Over recent years, the use of antibiotics worldwide has afforded considerable improvement in the treatment of infectious diseases and agricultural productivity ^[1]. Antibiotics are a type of antimicrobial used for treating and preventing bacterial infections ^[2]. Based on the pharmacological properties, the main categories of antibiotics include aminoglycoside,β-lactam, glycopeptide, macrolide, quinolone, sulphonamide, and tetracycline (TC) ^[3]. Keiji Fukuda once said that “A world without antibiotics would look very different to the world we live in now”. However, with the deliberate release of antibiotics in the environment and lack of effective treatments of antibiotic wastewater, antibiotic pollution poses a serious environmental threat to both the aquatic and terrestrial ecosystems ^[4].

During the past decade, various strategies have been employed to address the problem of antibiotic wastewater. Biotic elimination processes, such as biodegradation by bacteria and fungi, and non-biotic processes, including sorption, hydrolysis, oxidation, and reduction, have attracted tremendous attention ^[5-7]. For example, to investigate the removal of trace antibiotics from wastewater using biodegradation methods, Yang and co-workers studied the adsorption, desorption, and biodegradation characteristics of sulfonamide antibiotics in the presence of activated sludge with and without being subjected to NaN3 biocide ^[8]. The experimental results showed that the antibiotics were eliminated by sorption and biodegradation by the activated sludge. Gao et al. ^[9] recently found that graphene oxide (GO) could be used as an adsorbent for the adsorption and removal of TC. Owing to the π-π interactions and cation-π bonding between TC and GO, TC strongly adsorbed on the surface of GO. The adsorption isotherm followed the Langmuir and Temkin models. GO exhibited a better adsorption rate constant (k = 0.065 g/(mg∙h)) than other studied adsorbents. Liu and co-workers selected four antibiotics i.e., norfloxacin, ofloxacin, roxithromycin, and azithromycin as target antibiotics that were then subjected to disposal treatments of nanofiltration by UV254 photolysis, ozonation, and UV/O3 process ^[10]. The UV/O3 process exhibited the highest efficiency (>87%) in eliminating the antibiotics. However, the high cost, low stability, and poor recycling ability limit the application of these methods. Therefore, the development of new methods to address antibiotic pollution remains active and challenging in the area of environment and chemistry.

As one of the most promising methods to treat antibiotic contaminants, photocatalysis has attracted tremendous attention because it is an economic, efficient, and green technology for degrading antibiotics using sunlight and ambient conditions ^{[11, 12]}. Though the basic mechanism for photocatalytic degradation of antibiotics is similar to that of the degradation of organic dyes (discussed in Section 2), most antibiotics are resistant to decomposition owing to their robust molecular structures. Hence, designing and fabricating appropriate photocatalysts with high photocatalytic activities requires considerable research. Among the diverse researched photocatalysts, TiO₂ has been the most studied semiconductor for photocatalysis since it was used as a photocatalyst by Fujishima and Honda in 1972 ^[13]. Though much work has been directed towards enhancing the photocatalytic performance of TiO₂ via size and crystal face adjustments and morphology regulation, its application remains limited because of its restricted light absorption capability to the UV light region only ^[14-17]. The latter corresponds to only about 4% of the solar light spectrum. Thus, to better exploit the solar light spectrum, it is highly desirable to develop and synthesise advanced photocatalyst systems, with enhanced photocatalytic activities, that can also be activated by visible-light irradiation ^[18]. In recent years, continuous breakthroughs have been made in the exploration and development of novel visible light-driven photocatalysts, and numerous photocatalytic materials have been synthesised. Somesignificant discoveries have been reported ^[19-26].

This mini review focuses on the recent progress in the design and synthesis of visible light-driven photocatalysts towards the photocatalytic degradation of antibiotic wastewater and provides some perspectives on the challenges and future developments in the area of photocatalytic degradation of antibiotics.

2 Fundamental mechanism of photocatalytic

degradation of antibiotics The steps involved in the photocatalytic degradation of antibiotics are illustrated in Fig. 1. The entire process includes the following: (Ⅰ) when light of energy higher than the band gap of the photocatalyst is absorbed by the photocatalyst, excited electrons and holes, with respectively high reducing and oxidising abilities, are generated. Thereafter, the photoinduced electrons and holes separate and migrate to the surface of the photocatalyst; (Ⅱ) some of the electrons and holes recombine to liberate heat during the migration process; (Ⅲ) following migration of the electrons to the surface of the photocatalyst, a reductive pathway is initiated, wherein O₂ is reduced to ·O_2⁻; (Ⅳ) conversely, an oxidative pathway is initiated when the holes migrate to the surface of the photocatalyst, and ·OH is generated upon oxidation of H₂O or the antibiotic molecules. Then, the antibiotic molecules are degraded by the photocatalytic active species ·O_2⁻ or ·OH. All these processes influence the final rate and efficiency of the photocatalytic degradation of antibiotics over the photocatalyst system ^{[27, 28]}. Therefore, broadening the light absorption spectrum ability of the photocatalysts, enhancing the excited charge separation and migration rate, and preventing the recombination of the excited charges are expected to effectively improve the activities of photocatalysts ^[29]. Taking these factors into consideration, there are two key methods of developing highly efficient photocatalysts for the photocatalytic degradation of antibiotics: (1) the photocatalyst should have a suitable narrow band gap, which would afford absorption not only of UV light but also visible light, and (2) a suitable separating medium or surface defect state is required, which is capable of trapping the photoexcited electrons or holes to prevent their recombination.

Fig. 1

Fig. 1. Schematic of the mechanism of photogenerated charge transfer in a type Ⅱ heterojunction photocatalyst system.

3 Advanced photocatalysis for photocatalytic

degradation of antibiotics In recent years, much effort has been directed to designing and fabricating visible light-driven photocatalysts, and diverse advanced photocatalytic materials have been developed for the treatment of antibiotic wastewater ^{[30, 31]}. Four main strate-gies have been employed: (1) doping of UV light-driven photocatalysts with metals and non-metals; (2) development of new visible light-driven semiconductor photocatalysts; (3) design and construction of heterojunction photocatalysts; and (4) design and construction of surface plasmon resonance (SPR)-enhanced photocatalytic systems.

3.1 Metal- and non-metal-doped photocatalysts

Doping of metal and non-metal ions can effectively expand the photo-response of photocatalysts to the visible-light spectrum. For instance, when metal ions enter the TiO₂ lattice, impurity levels form in TiO₂, which will narrow the band gap of TiO₂ and enhance the light absorption range. Furthermore, metal ion dopants can influence the photocatalyst photoreactivity by acting as photoinduced charge traps and regulating the recombination rate ^[32]. However, the presence of excess electrons (or holes) in recombination centres is likely to reduce the amount of photoinduced charges and decrease the photocatalytic activity. Therefore, metal ions should be doped near the surface of the photocatalyst and their concentration should be controlled accordingly to suppress the recombination of electrons and holes. Those processes can be described by the equations below ^[33]:

Generation of impurity levels

Charge trapping

where M and Mⁿ⁺ represent the metal and doped metal ion, respectively, and cb and vb denote the conduction and valence bands of the photocatalyst, respectively. Zhao et al. ^[34] synthesised Bi₅Nb₃O₁₅ nanoparticles doped with Y³⁺ by a combined sol-gel and impregnation method for the first time. The resulting Y^3+_-Bi₅Nb₃O₁₅ nanoparticles were then examined towards the treatment of ornidazole wastewater. The nanoparticle photocatalyst exhibited a high photocatalytic activity under visible-light irradiation. Recently, the doping of SrTiO₃, which is regarded as one of the most promising photocatalysts for the photocatalytic degradation of antibiotics, has also been examined. The Shi’s group synthesised Fe-doped SrTiO₃ and Cr-doped SrTiO₃ ^{[35, 36]}. Doping metal ions into SrTiO₃ led to the formation of new absorption bands in the visible-light region. Furthermore, the band gap of the photocatalysts was reduced. These improved properties enabled the use of the SrTiO₃ materials as visible light-driven photocatalysts for TC degradation. The established optimal amounts of dopants (wt%) Fe and Cr were 3 and 2 wt%, respectively.

When compared with metal ions, non-metal ions are less likely to form recombination centres. Therefore, non-metal ions dopants will not consume the photoinduced charges and are thus more effective for enhancing the photocatalytic activity ^[32]. Vaiano et al. ^[37] examined N-doped TiO₂ photocatalysts towards the photodegradation of spiramycin under visible- light irradiation. The feasibility of the photocatalytic treatment of spiramycin wastewater over N-doped TiO₂ materials under visible-light irradiation was confirmed by total organic carbon analysis. In consideration of the high efficiency and long-term stability, a visible light-mediated C-doped TiO₂ photocatalytic system was examined to degrade norfloxacin by Chen and co-workers ^[38]. The catalyst dosage, solution pH, and initial target compound concentration were key factors that influenced the photocatalytic activity of the C-TiO₂ materials. In another study, Wang et al. ^[39] fabricated C, N, S-tri-doped TiO₂ by a facile, economical, and easy-to-scale-up sol-gel method. The X-ray photoelectron spectroscopy analysis demonstrated that N and C were doped in the TiO₂ lattice, specifically in the O sites, and S⁶⁺ substituted in the Ti⁴⁺ sites. The C, N, S-tri-doped TiO₂ exhibited a higher photocatalytic TC degradation efficiency under visible-light irradiation when compared with non-metal co-doped photocatalyst. The higher photocatalytic efficiency was attributed to higher electron-hole separation efficiency in the tri-doped photocatalyst system.

3.2 Visible light-driven semiconductor photocatalysts

Layered bismuth-based complex oxides, such as BiVO₄, Bi₂WO₆, Bi₂MO₆, and BiFeO₃, possess narrow band gaps, fast charge transfer rates, and high stabilities, which make them highly promising candidates for application in the photocatalytic degradation of antibiotics ^[40-43]. The lone-pair distortion of Bi 6s orbital in bismuth-based complex oxides may lead to the overlap of O 2p and Bi 6s orbitals in the valence band, and this overlap is beneficial for the mobility of photoinduced charges, significantly improving the photocatalytic performance ^[44]. Accordingly, many studies have been devoted to developing Bi-based complex oxide photocatalysts for the photocatalytic degradation of antibiotics. Xiao et al. ^[45] reported a simple microwave heating method to synthesise hierarchical Bi₂₄O₃₁Br₁₀ nanoflakes and systematically investigated their visible-light photocatalytic activity towards the degradation of tetracycline hydrochloride. Owing to the hierarchical structure, Bi₂₄O₃₁Br₁₀ nanoflakes featured a high Brunauer-Emmett- Teller (BET) surface area and small band gap energy (2.51 eV), which is beneficial for visible-light absorption. Also, the negatively charged surface of Bi₂₄O₃₁Br₁₀ nanoflakes promoted the degradation of TC, thereby improving the photocatalytic efficiency. In another study, BiYO₃ nanorods were synthesised using a combined hydrothermal and annealing treatment for the first time by Wu and co-workers ^[46]. The BiYO₃ nanorods photocatalysts, with a narrow band gap of 2.36 eV, showed good photocatalytic activity under visible-light irradiation because of fast charge migration and effective electron-hole separation.

Other advanced oxide photocatalysts with visible-light response abilities have been extensively investigated as exemplified. Liu and co-workers successfully fabricated Zn₂GeO₄ hollow spheres via a template-driven route using zinc hydroxide carbonate (ZHC) as the template ^[47]. Upon dissolution of Zn²⁺ from the surface of the ZHC spheres, Zn₂GeO₄ hollow spheres were generated in situ. The spheres exhibited enhanced photocatalytic activity towards the degradation of antibiotic metronidazole. The results may be ascribed to the high BET surface area owing to the hollow structure, which enabled a higher visible light-harvesting efficiency and generated effective reaction sites. The Shi’s group synthesised MgFe₂O₄ nanofibres by direct annealing of electrospun precursor fibres, followed by a heat treatment process ^[48]. The nanofibres were examined towards the photocatalytic degradation of TC under visible light. The possible mechanism is illustrated in Fig. 2. The unique rod-in-tube structure afforded excellent photocatalytic activity towards the degradation of TC; the structure enhanced light utilisation by light refraction and reflection. Additionally, the authors demonstrated the potential of rhombohedral In₂O₃ with well-defined, three-dimensional, flower-like porous nanostructures as a visible-light photocatalyst for the photocatalytic degradation of TC ^[49]. When compared with bulk In₂O₃, the three-dimensional, flower-like porous rhombohedral In₂O₃ displayed a higher photocatalytic performance, which was attributed to the nature of the crystal phase and high charge separation efficiency.

Fig. 2

Fig. 2. Possible mechanism for the photocatalytic degradation of TC over MgFe2O4 under visible-light irradiation. Reprinted with permission from Ref. [48]. Copyright 2016, The Royal Society of Chemistry.

3.3 Heterojunction photocatalysts

Recently, heterojunction photocatalytic systems, which constitute a combination of two semiconductors with matching energy band gaps, have attracted increasing attention because such a structure is an efficient way to separate the photoinduced charges, thereby enhancing the photocatalytic performance ^[50]. Heterojunction architectures are classified into three main groups denoted as type Ⅰ, type Ⅱ, and type Ⅲ ^[51]. Of the three photocatalyst systems, type Ⅱ heterojunction is believed to show the most potential in realising the highest photocatalytic efficiency as the configuration can effectively prevent the recombination of electrons and holes. The typical mechanism for photogenerated charge transfer in type Ⅱ heterojunction photocatalyst systems is illustrated in Fig. 3. As observed, the photoexcited electrons can migrate from semiconductor Ⅰ to semiconductor Ⅱ in a type Ⅱ heterojunction owing to the more negative conduction band position of semiconductor Ⅰ. Simultaneously, because of the more positive valence band of semiconductor Ⅱ, photoexcited holes can migrate in the opposite direction to that of electrons, leading to complete effective charge separation ^[51]. Furthermore, type Ⅱ hetero-junction is conducive to regulating the range of light absorption. To date, numerous studies have focused on the development of type Ⅱ heterojunction photocatalyst systems, and many advanced heterojunction photocatalysts with enhanced photocatalytic antibiotic degradation efficiency under visible-light irradiation have been synthesised ^[52].

Fig. 3

Fig. 3. Schematic of the mechanism of photogenerated charge transfer in a type Ⅱ heterojunction photocatalyst system.

Since the pioneering study on graphitic carbon nitride (g-C₃N₄) by Wang et al. ^[53-56], g-C₃N₄-based photocatalysts have attracted increasing interest for application in the field of visible-light photocatalysis owing to their unique electronic band structure and high chemical stability. Hong et al. ^[57] synthesised a V₂O₅/g-C₃N₄ heterojunction photocatalyst system via a facile in situ growth strategy, and the photocatalytic activities of the photocatalysts were investigated towards the photocatalytic degradation of TC under visible-light irradiation. The V₂O₅/g-C₃N₄ heterojunction photocatalysts showed significantly enhanced photocatalytic performance when compared with pure g-C₃N₄ and V₂O₅.The heterojunction system can not only improve the photoinduced charge separation, but also exhibits strong redox ability, resulting in efficient degradation of TC. Furthermore, combining UV light-driven photocatalysts with g-C₃N₄ to form a heterojunction structure is a useful method to develop visible light-driven photocatalysts for the degradation of antibiotics. For instance, the possible growth strategy of V₂O₅/g-C₃N₄ heterojunction photocatalysts is illustrated in Fig. 4. In another study, Song et al. ^[58] reported an ordinary solid-phase calcination method for the fabrication of g-C₃N₄/NaNbO₃ composite. The g-C₃N₄/NaNbO₃ photocatalyst displayed an excellent degradation rate, which was 2.7 times higher than that obtained for pristine g-C₃N₄.

Fig. 4

Fig. 4. (a) Schematic of the in situ growth strategy of V2O5/g-C3N4 heterojunction photocatalysts. (b) High-angle annular dark field scanning transmission electron microscopy images of V2O5/g-C3N4 (V2O5/g-C3N4 = 1.0%) sample with maps of C-K, N-K, V-L, and O-K. Reprinted with permission from Ref. [57]. Copyright 2015, Elsevier.

Other examples of heterojunction photocatalysts include the deposition of PbS-CdS onto clinoptilolite nanoparticles. The later photocatalyst system was successfully prepared by Azimi and co-workers and displayed good photocatalytic activity towards the degradation of a mixed aqueous solution of TC and cephalexin ^[59]. The existence of the PbS-CdS heterojunction was important for realising efficient electron transfer and charge separation. The same results were also obtained by Ren and co-workers who fabricated NiFe₂O₄/Bi₂O₃ heterostructures by a combined hydrothermal and microwave-assisted method ^[60]. When compared with pure NiFe₂O₄ and Bi₂O₃, the NiFe₂O₄/Bi₂O₃ heterostructures exhibited twice higher photocatalytic activities.

3.4 SPR-enhanced photocatalysts

For the past few years, SPR-enhanced photocatalysts, involving metal nanoparticles that exhibit strong plasmon resonance (e.g., Au, Ag), have emerged and been considered as a new type of visible-light photocatalyst with improved photocatalytic performance ^[61]. Plasmons are generated upon collective oscillation of free charges in conductors. Particularly, surface plasmons are oscillations confined to the surfaces of conductors and exhibit strong interactions with light. When the real part of the dielectric function is zero, resonance absorption occurs, and strong electric fields at the surface of the nanoparticles are generated when the metal nanoparticles are irradiated by light under their plasmon frequency ^[62]. Fig. 5 schematically illustrates the possible mechanism of photogenerated charge transfer in a plasmonic photocatalyst system. Upon light irradiation, if the frequency of the photons matches the natural frequency of surface oscillating electrons and is enough to prevent the restoring force of positive nuclei, the valence electrons of metal nanoparticles are generated. When such metal nanoparticles combine with other semiconductors, electrons migrate to the semiconductor material, consequently enhancing the light absorption and photocatalytic performance of the nearby semiconductor photocatalyst ^[63]. Using the SPR technology, visible light-driven photocatalysts can be developed from native UV light-based photocatalysts.

Fig. 5

Fig. 5. Schematic of mechanism of photogenerated charge transfer in a plasmonic photocatalyst system.

For example, Wang et al. ^[64] synthesised Ag/Ag₃PO₄/active carbon composite photocatalysts using a simple coupling- deposition method. Owing to the plasmonic effects of the Ag nanoparticles, which were loaded on the surface of Ag₃PO₄, the composite photocatalysts exhibited high visible-light absorption activity. Therefore, the Ag/Ag₃PO₄/active carbon composite photocatalysts showed considerably improved photocatalytic activities towards the degradation of TC. The Shi’s group conducted in-depth research on the synthesis of SPR-enhanced photocatalysts towards the photocatalytic degradation of antibiotics. As exemplified, Ag nanoparticles were considered. Binary composite photocatalysts Ag/Bi_3.84W_0.16O_6.24 nanooctahedron, Ag/K₂Ta₂O₆, and Ag/Bi₃TaO₇ and ternary composite photocatalyst Ag/AgBr/AgIn(MoO₄)₂ were successfully prepared ^[65-68]. In these photocatalysis systems, Ag nanoparticles served as both an SPR initiator and a medium for electron transfer. Fig. 6 shows a possible mechanism of the photocatalytic reaction involving the Ag/AgBr/AgIn(MoO₄)₂ photocatalyst system. Au nanoparticles have also been used to fabricate plasmonic photocatalysts. An Au/Pt/g-C₃N₄ plasmonic photocatalyst with enhanced photocatalytic activity to-wards antibiotic degradation was fabricated by Xue and co-workers. The degradation rate was 3.4 times higher than that obtained for pristine g-C₃N₄ ^[69]. The authors proposed that the high photocatalytic performance was due to the SPR effect of the Au nanoparticles and the electron-sink function of the Pt nanoparticles. The above-discussed studies demonstrate that the SPR effect can effectively improve the light absorption range and photogenerated electron transfer rate of photocatalysts and enhance the photocatalytic performance towards the degradation of antibiotics. The studies provide far-sighted insight into the design and development of plasmonic photocatalysts for enhancing visible light-driven photocatalytic activity in the area of antibiotic degradation.

Fig. 6

Fig. 6. (a) Field-emission scanning electron microscopy image of AgIn(MoO4)2 nanosheets. (b) Transmission electron microscopy, (c) high-resolution transmission electron microscopy, and (d) energy-dispersive X-ray spectrum of Ag/AgBr/AgIn(MoO4)2. (e) Schematic of mechanism of the photocatalytic reaction occurring on the Ag/AgBr/AgIn(MoO4)2 photocatalyst system. Reprinted with permission from Ref. [68]. Copyright 2015, Elsevier.

4 Conclusions and outlook

In this mini review, four strategies for fabricating visible light-driven photocatalysts for the photocatalytic degradation of antibiotics have been discussed, including: (1) doping of metal and non-metal into UV light-driven photocatalysts; (2) development of new semiconductor photocatalysts; (3) construction of heterojunction photocatalysts; and (4) construction of SPR-enhanced photocatalytic systems. These methods afforded visible light-driven photocatalysts with enhanced photocatalytic activity by either narrowing the band gap of the photocatalyst or improving charge separation. The recently achieved progress in this field indicates that the application of visible light-driven photocatalysts is a potential method to treat antibiotic wastewater. The large-scale production of photocatalysts is necessary to realise the commercial application of photocatalysis in treating antibiotic waste. Therefore, the development of simple and effective methods for large-scale preparation is key to photocatalyst application.

To date, improvements in the photocatalytic performance of photocatalysts under visible-light irradiation remain challenging. Fast charge transfer and efficient charge separation are critical to enhance photocatalytic activity. Two-dimensional graphene nanostructures have attracted increasing research interests owing to their superior specific surface area and electrical conductivity. Construction of visible light-driven photocatalytic systems using graphene materials can effectively accelerate the migration of photogenerated electrons and holes and promote photoinduced charge separation to attain high photocatalytic efficiency. Some studies on the application of graphene-based photocatalysts in antibiotic degradation have been initiated ^{[70, 71]}, and over the next few years, graphene- based photocatalysts are expected to be developed as robust materials for antibiotic wastewater treatment.

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