Bismuth Sillenite Crystals as Recent Photocatalysts for Water Treatment and Energy Generation: A Critical Review

: Photocatalysis has been widely studied for environmental applications and water treatment as one of the advanced oxidation processes (AOPs). Among semiconductors that have been employed as catalysts in photocatalytic applications, bismuth sillenite crystals have gained a great deal of interest in recent years due to their exceptional characteristics, and to date, several sillenite material systems have been developed and their applications in photoactivity are under study. In this review paper, recent studies on the use of Bi-based sillenites for water treatment have been compiled and discussed. This review also describes the properties of Bi-based sillenite crystals and their advantages in the photocatalytic process. Various strategies used to improve photocatalytic performance are also reviewed and discussed, focusing on the speciﬁc advantages and challenges presented by sillenite-based photocatalysts. Furthermore, a critical point of certain bismuth catalysts in the literature that were found to be different from that reported and correspond to the sillenite form has also been reviewed. The effectiveness of some sillenites for environmental applications has been compared, and it has demonstrated that the activity of sillenites varies depending on the metal from which they were produced. Based on the reviewed literature, this review summarizes the current status of work with binary sillenite and provides useful insights for its future development, and it can be suggested that Bismuth sillenite crystals can be promising photocatalysts for water treatment, especially for degrading and reducing organic and inorganic contaminants. Our ﬁnal review focus will emphasize the prospects and challenges of using those photocatalysts for environmental remediation and renewable energy applications.


Introduction
Water is the most important natural resource on our planet; its resources are one of the essential aspects of every society's sustainable development [1,2]. The water quality is critical in addition to its quantity [3]. Even though water covers more than 71% of the Earth's surface, approximately only 2.5% of it is fresh water and only 0.002% is considered available to humans [4]. Although this quantity is limited, it is constantly contaminated by harmful chemicals generated by human activities such as agricultural discharge, poor sanitation, and rapid industrialization [5,6]. These contaminants are challenging to eliminate, and their presence in water has negative impacts on both health and environmental factors [7]. As a result of the fast advancement of science and technology, industries have risen and evolved, and the developing chemical industry has synthesized a large number of chemical compounds so far [8,9]. More and more hazardous compounds are being created and released into the water environment and their concentrations are continuously increasing [10].
Several pieces of research have provided a critical overview of the various treatment strategies for treating contaminated water [1]. Contaminated water cannot be adequately treated using conventional methods. Conventional physical and biological techniques, such as adsorption, reverse osmosis, and air stripping, can only extract soluble organic materials [11][12][13]. In actuality, they do not completely eliminate pollutants, they rather transfer them from one phase to another [14]. In general, these techniques have considerable drawbacks and disadvantages, one of which is contamination with treatment by-products, which can increase the risk of cancer and pollute the environment [15,16]. Therefore, developing innovative methods and approaches for treating water and wastewater is very necessary [6]. In contrast to conventional approaches, advanced oxidation processes (AOPs) have exhibited satisfactory performances in the past few decades. They have shown to be a better solution for the rapid removal of non-biodegradable compounds in water [17,18]. AOPs use hydroxyl radicals and other powerful oxidizing agents to efficiently remove dangerous contaminants via direct or indirect mechanisms [19,20]. These radicals may react with a wide range of contaminants and produce hydroxylated or dehydrogenated products, which can then be mineralized into non-toxic final products such as water (H 2 O) and carbon dioxide (CO 2 ), as well as inorganic ions [21]. Pollutant mineralization is the strong point of these processes because it completely removes environmental pollutants [22].
As one of the AOP techniques, photocatalysis is one of the best approaches, as it very efficiently opens new avenues for addressing global energy needs and pollution issues [23,24]. It has aroused the interest of the scientific community in recent decades, particularly as one of the most effective applications for the removal of organic and inorganic pollutants [25]. It is also known that photocatalysis is capable of producing chemical reactions that are highly thermodynamically demanding that would otherwise require energy-intensive inputs in terms of an elevated temperature (or pressure), which are potentially beneficial in energy generation and environmental cleaning applications [26]. Issues directly related to human health and environmental health such as the purity of water and air can be effectively addressed through photocatalysis as it will be one of the most important future technologies for all kinds of photocatalysts in the field of environmental treatment [27]. Heterogeneous photocatalysis using semiconductors as catalysts is more efficient in environmental applications because they are very efficient, easy to produce, and use a heterogeneous process that makes it easy to separate them from water [24,28]. Semiconductors also have other properties suitable for photocatalytic reactions, such as their optical absorption characteristics, their electronic structure, and the capacity to form charge carriers when energized with a light source by transforming light energy into chemical energy [17,21]. Highly effective and eco-friendly metal oxide semiconductor types have been developed. For example, TiO 2 (Yao et al. 2003b) is widely considered the most photoactive of all catalysts. However, TiO 2 's disadvantages, such as its high photogenerated electron-hole pair recombination rate and poor irradiation sensitivity due to its large energy band gap, limit its practical applicability [29]. To overcome these obstacles, it is necessary to develop innovative, efficient, and multifunctional semiconductors with excellent photocatalytic activities, an appropriate photo-generated pair recombination rate, and a low band gap [30][31][32].
The core metal ion for photocatalysts is considered to be one of the most critical aspects in the development of new semiconductors [33], with titanium (Ti), bismuth (Bi), zinc (Zn), and tin (Sn) being the best metals for semiconductor synthesis [34]. Bismuth semiconductors have become incredibly valuable recently due to their superior chemical stability, strong light absorption, low pair recombination rate, and low optical band gap [34,35]. "Bi" ions are indeed harmful and hazardous when they are free, especially in the form of nitrate.
However, their risks can be reduced by using them to synthesize chemically stable semiconductors without an ionic state. This contributes to waste management by lowering harmful ions and synthesizing valuable materials that can be used for environmental purposes. For this purpose, this review takes on bismuth semiconductors as an exciting topic.

Bismuth Sillenite Crystals
As is well known, semiconductors based on the transition-metal ion of Bi 3+ have become attractive because of the proprieties of those ions [24], such as: (i) Their d10 configuration in the oxide lattices makes photo-generated electron-hole pairs easier to separate [36]. (ii) Their 6s 2 orbital lone pair does not have any bonding or sharing with other atoms, which can potentially produce a structural disruption by inducing repulsive effects on neighbors' bonding. It can also create an internal polar electric field in crystallites, increasing the probability of charge separation due to the opposing movements of electron-hole pairs in the electric field [33,37]. (iii) Their ability to narrow the band gap by the orbital hybridization between O 2p and Bi 6s in the semiconductor's valence band allows photo-induced holes to move in the valence band [33,36].
Those phenomena can aid the photocatalytic activities in Bismuth-based semiconductors. Within the bismuth semiconductors category, the most exciting class is that of sillenite crystals. These crystals have attracted a great deal of scientific attention because of their unique structure of crystals [33], high photorefractive [38,39] and photochromic properties [40], dielectric properties [41], electro-optic properties [42], fast response [41,43], long optical storage time in the dark [44], recyclability, and narrow band gap (<2.9 eV) [36,45]. Recently, sillenite-type materials have been widely applied in a number of applications such as optical data processing and storage, electro-optics, acoustics, peizotechnics, optoelectronic, electronics, fiber optic sensor, image amplification, real-time and multiwavelength hologram imaging, metrological problems, and a variety of photocatalysis applications [46][47][48][49][50]. Bismuth oxide crystals with the sillenite structure, as defined by the general formula Bi 12 [M]O 20 (BMO), where M is the tetravalent ion of the elements from II-VIII groups of the Periodic Table, are a body-centered cubic crystal structure with the space group I23 [51][52][53]. Figure 1 shows the crystal structure of the perfect Bi 12 MO 20 sillenite and local structure around the Bi and the M sites. The general structure of the BMO structure is described by a bismuth atom (Bi 3 • ) surrounded by eight oxygen atoms that share corners with other comparable polyhedra BiO 8 and tetravalent ions MO 4 that occur both at the cube center and on the corners of the cube [54][55][56][57][58][59][60]. There is a critical point of certain bismuth catalysts in the literature that was different from those reported and corresponds to the sillenite stoichiometric model. In fact, bismuth catalysts generally form a sillenite phase when calcined in an air oven, where the probability of obtaining another phrase similar to spinel is very low. However, several studies considered their catalyst to be another phase, such as spinel, where they were sillenites,  There is a critical point of certain bismuth catalysts in the literature that was different from those reported and corresponds to the sillenite stoichiometric model. In fact, bismuth catalysts generally form a sillenite phase when calcined in an air oven, where the probability of obtaining another phrase similar to spinel is very low. However, several studies considered their catalyst to be another phase, such as spinel, where they were sillenites, but it was difficult to determine the phase in their samples due to low information in the X-ray diffraction database about sillenites. For example, there is a great number of works (Kumar et [69]) that reports the synthesis of ZnBi 2 O 4 ; however, their phases were more compatible with sillenite phase Bi 12 ZnO 20 (PDF #78-1325 [70]) and far from the spinel phase ZnBi 2 O 4 (JCPDS: 43-0449 [71]). Another example, the synthesis of CoBi 2 O 4 , was reported in two works [7,72], although the phases were more consistent with the sillenite phase Bi 12 CoO 20 [34,73].
There has recently been a surge in research on sillenite-phase semiconductors for various photocatalytic applications. Series of sillenite semiconductors Bi 12 CoO 20 [34], Bi 12 TiO 20 [25], Bi 12 NiO 19 [58], and Bi 12 ZnO 20 [32,70] were studied, and they showed high photoactivity against various kinds of pollutants. So, it was exciting to scan the literature and summarize recent works that dealt with sillenite crystals as photocatalysts for water treatment and environmental application.

Sillenites as Recent Photocatalysts
Semiconductor nanostructures have the potential to revolutionize the disciplines of photocatalytic oxidation and heterogeneous catalysis through the combined effects of quantum confinement and unique surface conformations [27]. The sillenite-type crystals Bi 12 MO 20 have been the subject of extensive research to understand and adapt the material's properties for a variety of applications [48]. Extensive study of the photocatalytic applications of sillenite-structured crystals began in the mid-2000s, with many new sillenites being used as photocatalysts in previous research [51]. Various types of pollutants have been used in the application of sillenites. Among them, there are organic pollutants such as dyes and antibiotics, and there are also inorganic pollutants such as hexavalent chromium; however, the most frequently used contaminant for sillenites application in the literature was the antibiotic cefixime. Table 1 lists bibliographical references using sillenite crystals for water photocatalytic treatment for various pollutants. Not only were sillenites with the formula Bi 12 MO 20 undertaken in this summary, but even Bi-based sillenites with other stoichiometric qualities were also taken. The majority of the research mentioned in the table was recent.  [76] Catalysts 2022, 12, 500 6 of 13 As can be seen from the table, the reported sillenites have proven to be effective at removing various pollutants; however, the application has only focused on organic pollutants such as antibiotics and dyes and inorganic contaminants such as heavy pollutants metals. This creates the need to test those photocatalysts for other uses, such as disinfecting water from bacteria and viruses, as photocatalysis has already proven to be an effective process for this [1,2]. Another note from the table is that all reported sillenites showed almost complete efficiency, but unfortunately in different conditions, so it would be interesting to compare some of these catalysts under the same conditions for the same pollutant.

Sillenites Catalysts for Hydrogen Generation
The depletion of non-renewable energies and the increase in environmental pollution in our time raises many concerns and insists on the development of renewable and clean energy technologies. Currently, researchers are putting high levels of effort into reducing dependence on non-renewable energy resources and trying to find effective alternatives to green energy. In this regard, H 2 has been recognized as a clean fuel due to its zeroemissions combustion nature, and could become the most widely used fuel in the future [77]. The production of hydrogen gas (H 2 ) is one of the most important alternatives and the cleanest energy carrier for these purposes. Hydrogen generation via photocatalytic water separation (PWS) is an important strategy to facilitate global clean energy and overcome the severe environmental challenges facing our society today [78]. Photocatalysis using active catalysts is a method that makes it possible to obtain an energy source from renewable sources, in other words, water and the sun, without producing waste. One of the unique advantages of the photocatalyst lies in the possibility of using it in water purification and hydrogen generation at the same time [79] it can also be used with solar energy to become an ecological and inexpensive process that is very easy to use on an industrial scale. In the context of the current status, there are several photocatalytic materials reported so far for H 2 evolution reaction. Generally, the examination of the performance of these photocatalysts is based on two factors, (i) the quantity of the H 2 evolved and (ii) the reusability and recyclability of the catalysts. Sillenites can be relied on ( Figure 2) as has already been demonstrated in a previous work showing that sillenites such as Bi 12 CoO 20 can be used in the hydrogen generation process as an alternative to green energy due to its favorable and excellent optical and electrochemical properties [34]. Another experimental study of catalytic hydrogen production also supports the prediction made by the previous study where the sillenite Bi 25 FeO 40 showed significantly better performance and much higher hydrogen production compared to other types of nanoparticles [80]. Although little work has been performed on the use of sillenite in the hydrogen generation process, its ease of synthesis and its excellent physical and photoelectric properties predict its effectiveness to become one of the most important photocatalysts in this field. Therefore, different sillenite compounds should be used as photocatalysts and tested in different techniques to improve the photocatalytic efficiency and thus increase hydrogen production or reduce the time required for wastewater treatment, especially to make full use of solar radiation and obtain higher photocatalytic activity. This will be one of the most important goals and work must be performed to find and improve the excellent and effective mechanisms using sillenite crystals that can benefit humanity in the future.
fore, different sillenite compounds should be used as photocatalysts and tested in different techniques to improve the photocatalytic efficiency and thus increase hydrogen production or reduce the time required for wastewater treatment, especially to make full use of solar radiation and obtain higher photocatalytic activity. This will be one of the most important goals and work must be performed to find and improve the excellent and effective mechanisms using sillenite crystals that can benefit humanity in the future.

Comparative Study of Some Sillenite Catalysts
Based on our recent research that dealt with the characterization and application of the sillenites Bi12TiO20 (BTO), Bi12CoO20 (BCO), Bi12ZnO20 (BZO), and Bi12NiO19 (BNO) for environmental applications, it is now feasible to compare their efficiency in terms of pollution removal. All those sillenites were synthesized by the sol-gel method, and the detailed method for each material can be found in the following works [25,34,58,70]. Cefixime (CFX) was chosen as the target pollutant since it was used in most of those works. A comparative test was conducted using the sillenites BTO, BCO, BZO, and BNO in order to remove CFX antibiotics from water. The photocatalytic tests were carried out in a double-walled stirred reactor (batch). The photocatalytic conditions were chosen based on past research [25,70], with a concentration CCFX = 10 mg/L, pH ~ 6, T = 25 °C, and dose of

Comparative Study of Some Sillenite Catalysts
Based on our recent research that dealt with the characterization and application of the sillenites Bi 12 TiO 20 (BTO), Bi 12 CoO 20 (BCO), Bi 12 ZnO 20 (BZO), and Bi 12 NiO 19 (BNO) for environmental applications, it is now feasible to compare their efficiency in terms of pollution removal. All those sillenites were synthesized by the sol-gel method, and the detailed method for each material can be found in the following works [25,34,58,70]. Cefixime (CFX) was chosen as the target pollutant since it was used in most of those works. A comparative test was conducted using the sillenites BTO, BCO, BZO, and BNO in order to remove CFX antibiotics from water. The photocatalytic tests were carried out in a double-walled stirred reactor (batch). The photocatalytic conditions were chosen based on past research [25,70], with a concentration C CFX = 10 mg/L, pH~6, T = 25 • C, and dose of catalyst = 1 g/L. Before exposing those tests to irradiation sources (UV light), all sets were combined in the dark for 120 min to avoid the adsorption effect and establish the adsorption-desorption equilibrium. Figure 3 demonstrated the CFX degradation tests using those sillenites under the same conditions. As observed, CFX degradation using BZO and BTO as catalysts achieved almost relative efficiency of 94.34% and 88.07%, respectively, within 180 min, while this was lower for the others. The sillenite BCO has also shown a good degradation performance, giving 71.23% within 180 min. However, the sillenite BNO has shown a very low degradation efficiency for CFX, although it has given a very efficient degradation for Basic blue 41. The low efficiency of the sillenite BNO may be explained by the fact that BNO photodegradation can be enhanced in the basic condition [58]; however, CFX photodegradation is favored in the neutral medium of approximately 6 [25]; therefore, the two are incompatible. This phenomenon may be due to the surface charge of both the catalyst and the pollutant. The differences in the efficiency of each sillenite may be due to the color and gap energy of the catalysts, which provides a higher level of light absorption for both BTO and BZO in the range of 200-600 nm, which is very close to the UV region. The degree of crystallinity and the purity of the phase also play a major role in determining the efficiency of the catalyst. In the end, BZO proved to be the most efficient catalyst for the removal of CFX, with a degradation rate of 94.34% in just 3 h. charge of both the catalyst and the pollutant. The differences in the efficiency of each sillenite may be due to the color and gap energy of the catalysts, which provides a higher level of light absorption for both BTO and BZO in the range of 200-600 nm, which is very close to the UV region. The degree of crystallinity and the purity of the phase also play a major role in determining the efficiency of the catalyst. In the end, BZO proved to be the most efficient catalyst for the removal of CFX, with a degradation rate of 94.34% in just 3 h.

Combining of Sillenite with Other Processes
Several strategies can be utilized to improve the photocatalytic activity of these interesting sillenites, such as ion doping, the presence of other reactive species, the addition of H2O2, hybrid systems, and morphology modification ( Figure 4). However, only a few studies of sillenites as photocatalysts coupled with other strong oxidants have been published, leaving a gap in our knowledge of these important materials. This section will address some of the available references in this field.

Combining of Sillenite with Other Processes
Several strategies can be utilized to improve the photocatalytic activity of these interesting sillenites, such as ion doping, the presence of other reactive species, the addition of H 2 O 2 , hybrid systems, and morphology modification ( Figure 4). However, only a few studies of sillenites as photocatalysts coupled with other strong oxidants have been published, leaving a gap in our knowledge of these important materials. This section will address some of the available references in this field.  According to the work of Liu et al. 2018 [81], a hydrothermal technique was used to create the sillenite bismuth ferrite (S-BFO) Bi 25 FeO 40 , which was then used to activate peroxymonosulfate (PMS) in the degradation of aquatic levofloxacin. It was confirmed that a decay mechanism could proceed as follows: (i) Bi 5+ is replaced by Bi 3+ , resulting in the formation of an oxygen vacancy in the lattice and active oxygen (O * ), which could produce reactive species such as SO 4 * through reactions with PMS. This study demonstrates that the photocatalytic activity of the Bi 25 FeO 40 catalyst in the presence of PMS as a source of the reactive species could be adopted alternatively in the decontamination of surface/groundwater. Moreover, from the work of Naghmash et al., 2021 [82], it was shown that the addition of hydrogen peroxide (H 2 O 2 ) with the sillenite Bi 12 SiO 20 (BS) and the doped sillenite M-Bi 12 SiO 20 with transition metal ions (M = Mn, Cu, Co, Ni or Fe) as catalysts allow for chemical decomposition to form reactive species such as OH * . The performance of the Bi 12 SiO 20 catalyst was markedly improved with the addition of H 2 O 2 .
It was also proven that the doping of Bi 12 MO 20 (M = Ge, Si, Ti) crystals with elements in different valence states has a significant effect on their optical, photoelectric, and acoustic properties [83]. Another study on Bi 12 ZnO 20 demonstrated that doping the sillenites with an ion such as Ag can increase the electronic life (photosensitization), which means a higher recombination rate of the photogenerated electron-hole pairs [70].

Summary and Outlooks
This review discusses the effects of advances in nanoscience and photocatalysis for present and future applications, focusing on bismuth sillenite crystals that have attracted much attention as photocatalysts in water photocatalytic treatment in recent years due to their excellent properties. Recent studies on the use of Bi-based sillenites for water treatment have been compiled and discussed; they exhibited efficient photocatalytic performances for a large type of pollutants, including organic pollutants such as antibiotics and inorganic contaminants such as heavy metals. The features of bismuth-based semiconductors have been reported, as well as their advantages in the photocatalytic process. Bismuth-based semiconductors have features due to the properties of Bi 3+ ions, which include (i) the d10 configuration in the lattices of the oxide, (ii) the 6s 2 orbital lone pair, and (iii) the orbital hybridization between O 2p and Bi 6s in the valence band of the semiconductor. The advantages that the Bi 3+ ions provide in terms of photocatalytic activity are (i) the decrease in the recombination of photo-generated charges, which facilities the interfacial charge transfer, (ii) the production of an internal polar electric field in crystallites, which increases the probability of charge separation due to the opposing movements of electron-hole pairs in the electric field, and (iii) the narrowing of the band gap. The advantages of the sillenite structure were also discussed. They are (i) the unique structures of the crystal, (ii) high photorefractive ability, (iii) fast response, (iv) photochromic, dielectric, and electro-optic properties, (v) long optical storage time in the dark, (vi) recyclability, and (vii) the narrow band gap, which is less than 2.9 eV. A critical point concerning certain bismuth catalysts in the literature has also been reviewed; they reported bismuth spinel, which was found to be different from that reported and conformed to the form of sillenite. Finally, the effectiveness of certain sillenites for environmental applications has been compared, and it has been demonstrated that the activity of sillenites varies depending on the metal from which they were produced. Bi 12 ZnO 20 proved to be the most efficient catalyst because the Zn ions enhanced the photocatalytic activity. Although there are reports on the photocatalytic activity of these materials, detailed studies on these compounds are limited. More efforts are needed to understand the properties and relationships of such bismuth sillenites-based compounds and their optimization of photocatalytic activity. Understanding the link between this research, especially concerning photocatalytic activity, will aid in the development of more effective photocatalysts. Up to now, there has been a great number of works that have dealt with bismuth sillenites as photocatalysts in photocatalysis applications; however, those works focused not only on organic pollutants such as antibiotics and dyes and inorganic contaminants such as heavy metals but also on the possibility of using sillenite in the process of hydrogen generation to become an effective alternative resource for green energy. More efforts are required for the practical application in other uses, such as disinfecting water from bacteria and viruses. Based on this updated literature, bismuth sillenite crystals appear to be promising photocatalysts for water treatment, particularly for degrading and reducing organic and inorganic contaminants.