Hydrothermal Growth of Periodic, Single‐Crystal ZnO Microrods and Microtunnels

Abstract

Controlled and extended growth of micro-and nanostructured material systems is becoming increasingly important because of the ever-decreasing dimensions of a variety of devices, including those used for chemical and biological sensing and diagnosis, catalysis, energy conversion and storage, lightemitting displays, and optical storage. There is also significant academic interest in these systems because their properties can be remarkably improved over those of the bulk material due to quantum-sized effects. Beginning with the preparation of oriented carbon nanotubes, [1,2] there has been a dramatic increase in the volume of research into tubular and rodlike nano-and microscale structures for a variety of materials, such as In 1-x Ga x P [3] and MnOOH [4] Zinc oxide has been considered as the most promising candidate material for a wide variety of applications, such as room-temperature UV lasing, [10] transparent conduction electrodes, [11] surface-acoustic-wave devices, [12] and gas sensors, [13,14] because of its unique properties of a wide bandgap (E g = 3.37 eV) [15] with a large exciton binding energy (60 meV) and a large piezoelectric constant.[16] Recent progress in processing and properties of ZnO has been reviewed. [17][18][19] Therefore, there has also been significant research into the growth and properties of ZnO nano/microstructures of wires, tubes, belts, and rods. [20,21] The synthesis of nano/microstructures of ZnO has been carried out using various methods, such as evaporation and condensation, [22][23][24][25] physical vapor deposition, [26][27][28] chemical vapor deposition, [29,30] and solvothermal [31] and hydrothermal methods. [32][33][34][35][36] Among them, the hydrothermal method is one of the most attractive candidates for industrial use because industrial processes generally require rapid, low-cost techniques, which are the advantages of the hydrothermal method. The morphology of ZnO crystals can be controlled by varying growth conditions such as pH, [37] temperature, [38] and adsorbing molecules. [32,39] Nano/micrometer-scale ZnO structures that only have out-of-plane orientation without in-plane orientation with a substrate have been successfully grown.[ [40][41][42] However, the direct fabrication of periodic arrays of nano/micrometer-scale ZnO morphologies as single crystals with inplane and out-of plane epitaxial relations with a substrate have not been achieved yet.It is common to use photolithographic techniques to fabricate patterned periodic arrays of micrometer to sub-micrometer scale features of interesting materials on a substrate. A novel technique called "channel stamping" has also been developed as a soft lithographic alternative to photolithography, where either a metallo-organic precursor solution or a polymer solution is stamped from the channels of a polydimethylsiloxane (PDMS) stamp. Channel stamping has been used to produce micrometer and sub-micrometer scale features. [43,44] Several years ago, Andeen et al. [45] demonstrated that epitaxial ZnO films could be grown ...

One-dimensional ZnO nanostructures: Solution growth and functional properties

“…Citrate ions are characterized by three negative charges under the normal growth environment. Experimental results in the literature as well as theoretical calculations suggest that citrate ions strongly and specifically adsorb to the Zn 2+ ions on the (0001) surface, and thus inhibit the growth along [0001] and forced to grow along the 〈 〉 0110 or 〈 〉 2110 directions [150,157,160]. With citrate ions, rather than long hexagonal nanowires, flat hexagonal nanoplates were produced, as shown in Fig.…”

“…With citrate ions, rather than long hexagonal nanowires, flat hexagonal nanoplates were produced, as shown in Fig. 11(c) [150,157]. Due to the high roughness factor and/or the large areas of Figure 11 (a) Schematic illustration of the adsorption of PEI molecules on the ZnO nanowire side surfaces [159].…”

“…(b) SEM image of the ZnO nanowires formed with the addition of PEI [155]. (c) Large arrays of well-aligned helical ZnO whiskers on top of ZnO rod base [150,157]. Reproduced with permission exposed 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].…”

“…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][146][147][148][149]), AlN, SiC, Al 2 O 3 , and MgAl 2 O 4 substrates [150], either by hydrothermal decomposition [151] or electrodeposition. In particular, ZnO and GaN have the same wurtzite-type structure with a low lattice mismatch of 1.8% [152], which is much smaller than that (12.7%) with Au(111).…”

“…(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 side surfaces and enhance the vertical growth, such as amines like polyethylenimine (PEI) [18,155,156] and ethylenediamine [67,137]; those that cap onto the basal plane of the ZnO nanostructures and promote lateral growth, such as Cl - [124] and C 3 H 5 O(COO) 3 3− (citrate ions) [150,157,158]. The isoelectric point of ZnO powder is at around pH = 9.5 [54].…”