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

Abstract

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 CO₂ 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.

1. 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 technology to control pollution, where photocatalysis is the most widely and potentially applied method used for demineralization and degradation of such pollutants [16,17].

Various light sources have been applied for the excitation of heterogeneous catalysts [18], but the photodegradation approach is more economical if sunlight can be used compared to ultraviolet light [16,19,20]. The evolution of the term “photocatalysis” shows the development of certain fundamental concepts of photochemistry. The point where photochemistry became a discipline was when it became differentiated from thermal chemistry. Indeed, several researchers saw irradiation as one of the many methods available to catalyze a response that makes it quicker by, for example, heating or processing it with certain chemicals until the beginning of the 20th century [21]. Ciamician, the first scientist to systematically understand the chemical effect of light, took great pains in finding out if he had “initiated heat” alone rather than “light” [22]. This was appropriately allotted the term “photochemical,” whilst the word “photocatalytic” applied to reactions caused by light, but with the same result as thermal reactions. Another step further was the identification of electronically excited states, which became a general idea in 1914 and were part of Bodenstein’s photochemical reactions along with reactivity and thermodynamics. In an early stage, more distinction was made in the thermochemistry of the process itself. This allowed for photosynthesis to occur when part of photon energy in the products rose [22,23].

Around 43% of visible-light energy is solar, so visible-light catalysts are chosen in photoelectrocatalysis and photocatalysis processes. Until now, several semiconductive products have been utilized, including metal oxides (Ag₂O, TiO₂, Cu₂O, ZnO, Fe₂O₃, Ta₂O₅), metal selenides (CdSe and MOSe₂), metal phosphides (Ni₂P), metal sulfides (Bi₂S₃, ZnS, MoS₂, and CdS), multi-structure oxides (Sr TiO₃WO), metal halides and oxyhalides (AgBr, BiOBr) and metal-free materials (SiC, Si and g-C₃N₄), [24,25,26,27,28]. Those with a bandgap (Eg) greater than 3 eV, e.g., SrTiO₃, TiO₂, ZnO, KTaO₃, ZnS, and SrTiO₃, are called wide-bandgap photocatalysts, whereas catalysts with an Eg of less than 4 eV, e.g., Si, SiC, Ag₂O, Bi₂WO₆, CdSe, InTaO₄, Ag₃VO₄, CoO, Fe₂O₃, Cu₂O, TaON, Ta₃N₅, CdS, Bi₂S₃, g-C₃N₄, and BiVO₄, are photocatalysts that react to visible light [25,29].

Heterogeneous catalysts play a vital role in environmental pollution control [30,31,32]. Powdered semiconductor photocatalysts are commonly used in various areas, such as carbon reduction [33], selective organic transformations, environmental remediation [34], and water splitting [35]. There has been, in numerous applications, a growing interest in the use of semiconductors as photocatalysts. In 2015, around 5500 documents about photocatalytic applications were published, indicating that interest in heterogeneous photocatalysis was enormous and highly important in diverse research fields. This number has recently grown to over 13,000. A country-specific view of the increase in the number of publications on “photocatalytic degradation” is listed in

Table 1. Country-wise publications growth on the photocatalytic degradation of organic pollutants. (Data acquired from SciFinder).

S. No.	Country	No. of Publications
1	China	8838
2	India	1090
3	Iran	676
4	South Korea	384
5	United States of America	178
6	Japan	175
7	Malaysia	158
8	Saudi Arabia	103
9	Pakistan	84
10	Italy	77
11	Australia	73
12	Spain	72
13	Brazil	57
14	United Kingdom	48

. No commercially accessible material can currently meet all application requirements, such as cost-effectiveness, stability, high visible-light quantum efficiency, and security [36]. For such tasks to be completed, a highly effective architecture and system for environmental remediation and energy supply are needed to examine new visible-light semiconductor materials.

The development of nanomaterials has progressed from the synthesis of single-particles to multicomponent assemblies or hierarchical structures, where two or more pre-synthesized nanomaterials are coupled to obtain multifunctionality. Such multicomponent assemblies are termed nanohybrids. The development and use of these nanohybrids requires interdisciplinary knowledge from the energy and environmental sectors, including the applications reported in references [37,38,39,40,41,42,43]. There are previously published review articles on some types and uses of nanohybrids, including gold-graphene oxide nanohybrids [39], organic/inorganic nanohybrids [44], polymer nanohybrids for oil recovery [45], nanohybrids of epoxy/polyamide with carbon nanotubes [46], protein-inorganic nanohybrids [47], gold-based inorganic nanohybrids [48] and polymer-inorganic supramolecular nanohybrids [49].

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 CO₂ conversion process has been overviewed. Finally, the challenges associated with bismuth and graphene and possible solutions have been discussed.

2. Bismuth-Graphene Based Photocatalytic Materials

2.1. 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 [86]. Therefore, the photogenerated electrons accumulated on the CB of graphene display strong reduction ability, and the photogenerated holes on the VB of Bi₂O_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 Bi₂O₃/graphene photocatalyst. Bi₂S₃ 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 Bi₂S₃ 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 (Bi₂S₃) 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 photogenerated holes and hydroxyl radicals (-OH) in the VB of Bi₂S₃ (1.62 eV) are mostly utilized in dye pollutant decomposition [92]. In combination with many other photocatalysts such as CdS [95], TiO₂ [28,96], and Bi₂WO₆ [97], the recombination rate of electron-hole pairs could be lowered. An increase in visible light absorption enhances the photocatalytic activity.

A graphene/Bi₂S₃ 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/Bi₂S₃ heterojunction could be tailored to increase the transfer and separation efficiency of photoinduced carriers and the visible light response. These graphene/Bi₂S₃ composites are effective photocatalysts for the photocatalytic degradation of environmental pollutants [74].

2.2. Bi₂MO₆ (M = Cr, Mo, W)/Graphene Composites

Bi₂MO₆ (M = Mo, Cr, W) is considered the most common member of the Aurivillius family, Bi₂A_n−1B_nO_n+3 (A = Sr, Ca, Ba, Bi, Pb, K, Na; B = Nb, Ti, Ta, Fe, W, Mo) is the general formula for Bi₂MO₆. The Bi₂MO₆ electronic structure is theoretically based on density functional theory (DFT) [98], while the Bi₂MO₆ crystal structure falls under orthorhombic space group Pca2(1). It was seen that both VB and CB of Bi₂MO₆ are composed of hybridized orbitals Bi₆p, O₂p, and M_nd (n = 3, 4, and 5) for Bi₂CrO₆, Bi₂MoO_6, and Bi₂MO₆, respectively [99]. Bi₂MO₆ compounds are suitable as visible-light-activated photocatalysts. Among all Bi₂MO₆ species Bi₂CrO₆ 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 Bi₂CrO₆ studies are available in the field of photocatalysis. For the preparation of Bi₂MoO₆ samples with a wider special surface area, smaller particles, and higher photocatalytic function, the solvothermal and hydrothermal methods are effective. Several Bi₂MoO₆ morphologies have been described, including floral hollow spheres (solvothermal process) and nanoplates (hydrothermal method). Moreover, microwave heating was applied to synthesize Bi₂MoO₆ samples with high photocatalytic activity in short periods [100,101]. Major applications of Bi₂MO₆ (and Bi₂MoO₆ and Bi₂WO₆) 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], CHCl₃ and CH₃CHO in wastewater [104], and NO in air [105]. Microorganisms, e.g., E. coli, were also destroyed by the addition of Bi₂WO₆ [106] and Bi₂MoO₆ [107] under visible light irradiation.

Current studies reveal the combined effect of plasmonic metals and graphene. The photocatalytic activity of semiconductors, e.g., TiO₂ and ZnO, can be efficiently improved by increasing their photo-absorption ability and suppressing photogenerated electron-hole recombination. Compared to Bi₂MoO₆, Bi₂MoO₆-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 Bi₂MoO₆ photocatalytic activity resulting from a combination of noble metals and graphene is expected. Bi et al. developed a rGO-Bi₂MoO₆/Au composite that displayed high catalytic activity for the photodegradation of rhodamine B [20]. Wang and Tian reported composites of GO-Bi₂MoO₆ and rGO-Bi₂MoO₆ [108,109]. These composites showed advanced phenol and rhodamine B degradation properties, respectively, compared to Bi₂MoO₆ alone [74].

2.3. BiVO₄/Graphene Nanocomposites

Bismuth vanadate (BiVO₄) 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 O₂ p- and V₃ d-orbitals are included in the BiVO₄ valence band. There are three forms of BiVO₄, namely monoclinic fergusonite, tetragonal zircon, and tetragonal scheelite. Reversible monoclinic fergusonite and tetragonal scheelite phase transitions occur at 255 °C. A wide range of methods have been reported for BiVO₄ preparation. Monoclinic BiVO₄ is obtained by both high temperature melting reactions and by solid-state reactions (SSR) [111]. Tetragonal BiVO₄ 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 BiVO₄ is 2.9 eV. This selective monoclinic BiVO₄ 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 BiVO₄ crystals in a simple water-based process [113]. A hydrothermal method has been used successfully in recent times for monoclinic BiVO₄ preparation [114]. There are numerous advantages to this hydrothermal approach to selectively produce BiVO₄ 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]. BiVO₄ was also used for the scission of water [117,118]. BiVO₄ was shown to be an active photocatalyst for O₂ evolution under visible light radiation since its conduction strip potential isn’t high enough to produce H₂ by H₂O reduction [119]. Booshehri et al., found BiVO₄ 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 BiVO₄/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 BiVO₄/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].

2.4. BiOX (X = F, Cl, Br, I)/Graphene Composites

Bismuth oxyhalides’ (BioXoptical)’s properties can work as a photocatalyst. The structure of BiOX crystals is comprised of layer structure slabs [Bi₂O₂] which are inserted in two halogen atoms [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 visible-light 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 graphene-based semiconductor photocatalysts. A new BiOCl-Bi-Bi₂O₃/rGO heterojunction with oxygen vacancies has been developed, which provided a solid-solid, close-fit interface and strong interaction between BiOCl, Bi, rGO, and Bi₂O₃. BiOCl-BI₂O₃/rGO heterojunctions showed high photocatalytic performance due to the synergistic effect caused by effective charge separation among Bi₂O₃, BiOCl, rGO, and Bi-bridges. The BiOCl-Bi-Bi₂O₃/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-Bi₂O₃/rGO heterojunctions. In addition, a promising BiOCl-Bi-Bi-Bi₂O₃/rGO photocatalytic mechanism was proposed to describe primary phenomena taking place during the process, depending on multiple charge transfer channels [141].

2.5. BiPO₄/Graphene Composites

BiPO₄ 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 BiPO₄ has also been reported [143]. The bandgap in BiPO₄ prepared by hydrothermal methods is about 3.85 eV, higher than that of TiO₂ (3.2 eV). BiPO₄ 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 TiO₂, BiPO₄ still has high photocatalytic degradation kinetics. This is because the VB of BiPO₄ is 3 eV, higher than that of TiO_2, and it generates more oxidative holes in its VB compared to TiO₂. Photocatalytic conversion of the gas-phase benzene into CO₂ by BiPO₄ 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 CO₂ was also reported during an aqueous phase organic pollutant degradation study [144,145]. BiPO₄ 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 BiPO₄ and subsequently limit its industrial-scale applications [146,147]. Consequently, it is urgent to create and design photocatalytic materials based on BiPO₄, with required and useful photocatalytic performance properties. To date, numerous efforts have been made to improve the photocatalytic activity of the BiPO₄ photocatalyst by doping with non-metals or metals, surface hybridization, reducing the crystal size, forming heterostructures, or combinations of μ-structure materials [148,149]. BiPO₄/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 BiPO₄/rGO hybrids is an efficient way to improve the visible light catalytic performance of BiPO₄. 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].

2.6. (BiO)₂CO₃/Graphene C omposites

Bismuth subcarbonate is a known solid carbonate in the BI₂O₃-CO₂-H₂O system ((BiO)₂CO₃ or Bi₂CO₅) [152]. The bandgap of (BiO)₂CO₃ is 3.4 eV, so wavelengths under 365 nm can therefore stimulate the bandgap [153,154]. The CB of (BiO)₂CO₃ generally includes hybridized p-orbitals (O₂ 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)₂CO₃ 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 TiO₂-Bi₂O₃/(BiO)₂CO₃-rGO nanocomposite has been synthesized and experimentally used for bisphenol A (BPA) photodegradation. The Bi₂O₃ was intended to be a visible light photosensitizer. The appropriate VB and CB’s positions TiO₂ and (BiO)₂CO₃ 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].

2.7. M(BiO₃)_n/Graphene Composites

Pentavalent bismuthates (M(BiO₃)_n (where n = 1, M = Li, Na, K, Ag; n = 2, M = Mg, Zn, Sr, Ba, and Pb) can be bought directly from commercial companies to synthesize additional Bi-based compounds, such as BiOX, as a Bi source [160]. The bandgaps of these compounds are MgBi₂O₆ 1.61 eV, ZnBi₂O₆ 1.53 eV, SrBi₂O₆ 1.93 eV, SrBi₂O₆ 1.93 eV, BaBi₂O₆ 1.93 eV, PbBi₂O₆ 1.92 eV [161], LiBiO₃ 1.8 eV, KBiO₃ 2.1 eV [162], NaBiO₃ 2.6 eV [163], and AgBiO₃ 2.5 eV, respectively [164]. The valency of Bi-based composites is +3 while the value of (BiO₃)_n is +5. The Bi³⁺ 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 NaBiO₃, LiBiO₃, BaBi₂O_6, and SrBi₂O₆. 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(BiO₃)n could be used for efficient visible light photocatalytic degradation of organic pollutants [161,165].

3. 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.

3.1. 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.

3.2. 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/Bi₂MoO₆ 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 Bi₂MoO₆. There is an extraordinary improvement in the photocatalytic activity for bacterial treatment over the Bi₂MoO₆–rGO nanocomposite compared to the pure Bi₂MoO₆. This enhancement is accredited to the high orientation of Bi₂MoO₆, 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 Bi₂MoO₆–rGO nanocomposites can be accomplished [107].

The Bi₂MoO₆ microsphere surface contains different sizes of Ag₃PO₄ particles. The Bi₂MoO₆ and Ag₃PO₄ microspheres on both sides of the layer rGO are also well connected. The Ag₃PO₄/rGO/Bi₂MoO₆ 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 Ag₃PO₄/rGO/Bi₂MoO₆ composite [178]. Ag₃PO₄/rGO/Bi₂MoO₆ 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 Ag₃PO₄/rGO/Bi₂MoO₆ nanohybrid, a photocatalytic mechanism for MB-degradation via Ag₃PO₄/rGO/Bi₂MoO₆ nanohybrid, and energy band structures of Ag₃PO₄ and Bi₂MoO₆.

For BiPO₄ 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 BiPO₄ may have desirable graphene and BiPO₄ properties. This will significantly improve the photocatalytic activity of the BiPO₄ system. The two-step method of preparing the BiPO₄/GO nanocomposites was first used to synthesize oleylamine-coated BiPO₄ 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 BiPO₄/rGO cuboids with low OH-related defects. Although nanocomposites are produced successfully with BiPO₄-GO or BiPO₄/rGO, the experiments still display a large number of inconveniences: (1) BiPO₄/GO or BiPO₄-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 BiPO₄ results from 2- or multi-step synthetic routes to BiPO₄/rGO, so a simple, efficient, and green approach has been used to synthesize nanocomposites.

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.

A simple one-pot hydrothermal route was used to synthesize nanocomposites of biPO₄/nitrogen-doped graphene hydrogel (BiPO₄) 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 BiPO₄ visible illumination pairs. The BiPO₄/3DNGH morphology has disclosed a cross-linked, porous structure, and 3DNGH nanorods are attached to the area. The 3DNGH surface was randomly dispersed with BiPO₄ nanorods [181].

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 molecules on sufficiently large surfaces. The sample produced for MoS₂ has an ultra-free nanosheet structure. In composites many BiPO₄ NPs are dispersed compactly and homogenously on the surface of MoS₂/graphene nanosheets. It is proposed to dispense, build, and attach the BiPO₄ nanocrystals in MoS₂/graphene by microwave-assisted techniques. There are distinct gate fringes on the BiPO₄-MoS₂/graphene composite. The gap from 0.328 nm to the monoclinic plane BiPO₄ (200) corresponds very well, while the gap from 0.62 nm to the plane of (002) MOS₂ can be assigned. The presence of close contact between MoS₂/graphene nanosheets and BiPO₄ NPs is predictable for building a necessary hetero structure [180]. BiPO₄/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 BiPO₄ NPs gave rise to an increase in steric hindrance. It thus stuck the BiPO₄ NPs electron transfer toward the electrode surface, causing an observable fall in photocurrent [182].

rGO/Bi₂MoO₆ nanosheets were successfully synthesized using rGO/Bi₂ (EG) precursors using a two-stage solvothermal method. The introduction of graphene supports the recombination of electrons and holes generated by photogenerated rGO/Bi₂MoO₆ nanocomposite exhibits plate-on-plate enhanced Cr (VI) photoreduction structures with radiation from sunlight. With an ideal photocatalytic activity, the 2.5% rGO/Bi₂MoO₆ composite and a reduction of 94% to Cr(VI) at about 30 min, roughly twice that of pure Bi₂MoO₆. The rGO, which mainly functions as an electron collector and meaningfully promotes the photoinduced carrier separation, accommodates the improved photocatalytic efficacy. Furthermore, rGO/Bi₂MoO₆ composites have excellent stability and can be recycled in an industrial process. The composite morphologies of 2.5% rGO/Bi₂MoO₆ are low-lying and non-regular plate-on-plate structures. This indicates that Bi₂MoO₆ nanoflocks are scattered to the surface of large graphene layers forming Bi₂MoO₆ 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. MoS₂ was used to improve the transition between graphene and BWO through the “stepping stone” approach. A positive synergetic effect between the graphene sheets and MoS₂ is believed to occur. The cocatalyst components on photo- degradation 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].

Bi₂WO₆/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 Bi₂WO₆. In addition, Bi₂WO₆/rGO is highly stable and essential for applications in environmental protection applications [188,189].

3.3. 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/BiPO₄ were synthesized using an easy self-assembly two-phase 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. Summary of bismuth/graphene-based photocatalyst fabrication methods, morphology, and applications.

Photocatalyst	Activities	Morphology	Method	Refs.
Bi2MoO6/Au/rGO	RhB	lattice fringes	solvothermal and photochemical reduction	[20]
BiPO4/nitrogen-doped graphene hydrogel	biomedical, food and environment analysis	porous structure	one-pot hydrothermal	[181]
BiPO4/GO	MB	sphere-like/rod	two-phase self-assembly	[196]
BiPO4/MoS2/graphene	RhB	lattice fringes with wrinkles and folds	one-pot microwave-assisted hydrothermal	[180]
Bi2MoO6/Pd-rGO	phenol	microspheres/flake-like particles	Solvothermal photoreduction method	[200]
BiOBr/Au/Graphene	phenol	flower-like microstructure	hydrothermal synthesis and reduction method	[141]
BiPO4/rGO	Chlorpyrifos	nanoparticles/nanosheets	solvothermal method	[182]
TiO2-Bi2O3/(BiO)2 CO3-rGO	bisphenol A	nanoplates/nanosheet/nanorod	hydrothermal procedure	[159]
TZB-Gr composite	NO	NPs/2D graphene sheets	sol-gel based electrospinning process	[175]
black BiOCl-Bi-Bi2O3/rGO	2-nitrophenol (2NP)	nanosheets	sonication and mechanical stirring, in situ Fe reduction	[141]
BiPO4-graphene	Methyl Orange MO	wrinkles and folds	one-step solvothermal	[183]
Bi2WO6-rGO	NO	microspheres/nanosheets	hydrothermal method	[188]
Bi2MoO6/2D-rGO	Cr(VI) reduction	wrinkled nanoflakes	hydrothermal method	[184]
BWO−RGO	bisphenol A degradation	uniform structure	hydrothermal treatment	[195]
Bi2MoO6–RGO	bacterial destruction	highly oriented morphology	hydrothermal process	[107]
Bi2MoO6/Ag3PO4/RGO	MB	microspheres/flakes/irregular-sphere	precipitation-solvothermal method	[178]
Bi/BiOBr/Graphene	Degradation of RhB	Nanosheets assemble into flower-like microspheres	One-step solvothermal	[201]
Bi-NPs/GO	Remove ppb-level NO	nanospheres	Solution-based sonication	[73]
Bi-NPs/Graphene	Disinfection and antibacterial activity towards Escherichia coli	nanospheres	Non-injection facile strategy	[202]
PbBiO2Br/GO	CO2 conversion to CH4	nanolayers	Hydrothermal synthesis	[203]
h-BiVO4/rGO	BPA degradation and H2 evolution	nanoplates embedded nanosheets	Ultrasonication	[204]
BiVO4/rGO	MB degradation	nanoparticles	Hydrothermal synthesis	[205]
BiFeO3/N-rGO	RhB degradation	nanoparticles	Sol-gel method followed by hydrothermal synthesis	[206]
BONPs-NG/NGO	Xylene removal	nanoplates embedded nanosheets	Carbon vapor deposition, stirring, and heating	[207]
Bi(PO4)/GO	Ciprofloxacin degradation	nanospheres embedded nanosheets	Cross-linker polymerization	[208]
BiVO4/rGO	Triethylamine (TEA) detection	nanosheets wrapped with particles	Hydrothermal synthesis	[209]

.

4. 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].

4.1. 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 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].

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.

4.2. 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 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].

4.3. Other Applications

4.3.1. 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 (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 lower-temperature [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 semi-catalysts 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 forms with different morphology can be synthesized by various approaches, like a cetyl- trimethylammonium 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 Bi₂WO_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 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.

4.3.2. 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 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, H₂O is considered an ideal solvent. However, the problem is that, under photocatalytic conditions, the semiconductor VB hole can oxidize H₂O into a highly active OH radical form, making the reaction system complicated. Bi₂WO₆ photocatalyst VB’s inherent reduction potential is +1.77 V vs. Ag/AgCl, which is negative to the H₂O/ANOH. H₂O as a solvent is possible when Bi₂WO₆ is used as a catalyst. Recently, a selective oxidation of benzyl alcohols into aldehydes has been effectively developed with a Bi₂WO₆/H₂O/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., WO₃, TiO_2, and ZnO) have strong oxidation power, VB holes tend to oxidize non-selectively 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.

4.3.3. 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].

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.

5. 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 Bi³⁺-containing compounds, like Bi₂O₃ BiOX (X = Cl, Br, I), BiPO₄, BiVO₄, BiFeO₃Bi₄Ti₃O_12, Bi₂WO₆, Bi₂O₂CO_3, Bi₁₂TiO_20, Bi_0.5K_0.5TiO₃, and Bi₃TiNbO₉. Among them, a majority of the compounds possess a plate-like appearance and layered structures. Visible light can excite Bi⁵⁺-containing compounds, e.g., KBiO₃, LiBiO₃, and NaBiO₃. Hybridized O 2p and Bi 6s₂ 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 Bi₆ 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 Bi₂O₃, Bi₂MO₆ (M = W, Mo, and Cr), BiVO₄, BiOX (X = I, Br and Cl), BiPO₄, pentavalent bismuthate and (BiO)₂CO₃, 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 CO₂ photoreduction [94]. During various studies, photocatalytic water division for generating O₂ and H₂ 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].

6. Summary and Outlook

Future developments would be part of the present start of this new century. Bi/graphene-based 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 H₂ 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.