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Reports: UNI10 49524-UNI10: Nanostructured ZnO Energy Generator: Influence of Dimensional and Mechanical Properties
1. Objectives
Research on energy materials is growing in importance as traditional sources of energy are becoming increasingly difficult and costly to obtain. Among the various types of energy materials, transition metal oxide nanostructures -- especially zinc oxide (ZnO) nanostructures -- have been heavily researched because they exhibit dual semiconducting and piezoelectric properties, and are bio-safe. These properties make ZnO nanostructures a suitable candidate for energy generation in applications such as bulk heterojunction photovoltaic cells and piezoelectric energy scavengers. While ZnO nanostructures hold tremendous potential to be a practical energy source, little is known about how its conversion efficiency and long-term reliability are influenced by fundamental properties such as dimensional and mechanical properties. In addition, time-consuming and costly fabrication techniques that often necessitate the use of catalysts are a critical obstacle that inhibits the broad applications of ZnO for energy usage. In this project, we (1) develop a catalyst-free, rapid synthesis method to fabricate ZnO nanostructures for energy generation, and (2) investigate how the dimensional and mechanical properties of ZnO nanostructures affect their energy conversion efficiencies.
2. Findings
During the reporting time period, our work focused on the development of an ambient-pressure, catalyst-free rapid thermal method to synthesize ZnO nanostructures and nanocrystalline films, and the characterization of the quality and crystallinity of these ZnO nanostructures and nanocrystalline films.
2.1 Catalyst-free, rapid synthesis of ZnO nanostructures on silicon
The majority of ZnO nanostructures are currently synthesized via a catalyst-assisted vapor-phase route using a thermal furnace. While this is a simple method, its major shortcomings include lengthy synthesis time (~ hours) and high vacuum (~ 10-5 torr). In order to synthesize ZnO at a significantly reduced time scale and at operating pressure close to ambient pressure, we proposed to research the use of high-frequency RF power as a means to quickly vaporize Zn by localizing heating to a targeted, small thermal mass. To date, we have been successful in demonstrating a rapid and catalyst-free method of synthesizing ZnO nanostructures on Si(100) using a low-power (as low as 65W) RF-induced inductive heating process. Our scanning electron micrographs (SEM) show that, by carefully controlling the reacting gases' partial pressures, source and substrate temperatures, and rate of heating and cooling, we are able to produce ZnO nanostructures of specific geometries ranging from (a) nano sheets, (b) tetrapods, (c) high-aspect ratio nanowires, (d) telescopic nanowires, (e) nanoporous nanorods, (f) solid nanorods, and (g) partially hollow nanorods on a silicon platform within several minutes. The growth of these nanostructures is achieved purely via a catalyst-free vapor-solid (VS) mechanism, without the use of metallic catalyst such as those common in the catalyst-assisted vapor-liquid-solid (VLS) mechanism. Using an oxygen and argon mixture as a reacting gas at a fixed O2 : Ar ratio of 0.1:99.9, x-ray diffraction (XRD) studies revealed the predominant crystallographic plane to be (002), indicating the growth direction of ZnO nanostructures to be c-axis oriented at this reacting gas ratio. SEM also shows the dominant species at this reacting gas ratio to be ZnO nanowires, with a growth rate approaching 100 µm/min along the c-axis.
2.2 Effect of Oxygen:Argon ratios on the morphologies of ZnO nanostructures
We investigated the influence of partial pressures of O2 and Ar on the morphologies of the nanostructures. In general, lower O2 concentration during synthesis corresponded to finer nanostructures grown. We studied the growth of ZnO nanostructures using 10:90 (10% O2), 1:99 (1% O2) and 0.1:99.9 O2: Ar mixtures. In terms of the dimensions of the nanostructures, the effective range in which there was an observable nucleation and deposition of structures smaller than one micron was 0% - 10% oxygen in an argon atmosphere. Structures with diameters less than 100nm commonly occurred with an estimated oxygen concentration of less than 1%. It was observed that O2 content significantly influenced the morphology of ZnO nanostructures: higher O2 (up to 10% O2) content encouraged the formation of large nanocrystals with nominal diameter of 500nm, while lower O2 content favored the formation of tetrapods (at 1% O2) and nanowires (at 0.1% O2).
2.3 Catalyst-free, rapid synthesis of nanocrystalline ZnO films
With a slightly modified experimental setup and a newly developed close-looped temperature control system, we are currently experimenting with synthesizing nanocrystalline ZnO films. So far, we have demonstrated conformal growth of dense, non-porous nanocrystalline ZnO films on both crystalline and amorphous substrates including silicon (100), sapphire (a-plane and c-plane), fused quartz, muscovite, glass, and tin-doped indium oxide. A major finding from this research is that, via XRD analyses, we observe the crystallographic orientations of the as-deposited nanocrystalline films to be predominantly c plane-oriented, indicating the growth of the films are independent of substrate type.
Student Involvement
During the first year of this grant, the PI was able to (1) secure a larger laboratory for the experimental work involved in this work, (2) use the funding to procure hardware and apparatus essential to the success of the project, and (3) hire 3 engineering undergraduate students as undergraduate student researchers during regular semesters, as well as provide normal semester and summer research opportunity for an additional 5 engineering undergraduate students. Of the 8 undergraduates, 2 are female students and 2 are minority students. One of the summer research interns was later offered a National Nanotechnology Infrastructure Network (NNIN) Research Experience for Undergraduates (REU) internship at Washington University in Saint Louis, MO. Funding from this research grant also allowed another undergraduate researcher to present his work at the 2009 NSTI Nanotech conference in Houston, TX and at the 2010 MRS Spring Meeting in San Francisco, CA. The third undergraduate researcher used part of this project (design of a close-looped temperature control system) as his senior design project and successfully graduated with a Bachelor of Science degree from SFSU in May 2010.
