Copper nano-clusters prepared by one-step electrodeposition and its application on nitrate sensing

Copper has attracted considerable interest as an ideal sensing material due to its good stability, excellent electricalconductivity, electrocatalytic properties and low cost compared with noble metal such as platinum, gold and silver.^1–3 As a sensing material for electrochemical detection, copper nanostructures have many unique properties such as the enhanced mass-transport rate, high surface to volume ratio, and the improved signal-to-noise ratio in electroanalytical measurements.⁴ These characteristics significantly affect the electrochemical reactions, and promote the potential applications for catalysis and detection of a number of biochemical substances such as glucose,^5,6 COD (chemical oxygen demand),^7,8 kojic acid,⁹ sulfite¹⁰ and nitrate.^11,12 A lot of work has been published in discussing the synthesis, characterization and application of copper nanoparticles.^13–15 However, the reported synthetic methods based on reduction with hydrazine in ethylene glycol under microwave irradiation,¹⁶ the seed-mediated growth,¹⁷ the pulsed electrodeposition¹⁸ and deposition on some specific substrates such as carbon nanotubes¹⁹ and boron-doped diamond electrodes²⁰ are still complex and time-consuming.

In this paper we introduce a fabrication method of copper nanostructures through a one-step and low cost electrochemical deposition in acidic copper sulphate electrolyte with no other additives. The performance of the deposited copper nano-clusters in nitrate sensing application was also demonstrated. The experimental results reveal that the morphologies and structures of deposited copper layers can be adjusted conveniently by changeing the deposition voltage. Under the optimized deposition voltage, the fabricated copper layers were porous, and they presented high sensitivity for nitrate detection within the concentration ranging from 0.1 mmolL⁻¹ to 4.0 mmolL⁻¹.

All chemicals were of analytical reagent grade. CuSO₄⋅5H₂O, Na₂SO₄, NaCl, NaNO₃ and H₂ SO₄ were purchased from Beijing Chemical Reagent Company (China). The supporting buffer solutions were 0.1 molL⁻¹ Na₂SO₄ solution, prepared by mixing sulphuric acid solution and sodium sulfate solution, and the pH values were accurately adjusted to 2.0. In almost all cases the solutions were prepared by dissolving the reagents in deionised water with a resistivity of 18 MΩ cm.

Experiments were carried out in a conventional electrochemical cell with a three-electrode-system, including an Ag/AgCl electrode as the reference electrode (RE) and a microelectrode chip on which a working electrode and a counter electrode were prepared using microfabrication techniques. Electrochemical characterization of the microelectorde was carried out with a Gamry Reference 600 electrochemical measurement system (Gamry Instruments Co., Ltd., USA). All experiments were carried out at room temperature. The surface morphology analysis was carried out using the S-4800 field emission scanning electron microscope (SEM) produced by Hitachi (Japan), and the operating voltage was 20.0 kV.

The microelectrode chip was fabricated by standard micro fabrication process. Firstly, a layer of platinum with the thickness of 200 nm is deposited and patterned on a glass substrate by photolithography and lift-off process to form the working electrode (WE) and counter electrode (CE). A layer of tantalum with the thickness of 30 nm was used as the adhesion layer material between the platinum layer and the glass substrate. Then a SU-8 layer was patterned around the platinum electrodes to form a micropool to define the sensitive area of working electrode. Finally, the glass wafer was diced into individual chips, which were subsequently wire-bonded and encapsulated on a customized print circuit board, then ready to be used. Fig. 1 shows the major fabrication process and photograph of the microelectrode chip. The sensitive area of the working electrode is 1 mm².

Firstly, the microelectrode chip was sonicated in acetone, ethanol and deionized water for 5 min sequentially, and electrochemically cleaned by cyclic voltammetry scan in 0.5 molL⁻¹ H₂SO₄ solution until a reproducible voltammogram was obtained. Secondly, the electrodeposition of copper material was performed by chronoamperometry scan in electrolyte of copper sulphate (pH=2.0). Various fixed potentials from -100 mV to -700 mV were applied on the working electrode versus RE for 160 s. Voltammograms of the process are shown in Fig. 2. It is found that the deposition current is almost constant when the applied voltage is lower than -300 mV. The current is cumulative during the deposition process when the applied voltage is higher than -500 mV. This phenomenon indicates that under the high depostion voltage the synthesis speed of copper crystalline grain increases gradually. Finally, the modified microelectrode was washed by pouring deionized water on the electrode surface for 2 minutes, and kept in deionised water for further use.

The surface morphology study of working electrodes deposited under different voltages was conducted by scanning electron microscopy (SEM). It was found that copper layers deposited under different voltages present different morphologies. Fig. 3(A) and 3(B) display the electrode surface morphologies of copper layers with a compact and explanate structure. By contrast, Fig. 3(C) and 3(D) show porous structures constructed by uniform copper nanoparticles on the surface of electrode. The size of nanoparticles was about 50-100 nm.

The reaction mechanism and kinetics of copper nucleation on platinum substrate are used to explain the phenomena that different morphologies could be achieved under different voltages. Since the electrodeposition process was performed in acidic electrolyte, the reduction process of copper ion on working electrode is probably accompanied by a hydrogen evolution reaction according to the following equations:

When the deposition voltage is lower than -300 mV, the speed of copper nucleation on platinum surface is very slow and the deposition process was not disturbed by other reactions such as hydrogen evolution reaction. In this condition, the copper atoms were stacked compactly onto the electrode surface. The deposited copper layer tends to be flat and compact. While when the deposition voltage is higher than -500 mV, the copper nucleation is speeded up, and the electrodepostion process is accompanied by hydrogen evolution reaction at the same time. The electrodeposition was performed by chronoamperometry scan, so the hydrogen evolution reaction is relatively stable and slight. The evolution of H₂ with constant speed makes it convenient to form tiny hydrogen flow as a dynamic template and thus influences the morphologies of deposited copper layers.¹⁴ The deposited copper will not be stacked compactly and tends to develop along three dimensional directions, which results in porous copper-clusters on working electrode.

Materials deposited on the surface of working electrode were characterized by X-ray diffraction (XRD). The result is shown in Fig. 4. Four peaks of crystal directions could be clearly recognized in the spectrum, and it could demonstrate that the material deposited on platinum working-electrode was copper mainly grown in Cu(111) and Cu(200) crystallographic orientation.

Previous literatures have demonstrated that under acidic conditions nitrate can be reduced to ammonium ions on a copper surface according to the following equation:²¹ 

The electrocatalytic activity of the deposited copper layers were verified by quantifying the nitrate standard solutions with various concentrations based on Eq. (3). Linear sweep voltammetry was chosen for the electroanalysis of nitrate. Different concentrations of nitrate solution were prepared in 0.1 molL⁻¹ Na₂SO₄ supporting buffer adjusted at pH 2.0. When the scan limits of the working potential were set from -250 mV to -650 mV, the current responses were increasing with the nitrate concentration from 0.1 mmolL⁻¹ to 4.0 mmolL⁻¹. The linear sweep voltammograms of a microelectrode modified under -300 mV for 160 s was shown in Fig. 5.

The sensitivities of the modified microelectrode with different deposition voltages were researched. Fig. 6 shows the calibration plots of current values measured at -600 mV in the voltammograms as a function of nitrate concentrations using the microelectrodes modified under different deposition voltages. As shown in Fig. 6, compared the sensitivity of the microelectrode modified under -300 mV (curve a, 12.72 μA⋅mmolL⁻¹), the sensitivity of the microelectrode modified under -700 mV (curve d, 39.31 μA⋅mmolL⁻¹) was enhanced about 3.2-fold. That is because that under higher voltage the deposited copper layer shows porous structure, which has a larger surface area, is a more effective electrocatalyst in facilitating nitrate reduction than the compact one.

The detailed comparison of electroanalytical results of microelectrodes modified under different deposition voltages was shown in Table I. The results proved that the microelectrode modified under -700 mV has a good performance for nitrate detection.

Copper layers with different micromorphologies were electrodeposited onto the platinum microelectrode with a simple chronoamperometry scan method. The crystal direction and morphologies of freshly deposited copper layers were examined by SEM and XRD. It was found that the deposited layer has porous structure when the deposition voltage is higher than -500 mV, and exhibits a larger effective surface area compared with the smooth flat one deposited under voltage less than -300 mV. Under the optimized voltage, copper clusters constructed by uniform nanoparticles with an average diameter of about 100 nm can be achieved. The experimental results reveal that the porous copper layer electrodeposited under -700 mV is a high-performance electrocatalyst in facilitating nitrate reduction, and it also has a higher sensitivity of 39.31 μA/mmolL⁻¹ for nitrate detection within the concentration ranging from 0.1 mmolL⁻¹ to 4.0 mmolL⁻¹.

This work is supported by the National High Technology Research and Development Program (863 Program, No. 2012AA040506) and National Natural Science Foundation (No. 61302034, and No. 61134010).