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Structural Defects Regulation of Bismuth Molybdate Photocatalyst

Yifan Zhao , Qiyun Mao , Xiaoya Zhai , Guoying Zhang

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (8) : 1331 -1343.

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Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (8) : 1331-1343 DOI: 10.7536/PC201236
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Structural Defects Regulation of Bismuth Molybdate Photocatalyst
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Abstract

As a novel type of Bi(Ⅲ)-based semiconductor photocatalyst, bismuth molybdate(Bi2MoO6) possesses the advantages of layered structure, low cost, cleanness and efficiency, narrow band gap and visible light response, etc. Resultantly, it exhibits widely potential applications in various photocatalytic areas such as degradation of water pollutants, air purification, antibacterial, water splitting, carbon dioxide reduction and nitrogen fixation. However, there are still two main bottleneck problems which would restrict the practical applications of Bi2MoO6 and needs to be addressed urgently. One is the low absorption efficiency for solar energy and the other is the fast recombination rate of photogenerated electron-hole pairs. Introduction and regulation of structure defects in Bi2MoO6 have been proved to be effective strategies to resolve the above problems. In the paper, the research progress of Bi2MoO6 defect engineering in recent years is comprehensively reviewed, including elemental doping, oxygen vacancy, synergistic effects, etc. The different methods to construct defects in Bi2MoO6 and the corresponding photocatalytic properties are extensively summarized. Also, the structure-activity relationship and action mechanism of the structural defects in different research fields are in-depth discussed and concluded. Finally, the present shortcomings of defective Bi2MoO6 photocatalyst are analyzed and the development direction and prospects in future are prospected.

Contents

1 Introduction

2 Elemental doping defect of bismuth molybdate

2.1 Rare earth ion doping

2.2 Transition metal ion doping

2.3 Non-metal ion doping

2.4 Ion co-doping strategy

3 Oxygen vacancy defect of bismuth molybdate

3.1 Ion doping associated oxygen vacancy

3.2 Individual oxygen vacancy

3.3 Oxygen vacancy synergy

4 Conclusion and outlook

Graphical abstract

Keywords

bismuth molybdate / defect modulation / ion doping / oxygen vacancy / synergistic effect / photocatalysis

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Yifan Zhao, Qiyun Mao, Xiaoya Zhai, Guoying Zhang. Structural Defects Regulation of Bismuth Molybdate Photocatalyst. Progress in Chemistry, 2021, 33(8): 1331-1343 DOI:10.7536/PC201236

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1 Introduction

Environmental pollution and energy shortage are two major global problems facing mankind, and the development of effective environmental purification and energy regeneration technologies has become the focus of attention of the scientific community[1⇓⇓~4]. Since the discovery of photocatalytic technology in 1972, it has become an important way to solve the environmental and energy problems because of its mild reaction conditions, clean and efficient, and deep oxidation[5⇓~7]. Traditional metal oxides, such as TiO2 and ZnO, have the advantages of high reactivity and good stability. However, these wide band gap semiconductors only respond to ultraviolet light in sunlight, which seriously limits their practical application in the field of photocatalysis[8,9]. Therefore, the development of novel narrow band gap semiconductor photocatalysts with visible light response has become a research hotspot in this field.
Bi (III) -based compounds are a new type of photocatalysts which have attracted much attention.Include BiVO4, Bi2WO6, Bi2MoO6, Bi2S3, BiOX (X = Cl,Br, I), Bi2O2CO3, Bi4NbO8Cl, etc[10][11⇓~13][14,15][16][17,18][19][20]. The valence band (VB) of these semiconductors is formed by the hybridization of Bi 6s and O 2p orbitals, which can reduce the effective mass of photo-generated holes and facilitate the transport of photo-generated carriers, and at the same time, the VB position can be raised to reduce the band gap, so as to achieve a wider range of photoresponse[21]. In addition, the compounds have an inherent layered structure, and the internal electric field generated by the compounds is also beneficial to the separation and migration of photogenerated charges[22,23]. Among them, bismuth molybdate Bi2MoO6 (BMO) has attracted much attention because of its special physical and chemical properties. It has a typical Aurivillius structure (Figure 1A), which is composed of [Bi2O2]2+ layers and MoO6 perovskite layers alternately. The dipole moment generated by the distorted MoO6 octahedron can promote the transfer of photogenerated carriers[24][25]. Not only does its VB come from the hybridization of Bi 6p and O 2p orbitals, but its conduction band (CB) also comes from the hybridization of Mo 4D and Bi 6p orbitals, thus having a relatively narrow band gap (2.5 ~ 2.8 eV) and an appropriate band position (Fig. 1b)[26]. The characteristics of the crystal and electronic structure make it show a wide range of applications in the fields of photocatalytic degradation, CO2 reduction, water photolysis and organic photoreaction[27,28][29][30][31].
Although the BMO photocatalyst has the theoretical and practical application basis, its absorption band edge is about 460 nm, the response range to visible light is limited, and the problem of fast recombination rate of photogenerated carriers in single-component semiconductors leads to its optical quantum efficiency and catalytic activity can not meet the practical application. Based on the above problems, domestic and foreign scholars have carried out a lot of research on the modification of BMO, including morphology control, structure control, metal deposition, heterostructure, carbon material composite and so on[32,33][34,35][36,37][38,39][40,41]. By reducing the particle size, assembling three-dimensional structure, adjusting the crystal structure and surface engineering, the light absorption range of BMO can be properly broadened, the specific surface area can be increased or the carrier separation can be promoted, so that the photocatalytic performance of BMO can be improved to a certain extent.
Among many factors, the intrinsic structure of photocatalyst is the key to determine its activity.By introducing crystal defects into semiconductor photocatalyst, the energy band structure can be changed by adjusting the crystal structure, which can broaden the light absorption range and promote the separation of photogenerated carriers[42]. In this paper, the research progress of defect engineering in BMO in recent years is reviewed, including element doping, oxygen vacancy and their synergistic effects.From the aspects of preparation methods, modification effects, structure-effect relationship and influence mechanism, the research and development trends of defect engineering in BMO are summarized, and the research prospects are prospected.

2 Study on the defects of bismuth molybdate doped with elements

Ion doping is one of the most efficient strategies for photocatalyst modification. Doping ions form impurity defects by occupying lattice sites or entering lattice gaps. This process can produce new donor or acceptor levels in the forbidden band, thus changing the electronic structure distribution of the photocatalyst, in which the non-equivalent ion doping also produces concomitant defects due to charge compensation[43][44,45]. The doping process can change the orientation of crystal growth, broaden the range of photoresponse, inhibit the recombination of photogenerated carriers, and increase the concentration of active sites or active species, thus improving the photocatalytic activity. According to the types of doping elements, BMO can be divided into four categories: rare earth ion doping, transition metal ion doping, non-metal element doping and co-doping.

2.1 Rare Earth Ion Doping

Rare earth ions are ideal metal dopants for improving the photocatalytic activity of BMO due to their gradually filled 4F electron configurations and unique optical properties. At present, the reported doping elements include La, Gd, Ho, Yb, Sm, Ce, Er, Dy and so on, among which La and Ce are more studied[46,47][48⇓~50][50][50][51][52,53][54][55].
For example, Dong et al. prepared La-BMO nanosheets by a one-pot hydrothermal method by taking advantage of the fact that the La3+(104 pm) and the Bi3+(103 pm) have the same charge and similar radius. When the atomic percentage of La is 0.5%, the rate constant of degradation of rhodamine B (RhB) is 2.0 times that of BMO (Figure 2a)[46]. It is mainly attributed to two factors: :La3+ doping makes the BMO layer gradually thinner, and the exposed crystal plane changes from (010) to (001) (Fig. 2b), which provides more active sites for photocatalytic reaction; La3+ can act as the capture and release center of photogenerated electrons, inhibit the recombination of carriers and produce superoxide radicals (·O2-) through the circulation of La3+/La2+. Alemi et al. Synthesized BMO doped with Gd3+, Ho3+ and Yb3+ by polymer-assisted sol-gel method[50]. Similar to the doping of La3+, these rare earth ions can also inhibit the grain growth of BMO, and the photoresponse range is red-shifted from 8 nm to 35 nm due to the charge-transfer transition between the 4 f electrons and BMO. Among them, Gd-BMO has the best photocatalytic activity for RhB degradation, which is attributed to the 4f7 stable electron configuration of the Gd3+. When the photogenerated electrons are captured, the half-full electron configuration is destroyed and the energy is increased, so that the electrons are quickly transferred to the surface to adsorb oxygen, and more ·O2- are generated while the stable state is restored, thereby further enhancing the photocatalytic activity.
In addition to the influence of 4 f electron configuration and crystal growth, the doping of rare earth ions can also effectively improve the energy band structure of BMO. Pettes et al. Found that Sm3+ doping can not only reduce the band gap energy of BMO, but also change the semiconductor properties with the doping amount (Fig. 2C)[51]. When the doping amount is less than 0.3, it is a quasi-direct band gap semiconductor, and when the doping amount is 0.5, it is an indirect band gap semiconductor. Rare earth doping can not only effectively improve the photocatalytic degradation of dyes, but also significantly improve the photocatalytic activity of removing carcinogenic drugs such as Cr (Ⅵ) and tetracycline (TC) in water. For example, the photocatalytic degradation rate of Ce3+ doped CeO2/BMO synthesized by molten salt method can be increased by 4. 2 times under the irradiation of 5 W LED lamp, in which the cycling of Ce3+/Ce4+ is the key factor to promote electron transport[53].
The above results indicate that the redox couple of rare earth elements plays an important role in the process of BMO photogenerated electron transfer, especially for the elements with stable electron configurations such as 4f0 or 4f7. However, rare earth doping can also cause the reduction of catalyst particle size, which may lead to secondary pollution in the use of photocatalysts, and there is a certain contradiction with the recycling practicability of photocatalysts, so rare earth doping in BMO superstructure is expected to be an effective way to solve this problem.

2.2 Transition metal ion doping

Compared with rare earth ions, transition metals, especially the first transition metal ions, are more abundant and cheaper, and are common objects in the study of photocatalyst doping. This kind of ion has a gradually filled d-electron configuration, which is easy to form impurity energy levels in the host crystal after doping, and can promote the migration of photogenerated charges[56,57]. At present, the transition metal ion doping of BMO mainly includes Ti4+, Mn2+, Cu2+,Zn2+ 、Fe3+ 、W6+ 、Co2+, Pd2+, Ru3+, etc.Most of them are the substitution of Bi3+ in BMO lattice, while W element is more likely to replace U Mo6+ due to the same group effect[58][58][58][58,59][60,61][61,62][63][64][24]. Table 1 summarizes the doping research of a series of transition metal ions on BMO, from which it can be seen that the main preparation strategies of such doping are hydrothermal, solvothermal or sonochemical methods, and its application fields are relatively wide. In addition to photocatalytic degradation of dyes and achromatic phenols, it can also be used in the fields of NO gas removal, Cr (VI) reduction and nitrogen fixation.
For example, in the Zn-BMO system, the photocatalytic activity was significantly improved when the Bi3+ in the [Bi2O2]2+ layer was replaced by Zn2+, and the kinetic constant of RhB degradation was 21. 3 times that of BMO when the Zn/Bi molar ratio was 0. 015[59]. The surface photovoltage analysis showed that the doping of Zn2+ effectively promoted the photogenerated electron migration, and the VB position of Zn-BMO moved forward by 0. 13 eV, which indicated that the oxidation ability of photogenerated holes was enhanced, and the photocatalytic activity was effectively improved under the synergistic effect of the above factors. Chen et al. Reported the Fe-BMO photocatalytic system, and discussed the nitrogen fixation behavior and mechanism of Fe3+ before and after doping[60]. The research shows that the light absorption threshold of BMO is shifted from 475 nm to 521 nm after part of the Bi3+ in BMO is replaced by Fe3+, and the photocatalytic nitrogen fixation activity is significantly improved compared with Fe-0.5% doping, and the visible light NH3 generation efficiency is 3.7 times of that before doping (Figure 3A), and BMO also shows excellent performance in the photocatalytic reduction of Cr (Ⅵ). They discussed the reasons for the increase in activity from multiple perspectives: as shown in Figure 3B, the CB bottom of the doped BMO forms an impurity level, which reduces the band gap energy and causes a red shift in light absorption; The Fe3+/Fe2+ pairs formed in the catalytic process can not only promote the separation of carriers by gaining or losing electrons, but also act as the activation center of N2. More importantly, the surface work function of the doped material is significantly reduced (Fig. 3C, d), which can promote the migration of photogenerated charges to the surface and participate in photocatalytic reactions.
Among the transition metal elements, W and Mo belong to the same group, and they have similar radius and properties under the influence of lanthanide contraction, so W is more likely to replace Mo6+ in the doping process. For example, in W-BMO system, due to the formation of band gap impurity energy level and W6+/W5+ pair, it shows excellent photoelectric water decomposition and photocatalytic degradation ability, respectively[61][62]. In addition, the self-doping of BMO has also been reported, and the substitution of Bi3+ for partial Mo6+ can be achieved by the hydrothermal strategy of excessive bismuth source. Theoretical calculations show that a new energy level composed of O 2p orbitals appears in the Bi-BMO band gap.And the band potential is negatively shifted by 0. 05 eV as a whole, so the reduction ability of photogenerated electrons is enhanced, the concentration of the generated ·O2- is 4. 0 times of that before doping, and the efficiency of photocatalytic removal of NO is effectively improved[65].
Obviously, the doping of transition metal ions can affect the energy band structure, position and work function of BMO, so as to improve its photocatalytic activity in different fields. However, due to the different radii, doping methods and actual concentrations of different doping ions, there is a certain degree of randomness in the improvement of catalytic performance, and there is still a lack of systematic comparative study. In addition, the effect of magnetic transition metal ions on the photocatalytic activity of BMO remains to be further studied and discussed.

2.3 Nonmetal ion doping

Since Asahi et al. First doped the nonmetal element N into the TiO2 and confirmed its excellent photocatalytic activity under visible light, the nonmetal element doping of photocatalysts has attracted much attention[66][67,68]. Our research group has systematically studied the Br-doped Bi2O2CO3 nanosheets. Crystal structure analysis and density functional theory calculation show that Br enters the lattice gap of the material and forms a delocalized impurity level near the Fermi level, which enhances the electron transport capacity of the Bi2O2CO3 and inhibits the carrier recombination (Figure 4A, B). The photocatalytic degradation rates of RhB and phenol are increased by 3.6 and 2.7 times, respectively[19]. Compared with the transition and rare earth metal ions, the non-metal elements such as C, B and F in the second period have smaller radii and are easier to enter the lattice of the photocatalyst[69,70][71][72⇓~74]. The doping of Cl, Br, I, Te and CO32- radicals have also been reported[74,75][74][74,76][77][78].
The atomic radius of C (77 pm) is similar to that of O (74 pm), which is confirmed to be the preferred element to improve the photocatalytic performance of BMO by experimental and theoretical studies. Ye et al. Achieved in-situ C doping of ultra-thin BMO in the amide acid-base system formed by oleylamine and oleic acid by using the coordination of organic matter and the hydrophobic effect of long chain[69]. XPS detection under argon ion beam bombardment confirms that C is uniformly doped in the whole nanosheet. This process elevates the CB site of the BMO, and thus the photogenerated electrons have a stronger reducing ability, significantly increasing the concentration of active oxygen (Fig. 4 C, d). In addition, the impurity levels introduced within the band gap can facilitate the transfer of photogenerated electrons. Due to the above changes in electronic structure and band position, the rate of photocatalytic removal of NO is 4.3 times that of BMO bulk. Using glucose as carbon source, the substitution doping of oxygen ions in BMO by C can also be realized by hydrothermal-calcination method, which can not only effectively degrade dyes and TC, but also be used as an effective photosensitizer for photodynamic therapy of tumors, showing potential applications in the field of biomedicine[70].
The doping of electron-deficient element B can not only prolong the carrier lifetime as an electron trap, but also induce some Bi5+ ions in BMO, thus enhancing the oxidation performance by using the inert electron pair effect[71]. Halogen is another important research object of non-metal doping. The F- with smaller radius is easy to replace the O2- and cause lattice contraction. The F-BMO system shows three changes in the electronic structure: first, the dispersion of CB energy levels is enhanced, which is conducive to the migration of photogenerated electrons[72]; Secondly, Mo 4d1 donor levels are formed below the Fermi level, which can be used as electron trapping centers to suppress the recombination of photogenerated charges. Third, the VB position is reduced, thus enhancing the oxidation ability of the hole. Under the synergetic action of the multiple effects, the photocatalytic degradation activity of RhB and dichlorophenol is obviously improved. The doping of Cl- and I- ions in BMO showed similar photocatalytic effects, in which the I- ions entered the lattice gap due to their larger radius, and became the capture center of photo-generated holes, and promoted the photo-redox by forming IO3-/I- pairs, which could be used to efficiently remove TC, ciprofloxacin, oxytetracycline drugs and reduce Cr (Ⅵ)[75][76]. (Compared with monatomic ion doping, the larger density of States and the increase of active oxygen concentration can promote the oxidation of NO. )
The above studies show that different preparation methods have different effects on the doping of the same element, and sometimes the band gap becomes larger. This should be closely related to the specific location and concentration of the doping ions, so it is of great significance to develop quantitative characterization methods combined with theoretical calculations to understand the effects of the location and concentration of non-metallic elements on the crystal structure and electronic structure for guiding the synthesis of BMO-based photocatalysts.

2.4 Ion codoping

Single element doping has different emphasis on photocatalytic modification. In order to broaden the absorption band edge of semiconductor and improve the separation efficiency of photogenerated carriers, the co-doping of two or more elements is an ideal solution. The synergistic effect of different doping ions can make up for the deficiency of single doping, so that the photocatalytic performance can be further improved[79⇓~81]. The co-doping of BMO mainly includes three types: rare earth-rare earth, rare earth-nonmetal and transition-transition, in which the co-doping of rare earth ions is dominant.
Li et al. Have carried out a series of studies on double and triple rare earth co-doping, and put forward a unique view on the selection of rare earth elements based on electronic configuration. Previous studies have shown that Gd3+ is the preferred single doping guest ion because of its stable 4f7 half-filled electron configuration. Therefore, on the basis of preliminary exploration of Tb/Eu co-doping, they selected two rare earth ions with Gd as the symmetry center, and systematically studied the co-doping of Tb/Eu, Dy/Sm and Er/Nd to BMO[50][82][83]. As shown in fig. 5A, the paired rare earth ions can achieve complementary effects through the compensatory allocation of orbital electrons, such as the transfer of a paired electron from the Tb3+4f8 orbital to the vacant orbital of the Eu3+4f6, so that both of them can achieve a stable 4f7 electron configuration. The formation of this hetero-nuclear pair can doubly promote the separation and transfer of photogenerated electrons, and shows better photocatalytic activity than Tb3+, Eu3+ and Gd3+. They continued to study the Gd/Er/Lu tri-doped BMO system and found that the three had obvious synergistic catalytic effects (Fig. 5B), in which the energy up-conversion effect of Er3+ could improve the light capture ability of BMO and make it produce more abundant photogenerated carriers[84]. Gd3+, similar to those discussed earlier, act as electron capture and release centers and promote the production of more reactive oxygen species; However, the doping of Lu3+ results in a large number of oxygen vacancies in BMO crystals, which become electron traps and inhibit the recombination of carriers. Compared with single doping and double doping, the performance of the multi-doping system for the photocatalytic degradation of RhB, TC and p-chlorophenol is obviously improved. A series of studies by Li et al. Have shown that co-doping of rare earth elements can not only skillfully utilize the complementary characteristics of orbital electrons of rare earth elements, but also cooperate with the energy upconversion and oxygen vacancy effect produced by single rare earth doping to ultimately optimize the photocatalytic performance of BMO, which provides a new idea for the development of co-doping. Hou's report on Ho/Yb and Tm/Yb co-doping further enriches the research in this field[85][86].
Rare earth-nonmetal co-doping also shows a certain degree of synergy. For example, F-La co-doped BMO can be prepared by a solvothermal method. The results show that the oxygen ion in the [MoO4]2- is replaced by the F-, while the bismuth site in the [Bi2O2]2+ is replaced by the La3+. The degradation rate constants of RhB in the optimized co-doped system are 6.54, 2.98 and 2.48 times of those in pure BMO and F-La co-doped BMO, respectively[87]. The synergistic effect of the two is that F doping makes a new energy level appear at the top of the valence band, the absorption band edge of BMO is red-shifted and more carriers are generated, and F- as an n-type impurity can also increase the electron density. The high concentration of photogenerated electrons produced by the above factors are effectively separated and transferred by forming La3+/La2+ pairs, which not only prolongs the carrier lifetime, but also produces more active oxygen. The transition-transition BMO co-doped system is relatively rare. Rao et al. Prepared Ni2+-Ti4+ surface co-doped BMO by the combination of isovolumetric impregnation technology and calcination treatment. Under the synergistic effect of the two, the specific surface area is increased, the visible light response is broadened and the carrier separation is promoted. The catalytic degradation efficiency of the Ni2+(0.18%)-Ti4+(0.192%) co-doped system is 1.82 and 1.52 times higher than that of the corresponding single doping, respectively[88].
To sum up, due to the differences in radius, electronegativity and electronic configuration of different elements, the co-doping system does show better application effect than single doping, but the current research on BMO co-doping is mainly based on rare earth ions.There is still much room to explore the multiple co-doping of transition metals and nonmetals, and the synergistic mechanism and law of co-doping are expected to be deeply analyzed by theoretical calculation.

3 Study on Oxygen Vacancy Defects in Bismuth Molybdate

Oxygen vacancies (Vo), formed by the escape of oxygen ions from the crystal lattice, are the most common and important crystal defects in oxygen-containing compounds. Since it was reported that surface Vo could improve the efficiency of photocatalytic hydrogen production of TiO2, the introduction and regulation of Vo defects has become an important strategy to improve the photocatalytic activity[89][90,91]. The results show that Vo is closely related to the physical and chemical properties of photocatalysts, such as geometric structure, electronic structure, light absorption properties and surface active sites, which affect the adsorption and desorption, light absorption and charge transport of materials, and ultimately change their photocatalytic activity[92][93][94][95]. BMO is an ideal photocatalyst for the study of Vo due to the presence of oxygen ions in both the [Bi2O2]2+ and the [MoO4]2- of BMO.

3.1 Oxygen vacancy defect associated with ion doping

During the doping process, the crystal structure of BMO will be distorted due to the differences in radius, electronegativity and coordination number between the doping ions and the host ions, especially when the charge imbalance is caused by doping, it is easier to form Vo for charge compensation. Some early studies speculated that Vo might be produced in the process of ion doping, while some recent defect studies confirmed the process of doping accompanied by Vo, and the dual effects of the two were discussed in depth[96⇓~98].
Dai et al. Prepared the Ce-Vo-BMO system by one-step hydrothermal method. The ICP-MS and XPS analysis of the system showed that the type and location of defects depended on the doping amount of Ce[99]. As shown in fig. 6a, when the doping amount is low, the bismuth sites in the [Bi2O2]2+ layer are mainly replaced by the Ce3+; When the doping amount is further increased, two new defect States appear in the crystal, namely, the substitution of Ce4+ for molybdenum sites in the [MoO4]2- layer and the introduction of Vo, and the latter is formed due to charge compensation. The ESR signal in Fig. 6B strongly confirms the existence of Vo defects. The vacancy defect not only destroys the octahedral configuration of the MoO6, increases its dipole moment and reduces the coordination number of molybdenum, but also effectively promotes the electronic excitation due to the defect level at the bottom of the conduction band. Under the combined effect of doping defects and Vo defects, the carrier mobility and active oxygen concentration are significantly improved, thus the performance of removing toxic nerve agents and bacteriostasis is significantly improved (Fig. 6 C, d). In the Lu-Vo-BMO system, besides the systematic analysis of the accompanying Vo of Lu doping, the stability of vacancies and their effects on crystal growth are also proposed[100]. The ESR signal of Vo does not decay significantly after illumination, which indicates that the doping of Lu3+ has a stabilizing effect on the metastable state of Vo. The oxygen vacancies located in the [Bi2O2]2+ layer increase the energy of { 010 } crystal plane, so the crystal plane is oriented to grow during the hydrothermal process, and the sample gradually transits from two-dimensional sheet to one-dimensional rod. In terms of electronic structure, a Vo level is formed near the top of the conduction band of BMO, which not only effectively broadens the visible light absorption due to the reduction of the band gap energy by 0. 34 eV, but also effectively improves the carrier separation efficiency as an electron transfer station participating in a new photoexcitation process. In addition, the accompanying Vo defects in B-BMO, F-BMOSm-BMO and other doped systems are also clearly discussed, which capture photogenerated electrons, increase the content of adsorbed oxygen or provide active sites, respectively.BMO plays a certain role in the different stages of the photocatalytic process, such as light absorption, charge transport and surface reaction, and effectively improves the photocatalytic application effect of BMO through the joint action of BMO and doping ions[71][73,74][98].
Although the process of doping the accompanying Vo does not require additional operations, there is uncertainty about its introduction. In addition, the focus of this work is still doping, and there is still a lack of understanding of the relationship between Vo concentration and doping strategy, doping amount and catalytic activity.

3.2 Ingle oxygen vacancy defect

In order to reveal the effect of Vo defect on the photocatalytic activity, the pure Vo-introduced BMO photocatalytic system is a more ideal research model. Table 2 summarizes the current construction methods and application fields of Vo-BMO systems, mainly including solvothermal, ion-induced, atmosphere calcination, chemical reduction, solution etching and template-directed strategies. Except for the one-pot synthesis of solvothermal method, the rest need to carry out various post-treatments on the pre-prepared BMO, of which calcination and chemical reduction are the most common. Its application also covers many fields such as photocatalytic degradation, bacteriostasis, nitric oxide removal, organic oxidation and CO2 reduction.
One-pot solvothermal method is the most convenient method to introduce Vo. Liang et al. Used CTAB-assisted solvothermal method (Figure 7A) to prepare Vo-BMO microspheres assembled by ultra-thin nanosheets. By systematically investigating the influence of reaction conditions, they found that high temperature and short time of 180 ℃ and 3 H were conducive to the generation of high concentration of Vo[101]. Compared with the sample with low concentration of Vo, the carrier lifetime increases from 1. 59 ns to 5. 18 ns, and the selective oxidation efficiency of benzyl alcohol increases by 5. 5 times. In addition to the electron trapping and oxygen adsorption effects of Vo, they believe that the key reason is the coordination adsorption of Vo on benzyl alcohol, which is strongly confirmed by the in-situ IR spectra in Fig. 7B. It is clear that Vo acts as an active site to reduce the activation energy of benzyl alcohol, thus accelerating the oxidation kinetics. However, Wang et al found that Br- could induce and stabilize the generation of Vo defects in { 001 } crystal plane during the solvothermal process. The optical absorption of Vo-BMO-001 sample extended to the near infrared region, significantly improving the activation of molecular oxygen. The removal rate and selectivity of NO reached 62.9% and 93.61%, respectively[102].
Calcination of BMO is the most common strategy to introduce Vo defects. Liu et al. Successfully introduced Vo on the surface of BMO mesoporous microspheres by heat treatment at 450 ℃ for 2 H in air atmosphere, and the catalytic degradation efficiency of the sample for TC was increased by 17% after 150 min of ultraviolet irradiation[103]. The results show that the oxygen-containing groups such as — OH on the surface of pure BMO preferentially adsorb water molecules, which not only competitively inhibits the adsorption of pollutants, but also sterically hinders their entry into the mesoporous interstices, while Vo-BMO preferentially adsorbs oxygen and captures electrons due to the presence of Vo. In addition, the introduction of Vo leads to an overall upward shift of the band position, in which the increase of VB is conducive to the generation of holes with high mobility, while the increase of CB enhances the reduction ability of photogenerated electrons. The changes of adsorption and electronic structure mentioned above together promote the improvement of the catalytic activity of BMO. Heat treatment of BMO at 250 ℃ and 375 ℃ in air can also introduce Vo[104,105][106]. Chemical reduction is a mild method for the preparation of Vo, and the Vo-BMO system can be effectively constructed by using NaBH4 as the reducing agent at room temperature, which has obvious advantages for the removal of NO and the degradation of phenol and dyes[107][108]. Shi et al. prepared the Vo-BMO @ BMO core-shell structure by the method of CaH2 reduction followed by heat treatment and re-oxidation[109]. Vo is mainly located in the core of the microsphere, while the shell is a disordered layer containing only a small amount of Vo. The defect energy level plays an important role in regulating the electronic structure and improving the light absorption.
In addition to the usual solvothermal, calcination and chemical reduction methods, there are also reports of etching treatment and template-guided introduction of Vo. Li et al. Immersed BMO in NaOH solution with appropriate concentration, and the charge imbalance of the crystal was caused by the dissolution of part of the [MoO4]2- layer by etching. In the process of reconstructing the charge balance, compensatory oxygen-rich vacancies appeared in the [Bi2O2]2+ layer, which significantly improved the degradation of neurogenic drugs and bacteriostatic effect[110]. Xia et al. Prepared ultra-thin BMO containing Bi-O vacancy pairs by ion exchange using BiOBr nanosheet template-guided method[111]. The study shows that the escape energy of atoms in the ultra-thin two-dimensional lattice is reduced, and the divacancies are confirmed by ESR and dark-field TEM (Fig. 7 C, d). The multiple defects can not only control the local atomic and electronic structure, but also make a new density of States appear in the forbidden band. Vacancy pairs not only act as charge separation centers, but also facilitate the adsorption of CO2 and the desorption of reduction products, and the rates of photocatalytic reduction of CO2 and oxygen production are increased by 2.55 and 3.17 times, respectively.
Obviously, Vo defects play an important role in improving the electronic structure, adsorption and chemical activity of BMO, but most of the current construction methods are cumbersome, so it is more practical to develop a green and economical synthesis route. In addition, the quantitative characterization of the location and concentration of Vo defects is the basis for the study of structure-activity relationship, and more effective characterization techniques need to be explored and developed to provide design guidance for the regulation of photocatalytic materials on the basis of defining the precise existence state of Vo.

3.3 Synergistic mechanism of oxygen vacancy defect

The synergistic effect of Vo defects and other modification mechanisms can further optimize the photocatalytic effect. At present, the main reports are the synergistic effect of Vo-BMO and noble metal deposition or heterostructure layer, in which Fu et al. Have carried out extensive research in this field[112⇓~114].
They first obtained Vo-BMO by thermal treatment, and then achieved Ag loading on its surface by photoreduction deposition[104]. The study shows that Vo originates from the Bi-O layer and acts as a shallow donor level partially overlapping or above VB, which can reduce the band gap energy of BMO and broaden the light absorption range (Fig. 8A), so that more photogenerated carriers can be generated in the system. Not only the Vo defect can capture the photogenerated electrons and activate the molecular oxygen, but also the Schottky barrier formed between Ag and BMO can make the photogenerated electrons transfer to Ag unidirectionally. Under the synergistic effect of both, the recombination probability of photogenerated electron-hole pairs is greatly reduced (Fig. 8B). The photocatalytic degradation rate of RhB in the optimal Ag-Vo-BMO sample is 4.6 and 1.4 times of that in the presence of Ag and Vo alone, respectively. They also took advantage of the thermal instability of silver nitrate to achieve both Vo introduction and surface Ag loading during thermal treatment at 375 C[106]. Compared with BMO and Vo-BMO, the photocatalytic degradation rate of the sample prepared by this method was increased by 183 and 4. 9 times, respectively.Compared with the two-step method of thermal treatment followed by photoreduction, the photocatalytic degradation rate of the sample prepared by this method was also increased by about 26. 9%. They also carried out further research on the synergistic regulation of Au-Vo-BMO, and obtained Vo defects and surface Au deposition at the same time through the "two birds with one stone" NaBH4 low-temperature reduction strategy, which showed a significant synergistic effect on the degradation of dyes and phenol[108].
In addition to studying the synergy between Vo defects and noble metal deposition, Fu et al. Also reported the synergy mechanism between Vo and surface Fe (Ⅲ) clusters[105]. By calcining BMO at 250 ℃ and impregnating it with ferric nitrate solution, they obtained the Vo-BMO system complexed by Fe (Ⅲ) ion clusters. Among them, Vo plays a role in regulating the energy band structure and broadening the visible light absorption due to its abundant delocalized electron effect, while Fe (Ⅲ) ion clusters dispersed on the BMO surface in the form of Fe2O3 or FeOOH (Fig. 8C) play a role in direct interfacial charge transfer. As shown in Fig. 8d, the photogenerated electrons can not only transfer from the conduction band of BMO to the Fe (Ⅲ) cluster, but also transfer directly from the valence band top to the Fe (Ⅲ) cluster, and form the dynamic equilibrium of Fe (III)/Fe (II) in the process of being captured by adsorbed oxygen. Under the synergistic effect of Vo defects and Fe (Ⅲ) clusters, the photocatalytic activity of 15% Fe-Vo-BMO was 80 times higher than that of pure BMO, and 6. 7 and 5. 1 times higher than that of Vo defects and Fe (Ⅲ) clusters, respectively. Among them, their strategy of simultaneous Vo introduction and noble metal deposition through appropriate heat treatment and chemical reduction has important reference significance for the preparation of other synergistic systems.
In addition, Gao et al. Also carried out the composite research of Vo-BMO and metal organic framework material MIF-88B (Fe)[115]. The introduction of Vo defect increases the lifetime of BMO from 2. 4 ns to 3. 47 ns, while the recombination of MIF-88B (Fe) further prolongs the carrier lifetime to 4. 60 ns, which indicates that the photogenerated carriers are separated and migrated to a greater extent under the dual mechanism. They believe that the synergistic effect of the two is mainly reflected in the promotion of bulk charge separation by Vo, while the promotion of interface charge transfer by II-type heterojunction.
It is true that the establishment of effective Vo defect synergy can further optimize the catalytic activity of BMO, but most of the current strategies need multiple steps to complete, and the current research on synergistic systems is relatively small, mainly focusing on noble metal deposition and very few interface interactions, and other studies such as active crystal faces and Z-type heterojunctions are relatively rare.

4 Conclusion and prospect

In this paper, the recent progress in the engineering of structural defect control of BMO photocatalysts is summarized from the perspectives of ion doping and oxygen vacancy introduction. By introducing defects such as hetero ions or oxygen vacancies into the crystal structure of BMO, not only the electronic structure, energy band position or work function of BMO can be adjusted to a certain extent, but also the adsorption of substrates, the increase of active sites for catalytic reactions, the decrease of activation energy and the increase of active oxygen concentration can be facilitated. The changes caused by the crystal structural distortion play an active role in broadening the visible light response range of BMO and promoting the separation and migration of photogenerated carriers, thus improving the application of BMO in the fields of photocatalytic degradation of organic pollutants, removal of nitric oxide, reduction of CO2, nitrogen fixation and bacteriostasis to varying degrees.
Although the research work in this field has made fruitful progress, there are still many problems that are not clear and need to be further studied. (1) In the aspect of defect introduction, ion doping may cause the reduction of catalyst particle size, which is contradictory to the recyclability in the practical process. The hierarchical superstructure BMO assembled by ultra-thin nanosheets is expected to solve this problem. In addition, most of the introduction methods of oxygen vacancies are cumbersome, and more green and economical synthesis routes need to be developed. (2) In the aspect of characterization, the position and concentration of defects are basically in a qualitative or semi-quantitative characterization state, and the effects of the type and concentration of ion doping and the introduction strategy and position of oxygen vacancies on the catalytic performance are lack of quantitative comparison.Therefore, it is urgent to develop effective instrument detection technology to "see through" the precise existence state of defects, systematically study the structure-activity relationship of BMO, and provide targeted design for defect regulation of photocatalytic materials. (3) In terms of performance research, BMO defect system is mainly used for photocatalytic degradation of water pollutants such as typical dyes, phenols or antibiotics.There is still a lack of exploration on the treatment of actual wastewater with complex components, and the research on the application of hydrogen production, nitrogen fixation and other energy sources is relatively rare, so there is still a lot of room for development in the breadth and depth of its application field. (4) In the aspect of mechanism research, it is necessary to strengthen the close combination of theoretical calculation and experimental results, and to understand the changes of microscopic parameters such as defect structure, electronic structure, energy band and density of States of catalysts with the help of theoretical calculation.Simulate and reveal the influence of defects on the thermodynamics or kinetics of photocatalytic reaction, so as to provide theoretical basis and guidance for defect regulation and modification of materials, and design BMO-based photocatalysts with high activity and stability.

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Funding

Open Foundation of Key Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong Normal University(2018KLMNP05)

Open Foundation of Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education)

College of Chemistry, Nankai University and the Postgraduate and Doctoral Innovation Project of Tianjin(2020YJSS049)

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