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A titanium nitride nanotube array for potentiometric sensing of pH† Mengyang Liu,a Yanling Ma,a Lei Su,b Kuo-Chih Choua and Xinmei Hou*a A titanium nitride nanotube array (TiN NTA) electrode was fabricated through anodic oxidation of titanium and reduction and nitridation of TiO2 NTA. The microstructure of TiN NTA was characterized to be uniform with inner diameters of about 120 nm, a wall thickness of 15–20 nm and an average length of 10 μm. Open-circuit potentials were measured to evaluate the TiN TNA electrode related to pH sensitivity, response time, stability, selectivity, hysteresis and reproducibility in the pH range of 2.0–11.0 at 20 ± 1 °C.
Received 30th December 2015, Accepted 14th January 2016
The prepared TiN NTA electrode exhibits a near-Nernstian slope of 55.33 mV per pH with the correlation
DOI: 10.1039/c5an02675j
coefficient value of 0.995. It shows good selectivity for H+ ions in the presence of cations and anions, especially in fluoride-containing media. It also has good stability and reproducibility with a response time
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of 4.4 s. These make it a promising candidate as a pH electrode sensor.
Introduction The accurate and reliable measurement of pH in chemical, biological, clinical, industrial and environmental samples is extensively required.1,2 Measuring pH is also essential for determining the chemical characteristic of a substance because it is the first step toward managing chemical reactions. Due to the importance of this parameter, many different options exist for determining the pH level of a solution, ranging from the simple and inexpensive pH strip test to the much more complex and expensive pH meters.3,4 The most abundant electrochemical systems for pH sensing are based upon either potentiometric5 or amperometric measurements. For instance, ion selective electrodes,6,7 ion-selective field effect transistors,8–10 metal oxides including IrO2,11–15 RuO2,16 WO3,17–23 TiO2 and SnO2 etc.,24–27 and conducting polymer pH sensors,28–30 have been developed to impart high selectivity by virtue of different proton-recognition processes. Typically the glass pH electrode has had remarkable success for several decades due to an ideal Nernstian response independent of redox interference, a broad response range and a fast and stable response.15,25,26 One major drawback with these types of devices is that they often suffer from instability or potential drift and therefore require constant recalibration.14,16 One tech-
a State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China. E-mail: [email protected] b Research Center for Bioengineering and Sensing Technology, School of Chemistry and Biological Engineering, University of Science and Technology Beijing, Beijing 100083, China † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c5an02675j
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nique which has been utilized to overcome this potential drift is to measure the potential difference between two redox peaks, the first being the alkyl ferrocene with a pH insensitive redox wave and the second a hydroquinone alkylthiol with a pH sensitive redox wave.28 While it has been possible to overcome the design complications inherent in the self-assembled monolayer, these systems require multiple components to be mixed separately, which makes the manufacturing process complex. Nitrides have shown greater chemical inertness and electron mobility compared with most chemical sensor materials. Different from other semiconductor materials, nitrides have a greater energy gap. As a kind of typical nitride, titanium nitride (TiN) possesses many super properties including highmelting, high hardness, high chemical stability, excellent thermal conductivity and electrical conductivity of about 102 S m−1.31–34 It has been proved that TiN can act well as a suitable conducting material for potential applications of electronic devices, field emission and electrochemical capacitors.35–39 It is also found that nano-TiN based composites exhibit enhanced electro-catalytic performance.40 In principle, TiN has a typical crystal structure. Due to the existence of interstitial atoms, there are many holes in the lattice. A hydrogen ion can get into the lattice of TiN and diffuse between holes so that the potential difference appears inside and outside the crystal. Therefore, TiN can be considered as a potentiometric pH sensor. In this work, a titanium nanotube array (NTA) for pH sensing was fabricated via two steps. Firstly, vertically aligned TiO2 nanotubes through anodic oxidation of titanium were obtained. Then TiO2 nanotubes were directly reduced and nitrided under a nitrogen/hydrogen (5% hydrogen) flow to obtain TiN nanotubes. To the best of our knowledge, this is the first pH sensor based on a nitride array reported in the
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literature. Considering its chemical stability even at high temperatures, this work would open up the door to the fabrication of a miniaturized solid state pH sensor, which is crucial for both clinical and environmental applications.
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Experimental section Fabrication of TiN NTA electrode TiN NTA was synthesized by reduction and nitridation of TiO2 nanotubes. Firstly, a TiO2 nanotube array was fabricated by the anodic oxidation process at 30 V for 2 h in a two-electrode reaction system, which consisted of a working electrode of a titanium piece with the dimensions of 10 × 10 × 0.5 mm and a counter electrode of platinum foil in an aqueous solution containing glycol (97 vol%) and NH4F (0.5 wt%) solution. The obtained TiO2 was annealed at 450 °C for 2 h in air to obtain the anatase phase. Then the anatase TiO2 nanotube array was reduced and nitrided at 900 °C for 2 h under a nitrogen/hydrogen (5% hydrogen) flow in a tubular furnace. Finally, the sample was washed with deionized water and then dried to obtain the TiN NTA electrode with the dimensions of 10 × 10 × 0.5 mm. To investigate the effect of the morphology of TiN on the electrochemical behavior, a TiN nanopowder (TiN NP) electrode was also fabricated. TiN nanopowder was prepared involving two sequential reaction steps41: (1) the formation of a complex precursor through low temperature combustion synthesis (LCS) using TiCl4, HNO3, NH3·H2O, urea and glucose as raw materials. (2) Carbon-thermal reduction and nitridation of the precursor to obtain TiN nanopowder at 1050–1200 °C for 2 h under a flowing nitrogen atmosphere. The TiN nanopowder electrode was prepared by mixing TiN nanopowder and polyvinylidene fluoride (PVDF) with the mass ratio of 8 : 2 in N-methyl-2pyrrolidone (NMP) to form a stable slurry. Then the slurry was coated on a Ti sheet with the coating mass of ∼0.1 mg (10 × 10mm). Then the electrode was heated at 120 °C for 3 h to evaporate the solvent for electrochemical measurements.
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with the scan rate ranging from 10 to 500 mV s−1. Electrochemical impedance spectra (EIS) were recorded from 10 mHz to 100 kHz with an alternate potential amplitude of 5 mV. Opencircuit potential measurements were conducted to evaluate the pH response of the electrodes at various pH levels at room temperature. For the preparation of pH solution, Britton– Robinson (B–R) buffer solution containing 0.04 mol L−1 phosphoric acid, boric acid and acetic acid was compounded. The solution pH values were adjusted by addition of 0.2 mol L−1 NaOH solution and evaluated using a commercial pH meter.
Results and discussion Microstructure characterization The XRD patterns of TiO2 NTA and TiN NTA supported on the Ti sheet are shown in Fig. S1.† Both of the XRD patterns reveal the characteristic diffraction peaks at 40.2°, 53° and 70°, which can be ascribed to the Ti substrate. In view of TiO2 NTA, the peaks at 25.3°, 37.8°and 55.1° can be indexed as the (101), (004) and (211) crystal planes of anatase TiO2. While the peaks for TiN NTA at 36.8°, 43.3° and 62.5° can be indexed as the (111), (200) and (220) crystal planes of cubic TiN. The structures of TiO2 NTA and TiN NTA are characterized using Raman spectra. As shown in Fig. S2,† the TiO2 NTA sample has the characteristic Raman peaks at 400 cm−1, 515 cm−1 and 640 cm−1, which can be indexed as the vibration mode of B2g of anatase TiO2.42 On comparison, TiN NTA has the characteristic peak at 576 cm−1,43 indicating that anatase TiO2 is completely converted to cubic TiN. The SEM photographs of TiN NTA supported on the Ti sheet are shown in Fig. 1. It can be seen that TiN NTA is vertically oriented and highly ordered with an inner diameter of about 120 nm and a wall thickness of 15–20 nm (Fig. 1a and b). Fig. 1c shows the cross-sectional SEM image of TiN NTA. The total length of the TiN nanotubes is approximately 10 μm. To further investigate the microstructure of TiN NTA, TEM
Characterization and measurement X-ray diffraction patterns were performed using an X-ray diffractometer (XRD, DMAX-RB, Japan) with the use of a Cu-Kα radiation source. The surface morphology and microstructure of TiN NTA were characterized by using a field emission scanning electron microscope with energy dispersive spectroscopy (FESEM and EDS, JSM-6701F, Japan) and transmission electron microscopy (TEM, Tecnai F30, America). The Raman spectra were measured on a Raman spectrometer (Raman, LabRAM HR Evolution, France) using a He–Ne laser that emitted the sample at 785 nm excitation and the integration time for each sample was 10 s. Electrochemical measurements were carried out in a threeelectrode system using CHI660D electrochemical workstation. A platinum wire and Ag/AgCl electrode were used as the counter electrode and reference electrode, respectively. Cyclic voltammetry (CV) was measured in 0.5 mol L−1 Na2SO4 solution containing 0.01 mol L−1 ferrocyanide and ferricyanide
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Fig. 1 Morphology of TiN nanotube array: (a) and (b) a typical top view; (c) a cross-sectional view; (d) TEM images; (e) SEAD pattern; (f ) EDS spectra.
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with selected area electron diffraction (SEAD) was carried out. It clearly exhibits the character of the tube (Fig. 1d). The SAED pattern (Fig. 1e) shows three characteristic diffraction rings from inner to outer corresponding to (111), (200) and (220) planes of TiN. Mapping EDS analysis of TiN NTA shows that the formation of TiN NTA and no other phase exists (Fig. 1f ). The hollow structure can provide a large electrochemical surface and thus an excellent ion diffusion path. Electrochemical characterization of pH electrode The CV curves of TiN NTA, TiO2 NTA and Ti in 0.5 mol L−1 Na2SO4 electrolytes containing 0.01 mol L−1 Fe(CN)63− are shown in Fig. 2. On comparison, the anodic and cathodic peaks associated with the oxidation and reduction of the ferricyanide–ferrocyanide couple obviously appear at the TiN NTAsolution interface. While no obvious anodic and cathodic peaks are observed at either the TiO2 NTA-solution interface or Ti-solution interface. In addition, CV curves of TiO2 NTA and Ti are similar because Ti is prone to be oxidized. This indicates that TiN NTA possesses better electrical conductivity. The CV curves of TiN NTA at different scan rates in 0.5 mol L−1 Na2SO4 electrolytes containing 0.01 mol L−1 Fe(CN)63− were further measured. As shown in Fig. 3, the anodic and cathodic peaks associated with the oxidation and reduction of the ferricyanide–ferrocyanide couple respectively obviously appear at the TiN NTA-solution interface. The anodic and cathodic peak currents increase linearly with the square of scan rates as shown in the inset of Fig. 3, exhibiting that the electrode reaction is diffusion-controlled. The peak potential increases with the scan rates. However, at a scan rate of 0.01 V s−1, the peak current of anodic-to-cathodic ratio is roughly 1 and the potential difference between the anode and cathode is 65 mV, almost fitting for ΔEp = Epa − Epc = 59/n (Epa and Epc represent the anodic and cathodic peak potential respectively, n represents the number of electrons transferred). We can
Fig. 2 CV curves of Ti, TiO2 NTA, TiN NTA electrode at the scan rate of 0.1 V s−1 in 0.5 mol L−1 Na2SO4 electrolytes containing 0.01 mol L−1 Fe (CN)63−.
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Fig. 3 CV curves of TiN NTA electrode at different scan rates in 0.5 mol L−1 Na2SO4 electrolytes containing 0.01 mol L−1 Fe(CN)63−; The inset is the plot of the anodic/cathodic peak current versus the square of the scan rate.
deduce that the reaction is quasi-reversible and the electrode shows good electrical conductivity. The EIS plots of TiO2 NTA, Ti and TiN NTA in 1 mol L−1 KOH44 are shown in Fig. 4. The resistance values include the solution resistance (Rs) and the charge transfer resistance (Rct). It can be seen that the EIS of TiO2 NTA shows a typical semicircle at both high and low frequency. The large diameter of the semicircle represents a high charge transfer resistance. On comparison, the EIS result of Ti exhibits a similar semicircle with a larger diameter due to its prone to be oxidized. As for the EIS result of TiN NTA, it consists of a nearly semicircle with a small diameter in the high frequency range and a near vertical line to the abscissa axis in the low frequency range. The semicircle with a small diameter in the high frequency range indicates that the charge transfer resistance at the interface of TiN NTA-solution is very low and the near vertical line to the abscissa axis in the low frequency range is caused by a diffusion-controlled process. From the above results, TiN NTA exhibits a small value of Rct (0.48 Ω) and Rs (3.2 Ω).44
Fig. 4 EIS plots of Ti (black line), TiO2 NTA (red line) and TiN NTA (blue line) in 1 mol L−1 KOH.
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The EIS of TiN NTA in 1 mol L−1 H2SO4 (blue line), 1 mol L KOH (black line) and 1 mol L−1 Na2SO4 (red line) are measured and the results are shown in Fig. S3.† It can be seen that the EIS behavior of TiN NTA in the three solutions are similar, consisting of a nearly semicircle in the high frequency range and a near vertical line to the abscissa axis in the low frequency range. The absence of the complete semicircles indicates that the charge transfer resistance at the interface of TiN NTA is so low that its electrochemical reaction resistance becomes almost negligible in the three different electrolytes. This is ascribed to the superior electrical conductivity of TiN NTA. The analogous slope of the beelines indicates a similar resistance and capacitance character in the three different kinds of electrolytes. The EIS results demonstrate the electrochemical behavior of TiN NTA is diffusion controlled, which coincides with the CV curves. Herein, the Rs of the TiN NTA electrode under the three conditions are calculated to be 1.5 Ω (1 mol L−1 H2SO4), 3.2 Ω (1 mol L−1 KOH), 6.3 Ω (1 mol L−1 Na2SO4) and the Rct are 0.52 Ω (1 mol L−1 H2SO4), 0.48 Ω (1 mol L−1 KOH), and 0.76 Ω (1 mol L−1 Na2SO4) respectively.44
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−1
Sensor performance Sensitivity. It is known that TiN belongs to a kind of ionic selectivity electrode.45 The potential difference appears as a result of different ion activities at the interface. The electrode potential between the interface and solution can be expressed as follows according to the Nernst equation: E ¼ E° þ ðRT=nF Þ ln
aH aH ′
Fig. 5 pH sensitivity of TiN NTA, TiN NP and TiO2 NTA electrodes tested in B–R solutions in the pH range of 2.0 to 11.0.
measurement in B–R buffer solutions in all pH range of 2.0 to 11.0 was conducted to calculate the response time. The data are obtained at 0.05 s interval. The dynamic response in all pH range of 2.0 to 11.0 is shown in Fig. 6. The dynamic response
ð1Þ
where E is the potential difference between the working electrode and reference electrode, E° is the standard reduction potential, R is the gas constant, T is the temperature, F is Faraday’s constant, n is the number of moles of electrons transferred, aH and a′H are the activities of the hydrogen ion outside and inside the membrane respectively. The sensitivity of the TiN electrode was validated by immersing the electrode in B–R buffer solution of pH 2.0–11.0 at 20 °C and the data are repeated 3 times. For comparison, the pH sensitivity of TiN nanopowder (NP)41 synthesized through a low-temperature-self-propagating-process, TiO2 NTA were also tested. As for the Ti electrode, it tends to be oxidized and its pH sensitivity is not measured. The pH sensitivity of TiN NTA, TiN NP and TiO2 NTA electrodes was determined as shown in Fig. 5. The slopes of the curves of TiN NTA, TiN NP and TiO2 NTA are calculated to be 55.33 mV per pH, 46.48 mV per pH and 44.8 mV per pH with the correlation coefficient to be 0.995, 0.992, and 0.993 respectively. On comparison, the TiN NTA electrode possesses the highest pH sensitivity. Response time The response time of a pH sensor is defined as the transit time required for its potential to reach 90% of an equilibrium value after immersing the sensor in test solution. The response time of an ion-selective electrode is an important factor for any analytical application. In this experiment, continuous
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Fig. 6 Dynamic responses of the TiN NTA and TiN NP electrode for step changes of all pH range from 2.0 to 11.0.
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time thus obtained was about 4.4 s as for TiN NTA and 5.2 s for TiN nanopowder (TiN NP). The response time almost keeps constant in the pH range of 2.0 to 11.0. Both the values are comparable to, even smaller than that of other pH electrodes reported in the literature, such as WO3 nanoparticles (3–8 s),1 and poly vinyl chloride membrane (6–7 s).27 They were a little larger than that of iridium oxide based (within 2 s)14 and ruthenium oxide (3 s).16 The response time depends on the porous properties of the sensing film which traps the ions to a certain extent. However, the cleanliness of the electrode also makes sense. Stability For stability testing, the electrode was immersed in the buffer solutions with three different pH values for 200 s. Fig. 7 shows the measured potential versus time for different pH values. It can be seen that the pH value fluctuated slightly as the time goes on but tends to be stable finally. A similar phenomenon can be observed in other pH values. This potential drift can be attributed to the diffusion and trapping of hydrogen ions through the holes and at the boundaries of the electrode. In addition, the potential response still retained more than 94% of the initial response after being stored at 25 °C for a month. Selectivity Considering various ions and compounds typically exist randomly in practice, the electrode selectivity among possible cations and anions interference is investigated. The selectivity tests of the electrode were conducted with the fixed interference method. The interference effects can be calculated according to the following equation:46 E ¼ E° þ
i RT h ln ai þ Kij pot aj ðzi =zj Þ zi F
ð2Þ
where E is the observed potentiometric response, E° is the standard potential, R is the gas constant, T is the temperature,
Fig. 7
The stability of the TiN NTA electrode.
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Table 1 Selectivity coefficients determined by the use of the fixed interference method (FIM)
Ion
Concentration
ΔV
Na+ K+ Cl− F−
1 mol L−1 1 mol L−1 1 mol L−1 1 mol L−1
8 mV 5 mV 7 mV 3 mV
F is Faraday’s constant, z is the charge of ion i or j, ai and aj are the activities of the primary and interfering ions, respectively. Kijpot is the Nikolskii ( potentiometric selective) coefficient. The selectivity coefficients of the TiN NTA electrode among possible cations and anions interference are summarized in Table 1. It shows that the potential changes of the TiN NTA electrode are less than 8 mV, especially for F−, demonstrating better selectivity. Hysteresis The electrode potential may be different even if the ion activity is the same because it is related to the solution composition that was brought in contact before. In the experiment, the pH value was tested continuously from 2.02 to 11.00 in three cycles for the same TiN NTA electrode. The result is shown in Fig. 8. It can be seen that the three curves is almost coincidence. The hysteresis appears more obviously at the range of pH = 2.02–3.00 and pH = 10.00–11.00. The main reason is possibly that H+ in the hole of TiN will achieve a new balance with H+ in the solution when the electrode is shifted among different solutions. In addition, the experimental conditions and testing area of the surface of the electrode also cause hysteresis. Reproducibility The solid-state pH electrode tends to have poor reproducibility due to the difference of the solid surface. The reproducibility
Fig. 8 Continuous testing of the pH value in the range of 2.02–11.00 by using the TiN NTA electrode.
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Analyst Potentials tested in the pH value of 6.02 in cycles
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TiN NTA 1 2 3 4 5 6 AVG RSD%
36.9 36.8 36.3 36.3 36.9 36.8 36.5 0.32
was tested in B–R buffer solution with a pH of 6.02 in six cycles and the result is shown in Table 2. It shows that the relative standard deviation is about 0.32%, corresponding to the difference of potential less than 1 mV. In addition, the reproducibility of the electrochemical experiment is evaluated using three different TiN NTA electrodes in B–R solutions ( pH = 6.02), the resulting RSD ranges from 0.28 to 0.32%. This exhibits good reproducibility. As for the application of TiN NTA with a real sample, the tap water as the sample and its pH value is measured using a commercial pH meter and TiN NTA electrode respectively. The potential result tested using the TiN NTA electrode is 23.1 mV, corresponding to the pH value of 6.20, which is close to the pH value of 6.28 measured by using the commercial pH meter. From the above experiments, TiN NTA exhibits the promising application as the pH electrode. The favorable performance may be attributed to the following factors. First, TiN possesses super properties such as thermostability, high chemical stability and excellent electrical conductivity. These features make it possible to be used in a wide range of conditions. Second, the hollow structure of the nanotube provides a large electrochemical surface and thus an excellent ion diffusion path.
Conclusion A novel potentiometric pH sensor based on TiN NTA was fabricated. The electrode was characterized by SEM, TEM, EDS, XRD and Raman spectroscopy. The obtained TiN NTA was vertically oriented and highly ordered with inner diameters of about 120 nm and a wall thickness of 15–20 nm. The electrode exhibited a promising sensing performance with a linear-Nernstian response of 55.3 mV per pH in the pH range between 2.0 and 11.0 at 20 ± 1 °C. It has shown a shorter response time and better reproducibility compared with some oxide pH electrodes and TiN nanopowder. It also showed good stability. Therefore, the fabrication and potential application of new pH electrodes based on titanium nitride can be expected.
Acknowledgements The authors express their appreciation to the National Nature Science Foundation of China (No. 51174022 and 51474141).
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The authors also appreciate the National Science Fund for Excellent Young Scholars of China (No. 51522402) for financial support.
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A titanium nitride nanotube array for potentiometric sensing of pH† Mengyang Liu,a Yanling Ma,a Lei Su,b Kuo-Chih Choua and Xinmei Hou*a A titanium nitride nanotube array (TiN NTA) electrode was fabricated through anodic oxidation of titanium and reduction and nitridation of TiO2 NTA. The microstructure of TiN NTA was characterized to be uniform with inner diameters of about 120 nm, a wall thickness of 15–20 nm and an average length of 10 μm. Open-circuit potentials were measured to evaluate the TiN TNA electrode related to pH sensitivity, response time, stability, selectivity, hysteresis and reproducibility in the pH range of 2.0–11.0 at 20 ± 1 °C.
Received 30th December 2015, Accepted 14th January 2016
The prepared TiN NTA electrode exhibits a near-Nernstian slope of 55.33 mV per pH with the correlation
DOI: 10.1039/c5an02675j
coefficient value of 0.995. It shows good selectivity for H+ ions in the presence of cations and anions, especially in fluoride-containing media. It also has good stability and reproducibility with a response time
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of 4.4 s. These make it a promising candidate as a pH electrode sensor.
Introduction The accurate and reliable measurement of pH in chemical, biological, clinical, industrial and environmental samples is extensively required.1,2 Measuring pH is also essential for determining the chemical characteristic of a substance because it is the first step toward managing chemical reactions. Due to the importance of this parameter, many different options exist for determining the pH level of a solution, ranging from the simple and inexpensive pH strip test to the much more complex and expensive pH meters.3,4 The most abundant electrochemical systems for pH sensing are based upon either potentiometric5 or amperometric measurements. For instance, ion selective electrodes,6,7 ion-selective field effect transistors,8–10 metal oxides including IrO2,11–15 RuO2,16 WO3,17–23 TiO2 and SnO2 etc.,24–27 and conducting polymer pH sensors,28–30 have been developed to impart high selectivity by virtue of different proton-recognition processes. Typically the glass pH electrode has had remarkable success for several decades due to an ideal Nernstian response independent of redox interference, a broad response range and a fast and stable response.15,25,26 One major drawback with these types of devices is that they often suffer from instability or potential drift and therefore require constant recalibration.14,16 One tech-
a State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China. E-mail: [email protected] b Research Center for Bioengineering and Sensing Technology, School of Chemistry and Biological Engineering, University of Science and Technology Beijing, Beijing 100083, China † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c5an02675j
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nique which has been utilized to overcome this potential drift is to measure the potential difference between two redox peaks, the first being the alkyl ferrocene with a pH insensitive redox wave and the second a hydroquinone alkylthiol with a pH sensitive redox wave.28 While it has been possible to overcome the design complications inherent in the self-assembled monolayer, these systems require multiple components to be mixed separately, which makes the manufacturing process complex. Nitrides have shown greater chemical inertness and electron mobility compared with most chemical sensor materials. Different from other semiconductor materials, nitrides have a greater energy gap. As a kind of typical nitride, titanium nitride (TiN) possesses many super properties including highmelting, high hardness, high chemical stability, excellent thermal conductivity and electrical conductivity of about 102 S m−1.31–34 It has been proved that TiN can act well as a suitable conducting material for potential applications of electronic devices, field emission and electrochemical capacitors.35–39 It is also found that nano-TiN based composites exhibit enhanced electro-catalytic performance.40 In principle, TiN has a typical crystal structure. Due to the existence of interstitial atoms, there are many holes in the lattice. A hydrogen ion can get into the lattice of TiN and diffuse between holes so that the potential difference appears inside and outside the crystal. Therefore, TiN can be considered as a potentiometric pH sensor. In this work, a titanium nanotube array (NTA) for pH sensing was fabricated via two steps. Firstly, vertically aligned TiO2 nanotubes through anodic oxidation of titanium were obtained. Then TiO2 nanotubes were directly reduced and nitrided under a nitrogen/hydrogen (5% hydrogen) flow to obtain TiN nanotubes. To the best of our knowledge, this is the first pH sensor based on a nitride array reported in the
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literature. Considering its chemical stability even at high temperatures, this work would open up the door to the fabrication of a miniaturized solid state pH sensor, which is crucial for both clinical and environmental applications.
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Experimental section Fabrication of TiN NTA electrode TiN NTA was synthesized by reduction and nitridation of TiO2 nanotubes. Firstly, a TiO2 nanotube array was fabricated by the anodic oxidation process at 30 V for 2 h in a two-electrode reaction system, which consisted of a working electrode of a titanium piece with the dimensions of 10 × 10 × 0.5 mm and a counter electrode of platinum foil in an aqueous solution containing glycol (97 vol%) and NH4F (0.5 wt%) solution. The obtained TiO2 was annealed at 450 °C for 2 h in air to obtain the anatase phase. Then the anatase TiO2 nanotube array was reduced and nitrided at 900 °C for 2 h under a nitrogen/hydrogen (5% hydrogen) flow in a tubular furnace. Finally, the sample was washed with deionized water and then dried to obtain the TiN NTA electrode with the dimensions of 10 × 10 × 0.5 mm. To investigate the effect of the morphology of TiN on the electrochemical behavior, a TiN nanopowder (TiN NP) electrode was also fabricated. TiN nanopowder was prepared involving two sequential reaction steps41: (1) the formation of a complex precursor through low temperature combustion synthesis (LCS) using TiCl4, HNO3, NH3·H2O, urea and glucose as raw materials. (2) Carbon-thermal reduction and nitridation of the precursor to obtain TiN nanopowder at 1050–1200 °C for 2 h under a flowing nitrogen atmosphere. The TiN nanopowder electrode was prepared by mixing TiN nanopowder and polyvinylidene fluoride (PVDF) with the mass ratio of 8 : 2 in N-methyl-2pyrrolidone (NMP) to form a stable slurry. Then the slurry was coated on a Ti sheet with the coating mass of ∼0.1 mg (10 × 10mm). Then the electrode was heated at 120 °C for 3 h to evaporate the solvent for electrochemical measurements.
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with the scan rate ranging from 10 to 500 mV s−1. Electrochemical impedance spectra (EIS) were recorded from 10 mHz to 100 kHz with an alternate potential amplitude of 5 mV. Opencircuit potential measurements were conducted to evaluate the pH response of the electrodes at various pH levels at room temperature. For the preparation of pH solution, Britton– Robinson (B–R) buffer solution containing 0.04 mol L−1 phosphoric acid, boric acid and acetic acid was compounded. The solution pH values were adjusted by addition of 0.2 mol L−1 NaOH solution and evaluated using a commercial pH meter.
Results and discussion Microstructure characterization The XRD patterns of TiO2 NTA and TiN NTA supported on the Ti sheet are shown in Fig. S1.† Both of the XRD patterns reveal the characteristic diffraction peaks at 40.2°, 53° and 70°, which can be ascribed to the Ti substrate. In view of TiO2 NTA, the peaks at 25.3°, 37.8°and 55.1° can be indexed as the (101), (004) and (211) crystal planes of anatase TiO2. While the peaks for TiN NTA at 36.8°, 43.3° and 62.5° can be indexed as the (111), (200) and (220) crystal planes of cubic TiN. The structures of TiO2 NTA and TiN NTA are characterized using Raman spectra. As shown in Fig. S2,† the TiO2 NTA sample has the characteristic Raman peaks at 400 cm−1, 515 cm−1 and 640 cm−1, which can be indexed as the vibration mode of B2g of anatase TiO2.42 On comparison, TiN NTA has the characteristic peak at 576 cm−1,43 indicating that anatase TiO2 is completely converted to cubic TiN. The SEM photographs of TiN NTA supported on the Ti sheet are shown in Fig. 1. It can be seen that TiN NTA is vertically oriented and highly ordered with an inner diameter of about 120 nm and a wall thickness of 15–20 nm (Fig. 1a and b). Fig. 1c shows the cross-sectional SEM image of TiN NTA. The total length of the TiN nanotubes is approximately 10 μm. To further investigate the microstructure of TiN NTA, TEM
Characterization and measurement X-ray diffraction patterns were performed using an X-ray diffractometer (XRD, DMAX-RB, Japan) with the use of a Cu-Kα radiation source. The surface morphology and microstructure of TiN NTA were characterized by using a field emission scanning electron microscope with energy dispersive spectroscopy (FESEM and EDS, JSM-6701F, Japan) and transmission electron microscopy (TEM, Tecnai F30, America). The Raman spectra were measured on a Raman spectrometer (Raman, LabRAM HR Evolution, France) using a He–Ne laser that emitted the sample at 785 nm excitation and the integration time for each sample was 10 s. Electrochemical measurements were carried out in a threeelectrode system using CHI660D electrochemical workstation. A platinum wire and Ag/AgCl electrode were used as the counter electrode and reference electrode, respectively. Cyclic voltammetry (CV) was measured in 0.5 mol L−1 Na2SO4 solution containing 0.01 mol L−1 ferrocyanide and ferricyanide
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Fig. 1 Morphology of TiN nanotube array: (a) and (b) a typical top view; (c) a cross-sectional view; (d) TEM images; (e) SEAD pattern; (f ) EDS spectra.
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with selected area electron diffraction (SEAD) was carried out. It clearly exhibits the character of the tube (Fig. 1d). The SAED pattern (Fig. 1e) shows three characteristic diffraction rings from inner to outer corresponding to (111), (200) and (220) planes of TiN. Mapping EDS analysis of TiN NTA shows that the formation of TiN NTA and no other phase exists (Fig. 1f ). The hollow structure can provide a large electrochemical surface and thus an excellent ion diffusion path. Electrochemical characterization of pH electrode The CV curves of TiN NTA, TiO2 NTA and Ti in 0.5 mol L−1 Na2SO4 electrolytes containing 0.01 mol L−1 Fe(CN)63− are shown in Fig. 2. On comparison, the anodic and cathodic peaks associated with the oxidation and reduction of the ferricyanide–ferrocyanide couple obviously appear at the TiN NTAsolution interface. While no obvious anodic and cathodic peaks are observed at either the TiO2 NTA-solution interface or Ti-solution interface. In addition, CV curves of TiO2 NTA and Ti are similar because Ti is prone to be oxidized. This indicates that TiN NTA possesses better electrical conductivity. The CV curves of TiN NTA at different scan rates in 0.5 mol L−1 Na2SO4 electrolytes containing 0.01 mol L−1 Fe(CN)63− were further measured. As shown in Fig. 3, the anodic and cathodic peaks associated with the oxidation and reduction of the ferricyanide–ferrocyanide couple respectively obviously appear at the TiN NTA-solution interface. The anodic and cathodic peak currents increase linearly with the square of scan rates as shown in the inset of Fig. 3, exhibiting that the electrode reaction is diffusion-controlled. The peak potential increases with the scan rates. However, at a scan rate of 0.01 V s−1, the peak current of anodic-to-cathodic ratio is roughly 1 and the potential difference between the anode and cathode is 65 mV, almost fitting for ΔEp = Epa − Epc = 59/n (Epa and Epc represent the anodic and cathodic peak potential respectively, n represents the number of electrons transferred). We can
Fig. 2 CV curves of Ti, TiO2 NTA, TiN NTA electrode at the scan rate of 0.1 V s−1 in 0.5 mol L−1 Na2SO4 electrolytes containing 0.01 mol L−1 Fe (CN)63−.
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Fig. 3 CV curves of TiN NTA electrode at different scan rates in 0.5 mol L−1 Na2SO4 electrolytes containing 0.01 mol L−1 Fe(CN)63−; The inset is the plot of the anodic/cathodic peak current versus the square of the scan rate.
deduce that the reaction is quasi-reversible and the electrode shows good electrical conductivity. The EIS plots of TiO2 NTA, Ti and TiN NTA in 1 mol L−1 KOH44 are shown in Fig. 4. The resistance values include the solution resistance (Rs) and the charge transfer resistance (Rct). It can be seen that the EIS of TiO2 NTA shows a typical semicircle at both high and low frequency. The large diameter of the semicircle represents a high charge transfer resistance. On comparison, the EIS result of Ti exhibits a similar semicircle with a larger diameter due to its prone to be oxidized. As for the EIS result of TiN NTA, it consists of a nearly semicircle with a small diameter in the high frequency range and a near vertical line to the abscissa axis in the low frequency range. The semicircle with a small diameter in the high frequency range indicates that the charge transfer resistance at the interface of TiN NTA-solution is very low and the near vertical line to the abscissa axis in the low frequency range is caused by a diffusion-controlled process. From the above results, TiN NTA exhibits a small value of Rct (0.48 Ω) and Rs (3.2 Ω).44
Fig. 4 EIS plots of Ti (black line), TiO2 NTA (red line) and TiN NTA (blue line) in 1 mol L−1 KOH.
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The EIS of TiN NTA in 1 mol L−1 H2SO4 (blue line), 1 mol L KOH (black line) and 1 mol L−1 Na2SO4 (red line) are measured and the results are shown in Fig. S3.† It can be seen that the EIS behavior of TiN NTA in the three solutions are similar, consisting of a nearly semicircle in the high frequency range and a near vertical line to the abscissa axis in the low frequency range. The absence of the complete semicircles indicates that the charge transfer resistance at the interface of TiN NTA is so low that its electrochemical reaction resistance becomes almost negligible in the three different electrolytes. This is ascribed to the superior electrical conductivity of TiN NTA. The analogous slope of the beelines indicates a similar resistance and capacitance character in the three different kinds of electrolytes. The EIS results demonstrate the electrochemical behavior of TiN NTA is diffusion controlled, which coincides with the CV curves. Herein, the Rs of the TiN NTA electrode under the three conditions are calculated to be 1.5 Ω (1 mol L−1 H2SO4), 3.2 Ω (1 mol L−1 KOH), 6.3 Ω (1 mol L−1 Na2SO4) and the Rct are 0.52 Ω (1 mol L−1 H2SO4), 0.48 Ω (1 mol L−1 KOH), and 0.76 Ω (1 mol L−1 Na2SO4) respectively.44
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−1
Sensor performance Sensitivity. It is known that TiN belongs to a kind of ionic selectivity electrode.45 The potential difference appears as a result of different ion activities at the interface. The electrode potential between the interface and solution can be expressed as follows according to the Nernst equation: E ¼ E° þ ðRT=nF Þ ln
aH aH ′
Fig. 5 pH sensitivity of TiN NTA, TiN NP and TiO2 NTA electrodes tested in B–R solutions in the pH range of 2.0 to 11.0.
measurement in B–R buffer solutions in all pH range of 2.0 to 11.0 was conducted to calculate the response time. The data are obtained at 0.05 s interval. The dynamic response in all pH range of 2.0 to 11.0 is shown in Fig. 6. The dynamic response
ð1Þ
where E is the potential difference between the working electrode and reference electrode, E° is the standard reduction potential, R is the gas constant, T is the temperature, F is Faraday’s constant, n is the number of moles of electrons transferred, aH and a′H are the activities of the hydrogen ion outside and inside the membrane respectively. The sensitivity of the TiN electrode was validated by immersing the electrode in B–R buffer solution of pH 2.0–11.0 at 20 °C and the data are repeated 3 times. For comparison, the pH sensitivity of TiN nanopowder (NP)41 synthesized through a low-temperature-self-propagating-process, TiO2 NTA were also tested. As for the Ti electrode, it tends to be oxidized and its pH sensitivity is not measured. The pH sensitivity of TiN NTA, TiN NP and TiO2 NTA electrodes was determined as shown in Fig. 5. The slopes of the curves of TiN NTA, TiN NP and TiO2 NTA are calculated to be 55.33 mV per pH, 46.48 mV per pH and 44.8 mV per pH with the correlation coefficient to be 0.995, 0.992, and 0.993 respectively. On comparison, the TiN NTA electrode possesses the highest pH sensitivity. Response time The response time of a pH sensor is defined as the transit time required for its potential to reach 90% of an equilibrium value after immersing the sensor in test solution. The response time of an ion-selective electrode is an important factor for any analytical application. In this experiment, continuous
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Fig. 6 Dynamic responses of the TiN NTA and TiN NP electrode for step changes of all pH range from 2.0 to 11.0.
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time thus obtained was about 4.4 s as for TiN NTA and 5.2 s for TiN nanopowder (TiN NP). The response time almost keeps constant in the pH range of 2.0 to 11.0. Both the values are comparable to, even smaller than that of other pH electrodes reported in the literature, such as WO3 nanoparticles (3–8 s),1 and poly vinyl chloride membrane (6–7 s).27 They were a little larger than that of iridium oxide based (within 2 s)14 and ruthenium oxide (3 s).16 The response time depends on the porous properties of the sensing film which traps the ions to a certain extent. However, the cleanliness of the electrode also makes sense. Stability For stability testing, the electrode was immersed in the buffer solutions with three different pH values for 200 s. Fig. 7 shows the measured potential versus time for different pH values. It can be seen that the pH value fluctuated slightly as the time goes on but tends to be stable finally. A similar phenomenon can be observed in other pH values. This potential drift can be attributed to the diffusion and trapping of hydrogen ions through the holes and at the boundaries of the electrode. In addition, the potential response still retained more than 94% of the initial response after being stored at 25 °C for a month. Selectivity Considering various ions and compounds typically exist randomly in practice, the electrode selectivity among possible cations and anions interference is investigated. The selectivity tests of the electrode were conducted with the fixed interference method. The interference effects can be calculated according to the following equation:46 E ¼ E° þ
i RT h ln ai þ Kij pot aj ðzi =zj Þ zi F
ð2Þ
where E is the observed potentiometric response, E° is the standard potential, R is the gas constant, T is the temperature,
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The stability of the TiN NTA electrode.
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Table 1 Selectivity coefficients determined by the use of the fixed interference method (FIM)
Ion
Concentration
ΔV
Na+ K+ Cl− F−
1 mol L−1 1 mol L−1 1 mol L−1 1 mol L−1
8 mV 5 mV 7 mV 3 mV
F is Faraday’s constant, z is the charge of ion i or j, ai and aj are the activities of the primary and interfering ions, respectively. Kijpot is the Nikolskii ( potentiometric selective) coefficient. The selectivity coefficients of the TiN NTA electrode among possible cations and anions interference are summarized in Table 1. It shows that the potential changes of the TiN NTA electrode are less than 8 mV, especially for F−, demonstrating better selectivity. Hysteresis The electrode potential may be different even if the ion activity is the same because it is related to the solution composition that was brought in contact before. In the experiment, the pH value was tested continuously from 2.02 to 11.00 in three cycles for the same TiN NTA electrode. The result is shown in Fig. 8. It can be seen that the three curves is almost coincidence. The hysteresis appears more obviously at the range of pH = 2.02–3.00 and pH = 10.00–11.00. The main reason is possibly that H+ in the hole of TiN will achieve a new balance with H+ in the solution when the electrode is shifted among different solutions. In addition, the experimental conditions and testing area of the surface of the electrode also cause hysteresis. Reproducibility The solid-state pH electrode tends to have poor reproducibility due to the difference of the solid surface. The reproducibility
Fig. 8 Continuous testing of the pH value in the range of 2.02–11.00 by using the TiN NTA electrode.
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Analyst Potentials tested in the pH value of 6.02 in cycles
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TiN NTA 1 2 3 4 5 6 AVG RSD%
36.9 36.8 36.3 36.3 36.9 36.8 36.5 0.32
was tested in B–R buffer solution with a pH of 6.02 in six cycles and the result is shown in Table 2. It shows that the relative standard deviation is about 0.32%, corresponding to the difference of potential less than 1 mV. In addition, the reproducibility of the electrochemical experiment is evaluated using three different TiN NTA electrodes in B–R solutions ( pH = 6.02), the resulting RSD ranges from 0.28 to 0.32%. This exhibits good reproducibility. As for the application of TiN NTA with a real sample, the tap water as the sample and its pH value is measured using a commercial pH meter and TiN NTA electrode respectively. The potential result tested using the TiN NTA electrode is 23.1 mV, corresponding to the pH value of 6.20, which is close to the pH value of 6.28 measured by using the commercial pH meter. From the above experiments, TiN NTA exhibits the promising application as the pH electrode. The favorable performance may be attributed to the following factors. First, TiN possesses super properties such as thermostability, high chemical stability and excellent electrical conductivity. These features make it possible to be used in a wide range of conditions. Second, the hollow structure of the nanotube provides a large electrochemical surface and thus an excellent ion diffusion path.
Conclusion A novel potentiometric pH sensor based on TiN NTA was fabricated. The electrode was characterized by SEM, TEM, EDS, XRD and Raman spectroscopy. The obtained TiN NTA was vertically oriented and highly ordered with inner diameters of about 120 nm and a wall thickness of 15–20 nm. The electrode exhibited a promising sensing performance with a linear-Nernstian response of 55.3 mV per pH in the pH range between 2.0 and 11.0 at 20 ± 1 °C. It has shown a shorter response time and better reproducibility compared with some oxide pH electrodes and TiN nanopowder. It also showed good stability. Therefore, the fabrication and potential application of new pH electrodes based on titanium nitride can be expected.
Acknowledgements The authors express their appreciation to the National Nature Science Foundation of China (No. 51174022 and 51474141).
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The authors also appreciate the National Science Fund for Excellent Young Scholars of China (No. 51522402) for financial support.
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