Zinc oxide micro- and nanostructures as multifunctional materials
Zinc oxide (ZnO) is emerging as a useful multifunctional material due to its toughness, but it has the drawback that it is also very brittle. This brittleness can be overcome by incorporating ZnO nanostructures into polymer matrices to produce materials such as heavy-duty lacquers. However, ZnO also has special sensing, photocatalytic, and antiviral behavior, all of which can be enhanced by forming nano- and microstructures to increase the ratio of surface area to volume.
One main problem for these applications is how to control the synthesis parameters—temperature and pressure—which affect not only the quantity of particles or assemblies produced but also their shape. The shape in turn influences the physical and chemical properties of the material. Depending on the parameters, different crystal-growth directions are favored during ZnO particle synthesis, facilitating formation of one of these structural variations.1 Previously, precursors such as ZnO micropowder or dimethyl zinc were used as starting materials. Now, through the use of vacuum systems, plasma sources, and optimization of carrier and reactive gases, synthesis has moved toward higher throughput.
However, for many commercial applications these methods are still too complex or too slow. For our investigation, we made ZnO tetrapod structures by two methods: sputter deposition, and an alternative, high-throughput method we have developed based on oxidation of zinc powder by heating in a furnace in air.2 The high-throughput method gives us the possibility to synthesize a large quantity (up to the kilogram scale) of nanostructured particles at low cost.
The large quantities of particles produced by our high-throughput method allow them be used as a filler material in silicone rubber, creating a tough yet flexible composite. We are able to adjust the wetting behavior of the composite surface by varying the filling factor of tetrapod particles, allowing control of, for example, adhesion between the composite and a surface it will be applied to. The hydrophilicity of ZnO particles is governed by the prevalence of vacant oxygen sites within their crystal structure. Consequently, it is possible to adjust the wetting behavior of individual particles by controlling the number of oxygen vacancies within them. Coupled with the additional control afforded by varying the filling factor, the wetting behavior of composites can be finely tuned.
To control the number of oxygen vacancies during synthesis, ambient oxygen concentration is crucial: the lower it is during combustion, the greater the number of oxygen vacancies in the ZnO tetrapod particles or assemblies and the more hydrophilic they become. Keeping the material for several weeks in air results in filling of the vacancies by environmental oxygen, and wetting behavior subsequently switches from hydrophilic to hydrophobic. It is possible to give a hydrophobic character to the material by putting it into an oxygen atmosphere at elevated temperature. Conversely, we can increase the number of oxygen vacancies and switch the material to a hydrophilic state, either by irradiating with UV light or by heating the sample in a reducing atmosphere. We have demonstrated that a 90/10%-N2/H2 gas mixture stabilizes elevated oxygen vacancy levels in particles in hydrophilic surfaces and slows their tendency to switch back to a hydrophobic state compared with untreated particles.3
ZnO tetrapod particles are useful in a number of diverse applications. For example, we showed in collaboration with our research partners4 that they bind herpes simplex virus type-1 and neutralize viral infectivity. Increasing oxygen vacancies by irradiating with UV light lowers viral infectivity at microgram levels of ZnO particles, while also reducing the normal cellular toxic response to ZnO. This simple, low-cost synthesis method and potential medical application is just one example of numerous possible uses of ZnO micro- and nanoparticles.4
Another well-known property of ZnO is its photocatalytic behavior. Accordingly, particles of the same type as those described above have also been tested as photocatalysts of methylene blue. This intense blue dye serves as a useful model for degradable dyes that can alleviate the ecosystem-polluting effects of dye industry effluent. In the presence of certain metal oxides, including ZnO, methylene blue photodegrades to less harmful sulfate, ammonium and nitrate ions, and carbon dioxide gas. In our experiment we irradiated 50ml samples of a 100μmol/l methylene blue solution, each containing 0.1g of ZnO tetrapod microparticles, with UV light at a power density of 0.25W/cm2. This causes the methylene blue to photodegrade and the color of the solution to become less intense: see Figure 1(a–d). Measurements with a UV/visible spectrometer indicate a concentration decrease of approximately 1.15%/min. When we used smaller nanosized ZnO particles, produced by modifying the high-throughput method, we observe an improvement in the photocatalytic effect. This is due to the higher surface-to-volume ratio of the nanosized particles.
ZnO nanowires perform well as sensors.1 They are most often made by sputter deposition, a process consisting of several steps. First, the mask material is deposited onto a surface and fractures are introduced, causing cracks that expose it. A standard sputter deposition method is used to fill the cracks with ZnO, leading to a well-organized ZnO nanowire array connecting two contacts.5 However, the conductivity of such wires is too low for their use in sensors due to poor crystal quality, and requires irradiation by UV light to improve it. Wires produced by our high-throughput method, conversely, exhibit excellent crystal quality and high conductivity. This removes the need for UV treatment and makes them inherently better-suited for use in sensors.
In summary, ZnO micro- and nanostructures have physical, electronic, and antiviral properties that make them appropriate for use in fields ranging from sensing to biomedical engineering. As we are now able to produce large quantities of micro- and nanostructures, all of these applications can be further investigated and scaled up. Currently, we are developing a continuous process to further increase the throughput of our approach.2