2.1. Characterization
The synthesis process of SnO
2-Mn
2O
3@CC nanosheets grown on carbon cloth (CC) is shown in
Figure 1a, wherein self-supported SnO
2-Mn
2O
3@CC was synthesized using a simple hydrothermal reaction, followed by an annealing treatment at 400 °C. The crystal structure of these electrode materials was characterized using X-ray diffraction (XRD). In
Figure 1b, the diffraction peaks at 26.2° and 44.2° are corresponded to the (002) and (101) crystal planes of C substrate (JCPDS No. 75-1621), respectively. The diffraction peaks located at 32.9°, 38.2°, 45.2°, 49.3° and 55.2° can be, respectively indexed to the (222), (400), (332), (431) and (440) crystal planes of Mn
2O
3 (JCPDS No. 41-1442). The residual diffraction peaks at 26.6°, 33.9°, 37.9° and 51.8° match well with the (110), (101), (200) and (211) crystal planes of SnO
2 (JCPDS No. 72-1147), respectively. The above results confirm that the prepared sample is composed of SnO
2, Mn
2O
3 and carbon substrate. Additionally, XRD pattern of the precursor in
Figure S1 only detects the existence of SnO
2 and MnF
2, which further confirm the absence of Sn-MnO
x compound in the one-step preparation of SnO
2-Mn
2O
3/CC electrode materials using the hydrothermal method. For comparison, the similar hydrothermal reaction and annealing process are repeated based on the single Sn or Mn source as precursor; the corresponding samples are, respectively denoted by SnO
2@CC and Mn
2O
3@CC. XRD patterns in
Figure S2 verify the successful preparation of SnO
2@CC (
Figure S2a) and Mn
2O
3@CC (
Figure S2b) samples.
SEM images in
Figure 1c,d and
Figure S3 show that both Mn
2O
3@CC and SnO
2@CC are composed of small and compact particles, while the surface of SnO
2@CC sample presents a more tight-packed particles and seems to have a dense cover layer, which is verified by a broken surface with an intentional scratch (
Figure S4). It is well known that a compact surface can block the contact between electrode and electrolyte, unavailing electrocatalytic activity. In contrast, the synthesized SnO
2-Mn
2O
3@CC sample (
Figure 1e) exhibits an obviously different morphology, which is composed of nanosheets with a thickness of 5 nm. As depicted in
Figure 1e, the interlaced nanosheets are vertically aligned on the carbon fiber with irregular orientation. Compared with the tight-packed particles, the nanosheets-like morphology of SnO
2-Mn
2O
3@CC sample offers an enhanced surface area, availing the connectivity of catalytic active sites on electrode and electrolyte. It is worth noting that the self-supported nanosheets on CC substrate show a robust stability in comparison with the common electrode via the multiple dip/brush coating accompanied by the thermal decomposition method. A hydrothermal and annealing treatment can promote the adhesion between active materials and the substrate scaffold, and avoid the active materials peeling from the substrate, thus contributing to good stability. In addition, the self-supported nanosheets structure can ensure a rapid electron transfer between the electrode and the active layer, which shortens the charge transfer pathway compared with the aggregated particles.
Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were use to investigate the morphology and structure of SnO
2-Mn
2O
3 nanosheets, which was prepared using the scratching from the carbon cloth framework. As shown in
Figure 1f, SnO
2-Mn
2O
3 presents a nanosheet morphology, consistent with SEM results. HRTEM image displays that the SnO
2-Mn
2O
3 nanosheets are composed of crystalline nanoparticles (
Figure 1g). The lattice fringes with a distance of 0.334 and 0.271 nm are corresponded with (110) plane of SnO
2 and (222) plane of Mn
2O
3, respectively, which corroborates the fact that the nanosheet is composed of SnO
2 and Mn
2O
3. The scanning TEM (STEM) image and energy dispersive X-ray spectroscopy (EDS) elemental map** images (
Figure S5) confirms the existence and homogeneous distribution of Sn, Mn and O elements on SnO
2-Mn
2O
3 hybrid nanosheets. Additionally, the content of Mn
2O
3 is 57.8 wt% in the mixed oxide nanosheets (
Figure S6 and Table S1). Based on the above result, we can deduce that the additive of Mn precursor source can not only regulate the morphology to form three-dimensional intersected nanosheets, but also promote the hybrid Mn
2O
3 and SnO
2 composition.
X-ray photoelectron spectroscopy (XPS) measurement was performed to analyze the chemical composition and the valence state of the SnO
2-Mn
2O
3@CC sample. In
Figure 2a, the characteristic peaks of Sn, C, Mn and O can clearly be observed in the XPS survey, confirming the existence of Sn, C, Mn and O in SnO
2-Mn
2O
3@CC. In Sn 3d XPS spectrum (
Figure 2b), the peaks located at 487.2 and 495.6 eV are ascribed to Sn 3d
5/2 and Sn 3d
3/2, respectively. Additionally, binding energy difference between Sn 3d
3/2 and Sn 3d
5/2 is about 8.4 eV, indicating that Sn
4+ exists in SnO
2-Mn
2O
3@CC [
29]. The Mn 2p XPS spectrum can be deconvoluted into three characteristic peaks (
Figure 2c). The peaks at 641.5 and 653.1 eV, respectively correspond to Mn 2p
3/2 and Mn 2p
1/2, suggesting the existence of Mn
3+ in Mn
2O
3, and the peak at 645.5 eV is the satellite peak (marked by Sat.). The O 1s spectrum (
Figure 2d) can be fitted into two characteristic peaks. The O 1s peak at 530.75 eV is associated with the lattice oxygen (O
L) existing in the metal oxide crystal. Peaks located at 532.26 eV is attributed to the adsorbed oxygen (O
ad). In general, O
ad can promote the generation of ·OH during the electrooxidation process, owing to the easy exchange between O
ad and oxygen adsorbed molecules on the catalytic layer [
38]. The aforementioned results indicate that SnO
2 and Mn
2O
3 successfully coexisted in the prepared SnO
2-Mn
2O
3@CC sample.
2.2. Electrochemical Characterization
Linear voltammetry scanning (LSV) curves of different electrodes were tested in 0.5 M NaCl solution to evaluate the OEP of these electrodes, which can be obtained by the intersection value of the horizontal line and the tangent of LSV curve. OEP is an important factor to determine the electrochemical oxidation capacity of electrodes, because a large OEP value can restrain the oxygen evolution side reaction, thus providing more ·OH and larger current efficiency for electrooxidation reaction; meanwhile, it also restrains the oxidation of substrate produced by oxygen. Consequently, a larger OEP value usually suggests a superior electrooxidation catalytic activity of the electrode. As shown in
Figure 3a, the OEP value of SnO
2-Mn
2O
3@CC electrode is 1.73 V vs. the saturated calomel electrode (SCE), exceeding SnO
2@CC (1.68 V vs. SCE) and Mn
2O
3@CC (1.70 V vs. SCE). These results suggest the SnO
2-Mn
2O
3@CC electrode induced by Mn precursor can hinder the oxygen evolution side reaction on anode and contribute to a higher current efficiency in comparison with the single metal oxide electrode.
Interfacial impedance of different electrodes was evaluated using the electrochemical impedance spectroscopy (EIS). Nyquist plots and fitting results of different electrodes are displayed in
Figure 3b and
Table 1. In the equivalent circuit (the inset in
Figure 3b), R
s, R
f and R
ct, respectively represent the solution resistance, oxide film resistance and charge transfer resistance on the interface of electrolyte and electrocatalyst; and Q
f and Q
dl depict the film capacitance and the double layer capacitance, respectively. R
ct of SnO
2-Mn
2O
3@CC electrode is 8.51 Ω cm
2, which is far less than SnO
2@CC (70.65 Ω cm
2) and Mn
2O
3@CC (34.53 Ω cm
2). Moreover, the SnO
2-Mn
2O
3 hybrid nanosheets (
Table S2) also present the optimal intrinsic electrical conductivity. The aforementioned result verifies that the SnO
2-Mn
2O
3 hybrid nanosheets induced by the Mn precursor additive can improve the charge transfer kinetics, which favors the electrochemical/electrooxidation catalytic activity of electrodes.
A large electrochemical active surface area (ECSA) is also an important factor in evaluating the catalytic activity area for a good electrode, which can afford more exposed catalytic active sites to promote electrocatalytic oxidation. According to ECSA = C
dl/C
s, ECSA is proportional to the double-layer capacitance (C
dl), thus C
dl can be used to assess the ECSA and can be calculated using cyclic voltammetry (CV). In
Figure S7, CV curves of different electrodes are tested in the non-Faraday region from 0.5 to 0.7 V vs. SCE in 0.5 M NaCl solution at a scanning rate ranging from 20 to 100 mV s
−1. As shown in
Figure 3c, SnO
2-Mn
2O
3@CC exhibits the largest C
dl value (6.19 mF cm
−2), exceeding SnO
2@CC (3.23 mF cm
−2) and Mn
2O
3@CC (1.37 mF cm
−2). The result suggests that hybrid SnO
2-Mn
2O
3@CC can afford the largest catalytic surface area, matched well with SEM results.
Voltammetric charge (
q*) is also a critical parameter to assess the active surface area of electrodes, which is relevant to the specific electroactivity of the sites and can be obtained via CV curves [
39]. It is worth noting that the total voltammetric charge (
q*
T) contains the outer voltammetric charge (
q*
O) and the inner voltammetric charge (
q*
I), which can be calculated from the relationship of
q* and scan rate (
v). The detailed equations are as follows:
At a very low scan rate, the total active surface containing the inner and outer of electrode can participate in the reaction, thus the total voltammeric charge
q*
O can be calculated by Equation (1). The intercept of the straight line in
Figure 3d is the reciprocal of
q*
T, whereas at a high scan rate, especially when
v approaches to ∞, electrolyte ions only contact with the outer surface of electrode, and has no time to permeate into the inside of the electrode, thus only
q*
O contributes to the charge. According to Equation (2),
q*
O is equal to the intercept of
q* vs.
v−1/2 (
Figure 3e). As listed in
Table 2, SnO
2-Mn
2O
3@CC electrode presents the largest total voltammetric charge, which is, 2.2 and 6.2 times of SnO
2@CC and Mn
2O
3@CC, respectively. Similarly, both
q*
O and
q*
I value of SnO
2-Mn
2O
3@CC are still greatly larger than SnO
2@CC and Mn
2O
3@CC electrodes. These above results further confirm that the active surface area of hybrid SnO
2-Mn
2O
3@CC is larger than the single metal oxide electrode, which can ascribe to the three-dimensional nanosheets architecture produced by the addition of Mn precursor.
Service life of electrode is another pivotal factor to estimate the catalytic activity, which determines its further practical application. An accelerated life test of different electrode was carried out at 100 mA cm
−2. As manifested in
Figure 3f, SnO
2-Mn
2O
3@CC electrode exhibits the longest service life, exceeding that of SnO
2@CC and Mn
2O
3@CC, which indicates that the hybrid SnO
2-Mn
2O
3@CC electrode induced by the addition of Mn precursor source can improve electrode service life and stability. Interestingly, the service life of these electrodes is superior than the traditional dip/brush-coating electrodes [
40], which may be ascribed to the tight adhesion between catalytic active layer and substrate generated using the hydrothermal and annealing process. Therefore, the self-supported structure on substrate seems as a good candidate for a robust electrooxidation catalyst.
2.3. Electrochemical Degradation of Cationic Blue X-GRRL
Electrooxidation ability of these prepared electrode materials were evaluated based on the degradation of the cationic blue X-GRRL dye. The time-dependent Ultraviolet Spectrophotometer (UV–vis) absorbance spectra of cationic blue X-GRRL on SnO
2-Mn
2O
3@CC electrode are shown in
Figure 4a, which is a pivotal factor to estimate the degradation efficiency of cationic blue X-GRRL. The azo conjugated chromogenic system in cationic blue X-GRRL dye molecule corresponds to the absorption peak of 608 nm [
41]. In
Figure 4a, the intensity of characteristic adsorption peak at 608 nm decrease with time and disappears after 50 min, indicating that cationic blue X-GRRL has been completely decomposed in 50 min. Meanwhile, a visually gradual color fading of cationic blue X-GRRL dye is depicted in the time-dependent photos (
Figure 4b). The color is near to colorless at 40 min, suggesting that cationic blue X-GRRL dye has been successfully degraded in the electrochemical oxidation process. Moreover, the time-dependent UV–vis absorbance spectra of cationic blue X-GRRL on SnO
2@CC and Mn
2O
3@CC electrodes are also tested (
Figure S8a,b). The comparative intensity of time-dependent characteristic adsorption peaks at 608 nm (
Figure S8c) indicate a faster degradation process of cationic blue X-GRRL on SnO
2-Mn
2O
3@CC electrode.
The electrocatalytic oxidation performances of different electrodes for degrading cationic blue X-GRRL with an initial concentration of 20 mg L
−1 were investigated. In
Figure 5a, after 50 min of electrolysis, the removal efficiency of cationic blue X-GRRL on SnO
2-Mn
2O
3@CC is 97.55%, which is superior than that of Mn
2O
3@CC (92.74%) and SnO
2@CC electrode (88.3%). Meanwhile, the relationship between time and electrochemical degradation of cationic blue X-GRRL on different electrodes follows the pseudo-first-order kinetics model, which can be described as the following equation:
where, A
0 is the initial absorbance, A
t is the absorbance at given time t and
k is the kinetic rate constant. As displayed in
Figure 5b, the reaction kinetic rate constant
k1 of SnO
2-Mn
2O
3@CC is larger than Mn
2O
3@CC and SnO
2@CC electrodes, indicating a faster cationic blue X-GRRL degradation rate. The detailed values of k on different electrodes are listed in
Table S3. Different kinds of supporting electrolyte have varied influences on the electrocatalytic degradation efficiency. Therefore, different electrolytes containing NaCl, Na
2SO
4 and Na
2CO
3 were used to investigate the effect of electrolyte ions on degrading cationic blue X-GRRL. As displayed in
Figure 5c, the degradation efficiency of cationic blue X-GRRL on SnO
2-Mn
2O
3@CC electrode in three electrolytes increase along with time. Additionally, the dye degradation efficiency in 0.5 M NaCl electrolyte can be as high as 97.55% after 50 min, which greatly exceeds that of Na
2SO
4 (17%) and Na
2CO
3 (79.23%). During the electrooxidation process, HO· free radicals are responsible for the dye degradation, which is verified by the time-dependent fluorescence spectrometry (
Figure S9), moreover, chlorine species also plays an important role. Since the redox potential of Cl
−/Cl
2 (U = 1.36 V vs. RHE) is low than that of HO·/H
2O (U = 2.2 V vs. RHE) [
42], Cl
2 is more easily generated in comparison with HO· in wastewater, along with a series of the following reactions [
43]:
Therefore, SnO
2-Mn
2O
3@CC electrode in 0.5 M NaCl electrolyte can contribute to the excellent electrocatalytic oxidation efficiency. Moreover, the degradation of cationic blue X-GRRL in different electrolytes follows the pseudo-first-order kinetics (
Figure 5d). SnO
2-Mn
2O
3@CC electrode displays the fastest reaction degrading rate in 0.5 M NaCl electrolyte, and the corresponding reaction rate constant is listed in
Table S4.
Current density is another critical factor affecting the electrocatalytic degradation ability of electrodes.
Figure 5e shows the degradation efficiency of cationic blue X-GRRL on SnO
2-Mn
2O
3@CC electrode in 0.5 M NaCl electrolyte by applying 5, 10, 15 and 20 mA cm
−2. The degradation efficiency of cationic blue X-GRRL shows a remarkable enhancement with the increased current density ranging from 5 to 15 mA cm
−2, while the degradation efficiency decreased when the current density is 20 mA cm
−2. An excessive current density along with a higher potential can impel the oxygen evolution side reaction, restraining electrochemical oxidation reaction. Moreover, excessive current density is a waste of electric energy and economic cost. Therefore, 15 mA cm
−2 is an optimal condition for degrading cationic blue X-GRRL.
The recyclability of electrode is a pivotal issue in its practical application. SnO
2-Mn
2O
3@CC electrode is used for recyclable degradation of cationic blue X-GRRL for six times. As shown in
Figure 5f, the removal efficiency of cationic blue X-GRRL almost remains constant for six cycles. The successive degradation rates at the 50th min are 97.55%, 96.83%, 95.4%, 94.52%, 94.03%, and 94.12%, respectively. The above result confirms that SnO
2-Mn
2O
3@CC electrode as an electrooxidation catalyst for cationic blue X-GRRL dye possesses a good recyclability.
As shown in
Figure 6, the electrocatalytic degradation mechanism of cationic blue X-GRRL on SnO
2-Mn
2O
3@CC is proposed based on the aforementioned results in the degradation experiments. Firstly, H
2O and Cl
− on the surface of SnO
2-Mn
2O
3@CC electrode lose electrons to form hydroxyl radical (·OH) and active chlorine in the NaCl solution, and the active chlorine further hydrolyzes into ClO
−. Then, the ·OH radical and ClO
− combine with cationic blue X-GRRL dye molecule and degrade the dye molecule to produce CO
2 and H
2O [
41]. In the electrocatalytic degradation process of cationic blue X-GRRL on SnO
2-Mn
2O
3@CC, active chlorine may be the main factor for the rapid degradation.