Water treatment and energy production are two critical concerns in ensuring a healthy and sustainable environment, and their demand has soared in recent years [1,2]. The United Nations estimates that a quarter of the worldwide people are suffering from a water shortage due to economic reasons, and by 2030, the water demand will have significantly increased [3,4]. Wastewater reuse and recycling are proposed as possible means of ensuring a reliable supply of fresh water; however, traditional wastewater treatment systems face substantial limitations due to their high energy consumption and operational costs [5,6]. It has been demonstrated that the energy consumed for the treatment process amounts to approximately 3% of the world's energy consumption and is expected to almost double in the coming years [7,8]. This makes energy supply another worldwide issue, along with the problem of water scarcity. The increase of carbon dioxide levels in the atmosphere, along with the depletion of fossil fuel supplies, generate also severe worries about the consequences for the global climate and future energy supply [9]. Finding alternative fuels with high energy density and carbon-free emissions such as hydrogen (H2) and ammonia (NH3) would be a potential solution. To meet those issues, nexus thinking is necessary [10,11]. This is an approach to natural resource management that emphasizes interconnection instead of an isolated system [12,13]. The water-energy nexus is always being applied from two perspectives: energy consumption by water systems and water usage by energy systems [14]. Numerous recent studies have demonstrated the viability of sustaining the water-energy nexus, in which certain water treatment procedures can generate different energy types that can be leveraged to reduce treatment expenses and the damage of fossil fuels [15]. Moreover, effective treatment can result in lower energy consumption and higher energy production, particularly when renewable energy sources are employed [13,16].

There has been an increase in interest in advanced treatment systems that allow simultaneous wastewater cleaning and the production of energy, especially when that energy is ecologically benign and sustainable [17]. It is critical to invest in renewable energy sources as well as treatment techniques that safeguard the environment and decrease the danger of pollution [18]. Water treatment techniques that use solar energy and fuel are appealing forms of sustainable treatment and energy production owing to their efficacy and long durability [19]. Photocatalysis one of the advanced processes has acquired a lot of attention as an environmentally friendly and green approach for water treatment and energy production [20]. That is because it uses solar energy and that has a significant impact by reducing energy consumption and concurrently reducing the effects of anthropogenic pollutants [21,22]. Photocatalysis has been extensively investigated in diverse fields and areas, including water splitting [23], solar energy conversion [24], water/air purification [25], CO2 reduction [26], N2 fixation [27] and H2 production [28]. Photocatalysis using heterogeneous catalysts based on semiconductors has received a lot of interest in recent years, notably in water and energy applications because it is effective, simple to conduct, and used as a powder (suspension) that is simpler to separate from water [29,30]. Semiconductor materials qualified for heterogeneous photocatalysis should have essential qualities such as optical absorption, electronic structure, and the ability to create charge carriers when powered by a light source by converting light energy into chemical reactions [[31], [32], [33]]. Despite the development of highly various metal oxide semiconductors such as TiO2 and ZnO, their practical applicability is limited by their low irradiation sensitivity, high photo-electron pair recombination rate, and large band gap [[34], [35], [36], [37]]. To solve these challenges, novel, efficient, and multifunctional semiconductors are required.

One of the most crucial factors in the development of efficient photocatalysts is the core metal ion of the semiconductors [38]. It has been proved in previous works, that the metal ions of bismuth (Bi), titanium (Ti), tin (Sn), and zinc (Zn) have been classified as the best ions to generate efficient catalysts for numerous photocatalytic applications [39,40]. Bismuth ions have recently drawn a lot of researchers as the metal core of catalysts, due to a variety of unique characteristics, most notably their remarkably high chemical stability, high light absorption, low rate of pair recombination, and narrow band gap [41]. Bismuth sillenites with the chemical formula Bix[A]Oy are current materials with high photocatalytic potential, in which Bi12[A]O20 has been most widely investigated [42]. Sillenites have lately intrigued researchers owing to their nonlinear optical characteristics and photocatalytic capabilities [41,43]. They are regarded as attractive materials for a variety of potentially promising applications, including photocatalysis and other photonic domains, due to their combination of strong nonlinear characteristics, the relative simplicity of manufacture, and inexpensive production cost [44,45]. It is true that bismuth ions are harmful and dangerous in their ionic form, particularly in the nitrate form; nevertheless, by using their ions to produce chemically stable catalysts, their risk can be reduced, as a previous paper demonstrated that synthesizing sillenites using bismuth ion reduces their risk [40]. This provides additional benefit to sillenite by improving waste management by lowering harmful ions while also providing valuable and useful components that may be used in a variety of practical applications.

Although bismuth sillenites have been synthesized and studied in many recent papers, the majority of those investigations focused exclusively on the characterizations and efficacy of those materials in photocatalytic applications, without a detailed discussion of their interesting properties in comparison to other catalysts, which left a lack of understanding of this material class. The different properties or synthesis techniques of those intriguing materials have not been yet the subject of a systematic review. Additionally, there has only been a cursory investigation of the use of sillenite photocatalysts for water filtration, water splitting, CO2 abatement, and N2 fixation. Furthermore, there has never been a thorough analysis of the numerous approaches to enhancing sillenite-type photocatalysts. Only a short critical review has dealt with sillenite-type materials for some photocatalytic applications, and it has concentrated on the stoichiometry “Bi12MO20” [33]. For this purpose, this research paper aims to comprehensively and thoroughly review sillenite crystals as an interesting material type. Various procedures for the synthesis of sillenites, their advantages and conditions have been described. The structure, electronic properties as well as optical characteristics of sillenites have also been studied, along with a detailed discussion of the potential of those sillenite catalysts for photocatalytic applications. Furthermore, current advances in designing sillenite-based materials to enhance the efficiency of those materials based on the literature have been summarized and analyzed, to elucidate the advantages of various methods in the improvement of the proprieties and efficiency of those materials. In the end, this study has concluded with perspectives, in which general opinions have been proposed.