Review on nanoscale Bi-based photocatalysts

Abstract

Nanoscale Bi-based photocatalysts are promising candidates for visible-light-driven photocatalytic environmental remediation and energy conversion. However, the performance of bulk bismuthal semiconductors is unsatisfactory. Increasing efforts have been focused on enhancing the performance of this photocatalyst family. Many studies have reported on component adjustment, morphology control, heterojunction construction, and surface modification. Herein, recent topics in these fields, including doping, changing stoichiometry, solid solutions, ultrathin nanosheets, hierarchical and hollow architectures, conventional heterojunctions, direct Z-scheme junctions, and surface modification of conductive materials and semiconductors, are reviewed. The progress in the enhancement mechanism involving light absorption, band structure tailoring, and separation and utilization of excited carriers, is also introduced. The challenges and tendencies in the studies of nanoscale Bi-based photocatalysts are discussed and summarized.

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Conceptual insights

Photocatalytic technology is effective for environmental remediation and energy conversion. Nano Bi-based photocatalysts are an important family of visible-light-excitable photocatalysts. Therefore, enhancing the activity of these photocatalysts by adjusting their nanostructure is one of the hot topics in photocatalysis. In this paper, we have reviewed the recent development on the improvement of the performance of nanoscale Bi-based photocatalysts to provide general and new insights into this family.

Introduction

Escalating serious environment and energy crises have resulted in a growing demand for effective environmental remediation and energy conversion techniques. These needs can be addressed using photocatalysis, which is an increasingly developed photochemical strategy. In a photocatalytic procedure, electrons and holes are generated from semiconductors under light irradiation and participate in reduction and oxidization reactions, respectively. Thus, photocatalysis promotes redox reactions, such as pollutant degradation, water oxidization, H₂ evolution, CO₂ reduction and N₂ fixation.^1–19 Consequently, photocatalysis is regarded as a green technique for removing organic pollutants and producing fuel or electricity. Given that visible-light energy constitutes about 43% of solar energy, visible-light-responsive photocatalysts are preferred in photocatalysis and photoelectrocatalysis.

Up to now, many types of semiconductors, including metal oxides (TiO₂, ZnO, Ag₂O, Fe₂O₃, Cu₂O, Ta₂O₅), metal sulfides (ZnS, CdS, MoS₂, Bi₂S₃), multi-component oxides (Bi₂WO₆, InTaO₄, BiVO₄, Ag₃VO₄, SrTiO₃), metal selenides (MoSe₂, CdSe), metal phosphides (Ni₂P), metal phosphates (Ag₃PO₄), metal halides and oxyhalides (AgBr, BiOBr) and metal-free materials (SiC, g-C₃N₄, and Si), have been employed as photocatalysts.^3,4,20–24 Among them, those that possess a band gap of E_g > 3 eV are called wide-band-gap photocatalysts, such as TiO₂, ZnO, ZnS, SrTiO₃, and KTaO₃. In contrast, those with a band gap of E_g ≤ 3 eV are called visible-light-responsive photocatalysts, such as Ag₂O, Bi₂WO₆, InTaO₄, CoO, BiVO₄, Fe₂O₃, Cu₂O, Ag₃VO₄, TaON, CdS, Ta₃N₅, CdSe, Bi₂S₃, SiC, g-C₃N₄, and Si. Bi-Based photocatalysts are important visible-light-responsive photocatalysts and have recently attracted rapidly increasing attention, shown not only from the number of publications, but also from the ratio of publications in the field of photocatalysis (see Fig. 1a and b). Considering the stability of Bi³⁺, most studies have focused on Bi³⁺-containing compounds, such as Bi₂O₃,²⁵ BiVO₄,²⁶ Bi₄Ti₃O₁₂,²⁷ Bi₁₂TiO₂₀,²⁸ Bi₂O₂CO₃,²⁹ Bi₂WO₆,³⁰ BiPO₄,³¹ BiFeO₃,³² BiOX (X = Cl, Br, I),³³ Bi₃TiNbO₉,³⁴ and Bi_0.5K_0.5TiO₃.³⁵ The majority of these compounds possess a layered structure and plate-like appearance. Bi⁵⁺-Containing compounds, such as LiBiO₃, NaBiO₃, and KBiO₃, can also be excited by visible light. However, such types are less reported than the others because of the instability of Bi⁵⁺. In Bi(III) compounds, the hybridized O 2p and Bi 6s² orbitals may cause an upshift of the valence bands (VBs).³³ Therefore, the band gaps of Bi compounds are usually smaller than 3.0 eV and can be excited by visible light. Bi-based photocatalysts are regarded as promising candidates for removing toxins from water and air^3,36–38 as well as for the production of fuel.^2,39–41 However, the photocatalytic performance of bulk Bi-based semiconductors is not as high as those of nanoscale Bi-based photocatalysts, because the photogenerated electrons and holes of these materials have not been easily exploited and utilized. Also, bulk photocatalysts have weaker light absorption and smaller surface areas than nanoscale photocatalysts (see Fig. 2). Numerous attempts have been made to enhance bulk Bi-based semiconductors to achieve ideal photocatalytic activity. These studies have focused on nanoscale component adjustment, morphology control, heterojunction construction, and surface modification^2,37,38,42 (Fig. 3).

Fig. 1 (a) Number of publications of Bi-based photocatalysts during the last decade (source: web of science; date: 8th April 2018; key word: bismuth and photocatalytic), (b) the ratio of publications of Bi-based photocatalysts in the field of photocatalysis (source: Web of science; date: 8th April 2018; key word: photocatalytic).

Fig. 2 Schematic illustration showing the superiority of nanoscale photocatalysts.

Fig. 3 Enhancement strategies for nanoscale Bi-based photocatalysts.

Component adjustment

The band structure parameters, such as VB, conduction band (CB), and band gap (E_g), are crucial to the photocatalyst activity. Furthermore, component adjustments, such as doping, solid-solution preparation and stoichiometry alteration, are effective methods that can be used to tune the band structures of bismuthal semiconductors. Therefore, suitable component adjustment is favorable for bismuthal photocatalysts.

Doping

Heteroatom doping is widely adopted to increase the visible-light absorption of photocatalysts, because this process can generate a doping level between the CB and VB (Fig. 4). Consequently, the energy required to excite electrons decreases, and the light response of semiconductors increases.^43–47 Doping can also improve the charge transmission properties of semiconductors and lead to the augmented transfer efficiency of carriers. Doping is commonly employed to enhance the activity of Bi-based semiconductors.

Fig. 4 The energy level mechanisms for metal-doped and nonmetal-doped photocatalysts.

Many bismuthal compounds are visible-light responsive. Thus, doping has been performed more frequently to enhance the visible-light absorption of bismuthal compounds with a wider band-gap, such as in Bi₂O₂CO₃,⁴⁸ BiPO₄,⁴⁹ BiOCl,⁵⁰ and Bi₃TiNbO₉.³⁴ For example, the band gap of Bi₃TiNbO₉ can be reduced from 3.1 eV to 2.6 eV when it is doped with a 10% molar ratio of Ni²⁺.³⁴ Zheng et al.⁵¹ found that the band gap of Bi₂WO₆ can be narrowed by doping a small amount of Br with the surfactant cetyltrimethylammonium bromide (CTAB) during preparation. Doping by up to approximately 3% can also narrow the band gap of Bi₂MoO₆ from 2.96 eV to 2.69 eV and consequently improve the performance of Bi₂MoO₆ in the photocatalytic degradation of Rhodamine B (RhB).⁵² Zhang et al.⁵³ studied the effects of doping on the electronic structures and optical properties of BiOCl using first-principle calculations. Studies have also proposed that codoping with Sb and I can substantially narrow the band gaps and increase the light absorption of BiOCl because of the difference in the electronegativities between the Sb/I and Bi/Cl atoms.

Doping can further narrow the band gap of visible-light-responsive bismuthal compounds.^54,55 However, the improvement resulting from a narrowed band gap is limited, because visible light response is not the main barrier to the photocatalytic performance of bismuthal compounds. Low utilization efficiency of the generated carriers is the primary issue to be solved for these bismuthal photocatalysts. In this case, doping can also enhance the photocatalyst activity if the dopant can reduce impedance or improve charge separation. For instance, F-doped β-Bi₂O₃ has been shown to exhibit higher photocatalytic activity than that of pure β-Bi₂O₃ in the degradation of methyl orange (MO).⁵⁶ This can be attributed to the higher separation efficiency of electron–hole pairs and lower VB position (indicating stronger oxidation capacity). B-Doping can increase the photocatalytic activity of BiOBr on the inactivity of Escherichia coli, which is attributed to the role of B as the electron acceptor for the effective separation of electrons and holes.⁵⁷ Such a role may have been caused by the empty p-orbital and unpaired electrons in B. Br doping could reduce the effective mass of electrons in Bi₂WO₆, by which the transfer and separation of the carrier can be facilitated.⁵⁸ Aurivillius-type Ce-doped SrBi₂Ta₂O₉ fabricated by Senthil and co-workers⁵⁹ exhibited a higher photocatalytic activity than SrBi₂Ta₂O₉ for H₂ generation. The improvement comes from the enhanced separation of electrons and holes through a dipole moment derived from the bonding of Ce⁴⁺ with the surrounding oxygen atoms. Zhang and colleagues⁶⁰ found that carbon doping increases the internal electric field by 126-fold as a result of the enlarged differences in the electrostatic potentials between the [Bi₃O₄] and [Cl] layers induced by the carbon doping (Fig. 5). Consequently, the separation of holes and electrons was improved. Er-Doped Bi₂₄O₃₁Br₁₀,⁶¹ carbon-doped Bi₂WO₆,⁶² Mo-doped BiVO₄,⁶³ and Er/Yb codoped BiVO₄⁶⁴ also showed enhanced photocatalytic activities relative to those of the original bismuthal compounds due to increased carrier transfer and separation.

Fig. 5 (a) The performance of visible-light-driven oxygen and hydrogen evolution; (b) internal electric field intensity of C-doped Bi₃O₄Cl (assuming the intensity of BOC to be “1”); and (c) schematic diagram of the carrier migration of pure and C-doped Bi₃O₄Cl; BOC-C1, BOC-C2, and BOC-C3 present C-doped Bi₃O₄Cl with carbon concentrations of 0.92%, 1.86%, and 3.16%, respectively. Reproduced with permission from ref. 60. Copyright 2016 John Wiley and Sons.

Anion and mixed-valence cation dopants can also improve the photocatalytic performance of bismuthal semiconductors by facilitating the transfer and separation of carriers. For instance, doping with IO₃⁻ can increase the activity of BiOI in the photocatalytic degradation of MO because of the key role played by IO₃⁻ in improving the transfer efficiency of the carriers.⁶⁵ In-Doped BiOI exhibited augmented activity in the photocatalytic degradation of p-chloroaniline, because the introduced In resulted in the formation of a doping energy level that acted as a scavenger of photo-induced electrons and consequently prevented the recombination of charges.⁶⁶ Jiang et al.⁶⁷ fabricated Sn⁴⁺-doped Bi₂S₃ through a solvothermal method, in which Sn⁴⁺ acted as an electron scavenger by interconversion between Sn⁴⁺ and Sn²⁺ and facilitated O₂ reduction. Consequently, the photocatalytic performance of Sn⁴⁺-doped Bi₂S₃ was enhanced. Similarly, the dopant Tb acted as an electron-trapping agent through interconversion between Tb⁴⁺ and Tb³⁺ and improved the photocatalytic activity of Bi₂MoO₆ in RhB degradation.⁶⁸

Although the above conventional doping of heteroatoms can enhance the activities of bismuthal compounds, the introduced hetero element may alter the space lattice and cause additional stress. This process suppresses the introduction of the dopant and limits the improvements of visible-light absorption and carrier transfer derived from doping. Hence, doping by substituting the original element, rather than interstitial doping, is desired.

For example, when Bi and Ti in Bi₄Ti₃O₁₂ are partially replaced by Cr, the doped Bi₄Ti₃O₁₂ exhibits a higher activity in H₂ generation because of the considerably narrowed band gap and enlarged light absorption of Bi₄Ti₃O₁₂.⁶⁹ The cosubstitution of Gd and Mn in BiFeO₃ nanoparticles (NPs) reduces the band gap and electrical resistivity of BiFeO₃.⁷⁰ The band gap of BiOCl was narrowed when Bi³⁺ was partially substituted by Sn²⁺, and this process consequently endowed BiOCl with increased photocatalytic activity for degrading RhB.⁷¹ When O in BiVO₄ was partially substituted by F through a solid–vapor reaction, the photoelectrochemical water oxidation on BiVO₄ was strongly promoted.⁷² This effect may be attributed to the more negative flat band potential (an upshift of approximately 0.1 V) and increased carrier density resulting from the doping of F. The moderate upshift of the flat band potential makes the consumption of electrons easier, which is favorable to the holes in the VB for oxidation reactions. Bi₃FeMo₂O₁₂ was found to be superior to Bi₃GaMo₂O₁₂ in the photocatalytic degradation of RhB under visible-light irradiation. This result was due to the narrower band gap of Bi₃FeMo₂O₁₂ (2.3 eV) than that of Bi₃GaMo₂O₁₂ (2.95 eV) as a result of the contribution of Fe 3d orbitals to the CB.⁷³ Nevertheless, not all elemental substitutions lead to a reduced band gap in bismuthal compounds. If Bi and Fe in BiFeO₃ NPs are cosubstituted with Ca and Ti using a sol–gel method, the band gap will increase upon an increase in the amount of Ca and Ti.⁷⁴

Elemental substitution can also improve the separation of carriers and thus boost the performance of photocatalysts. Gu et al.⁷⁵ obtained Ce-doped BiVO₄ by substituting Bi³⁺ with Ce ions in a homogeneous precipitation synthesis. The introduction of Ce enhanced the photocatalytic performance of MB and phenol removal by effectively hindering the recombination of photo-generated carriers. BiV_0.8Mo_0.2O₄ exhibited an improved photoelectrochemical activity in water oxidation because of the more efficient charge transport and electron–hole separation derived from the formation of cation vacancies (Bi⁵⁺)/oxygen interstitials as a result of high Mo doping.⁷⁶

Solid solution

Fabricating solid solutions is an effective technique to adjust the band structure of photocatalysts by hybridizing two or more crystalline phases at the atomic scale. A solid solution can be regarded as a special doped semiconductor, in which the anions or cations of the host semiconductor are selectively replaced by introduced ones over the whole range of the composition.⁷⁷ By adjusting the concentration of cation and/or anion substitution (by controlling the components and composition), the band structure of a solid solution can be tuned.^78,79 Although the aforementioned doping methods can enhance the performance of bismuthal photocatalysts to some extent, the shortcomings of doping are obvious. The formed doping levels are generally discrete (Fig. 3), and the dopant may also act as the recombination center of carriers and increase the number of recombination sites for electrons and holes.^78,79 Contrary to doping, solid solutions contain no discrete energy levels. The band structures of solid solutions are uniform, and their band gaps usually fall in the region between those of original semiconductors (Fig. 6). Consequently, the electron excitation can be improved with a lower increase in carrier recombination.

Fig. 6 Schematic diagram of the band structure tuning through solid-solution fabrication.

Bismuth oxyhalide solid solutions are the most frequently studied among Bi-based solid solutions because of the similar structures of BiOX (X = Cl, Br, I) and easy fabrication of solid solutions comprising these compounds. Hence, several studies have also considered BiOX as a kind of heterooxyhalide-substituted bismuth oxyhalide.⁸⁰ Furthermore, along with the ratio of oxyhalides, the absorption edges of bismuth oxyhalide solid solutions usually vary gradually between the absorption edges of the pristine compounds (Fig. 7).⁸¹ This phenomenon renders the tailoring of the band structure as highly convenient. For example, hierarchical Bi-rich Bi₄O₅Br_xI_2−x solid-solution nanosheets can be prepared via a precursor method and show improved photocatalytic activity for CO₂ conversion and Cr(VI) removal.⁸² Flower-like BiOCl_0.5Br_0.5 hierarchical solid solutions can be fabricated through reacting Bi₂O₃ with a mixed solution of KCl and KBr at room temperature in the presence of glacial acetic acid and exhibit enhanced photocatalytic activity for RhB degradation under visible-light irradiation, compared to pure BiOCl and BiOBr.⁸³ Similarly, 3D flower-like BiOCl_xBr_1−x,⁸⁴ BiOCl_xI_y,⁸⁵ and BiOCl_1−xBr_x hierarchical microspheres⁸⁶ have also been prepared by simple one-pot methods and have exhibited better activities than pure BiOCl, BiOBr, and BiOI. When the three latter halogens were introduced during fabrication, ternary solid solutions BiOCl_xBr_yI_z (x + y + z = 1) could be obtained, and the as-prepared solid solutions generally possessed a controllable band gap and highly enhanced visible-light photocatalytic activity.⁸⁷ F-Containing bismuth oxyhalide solid solutions are less reported in solid-solution preparations because of the partial miscibility of BiOF_1−xY_x, as proposed by Zhao et al.⁸⁸ They examined the properties of BiOX_1−xY_x (X, Y = F, Cl, Br, I) solid solutions through density functional theory calculations. The results showed that several properties of BiOCl_1−xBr_x, BiOBr_1−xI_x, and BiOCl_1−xI_x solid solutions changed almost linearly with x, while the properties of BiOF_1−xY_x (Y = Cl, Br, I) solid solutions are partially miscible. In addition, the related calculated data were difficult to fit. Therefore, this behavior has been attributed to the partial miscibility of BiOF_1−xY_x.

Fig. 7 (a) UV-visible light diffuse reflectance spectra and (b) RhB degradation efficiency of BiOCl_xBr_1−x, BiOCl_xI_1−x, and BiOBr_xI_1−x solid solutions. Reproduced with permission from ref. 81. Copyright 2013 Royal Society of Chemistry.

In addition to bismuth oxyhalides, other Bi-containing solid solutions were found to be more efficient than pure bismuthal compounds for visible-light-driven photocatalysis. For instance, Terebilenko et al.⁸⁹ fabricated Bi_1−x/3V_1−xMo_xO₄ solid solutions for photocatalytic water oxidation. The resulting solid solutions possessed the same band gap of 2.25 eV (narrower than that of BiVO₄), which endowed such solid solutions with high visible-light absorption. A Sr_0.9Bi_0.1Ti_0.9Fe_0.1O₃ solid solution synthesized by Lu et al.⁹⁰ performed more effectively than SrTiO₃ in H₂ production under full-range irradiation (λ ≥ 250 nm). This finding was attributed to the suitable band gap of the former compound. The resulting band gap can be tuned by adjusting the ratio of Bi to Sr (see Fig. 8).

Fig. 8 (a) UV-visible diffuse reflectance spectra and (b) photocatalytic H₂ production of Sr_0.9Bi_0.1Ti_0.9Fe_0.1O₃ solid solutions under full-range irradiation (λ ≥ 250 nm). Reproduced with permission from ref. 90. Copyright 2017 Elsevier.

Stoichiometry alteration

The layered structures of bismuthal compounds render the component adjustment convenient. Even without introducing a foreign element, the band structures can also be tuned by changing the stoichiometry of the bismuthal semiconductor. In this field, similar to solid solutions, bismuth oxyhalides have been extensively studied because the band gap, VB, and CB of oxygen-rich (bismuth-rich) or oxygen-deficient compounds usually changes gradually along with the atomic ratio. Consequently, the compounds can be enhanced by stoichiometric variation. In a bismuth oxyhalide, both the O 2p and X np (n = 3, 4, and 5 for X = Cl, Br, and I, respectively) states have a major contribution to the VB, while the Bi 6p states dominate the CB. When the O content increases, the O 2p states of bismuth oxyiodide become more significant than the X np state in the VB, therefore the band gap energies of O-rich bismuth oxyiodides are closer to those of Bi₂O₃.⁹¹ That is, the VB can be tuned by changing the O/X ratio.

The band gap of BiOI is very narrow, but its CB is less negative, and its VB is less positive. These characteristics result in the low reduction and oxidation capacities of BiOI. Consequently, the CB upshift and VB downshift become more favorable, but excessive shift in the CB and VB may result in a wide band gap, which is unfavorable for photocatalysis. That is, a moderate upshift of CB and downshift of VB are required. Generally, the VB of bismuth oxyiodides downshifts to a more positive level because of the increased contribution of the O 2p states, while the CB upshifts to a more negative position owing to the additional contribution of I 5p, thus the band gap of the bismuth oxyiodides increases with an increase in the bismuth and oxygen content. Hence, bismuth oxyiodides with moderate overdoses of bismuth and oxygen, such as Bi₄O₅I₂ or Bi₆O₉I₃, are more advisable. For instance, Bi₄O₅I₂ nanosheets (fabricated using a solvothermal method) exhibited a higher activity than BiOI in the photocatalytic degradation of RhB because of the more positive VB of the former.⁹² For BiOCl, narrowing the wide band gap is the primary aim of the atomic ratio adjustment. Oxygen-rich bismuth oxychlorides, such as Bi₂₄O₃₁Cl₁₀ (E_g = 2.71 eV, ref. 93) and Bi₁₂O₁₇Cl₂ (E_g = 2.57 eV, ref. 94), possess narrower band gaps and can be excited by visible light. Therefore, the visible-light photocatalytic performances of these bismuth oxychlorides are better than that of BiOCl. Moreover, Cl-rich bismuth oxychloride was also found to possess a narrower band gap. For instance, the band gap of Bi₁₂O₁₅Cl_l6 nanosheets fabricated by Wang et al.⁹⁵ was estimated to be 2.36 eV, and the photocatalytic performance of Bi₁₂O₁₅Cl_l6 nanosheets for bisphenol A (BPA) removal was superior to that of BiOCl nanosheets. Studies on oxygen-rich bismuth oxybromides have mainly focused on Bi₄O₅Br₂ because of its narrow band gap. In reports by Xia and co-workers,⁹⁶ Bi₄O₅Br₂ nanosheets fabricated using a solvothermal method were shown to possess a narrower band gap (2.37 eV) and lower carrier transfer resistance than those of BiOBr (2.82 eV) and were more effective than BiOBr in the photocatalytic degradation of BPA. For similar reasons, Bi₄O₅Br₂ showed much higher activity than BiOBr in the photocatalytic degradation of resorcinol⁹⁷ and ciprofloxacin (CIP)⁹⁸ under visible light. Furthermore, Bi₄O₅Br₂ nanosheets were also discovered to be more suitable than BiOBr nanosheets in the photoreduction of CO₂.⁹⁹ These results are due to the CB position of Bi₄O₅Br₂ being higher than that of BiOBr and thus more favorable for the reduction reaction of CO₂.

The band structures of other Bi compounds, such as BiVO₄, Bi₂SiO₅, and Bi₂O₃, can also be altered by varying the stoichiometry for activity enhancement. Batool et al.¹⁰⁰ found that Bi₄(SiO₄)₃ nanofibers possess lower impedance and exhibit higher photocatalytic performance than Bi₂SiO₅ nanofibers in the degradation of MO. Huang and co-workers¹⁰¹ prepared Bi₄V₂O₁₁ through a solvothermal method. The synthesized Bi₄V₂O₁₁ had a narrower band gap (2.15 eV) than that of BiVO₄ (2.4 eV) and showed high water oxidation activity under 300 W Xe lamp irradiation. In Kalyan's research, a BiO_2−x phase with a narrow band gap (1.84 eV) formed on the surface of a Bi₂O₃ photoanode during photoelectrochemical testing in the presence of KOH and H₂O₂.¹⁰² The formed BiO_2−x phase improved the photocurrent because of its narrow band gap.

Morphology control

The photocatalytic properties of semiconductors are highly dependent on their morphologies and sizes.¹⁰³ High specific surface area, low thickness, and hierarchical and hollow structures can increase light absorption and accessibility of photocatalysts. Such properties enable the accelerated migration of carriers to the surface and reduce the dosage of photocatalysts. Accordingly, the performance of Bi-based photocatalysts can be enhanced by building ultrathin and hierarchical and hollow structures or by immobilization on substrates with specific morphologies.

Thin morphologies

Thickness greatly affects the activities of bismuthal compounds because of the influence of such a variable on the efficiency of light absorption and the distance of the photo-generated carriers from the surfaces. A smaller sized bismuthal compound usually leads to a higher specific surface area and better photocatalytic and photoelectrochemical performance.^104,105 Considering unique layered plate-like structures, the exposed-facet ratio of bismuthal compounds usually changes along with the thickness of the compounds. Therefore, the thickness effect has often been discussed in related reports. Thinner structures can endow bismuthal compounds with a higher specific surface area, increased light absorption efficiency, and enabled utilization of photo-generated electrons and holes. Ultimately, such properties may enhance photocatalytic activity. With a decrease in the thickness, the surface energy of the nanoplates would also largely increase, single plates become thermodynamically unstable and these nanoplates prefer to stack together into one larger particle to reduce surface energy during growth. As a result, many Bi-based photocatalysts usually possess a granular appearance comprising nanoplates. The thin morphology discussed in this section refers to these nanoplates.

Chen and colleagues¹⁰⁶ found that reducing the thickness of BiOBr nanosheets can significantly increase the exposed (001) facets and the photocatalytic activity of the BiOBr nanosheets. The enhanced photocatalytic activity of the BiOBr nanosheets was ascribed mainly to the enhanced absorption of visible light and improved separation efficiency of charge carriers due to the ultrathin structure. Li et al.¹⁰⁷ proposed that the photoactivity of the BiOBr nanosheets depends on nanosheet thickness. Similarly, reducing the thickness of Bi₂O₂CO₃ [accompanied by an increase in the (001) facets] improved its photocatalytic activity in the degradation of RhB,¹⁰⁸ and the activities of Bi₂WO₆ were augmented along with a reduction in thickness.¹⁰⁹

Thinner structures sometimes result in additional facet and structure differences, which affects the performance of bismuthal photocatalysts. For example, increasing the exposure of (001) facets (accompanied by a reduction in the thickness) can improve the efficiency of RhB degradation on Bi₂Fe₄O₉ nanoplates, because a larger exposed (001) surface can provide more Fe³⁺ cations to generate more hydroxyl radicals.¹¹⁰ A reduction in the thickness can also improve the photocatalytic performance of BiVO₄ in O₂ evolution, owing to the increased ratio of reduction functional (010) facets versus dominating oxidation functional (110) facets (Fig. 9).¹¹¹ Hu et al.¹¹² noted that reducing the thickness of BiTaO₄ not only increased the specific surface area but also caused upshifts in the VB and CB. Density of states (DOS) calculations showed that the energy levels of the VB and CB of the (010) facets of BiTaO₄ are more negative than those of bulk BiTaO₄. The (010) facets in BiTaO₄ increase along with a reduction in the thickness, therefore, thinner BiTaO₄ single-crystal nanoplates possess more {010} facets and higher VB and CB positions.

Fig. 9 (A) Photocatalytic O₂ evolution from an AgNO₃ solution; (B) pseudo-first-order-fitted photocatalytic degradation curves of 2,4-dichlorophenoxyacetic acid (2,4-D) on truncated bipyramid (red) and plate-like (black) BiVO₄ under visible light (λ > 420 nm); SEM images of bare (C-a) truncated bipyramid and (C-b) plate-like BiVO₄ and the corresponding Ag-deposited (C-c) truncated bipyramid and (C-d) plate-like BiVO₄. (D) Schematic diagram of the promotion induced by increasing the size of the (010) facets in BiVO₄. Reproduced with permission from ref. 111. Copyright 2016 American Chemical Society.

When the thickness of a bismuthal semiconductor is reduced to below 10 nm, the specific surface area, light absorption, and carrier migration will be further enhanced. Sometimes, new characteristics might also arise. Ultrathin square-like BiOCl nanosheets (3–7 nm in thickness) have been shown to exhibit a higher activity than thicker nanosheets in the photocatalytic degradation of RhB due to a larger specific surface area.¹¹³ Di et al.¹¹⁴ reported the preparation of ultrathin BiOI nanosheets and their composites, which displayed a uniform thickness of approximately 0.9 nm (Fig. 10), approaching the thickness of one [I–Bi–O–Bi–I] unit (0.9149 nm). The CB position of the BiOI nanosheets was determined to be −0.95 eV (NHE), negative to reduce O₂ to ˙O₂⁻. This characteristic is beneficial for H₂ generation or CO₂ reduction. However, the researchers did not provide results on the reduction application, although their reports on the surface modification of BiOI nanosheets still exhibited improved activity for the photocatalytic degradation of RhB. The ultrathin H_1.78Sr_0.78Bi_0.22Nb₂O₇ nanosheets (Fig. 11) prepared by calcination exhibited 5.5 and 26.2 times higher activity than H_1.78Sr_0.78Bi_0.22Nb₂O₇ plates and SrBi₂Nb₂O₉ platelets for photocatalytic H₂ generation, respectively. This result can be ascribed to the higher separation efficiency of the photo-generated carriers and specific surface area.¹¹⁵

Fig. 10 (a) Atomic force microscopy (AFM) image and corresponding height of the atomically thin BiOI nanosheets; (b) valence-band X-ray photoelectron spectra and (c) pseudo-first-order-fitted kinetic curves (for the RhB degradation) of pure BiOI and N-carbon quantum dot (CQD) decorated BiOI. Reproduced with permission from ref. 114. Copyright 2016 Royal Society of Chemistry.

Fig. 11 (a) Transmission electron microscopy (TEM) and (b) high-resolution TEM (HRTEM) images of H_1.78Sr_0.78Bi_0.22Nb₂O₇ nanosheets (HSN Ns); (c) AFM image; (d) corresponding height of the HSN Ns; and (e) the H₂ evolution curves of the SrBi₂Nb₂O₉ platelets (SBN Ps), H_1.78Sr_0.78Bi_0.22Nb₂O₇ platelets (HSN Ps), and HSN Ns. Reproduced with permission from ref. 115. Copyright 2017 Elsevier.

Preparation of fabric or quantum sized materials is another method for obtaining thin morphologies and high specific surface areas. Bharathkumar et al.¹¹⁶ fabricated BiFeO₃ fibers using an electrospinning method. The obtained BiFeO₃ fibers then exhibited improved relative photocatalytic activity compared with that of BiFeO₃ particles for degrading RhB, due to their high specific surface area. Zhao and co-workers¹¹⁷ prepared metal Bi nanofibers using an aqueous reduction procedure and found that Bi nanofibers exhibited a higher photocatalytic activity than Bi nanoplates and NPs in degrading RhB. Yang et al.¹¹⁸ fabricated bismuth oxide formate (HCOOBiO) nanowires (Fig. 12) with a diameter of 20 nm and a length of up to several hundreds of micrometers in N,N-dimethylformamide (DMF) solution via a solvothermal route. These HCOOBiO nanowires showed a higher activity than HCOOBiO microspheres in the photocatalytic degradation of MO. In the studies of Lu and colleagues,¹¹⁹ BiVO₄ nanofibers were prepared using an electrospinning and calcination method and showed a higher efficiency than BiVO₄ rods in the photocatalytic removal of MB under visible-light irradiation. Additionally, Bi₂S₃ with a high length-to-width ratio was found to be highly effective in the photocatalytic reduction of CO₂ to CH₃OH.¹²⁰ The high length-to-width ratio not only increases the specific surface area but also enlarges the band gap (by a downshift of the VB and an upshift of the CB) owing to the increased influence of the surface state resulting from the smaller size. And as it happens, the CB is upshifted to a level sufficient enough for CO₂ reduction. Bismuth monoxide quantum dots (QDs) obtained using a hydrothermal method exhibited high efficiency in the photocatalytic reduction of N₂.¹²¹ The ammonia production rate reached 1226 μmol g⁻¹ h⁻¹ in the absence of a sacrificial agent and a cocatalyst. As Bi₅O₇Br was tailored into nanotubes with a diameter of 5 nm, its CB (−1.14 eV) was more negative than that of BiOBr. Though such a CB is still not thermodynamically accessible for direct free N₂ fixation, N₂ fixation was achieved because the formed oxygen vacancy in samples injects trapped photogenerated electrons directly into the chemically adsorbed N₂ and weakens the N≡N triple bond.¹²²

Fig. 12 Scanning electron microscopy (SEM) images (a) of ultra-long HCOOBiO nanowires. Removal of (b) MO and (c) Cr(VI) at various initial concentrations from ultra-long HCOOBiO nanowires. Reproduced with permission from ref. 118. Copyright 2015 Elsevier.

Hierarchical structures

Photocatalysts with hierarchical structures usually exhibit better performances than bulk photocatalysts. Such superiority is due to the high specific surface area, efficient light harvesting, accessibility, and easy transport of the reactants of the former catalysts. Yang et al.¹²³ accomplished the early work on hierarchical ordered oxides. Since then, the design and synthesis of hierarchical-structured nanomaterials have attracted significant attention.^124–126

Specific surface area is one of the most important factors that results in the higher activity of bismuthal compounds. Generally, photocatalysts with a hierarchical structure possess relative higher specific surface areas than bulk photocatalysts or individual rods and plates, and work better in photocatalytic reactions. For example, grain-like Bi₂₄O₃₁Br₁₀ hierarchical architectures (Fig. 13) fabricated using a solvothermal method in the presence of starch also showed improved photocatalytic activity in the degradation of RhB.¹²⁷ Qin and co-workers¹²⁸ prepared BiYO₃ with a high specific surface area using a hydrothermal method with soft templates. The BiYO₃ prepared using disodium ethylenediaminetetraacetate (EDTA) as a template possessed the highest specific surface area and photocatalytic activity for reducing CO₂. Jia et al.¹²⁹ found that hierarchical Bi₂S₃ produced using a hydrothermal method [with cetyltrimethylammonium bromide (CTAB) as a surfactant] exhibited higher photocatalytic activity than Bi₂S₃ nanorods in the degradation of RhB. This performance was ascribed to the synergetic effect of the shape, surface area, band gap, crystallinity, and size resulting from the 3D hierarchical structure. Han's group¹³⁰ fabricated flower-like δ-Bi₂O₃ by thermally treating bismuthyl acetate CH₃COO(BiO), which was prepared from Bi₂O₃ and glacial acetic acid (HAc) or Bi(NO₃)₃·5H₂O and HAc. The obtained δ-Bi₂O₃ showed superior activity over that of δ-Bi₂O₃ sheets in the photocatalytic degradation of RhB. Zhou et al.¹³¹ prepared uniform cylindrical BiFeO₃ samples by adjusting the ratio of HNO₃ : NaOH to 8 : 12 in the hydrothermal process. The uniform cylindrical BiFeO₃ exhibited enhanced photocatalytic activities for removing RhB under visible-light irradiation, because the uniform structures favored charge transport and increased the accessibility to organic matter. Li et al.¹³² prepared BiOBr strips accumulated from tiny pieces using a solvothermal method with bismuth subsalicylate as a template. They found that the BiOBr strips showed an enhanced performance for the photocatalytic degradation of RhB. Furthermore, the hierarchical structure of bismuthal oxyiodide obviously promoted the performance of the compound in phenol removal.¹³³

Fig. 13 SEM images of Bi₂₄O₃₁Br₁₀ hierarchical architectures (BOB) (a) BOB-S0, (b) BOB-S0.2, (c) BOB-S0.4, (d) BOB-S0.8 (grain-like), and (e) BOB-S1.2 (e.g., -S1.2 indicates that the mass of starch used was 1.2 g); (f) EDS pattern of BOB-S0.8; (g) photocatalytic degradation curves of RhB; and (h) pseudo-first-order-fitted kinetic curves over Bi₂₄O₃₁Br₁₀ samples under visible-light irradiation. Reproduced with permission from ref. 127. Copyright 2016 Elsevier.

Even in the construction of a hierarchical structure, the morphologies of bismuthal photocatalysts can be further tuned to different forms by adjusting the solvent, pH, and surfactants, as well as by adding capping agents. For instance, Wang et al.¹³⁴ synthesized 3D hierarchical flower-like α-Bi₂O₃ microspheres with various morphologies (Fig. 14) in an ethanol–water mixture. They heated the mixture in an oil bath at 95 °C, and the morphology was controlled by adjusting the amount of added glycerol and oleic acid (capping agents). Sarka et al.¹³⁵ fabricated Bi₂S₃ in different shapes (Fig. 15) using a solvothermal method with various solvents. The performance of Bi₂S₃ nanocrystals in removing MB was better than those of the Bi₂S₃ nanorods and nanoplates due to the higher specific surface area and suitable band structure of the Bi₂S₃ nanocrystals. Zhao et al.¹³⁶ altered the morphologies of BiVO₄ from olive-shaped to primrose-like, leaf-shaped, or strip-like morphologies (Fig. 16), by adjusting the concentration of glycerol in the aqueous solution during template-free solvothermal synthesis. In addition to solvent control, the morphologies of BiVO₄ can also be modified by pH adjustment. Besides this, the hierarchical morphology alteration of BiVO₄ can also be achieved through adjusting the pH.¹³⁷ The relationship between pH and the morphologies of BiVO₄ is illustrated in Fig. 17. If NaHCO₃ is added in the solid-state synthesis of BiVO₄, the morphology can be turned from polyhedral to a rice-like shape, by increasing the nucleation rate and hindering the continual growth of the original product particles.¹³⁸ In the preparation of Bi₂WO₆, its morphology can be tuned from accumulated sheets to a complex morphology by adjusting the molecular weight of the added polyvinylpyrrolidone (PVP).¹³⁹ Examples of such complex morphology include flower-like, red blood cell-like, and square pillar-like morphologies. Bi₂WO₆ with a thin sheet-like appearance showed the best photocatalytic performance in the decomposition of RhB under visible-light irradiation.

Fig. 14 3D hierarchical flower-like α-Bi₂O₃ microspheres fabricated using different solvents: A1 (0.5 mL of glycerol), A2 (1 mL of glycerol), A3 (1 mL of glycerol and 1 mL of oleic acid), and A4 (1 mL of glycerol and 2 mL of oleic acid). Reproduced with permission from ref. 134. Open Access 2016 World Scientific.

Fig. 15 (a) Synthesis pathways of Bi₂S₃ NPs with tuned morphologies from Bi(ACDA)₃ (ACDA = 2-aminocyclopentene-1-dithiocarboxylic acid radical) using solvent comprising glucose, oleylamine (OAM), hexadecylamine (HDA), dodecanethiol (DT), trioctylphosphine oxide (TOPO), and dimethylformamide (DMF); (b) photocatalytic degradation curves of MB under visible-light irradiation and (c) the Tauc plot of the Bi₂S₃ NPs using a mixed solvent composed of OAM and TOPO. Reproduced with permission from ref. 135. Copyright 2016 Elsevier.

Fig. 16 SEM images of the BiVO₄ samples prepared with the following volume fractions of glycerol aqueous solution: (a) 100%, (b) 75%, (c) 50%, (d) 25%, (e) 15%, and (f) 10%; (g) relationship between the glycerol fraction, the (040)/(1̄21) intensity ratio, the morphology, and the photocatalytic performance of the BiVO₄ samples. Reproduced with permission from ref. 136. Copyright 2016 Elsevier.

Fig. 17 (a) Schematic diagram of the possible formation mechanism and (b) powder X-ray diffraction patterns (PXRD) patterns of BiVO₄ samples with various morphologies prepared at different pH values; (c) photocatalytic activities of the BiVO₄ samples in the degradation of MB under visible light. Reproduced with permission from ref. 137. Copyright 2016 Elsevier.

Hollow and porous structures

The hierarchical structure of Bi compounds can be transformed to a hollow structure, which is more preferable for improving the photocatalytic performance, using a suitable method. For instance, Qian et al.¹⁴⁰ fabricated mesoporous Bi₄Ti₃O₁₂ by calcinating a mixture of Bi(NO₃)₃ and tetrabutyl titanate. The product had a markedly higher specific surface area (109.4 m² g⁻¹) and exhibited a better performance than that of the bulk compound. Gong and co-workers¹⁴¹ prepared porous BiVO₄ by electrospinning and calcination in the presence of PVP. The obtained porous BiVO₄ had a larger specific surface area and enhanced photocatalytic activity for the degradation of MB. Kashfi-Sadabad et al.¹⁴² synthesized Sm-doped Bi₂MoO₆ hollow spheres by developing a solvothermal pathway in the presence of pluronic P123 copolymer. During the synthesis, no spherical structures formed in the absence of P123 (Fig. 18b). At a low dosage of P123, the products became solid spheres without hollow structures (Fig. 18c). Hollow spheres formed gradually and became enlarged upon further P123 addition. Finally, Bi₂MoO₆ hollow spheres with diameters of 1–1.5 μm and a thickness of approximately 100 nm (Fig. 18d) were obtained when the P123 amount reached 3 g. The formation of these spheres is illustrated in Fig. 18a. Hollow spherical bismuthal photocatalysts can also be fabricated using a solvothermal method in the absence of a template. Zhou and colleagues¹⁴³ fabricated hollow Bi₂WO₆ microspheres (Fig. 19) using a template-free solvothermal route with a mixed solvent composed of ethylene glycol (EG) and ethanol. The well-defined hollow Bi₂WO₆ microspheres exhibited considerably better photocatalytic activity in the degradation of RhB than Bi₂WO₆ with other morphologies under visible-light irradiation. Li et al.¹⁴⁴ synthesized hollow Bi₂MoO₆ spheres using a solvothermal route in a Bi₂MoO₆ precursor solution. The obtained yolk shell-like Bi₂MoO₆ displayed markedly higher efficiency in removing RhB. Such improvement was attributed to the hollow structure accompanied by enhanced light absorption, increased specific surface area, and augmented charge transfer. If a mixed solution containing glycerol, ethanol, and deionized water was adopted, hollow Bi₂MoO₆ was still obtained during the solvothermal synthesis.¹⁴⁵ Multishell hollow structures are more attractive than other structural types because of their higher surface area and unique nested structure, which ensure highly efficient light utilization. Zong et al.¹⁴⁶ achieved Bi–V–O heterostructured multishell hollow spheres (composed of interconnected BiVO₄ and Bi₄V₂O₁₁) (Fig. 20) using NaOH-treated carbonaceous microsphere templates and a suitable heating rate. The double-shell Bi–V–O hollow spheres were more effective than their single-shell counterparts in removing MB under visible-light irradiation because of the multiple light reflections and effective light utilization achieved through the multishell hollow structure.

Fig. 18 (a) Schematic diagram of the formation of hollow-structured Bi₂MoO₆ and SEM images of Bi₂MoO₆ fabricated (b) without P123, (c) with 1 g of P123, and (d) with 3 g of P123. Reproduced with permission from ref. 142. Copyright 2016 American Chemical Society.

Fig. 19 (a) SEM images of Bi₂WO₆ hollow spheres prepared using a mixed solvent of EG and ethanol (EA) (V_EG : V_EA = 1 : 2) at 160 °C for 2 h; (b) photocatalytic degradation curves of RhB on Bi₂WO₆ samples prepared using solvents with different V_EG : V_EA ratios, and (c) schematic diagram of the formation of hollow structures, involving the formation (1), surface roughening (2) and hollowing (3) of amorphous solid microspheres. Reproduced with permission from ref. 143. Open Access 2016 MDPI.

Fig. 20 (a) Schematic diagram of the formation of Bi–V–O multishell hollow spheres and (b) photoabsorption and photocatalytic mechanism of the Bi–V–O multishell hollow spheres. Reproduced with permission from ref. 146. Copyright 2017 Elsevier.

Hollow structures are not restricted to hollow spheres. Hollow tubes and cross-linked net-like structures are also favorable morphologies for bismuthal photocatalysts. These alternative structures can be constructed with assistance from directed surfactant agents or substrates. For example, Bi₂O₂CO₃ nanotubes (Fig. 21) can grow on a graphene substrate through hydrolysis by adjusting the pH to 10.5 using urea.¹⁴⁷ In the presence of CTAB (as a morphology director), BiVO₄ can grow into nanotubes during hydrothermal synthesis.¹⁴⁸ The formed BiVO₄ nanotubes exhibit improved activity relative to that of BiVO₄ nanorods in the photocatalytic degradation of MO. When dual templates of polystyrene latex spheres (PS) and P123 are adopted in the preparation of Bi₂O₃/TiO₂ composites (sol–gel method), a series of 3D-ordered macroporous profiles are produced (Fig. 22).¹⁴⁹ The composites present a significantly higher activity than P25 and Bi₂O₃ in the photocatalytic removal of crystal violet. Such an advantage can be ascribed to the formed porous structure and heterojunction. Lee et al.¹⁵⁰ fabricated a type of porous β-Bi₂O₃ from Bi(NO₃)₃ using a rapid phase and surface transformation with structure-guided combustion waves. The compound showed a higher photocatalytic performance in the degradation of RhB than those of α-Bi₂O₃ rods and Bi(NO₃)₃·H₂O rods.

Fig. 21 (a) TEM and (b) HRTEM images and (c) formation mechanism of Bi₂O₂CO₃ nanotubes on a graphene substrate; (d) photocatalytic degradation behavior of samples toward reactive red (X-3B). Reproduced with permission from ref. 147. Copyright 2017 Elsevier.

Fig. 22 SEM images of (a–c) polystyrene latex spheres, (d–f) 3DOM Bi₂O₃/TiO₂-1 (n_B : n_Ti = 0.015 : 1), (g–i) 3DOM Bi₂O₃/TiO₂-2 (n_Bi : n_Ti = 0.03 : 1), (j–l) 3DOM Bi₂O₃/TiO₂-3 (n_Bi : n_Ti = 0.06 : 1) and (m–o) 3DOM Bi₂O₃/TiO₂-4 (n_Bi : n_Ti = 0.09 : 1). Reproduced with permission from ref. 149. Copyright 2015 Elsevier.

Immobilization

During an application, the undesired aggregation of NPs would seriously affect the performance of Bi-based photocatalysts, so an effective separation method should be employed to reduce the aggregation. The cyclic utilization of Bi-based photocatalysts should also be considered. Immobilization has been considered as an effective means to solve these two issues. Furthermore, with the aid of substrate materials, the profile of bismuthal photocatalysts can be tuned to achieve photocatalytic reactions. Hence, suitable immobilizations have been explored for improving the performance of Bi-based photocatalysts.

Immobilization requires the increased dispersion of bismuthal photocatalysts, which increases the availability of photocatalysts and improves the attachment between the photocatalysts and reactants. Wang and co-workers¹⁵¹ prepared a BiOCl film on a Bi plate using a room-temperature reaction, and the product exhibited a high photocatalytic activity in the degradation of RhB because of the good dispersion of the formed BiOCl nanonuclei. Sonawane and colleagues¹⁵² immobilized Bi₂O₃ on bentonite using an intercalation method. The obtained Bi₂O₃/bentonite composite exhibited improved performance relative to the separate use of Bi₂O₃ and bentonite in the photocatalytic degradation of RhB because of the increased adsorption capacity brought about by the use of bentonite. Similarly, Wang et al.¹⁵³ immobilized phosphotungstic acid-modified BiOBr on the surface of a zeolite. This process endowed the composites with enhanced photocatalytic activity for the degradation of MO. Li et al.¹⁵⁴ immobilized BiOI on diatomite and obtained composites with improved photocatalytic performance in the degradation of RhB. Tong and co-workers¹⁵⁵ used nickel foam as a substrate to immobilize a Bi₂WO₆/TiO₂ composite and increase the efficiency of Bi₂WO₆/TiO₂ in the photocatalytic degradation of RhB. Shi et al.¹⁵⁶ found that the performance of BiVO₄ in the photocatalytic removal of tetracycline hydrochloride could be enhanced when supported on palygorskite. Zhang and colleagues¹⁵⁷ demonstrated that Bi₄Ti₃O₁₂ immobilized on SiO₂ spheres could more effectively decolorize Brilliant Red-X3B because of the good dispersion and smaller size of Bi₄Ti₃O₁₂.

If the substrates are semiconductor materials, a heterojunction will form and facilitate the separation of electrons and holes. Mu and co-workers¹⁵⁸ deposited spindle-like BiVO₄ on TiO₂ nanofibers (Fig. 23) using a solvothermal method, and the composite exhibited enhanced visible-light activity than that of a mechanical mixture of BiVO₄ and TiO₂ in the photocatalytic degradation of RhB. This result could be ascribed not only to the thorough dispersal of the BiVO₄ NPs by immobilization, but also to the improved separation of electrons and holes by the formed heterojunction between BiVO₄ and TiO₂. Zhang and co-workers¹⁵⁹ reported the enhanced performance of BiOCl nanosheets in the photocatalytic degradation of RhB by immobilizing the BiOCl sheets on TiO₂ arrays over FTO (Fig. 24). This effect was achieved, because the reflection within the ordered array structure endowed the composites with enhanced light absorption. Moreover, the heterojunction between BiOCl and TiO₂ significantly decreased the charge transfer resistance and facilitated the separation of photo-generated holes and electrons. Kumar et al.¹⁶⁰ prepared Bi₂S₃/TiO₂ composites from Bi₂S₃ nanotubes and TiO₂ NPs. The acquired composites showed improved photocatalytic performance in the degradation of amaranth.

Fig. 23 SEM images of (a) TiO₂ nanofibers (VT0), (b and c) BiVO₄ immobilized on TiO₂ nanofibers using a low (VT1) and high (VT2) concentration of raw material solution, and (d) the degradation curves of RhB over different photocatalysts under visible light. Reproduced with permission from ref. 158. Copyright 2016 Elsevier.

Fig. 24 (A) Schematic diagram of BiOCl/TiO₂ composite formation; (B) FESEM images of a BiOCl/TiO₂ composite (BCTO-3); (C) kinetic curves of RhB photocatalytic degradation on TiO₂, BiOCl, and BCTO-3; (D) schematic diagram of the reaction mechanism over the immobilized BiOCl/TiO₂ composite under visible light. Reproduced with permission from ref. 159. Copyright 2017 Elsevier.

Conductive substrates not only affect the morphologies of the composites, but also act as electron acceptors. For example, when a BiOCl sheet was immobilized on carbon fibers, the performance of the sheet in the photocatalytic degradation of 4-nitrophenol was enhanced due to the good dispersion of BiOCl nanoplates and the electron trapping role of the carbon fibers.¹⁶¹ Likewise, Weng and co-workers¹⁶² loaded Bi₄Ti₃O₁₂ on carbon fibers using a hydrothermal method with carbon-fiber-supported TiO₂ nanosheets as a precursor. The prepared composite exhibited better performance than that of Bi₄Ti₃O₁₂ nanosheets in the photocatalytic degradation of MO. Di et al.¹⁶³ deposited small Bi₄O₅Br₂ nanosheets on multi-walled carbon nanotubes (MWCNTs) via an ionic liquid-assisted solvothermal method and found that the activity of the composites in the photocatalytic degradation of tetracycline hydrochloride was obviously improved.

Immobilization plays a more important role in gaseous reactions than in other reactions, because well-dispersed photocatalysts are crucial to the external and internal diffusions of gaseous molecules. Dong's group¹⁶⁴ immobilized Bi NPs on g-C₃N₄ and observed that an NO removal efficiency of 60.8% was achieved when Bi NPs of 12 nm were decorated on g-C₃N₄. This efficiency was higher than that of g-C₃N₄ (38.6%). Meanwhile, Dong's group¹⁶⁵ deposited metal Bi NPs on TiO₂ for NO removal and noted that Bi can act as a cocatalyst, similar to a noble metal, due to surface plasmon resonance and can effectively photoactivate TiO₂ during NO oxidation under visible-light irradiation. Xia et al.¹⁶⁶ loaded BiOI on porous Al₂O₃ using a hydrothermal method and observed that the obtained BiOI/Al₂O₃ composite exhibited photocatalytic performance by simultaneously eliminating gaseous NO and SO₂.

Heterojunction construction

Heterojunction construction is an effective pathway for enhancing the performance of photocatalysts, because photo-generated electrons and holes can be effectively separated by a heterojunction.^{29,42,167–175} For most Bi-based photocatalysts, a narrow band gap ensures that electrons can be excited by visible light. However, the excited electrons recombine with holes soon after. Therefore, heterojunction construction plays a key role in improving the performance of Bi-based photocatalysts. Bi-Based heterojunctions include conventional and Z-scheme heterojunctions. Among the conventional heterojunctions, the type II junction is the most common one, while in Z-scheme systems, the newly-emerged direct Z-scheme heterojunction appears to be the most effective junction structure used for exploring the capacity of photo-generated carriers.

Conventional type II heterojunctions

Conventional heterojunctions are the most commonly reported in Bi-based photocatalysts, and the two main carrier migrations are illustrated in Fig. 25. In the type I mode, photo-induced holes migrate from the lower VB of semiconductor B (SC-B) to the higher VB of semiconductor A (SC-A), whereas electrons migrate from the higher CB of SC-B to the lower CB of SC-A. During the process, electrons and holes migrate in the same direction, but electrons and holes migrate at different speeds due to their different effective mass or other factors. Consequently, holes and electrons are separated. In the type II mode, holes migrate from the lower VB of SC-B to the higher VB of SC-A, while electrons migrate from the higher CB of SC-A to the lower CB of SC-B. During the process, electrons and holes migrate in opposite directions, resulting in their separation. In any of these two heterojunctions, the electrons and holes can be separated, and the photocatalytic activity of the photocatalysts can be enhanced.

Fig. 25 Schematic diagram of the carrier migrations in conventional (a) type I and (b) type II heterojunctions between semiconductor A (SC-A) and semiconductor B (SC-B).

Among the different types of heterojunctions, the type II junction is the most reported one. This kind of heterojunction usually forms between bismuthal compounds and semiconductors with a narrow band gap, when the CB and VB of one semiconductor are lower than that of the coupled one. For example, Zhu and co-workers¹⁷⁶ reported the fabrication of a binary Bi and non-Bi Bi₄Ti₃O₁₂/CeO₂ composite by coupling Bi₄Ti₃O₁₂ with CeO₂ using a molten salt and ion-impregnation method. The obtained Bi₄Ti₃O₁₂/CeO₂ composite exhibited enhanced activity in the photocatalytic degradation of BPA. This phenomenon can be attributed to the important role of the heterojunction between Bi₄Ti₃O₁₂ and CeO₂, which facilitates carrier separation (Fig. 26). In addition, Fan et al.¹⁷⁷ constructed a binary Bi-based Bi₂MoO₆/BiOI composite heterojunction (Fig. 27a and b) using an anion exchange method with BiOI as a precursor. The as-prepared Bi₂MoO₆/BiOI exhibited improved photocatalytic activities in the degradation of RhB (Fig. 27c). The sample with a Mo/I molar ratio of 50% exhibited the best activity under visible light excitation due to the formation of a type II heterojunction between Bi₂MoO₆ and BiOI (Fig. 27d). This type of charge migration and separation were also observed for heterojunctions formed between ZnO/BiOI and BiOCl/Bi₃PO₄ couples.^178,179 If three matched bismuthal semiconductors are coupled together, a ternary heterojunction such as Bi₂S₃/Bi₂O₃/Bi₂O₂CO₃ can be constructed that exhibits a better performance.¹⁸⁰ The enhancement in the activity of Bi₂S₃/Bi₂O₃/Bi₂O₂CO₃ is ascribed to the increased light absorption and efficient charge separation by the double type II heterojunction (Fig. 28). Type II carrier migrations can also occur in homojunctions. In a homojunction, the interface between two homogenous semiconductors forms due to different phase structures.^181,182

Fig. 26 (a) SEM and (b) TEM images of CeO₂/Bi₄Ti₃O₁₂ composites (1%C–BTO); (c) comparison of pseudo-first-order kinetic constants of different photocatalysts for BPA; and (d) a schematic illustration of the charge migration in the 1%C–BTO composites under light irradiation. Reproduced with permission from ref. 176. Copyright 2016 Elsevier.

Fig. 27 (a) TEM images and (b) the corresponding HRTEM images of Bi₂MoO₆/BiOI composites (BMO/BOI = 50%); (c) reaction rate constants for RhB degradation on BiOI (BOI), Bi₂MoO₆ (BMO) and BMO/BOI; and (d) a schematic diagram showing the charge transfer of a BMO/BOI heterostructure. Reproduced with permission from ref. 177. Open Access 2016 World Scientific.

Fig. 28 (a) SEM and (b) TEM images of a ternary Bi₂S₃/Bi₂O₃/Bi₂O₂CO₃ composite photocatalyst; (c) UV-visible absorption spectra of Bi₂O₂CO₃(BOC), Bi₂O₃/BOC, and Bi₂S₃/Bi₂O₃/BOC; (d) comparison of the photocatalytic HCHO removal of Bi₂O₂CO₃(BOC), Bi₂O₃/BOC, Bi₂S₃/BOC, and Bi₂S₃/Bi₂O₃/BOC; and (e) a schematic diagram showing the charge-transfer process in ternary Bi₂S₃/Bi₂O₃/Bi₂O₂CO₃ double heterojunctions. Reproduced with permission from ref. 180. Copyright 2016 Elsevier.

Numerous preparation strategies for conventional heterojunctions (mainly Type II) have been reported, and most of these techniques have revealed enhanced photocatalytic activities (Table 1). Nevertheless, conventional heterojunctions have evident limitations. Photo-generated electrons migrate from a more negative CB to a less negative CB and the holes from a more positive VB to a less positive VB. This type of migration for electrons and holes reduces the redox capacity of the photocatalyst. Consequently, any improvement in the performance of the photocatalysts will be significantly limited.

Table 1 Conventional heterojunction strategies of nanoscale Bi-based photocatalysts

Bismuthal compound	Coupled material	Method	Application	Ref.
Rhodamine B, RhB; methylene blue, MB; methyl orange, MO; bisphenol A, BPA; ciprofloxacin, CIP; PEDOT:PSS = Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate).
Binary Bi and non-Bi heterojunction
Bi2O3	FeVO4	Grounding and calcination	Malachite green	186
Bi2O3	TiO2	Hydrothermal synthesis and calcination	MB	187
Bi2O3	g-C3N4	Self-assembly	RhB	188
Bi2O3	g-C3N4	Mixing-calcination	RhB	189
Bi2S3	ZnS	Cation exchange	MB	190
Bi2S3	ZnS	Solvothermal method	H2 generation	191
Bi2S3	Pd4S	Thermal reduction and cation exchange	Atrazine	192
Bi2O2CO3	CdS	Reflux	MB	193
Bi2Ti2O7 nanosheets	TiO2 fibers	Hydrothermal method	RhB	194
Bi12TiO20	g-C3N4	Hydrothermal method	Gaseous HCHO	195
Bi4Ti3O12	g-C3N4	Ball milling	Acid orange II (AO-7)	196
Bi4Ti3O12	CeO2	Molten salt and ion impregnation	BPA	176
Bi2O2CO3	g-C3N4	Calcination	Dibutyl phthalate and MO	197
Bi2O2CO3	Ag3PO4	Hydrothermal method and precipitation	RhB	198
BiFeO3	g-C3N4	Hydrothermal method	Guaiacol	199
BiFeO3	CuO	Hydrothermal synthesis and impregnation	MO	200
Bi2Sn2O7	In2O3	Impregnation	RhB	201
BiOCOOH	Ag2O	Solvothermal deposition	RhB and p-chlorophenol	202
BiVO4	TiO2 fibers	Hydrothermal method	Brilliant red X-3B	203
BiVO4	3DOM TiO2	Hydrothermal method	RhB	204
BiVO4	g-C3N4	Ultrasonic assembly	CO2 reduction	205
BiVO4	CeO2	Coprecipitation and subsequent annealing	RhB	206
BiVO4	Ag2O	Impregnation and evaporation	MO	207
BiVO4	ZnO	Annealing of mixture	RhB	208
BiVO4	SnO2	Hydrothermal method	MB	209
BiVO4	Co3O4	Drop-casting method	Water oxidation	210
BiVO4/FTO	PEDOT:PSS	Electrodeposition	Photo-photocurrent	211
Bi2WO6	TiO2	Hydrothermal method	RhB and MO	212
Bi2WO6	TiO2 fibers	Hydrothermal method	RhB and phenol	213
Bi2WO6(001)	TiO2(001)	Hydrothermal method	MB	214
Bi2WO6	WO3	Hydrothermal method	RhB	215
Bi2WO6	α-Fe2O3	Electrospinning with sintering	RhB	216
Bi2WO6	CeO2	Hydrothermal method	RhB	217
Bi2WO6	AgCl	Hydrothermal method	RhB	218
Bi2WO6	Ag2O	Chemical precipitation	RhB	219
Bi2WO6	CdS and ZnS	Surface functionalization	RhB	220
Bi2MoO6	TiO2	Solvothermal method	Phenol and nitrobenzene	221
Bi2MoO6	g-C3N4	Liquid chemisorption and thermal treatment	RhB	222
Bi2MoO6	Ag3VO4	Hydrothermal method and precipitation	RhB	223
Bi2MoO6	AgBr	Precipitation–deposition	RhB	224
Bi2MoO6	AgI	Precipitation–deposition	RhB and BPA	225
Bi2MoO6	MoO3	Chemical vapor deposition	O2 evolution and glycerol oxidation	226
Bi2MoO6	MoO3	Hydrothermal method	Photoanode	227
Bi2SiO5	AgI	Precipitation	Acid red G and gaseous HCHO	228
BiOCl	g-C3N4	Solvothermal method	RhB	229
BiOCl	CuS	Hydrothermal method	RhB	230
BiOCl	TiO2	Solvothermal method	Benzene	231
BiOBr	N doped graphene	Wet chemical method	Chlorpyrifos detection	232
BiOBr	CoFe2O4	Solvothermal method	Congo red	233
BiOBr	NiFe2O4	Hydrothermal method	MB	234
p-Type BiOBr	n-Type La2Ti2O7	Refluxed in oil bath	RhB	235
BiOBr	CdWO4	Hydrothermal synthesis and precipitation	RhB	159
BiOBr	CeO2	Precipitation–deposition	RhB	236
BiOBr	Ag3PO4	Precipitation–deposition	RhB	237
BiOI	TiO2 nanotube	Impregnating-hydroxylation	MO	238
BiOI	TiO2 fibers	Successive ionic layer adsorption and reaction	MO	239
BiOI	Fe2O3	In situ hydrolysis	RhB	240
BiOI	La(OH)3	Chemical impregnation	NO	241
BiOX (X = Cl, Br, I)	AgX (X = Cl, Br and I)	Precipitation	RhB	242
Bi4O5I2	g-C3N4	Solvothermal method using an ionic liquid	RhB and endocrine	243
Binary Bi-based heterojunctions
α-Bi2O3/β-Bi2O3		Solid-state reaction	Indigo carmine and RhB	244
α-Bi2O3/β-Bi2O3		In situ phase transformation by calcination	RhB	245
Bi2O3-Bi2S3		Hydrothermal method	RhB	184
Bi2O3/BiVO4		Solvothermal synthesis followed by annealing	Phenol	246
Bi2O3/BiVO4		Hydrothermal method	RhB	247
Bi2O3 QDs/BiVO4 fibers		Direct heat treatment	RhB	248
Bi2WO6/Bi2O3		Solid-state reaction	RhB	249
Bi2O3/BiOCl		Alkaline treatment	MO	250
β-Bi2O3/BiOI		In situ reaction	MO	251
Bi5O7I/Bi2O3		Chemical etching	Malachite green	252
Bi2S3/BiOCl		Solvothermal synthesis	Salicylic acid, RhB	253
Bi2S3/Bi2WO6		Anion exchange approach	RhB	254
Bi2S3/Bi2WO6		Hydrothermal method	Reduction of Cr(VI)	255
Bi2S3/Bi4Ti3O12 nanofibers		In situ ion exchange	RhB	256
Bi2S3/Bi2O2CO3		Anion exchange	Gaseous NO	257
Bi2S3/Bi2O2CO3 hollow microspheres		One-pot room temperature route	RhB	258
Bi2Ti2O7/Bi4Ti3O12		One-step molten salt method	MO, RhB	259
Bi12TiO20/Bi2WO6		Hydrothermal method	RhB	260
BiVO4/Bi2O2CO3		Hydrothermal method	RhB	261
BiPO4/BiOBr		Mixing in solvent	Gaseous o-dichlorobenzene	262
Bi2MoO6/BiVO4		Spin-coating	Photoelectrode	263
Bi2MoO6/BiPO4		Hydrothermal method	RhB	264
Bi2WxMo1−xO6/BiOCl		Solvothermal method	RhB	265
Bi3.64Mo0.36O6.55/Bi2MoO6		Hydrothermal method	RhB	266
BiOI/BiVO4		Coprecipitation	Pseudomonas aeruginosa	267
BiOI/BiVO4		Precipitation–deposition	MO	268
BiOCl/BiVO4		Coprecipitation method	RhB	269
BiOCl/Bi12O17Cl2		Hydrothermal method	MO	270
BiOI/Bi2MoO6		Precipitation–deposition	BPA	271
Bi2MoO6/BiOI		hydrothermal method	RhB	177
Bi24O31Cl10/BiOCl		Phase transformation by annealing	Conversion of benzyl alcohol	272
Bi4O5I2/Bi5O7I		In situ phase transformation	BPA and RhB	273
Bi4O5I2/Bi5O7I		Hydrothermal method	Propylparaben	274
Bi2O2CO3/BiOI		Pore impregnation	RhB	275
Bi2O4-Decorated BiOBr		Alkali posttreatment assisted light irradiation	MO	276
Monoclinic BiVO4/tetragonal BiVO4		Hydrothermal method	RhB	277
BiVO4/Bi4V2O11		Precursor conversion	Photoelectrodes	278
Ternary heterojunctions
Bi2O3/Bi2S3/MoS2		Hydrothermal method	O2 evolution and MB degradation	279
AgI/BiOI–Bi2O3		Etching-deposition	Cr(VI) reduction	280
BiOClx/BiOBry/BiOIz		Electrospinning and the sol–gel methods	Trichloroethylene	281
Bi2S3/Bi2O3/Bi2O2CO3		Heat treatment and ion exchange	HCHO, MO, and phenol	180
Bi7O9I3/AgI/AgIO3		Chemical deposition	MO and gaseous NO	282
Bi7O9I3/Bi5O7I/g-C3N4		Hydrothermal method	Crystal violet	283
BiOBr/Co(OH)2/PVP		Solvothermal synthesis	MO	284

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Given the limited improvement of a simple conventional heterojunction, several strategies combining heterojunction and morphology control, such as the preparation of a MoS₂/Bi₁₂O₁₇C_l2 2D bilayer heterostructure¹⁸³ and a Bi₂S₃ nanorods/Bi₂O₃ microtubes composite,¹⁸⁴ have been developed to further enhance the performance of bismuthal photocatalysts. In our previous work, we found that heterojunctions can form between two adjacent (001) and (110) facets of BiOI, whereas their CBs and VBs are located in different positions. Consequently, BiOI with a hierarchical morphology and an optimal ratio of (001) and (110) facets was found to possess the highest photocatalytic activity.¹⁸⁵ However, the combination of heterojunction and morphology control is still unable to overcome the inherent limitations of a conventional heterojunction, although this characteristic results in the better performance of as-prepared photocatalysts.

Direct Z-scheme heterojunctions

In 2013, Yu et al. proposed a direct Z-scheme heterojunction concept to explain the enhancement in the photocatalytic activity of a TiO₂/g-C₃N₄ composite photocatalyst.²⁸⁵ This direct Z-scheme heterojunction is different from conventional liquid-phase Z-scheme heterojunctions and all-solid-state Z-scheme heterojunctions. The direct Z-scheme heterojunction does not require an electron medium.^286–290 The transfer of charge carriers in this heterojunction is through the built-in electric field between the interface of SC-A and SC-B (Fig. 29a). However, the transfer of charge carriers in the liquid-phase (Fig. 29b) and all-solid-state (Fig. 29c) Z-scheme heterojunction is through the use of ions in the solution and noble metal NPs as electron conductors, respectively.^291–295

Fig. 29 Schematic illustration of (a) direct Z-scheme, (b) liquid-phase Z-scheme and (c) all-solid-state Z-scheme heterojunctions (SC represents semiconductor).

The structure of a direct Z-scheme heterojunction is similar to that of a type-II heterojunction (Fig. 25b) but with different charge carrier transport mechanisms. In a typical type-II heterojunction photocatalyst, SC-A has higher CB and VB positions than SC-B. Under light irradiation, the electrons in SC-A and holes in SC-B will transfer to the CB of SC-B and VB of SC-A, respectively. This process results in the spatial separation of the electron–hole pairs. However, the reduction ability of photo-generated electrons in the CB of SC-B and the oxidation ability of holes in the VB of SC-A are greatly reduced.

In contrast, the direct Z-scheme heterojunction exhibits a completely different mechanism of charge carrier migration. A photo-generated electron with a low reduction potential in SC-B will recombine with a photo-generated hole with a low oxidation potential in SC-A (Fig. 29a). Finally, the photo-generated electron and hole will remain in the CB of SC-A and VB of SC-B, respectively. The direct Z-scheme heterojunction will result in the spatial separation of useful charge carriers, remove useless charge carriers, and optimize the redox ability of the SC-A and SC-B composition system. Therefore, an enhanced photocatalytic performance can be observed in a direct Z-scheme heterojunction photocatalyst. In addition, the transfer mechanism of the charge carriers in a direct Z-scheme heterojunction is also different from that of conventional liquid-phase and all solid-state Z-scheme heterojunctions. No electron conductor is needed in a direct Z-scheme heterojunction, and the charge carrier can directly migrate using the built-in field at the SC-A and AC-B interfaces. Moreover, the fabrication cost of the direct Z-scheme photocatalyst is greatly reduced. Significantly, the direct Z-scheme heterojunction offers at least two obvious merits, namely, a low fabrication cost and high redox ability.

Many Bi-based direct Z-scheme heterojunction photocatalysts have been prepared and reported. For example, Liu et al.²⁹⁶ prepared a CdTe/Bi₂S₃ direct Z-scheme heterojunction photocatalyst using an electrostatic adsorption and self-assembly method. The obtained CdTe/Bi₂S₃ exhibited higher photocurrent responses and microcystin-LR sensitivity than the individual CdTe and Bi₂S₃ nanorod components. Meanwhile, Dai and colleagues²⁹⁷ formed a g-C₃N₄/Bi₂MoO₆ direct Z-scheme heterojunction between g-C₃N₄ and Bi₂MoO₆ using a hydrothermal method. This g-C₃N₄/Bi₂MoO₆ photocatalyst exhibited a higher activity than the individual g-C₃N₄ and Bi₂MoO₆ components in the photocatalytic degradation of MB (Fig. 30). Ullah and co-workers²⁹⁸ reported the synthesis of a Se/BiVO₄ direct Z-scheme heterojunction photocatalyst and its higher photocurrent density than that of its individual Se and BiVO₄ components. Huang and colleagues²⁹⁹ demonstrated a direct Z-scheme electron migration mechanism between g-C₃N₄ and BiOIO₃ in their prepared g-C₃N₄/BiOIO₃ composite under UV-visible light irradiation. The photocatalytic activity of the g-C₃N₄/BiOIO₃ composite was shown to be markedly higher than those of g-C₃N₄ and mechanically mixed g-C₃N₄/BiOIO₃ for the removal of RhB. We recently fabricated a Bi₂O₃/g-C₃N₄ direct Z-scheme heterojunction photocatalyst using an in situ room-temperature approach, including photoreduction deposition of Bi³⁺ and subsequent air-oxidation of the resultant metallic Bi.³⁰⁰ In the prepared composite, Bi₂O₃ quantum dots were uniformly distributed on the surface of g-C₃N₄ sheets (Fig. 31a), and the photocatalytic activity of Bi₂O₃/g-C₃N₄ was found to be higher than those of pure Bi₂O₃ and g-C₃N₄ in the photocatalytic degradation of phenol under visible light (Fig. 31b). This phenomenon can be attributed to the formed direct Z-scheme heterojunction (Fig. 31c). More reports on direct Z-scheme heterojunctions are listed in Table 2 for reference.

Fig. 30 (a) TEM and (b) HRTEM images of a g-C₃N₄/Bi₂MoO₆ composite; (c) pseudo first-order fitted kinetic curves of MB degradation on g-C₃N₄/Bi₂MoO₆ composites (25%g-C₃N₄/Bi₂MoO₆); and (d) a schematic illustration of electron and hole migration in a g-C₃N₄/Bi₂MoO₆ direct Z-scheme heterojunction. Reproduced with permission from ref. 297. Copyright 2015 Elsevier.

Fig. 31 (a) TEM images of a Bi₂O₃/g-C₃N₄ composite; (b) comparison of the photocatalytic performance of Bi₂O₃, g-C₃N₄, and Bi₂O₃/g-C₃N₄ in the degradation of phenol; and (c) the photocatalytic mechanism of a Bi₂O₃/g-C₃N₄ direct Z-scheme heterojunction. Reproduced with permission from ref. 300. Copyright 2018 Elsevier.

Table 2 Nanoscale Bi-based direct Z-scheme heterojunction photocatalysts

Bismuthal compound	Coupled material	Method	Application	Ref.
Rhodamine B, RhB; methylene blue, MB; methyl orange, MO; bisphenol A, BPA; ciprofloxacin, CIP.
Bi2O3	g-C3N4	Ball milling and heat treatment	MB	301
Bi2Sn2O7	g-C3N4	High-temperature solid-state reaction	MB and acid red 18	302
Bi2MoO6	g-C3N4	Hydrothermal method	MB	297
BiOBr	g-C3N4	Reflux process	RhB and BPA	303
BiOIO3	g-C3N4	Hydrothermal method	MO, RhB, and dichlorophenol	299
BiOI	CdS	Hydrothermal method	RhB	304
Bi2WO6	MoS2	Hydrothermal method	RhB	305
Bi2MoO6	CuO, Co3O4, NiO	Precipitation	RhB	306
Bi2O3	NaNbO3	Ball milling method	RhB	307
BiVO4	Se film	Chemical vapor deposition	Photocurrent enhancement	298
BiVO4	SiC	Precipitation followed by calcination	O2 evolution	308
BiOBr	Bi2MoO6	Two-step coprecipitation method	RhB and CIP	309
BiO1−xBr	Bi2O2CO3	Solvothermal method	CIP	310
BiPO4	Bi2O2(OH)(NO3)	Hydrothermal method	Dichlorophenol	311
MoS2/BiOI/AgI		Precipitation	RhB	312
BiVO4/ZnIn2S4/g-C3N4		Impregnation and calcination	Congo red and metronidazole	313

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Surface modification

Surface modification is an effective and widely used method for enhancing the activity of a photocatalyst.³¹⁴ It also plays an important role in improving the performance of Bi-based photocatalysts. An extensive range of materials can be adopted to modify surfaces. Carbon-based materials, metals, ions, polymers, and semiconductors can be used to modify bismuthal photocatalysts. If a semiconductor is used as a modifier, a nanosized heterojunction will be formed, and the carriers will transfer faster in the tiny heterojunction than in bulk junctions because of the larger interface ratio. For example, the dandelion-like MoS₂-decorated Bi₂S₃ prepared by Li et al.³¹⁵ exhibited improved photocatalytic performance in the degradation of RhB due to the tiny heterojunctions formed between MoS₂ and Bi₂S₃ (Fig. 32). Further to this, SnO₂ modified BiVO₄ was found to be able to reduce CO₂ to CH₄, with the introduced SnO₂ acting as a redox reaction platform capable of accepting the photogenerated electrons from BiVO₄ nanoplates during the conversion of CO₂.³¹⁶ AgBr, graphene-like BN (g-BN), and some bismuthal compounds with a narrow band gap have also been reported as semiconductor modifiers for Bi-based photocatalysts (Table 3).

Fig. 32 (a) Schematic illustration of the formation of MoS₂-decorated Bi₂S₃ (BMS); (b–d) TEM and (e) HRTRM images of MoS₂-decorated Bi₂S₃; (f) comparison of the photocatalytic performance of RhB on MoS₂, Bi₂S₃ and BMS (2MBS, 5MBS, and 8MBS present in molar ratios of Mo⁴⁺-to-Bi³⁺ of 20%, 50%, and 80%) under visible-light irradiation; and (g) a schematic diagram of the charge transfer at a MoS₂/Bi₂S₃ heterojunction. Reproduced with permission from ref. 315. Open Access 2017 Springer Nature.

Table 3 Surface modification strategies for nanoscale Bi-based photocatalysts

Modifier	Bi-Based material	Method	Target material of enhanced performance	Ref.
GO: graphene oxide; RGO: reduced graphene oxide; MOF: metal–organic framework; CQDs: carbon quantum dots; GQDs: graphene quantum dots; Rh6G: Rhodamine 6G; MB: methylene blue; PEDOT:PSS: poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate).
Semiconductor
MoS2	Bi2S3	Hydrothermal method	RhB	315
g-C3N4	Bi2O4	Hydrothermal method	MO	335
CdS	BiVO4	Solvothermal method	H2 generation	336
Cu2O	Bi2WO6	Interfacial self-assembly	MB	337
AgBr QDs	Bi2WO6	Precipitation–deposition	MB and phenol	338
g-BN	BiOI	Solvothermal method	RhB	339
g-BN	BiOCl	Microwave-assisted method	Cr(VI) reduction	340
g-BN	BiPO4	Solvothermal method	Enrofloxacin	341
Bi2O3 QDs	BiVO4	Calcination	RhB	248
Bi2O4	BiOBr	Alkali posttreatment under light irradiation	MO	276
BiOI	Bi2MoO6	Deposition–precipitation	MO	342
BiOBr	BiPO4	Hydrothermal method	MB	343
Carbon material
Carbon	Bi4Ti3O12	Coprecipitation	MO	344
Carbon	Bi2MoO6	Two-step hydrothermal method	RhB	345
Carbon microspheres	BiFeO3	Hydrothermal method	RhB	346
Carbon microspheres	Bi0.5Dy0.5VO4	Self-assembly	H2 generation	347
Carbon nanotubes	Bi4O5Br2	Solvothermal method	Tetracycline hydrochloride and RhB	163
Carbon dots	BiOI	Immersion	MO	348
CQDs	Bi20TiO32	Oil bath	Isoproturon	349
CQDs	Bi4O5I2	Solvothermal method	RhB	350
CQDs	BiOI	Hydrothermal method	RhB	351
GO	BiVO4	Hydrothermal method	RhB	352
GO	BiOCl	Two-step synthesis	RhB	353
GO	BiOxIy	Hydrothermal method	Crystal violet	354
GO	BiFeO3	Ultrasonic treatment	BPA	355
GO	Bi2O2CO3	Precipitation	Gaseous NO	356
GO	Bi2WO6	Liquid nitrogen freezing and freeze-drying dehydration	RhB	323
GO	BiOI	Self-assembly	Phenol	357
RGO	BiOI	Hydrothermal method	MO	358
RGO	BiOBr	Two-step hydrothermal method	Higher activity for MO than for RhB	359
RGO	Bi25FeO40	Hydrothermal method	MO	360
RGO	Bi2WO6	Wrapped	RhB	361
RGO	Bi2S3	Hydrothermal method	2,4-Dichlorophenol	362
RGO	BiPO4	Solvothermal method	Chlorpyrifos detection	363
Graphene	Ag/BiOBr0.2I0.8	Combined solvothermal method and photodeposition	RhB	364
Graphene	BiOCl0.7Br0.3	Solvothermal method	RhB	365
Graphene	Bi2O2CO3	Hydrothermal method	RhB	366
Ag and graphene	Bi2WO6	Hydrothermal method followed by photodeposition	RhB	367
GQDs	Bi2MoO6	Self-assembly	BPA, MB, TC, CIP, and phenol	324
Au and RGO	Bi2MoO6	Solvothermal synthesis and photochemical reduction	RhB	368
Ag and graphene	Bi2Fe4O9	Multistep synthesis	MB	369
Metal and ion
Pd	Bi2MO6	Photoreduction	Phenol	370
Pd	Bi2WO6	Chemical deposition	RhB	371
Pd	BiOBr	Solvothermal method	RhB and CIP	372
Pd	m-BiVO4/BiOBr	Reduction in the dark	Polychlorinated biphenyls	373
Pt	BiOBr	Photodeposition	p-Nitrophenol	374
Pt	BiFeO3	Impregnation and thermal reduction	MO	375
Pt	Bi2WO6	Chemical reduction	Rhodamine 6G	376
Pt	Bi2WO6/TiO2	Photodeposition	CO2 reduction	377
Au	BiOCl	Photodeposition	Formaldehyde	378
Au	BiVO4	Pulsed laser deposition	Congo red and water splitting	379
Au	BiPO4	Solvothermal method	CO oxidation	380
Au	Bi2O2CO3	Hydrothermal method	NO	381
Ag	BiFeO3 film	Sol spin coating and annealing	O2 evolution	382
Ag	BiOBr	Solvothermal method	Tetracycline hydrochloride	383
Ag	BiVO4	Hydrothermal method	Reduced electron-transfer resistance	384
Ag	Bi4Ti3O12	Sonochemical method	RhB	385
Ag	Bi2WO6	Irradiation	RhB	386
Bi	Bi2WO6	In situ reduction	Phenol	319
Bi	Bi2WO6 nanorod	Hydrothermal reaction	Rh6G	387
Bi	Bi2MoO6	Solvothermal reduction	Rh6G	388
Bi	BiPO4	Solvothermal treatment	MB	389
Bi	BiOBr	Reduction reaction at room temperature	MO	390
Bi	BiOI/CNFs	Solvothermal method	MO	391
Cr	Bi4Ti3O12	Sol–gel hydrothermal process	H2 generation	69
Ni	BiOI	Photo-assisted deposition	Reducing Cr(VI) to Cr(III)	392
Fe(III)	Bi2O2CO3	Impregnation	MO	393
Fe	Bi2Ti2O7	Precipitation	MO	394
Fe2O3	Macroporous BiVO4	Impregnation	4-Nitrophenol	395
CoOx	BiFeO3	Photodeposition	O2 evolution	396
Rh(III)	BiOCl	Impregnation	Gaseous acetaldehyde (Vis)	397
Organic material
Polythiophene	Bi2MoO6	In situ chemical oxidative polymerization	RhB	326
Polyaniline	Bi2O2CO3	Low-temperature chemical method	RhB	327
PVP	BiOBr	Solvothermal method	RhB	328
MIL-101 (MOF)	BiVO4	Hydrothermal method	RhB	398
Ionic liquid	BiPO4	Hydrothermal method	RhB	399
Surface defect
Bi defects	Bi6S2O15	Hydrothermal method	Phenol	330
Dimer vacancy	Bi2WO6	Solvothermal method	Diclofenac	332
Defect	BiPO4	Ball milling	MB	400

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Coupling bismuthal photocatalysts with conductive materials is another effective surface modification strategy. The conductive materials usually act as electron acceptors to promote the migration of photo-generated electrons and consequently facilitate the separation of carriers. Metals, carbon-based materials, conductive polymers, and metallic Bi have been adopted as electron scavengers to enhance the photocatalytic activities of bismuthal compounds (Table 3). Commonly employed noble metals include Au, Ag, Pt, and Pd. Hirakawa et al.³¹⁷ obtained a composite by loading Au NPs on BiVO₄. The prepared Au/BiVO₄ exhibited an enhanced production of H₂O₂ from water than that of pure BiVO₄ under visible-light irradiation due to the selective two-electron reduction of O₂ by Au. In addition to noble metals, other metals can also serve as electron trapping agents. Park and co-workers³¹⁸ decorated Ni NPs on W-doped BiVO₄ nanofibers and obtained a fibrous composite (Ni@NiO/W:BiVO₄ NFs). The process was followed by a calcination method, during which Ni NPs were partly oxidized to NiO. The as-prepared composite exhibited a higher photocurrent and O₂ evolution than W-doped BiVO₄ nanofibers and Pt-decorated fibers in the photocatalytic oxidation of water. The enhancement was attributed to the unique structure of the Ni@NiO decoration, in which the Ni metal traps the photo-generated electrons and NiO accepted holes. Moreover, the metal Bi-decorated Bi₂WO₆ exhibited a photocatalytic efficiency three times higher than that of pure Bi₂WO₆ in the degradation of phenol under visible light, because metal Bi not only acts as an electron acceptor, but also promotes the separation of carriers.³¹⁹ Finally, the surface plasmon resonance of Bi was also embodied in the Bi/BiOBr composite, which possessed a higher activity for NO oxidation than that of pure BiOBr.³²⁰

Carbon materials, such as carbon dots (CDs), CNTs, graphene, graphene oxide (GO), reduced graphene oxide (RGO), carbon quantum dots (CQDs), and graphene quantum dots (GQDs), can be used as modifiers. Among these materials, graphene-based materials are more favorable because of their special graphitic structure and ideal conductivity.³²¹ Priya et al.³²² loaded a Bi₂O₃/BiOCl heterojunction on a prepared graphene sand composite using a wet impregnation method. The obtained composite showed an improved performance over that of Bi₂O₃/BiOCl for the mineralization of ampicillin and oxytetracycline. The improvement was attributed to the electron trapping role of the graphitic carbon on the graphene sand composite. When Bi₂WO₆ microspheres were wrapped with GO using a freeze-drying dehydration method, they exhibited a higher activity than that of pure Bi₂WO₆ on the photocatalytic degradation of RhB due to the improved separation of the electrons and holes by GO.³²³ GQDs possess a small particle size and high specific surface area. Therefore, GQDs are more favorable for the surface modification of photocatalysts. For example, Bi₂MoO₆ decorated with GQDs via a self-assembly approach, showed a remarkable increase in photocatalytic degradation efficiency of different target pollutants, including BPA, MB, TC, CIP, and phenol.³²⁴ Nevertheless, GO or RGO materials may suffer from instability in long-term reactions. Subramanian and colleagues³²⁵ found that RGO coupled with Bi₂Ti₂O₇ might suffer from decomposition and result in a reduced performance of the composite after long-term illumination in an oxidation environment (Fig. 33a and b). The degradation of RGO was attributed to the oxidation by the formed ˙OH (Fig. 33c). Several conductive polymers, such as polyaniline, polypyrrole, and polythiophene, can play the role of electron scavenger and promote the electron migration in bismuthal photocatalysts.^326–328 However, the conductivities of these polymers are much weaker than those of noble metal and carbon materials. Thus, the related research is less reported. Usually, amorphous carbon is not suitable for electron trapping because of its poor conductivity. Nevertheless, it also plays a positive role in the enhancement of photocatalytic activity due to it enhancing the visible light absorption and carrier separation of the photocatalyst.³²⁹

Fig. 33 (a) I/t and (b) V/t curves of the Bi₂Ti₂O₇/RGO composite (BTO/RGO) during long-term light irradiation and (c) the degradation mechanism of the RGO on a BTO/RGO film under light illumination for 3 h in an air-equilibrated solution. Reproduced with permission from ref. 325. Copyright 2016 Electrochemical Society.

Introducing vacancies or elements with different chemical valence states may also improve the performance of photocatalysts. Wang et al.³³⁰ introduced vacancies into the surface of Bi₆S₂O₁₅ nanowires by increasing the molar ratio of Na₂SO₄ and Bi(NO₃)₃ from 1 : 3 to 3 : 1 during hydrothermal synthesis. The light absorption of vacancy Bi₆S₂O₁₅ nanowires at 370–500 nm was stronger than that of pure Bi₆S₂O₁₅ nanowires. Vacancy Bi₆S₂O₁₅ nanowires also showed improved photocatalytic activity for the degradation of phenol and MB. The enhancement was attributed to the improved visible-light response induced by the surface vacancies (Fig. 34). Zhang and colleagues³³¹ treated BiVO₄ at different temperatures under different atmospheres. The sample treated at 973 K in N₂ exhibited the best activity for the formation of EG from aldehyde. The enhancement was attributed to the formation of V⁴⁺. Bi₂WO₆ with a large fraction of {100} high-energy facets showed relatively high efficiency in the photocatalytic degradation of diclofenac, because “Bi–O” dimer vacancy pairs formed on the {100} facets could bring about a narrower band gap and less photoexcited charge recombination.³³² Moderate oxygen-deficient defects in BiOBr also play an indispensable role for superior photocatalytic CO₂ reduction, by acting as the active sites for CO₂ adsorption and activation, trapping photogenerated electrons and thus impact upon the recombination of the electron–hole pairs.³³³ Oxygen vacancies were also found to have a positive influence on the performance of BiOCl on photocatalytic CO₂ reduction because oxygen vacancies can induce exciton dissociation and provide more photo-induced electrons for CO₂ reduction.³³⁴

Fig. 34 (a) UV-visible diffuse reflectance spectra of pure Bi₆S₂O₁₅ and vacancy Bi₆S₂O₁₅ nanowires; (b) degradation rate constant k of pure Bi₆S₂O₁₅ and vacancy Bi₆S₂O₁₅ nanowire photocatalysts; (c) schematic diagram of the carrier separation and photocatalytic reaction of pure Bi₆S₂O₁₅ and vacancy Bi₆S₂O₁₅ nanowires under UV-visible irradiation. Reproduced with permission from ref. 330. Copyright 2015 Elsevier.

Conclusions and prospects

A layered structure and excellent visible-light response endow Bi-based photocatalysts with excellent photocatalytic activity and promising prospects for application in the fields of environmental science and energy. Studies centered on Bi-based photocatalysts will be beneficial for the future development of photocatalysts. Considerable work has been performed to control the composition, morphology, and structure of these catalysts to improve the photocatalytic performance of Bi-based photocatalysts. These photocatalysts are used for the visible-light photocatalytic degradation of organic pollutants, H₂ generation, water splitting, and CO₂ reduction. Although significant progress has been achieved, great efforts are still required to further explore the advantages of Bi-based photocatalysts as visible-light photocatalysts. The following five aspects deserve special attention:

(1) Previous studies on Bi-based photocatalysts have mainly focused on the photocatalytic degradation of organic pollutants. Few studies have reported on the application of these photocatalysts in photocatalytic H₂ generation, water splitting, and CO₂ reduction. Several investigations have focused on BiVO₄. The main obstacle in the study of Bi-based photocatalysts is the less negative CB and the poor reducing abilities of bismuthal compounds. More bismuthal materials should be extensively explored to overcome these obstacles. The construction of nanosized direct Z-scheme heterojunctions by coupling bismuthal semiconductors with semiconductors that have a more negative CB is a promising method for exploring Bi-based photocatalysts for reduction applications.

(2) Bi-based photocatalyst photodegradation has been primarily evaluated in aqueous solutions instead of under gaseous conditions. The latter condition is favorable for the use of Bi-based photocatalysts because visible light can be more fully utilized in the gas phase. The removal of formaldehyde and benzene from air is important for environmental remediation and indoor air purification. Therefore, extending the application of Bi-based photocatalysts to the degradation of gaseous contaminants is meaningful.

(3) The aim of photocatalysis is to solve serious environmental and energy problems in the world. We hope that this review can further stimulate the research into and application of Bi-based photocatalytic materials in the near future. We believe that the understanding and knowledge on Bi-based photocatalysts can be greatly enhanced and the experimental and characterization methods improved.

(4) Theoretical simulations can provide new insight into understanding the photocatalytic mechanism and relationship between the microstructure and performance. The theoretical investigations on Bi-based photocatalytic materials should be further strengthened, which will contribute to a deep understanding on the surfaces, interfaces, nanostructures and activity at the atomic and molecular level of the material properties.

(5) Although Bi-based photocatalysts have been continuously investigated over the past ten years, their photocatalytic performance is still far from the requirements for practical application. More research work is still needed to reach their real application.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the NSFC (21573170, 51320105001, U1705251 and 21433007), the Self-determined and Innovative Research Funds of SKLWUT (2017-ZD-4), and the Natural Science Foundation of Hubei Province of China (No. 2015CFA001). The project was also supported by the Internal Research Grant (RG 11/2016-2017R), and the EdUHK and General Research Fund (GRF 18301117) by RGC of the HK government. The work was also supported by the Natural Science Foundation of Hunan Province of China (2018JJ2457).

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