Study of TiO 2 Nanotubes for Sensors and Integrated Devices

Transcription

1 Study of TiO 2 Nanotubes for Sensors and Integrated Devices K.F.Albertin *1, T.M.Fraga 2, M.Z. Mielli 3, M.N.P. Carreño 4, I. Pereyra 5 1 CECS, Universidade Federal do ABC - UFABC, CEP , Santo André, SP, Brazil 2, 3, 4, 5 Universidade de São Paulo - LME/PSI/EPUSP, CEP , São Paulo, SP, Brazil *1 katia.torres@ufabc.edu.br; 2 tmfraga@usp.br; 3 mzmielli@gmail.com; 4 carreno@lme.usp.br; 5 ipereyra@lme.usp.br Abstract- TiO 2 nanotube arrays obtained by Ti foil anodization are tested as ph electrodes, showing performance close to standard glass electrodes. The TiO 2 nanotube arrays were obtained by Ti foil anodization in a NH4F organic solution with different anodization voltages and times. Annealing treatment to promote nanotubes crystallization in anatase phase is also done. The obtained nanotubes samples were characterized by Raman spectroscopy and Scanning Electron Microscopy (SEM). In order to assess the viability of integrating the nanotubes to microfluidic devices, patternig of the nanotubes arrays is demonstrated and experiments showing the growth within microchannels and microcavities are also performed. Although preliminar, these results are promising for development of TiO 2 nanotube based biosensors integrated in microfluidic Lab-on-Chip systems. Keywords- TiO 2 Nanotubes; Ph Sensors; Photolithography Pattern I. INTRODUCTION Titanium dioxide is a very interesting material for a wide range of applications, varying from UV radiation skin protectors and self-cleaning coatings, to optoelectronic and electrochemical sensors. Commonly it is utilized in a nanostructured form as nanoparticles or nanotubes, due to the attainable very high effective surface area. In that way, they have been utilized in effluent treatment and photo-water splitting processes as well as in dye sensitized solar cells, hydrogen and ph sensors and biological applications among others, due to its unique photo-catalytic, optical and electrical properties [1, 2, 3, 4, 5]. TiO 2 presents three crystalline phases: rutile, anatase and brookite. The rutile phase is interesting for optical and electrical applications [4] due to its hardness and thermal stability, while anatase phase is interesting for effluent treatment, photovoltaic and biochemical applications due to its enhanced photocatalytic activity [6, 7, 8]. From the fabrication point of view, titanium dioxide nanotubes are obtained through several processes as sol-gel technique, geminated growth and hydrothermal process. Among them Ti foil anodization in fluorinated solutions has demonstrated to be one of the best techniques to obtain highly ordered and high aspect ratio structures with a well-defined and controllable diameter, wall thickness and tube length [5, 9]. From the application point of view, TiO 2 is an extremely versatile material due to its high refractive index, photo-stability, biocompatibility and dependence of its electrical and optical properties upon adsorption of specific molecules on its surface. In this way, TiO 2 has been utilized in photocatalysis, photo-splitting of water, photovoltaic and gas sensing devices, among others [3, 4, 5]. Also, due to its n-type semiconductor character and high chemical stability, TiO 2 thin films have been proposed for application as ph and hydrogen sensors [9]. Even more, its large specific surface area makes TiO 2 nanotubes, instead of bulk or thin TiO 2 films, a more attractive material for application in ph detection [10] and other electrochemical devices, where the self-cleaning properties under UV irradiation of TiO 2 nanotube arrays [11] can be also important. In this context, the development of miniaturized and high sensitive biosensors based on TiO 2 nanotubes are of great technological interest for utilization as individual devices or integrated in a more complex Lab on a Chip system [12]. For this, a challenge is the compatibility between the nanotubes fabrication process itself with the microfluidic and microelectronic fabrication techniques. Traditionally, ph measurements have been performed using glass type electrodes due to their sensitivity, selectivity, stability and long lifetime, however miniaturizing the glass type electrodes is difficult and the electrodes are fragile in the manipulation. Motivated by this, in this work TiO 2 nanotube arrays obtained by Ti foil anodization were tested as ph electrodes. Furthermore, in order to assess the viability of integrating the nanotubes either to microelectrodes or to microfluidic devices, different methods of patterning the nanotube arrays were also tested. For this purpose, nanotube samples were also produced by anodization of Ti thin films previously deposited on glass substrates. In both cases (Ti foil or Ti films on glass) the nanotubes were obtained by anodization in NH 4 F organic solutions. In order to evaluate the effect of the anodization voltage and time on the morphology and structure of the nanotubes, as well as on the ph sensing sensitivity, different values of these parameters were utilized

2 II. EXPERIMENTAL PROCEDURES A. Nanotubes Fabrication and Characterization In Figure 1 a scheme of the electrolytic cell utilized for the growth of the nanotubes is shown. Fig. 1 Electrolytic cell utilized for the anodization processes The platinum cathode and the anodic substrates (1x1 cm of 0, 5 mm Ti foil), where the TiO 2 nanotube arrays were grown, were polarized with a DC voltage source (Agilent E3649). As bath a solution of NH4F in ethylene glycol (0, 5 % in weight) and 2 % H2O, magnetically stirred during the process, was utilized. The anodization processes were carried out with different voltages and times in order to evaluate the influence of these parameters on the nanotubes morphology and structure. The anodization parameters are depicted in Table I. Since as grown nanotubes are amorphous, heat treatments at 300 C for 2 h were performed to crystallize them and improve their electrical and photocatalytic properties. TABLE I ANODIZATION VOLTAGE AND TIMES Voltage (V) Anodization Time (hours) 20 1; 3; ; ,16; 0,75; 1; 14; 19 The obtained nanotubes were characterized by scanning electron microscopy (SEM) to analyze the tubes morphology (diameter and length). Raman Spectroscopy measurements (utilizing Ar laser, 514 nm), were also performed to investigate the crystalline phase of the nanotubes. B. ph Sensors Fabrication and Characterization For the ph sensors, nanotubes were grown by anodization with voltages of 20 V (60 and 240 min), 40 V (60 min) and 60 V (10, 45 and 60 min). In sequence, the as grown nanotubes were annealed in air at 300 o C for 2 hour to obtain the anatase crystalline phase. After wire bonding, the Ti foils containing the nanotubes were encapsulated in a PDMS (polydimethylsiloxane) coating, where a small window was opened through which the test solution could contact the electrode/nanotubes. The measurements were performed using the TiO 2 nanotubes as the working electrode of a bench ph meter (HI 221, Hanna Instruments). An acid solution (HCl 0.26 M) and buffer solutions of ph 4, 7 and 10.0 were used as test solutions and the sensor s electrical response (voltage between nanotube electrode and Ag/AgCl reference electrode) was obtained. C. Patterning Experiments and Growth within Microchannels The patterning experiments on the nanotube arrays were performed to assess the feasibility of integration with microelectrodes and also to produce them inside fluidic devices. For this, a sample set of nanotubes was produced on 1 x ½ in, 2 mm thick glass slices covered with Ti films, 1 um and 0, 5 um thick, deposited by RF magnetron sputtering. In these substrates the anodization processes were carried out at 60 V for a time sufficient to consume completely the Ti film. The resulting nanotube morphology was evaluated by SEM analysis. The patterned regions are long interconnected lines with width varying from 10 to 90 m defined by photolithography utilizing three different methods: (1) anodization of a previously patterned Ti film (by photolithography), (2) photolithography of a previously produced nanotube layer and subsequent chemical etch and (3) Lift-off of a previously patterned photoresist film onto which a Ti thin film was deposited and the nanotubes were grown. Preliminary tests on nanotube growth inside microchannels were also performed. A 60 um thick SU-8 film was deposit on Ti foil and patterned by conventional photolithography, exposing selected areas of the underneath Ti, defining in this way, the microchannels. In sequence, the samples were anodized to grow the nanotubes in the exposed areas

3 III. RESULTS AND DISCUSSION A. Nanotubes Fabrication The SEM images for the nanotubes obtained with anodization voltages of 20, 40 and 60 V for 19 h are shown in Figure 2. It can be appreciated that the nanotubes are well defined with smooth walls and the diameter of the tubes increases for higher voltage values. In Figure 2 (a) a side view of the tubes grown with 20 V is shown, these tubes present a diameter varying from ~ nm and length of approximately 2 m. The tubes grown with 40 V are shown in Figure 2 (b) indicating an external diameter varying from ~ nm, these tubes have a length of ~7 m. The nanotubes grown with 60 V, shown in Figure 2 (c) presented diameter from ~ 95.7 to nm and 60 m length. (a) (b) (c) Fig. 2 SEM image (inclination 45 ) for the nanotubes: (a) grown at 20 V (diameter ~ nm), (b) grown at 40 V (diameter ~ nm). (c) grown at 60 V bottom view (diameter ~177.3 nm) A graph of nanotube diameter (obtained from SEM images) as function of the anodization voltage is presented in Figure 3, as it can be observed the diameter increases linearly with the anodization voltage. The NT formation in acidic electrolyte containing F ions is generally agreed to occur via the field assisted formation and dissolution of oxidized titanium surface. It involves two critical steps that occur simultaneously: formation of TiO 2 on the titanium surface and the dissolution of oxide. So, the growth depends on the voltage, the ph of the solution and the process time, in a way that the tubes length increases till a balance between growth and dissolution is achieved, saturating at values dependent on anodization parameters (ph, applied voltage and process time). In this work the solution ph was kept constant, so the nanotube length was controlled just with the process time and anodization voltage. Fig. 3 Nanotubes diameter values as function of anodization voltage In relation to the nanotube length it was observed, in accordance with the Literature [13], that it is controlled by the anodization time as well as by the anodization voltage. In this way for the same anodization time but different anodization voltages very different NT lengths are obtained, as shown in Figure 4a while for the same anodization voltage larger anodization times led to increasing nanotube length, as can be seen in Figure 4b

4 20 V, 19 h 40 V, 19 h 60 V, 19 h (a) 60V 1h 60V 4h 60 19h (b) Fig. 4 SEM micrograph indicating nanotubes length for (a) the same process time and different anodization voltage and (b) the same anodization voltage and different process time In Figure 5 (a) and (b) Raman spectroscopy results for nanotubes grown with 60 V for 4 h, before and after annealing in air at 300 C for 2 h, are shown. In Figure 4 (a) the spectrum before annealing is depicted, it can be observed that grown nanotubes present an amorphous structure. Fig. 5 Raman spectra for nanotubes grown with 60 V: (a) before and (b) after annealing at 400 o C On the other hand as shown in Figure 4 (b) after annealing at 300 C for 2 h the nanotubes crystallize in anatase phase (spectral bands at: 391, 511 e and 631 cm-1), since it presents the best performance for solar cells and sensors applications. B. ph Sensors Working electrodes were fabricated with the different diameter nanotube arrays produced as indicated in the experimental procedures section (A). ph measurements were performed in an acid solution of 0.26 mol/l of HCl in water (ph=3) and in buffer solutions of ph values 4, 7 and 10. In Figure 6 a typical device used as working electrode is shown, notice that a small window, through which the test solution could contact the electrode, was also defined

5 PDMS encapsulation Ti foil Nanotubes Exposed Nanotubes Fig. 6 ph Sensor: encapsulated (in PDMS) test device used as working electrode In Figure 7 the ph sensor response for the different tested working electrodes is shown. It is observed that all electrodes present almost linear behavior, except the one fabricated with NTs obtained with 20 V for 240 min, which did not respond to ph variation. Since this nanotubes exhibit the smallest diameter (between nm), this result was attributed to the poor solution exchange inside the tubes for these dimensions. Fig. 7 ph sensor response The sensitivity values (mv/ph) obtained from data in Figure 7 are shown in Table II. As it can be observed, the electrode with TiO 2 NTs grown for 60 V and for 45 min exhibits the best sensitivity value (52 mv/ph), which is close to the obtained with the standard glass electrode (57 mv/ph) and Nerst equation (59 mv/ph). Sample TABLE II ph SENSORS SENSITIVITY mv/ph ph ph ph V 60 min V-240 min V-60 min V-10 min V-45 min V-60 min These results are in accordance with the previous comment, since the NTs grown with 60 V exhibit the larger nanotube diameter. These nanotube dimensions are sufficiently large to allow the solution to penetrate inside and outside the tubes, promoting a larger surface contact area in comparison to the other samples. C. Pattern Experiments Among the different pattern procedures, the best results were obtained by Method (1), anodization of a previously patterned Ti film. A typical result is shown in Figure 8. Note that, although the anodization occurs along a thin and long line shape Ti electrode, the nanotubes grow with an excellent uniformity and the contours of the nanotube regions are pretty well defined, even for the thinner line with 10 um (see Figure 8a). In Figure 8 (b, c and d) a closer view permit to appreciate the uniformity of the nanotubes array

6 (a) (b) (c) Fig. 8 (a) SEM image of the linear regions with nanotubes obtained on Ti lines defined by standard photolitography etching the Ti film with DLV solution. (b), (c) and (d) closer view of the patterned line of TiO 2 nanotubes array For Method (2), photolithography of a previously produced nanotube layer, the results were not satisfactory. This occurs due to the porous character of the nanotubes layers, which permits the HF based etching solution penetrate underneath the photoresist, destroying the nanotube array, as shown in Figure 9a. The results for Method (3), nanotubes patterned by Liftoff photolithography process, were also satisfactory although the contours were not as well defined as by Method (1). See Figure 9b. (d) (a) Patterning method 2 (b) Patterning method 3 Fig. 9 SEM image of patterned nanotubes utilizing patterning methods (2) and (3). (a) In method (2) the photoresist is not a suitable masking material and the nanotubes are destroyed at the etching edge. (b) In method (3) the nanotubes are preserved but contours still are not well defined

7 The success of the patterning experiments leads to preliminary tests on nanotubes growth inside microchannels, defined on a Ti foil by a thick (60 um) pre patterned SU-8 film (a negative photoresist). The SU-8 film was deposit on the Ti foil and patterned by conventional photolithography, exposing selected areas of the underneath Ti. In sequence, this sample was anodized to grow the nanotubes in the exposed areas. Some images of the final result are shown in Figure 10 (a, b, c and d). In Figure 10a, an overview of the microfluidic device, composed by a microchannel and two circular microchambers, is shown. In Figure 10b it can be appreciated that the entire chamber base is filled with a uniform layer of nanotubes. However, in the closer view shown in Figures 10c and 10d, cracks can be observed on the nanotube array as well as small defects on its surface. These cracks are attributed to the crystalline domains on the Ti foil, which induce defects during the nanotube formation, and represent a disadvantage to utilize this type of substrate. Although preliminary, these are very encouraging and novel results towards TiO 2 nanotube based integrated devices. To our knowledge, up to date the literature reports just one similar result on NT arrays grown on Ti lines (deposited by e-beam) that has been reported in the Literature [14] whilst no reports on TiO 2 NT arrays growth within microchannels can be found. (a) (b) (c) (d) Fig. 10 SEM image of nanotubes process by alternative method utilizing SU-8 IV. CONCLUSION Ordered TiO 2 nanotube arrays with diameter and length controlled by the anodization voltage and time were obtained with the presented experimental process. Even more, the nanotubes diameter depends linearly on the utilized voltage. It was shown that these NT arrays present good sensitivity as working electrode in ph measurements, the best value (52.25 mv/ph) was obtained for 60 V grown NT. This result is attributed to the larger nanotube diameter allowing solution penetration inside the tubes and thus promoting a larger surface contact area. Even though this sensitivity is a little lower than the obtained of standard glass electrodes, the results are still promising since it was also shown that the nanotube arrays can be efficiently patterned, that is, fabricated in a controlled way in selected areas of a glass substrate. Furthermore, preliminary tests also

8 showed that the nanotubes can be grown inside microfluidc circuits, which is a very promising result towards TiO 2 nanotube based sensors integrated in microfluidic systems ( Lab-on-Chip ). ACKNOWLEDGEMENTS The authors are grateful to CNPq (Process / and Process /90-8) and FAPESP for financial support, to LEM-IQUSP for Raman measurements and to Adir José Moreira of LSI/POLI-USP for the SEM measurements. REFERENCES [1] Yang L., Luo S., Cai Q. and Yao S., A review on TiO 2 nanotube arrays: Fabrication, properties, and sensing applications, Chinese Science Bulletin, Vol.55, Issue 4-5, (2010) 331. [2] S. M. Gupta and M. Tripathi, A review of TiO 2 nanoparticles, Chinese Science Bulletin, Vol. 56, Issue 16, (2011) [3] G. K. Mor, O. K. Varghese, M. Paulose, K. Shankar and C. A. Grimes, A Review on Highly Ordered, Vertically Oriented TiO 2 Nanotube Arrays: Fabrication, Material Properties and Solar Energy Applications, Solar Energy Materials & Solar Cells 90, (2006) [4] L. K. Than, M. K. Kumar, W. W. An and H. Gao, Transparent, Well-Aligned TiO 2 Nanotube Arrays with Controllable Dimensions on Glass Sbstrates for Photocatalysis Applications, Applied Materials & Interfaces 2, (2010) 498. [5] A.W. Tan, B. Pingguan-Murphy, R. Ahmad and S.A. Akbar, Review of titania nanotubes: Fabrication and cellular response, Ceramics International, 38 (2012) [6] M. Gratzel, Photoelectrochemical cells, Nature 414, (2001) [7] G. Yu, X.J. Zhao, Q.N. Zhao, Photocatalytic activity of nanometer TiO 2 thin films prepared by the sol-gel method, Mater. Chem. Phys. 69 (2001) 25. [8] P. Roy, S. Berger and P. Schmuki, TiO 2 Nanotubes: Synthesis and Applications, Angew. Chem. Int., Ed. 2011, 50 (2011) [9] Nanotubes: Synthesis and Applications, Angew. Chem. Int., Ed. 2011, 50 (2011) [10] T. Pan, J. Lin, A TiO 2 /Er2O3 stacked electrolyte/insulator/semiconductor film ph-sensor for the detection of urea, Sens. Actuators B: Chem. 138 (2009) 474. [11] R. Zhao, M. Xu, J. Wang, G. Chen, A ph sensor based on the TiO 2 nanotube array modified Ti electrode, Electrochimica Acta 55 (2010) [12] G.K. Mor, M. A. Carvalho, O.K. Varghese, M.V. Pishko, C. A. Grimes, A room-temperature TiO 2 nanotube hydrogen sensor able to self-clean photoactively from environmental contamination, J. Mater. Res., Vol.19, No.2 (2004) [13] Chen-Fu Lin et al., Microfluidic ph-sensing chips integrated with pneumatic fluid-control devices, Biosensors and Biolectronics, 21 (2006) [14] Q. Cai, M. Paulose, O. K. Varghese and C. A. Grimes, The Effect of Electrolyte Composition on the Fabrication of Self-Organized Titanium Oxide Nanotube Arrays by Anodic Oxidation, Journal of Materials Research, Vol. 20, Issue 01 (2005) [15] K. N. Chappanda, Y. R. Smith, M. Misra and S. K. Mohanty, Site-specific and patterned growth of TiO 2 nanotube arrays from e-beam evaporated thin titanium film on Si wafer, Nanotechnology 23 (2012) (8pp). Katia F. Albertin, Graduated in Materials, Processes and Electronic Components at Technology College of São Paulo (2000), Ph.D. in Electrical Engineering (Microelectronics) by Polytechnical School at University of São Paulo (2007). Currently teaches and researches at the Center for Engineering, Modeling and Applied Social Sciences (CECS) of Federal University of ABC, São Paulo, Brasil. She has worked with PECVD thin films, fabrication and characterization of semiconductor devices and currently is working with sensors based on nanostructural materials (TiO 2 and TiO x N y ). Tiago M. Fraga, Graduated in Materials, Processes and Electronic Components for the Technology College of São Paulo (2009), obtained her Masters degree in Electrical Engineering (Microeletronics) by Polytechnic School of the University of São Paulo University (2012) working with TiO 2 nanotube. Currently teaches at ENIAC College, Guarulhos, Brasil. Murilo Z. Mielli, Electrical Engineer (2009) by the Polytechnic School of the University of São Paulo, obtained his Masters degree in Electrical Engineering (Microelectronics) in 2012, at the same Polytechnic School of the University of São Paulo. He has worked with the development of embedded electronics circuits, microfluidic sensors and systems (Lab-on-Chip) and aeronautics. Currently works at brazilian aerospace company EMBRAER. Marcelo N.P. Carreño, is B.S. in Physics (1985) by the Institute of Physics of the University of São Paulo and Ph.D. (1994) in Electrical Engineering (Microelectronics) by Polytechnical School at the same University. Currently he is Associated Professor in the Electronic Systems Engineering Department at Polytechnical School of the University of São Paulo, where conducts research in the development and applications on Microfluidic and Lab on Chip Systems, Micro-Electro-Mechanical Systems (MEMS), Sensors and integrated systems for Electronic Instrumentation and Software Development (scientific computing and technology of information). He is member of IEEE. Inés Pereyra, graduated in Physics (1973) at the University of Buenos Aires, Argentina, is PhD - Physics from the University of Delaware, USA, (1980). She did postdoctoral studies at the Institute of Energy Conversion (at Delaware) and has extensive experience working with solar cells, semiconductors devices, semiconductor and dielectrics thin films, and more recently, nanostructures materials. Currently is full professor at the Polytechnic School of the University of São Paulo, where he founded and led the Group of New Materials and Devices. She is also, general coordinator of Microelectronics Laboratory (LME) of the Polytechnic School of USP, and also Vice Chair of the Department of Electronic Systems Engineering from EPUSP. She is member of IEEE