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.

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One-dimensional ZnO nanostructures: Solution growth and functional properties

The synthesis of semiconductor nanowires has been studied intensively worldwide for a wide spectrum of materials. Such low-dimensional nanostructures are not only interesting for fundamental research due to their unique structural and physical properties relative to their bulk counterparts, but also offer fascinating potential for future technological applications. Deeper understanding and sufficient control of the growth of nanowires are central to the current research interest. This Review discusses the various growth processes, with a focus on the vapor-liquid-solid process, which offers an opportunity for the control of spatial positioning of nanowires. Strategies for position-controlled and nanopatterned growth of nanowire arrays are reviewed and demonstrated by selected examples as well as discussed in terms of larger-scale realization and future prospects. Issues on building up nanowire-based electronic and photonic devices are addressed at the end of the Review, accompanied by a brief survey of recent progress demonstrated so far on the laboratory level.

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Semiconductor nanowires: from self-organization to patterned growth

An efficient antireflection coating (ARC) can enhance solar cell performance through increased light coupling. Here, we investigate solution-grown ZnO nanostructures as ARCs for Si solar cells and compare them to conventional single layer ARCs. We find that nanoscale morphology, controlled through synthetic chemistry, has a great effect on the macroscopic ARC performance. Compared with a silicon nitride (SiN) single layer ARC, ZnO nanorod arrays display a broadband reflection suppression from 400 to 1200 nm. For a tapered nanorod array with average tip diameter of 10 nm, we achieve a weighted global reflectance of 6.6%, which is superior to an optimized SiN single layer ARC. Calculations using rigorous coupled wave analysis suggest that the tapered nanorod arrays behave like modified single layer ARCs, where the tapering leads to impedance matching between Si and air through a gradual reduction of the effective refractive index away from the surface, resulting in low reflection particularly at longer wavelengths and eliminating interference fringes through roughening of the air-ZnO interface. According to the calculations, we may further improve ARC performance by tailoring the thickness of the bottom fused ZnO layer and through better control of tip tapering.

ZnO nanostructures as efficient antireflection layers in solar cells.

A novel ZnO hierarchical micro/nanoarchitecture is fabricated by a facile solvothermal approach in an aqueous solution of ethylenediamine (EDA). This complex architecture is of a core/shell structure, composed of dense nanosheet-built networks that stand on a hexagonal-pyramid-like microcrystal (core part). The ZnO hexagonal micropyramid has external surfaces that consist of a basal plane (000) and lateral planes {011}. The nanosheets are a uniform thickness of about 10 nm and have a single-crystal structure with sheet-planar surfaces as {20} planes. These nanosheets interlace and overlap each other with an angle of 60° or 120°, and assemble into a discernible net- or grid-like morphology (about 100 nm in grid-size) on the micropyramid, which shows a high specific surface area (185.6 m2 g−1). Such a ZnO micro/nanoarchitecture is new in the family of ZnO nanostructures. Its formation depends on the concentration of the EDA solution as well as on the type of zinc source. A two-step sequential growth model is proposed based on observations from a time-dependent morphology evolution process. Importantly, such structured ZnO has shown a strong structure-induced enhancement of photocatalytic performance and has exhibited a much better photocatalytic property and durability for the photodegradation of methyl orange than that of other nanostructured ZnO, such as the powders of nanoparticles, nanosheets, and nanoneedles. This is mainly attributed to its higher surface-to-volume ratio and stability against aggregation. This work not only gives insight into understanding the hierarchical growth behaviour of complex ZnO micro/nanoarchitectures in a solution-phase synthetic system, but also provides an efficient route to enhance the photocatalytic performance of ZnO, which could also be extended to other catalysts, such as the inherently excellent TiO2, if they are of the same hierarchical micro/nanoarchitecture with an open and porous nanostructured surface layer.

ZnO Hierarchical Micro/Nanoarchitectures: Solvothermal Synthesis and Structurally Enhanced Photocatalytic Performance

Research applications in biomedical science and technology usually require various portable, wearable, easy-to-use, and/or implantable devices that can interface with biological systems. Organic or hybrid organic–inorganic microelectronics and nanoelectronics have long been a possibility. However, these devices require a power source, such as electrochemical cells or piezoelectric, thermoelectric, and pyroelectric transducers, to generate or store the electrical energy created through chemical, mechanical, or thermal processes. Finding a suitable power source has remained a major challenge for many devices in bioengineering and medical fields. ZnO is a typical piezoelectric and pyroelectric inorganic semiconducting material used for electromechanical and thermoelectrical energy conversion. Nanostructures of ZnO, such as nanowires (NWs), nanobelts (NBs), nanotubes, nanorings, nanosprings, and nanohelices, have attracted extensive research interest because of their potential applications as nanoscale sensors and actuators. While most of the current applications focus on its semiconducting properties, only a few efforts have utilized the nanometerscale piezoelectric properties of ZnO. Using ZnO NW arrays grown on a single-crystal sapphire substrate, we have successfully converted mechanical energy into electrical energy at the nanoscale. A conductive atomic force microscopy (AFM) tip was used in contact mode to deflect the aligned NWs. The coupling of piezoelectric and semiconducting properties in ZnO creates a strain field and charge separation across the NWs as a result of their bending. The rectifying characteristic of the Schottky barrier formed between the metal tip and the NW leads to electrical current generation. This is the principle behind piezoelectric nanogenerators. The ceramic and semiconducting substrates used for growing ZnO NWs are hard and brittle and cannot be used in applications that require a foldable or flexible power source, such as implantable biosensors. In this Communication, by using ZnO NW arrays grown on a flexible plastic substrate, we demonstrate the first successful flexible power source built on conducting-polymer films. This approach has two specific advantages: it uses a cost-effective, large-scale, wet-chemistry strategy to grow ZnO NW arrays at temperatures lower than 80 °C, and the growth of aligned ZnO NW arrays can occur on a large assortment of flexible plastic substrates. The latter advantage could play an important role in the flexible and portable electronics industry. Various dimensions, shapes, and orientations of ZnO NWs and microwires on flexible plastic substrates have been shown to be capable of producing piezoelectric voltage output, giving a real advantage for energy harvesting using large-scale ZnO NW arrays. The voltage generated from a single NW can be as high as 50 mV, which is large enough to power many nanoscale devices. The ZnO NWs were grown in solution using a synthetic chemistry approach. Figure 1 shows a series of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of typical ZnO NW arrays grown on a conductive plastic substrate. Figure 1a shows a low-resolution, topdown view of the densely aligned NW arrays. The aligned NWs have a uniform diameter of 200–300 nm and a hexagoC O M M U N IC A IO N

http://www.nanoscience.gatech.edu/paper/2006/06_AM_5.pdf

Nanowire Piezoelectric Nanogenerators on Plastic Substrates as Flexible Power Sources for Nanodevices

Nanostructured films with controlled architectures are desirable for many applications in optics, electronics, biology, medicine, and energy/chemical conversions. Low-temperature, aqueous chemical routes have been widely investigated for the synthesis of continuous films, and arrays of oriented nanorods and nanotubes. More recently, aqueous-phase routes have been used to produce films composed of more complex crystal structures. In this paper, we discuss recent progress in the synthesis of complex nanostructures through sequential nucleation and growth processes. We first review the use of multistage, seeded-growth methods to synthesize a wide range of nanostructures, including oriented nanowires, nanotubes, and nanoneedles, as well as laminated films, columns, and multilayer heterostructures. We then describe more recent work on the application of sequential nucleation and growth to the systematic assembly of large arrays of hierarchical, complex, oriented, and ordered crystal architectures. The multistage aqueous chemical route is shown to be applicable to several technologically important materials, and therefore may play a key role in advancing complex nanomaterials into applications.

Yingbing Jiang

Papers

Sequential Nucleation and Growth of Complex Nanostructured Films