Bismuth-Graphene Nanohybrids: Synthesis, Reaction Mechanisms, and Photocatalytic Applications—A Review

: Photocatalysis is a classical solution to energy conversion and environmental pollution control problems. In photocatalysis, the development and exploration of new visible light catalysts and their synthesis and modiﬁcation strategies are crucial. It is also essential to understand the mechanism of these reactions in the various reaction media. Recently, bismuth and graphene’s unique geometrical and electronic properties have attracted considerable attention in photocatalysis. This review summarizes bismuth-graphene nanohybrids’ synthetic processes with various design considerations, fundamental mechanisms of action, heterogeneous photocatalysis, beneﬁts, and challenges. Some key applications in energy conversion and environmental pollution control are discussed, such as CO 2 reduction, water splitting, pollutant degradation, disinfection, and organic transformations. The detailed perspective of bismuth-graphene nanohybrids’ applications in various research ﬁelds presented herein should be of equal interest to academic and industrial scientists. A BiPO 4 NPs with MoS 2 /graphene-layered hybrid is manufactured via an easy hydrothermal, microwave-assisted method, and the ternary BiPO 4 -MoS 2 /graphene photocatalyst optimizes the activity of each component. This study demonstrates that the graphene and MoS 2 nanoparticles as catalysts in the photocatalyst of BiPO 4 can improve transport charges, eliminate the pair electron hole’s photogenerated recombination rate, and provide highly reactive locations for a photodegradation reaction. This results in signiﬁcantly improved photocatalytic activity for organic pollutant photodegradation by the attained BiPO 4 -MoS 2 /graphene photocatalyst. The GO, BiPO 4 , and MoS 2 composite microstructure and morphology were characterized in the sense that GO has a layered stacking structure with some folds and wrinkles that can adsorb and photodegrade the color molecules on sufﬁciently large surfaces. The sample produced for MoS 2 has an ultra-free nanosheet structure. In composites many BiPO 4 NPs are dispersed compactly and homoge-nously on the surface of MoS 2 /graphene nanosheets. It is proposed to dispense, build, and attach the BiPO 4 nanocrystals in MoS 2 /graphene by microwave-assisted techniques. There are distinct gate fringes on the BiPO 4 -MoS 2 /graphene composite. The gap from 0.328 nm to the monoclinic plane BiPO 4 (200) corresponds very well, while the gap from 0.62 nm to the plane of (002) MOS 2 can be assigned. The presence of close contact between MoS 2 /graphene nanosheets and BiPO 4 NPs is predictable for building a necessary hetero structure [180]. BiPO 4 /rGO NCs were successfully synthesized by a simple solvothermal method. This composite possessed much advanced and best photocurrent performance. The as-prepared PEC sensor revealed a broader lower detection limit, linear range, and an excellent anti-interference capacity. In the formation of chlorpyrifos,


Introduction
The increase in pollution due to urbanization and industrialization has become a significant challenge for the sustainability of human society. The waste generated in different industries during crude oil storage, transportation, and refinery has become a global problem [1,2]. The water and soil pollution caused by several pollutants' discharge is a critical public health concern due to their toxicity. These pollutants can cause many health effects such as neurological toxicity, lung cancer, lethargy, fatigue, depression, headaches, nausea, dizziness, throat and eye irritation, and acute and chronic respiratory effects [3]. Toluene, benzene, xylene, ethyl benzene, and phenolic compounds some of the main compounds categorized as pollutants posing severe threats to our environment [4][5][6]. In the present situation, environmental pollution has increased several-fold due to the mismanagement of industrial waste. This can negatively affect the ecosystem and make lands unusable for agriculture and many other purposes [7]. Therefore, it is essential to remediate these toxic pollutants in our environment [8][9][10].
To eliminate organic pollutants from the environment, numerous technologies have recently been established for their degradation. Organic pollutants can be degraded by different methods, such as physical, chemical, biological treatments and advanced oxidation techniques [9,[11][12][13][14][15]. Organic pollutant photodegradation is an attractive "green" chemical Brazil 57 14 United Kingdom 48 Graphene is the basic structure of all other carbon allotropes. It is well noted that the potential applications of graphene its derivatives are mainly driven by progressive production of different graphene materials such as graphene oxide (GO), reduced graphene oxide (rGO), functionalized graphene oxide (fGO), and functionalized reduced graphene oxide (frGO) with specific attention to precise applications and this is expected to continue for at least a couple of decades as promising applications and requirements are disclosed [50,51]. Various literature reports on the synthesis, modification and application of photocatalysts based on graphene for energy and environment solutions have already been published [52]. Graphene, graphene and its derivatives [53,54], graphene in photocatalysis [55], graphene doping [56], graphene and graphene oxide sponge [57], nitrogen-doped graphene [58], structure of graphene and its disorders [59], strain engineering of graphene [60], mechanics of graphene nanocomposites [61], chemical vapor deposition of graphene [62], functional modification of graphene/graphene oxide [63], graphene-based fibers [64], and graphene-based electrochemical micro-supercapacitors [65] are some of the subjects that have been reviewed.
Considering the stability, reactivity, reusability, and light-responsive effect of bismuth (Bi) it has been widely used as a photocatalyst. Several state-of-the-art review articles on topics including barium potassium bismuth oxide [66], bismuth-based composite oxides [67], bismuth ferrite nanoparticles [68], bismuth vanadate-based materials [69], bismuth tungstate photocatalysts [70], and bismuth oxyhalides [71] have been published. Annual numbers of publications on graphene photocatalysts in the last ten years are shown in Figure 1a. Similarly, bismuth-containing compounds are significant photocatalysts that react to visible light and fascinating research has been published in the field of bismuth photocatalysis over the last ten years (Figure 1b). This review, therefore, summarizes and discusses recent Bi-graphene photocatalysts and their energy and environmental sector applications. The choice of bismuth with graphene is due to the vast available literature, as shown in Figure 1. Furthermore, most bismuth-based photocatalysts are stable, reusable, photoactive, cheaper, and more environmentally friendly that other alternatives. Besides, due to some shortcomings of the pristine photocatalysts, such as charge carrier recombination, slow migration of charge carriers, and low visible light absorption [72,73], we discuss modification of graphene with bismuth species to produce improved photocatalysts for practical applications.
The discussion above highlights the vital roles of graphene, bismuth, and nanohybrids. Structural, chemical synthesis and mechanistic aspects of these nanohybrids are discussed, as are the suggested industrial applications of Bi-graphene. Recent literature on energy conversion, degradation of various pollutants, and the CO 2 conversion process has been This review, therefore, summarizes and discusses recent Bi-graphene photocatalysts and their energy and environmental sector applications. The choice of bismuth with graphene is due to the vast available literature, as shown in Figure 1. Furthermore, most bismuth-based photocatalysts are stable, reusable, photoactive, cheaper, and more environmentally friendly that other alternatives. Besides, due to some shortcomings of the pristine photocatalysts, such as charge carrier recombination, slow migration of charge carriers, and low visible light absorption [72,73], we discuss modification of graphene with bismuth species to produce improved photocatalysts for practical applications.
The discussion above highlights the vital roles of graphene, bismuth, and nanohybrids. Structural, chemical synthesis and mechanistic aspects of these nanohybrids are discussed, as are the suggested industrial applications of Bi-graphene. Recent literature on energy conversion, degradation of various pollutants, and the CO2 conversion process has been overviewed. Finally, the challenges associated with bismuth and graphene and possible solutions have been discussed.

Bi2O3 and Bi2S3/Graphene Composites
A significant and the simplest bismuth compound is bismuth trioxide (Bi2O3). It can be used in various ceramics, fuel cells, and gas sensors [74,75]. It has also been used as a photocatalyst in organic pollutant decomposition and water splitting [76]. Bi2O3 is a visible-light-responding photocatalyst when acting as a semiconductor, and its bandgap ranges between 2.1 eV and 2.8 eV. Doping with noble materials and combination with other components have been used to increase graphene's activity in photocatalytic (PC) form [77,78].
In recent times, the PC activity of some Bi-based semiconductors, e.g., BiVO4 [79], Bi2MoO6 [80,81], BiOX (X = Cl, Br, I), Bi2Sn2O7 [82], Bi2O3 [83], and BiSbO4 [84] in the degradation of pollutants has been described. Bismuth oxide was shown to be a strong candidate among the various Bi-based semiconductors because of its good PC and appropriate bandgap properties. Bi2O3's PC activity is however restricted by quick recombination of the photogenerated carriers and by its susceptibility to photocorrosion. Because of the short distance between the conduction band (CB) of Bi2O3 and the valence band (VB), graphene can be designed for the sharing of Bi2O3 and graphene [85]. Under such conditions, electrons generated in the CB of Bi2O3 would quickly be coupled with graphene VB holes

Bi 2 O 3 and Bi 2 S 3 /Graphene Composites
A significant and the simplest bismuth compound is bismuth trioxide (Bi 2 O 3 ). It can be used in various ceramics, fuel cells, and gas sensors [74,75]. It has also been used as a photocatalyst in organic pollutant decomposition and water splitting [76]. Bi 2 O 3 is a visible-light-responding photocatalyst when acting as a semiconductor, and its bandgap ranges between 2.1 eV and 2.8 eV. Doping with noble materials and combination with other components have been used to increase graphene's activity in photocatalytic (PC) form [77,78].
In recent times, the PC activity of some Bi-based semiconductors, e.g., BiVO 4 [79], Bi 2 MoO 6 [80,81], BiOX (X = Cl, Br, I), Bi 2 Sn 2 O 7 [82], Bi 2 O 3 [83], and BiSbO 4 [84] in the degradation of pollutants has been described. Bismuth oxide was shown to be a strong candidate among the various Bi-based semiconductors because of its good PC and appropriate bandgap properties. Bi 2 O 3 's PC activity is however restricted by quick recombination of the photogenerated carriers and by its susceptibility to photocorrosion. Because of the short distance between the conduction band (CB) of Bi 2 O 3 and the valence band (VB), graphene can be designed for the sharing of Bi 2 O 3 and graphene [85]. Under such conditions, electrons generated in the CB of Bi 2 O 3 would quickly be coupled with graphene VB holes [86]. Therefore, the photogenerated electrons accumulated on the CB of graphene display strong reduction ability, and the photogenerated holes on the VB of Bi 2 O 3, exhibit excellent oxidation ability [87,88]. The Z-Scheme PC activities are more effective than one component in terms of reduction and oxidation and advanced photocatalytic performance in the traditional photocatalysts [89,90]. Cui has reported a novel Z-scheme Bi 2 O 3 /graphene photocatalyst. Bi 2 S 3 has a 1.7 eV bandgap and is a perfect photocatalytic material for light-harvesting due to its near-IR and visible light activation [91]. A number of Bi 2 S 3 nanocrystal forms ranging from 1D nanorods and 2D nanosheets have been created with hot injection and standard non-oxidation techniques [92,93], while a solvothermal method produces 3D sea-urchin-like spheres [94].
Bismuth sulfide (Bi 2 S 3 ) is a priviledged nontoxic inorganic semiconductor with excellent photocatalytic activity and chemical stability because of its good visible light response. It has been exploited and investigated mostly for optoelectronic applications. The photogen-  [92]. In combination with many other photocatalysts such as CdS [95], TiO 2 [28,96], and Bi 2 WO 6 [97], the recombination rate of electron-hole pairs could be lowered. An increase in visible light absorption enhances the photocatalytic activity.
A graphene/Bi 2 S 3 nanocomposite with narrow bandwidth was recently synthesized. Compared with the individual components, the PC of this nanocomposite was much higher. Zhou et al. stated that the well-matched bandgap of graphene/Bi 2 S 3 heterojunction could be tailored to increase the transfer and separation efficiency of photoinduced carriers and the visible light response. These graphene/Bi 2 S 3 composites are effective photocatalysts for the photocatalytic degradation of environmental pollutants [74].
2.2. Bi 2 MO 6 (M = Cr, Mo, W)/Graphene Composites Bi 2 MO 6 (M = Mo, Cr, W) is considered the most common member of the Aurivillius family, Bi 2 A n−1 B n O n+3 (A = Sr, Ca, Ba, Bi, Pb, K, Na; B = Nb, Ti, Ta, Fe, W, Mo) is the general formula for Bi 2 MO 6 . The Bi 2 MO 6 electronic structure is theoretically based on density functional theory (DFT) [98], while the Bi 2 MO 6 crystal structure falls under orthorhombic space group Pca2(1). It was seen that both VB and CB of Bi 2 MO 6 are composed of hybridized orbitals Bi 6 p, O 2 p, and M n d (n = 3, 4, and 5) for Bi 2 CrO 6 , Bi 2 MoO 6, and Bi 2 MO 6 , respectively [99]. Bi 2 MO 6 compounds are suitable as visible-light-activated photocatalysts. Among all Bi 2 MO 6 species Bi 2 CrO 6 has a narrower bandgap, thus, it easily undergoes recombination of photogenerated holes and electrons and is thus not considered suitable as a photocatalyst and consequently few Bi 2 CrO 6 studies are available in the field of photocatalysis. For the preparation of Bi 2 MoO 6 samples with a wider special surface area, smaller particles, and higher photocatalytic function, the solvothermal and hydrothermal methods are effective. Several Bi 2 MoO 6 morphologies have been described, including floral hollow spheres (solvothermal process) and nanoplates (hydrothermal method). Moreover, microwave heating was applied to synthesize Bi 2 MoO 6 samples with high photocatalytic activity in short periods [100,101]. Major applications of Bi 2 MO 6 (and Bi 2 MoO 6 and Bi 2 WO 6 ) photocatalysts involve the removal of organic pollutants from polluted air and water. The key pollutants that have been tested in different studies include phenol [102], dyes [103], CHCl 3 and CH 3 CHO in wastewater [104], and NO in air [105]. Microorganisms, e.g., E. coli, were also destroyed by the addition of Bi 2 WO 6 [106] and Bi 2 MoO 6 [107] under visible light irradiation.
Current studies reveal the combined effect of plasmonic metals and graphene. The photocatalytic activity of semiconductors, e.g., TiO 2 and ZnO, can be efficiently improved by increasing their photo-absorption ability and suppressing photogenerated electron-hole recombination. Compared to Bi 2 MoO 6 , Bi 2 MoO 6 -graphene binary composites have been developed and show improved photocatalytic performance. Graphene-based nanocomposites display desirable photocatalytic properties that their individual components do not have, therefore, improved Bi 2 MoO 6 photocatalytic activity resulting from a combination of noble metals and graphene is expected. Bi et al. developed a rGO-Bi 2 MoO 6 /Au composite that displayed high catalytic activity for the photodegradation of rhodamine B [20]. Wang and Tian reported composites of GO-Bi 2 MoO 6 and rGO-Bi 2 MoO 6 [108,109]. These composites showed advanced phenol and rhodamine B degradation properties, respectively, compared to Bi 2 MoO 6 alone [74].

BiVO 4 /Graphene Nanocomposites
Bismuth vanadate (BiVO 4 ) presents interesting physicochemical properties, including ionic conductivity and ferroelasticity. A theoretical bandgap of 2.047 eV was calculated by DFT for visible-light-driven photocatalysis [110]. Both O 2 p-and V 3 d-orbitals are included in the BiVO 4 [111]. Tetragonal BiVO 4 has been synthesized at room temperature by a precipitation method [112]. The bandgap for the monoclinic form is 2.4 eV, while the bandgap for BiVO 4 is 2.9 eV. This selective monoclinic BiVO 4 preparation is advantageous for assembling effective photocatalysts with visible light shifts. There has been a report of an additional method for synthesizing monoclinic and tetragonal BiVO 4 crystals in a simple water-based process [113]. A hydrothermal method has been used successfully in recent times for monoclinic BiVO 4 preparation [114]. There are numerous advantages to this hydrothermal approach to selectively produce BiVO 4 structures, i.e., mild experimental conditions, controllable conditions and simple experimental setups.
Photocatalytic degradation under visible light is commonly used to decompose organic pollutants (e.g., phenol and RhB) [115], and increased removal efficiency has been demonstrated [116]. BiVO 4 was also used for the scission of water [117,118]. BiVO 4 was shown to be an active photocatalyst for O 2 evolution under visible light radiation since its conduction strip potential isn't high enough to produce H 2 by H 2 O reduction [119]. Booshehri et al., found BiVO 4 to be a mild candidate for photocatalytic inactivation of bacteria in water under visible light irradiation [120]. For photocatalytic bactericidal activity, surface redox reactions are essential for reactive species generation [121]. In addition, the interface for charge separation and transfer in hybrid catalysts is to be considered for two components [122]. The BiVO 4 /Ag/graphene photocatalyst showed improved activity for photocatalytic degradation of organic pollutants [123,124] or oxidation of nitrogen monoxide and water [125]. The probability of photocatalytic wastewater or disinfection of water by the Z-scheme BiVO 4 /graphene is however still unknown to the best of our knowledge. Moreover, at the molecular level the photocatalysis consistency is clearly not yet investigated [74].  [126,127]. Biox contain X np (n = 2-5 for Cl, F, I, and Br respectively), O 2p, and Bi 6 p-orbitals both in the valence band (VB) and conduction band (CB). In theoretical terms, the bandgaps of BiOI, BiOF, BiOBr, and BiOCl are calculated to be 1.38 eV, 2.79 eV, 1.99 eV, or 2.34 eV, while experimentally, their bandgaps are estimated to be 1.77 eV, 3.64 eV [128], 2.64 eV, and 3.22 eV [129]. There are restrictions within the GGA method that cause these differences between the experimental and calculated bandgap results. However, both indicate the general decreasing tendency of the bandgaps as the atomic number increases. BIOF was used as a photocatalyst only under UV light, while BiOI was photocatalytically active both under near-IR and visible light. Because of their appropriate bandgaps, both BiOCl and BiOBr are therefore commonly tested. For BiOX synthesis with different morphologies, several methods can be effectively applied. In addition to direct precipitation techniques, the primary methods used to synthesize the BiOX with controlled nanostructures such as nanosheets, microsphere, and nanofibers include hydrolysis, solvothermal and hydrothermal methods [130]. By adjusting the precursor pH, controlling hydrothermal treatment duration time and temperature, and by adding a template structure that can be selectively controlled, one can directly affect the photocatalytic performance. An extensive review of BiOX nanostructures was previously published [131].
Significant efforts have been carried out to design innovative photocatalysts [132,133]. Because of their excellent catalytic activity under visible light, the sequence of ternary bismuth oxyhalides (BiOX, X = Cl, Br, or I) has been commonly studied [134]. The charge separation and atomic polarization efficiency of the layered BiOX structures can be improved. BiOBr, with its crystalline PbFCl layer structure has been a big consideration among BiOX photocatalysts because of its excellent photocatalytic activity, appropriate bandgap, and high stability. The binary component and multi-component counterparts showed improved photocatalytic activity compared to single-component semiconductors. Multi-component synergies may overcome the single-component shortcomings, e.g., insufficient charge separation ability and wide-bandgap. Consequently, the BiOBr photocatalytic activity [135,136] with an indirect-transition bandgap (2.75 eV) may be efficiently enhanced by incorporating other materials.
Graphenes are currently used as a promising support platform for anchoring host NPs as well as acceptors for charge separation and superb electron transfer mediation with peculiar characteristics such as low density, high conductivity, and large surface areas [137][138][139]. The hydrothermal method has been used for the synthesis of Au/BiOBr/graphene composites [140,141].
A practical approach to shrink the bandgap, increase the catalytic activity and visiblelight absorption was taken using black BiOCl material with the formation of oxygen vacancies. Although the black BiOCl is still subject to recombination of fast photocatalytic charge carriers, its photocatalytic activity is still not satisfactory. A simple and effective approach to resolve the above-related problems has been taken as the construction hetero-structures of BiOCls with the other appropriate photocatalysts. Thanks to their high electron mobility and a large surface area, the above issues could be well addressed by functional graphenebased semiconductor photocatalysts. A new BiOCl-Bi-Bi 2 O 3 /rGO heterojunction with oxygen vacancies has been developed, which provided a solid-solid, close-fit interface and strong interaction between BiOCl, Bi, rGO, and Bi 2 O 3 . BiOCl-BI 2 O 3 /rGO heterojunctions showed high photocatalytic performance due to the synergistic effect caused by effective charge separation among Bi 2 O 3 , BiOCl, rGO, and Bi-bridges. The BiOCl-Bi-Bi 2 O 3 /rGO heterojunction displayed high efficiency for photocatalytic degradation of 2-nitrophenol in industrial wastewater treatment. The significant task is to demonstrate the superior long-term photostability of the BiOCl-Bi-Bi 2 O 3 /rGO heterojunctions. In addition, a promising BiOCl-Bi-Bi-Bi 2 O 3 /rGO photocatalytic mechanism was proposed to describe primary phenomena taking place during the process, depending on multiple charge transfer channels [141].

BiPO 4 /Graphene Composites
BiPO 4 with high photocatalytic activity for organic pollutant degradation was fabricated for the first time by a hydrothermal approach [142]. A faster hydrothermal way of synthesizing BiPO 4 has also been reported [143]. The bandgap in BiPO 4 prepared by hydrothermal methods is about 3.85 eV, higher than that of TiO 2 (3.2 eV). BiPO 4 nanocrystals synthesized with standard oxygen-free procedures have a bandwidth of around 4.6 eV. Only UV light can be used as a light source for large bandgap semiconductors. Although its bandgap is broader than that of TiO 2 , BiPO 4 still has high photocatalytic degradation kinetics. This is because the VB of BiPO 4 is 3 eV, higher than that of TiO 2, and it generates more oxidative holes in its VB compared to TiO 2 . Photocatalytic conversion of the gas-phase benzene into CO 2 by BiPO 4 has also been reported in addition to the degradation of the organic pollutant in an aqueous phase. A photocatalytic gas-phase transformation of benzene to CO 2 was also reported during an aqueous phase organic pollutant degradation study [144,145]. BiPO 4 photocatalysts still have several drawbacks as photocatalysts however, such as low photocatalytic activity, and comparatively rapid recombination of charge carriers, wide bandgaps, low adsorption ability, and large size, which would decrease the photocatalytic activity of BiPO 4 and subsequently limit its industrial-scale applications [146,147]. Consequently, it is urgent to create and design photocatalytic materials based on BiPO 4 , with required and useful photocatalytic performance properties. To date, numerous efforts have been made to improve the photocatalytic activity of the BiPO 4 photocatalyst by doping with non-metals or metals, surface hybridization, reducing the crystal size, forming heterostructures, or combinations of µ-structure materials [148,149]. BiPO 4 /rGO nanocomposites exposed the importance of graphene as the support of separating electron-hole pairs, which leads to a high photocurrent. Thus, the development of BiPO 4 /rGO hybrids is an efficient way to improve the visible light catalytic performance of BiPO 4 . Extensive research has established a trend towards research in carbon-nanomaterials by doping with heteroatoms as they can adapt their fundamental properties successfully [150,151].

(BiO) 2 CO 3 /Graphene C omposites
Bismuth subcarbonate is a known solid carbonate in the BI 2 O 3 -CO 2 -H 2 O system ((BiO) 2 CO 3 or Bi 2 CO 5 ) [152]. The bandgap of (BiO) 2 CO 3 is 3.4 eV, so wavelengths under 365 nm can therefore stimulate the bandgap [153,154]. The CB of (BiO) 2 CO 3 generally includes hybridized p-orbitals (O 2 p and Bi 6p), while its VB consists of p-orbitals (O 2p, Bi 6p, and C 2p). A hydrothermal, template-free method has been used to efficiently synthesize (BiO) 2 CO 3 with hollow microsphere orders whose structure is dependent on Ostwald's growing properties. The compound showed photocatalytic activity for pollutant oxidation or disinfection of air and wastewater contamination [155,156]. Several articles have described p-n heterojunctions that exhibited enhanced photocatalytic activity [74,157].
An innovative multi-component TiO 2 -Bi 2 O 3 /(BiO) 2 CO 3 -rGO nanocomposite has been synthesized and experimentally used for bisphenol A (BPA) photodegradation. The Bi 2 O 3 was intended to be a visible light photosensitizer. The appropriate VB and CB's positions TiO 2 and (BiO) 2 CO 3 were used as selective sinks for photogenerated holes and electrons, and rGO acted as a channel for charge carrier transport that extended the lifetime of the catalysts. BPA is an endocrine disruptive compound commonly used for the production of many common packaging materials [158]. These materials typically end up in waste dumps, leading to the slow leaching of BPA into water bodies. Accordingly, BPA has been chosen as the model for the photocatalytic activity of the designed photocatalysts based on environmental issues [158,159].  [162], NaBiO 3 2.6 eV [163], and AgBiO 3 2.5 eV, respectively [164]. The valency of Bi-based composites is +3 while the value of (BiO 3 ) n is +5. The Bi 3+ cation consists of two orbitals (10 d and 6 s). This indicates that the electronic structure of pentavalent bismuthates is different. Takei et al. tested nine bismuthates for degrading phenol and methylene blue [161]. High photo-catalytical activity under visible light irradiation was shown by NaBiO 3 , LiBiO 3 , BaBi 2 O 6, and SrBi 2 O 6 . The d-electrons from Zn, Pb and Ag produce a large conduction range as well as consequently poor photocatalytic performance. Electronic systems greatly affect catalytic performance. Excellent visible-light photocatalytic activity recommends pentavalent bismuthates for different photocatalytic applications. M(BiO 3 )n could be used for efficient visible light photocatalytic degradation of organic pollutants [161,165].

Synthesis of Bismuth/Graphene Nanohybrid Materials
In composites based on bismuth graphene, the graphene acts as a substrate for immobilization and the other composites as a functional component. The robust conductive structure and wide graphene surfaces often facilitate the redox reaction, charge transfer, and the enforcement of the resulting composites' mechanical strengths. The coupling of metal oxides with graphene will therefore enhance the efficiency for numerous energy conversion, storage, and catalytic reactions [166,167]. This section mainly focused on the recent progress to develop practical approaches to fabricate Bi-graphene nanocomposites.

Sol-Gel Method
In this section, we focus on recent progress in the development of practical approaches for the fabrication of Bi-graphene nanocomposites [168][169][170]. The robust coupling offers Energies 2021, 14, 2281 9 of 36 many applications for hybrids, such as photocatalysis [171][172][173]. Anchoring and reactive areas for growth and the nucleation of NPs can be found in functional groups based on reduced graphene oxides (GO/rGO), which allow metal oxide nanostructures to be chemically attached to GO/RGO surfaces.
A new sol gel-based electro-spinning process configuration was adopted for the fabrication of TiO 2 /ZnO/Bi 2 O 3 -Gr (TZB-Gr) composites photocatalyst. With this technique, the rim effect was removed by rolling graphene into 'spiral rolls' implanted in TiO 2 /ZnO/Bi 2 O 3 (TZB) nanofibers, which allowed free electrons to move in the axis of nanofibers on the graphene rolls unidirectional [174]. This new configuration significantly reduced the energy bandgap, enhanced the specific surface area, accelerated charge transport and delayed electron-hole pair recombination. In this unique configuration, the electrons' mobility and lifetime were enhanced [175]. The scheme of TZB-Gr nanofibers is shown in Figure 2. metal oxides with graphene will therefore enhance the efficiency for numerous energy conversion, storage, and catalytic reactions [166,167]. This section mainly focused on the recent progress to develop practical approaches to fabricate Bi-graphene nanocomposites.

Sol-gel Method
In this section, we focus on recent progress in the development of practical approaches for the fabrication of Bi-graphene nanocomposites [168][169][170]. The robust coupling offers many applications for hybrids, such as photocatalysis [171][172][173]. Anchoring and reactive areas for growth and the nucleation of NPs can be found in functional groups based on reduced graphene oxides (GO/rGO), which allow metal oxide nanostructures to be chemically attached to GO/RGO surfaces.
A new sol gel-based electro-spinning process configuration was adopted for the fabrication of TiO2/ZnO/Bi2O3 -Gr (TZB-Gr) composites photocatalyst. With this technique, the rim effect was removed by rolling graphene into 'spiral rolls' implanted in TiO2/ZnO/Bi2O3 (TZB) nanofibers, which allowed free electrons to move in the axis of nanofibers on the graphene rolls unidirectional [174]. This new configuration significantly reduced the energy bandgap, enhanced the specific surface area, accelerated charge transport and delayed electron-hole pair recombination. In this unique configuration, the electrons' mobility and lifetime were enhanced [175]. The scheme of TZB-Gr nanofibers is shown in Figure 2.

Hydrothermal/Solvothermal Methods
Hydrothermal/solvothermal methods are key tools for synthesizing inorganic nanocrystals that work at a high temperature in a limited volume under high pressure. With a one-pot hydrothermal/solvothermal approach, highly crystalline nanostructures can be prepared without post-synthetic calcination, and at the same time GO is reduced to rGO. The rational design of nanomaterials and fabrication with distinctive morphology has received a great deal of consideration because the material properties depend not only on the chemical phase and its composition, but also on its size and shape. The synthesis of nanomaterials with different sizes has inspired many researchers due to its potential applications and the size-dependent properties [176]. Consequently, numerous approaches have been developed to make nanocrystals with controlled morphology. Among them, the hydrothermal method is considered to be effective because it is useful for controlling the size and shape of nanomaterials [177]. rGO/Bi 2 MoO 6 nanocomposites are effectively synthesized via a simple hydrothermal process, with virtual uniformity and high-order direction. The rGO had also been added to the surface of Bi 2 MoO 6 . There is an extraordinary improvement in the photocatalytic activity for bacterial treatment over the Bi 2 MoO 6 -rGO nanocomposite compared to the pure Bi 2 MoO 6 . This enhancement is accredited to the high orientation of Bi 2 MoO 6 , which efficiently improved photogenerated electrons-holes pair's separation.
At the generation site, these electrons are quickly inserted into graphene, thus reducing charge recombination. Improved visible light catalytic wastewater treatment performance of Bi 2 MoO 6 -rGO nanocomposites can be accomplished [107].
The Bi 2 MoO 6 microsphere surface contains different sizes of Ag 3 PO 4 particles. The Bi 2 MoO 6 and Ag 3 PO 4 microspheres on both sides of the layer rGO are also well connected. The Ag 3 PO 4 /rGO/Bi 2 MoO 6 structure can be established with a closed interface, which is beneficial during the photocatalytic process to accelerate charge transfer. The appropriate porous structures and storage surface can offer substantial active surface sites to easily absorb more organic pollutants, which would favor an increase in the photocatalytic activity of the Ag 3 PO 4 /rGO/Bi 2 MoO 6 composite [178]. Ag 3 PO 4 /rGO/Bi 2 MoO 6 shows the broadest absorption edge and the highest absorption intensity in the visible light region. This suggests that this ternary composite can absorb a broad spectrum of visible light [179]. Figure 3 describes the synthesis process for Ag 3 PO 4 /rGO/Bi 2 MoO 6 nanohybrid, a photocatalytic mechanism for MB-degradation via Ag 3 PO 4 /rGO/Bi 2 MoO 6 nanohybrid, and energy band structures of Ag 3 PO 4 and Bi 2 MoO 6 .  The full GO reduction to graphene, the formation of BiPO4-nanorods, and appropriate mixing are carried out in a one-stage synthetic route using these two materials. As an essential agent for GO reduction, ethylene glycol (EG) plays an important role and does not require any additional agents. Besides, ethylene glycol is compatible with BiPO4 nanorod preparation. BiPO4-2% rGO is far more photocatalytic than pure BiPO4, and graphene for photodegradation of methyl orange under UV radiation is accredited to a wider surface area, efficient cargo transportation, the graphene introduction, and the close interfacial contact between graphene and BiPO4 have contributed to a much-increased adsorption and separation capacity [182,183]. BiPO4/rGO and BiPO4/GO composites synthesized, and simulated images are shown in Figure 4. For BiPO 4 and graphene composite formation, two approaches are used. Because two-dimensional high-surface graphene platforms and exceptionally high conductivity can properly contact the target pollutants to provide plenty of reactive sites and efficiently accelerate the process of transferring photo-induced electrons from photocatalyst to reactant sites to suppress the photo-induced pair of electron-holes, graphene and nanocomposite integration with the appropriate graphene and BiPO 4 may have desirable graphene and BiPO 4 properties. This will significantly improve the photocatalytic activity of the BiPO 4 system. The two-step method of preparing the BiPO 4 /GO nanocomposites was first used to synthesize oleylamine-coated BiPO 4 and then assemble it onto a GO nanosheet at the water/toluene interface in the second step [180,181].
A two-step hydrothermal approach was used to synthesize BiPO 4 /rGO cuboids with low OH-related defects. Although nanocomposites are produced successfully with BiPO 4 -GO or BiPO 4 /rGO, the experiments still display a large number of inconveniences: (1) BiPO 4 /GO or BiPO 4 -rGO nanocomposite synthesis requires two or several steps that are tedious and time-consuming; (2) toxic organic solvents (toluene), hazardous reducing agents (oleylamine) and other additives may cause many environmental protection problems and in the product post-treatment; (3) the weak interaction between graphene nanosheets and BiPO 4 results from 2-or multi-step synthetic routes to BiPO 4 /rGO, so a simple, efficient, and green approach has been used to synthesize nanocomposites.
The full GO reduction to graphene, the formation of BiPO 4 -nanorods, and appropriate mixing are carried out in a one-stage synthetic route using these two materials. As an essential agent for GO reduction, ethylene glycol (EG) plays an important role and does not require any additional agents. Besides, ethylene glycol is compatible with BiPO 4 nanorod preparation. BiPO 4 -2% rGO is far more photocatalytic than pure BiPO 4 , and graphene for photodegradation of methyl orange under UV radiation is accredited to a wider surface area, efficient cargo transportation, the graphene introduction, and the close interfacial contact between graphene and BiPO 4 have contributed to a much-increased adsorption and separation capacity [182,183]. BiPO 4 /rGO and BiPO 4 /GO composites synthesized, and simulated images are shown in Figure 4.  A BiPO4 NPs with MoS2/graphene-layered hybrid is manufactured via an easy hydrothermal, microwave-assisted method, and the ternary BiPO4-MoS2/graphene photocatalyst optimizes the activity of each component. This study demonstrates that the graphene and MoS2 nanoparticles as catalysts in the photocatalyst of BiPO4 can improve transport charges, eliminate the pair electron hole's photogenerated recombination rate, and provide highly reactive locations for a photodegradation reaction. This results in significantly improved photocatalytic activity for organic pollutant photodegradation by the attained BiPO4-MoS2/graphene photocatalyst. The GO, BiPO4, and MoS2 composite microstructure and morphology were characterized in the sense that GO has a layered stacking structure with some folds and wrinkles that can adsorb and photodegrade the color mol- A simple one-pot hydrothermal route was used to synthesize nanocomposites of biPO 4 /nitrogen-doped graphene hydrogel (BiPO 4 ) to serve as a visible light-responsive material. The porous 3DNGH structure significantly enhanced the photo-induced electron holes and the transfer and separation efficiency of BiPO 4 visible illumination pairs. The BiPO 4 /3DNGH morphology has disclosed a cross-linked, porous structure, and 3DNGH nanorods are attached to the area. The 3DNGH surface was randomly dispersed with BiPO 4 nanorods [181].
A BiPO 4 NPs with MoS 2 /graphene-layered hybrid is manufactured via an easy hydrothermal, microwave-assisted method, and the ternary BiPO 4 -MoS 2 /graphene photocatalyst optimizes the activity of each component. This study demonstrates that the graphene and MoS 2 nanoparticles as catalysts in the photocatalyst of BiPO 4 can improve transport charges, eliminate the pair electron hole's photogenerated recombination rate, and provide highly reactive locations for a photodegradation reaction. This results in significantly improved photocatalytic activity for organic pollutant photodegradation by the attained BiPO 4 -MoS 2 /graphene photocatalyst. The GO, BiPO 4 , and MoS 2 composite microstructure and morphology were characterized in the sense that GO has a layered stacking structure with some folds and wrinkles that can adsorb and photodegrade the color molecules on sufficiently large surfaces. The sample produced for MoS 2 has an ultra-free nanosheet structure. In composites many BiPO 4 NPs are dispersed compactly and homogenously on the surface of MoS 2 /graphene nanosheets. It is proposed to dispense, build, and attach the BiPO 4 nanocrystals in MoS 2 /graphene by microwave-assisted techniques. There are distinct gate fringes on the BiPO 4 -MoS 2 /graphene composite. The gap from 0.328 nm to the monoclinic plane BiPO 4 (200) corresponds very well, while the gap from 0.62 nm to the plane of (002) MOS 2 can be assigned. The presence of close contact between MoS 2 /graphene nanosheets and BiPO 4 NPs is predictable for building a necessary hetero structure [180]. BiPO 4 /rGO NCs were successfully synthesized by a simple solvothermal method. This composite possessed much advanced and best photocurrent performance. The as-prepared PEC sensor revealed a broader lower detection limit, linear range, and an excellent anti-interference capacity. In the formation of chlorpyrifos, the Bi-chlorpyrifos complex formation on BiPO 4 NPs gave rise to an increase in steric hindrance. It thus stuck the BiPO 4 NPs electron transfer toward the electrode surface, causing an observable fall in photocurrent [182].
rGO/Bi 2 MoO 6 nanosheets were successfully synthesized using rGO/Bi 2 (EG) precursors using a two-stage solvothermal method. The introduction of graphene supports the recombination of electrons and holes generated by photogenerated rGO/Bi 2 MoO 6 nanocomposite exhibits plate-on-plate enhanced Cr (VI) photoreduction structures with radiation from sunlight. With an ideal photocatalytic activity, the 2.5% rGO/Bi 2 MoO 6 composite and a reduction of 94% to Cr(VI) at about 30 min, roughly twice that of pure Bi 2 MoO 6 . The rGO, which mainly functions as an electron collector and meaningfully promotes the photoinduced carrier separation, accommodates the improved photocatalytic efficacy. Furthermore, rGO/Bi 2 MoO 6 composites have excellent stability and can be recycled in an industrial process. The composite morphologies of 2.5% rGO/Bi 2 MoO 6 are low-lying and non-regular plate-on-plate structures. This indicates that Bi 2 MoO 6 nanoflocks are scattered to the surface of large graphene layers forming Bi 2 MoO 6 nanoflocks and small ribs. Defects may cause wrinkles during the functioning of oxygen when GO was synthesized [184,185].
A newer BWO/MG ternary heterojunction photocatalyst was designed with an improved load carrier separation using the two-step hydrothermal method through a progressive load transfer route. MoS 2 was used to improve the transition between graphene and BWO through the "stepping stone" approach. A positive synergetic effect between the graphene sheets and MoS 2 is believed to occur. The cocatalyst components on photodegradation can efficiently improve the interfacial charge transfer, suppress the recombination of charges, and offer many photocatalytic reaction centers and active absorption sites [186]. The BWO/MG ternary hybrid facility is a visible and inexpensive environmental photocatalyst that expands the composite photocatalyst preparation range of MG hybrids and provides a prospective way to improve the performance of photocatalysts. The BWO catalyst has a microscopic structure and morphology with an average diameter of 3-4 microspheres. These microspheres consist of several hundred nanometers of lateral nanoplates. The BWO microspheres used the automatic spherical construction of nanosheet nanoplates. The SEM and TEM images of BWO and BWO/MG are shown in Figure 5. BWO nanosheets are not agglomerated during growth following MG modifications. The morphology of the BWO crystalline structures is controlled by the incorporation of MG, which has increased photocatalytic performance in a larger specific area. The photogenerated electrons should improve the photocatalytic efficiency and charging separation, a close relationship between BWO, graphene, and components achieved via hydrothermal processing [187].
tal photocatalyst that expands the composite photocatalyst preparation range of MG hybrids and provides a prospective way to improve the performance of photocatalysts. The BWO catalyst has a microscopic structure and morphology with an average diameter of 3-4 microspheres. These microspheres consist of several hundred nanometers of lateral nanoplates. The BWO microspheres used the automatic spherical construction of nanosheet nanoplates. The SEM and TEM images of BWO and BWO/MG are shown in Figure 5. BWO nanosheets are not agglomerated during growth following MG modifications. The morphology of the BWO crystalline structures is controlled by the incorporation of MG, which has increased photocatalytic performance in a larger specific area. The photogenerated electrons should improve the photocatalytic efficiency and charging separation, a close relationship between BWO, graphene, and components achieved via hydrothermal processing [187].
.  Bi 2 WO 6 /rGO photocatalysts have been synthesized by an easy hydrothermal method and with 2 wt % rGO content display the highest photocatalyst performance. Enhanced photocatalytic activity for more efficient cargo transport, maximum light absorption, and separation can be accredited to strong chemical bonds between rGO and Bi 2 WO 6 . In addition, Bi 2 WO 6 /rGO is highly stable and essential for applications in environmental protection applications [188,189].

Self-Assembly
Self-assembly is a useful and frequently favored method for assembling micro-and nano-substances into macroscopic systems [190][191][192]. It is used to produce functional materials such as composites, photonic crystals, and DNA structures. An innovative way of synthesizing ordered graphene-metal oxide hybrids via a surfactant-supported, ternary self-assembly process was established to achieve an interchangeable layer structure of final composites [193]. The efficient and easy electrostatic self-montage method is successfully used to produce BWO/rGO nanocomposites. BWO-nanocomposites RGO's have been synthesized with hydrothermal reduction through electrostatic self-assembly processes. The uniform, electronically interacting, and close interface contact can be achieved with nanocomposites from the BWO/rGO. The adjacent interface contact stimulates the separation of e/h + pairs and extends the lifetime of the photo-induced charge carrier [194]. The charging balance and electronic interaction between rGO and BWO lead to VB change and change in conductive electricity and the valence band holes [195].
Nanocomposites of GO/BiPO 4 were synthesized using an easy self-assembly twophase method. The GO presence can substantially improve the visible light absorption of the load transfer facilitators, catalysts, and the pair of electron holes [196]. The GO/BiPO 4 nanocomposites formation via a self-assembly method is shown in Figure 6. and change in conductive electricity and the valence band holes [195].
Nanocomposites of GO/BiPO4 were synthesized using an easy self-assembly twophase method. The GO presence can substantially improve the visible light absorption of the load transfer facilitators, catalysts, and the pair of electron holes [196]. The GO/BiPO4 nanocomposites formation via a self-assembly method is shown in Figure 6. An easy and fast approach to energy-generating chemical reactions is microwave irradiation. Graphene-metal oxide hybrids, for example, graphene-MnO2 have been synthesized using microwave irradiation [197] as has graphene-Co3O4 [198]. Direct electrochemical deposition of inorganic crystals on graphene substrates is an intelligent approach for thin film-based applications with no need for post-synthetic transfer of composite materials [199][200][201][202][203][204][205][206][207][208][209]. A summary of bismuth/graphene-based photocatalysts fabrication methods, morphology, and applications is presented in Table 2.

Applications of Bismuth/Graphene Nanohybrids
Bismuth-graphene-based composites have been used for the photodegradation of pollutants and also in many other domains, such as hydrogen production and photovoltaic cells linked to environmental preservation [210][211][212]. An easy and fast approach to energy-generating chemical reactions is microwave irradiation. Graphene-metal oxide hybrids, for example, graphene-MnO 2 have been synthesized using microwave irradiation [197] as has graphene-Co 3 O 4 [198]. Direct electrochemical deposition of inorganic crystals on graphene substrates is an intelligent approach for thin film-based applications with no need for post-synthetic transfer of composite materials [199][200][201][202][203][204][205][206][207][208][209]. A summary of bismuth/graphene-based photocatalysts fabrication methods, morphology, and applications is presented in Table 2.

Applications of Bismuth/Graphene Nanohybrids
Bismuth-graphene-based composites have been used for the photodegradation of pollutants and also in many other domains, such as hydrogen production and photovoltaic cells linked to environmental preservation [210][211][212].

Water Splitting
Hydrogen energy is considered as an ideal green energy source, and the product of hydrogen combustion is H 2 O, so hydrogen, when used as fuel, it both solves the future fossil fuel crisis and shortage and lessens the environmental pollution from fossil fuel consumption [150,210,[213][214][215]. In 1972, Fujishima et al. first described the TiO 2 photoelectrode water splitting phenomenon [216], and as a result, photocatalytic H 2 production has gained much attention [217][218][219][220]. Hydrogen is one of the crucial pure fuels [221][222][223]. Hydrogen production using the appropriate photocatalyst and solar power is an important factor not just because it is an excellent way to supply large-scale renewable and clean hydrogen but also to prevent probable energy-storage problems. One of the more convenient methods in this respect is photocatalytic water splitting. To date, some nanocomposites based on graphene have been used for the photocatalytic cleavage of water [220,224]. To transform this technology into an industrial application, the development and exploration of relevant photocatalysts with outstanding performance are vital. In the past four decades, therefore, several semiconductors were tested as photocatalysts. Graphene is considered to have a great performance in this research field [225,226]. In order to make a practical photocatalyst economically attainable, efforts have been made to improve the efficiency of the photocatalysts. Amal's group developed photocatalysts such as rGO/Ru/Sr, rGO/BiVO 4 , rGO/WO 3 , and TiO 3 , rGO/TiO 2 [117,227,228]. In the case of the BiVO 4 /rGO composite, the evolution of the O 2 and H 2 on BiVO 4 /rGO was 0.21 mm and 0.75 mmol h −1 , respectively, under visible light, while negligible gas production is detected in pure cells of BiVO 4 . This photocatalytic water splitting has been accredited to the longer electron life of provoked BiVO 4 electrons that promptly injected in rGO at the production site, leading to lower recombination of charges (Figure 7). In recent times, an inspired Z-scheme photocatalysis system for dividing water under visible light radiation has been established. Photocatalytic systems for the artificial Z-scheme offer a blossoming approach for enhancing the performance of PH 2, by imitating the natural photosynthesis in typical green leaves [229].

Water Splitting
Hydrogen energy is considered as an ideal green energy source, and the product of hydrogen combustion is H2O, so hydrogen, when used as fuel, it both solves the future fossil fuel crisis and shortage and lessens the environmental pollution from fossil fuel consumption [150,210,[213][214][215]. In 1972, Fujishima et al. first described the TiO2 photoelectrode water splitting phenomenon [216], and as a result, photocatalytic H2 production has gained much attention [217][218][219][220]. Hydrogen is one of the crucial pure fuels [221][222][223]. Hydrogen production using the appropriate photocatalyst and solar power is an important factor not just because it is an excellent way to supply large-scale renewable and clean hydrogen but also to prevent probable energy-storage problems. One of the more convenient methods in this respect is photocatalytic water splitting. To date, some nanocomposites based on graphene have been used for the photocatalytic cleavage of water [220,224]. To transform this technology into an industrial application, the development and exploration of relevant photocatalysts with outstanding performance are vital. In the past four decades, therefore, several semiconductors were tested as photocatalysts. Graphene is considered to have a great performance in this research field [225,226]. In order to make a practical photocatalyst economically attainable, efforts have been made to improve the efficiency of the photocatalysts. Amal's group developed photocatalysts such as rGO/Ru/Sr, rGO/BiVO4, rGO/WO3, and TiO3, rGO/TiO2 [117,227,228]. In the case of the BiVO4/rGO composite, the evolution of the O2 and H2 on BiVO4/rGO was 0.21 mm and 0.75 mmol h −1 , respectively, under visible light, while negligible gas production is detected in pure cells of BiVO4. This photocatalytic water splitting has been accredited to the longer electron life of provoked BiVO4 electrons that promptly injected in rGO at the production site, leading to lower recombination of charges ( Figure 7). In recent times, an inspired Zscheme photocatalysis system for dividing water under visible light radiation has been established. Photocatalytic systems for the artificial Z-scheme offer a blossoming approach for enhancing the performance of PH 2, by imitating the natural photosynthesis in typical green leaves [229].
PVRO (PRGO/BiVO4, PRGO) and Ru/SrTiO3 photographic graphene oxide blends (PRGO/Ru/SrTiO3:Rh) can be synthesized in the presence of the photocatalytic reduction of GO on both BiVO4 and Ru/SrTiO3:Rh, in the presence of methanol as a hole scavenger. PRGO functions as a solid-state electron mediator in this system and transports electrons from the BiVO4 CB to vacancies in the Ru/SrTiO3:Rh impurity levels. In Ru/SrTiO3 electrons, the water is reduced H2 by a Ru cocatalyst, and the water is oxidized into O2 by holes from BiVO4, thus producing a full water decomposition cycle. The O2 and H2 time cycles have demonstrated that after the second cycle, this system is constant. This important work provides a new entry to the use of g-C3N4 in the design of new and efficient water division systems [224]. Chong et al. [230] reported V2O5/rGO/BiVO4 heterojunction ( Figure 8) as an efficient photo-electrochemical water division photoanode. electrons, the water is reduced H 2 by a Ru cocatalyst, and the water is oxidized into O 2 by holes from BiVO 4 , thus producing a full water decomposition cycle. The O 2 and H 2 time cycles have demonstrated that after the second cycle, this system is constant. This important work provides a new entry to the use of g-C 3 N 4 in the design of new and efficient water division systems [224]. Chong et al. [230] reported V 2 O 5 /rGO/BiVO 4 heterojunction ( Figure 8) as an efficient photo-electrochemical water division photoanode. . Figure 8. Photoelectrochemical water splitting system design and electron transfer mechanism schematics in V2O5/rGO/BiVO4 heterojunction photoanode. Adapted from [230].

CO2 Reduction
Due to growing energy and environmental concerns, CO2 conversion into fuel is considered a favorable approach [231][232][233]. Solar energy is mainly used for this due to its capacity to imitate the natural photosynthesis process to transform solar energy into chemical energy. The photocatalytic reduction of CO2 into valued fuels like formic acid, methane, and methanol is of particular importance [234][235][236]. In the last decades, this has received great attention and we have become acquainted with the enhanced release of the greenhouse gas CO2 into our atmosphere and the potential and real power supply shortage. The conversion of solar power into chemicals by photoelectrochemically or photocatalytically reducing CO2, is also one of the most advantageous methods to solve environmental and energy problems simultaneously. CO2 molecules are chemically inert and therefore highly stable, with linear geometry and shell electronics [235]. The CO2 reduction by photosensitive semiconductor catalysts yields highly sought products, e.g., formic acid, methane, formaldehyde, and methanol, etc. Several compounds, including metal complexes, can function as electrocatalysts for CO2 reduction [235,236]. Bismuth and graphene's role is vital and has been studied widely in CO2 conversion to valued products. Bismuth is prominently used through electrochemical CO2 reduction reactions (ECRR), while there are several reports of photocatalysis by a bismuth-graphene nanohybrid catalyst. Sun el al. converted CO2 into formate using bismuth with bismuth oxides supported on graphene nanosheets (Bi/Bi2O3/NrGO). This hybrid electrocatalyst gives a high current density and low overpotential in ECRR due to the synergistic effect of bismuth and its oxides [237]. Similarly, a bismuth oxide-reduced graphene oxide quantum dots (rGO/BiO QDs) composite was synthesized, which provides excess photoelectrons and protons for CO2 reduction [238]. In another study, a nanoheterojunction electrocatalyst made of zinc phthalocyanine/graphene/BiVO4 showed higher performance than the BiVO4 nanocatalysts due to the modulating presence of graphene [239]. Using defect engineering, oxygen vacancy-rich electrocatalysts were prepared by Yang et al. [240]. The electrocatalysts were prepared by a precipitation method from bismuth oxide and bismuth sulfide supported on reduced graphene oxide. This hybrid nanocatalyst facilitates CO and formate formation during ECRR at low overpotential with high stability during on-stream analysis. A lead bismuth oxobromide/graphene oxide catalyst was prepared and studied for the conversion of CO2 into methane under light [203]. The graphene-supported catalyst activity was much higher than without graphene, reflecting the importance of graphene in future environmental and energy conversion and storage applications. More research on

CO 2 Reduction
Due to growing energy and environmental concerns, CO 2 conversion into fuel is considered a favorable approach [231][232][233]. Solar energy is mainly used for this due to its capacity to imitate the natural photosynthesis process to transform solar energy into chemical energy. The photocatalytic reduction of CO 2 into valued fuels like formic acid, methane, and methanol is of particular importance [234][235][236]. In the last decades, this has received great attention and we have become acquainted with the enhanced release of the greenhouse gas CO 2 into our atmosphere and the potential and real power supply shortage. The conversion of solar power into chemicals by photoelectrochemically or photocatalytically reducing CO 2 , is also one of the most advantageous methods to solve environmental and energy problems simultaneously. CO 2 molecules are chemically inert and therefore highly stable, with linear geometry and shell electronics [235]. The CO 2 reduction by photosensitive semiconductor catalysts yields highly sought products, e.g., formic acid, methane, formaldehyde, and methanol, etc. Several compounds, including metal complexes, can function as electrocatalysts for CO 2 reduction [235,236]. Bismuth and graphene's role is vital and has been studied widely in CO 2 conversion to valued products. Bismuth is prominently used through electrochemical CO 2 reduction reactions (ECRR), while there are several reports of photocatalysis by a bismuth-graphene nanohybrid catalyst. Sun el al. converted CO 2 into formate using bismuth with bismuth oxides supported on graphene nanosheets (Bi/Bi 2 O 3 /NrGO). This hybrid electrocatalyst gives a high current density and low overpotential in ECRR due to the synergistic effect of bismuth and its oxides [237]. Similarly, a bismuth oxide-reduced graphene oxide quantum dots (rGO/BiO QDs) composite was synthesized, which provides excess photoelectrons and protons for CO 2 reduction [238]. In another study, a nanoheterojunction electrocatalyst made of zinc phthalocyanine/graphene/BiVO 4 showed higher performance than the BiVO 4 nanocatalysts due to the modulating presence of graphene [239]. Using defect engineering, oxygen vacancy-rich electrocatalysts were prepared by Yang et al. [240]. The electrocatalysts were prepared by a precipitation method from bismuth oxide and bismuth sulfide supported on reduced graphene oxide. This hybrid nanocatalyst facilitates CO and formate formation during ECRR at low overpotential with high stability during on-stream analysis. A lead bismuth oxobromide/graphene oxide catalyst was prepared and studied for the conversion of CO 2 into methane under light [203]. The graphene-supported catalyst activity was much higher than without graphene, reflecting the importance of graphene in future environmental and energy conversion and storage applications. More research on bismuth graphene composites is needed in this field [241]. Figure 9 presents an electron transfer mechanism and reducing adsorption and formate formation from CO 2 molecules over the BiVO 4 quantum dots/rGO composite [242,243]. bismuth graphene composites is needed in this field [241]. Figure 9 presents an electron transfer mechanism and reducing adsorption and formate formation from CO2 molecules over the BiVO4 quantum dots/rGO composite [242,243].

NOx Conversion
In addressing environmental problems associated with water and air pollutants, photocatalytic processes in decomposition and inorganic compounds, along with the removal of dangerous gases, are of great importance [244][245][246][247]. The main pollutants caused by the combustion of industrial burners or fossil fuel in automotive engines are nitric oxide (NO) and nitric dioxide (NO2) [248]. Many catalytic processes for the transformation of nitrogen gases (e.g., NO and NO2) into nitrogen (N2), oxygen (O2), or nitrate (NO3 − ) have been established [249,250]. An ideal NOx conversion catalyst transforms NOx gases at lowertemperature [251]. TiO2 is one of the leading catalysts for the catalytic conversion of NOx gases into nitrous oxide (N2O) and N2 [252,253]. The majority of previous studies on the conversion of NOx gases have involved different lasers [254], spectroscopic (such as infrared (IR), [255], and chemiluminescence (CL) [256] or electrochemical techniques [257] for the detection of NOx reaction products. The use of high-resolution MS for biomedical applications to detect NO [230] and indirectly semiconducting metal oxides [231] has been described. The main cause of water pollution is industrial wastewater discharge. Drinking polluted water for a long time poses potential health risks, and can also cause cancer, teratogenicity and mutagenicity. For this reason, it is very important to develop suitable techniques for the treatment of industrial wastewater to meet emission standards. Photocatalysis is considered a sustainable and efficient water treatment technology. Old-photocatalysts (such as ZnO and TiO2) with a wide bandgap are only active in the UV light region and their quick recombination of photo-generated holes and electrons leads to low quantum efficiencies that limit their application for wastewater treatment. The traditional inconveniences of these photocatalysts requires the development of new Bi-based semicatalysts for real waste water treatment such as black BiOCl-Bi2O3/rGO nanocomposite with high photocatalytic efficiency [141].
With economic growth, pollution, primarily air pollution, is becoming a serious concern and must be treated instantaneously. NOx plays an important role in acid rain formation, diseases and photochemistry. Therefore, the elimination of NOx is a hot topic in the area of environmental protection [258][259][260][261]. The photocatalytic oxidation of NO to NO2 is a good way to remove NO from flue gas, as NO2 can be removed simply by reacting with hydrocarbons to release N2 or water [262]. The photocatalytic NO-NO2 oxidation is observed as an essential reaction, and a great deal of effort has been made to develop appropriate NO-removal photocatalysts [263,264]. Bi2WO6 has attracted considerable attention as an Aurivillius oxide semiconductor with a 2.66 eV narrow bandgap. Bi2WO6

NO x Conversion
In addressing environmental problems associated with water and air pollutants, photocatalytic processes in decomposition and inorganic compounds, along with the removal of dangerous gases, are of great importance [244][245][246][247]. The main pollutants caused by the combustion of industrial burners or fossil fuel in automotive engines are nitric oxide (NO) and nitric dioxide (NO 2 ) [248]. Many catalytic processes for the transformation of nitrogen gases (e.g., NO and NO 2 ) into nitrogen (N 2 ), oxygen (O 2 ), or nitrate (NO 3 − ) have been established [249,250]. An ideal NOx conversion catalyst transforms NOx gases at lower-temperature [251]. TiO 2 is one of the leading catalysts for the catalytic conversion of NOx gases into nitrous oxide (N 2 O) and N 2 [252,253]. The majority of previous studies on the conversion of NOx gases have involved different lasers [254], spectroscopic (such as infrared (IR), [255], and chemiluminescence (CL) [256] or electrochemical techniques [257] for the detection of NOx reaction products. The use of high-resolution MS for biomedical applications to detect NO [230] and indirectly semiconducting metal oxides [231] has been described. The main cause of water pollution is industrial wastewater discharge. Drinking polluted water for a long time poses potential health risks, and can also cause cancer, teratogenicity and mutagenicity. For this reason, it is very important to develop suitable techniques for the treatment of industrial wastewater to meet emission standards. Photocatalysis is considered a sustainable and efficient water treatment technology. Oldphotocatalysts (such as ZnO and TiO 2 ) with a wide bandgap are only active in the UV light region and their quick recombination of photo-generated holes and electrons leads to low quantum efficiencies that limit their application for wastewater treatment. The traditional inconveniences of these photocatalysts requires the development of new Bi-based semicatalysts for real waste water treatment such as black BiOCl-Bi 2 O 3 /rGO nanocomposite with high photocatalytic efficiency [141].
With economic growth, pollution, primarily air pollution, is becoming a serious concern and must be treated instantaneously. NOx plays an important role in acid rain formation, diseases and photochemistry. Therefore, the elimination of NOx is a hot topic in the area of environmental protection [258][259][260][261]. The photocatalytic oxidation of NO to NO 2 is a good way to remove NO from flue gas, as NO 2 can be removed simply by reacting with hydrocarbons to release N 2 or water [262]. The photocatalytic NO-NO 2 oxidation is observed as an essential reaction, and a great deal of effort has been made to develop appropriate NO-removal photocatalysts [263,264]. Bi 2 WO 6 has attracted considerable attention as an Aurivillius oxide semiconductor with a 2.66 eV narrow bandgap. Bi 2 WO 6 forms with different morphology can be synthesized by various approaches, like a cetyltrimethylammonium bromide-assisted bottom-up route, hydrothermal processes and solidstate reactions [265][266][267]. It was used in several applications, including the decomposition of pollutants [266,267]. However, the photocatalytic activity fades due to fast recombination of photogenerated carriers in Bi 2 WO 6, and its more practical applications are restricted. Graphene has been shown to successfully improve photocatalysts' photoactivities through further separation of the electron-holes generated and helping photoinduced electrons to migrate and preventing the recombination of electron-holes and increasing the efficiency of quantization [268][269][270]. Bismuth compounds have also been employed in combination with graphene to produce useful photocatalytic composites for NOx removal under visible light irradiation [271]. Zhihui et al. [272] prepared BiOBr-graphene nanocomposites for efficient removal of NO via visible-light photocatalytic activity. The improved photocatalytic activity of the BiOBr-graphene nanocomposite was ascribed to the efficient charge separation, and enhanced transfer is due to robust chemical bonding between graphene and BiOBr. Also, the N 2 -doped (BiO) 2 CO 3 /GO nanocomposites, reported by Chen et al., [273], play a pivotal role in higher photocatalytic performance for NOx removal under visible light irradiation. The rGO improved the electron-hole separation for pure Bi 2 WO 6 and fully degrading RhB [274]. Ma et al. described an improved composite performance of rGO/Bi 2 WO 6 photocatalytic in phenol and RhB degradations [275]. The selective photocatalytic 4-NP reduction on blank nanocomposites BWO, rGO and BWO/rGO after 30 min of irradiation is shown in Figure 10. forms with different morphology can be synthesized by various approaches, like a cetyltrimethylammonium bromide-assisted bottom-up route, hydrothermal processes and solid-state reactions [265][266][267]. It was used in several applications, including the decomposition of pollutants [266,267]. However, the photocatalytic activity fades due to fast recombination of photogenerated carriers in Bi2WO6, and its more practical applications are restricted. Graphene has been shown to successfully improve photocatalysts' photoactivities through further separation of the electron-holes generated and helping photoinduced electrons to migrate and preventing the recombination of electron-holes and increasing the efficiency of quantization [268][269][270]. Bismuth compounds have also been employed in combination with graphene to produce useful photocatalytic composites for NOx removal under visible light irradiation [271]. Zhihui et al. [272] prepared BiOBr-graphene nanocomposites for efficient removal of NO via visible-light photocatalytic activity. The improved photocatalytic activity of the BiOBr-graphene nanocomposite was ascribed to the efficient charge separation, and enhanced transfer is due to robust chemical bonding between graphene and BiOBr. Also, the N2-doped (BiO)2CO3/GO nanocomposites, reported by Chen et al., [273], play a pivotal role in higher photocatalytic performance for NOx removal under visible light irradiation. The rGO improved the electron-hole separation for pure Bi2WO6 and fully degrading RhB [274]. Ma et al. described an improved composite performance of rGO/Bi2WO6 photocatalytic in phenol and RhB degradations [275]. The selective photocatalytic 4-NP reduction on blank nanocomposites BWO, rGO and BWO/rGO after 30 min of irradiation is shown in Figure 10.

Organic Degradation
A dramatic surge in research in the visible light photocatalysis area was observed at the start of the 21st century, as evidenced by a promptly increasing number of publications. Using visible light in combination with catalysts is effective for producing selective and efficient chemical transformations. Nature remarkably reveals the power of photosynthesis by transforming CO2 and H2O into oxygen and carbohydrates, a process that is so far unequaled by any man-made chemical procedure [276].
The use in organic synthesis of solar energy as a motiving power is now beginning. Key solar energy components include UV (λ = 200-400 nm), visible light (λ = 400-800 nm), and infrared light (λ > 800 nm), accounting for almost 5%, 43%, and 52%, respectively. UV energy can directly trigger certain organic molecules to provide highly reactive intermediates, resulting in poor product selectivity. Furthermore, for the vast majority of organic reactions, the infrared wavelength with relatively low energy does not meet the energy demand. In comparison, UV and visible light are abundant, but the reactant molecules can usually not directly adsorb them to drive reactions. Therefore, it will be important for visible photocatalysts to work as bridging media for energy transfer between the substrate

Organic Degradation
A dramatic surge in research in the visible light photocatalysis area was observed at the start of the 21st century, as evidenced by a promptly increasing number of publications. Using visible light in combination with catalysts is effective for producing selective and efficient chemical transformations. Nature remarkably reveals the power of photosynthesis by transforming CO 2 and H 2 O into oxygen and carbohydrates, a process that is so far unequaled by any man-made chemical procedure [276].
The use in organic synthesis of solar energy as a motiving power is now beginning. Key solar energy components include UV (λ = 200-400 nm), visible light (λ = 400-800 nm), and infrared light (λ > 800 nm), accounting for almost 5%, 43%, and 52%, respectively. UV energy can directly trigger certain organic molecules to provide highly reactive intermediates, resulting in poor product selectivity. Furthermore, for the vast majority of organic reactions, the infrared wavelength with relatively low energy does not meet the energy demand. In comparison, UV and visible light are abundant, but the reactant molecules can usually not directly adsorb them to drive reactions. Therefore, it will be important for visible photocatalysts to work as bridging media for energy transfer between the substrate and visible light. These photocatalysts may be assigned to five different groups: plasmonic-metal NPs, homogenous photocatalysts, opposite heterogeneous semiconductor photocatalysts, other new photoelectric materials, and organic dyes. Various semiconductors show different widths and positions of the string so that there are different reduction and oxidation potential for the electrons and hole pairs created in situ. When the carriers (holes and electrons) travel to the catalyst surface, which lowers photo-catalytic efficiency, electron and hole pair recombination occurs frequently. Many approaches have been developed to improve the separation efficiency of electron-hole pairs, such as supporting a photocatalyst on graphene with a big surface or using a valuable metal materials such as Pt so photogenerated charge transfer could be accelerated.
In organic reactions, H 2 O is considered an ideal solvent. However, the problem is that, under photocatalytic conditions, the semiconductor VB hole can oxidize H 2 O into a highly active OH radical form, making the reaction system complicated. Bi 2 WO 6 photocatalyst VB's inherent reduction potential is +1.77 V vs. Ag/AgCl, which is negative to the H 2 O/ANOH. H 2 O as a solvent is possible when Bi 2 WO 6 is used as a catalyst. Recently, a selective oxidation of benzyl alcohols into aldehydes has been effectively developed with a Bi 2 WO 6 /H 2 O/air system [277]. Although the different synthetic applications of visible light photocatalysis are awe-inspiring, there is still scope for improvement. In several instances, the reaction times for many conversions are fairly long. In order to make photocatalytic changes faster and more energy-efficient, the quantum efficiency must be extremely enhanced. A better mechanistic consideration could benefit the rational design of new transformations and the expectation of the substrate scope. The reachable potential should be stretched for the exchange of chemically reduced single-electron or stoichiometric oxidizing reagents by photocatalytic reactions. There is no examination of the various photocatalytic energies of transformations, and chemists have just begun to produce organic conversions that are promising with additional light energies. Finally, we must find out how this can be extended to ions and carbenes and how the common visible light's common photocatalytic reactions continue through the radical intermediates. There are plenty of opportunities for future development in photocatalysis. We should have followed Ciamician's initial ideas for sustainable and innovative organic syntheses using visible light much earlier [278].
In comparison with applications such as organic contaminant degradation, heterogeneous semi-conducting photocatalysis addresses more complex problems. The photoinduced charging transfers resulting from semiconductor interfaces with holes or electrons used as reducers and oxidizers, respectively, are the basis of all types of photocatalytic applications. In photocatalytic selective organic synthesis, the critical problem is how to regulate the method of interfacial charge transfer to ensure only the selective transformation of specific functional groups in organic substrata while the remaining molecular structure remains intact [279]. Because the VB holes photogenerated as a stable photocatalyst (e.g., WO 3 , TiO 2, and ZnO) have strong oxidation power, VB holes tend to oxidize nonselectively and degrade whole molecules, respectively. For RhB degradation, BiOCl/rGO is considered an effective photocatalyst [280]. The mechanism is schematically shown in Figure 11. At present, there are several technical difficulties and knowledge gaps in the organic synthesis research field. The photocatalytic method is heterogeneous. It is expected that individual photocatalysts will offer enhanced selectiveness for selective reactions, similar to organic degradation processes. It is estimated that for individual organic synthesis reaction cases, each photocatalyst must be optimized as selectivity control depends on the molecular structure and the particular organic substrate characteristics as well as on the photocatalyst [281]. As an efficient, non-toxic, and stable method, photocatalytic disinfection was shown to be superior to traditional methods for water disinfection, including UV irradiation, ozonation and chlorination, since they form carcinogenic disinfection by-products, and are causes of global warming due to the formation of chemical-intensive or energy-intensive products. Highly successful and innovative wastewater disinfection approaches need to be implemented and maintained, that are less dependent on fossil fuels and chemicals [282]. In particular, rationally designed nanophotocatalyst nanomaterials have tremendous potential here to produce robust and adequate reactive species using solar light (the most plentiful, and accessible renewable energy source on Earth). The bactericidal activity of photocatalysts sextends to all reactive species formed during the photocatalytic process. In addition, visible light corresponds to the strongest solar irradiance range. A photocatalyst, which can efficiently absorb the visible light to produce reactive species, is a condition for achieving fast photocatalytic disinfection [283]. Jamshaid et al. [284] synthesized a BiOCl/GO composite and utilized it under visible light, full solar light, and UV photocatalytic degradation of diclofenac sodium (DCF) ( Figure 12).
As a photocatalyst, BiVO4/rGO nanocomposite exhibits efficient catalytic activity towards organic dye degradation [285,286]. The photodegradation results showed that the BiVO4-rGO nanocomposite catalyst could effectively degrade organic dyes in a variety of wastewaters. Similarly, a one-step hydrothermally synthesized Bi-TiO2/graphene nanocomposite is considered an efficient photocatalyst for remarkable organic pollutant degradation under visible light irradiation [287]. The Z-scheme photocatalyst systems provided a promising approach of simultaneously removing heavy metals and organic pollutants. Acong et al. [288] reported an all-solid-state Z-scheme system containing BiOI/Bi2S3/rGO composites for simultaneous removal of aqueous Cr(VI) and phenol [288]. A series of bismuth-graphene nanocomposite systems were summarized by Yu-Hsun et al. [289] for adequate catalytic activity and stability, acting as visible-light-driven photocatalysts in efficient organic pollutant degradation. single oxygen under the white LED irradiation. Adapted from [280].
As an efficient, non-toxic, and stable method, photocatalytic disinfection was shown to be superior to traditional methods for water disinfection, including UV irradiation, ozonation and chlorination, since they form carcinogenic disinfection by-products, and are causes of global warming due to the formation of chemical-intensive or energy-intensive products. Highly successful and innovative wastewater disinfection approaches need to be implemented and maintained, that are less dependent on fossil fuels and chemicals [282]. In particular, rationally designed nanophotocatalyst nanomaterials have tremendous potential here to produce robust and adequate reactive species using solar light (the most plentiful, and accessible renewable energy source on Earth). The bactericidal activity of photocatalysts sextends to all reactive species formed during the photocatalytic process. In addition, visible light corresponds to the strongest solar irradiance range. A photocatalyst, which can efficiently absorb the visible light to produce reactive species, is a condition for achieving fast photocatalytic disinfection [283]. Jamshaid et al. [284] synthesized a BiOCl/GO composite and utilized it under visible light, full solar light, and UV photocatalytic degradation of diclofenac sodium (DCF) ( Figure 12).
As a photocatalyst, BiVO 4 /rGO nanocomposite exhibits efficient catalytic activity towards organic dye degradation [285,286]. The photodegradation results showed that the BiVO 4 -rGO nanocomposite catalyst could effectively degrade organic dyes in a variety of wastewaters. Similarly, a one-step hydrothermally synthesized Bi-TiO 2 /graphene nanocomposite is considered an efficient photocatalyst for remarkable organic pollutant degradation under visible light irradiation [287]. The Z-scheme photocatalyst systems provided a promising approach of simultaneously removing heavy metals and organic pollutants. Acong et al. [288] reported an all-solid-state Z-scheme system containing BiOI/Bi 2 S 3 /rGO composites for simultaneous removal of aqueous Cr(VI) and phenol [288]. A series of bismuth-graphene nanocomposite systems were summarized by Yu-Hsun et al. [289] for adequate catalytic activity and stability, acting as visible-light-driven photocatalysts in efficient organic pollutant degradation.

Gas Sensing
The exploitation and design of photoelectrochemical (PEC) sensors with innovative nanomaterials are of great significance to attain the goal of inexpensive and sensitive detection. Therefore, BiPO4/rGO nanocomposite, a novel PEC sensor platform, can offer a delicate approach in chlorpyrifos detection and the resulting BiPO4/rGO nanocomposite is a potentially active catalyst for the PEC-related applications ( Figure 13) [182].

Gas Sensing
The exploitation and design of photoelectrochemical (PEC) sensors with innovative nanomaterials are of great significance to attain the goal of inexpensive and sensitive detection. Therefore, BiPO 4 /rGO nanocomposite, a novel PEC sensor platform, can offer a delicate approach in chlorpyrifos detection and the resulting BiPO 4 /rGO nanocomposite is a potentially active catalyst for the PEC-related applications ( Figure 13) [182].

Gas Sensing
The exploitation and design of photoelectrochemical (PEC) sensors with innovative nanomaterials are of great significance to attain the goal of inexpensive and sensitive detection. Therefore, BiPO4/rGO nanocomposite, a novel PEC sensor platform, can offer a delicate approach in chlorpyrifos detection and the resulting BiPO4/rGO nanocomposite is a potentially active catalyst for the PEC-related applications ( Figure 13) [182]. Figure 13. The PEC sensor illustration for chlorpyrifos. Adapted from ref [182]. Figure 13. The PEC sensor illustration for chlorpyrifos. Adapted from ref [182].
In the modern nanotechnology field, considerable attention has been given to an architecture-controlled combination of nanomaterials because of their astonishing chemical and physical properties and promising applications in different fields, e.g., optics, electronics, catalysis, and so on [290,291]. Similarly, using innovative configurations with implanted graphene for a broad surface, long electron life can be supported by other photonic devices such as solar cells and non-photonic devices, like lithium batteries and biochemical sensors. Low band-gap energy, reduced recombination rate, and fast charge transit e.g., spiral rolls-implanted graphene in the TiO 2 /ZnO/Bi 2 O 3 (TZB) nanofiber [175,292]. The BiPO 4 /3DNGH and BiVO 4 /rGO provide a new platform for specific biomedical, food, and environmental detection applications [181,293,294]. TEA and H 2 S are highly toxic gases that can pollute the atmosphere and damage the human respiratory system. Consequently, it is important to be able to easily detect low levels of TEA and H 2 S in our everyday lives. Shouli et al. [209] developed a pine dendritic BiVO 4 /rGO hybrid heterojunction, which improves not only BiVO 4 response and speeds up the response time but also has good selectivity and stability to 10 ppm TEA at 180 • C operating temperature. The formation of heterojunction and the integration of rGO are responsible for the change. Ketkaeo et.al. [295] investigated Bi 2 WO 6 nanoparticles loaded with rGO nanosheets for H 2 S gas sensing applications. The developed sensor exhibited high H 2 S selectivity against numerous volatile organic compounds and some other environmental gases. The H 2 S sensing mechanism via Bi 2 WO 6 /rGO composite is illustrated in Figure 14. In the modern nanotechnology field, considerable attention has been given to an architecture-controlled combination of nanomaterials because of their astonishing chemical and physical properties and promising applications in different fields, e.g., optics, electronics, catalysis, and so on [290,291]. Similarly, using innovative configurations with implanted graphene for a broad surface, long electron life can be supported by other photonic devices such as solar cells and non-photonic devices, like lithium batteries and biochemical sensors. Low band-gap energy, reduced recombination rate, and fast charge transit e.g., spiral rolls-implanted graphene in the TiO2/ZnO/Bi2O3 (TZB) nanofiber [175,292]. The BiPO4/3DNGH and BiVO4/rGO provide a new platform for specific biomedical, food, and environmental detection applications [181,293,294]. TEA and H2S are highly toxic gases that can pollute the atmosphere and damage the human respiratory system. Consequently, it is important to be able to easily detect low levels of TEA and H2S in our everyday lives. Shouli et al. [209] developed a pine dendritic BiVO4/rGO hybrid heterojunction, which improves not only BiVO4 response and speeds up the response time but also has good selectivity and stability to 10 ppm TEA at 180 °C operating temperature. The formation of heterojunction and the integration of rGO are responsible for the change. Ketkaeo et.al. [295] investigated Bi2WO6 nanoparticles loaded with rGO nanosheets for H2S gas sensing applications. The developed sensor exhibited high H2S selectivity against numerous volatile organic compounds and some other environmental gases. The H2S sensing mechanism via Bi2WO6/rGO composite is illustrated in Figure 14.

Drawbacks/Challenges Related to Bismuth and Graphene
Although, there has been diversified study on bismuth and graphene nanohybrids for large-scale applications of such photocatalysts, there remains several drawbacks/challenges such as the site of attachment of dopant, the overall efficient doping mechanism, assessment of integration, photocatalyst degradation, and visible light absorption that remain to be unraveled.
The improved Hummers process has been commonly used to synthesize graphene, which is the most recent and best method. However, despite the low experimental complexity, the experimental procedures to complete the graphene fabrication are time-consuming. As a result, the substitution or elimination of such chemicals must be studied further to reduce fabrication times and produce a better fabrication processes. Furthermore, the amount of chemicals used in the fabrication process or replacing them with less expensive alternatives could make the whole process more cost-effective and applicable to real-world applications.
Challenges also remain in the exploration of graphene-based nanohybrids for high performance practical applications. High-quality graphene nanohybrids with tailored functionalization, tunable structures, and optimized properties need to be fabricated in a more simple, effective, and economical approach. In graphene functionalization, attention must be paid to the control distribution, amount, and affinity to graphene nanosheets and the dispersibility and functionality of nanohybrids.
Graphene sheets tend to form aggregates in solution due to hydrogen bonding or strong van der Waals force interactions in polar solvents. Chemical functionalization [291,296] and electrostatic stabilization [297] are used to avoid this aggregation. Graphene reduction using simple methods facilitates graphene applications to synthesize composite materials in cost-effective, scalable approaches with low cost of production [167,298]. GOs may be synthesized using the Hummers and Offeman method and then by sonication exfoliated using strong graphite chemical oxidation. Most studies have concentrated on Bi 3+ -containing compounds, like Bi 2 O 3 BiOX (X = Cl, Br, I), BiPO 4 , BiVO 4 , BiFeO 3 Bi 4 Ti 3 O 12, Bi 2 WO 6 , Bi 2 O 2 CO 3, Bi 12 TiO 20, Bi 0.5 K 0.5 TiO 3 , and Bi 3 TiNbO 9 . Among them, a majority of the compounds possess a plate-like appearance and layered structures. Visible light can excite Bi 5+ -containing compounds, e.g., KBiO 3 , LiBiO 3 , and NaBiO 3 . Hybridized O 2p and Bi 6s 2 orbitals can influence the valence bands in Bi(III) compounds (VBs). Therefore, the Bi compounds' band gap is usually less than 3.0 eV and can easily be excited by visible light. However, the photocatalytic performance of bulk Bi-based semiconductors is not as high as the performance of photocatalysts from nano Bi-sources, like photogenerated holes and electrons have not been used and used efficiently. The photocatalysts in bulk are smaller in area and have less light absorption than photocatalysts in the nanoscale range. A variety of attempts to improve bulk semiconductors have been made to achieve the ideal photocatalytic activity. In addition, changes in components, e.g., doping, alteration of stoichiometry, and preparation of solid solutions, are current methods used to change the Bi-based semiconductor band structures. Therefore for Bi photocatalysts, a suitable component change is promising [25]. It has been studied that the Bi 6 s-orbital decreases the bandgap while increasing photogenerated charge carriers' mobility [299]. While a majority of the Bi-based compounds have about a 3.0 eV bandgap. Bi-based compounds, including Bi 2 O 3 , Bi 2 MO 6 (M = W, Mo, and Cr), BiVO 4 , BiOX (X = I, Br and Cl), BiPO 4 , pentavalent bismuthate and (BiO) 2 CO 3 , were tested as a large number of photocatalytic compounds. In environmental protection applications, Bi-based semiconductors have been used for the oxidation of gaseous pollutants, such as NO [105], organic dye degradation in wastewater [300], and CO 2 photoreduction [94]. During various studies, photocatalytic water division for generating O 2 and H 2 was reported [301].
An efficient strategy considered a new approach for improving bare photocatalysts' catalytic performance is by combining a new Z-scheme structure with the appropriate band position. The Z-scheme design can retain a high redox capacity to forgive both semiconductors, except for e-h pairs' recombination. Thanks to the band structure's adaption, environmental ease, and low cost, graphene was reported as another component by modifying Bi to change a Z-scheme system and doping [302].

Summary and Outlook
Future developments would be part of the present start of this new century. Bi/graphenebased semiconductors' fascinating physiochemical features have attracted researchers' attention and significantly motivated research, especially on visible-light photocatalytic activities. This review has discussed the most frequently studied bismuth/graphene photocatalysts. In addition, key challenges, including the broad bandwidth, high photogeneration carrier recombination rates, and low-capacity reduction in the conduction band, are outlined. The work reported has supported recommending achievable approaches to overcome these challenges. Though photocatalysts based on bismuth/graphene can considerably lessen the inconvenience, further efforts are still necessary to achieve significant advancements.
To date, these prepared bismuth/graphene materials' major applications are to purify polluted air and destroy pollutants in wastewater. Applying the formation of Z-scheme structures or modifying energy bands, improves the photocatalytic H 2 production. The work on these advanced nanocomposites should extend to other major areas, such as photocatalytic improvements, photocatalytic organic synthesis, and the recovery of heavy metals. The practical uses of photocatalysts using bismuth/graphene are seldom described. Integrating the application in different directions and many other areas with other suitable techniques, such as biotechnology, membrane technology, and electrochemistry, can lead to rapid advancements. Although many nanocomposites with bismuth/graphene have been reported to be active using visible and high photocatalytic light, the use of these advanced materials is still in the early stages of commercialization. Photocatalysts can selectively degrade pollutants.
Nanomaterial photocatalysis, especially nanophotocatalysts, exhibits huge potential because solar light can produce powerful and abundant reactive species. The visible light range is intended to achieve maximum photocatalytic decontamination and fast output rates. A photocatalyst capable of efficiently absorbing visible light is a prerequisite for producing reactive species. Two well-investigated visible-light-driven photocatalysts among various semiconductors are bismuth (Bi) and graphene. Due to their chemical stability, bulk availability, they have great potential for water disinfection applications and environmental friendliness. A bismuth/graphene hybrid effectively suppressed e − and h + pair recombination, promoted the interfacial electron transfer, and enhanced the photocatalytic process of reactive species generation. While this review is incomplete in the context of photocatalytic pollutant breakdown of bismuth/graphene nanocomposites, important aspects have been addressed concerning fundamental applications and principles.