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Electrochemistry Communications 47 (2014) 71–74 Contents lists available at ScienceDirect Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom Short communication A novel thermocouple microelectrode for applications in SECM and variable temperature electrochemistry Hailing Zhang a, Xiaojian Xiao a, Tongyu Su a, Xiaoxing Gu b, Tao Jin b,⁎, Lin Du a, Jing Tang a,⁎ a b Ministry of Education & Fujian Provincial Key Laboratory of Analysis and Detection of Food Safety, Department of Chemistry, Fuzhou University, Fuzhou 350116, PR China Department of Electrical Engineering, Fuzhou University, Fuzhou 350116, PR China a r t i c l e i n f o Article history: Received 4 May 2014 Received in revised form 27 June 2014 Accepted 27 June 2014 Available online 6 July 2014 Keywords: Thermocouple microelectrode Temperature Scanning electrochemical microscope a b s t r a c t This paper describes a novel method for the fabrication of a thermocouple microelectrode by melting Pt and Pt–Rh wires into a measuring junction using a hydroxygen flame. A thermocouple with a tip apex diameter of approximately 20 μm can be fabricated by sharpening the two wires by electrochemical etching, followed by merging of the two etched ends into a spherical joint to act as the measuring point. The as-fabricated microelectrode can work as a kind of thermocouple to measure the local temperature of the electrode. The microelectrode can be employed to study the electrocatalytic oxidation of methanol. Most importantly, the thermocouple microelectrode can be employed as an SECM tip, for example in the measurement of the reactivity of Cu patterns with bromine on a circuit board over a variable temperature range. © 2014 Elsevier B.V. All rights reserved. 1. Introduction The measurement of spatially localized rapid temperature changes has become a major issue in various research areas of materials science and nanotechnology. Various thermometers have been developed in order to measure the temperature in a small volume [1]. There are plenty of thermometer types, which can be adopted for temperature measurements, including commonly used resistance thermometers [2], those based on molecular fluorescence [3] or molecular IR measurements [4]. In comparison with other techniques, thermocouples are, in principle, independent of the shape and size of the measuring point and it is possible to improve the spatial resolution into the nanometer domain by decreasing the size of a thermocouple [5]. Scanning thermal microscopy uses an STM probe with a thermocouple junction of ~100 nm fabricated at the probe tip to perform localized temperature measurements. The STM tip behaves both as a tunneling source and as a temperature detector [6,7]. A Ti–Pt coated AFM cantilever is anchored to a thermal reservoir at room temperature and produces a well-defined and reproducible electrical conductance that depends only on the local temperature of the point contact [8]. However, the use of such an instrument is limited to conducting surfaces and to air environments [9]. SECM has been developed as an important electrochemical technique to investigate heterogeneous and homogeneous reactions in electrolytes or biological systems, which show significant temperature dependence [10]. Many of the processes and surfaces investigated ⁎ Corresponding authors. Tel./fax: +86 591 22866165. E-mail addresses: [email protected] (T. Jin), [email protected] (J. Tang). http://dx.doi.org/10.1016/j.elecom.2014.06.027 1388-2481/© 2014 Elsevier B.V. All rights reserved. using SECM show pronounced temperature dependence due to thermodynamic effects on reaction rates and mass transport [11]. Therefore, it is necessary to measure the temperature during an SECM investigation to improve accuracy and reproducibility. While a thermocouple microelectrode has been proposed by Baranski to measure electrode temperature when heated with high frequency AC current [12,13], the structure of this thermocouple is very bulky and not suitable for use as an SECM tip. Here, we propose a method to prepare a new kind of thermocouple microelectrode, which can be used as a working electrode in variable temperature electrochemistry. More importantly, this new thermocouple microelectrode can be used in SECM measurements at different temperatures. The integration of the thermocouple microelectrode with SECM can satisfy the requirement to measure the local temperature around the tip during electrochemical SECM measurements. 2. Experiment section Borosilicate capillaries (O.D.: 2.0 mm, I.D.: 1.56 mm) and type quartz glass capillaries (O.D.: 1.2 mm, I.D.: 0.90 mm) from Sutter Instruments were soaked in boiling H2SO4, repeatedly sonicated with ultrapure water, oven dried and stored in a clean vial. Cyclic voltammetry was performed in an isothermal half electrochemical cell with N2 bubbling. The reference electrode was a saturated calomel electrode (SCE), which was kept at constant temperature, while the working electrode was under thermostatic control with a water bath. The heating apparatus of SECM experiments is similar to that described by Schuhmann [10], the only difference is that a ceramic heating sheet was used instead of the Peltier element and the temperature can be adjusted above room temperature. 72 H. Zhang et al. / Electrochemistry Communications 47 (2014) 71–74 The etching solution for thinning of the Pt and PtRh wires was 60% (v/v) saturated CaCl2 + 4% HCl + 36% H2O [14]. The thermocouple microelectrode was characterized in a solution of 1 mM FcMeOH + 0.1 M KCl. The aqueous solution for studying the effect of temperature on methanol oxidation was 0.4 M CH3OH + 0.2 M H2SO4. In the SECM experiments, the aqueous solution was 0.1 M NaBr + 2 M H2SO4. A circuit board with parallel copper wires was used as the SECM substrate at variable temperatures. All aqueous solutions were prepared with ultrapure water. The morphology of the thermocouple microelectrode was characterized by SEM (NOVA NANO SEM 230) or a metallurgical microscope (Oto optics Inc., China). The chemical composition was analyzed by energy dispersive X-ray analysis (EDX). The SECM measurement at variable temperature was controlled with a CHI 920C (CH Instruments Inc.) potentiostat. The electrochemical measurements were performed with CHI 842B or CHI 614D. joint of Pt and Pt–Rh, was heated by electrical resistance of a Ni–Cr coil and the two kinds of capillaries were sealed together with epoxy resin. (6) The thermocouple microelectrode was polished carefully using emery paper and 0.05 μm alumina suspension before use as an SECM tip. The RG ratio of the diameter of (metal + glass) to the diameter of metal is 3–6. The morphology of the thermocouple microelectrode is shown in Fig. 1(b). Fig. 1(c) shows the EDX spectrum of the elemental composition of the microelectrode. It can be estimated roughly to be 85.59% Pt and 14.41% Rh. Fig. 1(b) shows the voltammogram of 1 mM FcMeOH in 0.1 M KCl solution. The sigmoidal shaped CV demonstrates that the electrochemical response is typical of a freely diffusing species at a microelectrode at slow scan rates. The size of the thermocouple microelectrode can be calculated from Eq. (1): 3. Results and discussion where F is Faraday's constant (96485 C·mol−1), D is the diffusion coefficient (7.8 × 10−6 cm2·s−1) [15], C0 is the bulk concentration of electroactive species, and r is the radius of the disk electrode. A steady-state current of (iT∞ = 3.1 × 10−9 A) obtained from CV was used to calculate a microelectrode diameter of 20.58 μm, and this value agrees with that measured from the SEM image (18.43 μm) in Fig. 1(c). The difference is likely due to surface roughness. In these experiments, the thermocouple is used as the working electrode and the temperature of the electrode is read from a thermocouple meter. The time constant of the thermometer is 3 ms, which is defined as the time required to reach 63.2% of an instantaneous temperature change. Fig. 2(a) shows the increased steady-state current of the thermocouple microelectrode in 1 mM FcMeOH + 0.1 M KCl at a scan rate 10 mV·s−1. A mercury thermometer was employed to check the deviation between the temperature measured by the thermocouple microelectrode (T2) and the Hg thermometer (T1). The difference is shown to be 1–2 °C as shown in Fig. 2(a). The main source of the error may be due to The fabrication process of the thermocouple microelectrode is shown in Fig. 1(a) and described as follows. (1) The borosilicate capillary was pulled with a capillary puller to decrease the size of the opening at one end. (2) Pt wire and 13% Pt–Rh wire (OMEGA part number SP13R-001), with diameters of 25 μm and lengths of almost 1 cm, were connected with two Cu wires using tin solder. (3) The thinning of the wire was achieved by anodic dissolution of the two wire ends in the electrochemical etching solution. A potential of 20 V was applied between the working electrode (Pt or Pt–Rh wire) and a carbon rod counter electrode. (4) The wires were isolated from one another by inserting them into Θ-type quartz capillary. The etched ends of the Pt and PtRh wires were merged into a spherical joint after melting in a hydroxygen flame. (5) The quartz capillary was inserted into the wide end of the borosilicate capillary and gently pushed to the pulled end. The outer borosilicate capillary, with the spherical iT∞ ¼ 4nFC 0 Dr ð1Þ Fig. 1. (a) Schematic diagram of the process for preparing a thermocouple micro-electrode. The insets are photos of the corresponding steps. (b) Voltammetric responses of a Pt–Rh thermocouple microelectrode in a 1 mM FcMeOH solution containing 0.1 M KCl (scan rate = 10 mV·s−1). The inset in (b) is a photo of a Pt–Rh thermocouple microelectrode. (c) EDX and FESEM (inset) of a Pt–Rh thermocouple microelectrode. H. Zhang et al. / Electrochemistry Communications 47 (2014) 71–74 73 Fig. 2. Cyclic voltammograms of a Pt–Rh thermocouple microelectrode in (a) 1 mM FcMeOH solution containing 0.1 M KCl (scan rate = 10 mV·s−1). (b) 0.4 M CH3OH and 0.2 M H2SO4 (scan rate = 50 mV·s−1) at different temperatures. T1 reading from the Hg thermometer and T2 reading from the thermocouple meter. temperature fluctuations immediately above the water bath, where the reference junction of the thermocouple is located. The thermocouple microelectrode was used to examine the effects of temperature on methanol oxidation at different temperatures. Fig. 2(b) shows that the PtRh thermocouple microelectrode was characterized in 0.4 M CH3OH + 0.2 M H2SO4 at different temperatures from 27 °C to 56 °C. The inset of Fig. 2(b) shows that the shape of the voltammogram of the PtRh microelectrode is similar as that of the pure Pt electrode. It is known that the oxidation products of CH3OH on Pt based alloy electrodes are to be CO, CO2, HCHO, HCOOH and HCOOCH3 [16]. The relative yields of the products depend on the initial methanol concentration, temperature, electrode roughness and alloy composition. In Fig. 2(b), the significant oxidation peak of CH3OH on PtRh alloy microelectrode is located at about 0.558–0.612 V and the Fig. 3. SECM images and scanning curves of a copper circuit board in 0.1 M NaBr + 2 M H2SO4 at (a) 25 °C, (b) 35 °C, (c) 45 °C, and (d) 55 °C (tip potential = 1.15 V, vs. Ag/AgCl; substrate potential = open-circuit potential; tip-to-substrate distance: 35 μm; scan rate: 20 μm·s−1). Temperature reading from the thermocouple meter. (e) The schematic illustration of SECM. (f) Lateral scanning curves over the circuit board at different temperatures. 74 H. Zhang et al. / Electrochemistry Communications 47 (2014) 71–74 renewed oxidation peak is at 0.508–0.515 V. Compared to at 27 °C, the activity for methanol oxidation at elevated temperatures is markedly enhanced, as indicated by a ca. 20 mV cathodic shift of onset potential. The maximum faradaic currents in the positive- and negative direction scans at 56 °C are 12.4 and 15.3 times as high as that at 27 °C, respectively. A circuit board with parallel copper wires was used as the substrate to detect the effects of elevated temperature on SECM measurement using the thermocouple microelectrode. The tip-generated Br2 can be reduced to bromide ions at the open circuit potential [17]. A positive feedback current can be observed on the conductive copper surface, while a negative feedback current can be observed on the insulated bakelite surface. Fig. 3(a–d) shows SECM images of the solution (0.1 M NaBr + 2 M H2SO4) before and after heating at temperatures between 25 °C and 55 °C. The largest width of the left and right copper wires was 276.80 μm and 244.01 μm at 25 °C, whereas the corresponding values are 290.61 μm and 257.66 μm at 55 °C. The widest two copper wires are shown in Fig. 3(a) and (d) and correspond to increases of 4.99% and 5.59%. The schematic illustration of SECM and the cross section analysis of the current are shown in Fig. 3(e, f). In Fig. 3(e), one of the Pt and Pt–Rh wires is connected with the bipotentiostat. ΔI was defined as the difference between the lowest and highest currents measured during a single SECM measurement. ΔI was 0.130 μA at 25 °C and 1.166 μA at 55 °C. The increase in the tip current minimum from 25 °C to 55 °C was less than the increase of the highest tip current. This difference arises from the reaction between the tip-generated bromine and the Cu substrate which regenerates the bromide ions. Hence the influence of temperature on the reaction rate between the bromine and Cu was more significant than the electrochemical reaction at the microelectrode. The effects of natural convection and forced convection on the resolution of SECM images at increased temperature require further detailed study. 4. Conclusions We describe a method to prepare a new type of thermocouple microelectrode for the efficient measurement of local temperature, which also can be used in SECM measurements. The thermocouple microelectrode was prepared by the merging of Pt and Pt–Rh wires into a measuring point. This microelectrode can be used to measure highly localized temperature fluctuations during an electrochemical experiment. 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