Kirankumar Venkatesan
Savunthari
abc,
Daneshwaran
Balaji
a,
Nivedita
Sudheer
a,
Mittal
Bathwar
a,
Manjunath
Rangasamy
b,
Ganesh Kumar
Dhandabani
d,
Alain R.
Puente Santiago
e,
Sumathi
Shanmugam
*a,
Kien-Voon
Kong
*b and
Vijayaraghavan
R
*a
aDepartment of Chemistry, School of Advanced Sciences, Vellore Institute of Technology, Vellore, Tamil Nadu 632014, India
bDepartment of Chemistry, National Taiwan University, Taipei 106, Taiwan
cDepartment of Chemistry and Chemical Biology, Northeastern University, Boston, Massachusetts 02115, USA
dDepartment of Pharmacy, National Taiwan University, Taipei 100, Taiwan
eWalker Department of Mechanical Engineering, University of Texas at Austin, Austin, TX 78712, USA
First published on 17th July 2023
Environmental pollution due to dye industries is a major concern as it affects both aquatic and human life. Among various processes, photocatalysis involving the advanced oxidation process (AOP) that employs semi-conductor based photocatalysis is popular and this process degrades dyes into CO2 and H2O. The currently used TiO2 based photocatalysis requires ultra-violet (UV) light irradiation for activation. There is always a need for a catalyst that works under natural sunlight so that the dye degradation process is economical. Towards this objective, we have developed Zn doped metal peroxides as efficient photocatalysts towards rhodamine B (RhB) dye degradation. A new family of zinc-doped barium peroxide (Ba1-xZnxO2x = 0, 0.2, 0.4, 0.6, and 0.8) and zinc-doped strontium peroxide (Sr1−xZnxO2x = 0, 0.2, 0.4, 0.6, and 0.8) photocatalysts were prepared by the co-precipitation technique for the photocatalytic degradation of RhB under UV and natural sunlight irradiation. In particular, Ba0.4Zn0.6O2 (x = 0.6) and Sr0.6Zn0.4O2 (x = 0.4) photocatalysts exhibit higher photodegradation efficiency than the pure peroxides both under UV and sunlight irradiation. Ba0.4Zn0.6O2 and Sr0.6Zn0.4O2 photocatalysts show a degradation efficiency of 99.9% and 99.8% within 15 min and 10 min under UV light, respectively. The rate constants of degradation by doped peroxides, in particular Ba0.4Zn0.6O2 and Sr0.6Zn0.4O2, are 100 times higher than those of the parent peroxides.
Sustainability spotlightDue to the rapid development of industry, environmental pollution has become a major topic worldwide. Water pollution is a key concern in the world today, posing a health risk to all living creatures. In particular, rhodamine B is commonly used as an organic dye and is extremely difficult to degrade under UV and natural sunlight. Consequently, developing novel catalysts to degrade rhodamine B under sunlight is an important topic. Here, we attempt to study effective photocatalysts (zinc-doped barium and strontium peroxide) for the degradation of rhodamine B under UV and sunlight. The synthesized photocatalysts exhibit a low bandgap and a high surface area, which helps degrade the rhodamine B dye in a short period under UV and sunlight irradiation. Furthermore, our research demonstrates an effective photocatalyst for persistent organic pollutants (POPs) that can be used for environmental remediation applications of sustainable development goals (SDGs). |
The photocatalytic activity of nanoparticles is particularly important for improving the degradation efficiency of many catalysts, including metal oxides, carbides, sulfides, chalcogenides, oxyhalides, halides, and hydroxides.18–20 Among all semiconductor metal oxides, SnO2,21 ZnO2,22 CeO2,23 SiO2,24 MnO2,25 ZrO2,26 and TiO2 (ref. 27 and 28) are described as developing photocatalysts due to their excellent photocatalytic activity. To improve photocatalytic performance, many researchers focus on metal or nonmetal doping, surface modification, heterostructure formation, and composite materials. In particular, doped nanomaterials can improve structural, optical, electrical, magnetic, catalytic, and antibacterial activities.21,26,29,30 There is always a need to develop photocatalysts that work under natural sunlight so that the process is economical. Towards this objective, we attempt to study the photocatalytic activity of barium peroxide and strontium peroxide through zinc doping.
In the present work, zinc-doped barium peroxide (Ba1−xZnxO2x = 0, 0.2, 0.4, 0.6, and 0.8) and zinc-doped strontium peroxide (Sr1−xZnxO2x = 0, 0.2, 0.4, 0.6, and 0.8) were developed as photocatalysts to improve the catalytic performance of barium peroxide and strontium peroxide. These photocatalysts were prepared by the co-precipitation method. The prepared compounds were analyzed by powder X-ray diffraction (XRD), UV-vis-differential reflectance spectroscopy (UV-DRS), Fourier transform infrared spectroscopy (FT-IR), high resolution-transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), and Brunauer–Emmett–Teller analysis (BET). The photocatalytic performance was determined by the degradation of the dye RhB under UV and sunlight. Zinc-doped barium peroxide (x = 0.6) and strontium peroxide (x = 0.4) photocatalysts show 99.9 and 99.8% degradation in about 15 and 10 minutes, respectively, under UV light. These compositions exhibit 99.5 and 99.8% degradation efficiency within 90 minutes under sunlight irradiation. Among the studied compositions, Ba0.4Zn0.6O2 and Sr0.6Zn0.4O2 photocatalysts are found to be more active and efficient than well-known catalysts for RhB degradation. Zn doping into Ba & Sr peroxides has enhanced the photocatalytic activity significantly.
The difference in lattice parameters results from the smaller ionic radius of Sr2+ (rSr = 1.18 Å) than Ba2+ (rBa = 1.35 Å).32 It is to be noted that we could obtain phase pure BaO2 and SrO2 without the carbonate impurity by carrying out the reaction in a nitrogen atmosphere. From Fig. 1a and b, for x = 0.2 and above, it can be seen that the ZnO phase is present in addition to BaO2 and SrO2. The peaks marked with a star symbol indicate the presence of ZnO and the products are a composite of peroxide and ZnO in the composition range of x = 0.2 to 0.8 The crystallite size of the synthesized compounds was calculated using Scherrer's equation.33 The sizes of pure BaO2 and SrO2 are 45 nm and 36 nm, respectively. The crystal structure of MO2 (M = Ba and Sr) is shown in Fig. S2† using the software VESTA. The crystal structures of both compounds are isostructural to those reported by Bernal et al.31 for BaO2 and SrO2.
The FT-IR spectra of zinc-doped barium peroxide (Ba1−xZnxO2, x = 0, 0.2, 0.4, 0.6, and 0.8) and zinc-doped strontium peroxide (Sr1−xZnxO2, x = 0, 0.2, 0.4, 0.6, and 0.8) are shown in Fig. 2a and b, respectively. The M–O stretching is observed in the frequency range of 400–900 cm−1. The bands at 1400–1475 cm−1 confirm the O22− species,34 830–864 cm−1 (O–Ba–O and O–Sr–O stretching modes),35–37 690–720 cm−1, and 400–430 cm−1 (Ba–O, Sr–O and Zn–O bending modes).34–37 ICP-OES confirms the bulk compositions (Table S2†).
The optical absorption spectra of zinc-doped barium peroxide (Ba1−xZnxO2, x = 0, 0.2, 0.4, 0.6, and 0.8) and zinc-doped strontium peroxide (Sr1−xZnxO2, x = 0, 0.2, 0.4, 0.6, and 0.8) were studied by UV-DRS analysis. The band gap (eV) of the synthesized products was calculated using Tauc's equation.34Fig. 3a and b show that the band gaps of Ba1−xZnxO2, x = 0, 0.2, 0.4, 0.6, 0.8 and Sr1−xZnxO2, x = 0, 0.2, 0.4, 0.6, 0.8 are in the range of 3.66 to 3.20 eV and 3.97 to 3.21 eV, respectively.
For further characterization (HR-TEM, XPS, and BET) we chose the low band gap materials Ba0.4Zn0.6O2 and Sr0.6Zn0.4O2. The HR-TEM analysis was carried out to investigate the size and shape of Ba0.4Zn0.6O2 and Sr0.6Zn0.4O2 particles (Fig. 4a–d). The morphology of Ba0.4Zn0.6O2 shows the shape of rods (Fig. 4a) and Sr0.6Zn0.4O2 has a flower-like morphology (Fig. 4c) respectively. The selected area electron diffraction (SAED) (Fig. 4b and d) image shows that the d-spacing is 0.39 nm and 0.27 nm and 0.36 nm and 0.32 nm, corresponding to Ba0.4Zn0.6O2 and Sr0.6Zn0.4O2 phases.
The surface chemical composition and bonding states of the Ba0.4Zn0.6O2 and Sr0.6Zn0.4O2 photocatalysts were analyzed by XPS. Fig. 5 and 6 show the XPS spectra of Ba0.4Zn0.6O2 and Sr0.6Zn0.4O2 samples showing peaks of Zn 2p, Ba 3d, Sr 3d, O 1s, and C 1s. The C 1s binding energy was used to calibrate all binding energies (C 1s: 283.4 eV). The deconvolution method is used for the Zn, Ba, Sr, and O elements. Ba 3d (Fig. 5b) contains two spin–orbit doublets Ba 3d5/2 (777.83 eV and 778.69 eV) and Ba 3d3/2 (793.32 eV and 794.19), confirming the presence of Ba2+ ions.38,39 The XPS spectrum of Sr 3d is shown in Fig. 6b, with four peaks at 131.37 eV and 132.32 eV and 133.24 eV and 134.09 eV related to Sr 3d5/2 and Sr 3d3/2 of Sr2+.37,40 Zn 2p is shown in Fig. 5c and 6c, and Zn 2p3/2 (1020.07 eV) and Zn 2p1/2 (1043.05 eV) confirm the presence of Zn2+ ions in both compounds.34,41 The binding energy of the O 1s peak (Fig. 5d and 6d) was found to be 529.28 eV, 530.24 eV, and 531.74 eV, which are related to the lattice oxygen in the metal–oxygen bonds, hydroxyl species, and O22− species, respectively.34,37,41,42
One of the main factors on which the photocatalytic performance depends is the surface area of the catalyst. BET surface areas of BaO2, SrO2, Ba0.4Zn0.6O2, and Sr0.6Zn0.4O2 samples are 2.2, 3.4, 13.5, and 18.3 m2 g−1, respectively. Zinc-doped barium peroxide and strontium peroxide have a larger surface area with smaller particles.
The present study deals with the degradation of RhB, an organic pollutant, using BaO2, Ba0.8Zn0.2O2, Ba0.6Zn0.4O2, Ba0.4Zn0.6O2, and Ba0.2Zn0.8O2 and SrO2, Sr0.8Zn0.2O2, Sr0.6Zn0.4O2, Sr0.4Zn0.6O2, and Sr0.2Zn0.8O2 photocatalysts under UV and direct sunlight. For the model reaction, 25 mg of the different photocatalysts and 50 ml of a 10 ppm RhB dye solution were examined with a photocatalytic instrument under UV light irradiation. Fig. S3†, 7a–d & Table 1 show the efficiency of RhB dye degradation using the Ba1−xZnxO2 photocatalysts.
Fig. 7 Influence of Zn doping on the photocatalytic degradation of RhB using (a) Ba0.8Zn0.2O2, (b) Ba0.6Zn0.4O2, (c) Ba0.4Zn0.6O2 and (d) Ba0.2Zn0.8O2 photocatalysts under UV light irradiation. |
Composition (x) | Ba1−xZnxO2 | Sr1−xZnxO2 | ||
---|---|---|---|---|
Time (min) | % of degradation | Time (min) | % of degradation | |
0 | 35 | 98.5 | 35 | 98.5 |
0.2 | 20 | 98.6 | 30 | 98.6 |
0.4 | 25 | 99 | 10 | 99.8 |
0.6 | 15 | 99.9 | 15 | 99.2 |
0.8 | 25 | 97.6 | 20 | 98.5 |
From Table 1, it is evident that all the compositions degrade the dye more than 95% within a short duration; in particular the compositions Ba0.4Zn0.6O2 and Sr0.6Zn0.4O2 exhibit faster degradation. This may be due to the high surface area. The amount of H2O2 released from the aqueous suspension is tabulated in Table S3.† It is to be noted that the best compositions Ba0.4Zn0.6O2 and Sr0.6Zn0.4O2 release the highest amount of H2O2 confirming that the photocatalytic activity is maximum in these compositions.
The photocatalytic performance of Sr1−xZnxO2 against the RhB dye under UV light is shown in Fig. S4†, 8a–d & Table 1. The composition x = 0.4 shows the fastest degradation. From the results, we conclude that zinc doping enhances the performance of the photocatalyst. Fig. S5† shows the first-order pseudo-kinetics for the photocatalytic degradation of RhB by BaO2, Ba0.4Zn0.6O2, and SrO2, Sr0.6Zn0.4O2 photocatalysts under UV light illumination. The rate constants, k, calculated for BaO2, Ba0.4Zn0.6O2 photocatalysts are 0.082 and 0.344 min−1, respectively (Fig. S5a†). As shown in Fig. S5b,† the rate constants for SrO2 and Sr0.6Zn0.4O2 are 0.099 and 0.394 min−1 respectively. Compared with the undoped peroxides, the Ba0.4Zn0.6O2 and Sr0.6Zn0.4O2 photocatalysts exhibit four times faster kinetics.
Fig. 8 Influence of Zn doping on the photocatalytic degradation of RhB using (a) Sr0.8Zn0.2O2, (b) Sr0.6Zn0.4O2, (c) Sr0.4Zn0.6O2, and (d) Sr0.2Zn0.8O2 photocatalysts under UV light irradiation. |
The photocatalysts Ba0.4Zn0.6O2 and Sr0.6Zn0.4O2 exhibit higher photocatalytic performance for the degradation of RhB (Table S4†) under UV light compared with other systems reported in the literature. The rate constants are two orders of magnitude higher in the peroxide composite system. This is associated with the interface of peroxide and ZnO. Therefore, BaO2, Ba0.4Zn0.6O2, SrO2, and Sr0.6Zn0.4O2 photocatalysts were selected for photocatalytic degradation of RhB under direct sunlight. According to Fig. 9a–d, the photocatalytic degradation of RhB with the photocatalysts BaO2 and Ba0.4Zn0.6O2, and SrO2 and Sr0.6Zn0.4O2 is 50.2% after 120 minutes and 99.8% after 90 minutes and 60.5% after 105 minutes and 99.5% after 90 minutes respectively under direct sunlight. This may be due to large surface area and a higher amount of H2O2. In addition, the photodegradation kinetics of RhB by BaO2, Ba0.4Zn0.6O2, and SrO2, Sr0.6Zn0.4O2 photocatalysts under sunlight are shown in Fig. 9e and f, respectively. The rate constants (k) for Ba0.4Zn0.6O2 and Sr0.6Zn0.4O2, are 2.480 and 2.340 min−1 respectively which are two orders of magnitude higher compared to those for BaO2 and SrO2 (0.018 and 0.020 min−1, respectively). The higher activity of the doped samples may also be due to the more efficient generation of electron–hole pairs under sunlight.43 From the results, we conclude that the compositions Ba0.4Zn0.6O2 and Sr0.6Zn0.4O2 are suitable photocatalysts for degradation by sunlight. These photocatalysts exhibit high photocatalytic performance in direct sunlight compared to previous reports (Table S5†). This finding can be used for a wide range of environmental remediation applications.
To understand the photocatalytic performance of Ba0.4Zn0.6O2 and Sr0.6Zn0.4O2 photocatalysts, a possible mechanism is proposed as shown in Scheme 1. The general mechanism involves excitation of electrons from the valence band (VB) to the conduction band (CB) resulting in holes in the VB and electrons in the CB.44 Holes and electrons react with water and oxygen producing •OH radicals and super oxide radicals respectively that could degrade the dyes. The various steps are given in eqn (1)–(7).
MO2 + hν → e− (CB) + h+ (VB) | (1) |
H2O + h+ → ˙OH + H+ | (2) |
O2 + e− → O2−˙ | (3) |
(4) |
(5) |
2˙OH → H2O2 | (6) |
H2O2 + e− → OH− + ˙OH | (7) |
Doping of Zn into the peroxides creates additional levels below the conduction bands of BaO2 and SrO2 similar to doped ZnO.45
Metal peroxides in aqueous suspensions release hydrogen peroxide. The released hydrogen peroxide could be decomposed under UV light and sunlight into hydroxyl radicals that degrade the dye. Thus, the composites of peroxide could degrade dyes through the chemical route and photochemical routes. Moreover, hydrogen peroxide is a scavenger of the e− from zinc that is present on the surface of BaO2 and SrO2, forming hydroxyl radicals and hydroxyl ions. The highly reactive hydroxyl radical and O2− are responsible for the photochemical degradation of RhB, which is degraded to non-toxic products such as carbon dioxide and water.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3su00091e |
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