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Rapid growth of zinc oxide nanotube-nanowire hybrid architectures and their use in breast cancer related volatile organics detection Giwan Katwal, Maggie Paulose, Irene A Rusakova, James E Martinez , and Oomman K Varghese Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b05280 • Publication Date (Web): 05 Apr 2016 Downloaded from http://pubs.acs.org on April 6, 2016
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Nano Letters
Rapid growth of zinc oxide nanotube-nanowire hybrid architectures and their use in breast cancer related volatile organics detection Giwan Katwal1, Maggie Paulose1, Irene A. Rusakova2, James E. Martinez3 and Oomman K. Varghese1* 1
Nanomaterials and Research Laboratory, Department of Physics, University of Houston,
Houston, Texas 77204, USA. 2
Department of Physics and Texas Center for Superconductivity, University of Houston,
Houston, Texas 77204, USA. 3
Jacobs Technology, Structural Engineering, NASA Johnson Space Center, Houston, Texas
77058, USA. Abstract: A simple direct method for the rapid fabrication of zinc oxide nanotube-nanowire hybrid structure in an environmentally friendly way is described here. Zinc foils were anodized in an aqueous solution of washing soda (Na2CO3) and baking soda (NaHCO3) at room temperature in order to obtain the hybrid architecture. At the beginning of the process nanowires were formed on the substrate. The wider nanowires transformed into nanotubes in about a minute and grew in length with time. The morphological integrity was maintained upon heat treatment at temperatures up to the melting point of the substrate (~ 400 °C) except that the nanotube wall became porous. The chemiresistor devices fabricated using the heat treated structure exhibited high response to low concentration volatile organic compounds (VOCs) that are considered markers for breast cancer. The response was not significantly affected by high humidity or presence of hydrogen, methane or carbon dioxide. The devices are expected to find use as breath sensors for non-invasive early detection of breast cancer. Keywords: zinc oxide, nanotube, anodization, volatile organic compound, breast cancer, breath sensor
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Very few binary materials have received the attention that zinc oxide earned due to its enormous potential as a viable material for a wide range of applications pertaining to everyday lives of people. The interest in ZnO originated essentially from its diverse properties that include antiseptic and astringent characteristics and biocompatibility useful for medical applications, catalytic activities appropriate for chemical processes such as methanol synthesis, unique optical and semiconducting characteristics attractive for current and new generation electronic, optoelectronic, environmental sensing and photovoltaic devices and piezoelectric, ferroelectric and thermoelectric behavior suitable for transducer and energy harvester development1-3.The technological relevance of ZnO as a semiconductor4 is linked to its wide direct band gap (~3.37 eV), high exciton binding energy (~ 60 meV) that facilitates excitonic emission processes above room temperature, high electron mobility (~ 400 cm2 V-1 s-1)5, the tunability of n-type conductivity from semi-insulating to near metallic by doping and the possibility of creating ptype conductivity6. It has been proposed for next generation optoelectronic devices such as ultraviolet (UV) light emitting diodes7 (LED) and detectors8 as well as lasers9. Nonetheless, the low material cost, higher abundance, ease in fabricating high crystal quality films even on the lattice mismatched substrates and compatibility with simple mild wet etching routes are real drivers for it being considered as an alternative material for gallium nitride and related materials4. Although zinc oxide has been in practical use for several decades, there has been a renewed interest in recent years that can be attributed primarily to the emergence of nanostructures exhibiting unique properties and availability of scalable technologies for fabrication. Due to the crystallographic peculiarity of zinc oxide, an unusually wide range of nano-architectures with different shapes and sizes could be fabricated2,10-12. Among these, nanowires (or nanocolumns or nanorods) of zinc oxide grown using various methods were the first to catch wide-spread attention13-15. Phenomena such as quantum confinement effects16 and room temperature ultraviolet lasing17 were demonstrated and a variety of devices that include dye sensitized solar cells18, broadband LEDs19, UV detectors20 and piezoelectric nanogenerators21 were developed using nanowires. The early interest was largely limited to nanowires as these were relatively easy to grow using a wide variety of methods2. While ZnO nanowires are attractive, especially for electronic and optoelectronc device applications, due to the high surface area and appropriate carrier transport properties, ZnO 2 ACS Paragon Plus Environment
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nanotubes are a more useful one-dimensional architecture. The nanotube geometry provides much higher surface area than wires, for a given diameter, as the tubes possess both inner and outer surfaces. Furthermore, unlike the case of wires, the walls of the nanotubes can be thinned without sacrificing the structural integrity so that the charge carriers get facile pathways to reach the surface to take part in reactions or travel along the length. As a result, the nanotube architecture could outperform nanowires particularly in applications such as environmental sensing and catalysis/photocatalysis where surface area and carrier availability for surface reactions are critical. The ZnO nanowire growth, in general, is energetically favored in most of the fabrication conditions and hence, specific processes involving elevated temperatures/pressures or multiple steps involving transformation of nanowires/particles have been used for the growth of ZnO nanotubes. Vayssieres et al22 used hydrothermal conditions to decompose a Zn2+ amino complex and fabricated ZnO microtube arrays on conducting glass substrates. Before long Zhang et al23 reported decomposing Zn(NH3)42+ at 180 °C hydrothermally in ethanol for growing dispersed tubular zinc oxide tubes. Since then, different forms of hydrothermal route, single as well as multistep processes, have been used for ZnO nanotube growth24-26. Pyrolysis of organic compounds of zinc27, vapor phase growth28, metal organic chemical vapor deposition (MOCVD)29, template assisted multistep processes30 and electrochemical deposition31 were also tried for the growth of zinc oxide nanotubes. She et al32 developed a two-step process involving electrochemical deposition of ZnO nanorod arrays on conducting glass in an aqueous solution of ZnCl2 and KCl at 85 °C followed by reversing the potential to give a small positive bias to the nanorod electrode so that the nanorods are electrochemically etched to form nanotubes. Others replaced electrochemical etching of electrodeposited nanowires with chemical etching at temperatures close to 80 °C and formed ZnO nanotubes33. All these techniques require elevated temperatures or high pressure or both for the nanotube growth. Here we describe a simple environmentally benign template and catalyst free single step method to grow ZnO nanotube/nanowire hybrid structures rapidly. The process involves anodization of zinc in an aqueous electrolyte consisting of small amounts of sodium bicarbonate (baking soda) and sodium carbonate (washing soda). We note that the previous attempts to anodize zinc using bicarbonate electrolyte failed to yield nanotubes 34-36 while other electrolytes such as hydrofluoric acid gave disrupted nanotubular structures or random architectures37,38. In 3 ACS Paragon Plus Environment
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contrast, we obtained nanotubes without involving any toxic, hazardous or highly corrosive chemicals and hence, the process is environmentally friendly. The process does not require high pressure or elevated temperature conditions and it is highly scalable. Furthermore, the nanotubes can be grown from zinc on any substrates. The ZnO nanotube-nanowire hybrid structure was found to be highly useful for device applications. ZnO is known to change electrical resistance in presence of gases and organic compounds39. Chemisorption of the gases on the surface of the semiconductor material is generally responsible for this phenomenon40. Upon chemisorption, the oxidizing gases such as oxygen trap the electrons near the surface region of the n-type zinc oxide and reduce the resistance while reducing gases such as hydrogen donate electrons into the lattice causing the resistance to decrease. The analyte gases/vapors are activated for direct chemisorption onto ZnO surface by the highly active surface sites or temperature or catalysts or a combination of these factors. Surface of ZnO exposed to air contains chemisorbed oxygen and the test gases can interact through these oxygen species also. Some gases, hydrogen for example, can diffuse into the lattice and change the carrier concentration. Desorption of these gases from the material brings the resistance back to the original level. The chemical sensors based on ZnO correlates the magnitude of resistance change with the concentration of the analyte gas/vapor. The ability of the sensor to detect the gas/vapor, called sensitivity, is defined as the ratio of the resistance change to the original resistance of the sensing element. While sensitivity of bulk ZnO is not adequate, in general, for detecting small amounts of gases/vapors, nanostructured ZnO has been reported to exhibit promising characteristics41. Recently, attempts have been made to use nanostructured ZnO for detecting volatile organic compounds (VOC) at low concentrations because of the realization that the presence of these compounds in exhaled gas can be related to disease state of the body42,43. As discussed in detail later, the nanotube-nanowire hybrid structure of ZnO fabricated by anodic oxidation exhibited very high sensitivity to low concentration VOCs that are considered markers for breast cancer. The initial anodization experiments were conducted in an aqueous solution consisting of only sodium bicarbonate (NaHCO3). Figure 1a shows the nanotubes obtained on a zinc foil (2 cm x 1 cm) when the anodization was conducted initially at 10 V in 6.8 mM NaHCO3 in water followed by a second anodization at 20 V in 0.2 mM NaHCO3 in water. The foil was covered predominantly by nanowires. The nanotubes were found only at the edges (
