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Article

Construction of the Photocatalytic Film of the Recyclable TaON/Nickel Foam with Ohmic Junction for Efficient Wastewater Treatment

by
Shaomang Wang
1,2,*,
Jie Wang
1,
Rui Liu
2,
Zhiqian Zheng
1,
Yuan Guan
3,* and
Juan Shen
4
1
School of Environmental Science and Engineering, Changzhou University, Changzhou 213164, China
2
School of Urban Construction, Changzhou University, Changzhou 213164, China
3
Jiangsu Province Key Laboratory of Fine Petrochemical Engineering, School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China
4
Jiangsu Longmem Environmental Technology Co., Ltd., Changzhou 213164, China
*
Authors to whom correspondence should be addressed.
Catalysts 2022, 12(10), 1160; https://0-doi-org.brum.beds.ac.uk/10.3390/catal12101160
Submission received: 9 September 2022 / Revised: 28 September 2022 / Accepted: 30 September 2022 / Published: 2 October 2022
(This article belongs to the Special Issue Synthesis and Application of Composite Photocatalysts)
Browse Figures
Versions Notes

Abstract

:
A recyclable photocatalytic film of TaON/Ni foam with ohmic junction is prepared by the electrophoretic deposition technology. The photocatalytic film of 60 mg TaON/Ni foam demonstrates excellent photocatalytic activity and recycling performance for the degradation of basic fuchsin from water. Around 80% of basic fuchsin (50 mL, 10 mg L−1) is removed over 60 mg TaON/Ni foam under irradiation of 72 W LED white light for 5 h. The photocatalytic activity of the film does not significantly decrease after three rounds of use. The active species for the photocatalytic degradation of basic fuchsin are ·O2−, h+ and ·OH.

1. Introduction

Photocatalytic technology applied in environmental contaminant purification has been a research hotspot [1,2,3,4,5] since TiO2 was found to be able to degrade harmful cyanide under irradiation of ultraviolet light in 1977 [6]. So far, a lot of photocatalysts, such as ZnO [7], CdS [8], Ag3PO4 [9], Bi2WO6 [10], g-C3N4 [11], WO3 [12], BiOCl [13], and BiVO4 [14], are successfully developed by researchers. Unfortunately, although these photocatalysts exhibit good photocatalytic activity in the experiment, it is still a long way from the practical applications. The photocatalysts with industrialization potential should have two key advantages. First of all, they should possess high quantum efficiency. Secondly, they should be produced with low cost and be easy to recycle and reuse.
Lately, TaON as the photocatalyst applied in pollutant degradation has attracted great attention [15,16,17]. However, the photocatalytic activity of single TaON is not prominent due to the low separation efficiency of carriers. Pei used nickel (Ni) metal particles to modify TaON, which effectively improves its photocatalytic activity [18]. Li prepared a core-shell heterojunction of TaON/Bi2MoO6 with good visible-light catalytic activity for the levofloxacin degradation and Cr (VI) reduction [19]. Wang developed a novel heterojunction of TaON/V2O5, which exhibits excellent photocatalytic activity for the degradation of gaseous toluene [20]. Yan synthesized an organic–inorganic photocatalyst of TaON/g-C3N4, which displays distinctly photocatalytic activity for RhB degradation [21]. However, the TaON-based powder photocatalyst has the problems of easy loss and difficult recycling in practical applications. Recently, researchers have paid attention to the photocatalytic performance of TaON film for pollutant degradation [22].
In this work, we use Ni foam to fix TaON powder and build a photocatalytic film of TaON/Ni foam by the electrophoretic deposition technology. An ohmic junction is constructed between TaON and Ni foam, which facilitates the photogenerated electron migration of TaON to Ni foam and thereby achieves efficient separation of TaON carriers. Furthermore, the TaON powder is loaded on Ni foam, which reduces the loss of TaON powder during recycling, maintaining the durable photocatalytic activity of TaON/Ni foam. Consequently, the photocatalytic film exhibits excellent photocatalytic activity and recycling performance for the degradation of basic fuchsin from water. The best content of TaON on Ni foam is optimized, and the main active species for the degradation of basic fuchsin is determined. The mechanism of carrier migration separation between TaON and Ni foam are revealed.

2. Experimental Section

2.1. Preparation of Samples

For the synthesis of TaON, 1 g of Ta2O5 power was put into a crucible and calcined at 900 °C for 3 h with the ramp rate of 10 °C min−1 under flowing NH3 of 100 mL min−1 in a tube furnace [23]. The yellow TaON was obtained after the sample was cooled to room temperature. To prepare TaON/Ni foam, 100 mg of TaON powder and 20 mg of iodine were dispersed in 50 mL of acetone. The suspension was mixed by ultrasound for 0.5 h. Two pieces of Ni foam (3 cm × 3 cm × 1 mm) were inserted into this suspension as cathode and anode, respectively. Then, 30 mg of TaON was deposited on Ni foam at 15 V for 90 s. The 40, 50, 60, and 70 mg TaON were deposited on Ni foam at 15 V for 120, 150, 180, and 210 s, respectively. There was a margin of error of plus or minus 3 mg in the content of TaON on Ni foam.

2.2. Characterization

X-ray diffraction (XRD) was carried out via an ARL X’TRA X-ray diffractometer with Cu Kα radiation (λ = 0.154 nm). Scanning electron microscope (SEM) images were obtained through a HITACHI SU8020 at an accelerating voltage of 2.0 kV. The UV–vis diffuse reflectance spectra (DRS) were collected via a Shimadzu UV2550 spectrophotometer. Electron spin resonance (ESR) signals were acquired on the JES FA200. Electrochemical impedance spectroscopy (EIS) was tested in an electrochemical workstation (CHI660D) with a standard three electrode system from 1 to 100 kHz at an open circuit potential of 0.26 V and an alternating current (AC) voltage amplitude of 5 mV. The electrolyte was a mixed aqueous solution of 0.0025 M of K3[Fe(CN)6] and 0.1 M KCl. Photoluminescence (PL) spectra were characterized by Fluoromax-4 (HORIBA) with an excitation wavelength at 350 nm.

2.3. Photocatalytic Activity Evaluation of Samples

The photocatalytic performance of TaON/Ni foam was evaluated by degrading basic fuchsin (BF) under irradiation of 72 W LED white light. The photocatalytic film was laid flat at a depth of 1 cm below the surface of 50 mL BF aqueous solution (10 mg L−1) and magnetically stirred for 1 h in the dark to achieve the equilibrium of adsorption–desorption. In the photocatalytic process, 4 mL of solution was taken out to analyze BF concentration at 543 nm by an UV-visible spectrophotometer. The used film was washed with ethanol and deionized water alternately and then recycled for the next round of BF degradation to evaluate its photocatalytic stability. Degradation efficiency (DE) of BF was calculated by the Equation (1).
DE = (Co − Ct)/Co
Herein, Co and Ct are the solution absorbance after adsorption balance and at t of irradiation time, respectively.

2.4. Calculation Method

The work functions of TaON and Ni were calculated through the CASTEP module in materials studio. The exchange-correlation effects were described by generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE) functional. The TaON (001) and Ni (001) were used as the calculated structure. The vacuum layer thickness was set to 10 Å along the Z axis, and the calculation precision adopted the software default fine.

3. Results and Discussion

3.1. Characterization of Samples

In Figure 1a, the diffraction peaks at 2θ values of 44.5, 51.8, and 76.4° belong to the (111), (200), and (220) crystal planes of the cubic nickel (PDF#04-0850), respectively. The diffraction peaks of TaON are clearly observed in TaON/Ni foam. The diffraction peaks locating to 29.1 and 32.7° are ascribed to the (−111) and (111) crystal planes of the monoclinic TaON (PDF#70-1193), respectively [20]. It can be clearly seen from Figure 1b that the diffraction peaks at 35.1, 35.6, and 36.7° are also assigned to the (002), (020), and (200) crystal planes of the monoclinic TaON. Furthermore, with the increase in TaON content in TaON/Ni foam, its characteristic peak intensity increases gradually. The XRD results suggest that the TaON is successfully deposited on Ni foam to obtain the TaON/Ni foam.
The fabrication process of TaON/Ni foam is depicted in Figure 2a. The Ta2O5 is treated to transform porous TaON by NH3 (Figure 2e). As displayed in Figure 2b–d, the TaON is uniformly deposited on the skeleton of gray Ni foam to obtain yellow TaON/Ni foam. The TaON is not stacked in the pores of Ni foam so that the Ni foam maintains the three-dimensional netlike structure. This three-dimensional hollow structure can ensure free flow of reactants and products, which is beneficial for the efficient degradation of pollutants. Figure 2f further reveals that the layer thickness of 60 mg TaON/Ni foam is around 6 µm. The SEM images clearly show that the porous TaON is successfully and evenly fixed on Ni foam.
As depicted in Figure 3, the light absorption performance of TaON/Ni foam is not distinctly affected by the content of TaON. All TaON/Ni foam can absorb photons with maximum wavelength of about 540 nm. The band-gap energy (Eg) of the TaON/Ni foam is calculated to be 2.3 eV by the Equation (2), which is in line with the literature [23,24].
Eg = 1240/λmax
The results demonstrate that the Ni foam has no obvious effect on the light absorption properties of TaON/Ni foam, and the as-prepared TaON/Ni foam should possess good visible-light catalytic activity.

3.2. Photocatalytic Performance Test

The concentration of BF gradually decreases with the extension of irradiation time (Figure 4a), indicating that all samples display photocatalytic activity for BF degradation. The degradation efficiencies of BF are 55.3, 57.3, 64.9, 76.8, and 74.1% over 30–70 mg TaON/Ni foam under light irradiation of 5 h, respectively (Figure 4a,c). The degradation rate constants obtained by the pseudo-first-order rate equation to fit are 0.15, 0.16, 0.19, 0.26, and 0.25 h−1 over 30–70 mg TaON/Ni foam, respectively (Figure 4b,c). The fitted correlation coefficients (R2) are 0.999, 0.998, 0.983, 0.984, and 0.991, respectively. The photocatalytic activity of TaON/Ni foam increase first and then decrease with increasing content of TaON in TaON/Ni foam. The 60 mg TaON/Ni foam exhibits the best photocatalytic activity. It is due to the low TaON content in 30, 40, and 50 mg TaON/Ni foam. For the 70 mg TaON/Ni foam, the TaON with high content will accumulate on Ni foam, leading to the decreased contact area between TaON and BF.
As listed in Table 1, compared to the literature reports, the degradation efficiency of basic fuchsin over 60 mg TaON/Ni foam is higher under irradiation of low-power LED lamp. In addition, the stability test for the 60 mg TaON/Ni foam is conducted because the catalyst stability is very important in practical applications. After the 60 mg TaON/Ni foam is reused three times, its activity does not decrease distinctly, and it can still degrade about 80% of BF under light irradiation of 5 h (Figure 4d). The results display that the 60 mg TaON/Ni foam has the excellent photocatalytic performance.

3.3. Carrier Transfer Resistance and Separation Efficiency

Figure 5a depicts that the sequence of electrochemical impedance is 60 mg TaON/Ni foam < 50 mg TaON/Ni foam < 30 mg TaON/Ni foam < 40 mg TaON/Ni foam < 70 mg TaON/Ni foam. The electrochemical impedance of the 60 mg TaON/Ni foam is minimal, which means that the photogenerated carriers of the 60 mg TaON/Ni foam migrate from bulk to surface with the minimal resistance. Therefore, it demonstrates the highest photocatalytic activity. The photocatalytic activity of 70 mg TaON/Ni foam with maximum impedance ranks second. It is due to the fact that the 70 mg TaON/Ni foam possesses the highest content of TaON producing more photo-induced carriers. Similarly, the impedance of 40 mg TaON/Ni foam is greater than 30 mg TaON/Ni while their photocatalytic activity is contrary.
The separation effect of Ni foam on photogenerated electron-hole pairs of TaON is characterized by PL spectroscopy. A photocatalyst demonstrates lower fluorescence peak intensity, implying that it has higher separation efficiency of photo-induced electron-hole pairs. As shown in Figure 5b, the intensity of the fluorescence peak emitted by TaON/Ni foam is obviously lower than that of TaON. This indicates that the Ni foam can effectively inhabit the recombination of TaON carriers.

3.4. Photocatalytic Active Species

It is well known that the primarily active species in the photocatalytic reaction are h+, ·OH, ·O2−, and so on [28]. ESR technology is used to detect active free radicals in the process of BF degradation. In the process of ·O2− detection, in order to avoid the interference of ·OH signal, methanol is used instead of water as solvent. Under light irradiation for 5 min, four peaks with the signal intensity of 1:2:2:1 (Figure 6a) in water solution and 1:1:1:1 (Figure 6b) in methanol solution are observed, which originate from the signals of ·OH and ·O2−, respectively [29]. No peaks are found for 5 min in the dark. The ESR results suggest that the radicals for BF degradation are ·OH and ·O2−. The potentials of the conduction-band and valence-band edges of TaON are around −0.34 and 2.16 V, respectively [30]. Due to the potential of ·O2−/O2 (−0.33 eV) [31], the free radical of ·O2− can be generated by the Equation (3).
O2 + e = ·O2−
In addition, the reduction potential of O2/H2O2 is 0.695 eV [31]. Consequently, the free radical of ·OH is from the Equations (4) and (5).
O2 + 2e + 2H+ = H2O2
H2O2 + e = ·OH + OH−
In order to ascertain the crucially active species for BF degradation, benzoquinone (BQ), isopropanol (IPA), and EDTA are added into BF solution as capture agents for superoxide radical (·O2−), hydroxyl radical (·OH), and hole (h+), respectively [32]. The concentration of scavengers is 0.1 mmol L−1. After the BQ, IPA and EDTA are added into BF solution, the degradation rates of BF over 60 mg TaON/Ni foam decrease to 39.7%, 57.2%, and 48.2%, respectively (Figure 7). The results imply that the ·O2−, h+, and ·OH participate in the degradation of BF. Among them, the free radical of ·O2− plays the most significant role.

3.5. Photocatalytic Degradation Mechanism

The work functions of TaON and Ni are calculated to study the mechanism of carrier migration separation for BF degradation. As illustrated in Figure 8a,b, the work functions of TaON (001) and Ni (001) are 6.22 and 2.8 eV, respectively. When TaON and Ni foam are in contact in the dark, the electrons of Ni foam will flow to TaON causing the interfacial energy band of TaON to bend downward. With the electron migration, the Fermi level of TaON shifts upward and that of Ni foam comes downward until their Fermi levels are same. Ultimately, an electric field is built from the surface to the bulk phase of TaON, thereby forming the ohmic junction between TaON and Ni. Under light irradiation, the photogenerated electrons of TaON will be driven to Ni foam by the interfacial electric field and then react with O2 to produce ·O2− and ·OH, which are used to decompose BF. The hole from the valence band of TaON will directly oxidize BF resulting in its degradation (Figure 8c).

4. Conclusions

The ohmic junction film of TaON/Ni foam is successfully prepared by depositing TaON powder on Ni foam. The TaON/Ni foam has good light-absorption property, and it can capture photons with a maximum wavelength of 540 nm. The optimal content of TaON on Ni foam (3 cm × 3 cm × 1 mm) is 60 mg. The 60 mg TaON/Ni foam has excellent performances in photocatalytic activity and recycling stability. In a word, the photocatalytic film of TaON/Ni foam has good application potential in the purification of environmental pollutants.

Author Contributions

S.W. and Y.G. designed this work and prepared the manuscript. J.W., R.L. and Z.Z. carried out the experiments and data analysis. J.S. participated in the discussion of the results. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The study did not report any data.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (21876015), the Natural Science Foundation of Jiangsu Province (BK20190934), and Student Innovation and Entrepreneurship Fund of Changzhou University (2022-Z15).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a,b) X-ray diffraction patterns of TaON/Ni foam and TaON.
Figure 1. (a,b) X-ray diffraction patterns of TaON/Ni foam and TaON.
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Figure 2. (a) Schematic diagram of the preparation of TaON/Ni foam; images of (b) Ni foam and (c–f) 60 mg TaON/Ni foam.
Figure 2. (a) Schematic diagram of the preparation of TaON/Ni foam; images of (b) Ni foam and (c–f) 60 mg TaON/Ni foam.
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Figure 3. UV-vis diffuse reflectance spectra of TaON/Ni foam.
Figure 3. UV-vis diffuse reflectance spectra of TaON/Ni foam.
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Figure 4. (a) Concentration curve of BF over TaON/Ni foam with time; (b) pseudo-first-order degradation kinetic curves; (c) photocatalytic degradation efficiency and rate constant; (d) photocatalytic stability of 60 mg TaON/Ni foam.
Figure 4. (a) Concentration curve of BF over TaON/Ni foam with time; (b) pseudo-first-order degradation kinetic curves; (c) photocatalytic degradation efficiency and rate constant; (d) photocatalytic stability of 60 mg TaON/Ni foam.
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Figure 5. (a) Electrochemical impedance spectroscopy of TaON/Ni foam; (b) PL spectra of TaON powder and 60 mg TaON/Ni foam.
Figure 5. (a) Electrochemical impedance spectroscopy of TaON/Ni foam; (b) PL spectra of TaON powder and 60 mg TaON/Ni foam.
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Figure 6. (a,b) ESR spectra of TaON utilizing DMPO as a radical scavenger.
Figure 6. (a,b) ESR spectra of TaON utilizing DMPO as a radical scavenger.
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Figure 7. Effect of different scavengers on photocatalytic activity of 60 mg TaON/Ni foam.
Figure 7. Effect of different scavengers on photocatalytic activity of 60 mg TaON/Ni foam.
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Figure 8. Work functions of (a) TaON (001) and (b) Ni (001); (c) migration separation pathway of photo-induced carriers for BF degradation.
Figure 8. Work functions of (a) TaON (001) and (b) Ni (001); (c) migration separation pathway of photo-induced carriers for BF degradation.
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Table
Table 1. Comparison of degradation efficiency of basic fuchsin on 60 mg TaON/Ni foam with references.
Table 1. Comparison of degradation efficiency of basic fuchsin on 60 mg TaON/Ni foam with references.
MaterialLight SourcePollutantPollutant Concentration
(mg L−1)		Photocatalyst
Dosage
(g L−1)		Irradiation
Time
(min)		Degradation
Efficiency (%)		Reference
Co-doped titania-silicaHalogen lamp
(500 W)		Basic fuchsin500.2524073.0%[25]
Ag/ZnO/TiO2Direct sun lightBasic fuchsin30.29069.8%[26]
H3PW12O40/TiO2Xenon lamp
(400 W) 		Fuchsin501.2524075%[27]
60 mg
TaON/Ni foam		72 W LED white lightBasic fuchsin50_30076.8%Our work
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Wang, S.; Wang, J.; Liu, R.; Zheng, Z.; Guan, Y.; Shen, J. Construction of the Photocatalytic Film of the Recyclable TaON/Nickel Foam with Ohmic Junction for Efficient Wastewater Treatment. Catalysts 2022, 12, 1160. https://0-doi-org.brum.beds.ac.uk/10.3390/catal12101160

AMA Style

Wang S, Wang J, Liu R, Zheng Z, Guan Y, Shen J. Construction of the Photocatalytic Film of the Recyclable TaON/Nickel Foam with Ohmic Junction for Efficient Wastewater Treatment. Catalysts. 2022; 12(10):1160. https://0-doi-org.brum.beds.ac.uk/10.3390/catal12101160

Chicago/Turabian Style

Wang, Shaomang, Jie Wang, Rui Liu, Zhiqian Zheng, Yuan Guan, and Juan Shen. 2022. "Construction of the Photocatalytic Film of the Recyclable TaON/Nickel Foam with Ohmic Junction for Efficient Wastewater Treatment" Catalysts 12, no. 10: 1160. https://0-doi-org.brum.beds.ac.uk/10.3390/catal12101160

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