Applied Mechanics and Materials
ISSN: 1662-7482, Vols. 138-139, pp 612-617
doi:10.4028/www.scientific.net/AMM.138-139.612
© 2012 Trans Tech Publications, Switzerland
Online: 2011-11-16
Preparation and Characteristic Measurement of Born Doped Diamond
Film Electrode on Tantalum Substrate and Its Properties for Degradation
of High COD Wastewater
Chengyao GAO1, a
1
Chinese Peoples Armed Police Forces Academy, Langfang,Hebei, P. R. China
a
[email protected]
Keywords:Diamond film; Electrode material; Electrochemical characterization; Wastewater
treatment
Abstract.The boron-doped diamond film electrode grown on tantalum substrate (BDD/Ta) was
prepared by hot filament chemical vapor deposition (HFCVD) technique. The morphology and
quality of BDD/Ta film electrode were investigated by SEM and Raman spectroscopy respectively.
The electrochemical behavior of the BDD/Ta film electrodes in Na2SO4 solution was also
investigated by cyclic voltammetry and the window potential of BDD/Ta film electrode in Na2SO4
solution is of 4.1V, the hydrogen and oxygen evolution potentials are of -1.8V and +2.3V
respectively. The characteristic measurements of BDD/Ta film electrode and its application to
degradation of high concentration organic wastewater indicated that BDD/Ta film electrode have a
series of advantages, including high overpotential for oxygen revolution, high current efficiency,
good removal of chemical oxygen demand (COD).
Introduction
Recent researches have demonstrated that electrochemistry offers an attractive alternative to
traditional methods for treating wastewaters containing organic compounds[1, 2]. It is well known
that the anodes material is the most important of the anodic oxidation process. The pollutants
oxidation efficiency depends highly on the material and properties of the anodes[3]. In this frame,
several anode materials have been tested, but some of them presented a rapid loss of activity due to
surface fouling (glassy carbon) others released toxic ions (PbO2)[4] and others showed a limited
service life (SnO2)[5]. Synthetic boron-doped diamond (BDD) thin film coating on a p-silicon
substrate has been paid great attention recently[6, 7]. The wide potential window and the high anodic
stability of the BDD film permit its applications in electroanalysis and preparation of powerful
oxidant. The electrochemical behavior of diamond electrodes has been also studied with the goal of
developing application in the electrochemical oxidation of organic pollutants for wastewater
treatment and for electrosynthesis. However，it is impossible to wide application of the BDD film
electrodes grown on Si to treat wastewater because of the poor mechanical strength and low
conductivity of Si[8]. BDD film electrode grown on Ta has good conductivity, high mechanical
strength, and electrochemical inertness.
In this paper, boron-doped diamond film grown on tantalum substrate (BDD/Ta) film electrode
was synthesized and characterized; the electrochemical properties of BDD/Ta film electrode were
investigated. The high removal ratios of COD, high efficiency, high chemical stability were obtained
in electrochemistrydegradation by using BDD/Ta film electrode.
Experimental
Electrode Preparation. Boron-doped diamond films were grown on Ta substrates in term of
HFCVD technique. The experimental apparatus is schematically shown in Fig.1. Ta substrates (0.5
mm thick, 30 cm2 in area) were pretreated by solvent washing in acetone. After rinsing with ultrapure
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Applied Mechanics and Materials Vols. 138-139
613
water and drying, the substrates were sonicated in diamond powder/acetone slurry for 30 min.
Acetone was placed in the container filled with water and ice compounds to keep temperature
permanent. Tantalum substrates were scratched by diamond powder with a diameter of 0.5µm for the
goal of easy nucleation and then cleaned by using ultrasonic method. Filament temperature
measurement is carried out by infrared thermometer. The specific craft parameters can be seen at
reference[9] .
reacter
1.mixture of ice and water
2.acetone
3.thermal couple
4.Ta substrate
copper
pipe
Ta filament
mass flow controller
3
4
Mo
hydrogen
mass flow controller
2
1
vacuum pump
Fig. 1 Schematic diagram of HFCVD apparatus
electrode
power supplier
+
-
pump
organic warter
Fig. 2 Scheme of the equipment used for degradation of wastewater with ultra high COD
Physicochemical Characterization. Scan electron microscope (SEM) and Raman spectroscopy
were employed to investigate the morphology and quality of the BDD/Ta film. Electrochemical
measurements were obtained using a conventional three electrodes cell in conjunction with a
computer-controlled potentiostat/galvanostat. BDD/Ta film was used as the working electrode,
commercial saturated calomel electrode (SCE) as a reference electrode, and stainless steel as a
counter electrode. Voltammetry experiments were performed in unstirred solutions. The electrode
was anodically polarized for 5 min with a 1 M H2SO4 solution at 10 ampere prior to experiment. The
cyclic voltammetry (CV) curves of the BDD/Ta film electrode in the solution of 0.5M Na2SO4 were
recorded. Furthermore, the CV curves at BDD/Ta film electrode at different scan rates in 0.1M KCl
solutions containing 0.01M [K3Fe(CN)6]/[K4Fe(CN)6] were recorded to test the reversibility of it.
The scan rates were set as : 62.5，125，250，500，1000mV/s. All solutions were prepared using
reagent grade chemicals in doubly distilled water. The solutions were purged by vacuum treatment
prior to the electrochemical measurements. The degradation of organic wastewater with ultra-high
COD (>130000mg/L) was performed in a one-compartment electrolytic flow cell (see Fig.2).
BDD/Ta film was used as anode and titanium as cathode with an interelectrode gap of 5 mm, the
614
Applied Mechanics and Mechanical Engineering II
geometric area of each rectangular 30 cm2. The wastewater electrolyte was stored in a 1000mL
thermoregulated glass tank and circulated through the electrolytic cell by means of a centrifugal
pump(see Fig.2). The flow rate of the electrolyte in the cell was 160 dm3 /h. COD was determined by
the dichromate method. The instantaneous current efficiency (ICE) was calculated according to Eq.1:
ICE = FV
COD t -COD t +∆t
8I ∆t
(1)
Where CODt and CODt+∆t are the COD values at times ” t” and” t+∆t” (g O2 dm-3) respectively, I
:the applied current (A), F : the Faraday constant (96 487 C mol-1) , V : the volume of electrolyte
(dm3), and coefficient 8: the oxygen equivalent mass (g equiv.- 1).
Results and Discussion
Morphological Characterization. In Fig. 3, SEM image of BDD/Ta film shows that the granularity
of diamond particles is about 3-7µm and the oriented growth direction is along (111). Fig. 4 shows
the Raman spectrum of BDD/Ta film electrode, in which the first broad peak (A) at 1334 cm-1 is
corresponds to sp3 diamond and the peak (B) at about 1560 cm-1 is assigned to the sp2impurities.
When comparing the both peak intensities of both sp3(diamond) and sp2(non-diamond) carbon, it is
necessary to consider that Raman signal of non-diamond carbon is 50 times more sensitive than that
of the diamond one .
Fig. 3 SEM image of a polycrystalline BDD electrode grown on Ta by HFVCD method
A
6000
Counts
5000
4000
B
3000
2000
1000
1000
1200
1400
1600
Wavenumber /cm
1800
2000
-1
Fig. 4 Raman spectrum of BDD film grown on Ta by HFVCD
Electrochemical Characterization.Background cyclic voltammetric I-E curves are useful for
examining the diamond film quality, because the electrochemical response is highly sensitive to
physicochemical properties of the surface. The magnitude of the background current, the working
potential window and the voltammetric features within the working potential window are all sensitive
Applied Mechanics and Materials Vols. 138-139
615
to the presence of non-diamond, including amorphous, graphitic and carbon impurities. Fig. 5 shows
CV curve at the BDD/Ta film electrode in the solution of 0.5M Na2SO4 at scan rate of 100mV/s over
the potential range of −3.0 to 3.0V. There is no evidence for surface redox processes. Therefore, the
film appears to be ideally polarizable in this potential region. A low and featureless background
current is observed .The BDD/Ta film electrode is electrochemically stable and there no significant
decomposition of water occur over a wide potential range. In Na2SO4 solution, the overpotential
ranges from +2.3V for oxygen evolution to - 1.8V for hydrogen generation (vs. SCE). Since oxygen
evolution is the main secondary reaction on the anode, the higher onset potential for oxygen evolution
indicates the electrodes will have higher CE for organic pollutant oxidation. The CV curves of
BDD/Ta film electrode at different scan rates in 0.1M KCl solutions containing 0.01M
[K3Fe(CN)6]/[K4Fe(CN)6] redox-couple are shown in Fig. 6, in which the symmetrical anodic and
cathodic peaks associated with the oxidation and reduction of the ferricyanide–ferrocyanide couple at
the diamond–solution interface. The anodic peak potential shifts positively with the rising of the scan
rate, while the peak current increases. The ratios of anodic/cathodic peak currents approach 1, which
indicates that both ferrocyanide and ferricyanide are stable in solution and the process is a
quasi-reversible one. The linear relationship between anodic peak current Ip and the square root of
scan rate v1/2,as shown in Fig. 7, indicates a diffusion-controlled process at the electrode.
0.08
0.06
Current /A
0.04
0.02
0.00
-0.02
-0.04
-0.06
-0.08
-3
-2
-1
0
1
2
3
Potential/ V
Fig. 5 Background cyclic voltammetric I-E curve for BDD/Ta film electrode in 0.5 M Na2SO4.
0.003
1000
(a)
0.002
500
250
Current /A
0.001
125
62.5
0.000
62.5
125
-0.001
(b)
250
500
-0.002
1000
-0.003
-0.004
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
Potential /V
Fig. 6 Cyclic voltammetric I-E curve curves for BDD/Ta film electrode in 0.1M KCl + 0.01M
K3Fe(CN)6/K4Fe(CN)6 at different scan rates
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Applied Mechanics and Mechanical Engineering II
-3
3.5x10
-3
3.0x10
-3
Current /A
2.5x10
-3
2.0x10
-3
1.5x10
-3
1.0x10
-4
5.0x10
0.2
0.3
0.4
0.5
0.6
1/2
0.7
1/2
V / V
s
0.8
0.9
1.0
1.1
-1/2
Fig. 7 Plots of peak currents versus square roots of scan rates in Fig.6
COD Remove of Organic Water. As shown in Fig.8, the high remove ratio of COD and high
current efficiency were obtained by using BDD/Ta film electrode. As it can be observed, the almost
complete removal of the COD in the organic water was obtained, no refractory compounds were
found as product at the end of the electrochemical treatment. At the beginning of electrolysis when
the COD is relatively high(>10000mg/L), the COD decreased linearly with time and the current
efficiency was almost 100% because the oxidation was under limiting current control and all the
electrogenerated hydroxyl radicals were consumed in the oxidation of the organics. Only when the
COD is below 10000mg/L, the COD decreased exponentially and the current efficiency was lower
than 100% because the process was under mass-transfer control and the secondary reaction of oxygen
evolution occurred.
140000
100
ICE /%
120000
COD/ mg/L
100000
80
60
40
80000
20
00 2 4 6 8 10 12 14 16 18 20 22
60000
Time /h
40000
20000
0
0
2
4
6
8
10
12
14
16
18
20
22
Electrolysis Time /h
Fig. 8 Degradation curve of wastewater with ultra high COD on BDD/Ta film electrode.ICE curve
was inserted
Conclusions
Diamond films could be successfully deposited on Ta substrates by HFCVD. Physicochemical and
electrochemical examinations demonstrated that the BDD/Ta films had compact microstructure, low
background current, high overpotential for oxygen evolution. The electrocatalytic activity of the
electrodes was found to be superior in oxidizing high concentration organic pollutants, including the
high remove ratio of COD, high current efficiency and high chemical stability.
Applied Mechanics and Materials Vols. 138-139
617
Acknowledgement
Wewould like to express our thanks to Prof. M.Chang at Tianjin University of technology for his
assistance anddiscussion on HFCVD experiments.
This work was supported by National Natural Science Founda-tion of China (no. 50972105) and
National high-tech research and development 863projects（no. 2009AA03Z444）.
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E50
Applied Mechanics and Mechanical Engineering II
10.4028/www.scientific.net/AMM.138-139
Preparation and Characteristic Measurement of Born Doped Diamond Film Electrode on Tantalum
Substrate and its Properties for Degradation of High COD Wastewater
10.4028/www.scientific.net/AMM.138-139.612