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Electrochemistry Communications 47 (2014) 71–74
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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.
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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
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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.
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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. The effect of temperature on the catalytic oxidation of
methanol on the microelectrode has been studied. Furthermore, the
microelectrode was used to characterize Cu patterns in an SECM
measurement. This microelectrode will be suitable for electrochemical
experiments in minimized volume cells and we anticipate that it will
be applied to more processes relating to temperature. Future work
will involve attempts to record a thermal image simultaneously during
SECM imaging.
Conflict of Interest
There is no conflict of interest among authors.
Acknowledgments
Support for this research by the National Natural Science Foundation
of China (21173048, 21073038) is gratefully acknowledged.
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