The hydro/solvothermal method is the most widely used for producing diverse micro/nanostructured BiOX. Specifically, a chemical reaction proceeds in a sealed pressure vessel at high temperature and pressure in water or an organic solvent. The products often possess high crystallinity without calcination. Huo et al. prepared BiOBr nanosheets by hydrothermally treating Bi(NO₃)₃∙5H₂O and KBr at 160°C for 12 h (Li K. L. et al., 2014). A similar procedure was extended to the preparation of BiOCl (Xiong et al., 2011b) and BiOI (Li H. et al., 2013). Different from the hydrothermal route which uses a precursor water solution, the solvothermal route adopts various solvents (e.g., methanol (Vadivel et al., 2014), ethylene glycol (Liu et al., 2014), and glycerol (Liu Z. et al., 2012)) to synthesize the precursor solution. This route allows the increment of pressure and temperature relative to the hydrothermal route (Liu Z. et al., 2012). Zhang et al. first produced 3D BiOX microspheres by using the solvothermal route with ethylene glycol (Figure 3) (Zhang et al., 2008) Furthermore, difference in solvents during this route impacts the growing direction of BiOX crystals. Xiong et al. studied the effect of alcohol chain length on the thermal preparation of BiOBr by using methanol, n-butanol, and n-hexanol as solvents at 180°C for 5 h in a stainless steel vessel (Xiong et al., 2016). When the carbon chain length of n-alcohol was prolonged, the characteristic diffraction peaks of BiOBr at the (102) and (110) facets were sharper and stronger, indicating the improvement of BiOBr crystallization.

The adoption of precursors during preparation directly affects the BiOX facets. As reported, BiOBr with exposed (001) facet is photocatalytically more active, because the (001) facet has much lower surface energy than other facets and BiOBr materials have an intraelectric field along the [001] direction (Zhao et al., 2012). The use of ionic liquid Br instead of KBr led to the generation of BiOBr with exposed (001) facet, because the ionic liquid is passivating and can lower the facet surface energy (Mao et al., 2014).

The addition of surfactants such as polyvinylpyrrolidone (PVP) and hexadecyl-trimethyl-ammonium bromide can reportedly regulate morphology. Liu et al. made BiOBr flower-nanospheres with 32.9 m²/g surface area via the solvothermal procedure using Bi(NO₃)₃⋅5H₂O, PVP (M = 45,000∼58,000), bromide [C₁₆min]Br, and ethylene glycol as precursors while keeping the reaction at 140°C for 24 h (Liu and Wu, 2017). Hao et al. compared the effect of PVP on BiOI photocatalytic activity (Hao et al., 2012). With PVP, the BiOI nanosheets self-aggregated to form 3D hierarchical microspheres, which exhibited a higher photocatalytic activity in tetracycline hydrochloride (TCHC) degradation than 2D-BiOI nanoplates (removal rate 94 vs. 44%).

Both hydrolysis and alcoholysis methods offer mild conditions for synthesis of BiOX, but the products have nonuniform dimensions. For instance, BiOCl nanostructures were synthesized from 6 h of BiCl₃ hydrolysis at 65°C using acetylacetone as an assisting solvent under acidic conditions (Armelao et al., 2012). Song et al. prepared 21- to 85-nm-thick BiOCl nanosheets from hydrolysis of Bi(NO₃)₃ with HCl and Na₂CO₃ by keeping it at ∼ pH2 for 30 min at ambient temperature (Song et al., 2017). Zhang et al. compared the photocatalytic activity of BiOBr prepared by hydrolysis and alcoholysis and added 0.5 g of BiBr₃ to 15 ml of solvent of water or isopropyl alcohol at room temperature (Li R. et al., 2015). Then the suspension was heated at 20, 40, or 60°C for 10 min, and the pH was kept at 9. The BiOBr from hydrolysis was nanosheet-shaped and composed of the best crystallinity and unique (102) facets, while the BiOBr from alcoholysis was flower-like with exposed (110) facets. Both showed a superior photocatalytic activity toward methyl orange (MO) degradation, but the BiOBr from hydrolysis had higher photostability after 4 cycles. Huang et al. prepared BiOBr with different shapes (0D spherical quantum dots, 1D nanorods, 2D nanosheets, and 3D hierarchical architectures) through room-temperature hydrolysis (Figure 4) (Han et al., 2020) Different bismuth or bromine sources, solvents, capping agents, or surfactants affected the morphology of BiOBr during hydrolysis. The direct hydrolysis of BiI₃ was used to prepare BiOI by Su et al., who used the as-obtained BiOI to degrade Rhodamine B under simulated solar light (Su et al., 2014).

This method alters the morphology of BiOX mainly by modulating the crystal nucleation and growth during the synthesis. Nanomaterial synthesis by the template method basically takes three steps (Xie et al., 2016). 1) A template is produced. 2) The target product is synthesized under the action of the template via precipitation, sol-gel, hydrothermal, or other common synthetic method. 3) The template is removed. Cui et al. adopted carbonaceous microspheres as a template to adsorb Bi³⁺ and Cl⁻, followed by sintering up to 400°C to form hollow BiOCl microspheres, which were in a shell thickness of 40 nm and a diameter of 200 nm (Cui et al., 2016). With a butterfly wing as a biological template, Yan et al. prepared hierarchical BiOCl (Yan et al., 2019). BiOI was also used as a self-sacrifice template to construct Bi₄O₅I₂/Bi₅O₇I and BiOI/Bi₄O₅I₂ heterojunctions from simple calcination (Cheng et al., 2020). The BiOBr self-sacrifice template was also used for the formation of BiOBr/Bi₄O₅Br₂ (Li P. et al., 2020).

Microwave, co-precipitation, and calcination can also be used to synthesize BiOX. For example, Chen et al. prepared BiOBr with a 3D flower-like structure after 27 min of microwave processing (Li et al., 2012). The BiOBr showed a Brunauer, Emmett and Teller (BET) surface area of 63.5 m²/g and an outstanding removal ability and fast adsorbing rate over Cr⁶⁺ in a wide pH range. Reportedly, the presence of surfactants during microwave processing improved the BiOBr photocatalytic activity (Chen et al., 2017). The BiOCl was also prepared by microwave with the same procedure as BiOBr and showed an excellent visible-light photocatalytic activity for dye Rhodamine B (RhB) (He et al., 2015). Ai et al. prepared the hierarchical porous BiOI using the microwave method. The as-obtained BiOI shows an excellent adsorption ability toward Congo red in water and it can be easily regenerated under light irradiation (Ai et al., 2014).

During preparation of BiOBr by co-precipitation (Lu et al., 2012), Bi(NO₃)₃ and KBr were mixed in acetic acid water solution, stirred, and aged for 6 h. The BiOCl microflowers were successfully prepared by using a simple co-precipitation method, and it had a better photocatalytic activity under acidic conditions than neutral and alkaline conditions (Wang and Zhang, 2020). The BiOI was also prepared by co-precipitation and EDTA was introduced as the retarder of the reaction and a structure-directing agent in aqueous media. The as-obtained BiOI exhibited a large BET surface area (∼47.5 m² g⁻¹) and thus had an excellent photocatalytic activity for NO removal (98%) in the gaseous phase (Montoya-Zamora et al., 2017).

The unstable halogen atoms in BiOX can easily escape due to the weak van der Waals forces. Therefore, phase transformation can be achieved by directly heating BiOX. Plate-like Bi₂₄O₃₁Br₁₀ was prepared via sintering of BiOBr at 750°C (Yu C. et al., 2012). A similar process was extended to the synthesis of hollow Bi₂₄O₃₁Cl₁₀ microspheres at 600°C (Cui et al., 2016). The 3D hierarchical bismuth oxyiodides was prepared using the ethylene glycol–assisted in situ hydrolysis method and the subsequent calcination at high temperature. The multiform bismuth oxyiodides obtained at different temperatures exhibit very distinct microstructure and band structure, and their photoabsorption was tuned from 700 to 400 nm in an orderly manner, rendering the adjustable oxidation and reduction ability of band energy levels (Huang et al., 2017).

Heterojunction basically refers to the band alignment caused by the interface between two types of semiconductors with unequal band structures. Heterojunction is divided by the band positions of the semiconductors into three types (Yang, 2021). Type II, the most widely used one, offers the best band positions for effective charge carrier isolation, which results in higher photocatalytic activity. The photoelectrons will be transported from the CB of semiconductor A to the CB of semiconductor B for favorable energetics of the relative positions of the CBs (Figure 5A). Holes are transferred at the same time from the VB of semiconductor B to the VB of semiconductor A. Under light illumination, the electrons and holes are spatially isolated from each other, lowering the recombination possibility (Ning et al., 2016). Yang et al. prepared type II Bi₂WO₆/BiOBr heterojunction via a one-step solvothermal route for degradation of rhodamine B (RhB), methylene blue, and colorless TCHC (Figure 6A) (Ren et al., 2019) Guo et al. prepared BiVO₄/BiOCl heterojunction with a higher charge separation rate than pure BiVO₄ or BiOCl (Liu et al., 2019). More research on BiOX-based Type II heterojunction can be found in Table 1.

Though type-II heterojunction can perfectly isolate electrons-holes in space, the improvement in such isolation across this heterojunction is unable to prevent the ultrafast hole-electron recombination on the semiconductor. Therefore, p-n junction is proposed for greatly effective charge acquisition and isolation. Generally, p- and n-type semiconductors upon contact will constitute a p-n junction with an interfacial space-charge zone due to the electron and hole diffusion and thus generate a built-in electrical potential that can guide the electron and hole migration in the opposite direction (Figure 5B) (Jiang L. et al., 2012; Li L. et al., 2014; Jiang and Li, 2015) When incident light with an energy at least equal to the bandgap values irradiates the p-n junction, both p- and n-type semiconductors can be excited to form electron-hole pairs (Lin et al., 2011; Cao J. et al., 2012; Peng et al., 2014). The electrons are stimulated by the electric field to move to the CB of the n-type semiconductor, and the holes to the VB of the p-type semiconductor. Consequently, the electron-hole pairs are separated in p-n heterojunctions more quickly than in type-II heterojunctions due to the cooperation between the inner electric field and band arrangement (Yu et al., 2010). Wang et al. prepared p-type BiOBr and interfaced with n-type Bi₂O₂CO₃ by precipitation conversion. The photoactivity of BiOBr/Bi₂O₂CO₃ p-n junction is 14.8 folds that of bare Bi₂O₂CO₃ (Wang J. J. et al., 2020). Sun et al. prepared BiOBr/SnO₂ p-n heterojunctions to eliminate NO in visible light. Importantly, the charge transfer channels and directions at the BiOBr/SnO₂ interface were determined by theoretical calculations (Figure 6B) and experiments (using XPS) (Wu et al., 2020). Similar research was extended to BiOCl and BiOI (Table 1).

Although the above heterojunctions are advantageous in separating photoelectron-hole pairs, the redox ability of these systems is weakened and thus cannot provide sufficient driving force for a specific photocatalytic reaction. Direct Z-scheme heterojunction photocatalysts have been proposed for spatial division of photoelectrons and holes whilst preserving the strong redox ability of two semiconductors (Figure 5C). Under irradiation, the CB photoelectrons of semiconductor A with strong reducing ability and the VB holes of semiconductor B with strong oxidizing ability are preserved. The CB photoelectrons of semiconductor B with low reducing ability and the VB holes of semiconductor A with low oxidizing ability recombine. The 4% CdS/BiOCl heterojunction prepared by Lin et al (Lin et al., 2017) showed a 10% higher photoactivity than pristine BiOCl in elimination of RhB, which was due to the direct Z-scheme mechanism (Figure 6C). In a new Z-scheme BiOBr/MnFe₂O₄ nanocomposite, spontaneous transfer of photoelectron carriers contributed to the photoelectron-hole division and decelerated the electron-hole recombination (Sin et al., 2020). Other Z-scheme photocatalysts were also reported before (Table 1).

Incorporation of transition metal ions into BiOX lattices to generate or modify the electronic states has gained much attention. Furthermore, it may induce lattice defects in BiOX or change crystallinity to prevent electron-hole combination, forming long-lasting carriers. Doping with special metal ions can also enhance light absorption region, which remarkably influences the photoelectrochemical (PEC) performance of semiconductors. Doping with transition metal ions (e.g., Fe³⁺, Cu²⁺, and Mn³⁺) will form impurity energy levels and may induce a redshift in the energy band, which is attributed to the electron transition from impurity bands to the CB or VB. Cu²⁺ was doped into BiOBr by using the solvothermal method to form the doping level (Figure 7), which strengthened both light absorption and electron-hole isolation efficiency (Lv et al., 2020). Consequently, the Cu²⁺-doped BiOBr showed a 2.28 times higher visible-light–driven photocatalytic activity relative to pristine BiOBr. A similar method was used to prepare Fe²⁺-doped BiOBr (Li W. et al., 2017). The doping of tungsten resulted in a bandgap reduction and the generation of an impurity band in the forbidden zone of BiOCl, as confirmed by first-principle theoretical computation and experimental findings. (Yang W. et al., 2014). Other commonly used transitional metal dopants reported in BiOX include Zn²⁺ (Li J. et al., 2015; Zhou et al., 2019b), Ti⁴⁺ (Jiang G. et al., 2012), Al³⁺ (Weng et al., 2014a), Sn⁴⁺ (Tu et al., 2015), and Mn³⁺ (Pare et al., 2011).

The incorporation of non-metal dopants can distribute extra extrinsic electronic levels in the band gap, which greatly affects light adsorption and charge carrier isolation. The main hypothesis is that the VB shifts upward to its maximum level after valence orbitals and local states hybridize from valence orbitals (Chen et al., 2015). Many non-metals such as N, Fen-Qiang Ma et al. (2017), Zhancheng Zhang et al. (2019), Obeid et al. (2020), Liu et al. (2021), and López-Velázquez et al. (2021) were doped into BiOX to improve the photocatalytic activity. Wang et al. prepared the boron-doped BiOBr nanosheets, in which the B dopants form an impurity level to accelerate charge separation (Figure 7G) (Wu et al., 2016) Carbon doping can enhance the internal electric field of Bi₃O₄Cl and thereby increase bulk-charge separation to 80% (Li et al., 2015). This homogeneous carbon-doped Bi₃O₄Cl was prepared by using a glucose-hydrothermal method, followed by thermal treatment to allow the diffusion of the carbon dopant in host lattices. A similar method was employed by Jin et al. to prepare C-doped Bi₂₄O₃₁Cl₁₀ from oxytetracycline hydrochloride as a carbon and chlorine source (Jin et al., 2018).

Rare earth metals with a 4f electron shell are rich in electronic energy levels, which function as electron trapping centers for photocharge carriers. Hence, doping with rare earth metals may promote charge separation and overall photocatalytic performance. In the La³⁺-doped BiOBr microspheres produced by using an ionic liquid ([C16mim] Br)-assisted solvothermal route, (Yin et al., 2017), the substitution of Bi³⁺ by La³⁺ was monitored by XRD peak shift, while no La₂O₃ or another phase of La was found. The La-doped BiOBr significantly increased the photodegradation of ciprofloxacin, which was because La may trap photoelectrons and thus inhibit the electron-hole recombination (Figures 7H–J). Similarly, Y-doped BiOCl improved the visible-light photocatalytic activity in tetracycline decomposition (Zhong et al., 2020). Moreover, europium and erbium were also used as dopants in BiOX recently (Xia et al., 2016).

Deposition of noble metals (e.g., Ag, Pt, Rh, or Pd) on the surface of BiOX to build a space-charge dividing region (namely, the Schottky barrier) is another effective method to promote photo-charge separation. At the interface between a noble metal and BiOX, the electron transfer from the higher-Fermi-level BiOX to the lower-Fermi-level metal will arrange the Fermi energy levels. The presence of the Schottky barrier endows the metal and the semiconductor with excessive negative and positive charges, respectively. The Schottky barrier also acts as an effective electron trap against electron-hole reconnection in photocatalysis, which often leads to a higher photocatalytic capability. More recently, the deposition of Ag nanoparticles (NPs) on BiOX was found to remarkably improve dye decomposition efficiency under UV or visible light illumination (Yu et al., 2011; Yu C. L. et al., 2012). Li et al. prepared Pd/BiOBr by photodeposition (Figure 8A) and found the existence of Pd initiated the formation of more oxygen vacancies on the surface of BiOBr due to the electron interaction at the Pd–BiOBr interface. The coexistence of Pd and oxygen vacancies on BiOBr facilitates the adsorption of O₂ and toluene molecules (Figure 8B). As a result, the Pd/BiOBr interface controls the charge division and activates O₂ and toluene, improving the photocatalytic activity of BiOBr by 1.5 times in 99% selective oxidation of toluene to benzaldehyde (Figure 8C) (Li X. et al., 2020) Yu et al. investigated how noble metals (Rh, Pd, and Pt) deposition affected the photocatalytic behaviors of BiOX. The impact of the noble metals ranks as Pt > Pd > Rh under UV and as Rh > Pt > Pd under visible light. The noble metal NPs also function as electron traps to quicken electron-hole division and thus improve the visible light activity (Yu et al., 2013).

Due to the excellent electrical conductivity, flexibility, and specific surface, carbon materials are ideal supporting scaffolds for homogeneous anchoring of functional nanomaterials. Xu et al. coated hierarchical BiOX (X = Cl or I) on the newly prepared functional carbon nanotubes through ionic layer adsorption and reaction. The nanotubes with outstanding mechanical properties and pollutant absorbing ability were used to tailor the energy gaps of BiOX via covalent bonds (Figure 9A) (Weng et al., 2014b). Similarly, the BiOI/multi-walled carbon nanotube composites prepared by Xiong et al. using the solvothermal method photocatalytically outperformed pure BiOI owing to the interfacial charge-transfer (Figure 9B) (Su et al., 2012). Graphene is another common carbon material to couple with BiOX. Li et al. prepared a BiOBr/graphene composite by an in situ solvothermal route using graphene oxide (Figure 9C). The graphene oxide was reduced to graphene, and BiOBr NPs were formed on the surface. This composite has higher photocatalytic activity in degrading RhB (Tu et al., 2012). Reduced graphene oxide (RGO)/BiOCl hybrids were synthesized by Zhang et al. with graphene oxide as the precursor (Li Z. et al., 2017). The RGO/BiOCl was 8.4 and 3.8 times more photocatalytically active than BiOCl in MO degradation and water oxidation, respectively. This was because the chemical bonding of BiOCl with RGO led to faster charge separation and longer carrier lifetime. It is indicated that chloride vacancies may act as recombination centers to impede photocharge separation (Figure 9D). Carbon quantum dots (CQDs) with quasi-spherical nanoparticles usually <10 nm in diameter have been introduced to BiOX recently. Li et al. prepared the CQD-modified BiOCl which showed the enhanced photocatalytic activity due to superior electron transfer ability of CQDs (Di et al., 2015). Furthermore, the nitrogen-doped CQD–modified BiOBr was prepared by the same group to boost the photodegradation toward ciprofloxacin, rhodamine B, tetracycline hydrochloride, and bisphenol A (Di et al., 2016).

The creation of defects permits the adjustment of micro/electron structure, atom coordination number, and charge transfer in BiOX. Zhang et al. prepared the [001] facet exposed BiOBr with oxygen vacancy (OV) by using an ethylene glycol–assisted solvothermal method and used it to fix atmospheric N₂ to NH₃ (Figures 10A,B) (Li et al., 2015d). The sample without OV was also synthesized by further sintering in an O₂ atmosphere, and the color turned white (Figure 10C). NH₃ was detected over BiOBr with OV, but not in BiOBr without OV (Figure 10D). This was because the OV in BiOBr was the catalytic activation center and can chemisorb N₂ on the surface. Similarly, Xue et al. used BiOBr nanosheets with OV to realize N₂ to NH₃ (Xue et al., 2018). The Bi₃O₄Br nanosheets with single-unit-cell thickness and confined defects have been controllably prepared by Di et al., who demonstrated that the bismuth vacancy can greatly facilitate the generation of oxygen defects, accounting for the formed oxygen defects in the bismuth oxyhalides (Di et al., 2019). Wang et al. prepared OV-rich S-doped BiOBr by one-step solvothermal treatment. The optimal BB-5S degraded 98% 4-chlorophenol (4-CP) under visible light within 120 min, which was 18.0 and 4.9 times those of OV-poor sulfur-doped BiOBr and pristine ultrathin BiOBr, respectively. Experiments and DFT computation showed a sub-band was formed under the synergy of OVs and S doping, which largely facilitated visible-light absorption capability and inhibited photocharge recombination, thus improving photocatalytic activity (Wang Q. et al., 2019). Cui et al. solvothermally prepared OV-rich BiOCl nanosheets (Cui et al., 2018). A new energy level from OV was verified both theoretically and experimentally. BiOCl nanosheets with OV were capable of photocatalytic oxygen evolution under visible light. Additionally, OV-rich BiOCl displayed a higher visible-light photocurrent and more efficient photocharge division and migration than BiOCl with fewer OVs.

Apart from the single vacancies, the effect of surface vacancy pairs in BiOX on photocatalytic activity has been explored recently. When the thickness of BiOX decreased to atomically thin, the atomic-escape energy will be smaller and then surface atoms will escape from the 2D lattice and create vacancies more easily (Di et al., 2018a). Liu and co-workers prepared the Bi₂₄O₃₁Br₁₀ atomic layer with “Bi-O” vacancy pairs by a mild polyvinylpyrrolidone (PVP) self-assembly strategy coupled with pH adjusting (see Figure 10E). In contrast to the perfect surface, Bi₂₄O₃₁Br₁₀ with Bi-O vacancy pairs own large local electric field intensity (Figures 10F,G). Furthermore, the presence of Bi-O vacancy pairs would create an electron delocalization hot spot (Figures 10H,I), where both inflow and outflow of electrons are better favored. Therefore, the separated charges can be effciently delivered to target reactants, significantly enhancing the catalytic reaction rate (Di et al., 2021).

Semiconductor photocatalysts with specific exposed crystal surfaces can improve photo-charge separation and transportation, which further strengthens the photocatalytic performances. In general, surfactants and H⁺ are used to modulate the exposed crystal surface. Glucose can be the capping and structure-directing agent in preparing one-dimensional rod-like BiOBr with exposed (110) facets (BiOBr-110) (Figure 11A) (Peng et al., 2017). Glucose can interact with the (001) facet of BiOBr NPs to prevent the development of these planes, leading to the generation of ultrathin BiOBr nanosheets with exposed (001) facets. In the meantime, glucose drives the ultrathin nanosheets to gather along the [001] orientation, forming BiOBr nanorods with exposed (110) facets. BiOBr-110 photocatalytically outperforms BiOBr-001 owing to the lower resistance and higher efficiency of charge transfer. Zhao et al. prepared the BiOCl with high ratios of [001] and (Li et al., 2012) facets (BiOCl-H1 and BiOCl-H2, respectively) under various acid conditions (Figure 11D) (Zhao et al., 2020), which remarkably raised the RhB photodegradation efficiency. The apparent rate constant of BiOCl-H2 was 0.0961 min⁻¹, about 2.7 times that of BiOCl-H1 (Figures 11E,F). The Z-scheme facet heterojunction was constructed to accelerate charge separation and transportation (Figure 11D). Zhang et al. found that single-crystal BiOCl nanosheets with exposed [001] facets were more active in direct semiconductor photoexcited MO removal under UV light, but the counterpart with exposed [010] facets was highly active in indirect dye photosensitive decomposition under visible light (Jiang J. et al., 2012).

Thickness tuning is another strategy for improving the photocatalytic activity of BiOX. When the thickness of BiOX is reduced, the quantum confinement can be realized to tune the band edge structure, which is favorable for meeting the potential requirement of various catalytic reactions. In addition, the charge migration distance can be shortened from the inside to the surface which improves the separation efficiency of charges. Low-coordinated atoms with dangling bonds can provide active sites to participate in the catalytic reactions (Di et al., 2018b; Xiong et al., 2018). The thickness-dependent photocatalytic activity toward salicylic acid degradation was reported by Li et al., who prepared the thinner and thicker Bi₃O₄Cl through the liquid phase exfoliation under isopropanol or further recrystallization (Li J. et al., 2013). The thinnest Bi₃O₄Cl endows the highest photodegradation activity due to the highest internal electric field. The thinner thickness of Bi₂₄O₃₁Br₁₀ was prepared when a larger amount of NH₄Br was used. The higher charge separation efficiency can be realized in thinner Bi₂₄O₃₁Br₁₀ nanosheets, thus leading to the formation of higher reactive oxygen species concentration, favoring the degradation of tetracycline hydrochloride (Wang C. Y. et al., 2017).

The morphology control is a classic but effective strategy for boosting the photocatalytic activity of BIOX (Garg et al., 2018b; Bárdos et al., 2019; Bárdos et al., 2020). It is well known that porous and hollow solids have excellent adsorptive properties, which is a prerequisite for highly efficiently utilizing photo-induced electron-hole pairs during photocatalysis. BiOI hollow microspheres were synthesized using a facile reactable ionic liquid 1-butyl-3-methylimidazolium iodine ([Bmim]I)-assisted microemulsion method at room temperature (Di et al., 2014). The as-obtained BiOI hollow microspheres shows enhanced photocatalytic activity, which was attributed to the shortened diffusion pathways, lower energy band gap, higher BET surface area, faster interfacial charge transfer rate, and higher separation efficiency of photogenerated electrons and holes. Liu and co-workers prepared the Bi₁₂O₁₇Br₂ nanotubes, exhibiting 14.4 times more activity than Bi₁₂O₁₇Br₂ nanoplates (Di et al., 2020). The enhanced activity is attributed to better CO₂ adsorption and activation, increased carrier separation, and CO desorption, which was induced by the surface tensile strain in Bi₁₂O₁₇Br₂ nanotubes.

By adjusting the elemental constituents, the energy band structures of BiOX materials can be tuned to meet the requirement of various applications. For example, the bismuth-rich BiOCl and BiOBr can narrow the band gap compared to pristine BiOCl and BiOBr while bismuth-rich BiOI shows an increased band gap (Li J. et al., 2017; Wang Z. et al., 2019). In addition, the bismuth-rich BiOX materials own more abundant tunable surface atomic configurations including surface atomic types, atomic quantity, and atomic distance. Bi₂₄O₃₁Br₁₀ materials display outstanding photocatalytic hydrogen evolution behavior, while the pristine BiOBr has been demonstrated to be inactive for photocatalytic hydrogen evolution (Shang et al., 2014).

Photooxidative degradation of many organic pollutants over modified BiOX has been extensively reviewed (Yang et al., 2018; Sharma K. et al., 2019; Garg et al., 2019). Both the decomposition mechanism and the number of intermediates depend on the properties of organic compounds. The abundant organic dyes in textile, apparel, and paper industrial wastewater cause severe environmental pollution. The dye-polluted wastewater carries refractory pigments that are highly harmful and carcinogenic to humans (Brown and De Vito, 1993; Chen et al., 2010). Dyes even at extremely low levels (below 1 ppm) are highly visible in water and severely worsen water environments (Ledakowicz and Gonera, 1999; Grzechulska and Morawski, 2002; Daneshvar et al., 2004). In general, organic dyes are either cationic or anionic according to their chemical structures. Cationic dyes (e.g., RhB, methylene blue, and malachite green) carry cationic functional groups (mostly N⁺) that can dissociate into cations in aqueous solutions. In contrast, anionic dyes including acid orange 7 and MO contain sulfonic or carboxylic acid groups. Both anionic and cationic functional groups can interact with BiOX with a hydrophilic surface, which is favorable for the oxidation by hole or hydroxyl radicals. For instance, RhB was mainly degraded by hole and hydroxyl radicals in BiOBr/BiOF (Jiang et al., 2017). However, the RhB degradation by BiOBr/TiO₂ heterojunction was mediated by superoxide radical anions (O²⁻), which was produced by single electron reduction of dissolved oxygen (Yang et al., 2021). Organic dye degradation was also investigated using different BiOX-based photocatalysts, namely, BiOCl/CeO₂ (Zhang Y. et al., 2020b), W-doped BiOCl (Mokhtari and Tahmasebi, 2021), Bi₂WO₆/BiOX (X = Cl, Br, I) (Derikvand and Tahmasebi, 2021), and BiVO₄/BiOX (X = F, Cl, Br, I) (Razavi-Khosroshahi et al., 2019).

The release of antibacterial agents into the aquatic environment has caused more concerns about their potential environmental risks, especially the occurrence and spread of antibiotic resistance and chronic toxicity to microbial species. Fluoroquinolones (e.g., norfloxacin, ciprofloxacin, moxifloxacin, and ofloxacin) are extensively used as human and veterinary medicine owing to their wide-spectrum activity against Gram- and Gram-negative bacteria (Brain et al., 2004; Liu C. et al., 2012). However, fluoroquinolones cannot be fully metabolised in the body and are largely excreted in their pharmacologically active forms, which make them the most-detected antibiotics in wastewater and surface water (Robinson et al., 2005). In general, the degradation of fluoroquinolones mainly involves amine side-chain degradative oxidation, defluorination, and decarboxylation (Chen and Chu, 2015). Of them, the degradation of the electron-rich amine side-chain to oxidized products is extremely important (Sturini et al., 2012). Different BiOX-based photocatalysts were also designed to reduce antibiotics. For example, Cu²⁺-doped BiOBr (Lv et al., 2020), Nb⁵⁺ -doped BiOBr (Wei et al., 2020), and N-doped BiOCl (Maimaitizi et al., 2020) were used in the degradation of norfloxacin. Type II heterojunctions such as BiVO₄/BiOCl (Ma X. et al., 2017), BiOBr/Fe₂O₃ (Guo et al., 2017), and BiOBr/CdS (Senasu et al., 2021) can reportedly enhance the photodegradation of antibiotics.

Apart from the oxidation degradation of organic dyes and antibiotics, modified BiOX can also be applied to other common organics, including pharmaceutical active compounds (Heidari et al., 2020; Gao et al., 2021) and endocrine-disrupting chemicals (López-Velázquez et al., 2021; Wu et al., 2021; Zhang H. et al., 2021). However, only organics transformation occurs in most cases. Therefore, future works shall focus on the development of modified BiOX with strong oxidation ability to degrade organics and mineralize their byproducts.

Heavy metal water pollution is a global environmental problem that threatens human health and the ecology. Hexavalent chromium (Cr(VI)) is an essential component in many industrial applications, including electroplating, leather tanning, and anti-corrosion steel, despite its carcinogenicity and mutagenicity (Shi et al., 1994; Salnikow and Zhitkovich, 2008). Compared to other oxidation states of Cr ions, Cr(III) is relatively benign with significantly lower toxicity since it can be excreted from the human body. Hence, photocatalytic reduction of Cr(IV) to Cr(III) is considered as an effective route. Long et al. prepared BiOBr-Bi₂S₃ heterojunctions via ion-exchange (Long et al., 2021). The performance of photo-reduced Cr(VI) was about 28.9 and 184.6 times higher than those of pure Bi₂S₃ and BiOBr, respectively. More importantly, the novel composite had a good Cr(III) adsorption efficiency, suggesting that it can be a bifunctional photocatalyst. Similar Cr (VI) removal was also finished with CoFe₂O₄/BiOBr/graphene, (Li et al., 2019), BiOCl/Bi₂S₃ (Lu et al., 2020), and BiOCl_xBr_1-x (Hussain et al., 2020). However, there is no example of using BiOX as photocatalysts into the reduction of other heavy metal ions.

Although numerous modified BiOX had been developed and used on photocatalytic degradation, they still faced limitation during the practical application. One of the major challenges that the photocatalysts confront is the inefficient separation and recovery from the aqueous medium after being utilized in the wastewater treatment process. An et al. collected the Cu-doped BiOBr from suspension by centrifuge and then regenerated it by heating at 200°C to remove the residual organic (Lv et al., 2020). Although this method is effective for the recovery of photocatalysts, the process is time consuming and that inevitably raises the treatment cost. The immobilization of photocatalysts on a certain supporting substrate provides a dual advantage; the first is the efficient separation and easy recovery of reused photocatalysts, and the second is the prevention of agglomeration of the photocatalysts for higher photocatalytic performance (Tettey et al., 2010; Cao C. Y. et al., 2012). A wide range of substrates have been employed for the immobilization of BiOX, such as activated carbon, ceramic fibers, and ceramic paper (Yadav et al., 2019; Kása et al., 2020; Yadav et al., 2020). The novel green BiOX (Cl, Br and I) was homogeneously immobilized on Al₂O₃-based ceramic fiber sheets, which exhibited an excellent photocatalytic performance and recyclability for MO, bisphenol A, and ampicillin disintegration under visible-light irradiation (Yadav et al., 2019).