Fabrication of TiN nanostructure as a hydrogen peroxide sensor by oblique angle deposition
© Xie et al.; licensee Springer. 2014
Received: 5 February 2014
Accepted: 21 February 2014
Published: 4 March 2014
Nanostructured titanium nitride (TiN) films with varying porosity were prepared by the oblique angle deposition technique (OAD). The porosity of films increases as the deposition angle becomes larger. The film obtained at an incident angle of 85° exhibits the best catalytic activity and sensitivity to hydrogen peroxide (H2O2). This could be attributed to its largest contact area with the electrolyte. An effective approach is thus proposed to fabricate TiN nanostructure as H2O2 sensor by OAD.
KeywordsTitanium nitride Nanostructure Oblique angle deposition technique Hydrogen peroxide Sensor
Nanostructured electrodes have stimulated great interests due to their potential applications in the areas of online real-time analysis and sensitive detection [1, 2]. To meet the demand in those applications, electrodes need to have some important criteria including large specific area, high electrochemical activity, and good biocompatibility. In recent years, nanorod arrays directly grown on a current collector have been investigated as nanostructured electrodes for biosensor application since their well-defined one-dimensional (1D) structure is favorable for electron conducting and ion accessing . Due to the exceptional combination of chemical, physical, mechanical, and electrical properties, titanium nitride (TiN) attracts much attention for their potential application in various fields such as protective coating , supercapacitors , and catalysis [6, 7]. Recent literature has also reported its potential use as electrodes for pH sensor  and hydrogen peroxide (H2O2) sensor . H2O2 is not only a byproduct of a wide range of biological processes but also an essential mediator in food, pharmaceutical, clinical, industrial, and environment analysis . Therefore, it is of great importance to achieve sensitive and accurate determination of H2O2. TiN nanorod arrays (NRAs) are expected to possess good conductivity and biocompatibility with unique 1D nanostructure, making a superb electrode for H2O2 sensor.
The TiN NRAs can be obtained by a great number of methods, such as electrospinning  and solvent-thermal synthesis . However, all the aforementioned methods need a nitridation treatment of TiO2 nanorods in ammonia atmosphere at a high temperature. Therefore, a facile and one-step fabrication method to prepare TiN NRAs is in demand. Oblique angle deposition (OAD) technique is an electron beam evaporation method, which has been used in the industry for fabricating one- or two-dimensional materials at large-scale production with relatively low cost. It provides a simple way to produce large area, uniformly aligned nanorods with controlled porosity. During the OAD process, the vapor flux is deposited onto a substrate at a large angle α with respect to the substrate normal, and a well-aligned and separated nanorod arrays can be obtained due to the self-shadowing effect [11, 12], with growth orientation toward the vapor flux direction . Moreover, the porosity can be readily tuned by varying the oblique angle, and various substrates such as glass, F-doped SnO2 (FTO), Si, etc., could be deposited on.
In this work, we report a one-step method, i.e., by OAD method using electron beam evaporation for fabricating TiN nanostructure with tunable morphologies and porosities. The TiN nanostructures are used as the electrodes for electrochemical sensing H2O2, exhibiting good performance.
Fabrication of TiN films by OAD
The TiN NRAs were deposited on silicon and FTO substrates using OAD described elsewhere . The substrates were sequentially cleaned in acetone and alcohol by ultrasonic washer and then rinsed in deionized water for 5 min each. The system was pumped down to a base pressure of 2 × 10−5 Pa, and then the TiN films were deposited at a deposition rate of 0.5 nm s−1, which was monitored by a quartz crystal microbalance. The deposition angle of TiN flux was set at ca. 0°, 60°, 70°, 80°, and 85° with respect to the substrate normal, respectively. The substrate temperature was maintained at ca. −20°C with liquid nitrogen.
The crystal structure of the TiN films was characterized by X-ray diffraction (XRD Rigaku 2500, Shibuya-ku, Japan ), which was conducted from 20° to 60° at a scanning speed of 6° min−1, using Cu Kα radiation (λ = 0.15406 nm). The morphology was characterized with a field emission scanning electron microscopy (SEM JEOL-7001 F, Akishima-shi, Japan) working at 20 kV. The microstructures of the prepared samples were characterized in detail with a transmission electron microscope (TEM JEOL-2010 F). The refractive index (ne) of the TiN films deposited at various oblique angels was measured by spectroscopic ellipsometry (J.A. Woollam, Co., Inc., Lincoln, NE, USA).
Electrochemical measurements were carried out in a 250-mL quartz cell connected to an electrochemistry workstation (CHI 660, Shanghai Chenhua Instrument, Shanghai, China). A three-electrode assembly was adopted for the test, with the TiN films as a working electrode, a Pt foil as a counter electrode, a saturated Ag/AgCl as a reference electrode, and phosphate buffer solution (PBS, pH 7.0) as the electrolyte. The current versus time was recorded at −0.2 V bias versus saturated Ag/AgCl.
Results and discussion
Herein, ne of a porous film is given by an average of air and material when the pore size is much smaller than the wavelength. Using the ne at 600 nm, the porosity of the above TiN films is calculated using the Bruggemann approximation, and the result is displayed in Figure 3b. When the deposition angle is increased, the porosity increases and reaches the maximum at the deposition angle of 85°, which is in accordance with that observed by SEM (see Figure 1).
TiN films with tunable porosity were fabricated by oblique angle deposition at different deposition angles. The porosity increases with the increase of the deposition angle due to the self-shadowing effect. All the TiN films show sensitive electrochemical catalytic property towards H2O2. The film of self-standing nanorods was obtained at the deposition angle of 85° and exhibits the best performance due to its highest porosity thus the largest effective contact area with the electrolyte. Therefore, oblique angle deposition provides a promising way to fabricate TiN nanostructure as a H2O2 sensor.
The authors are grateful to the financial support by the National Natural Science Foundation of China (grant nos. 51372135 and 51228101), the financial support by the National Basic Research Program of China (973 program, grant nos. 2013CB934301), the Research Project of Chinese Ministry of Education (grant no. 113007A), and the Tsinghua University Initiative Scientific Research Program.
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