechanical energy such as air flow vibrations and body movements [
Working principle and output measurement of the HONG. (a) Schematic diagram of the HONG structure without mechanical deformation, in which gold is used to form Schottky contacts with the ZnO nanowire arrays. (b) Demonstration of the output scaling-up when mechanical deformation is induced, where the “±” signs indicate the polarity of the local piezoelectric potential created in the nanowires. (c) Open circuit voltage measurement of the HONG. (d) Short circuit current measurement of the HONG. The measurement is performed at a strain of 0.1% and strain rate of 5% s−1 with a deformation frequency of 0.33 Hz. The insets are the enlarged view of the boxed area for one cycle of deformation (reproduced with permission from [
Hu et al. [
ZnO nanowires are considered as candidates of field emitters due to their high melting point and high stability under an oxygen environment as compared to CNTs [
A typical nanowires field effect transistor (FET) consists of a semiconductor nanowire (ZnO, etc.) connected with two electrodes at two ends and placed on a flat substrate that serves as a gate electrode. The current flow from the drain to the source along the nanowire is controlled by the applied gate voltage or the chemical/biochemical species adsorbed on the surface of the nanowires. Recently, piezoelectric-FET (PE-FET) has been demonstrated by coupling the semiconductive and piezoelectric properties of ZnO, which is defined as the piezotronics effect [
ZnO nanowires used as photocatalysts have been recently reported by many research groups [
Schematic of a continuous-flow photocatalytic water treatment system (reproduced with permission from [
Surface properties such as surface defects and oxygen vacancies of photocatalysts play a significant role in the photocatalytic activity. ZnO nanowire crystalline defects exist primarily due to oxygen vacancies [
To further increase the photoactivity of ZnO nanowires, our group has synthesized ZnO/Fe nanowires by the growth of ZnO nanowires on the Fe-doped ZnO seeding layer using the hydrothermal method. The photocatalytic activity was evaluated by photodegradation of dichlorobenzene (DCB) and methyl orange (MO) in water. The experiments were carried out under white light (60 W/m2 in visible plus 2 W/m2 in UV) or UV light (30 W/m2) irradiation. The results showed that the ZnO/Fe nanowires exhibited enhanced photocatalytic activity as compared with pure ZnO nanowires under different light irradiation as well as different contaminants. For the decontamination of DCB (Figures
Photocatalyst activity testing: (a) degradation of DCB under white light irradiation; (b) degradation of DCB under UV light irradiation; (c) photodegradation of MO in visible light irradiation; (d) photodegradation of MO in UV light irradiation. The control of DCB is also reduced due to the volatility of the DCB (from [
Doping transition metal ions is a strategy for improving the visible light photocatalytic activity, by lowering the band gap, resulting from the creation of dopant energy levels below the conduction band [
This paper provides an overview of the synthesis, characterization, and applications of ZnO nanowires. The hydrothermal synthesis method is simple and efficient and it has received increased attention. A mixture of zinc nitrate and hexamine as precursor is the most popular. Due to the unique properties of the material, ZnO nanowires are attractive for a number of potential applications such as photocatalysis, solar cells, sensors, and generators. Among the applications of ZnO nanowires, photocatalysis is being increasingly used for environmental protection. Further research is needed to improve the quality of ZnO nanowires and large-scale produce ZnO nanowires for practical industrial applications. Based on this paper, ZnO nanowires promise to be one of the most important materials in photocatalytic as well as others applications.
The research leading to this paper was funded by the State of Florida through the Florida Energy Systems Consortium (FESC) funds.