carbon fiber composite as anode in Brilliant Green dye electrochemical degradation

Diamond & Related Materials 103 (2020) 107708

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Singular properties of boron-doped diamond/carbon fiber composite as anode in Brilliant Green dye electrochemical degradation L.A. Pereira, A.B. Couto, D.A.L. Almeida, N.G. Ferreira

T

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Laboratório Associado de Sensores e Materiais - Instituto Nacional de Pesquisas Espaciais (INPE), Av. dos Astronautas, 1758, 12227 010, São José dos Campos, SP, Brazil

A B S T R A C T

Carbon fiber (CF) and boron doped diamond/carbon fiber (BDD/CF) were applied as anodes for the Brilliant Green dye electrochemical degradation process. CF substrates were obtained from polyacrylonitrile precursor heat treated at 1000 °C in a nitrogen atmosphere. BDD films were grown on CF substrates by hot filament chemical vapor deposition with boron source from an additional hydrogen line passing through a bubbler containing B2O3 dissolved in methanol solution with B/C ratio of 15,000 ppm. The electrodes were characterized by field emission gun-scanning electron microscopy images, Raman spectroscopy, and electrochemical impedance spectroscopy showing notable tridimensional characteristics besides their high diamond quality. They were applied to electrochemical degradation of Brilliant Green dye at different current densities using a concentration of 100 mg L−1. Their electrolysis efficiency was analyzed by UV/Vis spectrophotometry technique. The results showed that BDD/CF electrodes were efficient in the solution color removal, regardless of the current density value. BDD/CF electrode under the lowest current density of 10 mA·cm−2 presented the best performance with higher kinetic constant and color removal percentage associated to its low energetic consumption.

1. Introduction Wastewaters contamination by dyes has been causing great concern due to the large volumes that are daily discharged into rivers and effluents, promoting not only aesthetic problems but also toxic effects on aquatic organisms and humans. Thus, the disposal of considerable concentration of synthetic dyes in the environment represents an important challenge for scientific community. As dyes are difficult to degrade due to their complex structure and synthetic origin, they constitute object of large interest is the development of advanced oxidation processes (AOPs) for application in wastewater treatment. In this sense, electrochemical advanced oxidation processes (EAOPs) have received high attention for their removal capability of organic pollutants from wastewater not to mention their high efficiency in the organic degradation. EAOPs target the production of hydroxyl radicals (%OH), highly oxidative species, and therefore can be viewed as processes allied with the Twelve Principles of Green Chemistry regarding the use of catalysis [1,2]. Concerning EAOPs, boron-doped diamond (BDD) has appeared as a very effective electrode for organic pollutants degradation, which allows reaching a high anode potential to generate the hydroxyl radical (% OH). Kinetic processes and electrochemical reversibility strongly depend on the electrode surface making surface treatments commonly employed to improve the response on flat solid electrodes. These treatments include changing surface functional groups, surface

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crystallographic structure, and surface roughness. In the case of carbonaceous materials, heat treatments, acid chemical treatments, laser treatments, and surface polishing treatments have been employed in order to control surface roughness and increase the electrode activation area. Therefore, some preceding studies have demonstrated that the electro-catalytic activity of BDD is highly affected by its sp2/sp3 ratio [3] not to mention its surface termination [4], roughness, film thickness, and also film morphology as micro or nanocrystalline diamond [5,6]. Besides, other important contribution for BDD electrode may come up when it is grown on porous materials [7]. Historically, our group has dedicated special attention to growing and characterizing diamond films on porous substrates also aiming at their electrochemical applications. Contributions in this subject may be cited as micro and nanocrystalline diamond formation on reticulated vitreous carbon (RVC) substrate [8,9]. Moreover, micro and nanodiamond films growth on carbon of fibers (CF) [10–13] and porous silicon substrates were structurally and voltammetry studied for electrochemical applications [6,14,15], and diamond formation on 3D-porous titanium [15–17], among others. In addition, keeping in mind that among carbonaceous materials CF and activated CF have successfully been used as anode for degrading organic pollutants [18–20], we have shown that CF represents a very attractive substrate to grow BDD films in order to obtain a three-dimensional material, which exhibits high surface area and high active site density in a homogeneous porosity distribution by combining

Corresponding author. E-mail address: [email protected] (N.G. Ferreira).

https://doi.org/10.1016/j.diamond.2020.107708 Received 24 October 2019; Received in revised form 8 January 2020; Accepted 10 January 2020 Available online 10 January 2020 0925-9635/ © 2020 Published by Elsevier B.V.

Diamond & Related Materials 103 (2020) 107708

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used the same electrode at least two times to reproduce one specific measurement. To analyze the dye decolorization degree, aliquots were removed at time intervals of 0, 15, 30, 45, 60, 75, 90, 120, 150, 180, 240 and 300 min and evaluated by UV–vis spectrophotometer analyzer from Varian Cary 50 at λmax = 426 nm. The decolorization percentage was calculated using the following equation [22]:

our previous studies with the suitable literature. Also, these electrodes present good electrical conductivity, high overvoltage for oxygen and hydrogen evolution keeping their chemical inertness. Thus, BDD/CF as 3D anode for dye degradation may represent a contribution to the literature. CF and BDD/CF electrodes were studied regarding their morphological, structural, and electrochemical results in relation to their charge transfer resistance (Rct) from electrochemical impedance spectroscopy (EIS). Although these anodes required a relatively simple way to prepare them, BDD deposition on CF represents a competition process between diamond deposition and CF etching in the HFCVD (hot filament chemical vapor deposition) reactor. Finally, the electrodes were used for Brilliant Green (BG) dye electrochemical degradations at different current densities. The electrolysis results were assessed by UV/ Vis spectroscopy technique comparing the decay curve concentrations, apparent kinetic constants, and energetic consumptions.

%decolorization =

2. Experimental section

where ΔEc is the average cell voltage (V), I is the electrolysis current (A), t is the electrolysis time (h), and Vs is the cell solution volume (m3). The first-order kinetics may be express as [7]:

A0 − At × 100 A0

(1)

where A0 and At are the dye absorbance at the beginning and at the instant t of the electrolysis, respectively. The energetic consumption (EC) for each current density was evaluated using [23]:

EC =

2.1. Preparation of the BDD/CF composites

ln CF substrates were obtained from polyacrylonitrile precursor heat treated at 1000 °C in a nitrogen atmosphere, according to previous paper [21]. BDD films were grown on CF substrates with dimensions of 2.5 × 2.5 × 0.2 cm3. HFCVD technique was used for diamond film syntheses in a gas mixture of 198/2 sccm of H2/CH4 at 800 °C under pressure of 4 × 103 Pa for 18 h. Boron source was obtained by an additional hydrogen line passing through a bubbler containing B2O3 dissolved in methanol solution with boron/carbon ratio of 15,000 ppm. To avoid the substrate sp2 etching as well as to increase the diamond nucleation in the HFCVD environment, CF samples were pre-treated with a seeding methodology using a diamond powder with grain size of 250 nm dissolved in hexane solution and immersed in an ultrasonic bath for 2 h. In this procedure, a thin layer of diamond powder can be observed covering the carbon fibers. CF and BDD/CF electrodes were mounted using Teflon frame connected to a cylindrical Teflon support. A Pt wire was responsible for the electrical contact direct in the CF or BDD/CF passing through a hole in this cylindrical Teflon support avoiding Pt contact with the solution.

∆Ec ·I ·t Vs

C = −kapp t C0

(2)

(3)

where kapp is the apparent reaction kinetic constant whereas C0, and C are the concentrations of BG dye at initial stage and after degradation time t. 3. Results and discussions 3.1. Morphological characterizations of CF and BDD/CF The morphology results of 3D BDD/CF composites may present singular characteristics concerning diamond growth on carbon porous substrate as well as the oxygen presence on the gas phase in this CVD process. Firstly, diamond deposition on carbon substrate represents a challenge considering the rich atomic hydrogen environment in HFCVD reactor, which despite being the main growth precursor it causes um effective etching in sp2-bounded surface like graphitic and amorphous carbon. Thus, the substrate pre-treatment should be efficient to improve the diamond growth saturating CF surface of sp3 bonds to decrease the sp2-bond etching on CF substrate caused by atomic hydrogen present in the growth environment [24,25]. In addition, as we have already discussed in previous papers, microcrystalline diamond growth on reticulated vitreous carbon (RVC) depends on the growth time, sample depth, and carbon graphitization index. In these studies a microcrystalline morphology was observed on RVC surface whereas in deeper sample regions (at about 1.5 mm from sample top), nanocrystalline diamond was deposited as a result of poor concentration of atomic hydrogen due to its limited diffusion coefficient for sample deeper planes [8,9,26]. Secondly, concerning the doping process, from B2O3 dissolved in methanol source, it can promote a competition course to produce and to destroy sp3 bonds on growth surface because of the higher %OH formation, which is much more effective than atomic hydrogen to clean the diamond surface and may be positive in small amounts in this CVD process [27]. Consequently, for BDD/CF composites, a fine control is required to assure the diamond growth is a dominant process avoiding CF sp2 etching in addition to the diamond sp3 destruction by %OH presence. Therefore, from the above considerations the FEG-SEM images of CF substrate and BDD/CF composite may be analyzed with their remarkable appearance. CF substrate from PAN precursor (Fig. 1(a)) exhibits its typical top view morphology, with grooves parallel to the fiber axis resulting from the carbonization procedure [28]. Image (a) inset shows the fibers with lower magnification evidencing their random distribution. BDD/CF composite image (Fig. 1(b)) confirms the top CF surface completely covered by a polycrystalline diamond coating. The film

2.2. Characterization The morphological and structural characteristics of BDD/CF composites were investigated by field emission gun scanning electron microscopy (FEG-SEM) from TESCAN MIRA 3 microscope system and by Raman scattering spectroscopy from Horiba Scientific LabRAM HR Evolution microscope system with laser beam line of 514 nm. Electrochemical impedance spectroscopy curves (EIS) were acquired in a conventional three-electrode cell using Ag/AgCl(sat) electrode as reference electrode and a platinum sheet as counter-electrode in deaired 0.5 mol L−1 H2SO4 solution with N2, using a potentiostat PGSTAT 302 with FRA module (AUTOLAB–EcoChemie). The measurements were recorded at open circuit potential (OCP) with ± 10 mV of the potential amplitude in the frequency range from 105 to 10−3 Hz. 2.3. Electrochemical oxidation process For electrodegradation experiments, electrolysis was performed using a polypropylene home-made single cell containing 450 mL of 100 mg L−1 of Brilliant Green dye (supplied by Aldrich ~96% m/m) in 0.1 mol L−1 K2SO4 aqueous solution. All experiments were carried out under constant stirring, temperature at 25 °C, and total treatment time of 300 min at current densities of 10, 25, 50 and 100 mA cm−2. Considering the experiments repeatability, for CF and BDD/CF electrodes each electrolysis process was performed at least three times using three different electrodes obtained in similar conditions. In addition, we 2

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Fig. 1. FEG-SEM images of (a) CF and (b) BDD/CF composite top view; (c) BDD/CF cross-section image sample top plane; (d) BDD/CF cross-section image sample deep plane. Image (a) inset depicts CF sample with lower magnification.

grains are faceted with symmetric and smooth faces showing uniform and homogeneous morphology characteristic of microcrystalline diamond [29]. Detailed BDD/CF morphology analyses may be done using FEG-SEM cross section images where two regions of sample depth may be discussed. Fig. 1(c) and (d) depicted cross section images of BDD/CF sample considering two different depths related to the sample top surface. Image 1(c) shows one lateral sample view close to its top, considering the focus image of one fiber enwrapped by BDD film, with high polycrystalline texture. Moreover, the image depicts a central hole may be related to the broken and/or etched CF, where both processes could occur. On the other hand, Fig. 1(d) shows another cross section view of isolated fiber in a deeper plane concerning sample top. This image presents thin diamond coating with CF integrity well defined, which may represent a film in initial growth stage, comparable to that of nanocrystalline formation. These differences are well established for CVD process. As already mentioned for porous substrates, deeper planes are related to sample regions with poor atomic hydrogen presence which drastically decreases the diamond growth rate not to mention the CF etching maintenance.

1332 500

Intensity / (a.u.)

1220

1360

(b)

1580

(a) 400

800

1200

1600

-1

2000

Raman shift / cm

Fig. 2. Raman spectra of (a) CF and (b) BDD/CF electrodes. 3

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electrode presented lower Rct than that of CF (Table 1). It is noteworthy that its CPE 2 value is at about one order of magnitude higher than that of CF, which indicates a better electrochemical performance. This CPE 2 value associated with lower Rct is considered beneficial when the material is applied as anode for degradation of organic compounds in water treatment [7]. This behavior is ascribed to the fact that the lower Rct the higher the electrode reaction kinetics resulting in the fast electron transfer process that favor the photocatalytic activities promoting more efficiently the destruction of pollutants. Therefore, it has a direct relationship between the nature of the electrode and the degradation efficiency which is strongly influenced by the electrode selectivity in the final stage of pollutant degradation [48]. In addition, the Body plots illustrate the complementary EIS data, in which the smallest impedance modulus is presented by the BDD/CF electrode, as shown in Fig. 3(c). It depicts the largest phase angle, i.e., 85°, while for CF this value is at about 75°, as exposed in Fig. 3(d), with a profile similar to that of an ideal capacitor whose phase angle is 90°.

3.2. Raman characterizations Raman spectra of CF and BDD/CF composites are shown in Fig. 2. Fig. 2(a) exhibits the characteristics bands of carbonaceous materials, i.e., D and G bands [30]. D band, of around 1360 cm−1, is associated with the disorder on the crystalline structure, while that G band, around 1580 cm−1, is related with the ordered graphitic structure [31]. Besides, D widening is enhanced by the presence of defects, and it is more common in carbon material produced at low heat treatment as in the case of CF treated at 1000 °C [32,33]. Also, is possible verify that around 1500 cm−1 there is a high intensity signal between the D and G bands that can be attributed to the designated D3 or D″ band. These bands originate from vibrations associated the amorphous carbon, such as organic molecules, stacking faults, and non-graphitic carbon with a high density of functional groups and/or defects [34–36]. BDD/CF Raman spectrum in Fig. 2(b) evidences the well-defined peak at around 1332 cm−1, which corresponds to the diamond firstorder phonon vibration. This also shows two centered bands around 500 and 1230 cm−1, both characteristics of boron incorporation in the diamond crystal lattice. The first band has been explained as the vibration of boron pairs in diamond and the second band while the second has been attributed to the induced disorder in the diamond structure due to the boron incorporation [37,38]. Niu et al. have discussed the bands at 500 and 1230 cm−1 are both superposed including not only C vibrations but also BeB vibrations and BeC vibrations, respectively [39]. Other authors have pointed out that the 1200 cm−1 band appearance is because of the relaxation of the Raman wave vector conservation rule in high lattice disordering by bore introduction in the diamond lattice [40,41]. More recently, V. Motet et al. have discussed these two bands as the Fano-shape results from interaction between discrete diamond zone-center phonon (ZCP) and electronic Raman scattering effects ascribed to the maxima of the phonon density of states (PDoS) [42,43].

3.4. Electrochemical oxidation of BG dye The electrochemical oxidation analyzes for the two electrodes studied were evaluated taking into account the Brilliant Green dye organic pollutant. This dye presents two characteristics absorption bands at 317 and 426 nm where its green color is attributed to the latter associated to the azo group (n → π∗), while the former is related to the aromatic ring transitions (π → π∗) [49]. The results are presented in Fig. 4 from UV–Vis peak intensities at 426 nm at the lowest (10 mA·cm−2) and the highest (100 mA·cm−2) current density values for both electrodes. We can observe the color removal percentage evolution for CF and BDD/CF at fixed electrolysis times of 45, 90, 120, 180, and 300 min. A significant current density effect on CF electrode is observed with high color removal increase at 100 mA·cm−2, but it is smaller to those for all times when compared to BDD/CF at 10 mA·cm−2. This behavior is a strong indicator of the BDD film influence. On the othe hand, for BDD/ CF electrode the color removal percentage was only lightly influenced by a ten times higher current density value, where only a small difference at 90 min of electrolysis occurred. For a more detailed analyze, the whole set of UV–vis spectra was studied for both anodes. Using the peak at 426 nm in the electrolysis range for one anode at a fixed current density, the absorbance versus dye concentration was plotted and allowed a linear fitting. From this calibration curve (not shown) the dye concentration decay (C/C0) as a function of the electrolysis time was obtained for all experiments presented in this manuscript. Fig. 5(a) depicts C/C0 versus electrolysis time for CF and BDD/CF electrodes using current density values of 10, 25, 50, and 100 mA·cm−2, with linear and angular coefficients obtained from the calibration curve. A drastic change in C/C0 decay may be observed for all current densities when BDD/CF composite was used as anode. Nonetheless, for CF electrode some aspects may be pointed out where a small change occurred from 10 to 50 mA·cm−2 while for 100 mA·cm−2 the degradation process clearly presented a high gain with an almost color removal in the ending electrolysis time. In addition, for BDD/CF composite the C/C0 decays are very close for all current density values due to the superior response of diamond electrode for organic degradation. Even when 100 mA·cm−2 was used for CF electrode its decay is lower than that for BDD/CF at 100 mA·cm−2, mainly up to 100 min of electrolysis time. In general, dye concentration decays depicted an exponential performance, which is more pronounced up to 90 min. This behavior suggests a first-order kinetic process, where the reaction rate is proportional to the reagent concentration at t time. These linear plots are shown in Fig. 5(b) corroborating that the reactions evolved in the BG dye electrochemical degradation followed the pseudo first-order criterion [50]. To compare the kinetic responses, Table 2 presents the values of kapp and EC evaluated from Eqs. (3) and (2), respectively, taking into account both electrodes and the whole range of current density studied.

3.3. Electrochemical impedance spectroscopy analyses From EIS technique a more detailed study of the processes that occur at the electrode/solution interfaces may be carried out, mainly to evaluate the charge transfer resistance (Rct) of the electrodes. This parameter is directly related to the electronic transfer resulting from kinetic processes that occur during electrochemical measurements [44]. To further evaluate EIS measurements, the resulting Nyquist diagram is generally divided into three regions of high, medium, and low frequencies, so that the extrapolations from these regions with the real Z′ axis provide the values of the effective series resistances, which are evident for electrodes of porous carbonaceous materials. Fig. 3(a) shows the Nyquist graphs obtained for CF and BDD/CF samples. BDD/ CF electrode presented predominant capacitive behavior and smaller impedance at low frequencies when compared to those of CF electrode. In Fig. 3(b), the small semicircle presented in the high frequency regions of the Nyquist diagram for the composite electrode (BDD/CF) is characteristic of RC association, resulting from the charge transfer resistance (Rct) in parallel with the double layer capacitance, known as constant phase element (CPE). This association represents electronic transport at liquid-solid interfaces [45]. Table 1 displays the main electrochemical parameters obtained by fitting of EIS data simulating an equivalent circuit for CF and BDD/CF. The circuits are composed of the following components. Rct and Warburg impedance are in series both being in parallel with the capacitance electric double layer (Cdl). Then, this set is in series with the resistance of the solution (Rs) and the two in accordance with the model proposed by Randles [46]. Nonetheless, in the case of porous electrodes, the analysis is more complex and the physical interpretation of the Cdl cannot be described as a pure capacitance, being represented by a CPE1 [47]. The second RC association is related to the interface resistance in parallel with the limit capacitance (CPE2) of the film in the low frequency region. BDD/CF 4

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Fig. 3. Nyquist diagrams of the CF and BDD/CF composite (a–b), and Body plots (c–d).

have a huge difference of 57.3 against 2.1 kWh·m−3, respectively. The BDD/CF efficiency is noteworthy and deserves to be explored with respect in comparison with our previous paper where the growth of 2D BDD/Ti electrodes was systematically optimized concerning their sp2 amount and doping level in a large sample set [3]. This paper also discussed these 2D BDD/Ti electrodes applied to BG dye degradation and we found the best kapp value of 16 × 10−3 min−1, but using 100 mA·cm−2. Thus, in similar experiments, we have demonstrated the importance and the contribution to study 3D BDD/CF electrodes. Besides, from kapp values of Table 2, it is important to highlight that the current density increase did not improve the dye removal for BDD/CF electrode. In fact, this process with lower current densities (10 and 25 mA·cm−2) presented higher dye removal efficiency. This performance may be explained taking in mind that although higher current density values can promote the increase of %OH radical production, the competitiveness between pollutant oxidation and oxygen evolution

Table 1 Main parameter values obtained from the fitting of EIS Nyquist spectra for CF and BDD/CF electrodes. Electrode

CF BDD/CF

Rs (Ω cm2)

1.02 1.08

Rct (Ω cm2)

0.98 0.54

x2

CPE2

CPE1 Y0 × 10−4 (Ω−1 sn)

n

Y0 × 10−3 (Ω−1 sn)

n

10−2

113.7 1880

0.92 0.16

4.88 39.6

0.83 0.96

7 5

These results confirmed the superior performance of BDD/CF electrode regardless of the current density values. For CF electrode even using a current density 10 times higher (100 mA·cm2) compared to that for BDD/CF electrode at 10 mA·cm2 its kapp value is lower. That means 14.8 × 10−3 against 21.6 × 10−3 min−1. Concerning EC values we 5

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Table 2 kapp and EC values for BG dye electrochemical degradation using 10, 25, 50, and 100 mA·cm−2 for CF and BDD/CF electrodes. Sample

I (mA·cm−2)

R

kapp (×10−3 min−1)

EC (kWh·m−3)

CF

10 25 50 100 10 25 50 100

0.99894 0.99984 0.99607 0.99622 0.99443 0.99515 0.99895 0.99845

6.8 7.0 7.9 14.8 21.6 21.3 18.4 20.4

2.0 7.2 19.3 57.3 2.1 7.1 19.6 59.0

BDD/CF

4. Conclusions According to our preceding experience described in the introduction section, this manuscript was very important to sum up as well as to consolidate the study concerning growth, characterization, and application of 3D BDD/CF electrode. Once the challenge of growing diamond films on carbon substrate was overcome, this electrode proved to be highly efficient to color removal from electrochemical oxidation of BG dye. BDD film presented a high nucleation rate and a successful growth process enwrapping all carbon fibers not only in sample top surface, but revealing a real 3D formation. BDD/CF also presented high quality and doping control assured by its Raman spectrum. The degradation kinetic reaction did not show a strong dependence on the current density for BDD/CF electrode whereas CF electrode was competitive only for current density of 100 mA·cm2. In summary, BDD/CF electrode exhibited high electro-catalytic oxidation efficiency for BG dye degradation with singular properties such as high reaction rate, lightweight, small volume, and low energy consumption.

Fig. 4. Color removal percentage evolution for CF and BDD/CF at a fixed electrolysis times using 10 and 100 mA·cm−2.

CRediT authorship contribution statement L.A. Pereira: Writing - original draft, Data curation, Formal analysis, Conceptualization. A.B. Couto: Formal analysis, Methodology. D.A.L. Almeida: Methodology, Data curation, Validation. N.G. Ferreira: Supervision. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by Brazilian Agencies of FAPESP (grant number 2017/10118-0), CAPES, and CNPq. Special thanks to L. G. Vernasqui by FEG-SEM images. References [1] C.A. Martınez-Huitle, E. Brillas, Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods: a general review, Appl. Catal. B Environ. 87 (2009) 105–145, https://doi.org/10.1016/j.apcatb.2008.09.017. [2] E. Brillas, C.A. Martínez-Huitle, Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods. An updated review, Appl. Catal. B Environ. 166–167 (2015) 603–643, https://doi.org/10.1016/j.apcatb.2014.11.016. [3] F.L. Migliorini, J.R. Steter, R.S. Rocha, M.R.V. Lanza, M.R. Baldan, N.G. Ferreira, Efficiency study and mechanistic aspects in the Brilliant Green dye degradation using BDD/Ti electrodes, Diam. Relat. Mater. 65 (2016) 5–12, https://doi.org/10. 1016/j.diamond.2015.12.013. [4] A.F. Azevedo, M.R. Baldan, N.G. Ferreira, Doping level influence on chemical surface of diamond electrodes, J. Phys. Chem. Solids 74 (2013) 599–604, https://doi. org/10.1016/j.jpcs.2012.12.013. [5] A.F. Azevedo, F.A. Souza, J.T. Matsushima, M.R. Baldan, N.G. Ferreira, Detection of phenol at boron-doped nanocrystalline diamond electrodes, J. Electroanal. Chem. 658 (2011) 38–45, https://doi.org/10.1016/j.jelechem.2011.04.020.

Fig. 5. (a) Dye concentration decay curves and (b) ln(C0/C) plots as a function of electrolysis time for CF and BDD/CF electrodes at current densities of 10, 25, 50, and 100 mA·cm−2.

reactions also increases, decreasing dye removal efficiency [19,48,51,52]. In addition, for BDD/CF electrode, the CE for 100 mA·cm−2 is 30 times higher than that of 10 mA·cm−2 showing that this lowest current density is the most suitable value where BDD/CF showed its lowest CE associated to its highest reaction kinetic.

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