Bismuth-Based Multi-Component Heterostructured Nanocatalysts for Hydrogen Generation

Abstract

Developing a unique catalytic system with enhanced activity is the topmost priority in the science of H₂ energy to reduce costs in large-scale applications, such as automobiles and domestic sectors. Researchers are striving to design an effective catalytic system capable of significantly accelerating H₂ production efficiency through green pathways, such as photochemical, electrochemical, and photoelectrochemical routes. Bi-based nanocatalysts are relatively cost-effective and environmentally benign materials which possess advanced optoelectronic properties. However, these nanocatalysts suffer back recombination reactions during photochemical and photoelectrochemical operations which impede their catalytic efficiency. However, heterojunction formation allows the separation of electron–hole pairs to avoid recombination via interfacial charge transfer. Thus, synergetic effects between the Bi-based heterostructured nanocatalysts largely improves the course of H₂ generation. Here, we propose the systematic review of Bi-based heterostructured nanocatalysts, highlighting an in-depth discussion of various exceptional heterostructures, such as TiO₂/BiWO₆, BiWO₆/Bi₂S₃, Bi₂WO₆/BiVO₄, Bi₂O₃/Bi₂WO₆, ZnIn₂S₄/BiVO₄, Bi₂O₃/Bi₂MoO₆, etc. The reviewed heterostructures exhibit excellent H₂ evolution efficiency, ascribed to their higher stability, more exposed active sites, controlled morphology, and remarkable band-gap tunability. We adopted a slightly different approach for reviewing Bi-based heterostructures, compiling them according to their applicability in H₂ energy and discussing challenges, prospects, and guidance to develop better and more efficient nanocatalytic systems.

1. Introduction

As the demographic transition theory indicates, the world’s population has increased explosively and simultaneously galvanized many problems, such as the energy crisis, global warming, environmental pollution, and degradation of fossil fuels. In order to combat these problems, researchers envisage green energy resources will support economically sustainable development as an alternative to existing energy resources, propelling this as the ultimate challenge for the research community [1]. According to statistical analysis, it is speculated that by 2050, energy consumption will be nearly 27 terawatts (TW), which is not practically sustainable using fossil derivative fuels. Hence, the development of renewable energy sources (RES) is of critical importance, as some variables, such as population growth and global food supply, cannot be significantly modified [2]. According to the report by the International Energy Agency (IEA) on net-zero policy, there will be slowing in global temperature elevation by 2050 through taking green pathways instead of non-renewable energy sources [3]. Hydrogen (H₂) generation has attracted much attention, as it is the most viable replacement for fossil fuels in the future, owing to its escalated efficiency [4]. Precursors of clean H₂ fuel are available abundantly; however, the challenges associated with their cost of efficient production are multifarious. Innovative socio-economic policies and research developments are urgently needed for the sustainability of an H₂ economy. In order to harness H₂ as an energy source in the long term and to enhance the share of renewable energy in total energy consumption, several developing and developed nations are investing in the sustainable energy sector. Global accords that are internationally mutually agreed upon can serve as the building blocks of an environmentally viable H₂ pathway [5].

H₂ production from fossil fuels is a major concern, as it extensively depletes fossil fuels, causing chain reactions that influence the global climatic and exogenic phenomena. Furthermore, the ample use of non-renewable energy sources, such as burning fossil fuels, increases anthropogenic carbon dioxide (CO₂) emissions and is responsible for the greenhouse effect and climate change resulting from increasing global temperature. H₂ fuel is non-toxic, abundantly available and inexpensive, and has higher energy than natural gasoline, along with higher gravimetric energy density, i.e., high energy-to-weight ratio [6]. The significant challenge for the H₂ economy is the storage of H₂, as it is flammable due to a low ignition temperature that leads to explosion [7]. It is not stored underground, as it has a high escaping tendency because of its small size. The odorless, colorless, and tasteless properties of H₂ make it difficult to detect by physical methods. Currently, two main problems arise in the energy sector: the rise in global temperature and degradation of fossil fuels. Most of the needs for energy consumption are fulfilled by fossil fuels and natural gas, directly leading to the depletion of non-renewable energy sources and ultimately causing environmental hazards [8,9,10]. It is essential to raise awareness about renewable energy for sustainable development to minimize the world’s energy crisis.

There are many other renewable energy sources, such as wind energy, thermal energy, biomass, etc. [11], and amongst them, solar energy is the most prominent, abundantly available energy, a major reason for the substitution of non-renewable energy sources using solar energy. Solar energy is favorable for H₂ production because, unlike fossil fuels, it is a green and clean energy source with higher efficiency for energy conversion. Visible, light-activated materials account for about 43% of the solar spectrum, while UV light-active semiconductors make up only 5% [12]. This necessitates the importance of developing visible light-active catalytic systems. Fujishima and Honda carried out photoelectrochemical water splitting studies on TiO₂ in 1972 [13]. Since then, H₂ generation using overall water splitting (OWS) has attracted much attention, and has been considered as the most sustainable, renewable, and clean energy source. It is a very efficient approach, as water is the starting material and also the byproduct of the energy conversion process [14,15]. Various routes, such as photocatalytic (PC), electrocatalytic (EC), and photo-electrocatalytic (PEC) pathways, are preferred for OWS applications [16]. Multifunctional nanocatalysts for water splitting are urgently needed for carrying out diverse optoelectronic, multiferroic, and sustainable energy applications, as they possess more exposed sites than the bulk catalysts [17,18].

Among the fundamental approaches to water splitting, semiconductor photocatalysis has an immense potential to split water into H₂ and O₂. There are two reactions that occur: one is the H₂ evolution reaction (HER), in which H⁺ accepts an electron to produce H₂; and the O₂ evolution reaction (OER), in which holes participate in the evolution of O₂ by reacting with water. Water splitting is an endothermic process, with a very large free energy value of ΔG ~237 KJ/mol, and is known as an “up-hill” reaction [19]. Oxidation and reduction reactions occur at the respective reaction sites. Few photocatalytic systems are active under UV irradiation. However, for higher efficiency, one needs a catalyst with a narrow band range in the visible light region for enhanced light absorption, leading to effective OWS [20].

According to band theory, the semiconductor contains the conduction band (CB), valence band (VB), and energy levels between VB and CB. The band structure of the semiconductors has discontinuous energy bands [21,22,23]. The oxidation and reduction of photogenerated holes and electrons depend on the positioning of VB and CB, respectively. After irradiating with light, semiconductor photocatalysts exhibit photoexcitation, due to which electrons jump from the VB to the CB, leaving a hole in the VB and then diffusing the photogenerated charges at the surface of the semiconductor. In the case of water splitting, H⁺ accepts an electron to produce H₂, while in the case of pollutant degradation, electrons are captured by O₂ to produce superoxide radicals. The primary focus of the researchers is the development of nanocatalysts that can excellently carry out OWS operations [22]. Other important routes for green H₂ energy production are electrocatalysis and photoelectrocatalysis. In an electrocatalysis reaction, water splitting is composed of two electrodes, and HER and OER reactions occur at the respective electrode. A more efficient catalytic system in OWS is a promising candidate for replacing fossil fuels with traditional H₂ generation. However, a higher overpotential is needed to overcome the sluggish kinetics in EC applications. In photoelectrocatalysis, the cell comprises of photoanodes and photocathodes [14,20]. Generally, p-type and n-type semiconductors have been exploited for photocathodes and photoanodes, respectively. A high photocurrent density is needed for enhanced PEC efficiency [22,23].

The various Bi-based compounds are exploited in the PC and PEC applications owing to tremendous physicochemical features such as narrow band gap of less than 3 eV, appropriate band alignment, excellent optoelectronic properties, non-toxicity, low cost, and excellent light-driven ability, which preferably occurs in the visible region. Along with their exploitability in HER, OER, and pollutant degradation, Bi-based compounds also have plenty of applications in industrial sector [24]. In the past few years, many Bi-based nanocatalysts have been utilized in OWS applications, such as Bi₂O₃ [25], BiPO₄ [26], BiVO₄ [27], BiOCl [28], Bi₂Mo₆ [29], etc., but have exhibited nominal catalytic efficiencies because of the swift recombination of photogenerated charge carriers, photo-corrosiveness, and chemical instability due to exceptionally reductive electrons and oxidative holes. Some modifications have been developed to obtain the optimum catalytic efficiency of the Bi-based nanocatalysts. Semiconductor nanocatalysts synthesized by different methods have different sizes, structures, and optoelectronic properties and have their pros and cons [30]. Various methods, such as doping with other metals and nonmetals, building heterostructures, surface atom reconstructions/modifications, morphology control, etc., ensure the accelerated performance of Bi-based nanocatalysts [31]. There is expeditious research on Bi-based heterostructured nanocatalysis conducted year by year, as demonstrated in Figure 1.

Bi-based oxides contain 6s orbital of Bi and 2p orbital of oxygen in the VB making, making them more favorable than other oxides such as TiO₂, a well-known photocatalytic material that has a wide band gap of ~3.2 eV [32]. PEC water splitting with TiO₂ has been studied for a long time, but it is less effective as the band gap lies in the UV region. Metal nitrides, sulfides, and oxides are favorable photoelectrodes, but metal oxides are generally explored due to their low cost and stability. Bi-based composite oxides are cost-effective and contain a perovskite-type structure with the general formula ABX₃, in which the ‘A’ atom contains rare earth metals, and the ‘B’ atom has transition metals, and ‘X’ can be oxides or halides. Perovskite oxide is an excellent multiferroic material with optoelectronic properties. Bi₂WO₆ is an Aurivillius phase material with a band gap of ~2.8 eV and an orthorhombic structure [33]. It combines with other semiconductors to improve the visible range, separation of charge carriers, photo-corrosion, and stability of the catalysts.

Bi₂WO₆ forms heterojunction with ZnIn₂S₄ to improve the catalytic efficiency of OWS [34]. BiVO₄ shows exceptional photocatalytic activity with a band gap of ~2.4 eV but undergoes a recombination reaction. For an efficient H₂ generation, Z-scheme ZnIn₂S₄/BiVO₄ heterojunction is effectively employed. The rate of photocatalytic H₂ production is 5944 μmol h⁻¹, five times higher than that of bare ZnIn₂S₄ [35]. Bi₂MoO₆ exhibits excellent light-driven properties, showing a band gap of 2.7–2.8 eV. It is chemically stable, cost-effective, and non-toxic. It is a very effective catalyst showing excellent optoelectronic properties and is used for water splitting and for pollutant degradation applications [36]. The Bi₂S₃/Ni₃S₂/NF (Nickel Foam) heterostructure electrocatalyst exhibits a low overpotential of 268 mV at 10 mA cm⁻² and low Tafel slope of 82 mVdec⁻¹, leading to a high electrocatalytic performance with a strong synergistic effect [37]. High current density and low Tafel slope are the ultimate requirements for a better electrocatalyst. The BiVO₄/WO₃/SnO₂ ternary heterostructured photoelectrocatalyst delivers higher photocurrent density as high as 3.1 mAcm⁻², and maximum incident photon-to-current conversion efficiencies (IPCE) of ~70% with enhanced PEC activity [38]. Therefore, this review endeavors to summarize the various promising Bi-based heterostructured nanocatalysts such as TiO₂/Bi₂WO₆, Bi₂WO₆/Bi₂S₃, Bi₂WO₆/BiVO₄, Bi₂O₃/Bi₂WO₆, ZnIn₂S₄/BiVO₄, and Bi₂O₃/Bi₂MoO₆, which have made steady progress so far in the development of water splitting for effective H₂ generation.

2. Scope of the Review

Hitherto, there are many reviews on the various Bi-based nanocatalysts for organic degradation [39], environmental applications [40], fuel production [41], water treatment [42], air purification [43], and CO₂ reduction [44]. There are sporadic reviews which highlight the importance of Bi-based heterostructures towards green energy production [45,46]. Some reviews also have a proclivity for various modification strategies [47]. Due to their advanced physicochemical properties, Bi-based heterostructures attain a lot of interest in PC, EC, and PEC applications. However, we could not find a review that discusses their applicability in the PC, EC, and PEC pathways of H₂ generation under a single umbrella. Therefore, this review merely focuses on Bi-based heterostructured nanocatalysts for varied applications in PC, EC, and PEC for H₂ production.

3. Heterojunctions

Heterostructures are composite materials (conductor, semiconductor, or insulator) that contain an interface between two or more different materials known as heterojunctions. The schematic diagram of heterojunction formation is illustrated in Figure 2. Heterojunction formation tends to separate electron–hole pairs from restraining charge recombination. It helps retain the redox reactions at different sites and maintain energy band gaps [48]. Heterostructures constructed from two other semiconductors improve light absorption ability and enhance charge separation due to the interface effect [36]. Upon irradiation, the photogenerated electrons and holes are enforced between the interface, exhibiting an electric field. The photoexcited electrons and holes can only be transferred from a wide bandgap to a narrow band gap. The very conceptual framework of heterostructure comes from semiconductor physics. It is a very effective method for the modification of catalytic activity towards PC and PEC applications, as the heterostructure formation tremendously subdues the recombination of charge carriers for PC and PEC applications, improves the transport properties, modifies active sites, etc. The various heterojunctions show distinct catalytic applications. The four major types of heterojunctions for charge separations are (a) Type II heterojunctions, (b) Z-scheme heterojunctions, (c) p–n heterojunctions, and (d) Schottky heterojunctions [49,50], as demonstrated in Figure 3. The Z-scheme and p–n heterojunctions have an internal electric field (IEF) at the junctions of two different composite materials, which is productive in carrier charge transfer, relating to separation under excitation conditions.

3.1. Type II Heterojunctions

Type II heterojunctions are also known as staggered band systems due to the staggered alignment of bands between the two semiconductors [51]. The electrons transfer from the semiconductor with the higher CB to the semiconductor with the lower CB, and holes transfer from the semiconductor with the lower VB to the higher VB of the second semiconductor, respectively. In a type II heterojunction, electrons and holes migrate in opposite directions. Apart from type II, there are also type I and type III heterojunctions. Type I has a straddling band alignment, and type III is of broken band type. However, the type II heterojunction is the most promising for water splitting as it facilitates the effective separation of electron–hole pairs [52]. V₂O₅/BiVO₄ [53], BiVO₄/TiO₂/FTO [54], and BiVO₄/CeO₂ [55] are some of the Bi-based type II heterojunctions for feasible OWS applications. Yaw et al. [53] synthesized V₂O₅/BiVO₄ type II heterojunction photoelectrodes for PEC applications. Cheng et al. [54] synthesized a ternary type II heterostructured photocatalyst BiVO₄/TiO₂/FTO by constructing BiVO₄ on fluorine-doped tin oxide (FTO) and TiO_2, improving the charge separation ability of the catalyst. Wetchakun et al. [55] synthesized BiVO₄/CeO₂ type II heterojunction photocatalysts through hydrothermal synthesis. These are the few catalytically optimized type II heterostructured systems in Bi-based oxides.

3.2. Z-Scheme Heterojunctions

These can be classified into traditional and direct Z-scheme heterojunctions. In the traditional Z-scheme, there is a redox mediator that works as the charge carrier. It was proposed by Bard in 1979, and it improves charge separation efficiency and shows great redox ability. However, the major disadvantages of this method are that it is only confined to the solution phase and is prone to pH sensitivity and undesirable side reactions. While in the case of the direct Z-scheme, no intermediate is usually employed. This heterojunction is analogous with natural photosynthesis and exhibits high electron–hole separation propensity and redox ability [35,56]. In 2006, Tada et al. [57] introduced the solid-state Z-scheme photocatalyst, with two different photocatalytic semiconductors (PS), PS-I and PS-II, and an electron mediator. The two semiconductors must have a suitable band gap for oxidation and reduction potential for the effectual construction of Z-scheme heterojunction. The electrons have high reduction potential, and the holes have a high oxidation potential so that they cannot recombine. In this pathway, electrons and holes recombine at the interfacial region and possess remarkable redox ability compared with type II heterojunctions. The fabrication cost of Z-scheme heterojunction catalysts is lower than that of type II heterojunctions [58]. Bi₂WO₆/Ta₃N₅, black phosphorus/Bi₂WO₆, Bi₂WO₆/CuBi₂O₄, and Bi₂WO₆/Cu₃P are some Bi-based Z-scheme heterojunctions that exhibit enhanced catalytic performance and excellent characteristics towards OWS [59,60]. These Bi-based Z-scheme heterojunctions also possess remarkable responses towards environmental remediation, reported by Dutta et al. [61] who fabricated Z-scheme BiFeO₃/CuS/SiO₂ nanocatalysts for the degradation of methyl orange dye. Ternary Z-scheme systems deliver an effective pathway for the separation of photogenerated electron–hole pair by maintaining the redox potential and possessing excellent visible light-driven PC and PEC activities [62].

3.3. p–n Heterojunctions

The diffusion of charge carriers between two different semiconductors, specifically p- and n-type materials, develops an internal electrical field (IEF) ascribed to the distribution of electrons and holes in the heterojunction. This space charge region is known as a p–n heterojunction [63]. IEF emerges between Bi-based heterostructures during p–n heterojunction formation and infers the advanced catalytic efficiency towards overall water splitting [63,64,65,66,67,68,69]. Apart from p–n heterojunctions, there are also n–n type (consists of both n-type) semiconductors and p–p type (containing both p-type) semiconductors. Charge separation is faster in p–n type compared to the p–p type and n–n type heterojunctions [67]. The various Bi-based p–n heterojunctions such as Bi₂WO₆/BiOI [63], BiOBr/Bi₂WO₆ [64], and BiOCl/BiVO₄ [65] have been exploited to increase the catalytic efficiencies of pristine Bi-based compounds. Bi₂MoO₆ is an n-type semiconductor that forms a heterojunction with p-type Ag₂O, creating an Ag₂O/Bi₂MoO₆ p–n heterojunction [66]. Dai et al. [63] synthesized a BiOI/TiO₂ p–n junction by coating BiOI on TiO₂ through the hydroxylation method. BiOCl is a p-type semiconductor with a wide band gap of ~3.5 eV and exhibits negligible catalytic activity. Feng et al. [65] doped it with BiVO₄ to engineer a BiOCl/BiVO₄ p–n heterojunction which exhibited marvelous photocatalytic activity. Peng et al. [69] synthesized Bi₂O₃/Bi₂WO₆ p–n heterojunctions with enhanced charge separation. BiFeO₃ is a single perovskite photocatalyst with a band gap of ~2.1 eV that shows multiferroic properties, due to which Arunkumar et al. [70] fabricated BiFeO₃/SnS₂ p–n heterojunctions for pollutant degradation. The separation of photogenerated electron–hole charge carriers is thermodynamically attainable because the CB and the VB in p-type semiconductors are higher than those in n-type semiconductors, thus forming p–n heterojunctions. The charge carrier separation in p–n heterojunctions is faster than that of type II heterojunction photocatalysts due to the synergistic effect of IEF and band alignment [71].

3.4. Schottky Heterojunctions

This is the junction formed between a semiconductor and a metal. Due to unequal Fermi energy levels, a Schottky barrier is formed, which helps subdue the back recombination of photogenerated electron–hole pairs. In Schottky heterojunctions, the electrons are transferred from the surface plasmon of metal with higher work function (ɸ) to the semiconductor with lower ɸ or excited electrons from semiconductor to metal. Mainly, metals with high ɸ are doped with n-type semiconductors to form a Schottky junction. Various noble metals such as Pt, Pd, Au, and Ag are incorporated in Bi-based compounds to enhance their catalytic efficiency [72]. The synergistic effect of the metal nanoparticles on the surface of the semiconductor enhances the separation of the charge carriers [73]. In the past decade, various Bi-based Schottky heterojunction catalysts have been employed for OWS applications, such as Ag/Bi₃TaO₇ [72], Pt/Bi₂WO₆ [73], Ag/Bi₂S₃ [74], and Ag/BiVO₄ [75].

4. Bi-Based Heterostructures Exploited in Multi-Functional H₂ Generation

The synthesis of Bi-based heterostructures has made consistent improvement, and several advanced synthetic strategies have been developed and engaged in designing Bi-based heterostructures with distinct nanostructures. The conventional features of all the synthetic routes are to upgrade the physiochemical properties and to produce a significant number of catalytically active sites. As a result, to achieve nanocatalysts propelled by morphology and surface area, it is important to employ modern synthetic approaches such as solvothermal/hydrothermal methods [76,77,78,79], the polymeric citrate precursor (PCP) route [80], the reverse-micellar method [81], the chemical vapor deposition method (CVD) [82], the wet impregnation method [83], and electrochemical methods [84]. The most important Bi-based heterostructured nanocatalysts (Bi₂S₃, Bi₂WO₆, BiVO₄, and Bi₂O₃) which yield an excellent catalytic ability with other semiconductors for PC, EC, and PEC applications are studied in detail.

5. Bismuth Sulfite (Bi₂S₃)-Based Heterostructured Nanocatalysts

Bi₂S₃ is an excellent n-type semiconducting material with a distinctive catalytic efficiency and narrow visible range band gap of nearly 1.62 eV, making it an ideally suitable photocatalyst. Bi₂S₃ exhibits an excellent optoelectronic property, a high absorption coefficient of 10⁴–10⁵ cm⁻¹, and lesser toxicity [85], due to which pristine Bi₂S₃ and its heterostructures have been used in diverse photocatalytic applications such as solar cells [86], lithium-ion batteries [87], photodetectors [88], etc. Different morphologies of Bi₂S₃ nanostructures can be attained depending on various physicochemical parameters such as temperature, solvent system, and surfactant [89]. Among the conventional chemical routes generally employed, the hydrothermal method is by far the most versatile procedure for the fabrication of Bi₂S₃ nanorods [90], nanoflowers [91], and nanotubes [92]. Despite possessing tailored band energy, Bi₂S₃ has certain disadvantages, such as photo-corrosiveness and rapid back recombination of charge carriers during photocatalytic water splitting [93]. Bi₂S₃ forms effective heterojunctions with BiVO₄ [94], MoS₂ [95], CdS [96], TiO₂ [97], and ZnS/ZnO [98] for efficient H₂ generation via OWS. Mahadik et al. [94] synthesized Bi₂S₃/BiVO₄ through a PEC transformation approach, which had an enhanced optoelectronic property due to the synergistic effect between Bi₂S₃ and BiVO₄. It exhibited a remarkable photocurrent density of 3.3 mA cm⁻² at 0.67 V potential, and an H₂ production rate of ~417 µmol cm⁻² h⁻¹. Zhou et al. [95] fabricated Bi₂S₃/MoS₂ heterostructured electrocatalysts through a hydrothermal method which exhibited a lowered overpotential (ƞ) of 124 mV and a Tafel slope of 123 mV dec⁻¹ at the current density of 10 mA cm⁻². Bi₂S₃/MoS₂ yielded enhanced HER activity due to the synergistic effect between the two semiconductors in heterojunction with a substantial electrocatalytic surface area. Hao et al. [96] synthesized Bi₂S₃/CdS heterostructure through a sonochemical method for H₂ evolution via photocatalytic OWS. Bare CdS delivered a minimal photocatalytic efficiency of 0.04 mmol h⁻¹ g⁻¹. After forming heterojunctions with Bi₂S₃, the H₂ production rate was elevated to 5.5 mmol h⁻¹ g⁻¹ due to large active sites, unique structure, and effective separation of electron–hole pairs. Ahmad et al. [97] synthesized 3-D rosette–rod TiO₂/Bi₂S₃ heterojunctions, revealing a photocurrent density of 3.98 mA cm⁻² at 1.23 V. The CB of Bi₂S₃ was found to be more negative than the CB of TiO₂. Hence, electron transfer must have taken place from the CB of Bi₂S₃ to the CB of TiO₂, whereas the holes channeled from the VB of TiO₂ to the VB of Bi₂S₃ and resulted in the movement of electrons and holes in the opposite direction, which ultimately accelerated the charge separation and PEC activity of TiO₂/Bi₂S₃. Sadhasivam et al. [98] synthesized a Bi₂S₃/ZnS/ZnO ternary heterostructure, which depicted enhanced PEC properties and yielded a photocurrent density of 220 mAcm⁻² at 0.2 V vs. Ag/AgCl. The ternary heterostructure showed higher current density and charge separation efficiency than pristine Bi₂S₃ and Bi₂S₃/ZnO. Wei et al. [99] fabricated ternary BiVO₄/Bi₂S₃/NiOOH heterostructures through the electrodeposition of NiOOH over BiVO₄/Bi₂S₃ electrodes for evaluating the catalytic efficiency in OWS. BiVO₄/Bi₂S₃/NiOOH exhibited a higher photocurrent density of 0.91 mA cm⁻² at 1.23 V vs. RHE than that of pristine BiVO₄ (0.29 mA cm⁻² at 1.23 V vs. RHE) and BiVO₄/Bi₂S₃ (0.50 mA cm⁻² at 1.23 V vs. RHE). In the PEC operation, the electrons transferred from the CB of Bi₂S₃ to that of BiVO₄ for H₂ production as Bi₂S₃ has more negative CBs and VBs than BiVO₄. Bi₂S₃ acted as the light absorber, improving the separation kinetics and transfer of the photo-generated charge carriers. On the one hand, BiVO₄ worked as an electron conductor for direct charge transfer to Indium-doped tin oxide glass. On the other hand, NiOOH layers enhanced the PEC activity by providing stability to the BiVO₄/Bi₂S₃/NiOOH. Wang et al. [100] fabricated WO₃/Bi₂S₃ through the chemical bath deposition (CBD) method and reported the highest photocurrent density of 5.95 mAcm⁻² at 1.23 V potential, much higher than that of pristine WO₃ (0.17 mAcm⁻² at 1.23 V). The enhancement of catalytic activity was observed due to the formation of type II heterojunction that resulted in the easy transport of charge carriers and impeded the rate of back recombination reactions. Subramanyam et al. [101] developed a Bi₂S₃@rGO nanocomposite photoanode for PEC water splitting and yielded a photocurrent density of 6.06 mA cm⁻² at 1.23 V and higher H₂ generation efficiency of 68.5 µmol/h due to the synergistic effect between Bi₂S₃ and rGO. The mechanism of electron transfer in Bi₂S₃@rGO for PEC water splitting is depicted in Figure 4. These are some remarkable recent reports of Bi₂S₃-based heterostructures, which revealed their catalytic efficiency in H₂ production via OWS.

6. Bismuth Tungstate (Bi₂WO₆)-Based Heterostructured Nanocatalysts

Bi₂WO₆ is a cost-efficient, chemically stable, and environmentally benign Bi oxide with an Aurivillius phase perovskite structure composed of layers between [Bi₂O₂]²⁺ and WO₆. The Bi₂WO₆ has a narrow band gap of ~2.8 eV, and its band contains 2p, 6s, and 5d orbitals of O, Bi, and W, respectively [33]. Monolayer Bi₂WO₆ exhibits excellent catalytic efficiency for green H₂ production via OWS [102]. Bi₂WO₆ can be synthesized through various methods such as hydrothermal, solvothermal, microemulsion, sol–gel, and co-precipitation routes [103,104,105]. Despite its visible light-driven band energy, certain constraints impede its catalytic activity, such as the back recombination of electrons and sluggish kinetic reaction barriers in PC, PEC, and EC. Therefore, to obtain a higher efficiency via interfacial charge transfer, Bi₂WO₆ forms heterojunctions with BiO [106], Bi₂O₃ [107], BiVO₄ [108], AgInS₂ [109], ZnIn₂S₄ [34], TiO₂ [110], and graphene [111]. Parnicka et al. [109] developed AgInS₂/Bi₂WO₆ via a deposition method. The band energy of both semiconductors was well matched and resulted in an effective heterojunction for higher efficiency. Upon visible light irradiation, the electrons from the CB of Bi₂WO₆ combined with the holes in the VB of AgInS₂. In contrast, the holes and electrons from the more positive VB of Bi₂WO₆ and the more negative CB of AgInS₂, respectively, participate in oxidation and reduction reactions, respectively, as shown in Figure 5a,b. This increases the charge separation as well as the redox ability. The synthesized AgInS₂/Bi₂WO₆ depicts a small decrease after reuse and slight decline in activity after three months; furthermore, there were no structural changes confirmed by XRD analysis which showed the stability and reusability of the nanocatalysts, as shown in Figure 5c–e. AgInS₂/Bi₂WO₆ showed the optimum H₂ production rate of 152.75 µmol g⁻¹ h⁻¹, which is much higher than pristine Bi₂WO₆, as shown in Figure 5f. The hydrothermal method is a proven approach for synthesizing nanocatalysts [112]. Xiong et al. [113] proposed Z-scheme BP/monolayer Bi₂WO₆ heterojunctions for an efficient charge separation through the hydrothermal method. Black phosphorous (BP) has high charge mobility, tunability, and variant characteristics, and a high surface area. BP is an excellent catalyst for HER and OER in water splitting [114]. The CB and VB energies of BP are −0.8 V and 0.7 V, respectively. BP/Bi₂WO₆ had an H₂ evolution rate of 21,042 µmol/g with Pt as a cocatalyst, and triethanolamine as a hole sacrificial agent, which is 9.15 times higher than that of pure monolayer Bi₂WO₆. The graphene-based Bi₂WO₆ heterostructures are prepared through the hydrothermal method [115]. As reported, a single layer of Bi₂WO₆ shows a significant H₂ generation (5.6 μmol g⁻¹ h⁻¹), which showed a 14 times enhanced rate than that of bulk Bi₂WO₆ [116]. Two-dimensional materials exhibit vast applications due to their superior surface area and improved charge separation. Two-dimensional Bi₂WO₆ fabricated with the CTAB-assisted hydrothermal method revealed an excellent photocatalytic activity compared with its single monolayer counterpart [117]. Zhao et al. [118] synthesized Bi₂WO₆/RGO (reduced graphene oxide) nanocomposite through the electrospinning method with an H₂ production efficiency of ~935 µmol h⁻¹, much higher than that of Bi₂WO₆. RGO has an exceptional ability to accept and transfer electrons, but an excess amount of it can decrease the photocatalytic activity. The electrons on the CB of Bi₂WO₆ transferred through RGO, thus improving the separation efficiency of the photogenerated charge carriers. Arif et al. [119] fabricated Z-scheme Bi₂WO₆/CdS heterostructures with a two-step hydrothermal method. CdS has a band gap of ~2.4 eV, showing an enhanced optoelectronic property for water splitting [120]. The VB and CB of CdS are well matched with Bi₂WO₆ for constructing heterojunctions. Bi₂WO₆/CdS heterostructures fabricated with the H₂ evolution of ~167 mmol h⁻¹ and Pt loaded showed an increased H₂ efficiency of ~1223 mmol h⁻¹, as Pt is a very promising co-catalyst for the generation of H₂, and has a low overpotential and electron accepting ability. Chachvalvutikul et al. [34] constructed Z-scheme Bi₂WO₆/ZnIn₂S₄ heterostructured nanocatalysts via the hydrothermal method, using methanol as a sacrificial agent. The photocatalytic H₂ production rate of Bi₂WO₆/ZnIn₂S₄ at different compositions was measured, and the highest reported rate was found to be 395.5 µmol h⁻¹g⁻¹ at 12.5% Bi₂WO₆/ZnIn₂S₄, as shown in Figure 6a. The catalyst showed excellent reusability and stability after three successive runs, as depicted in Figure 6b. Upon irradiation, the electrons and holes were produced in the CB and VB of both the semiconductors, then the electrons in the CB of ZnIn₂S₄ channeled towards the CB of Bi₂WO₆, and the holes in the VB of Bi₂WO₆ streamed towards the VB of ZnIn₂S₄, as depicted in Figure 6c.

7. Bismuth Vanadate (BiVO₄) Based Heterostructured Nanocatalysts

This is an n-type semiconductor with a band gap of 2.4 eV, with controlled morphology. It is a very promising nanocatalyst with low toxicity, higher stability towards photo-corrosion, low cost, and excellent photocatalytic activity with a narrow visible range. BiVO₄ exists in different polymorphic forms, which are tetragonal zircon, tetragonal scheelite, and monoclinic scheelite [121]. The reported solar energy efficiency of pristine BiVO₄ is about 9%, and its photocurrent density is 6.47 mA cm⁻² at 1.23 V vs. Ag/AgCl [122]. Kudo et al. [27] reported BiVO₄ for water oxidation in 1998 for the very first time. BiVO₄ has certain disadvantages such as low charge separation efficiency and poor transport ability. To enhance its catalytic efficiency, BiVO₄ forms heterojunctions with ZnIn₂S₄ [35], Fe₂O₃ [123], V₂O₅ [52], BP [124], and WO₃ [125]. Xia et al. [123] constructed Fe₂O₃/BiVO₄ photoanodes with a higher H₂ production of ~28 μmol cm⁻² h^–1 through the spin-coating successive ionic layer adsorption and reaction (SILAR) method, which is considerably higher than that of bare BiVO₄ photoanode. Fe₂O₃/BiVO₄ photoanode exhibited a photocurrent density of 1.63 mA cm⁻² at 1.23 V vs. RHE, and is considered an excellent catalyst for PC and PEC applications. Yaw et al. [53] synthesized V₂O₅/BiVO₄ type II heterojunctions through a facile electrodeposition method with a current density of 1.53 mA cm⁻² at 1.5 V vs. Ag/AgCl, a much improved efficiency than that of individual V₂O₅ (0.21 mA cm⁻²) and BiVO₄ (0.22 mA cm⁻²). WO₃ is an n-type semiconductor with a band gap of ~2.5–2.8 eV. It is stable at a low pH and shows a higher catalytic efficiency than that of TiO₂ [126]. Sitaaraman et al. [125] synthesized WO₃/BiVO₄ heterostructures through spin coating and studied them for PEC applications. WO₃ has a low pH and a greater H₂ generation efficiency. The VB of WO₃ is positive for the oxidation of H₂O, whereas the CB is not negative enough for H₂ generation. Thus, during PEC water splitting, O₂ can be produced easily but H₂ cannot and requires an external potential. Therefore, WO₃ forms a heterojunction with another n-type semiconductor to produce both H₂ and O₂ at the respective sites. Kalanoor et al. [127] reported Mo-doped WO₃/BiVO₄ and showed a high photocurrent density of ~1.48 mA/cm² with high PEC applicability. Baek et al. [38] introduced BiVO₄/WO₃/SnO₂ ternary heterojunctions with improved charge transport and a photocurrent density of ~3.1 mA/cm². Combining SnO₂ with BiVO₄/WO₃ promoted electron transfer and the separation of charge carriers. The photoelectrocatalyst showed higher efficiency and fabricated type II heterojunctions using the sol–gel method. It is a triple-layer photoanode with improved charge separation. The structural and morphological analysis helped characterize the different layers. ZnIn₂S₄ is an excellent photocatalyst due to its remarkable energy band structure and outstanding optoelectronic properties, as discussed above. Hu et al. [35] synthesized ZnIn₂S₄/BiVO₄ through a self-assembly method. ZnIn₂S₄ has a smaller work function ɸ (3.54 eV) and a higher Fermi level. It forms type II heterojunctions with BiVO₄, which exhibits a higher ɸ (4.69 eV) and a lower Fermi level. The electrons migrate from the CB of BiVO₄, while holes remain in the VB, as displayed in Figure 7. ZIS/BVO heterostructured nanocatalysts infer the improved photocatalytic activity with an H₂ evolution of ~5944 μmol h⁻¹ g⁻¹ and stability over four consecutive cycles, as demonstrated in Figure 8. V₂O₅ is an n-type semiconductor with a band gap of 2.3 eV. Yaw et al. [128] synthesized V₂O₅/RGO/BiVO₄ through the electrodeposition method. It is a type II heterojunction with a photocurrent density of 2.1 mAcm⁻² at 1.5 V vs. Ag/AgCl, which is higher than that for pristine BiVO₄ (0.22 mAcm⁻²). On visible light irradiation, the electrons migrate from the CB of BiVO₄ to the CB of V₂O₅ for the H⁺/H₂ reaction, while holes migrate to evolve O₂. RGO has a high conductivity and higher charge mobility of photogenerated charge carriers, thus designing its ternary heterostructure with V₂O₅/BiVO₄ corroborating the high PEC performance. Optimized V₂O₅/RGO/BiVO₄ heterojunction photoanodes show a higher H₂ generation efficiency of ~32.7 mmol/h.

Majumder et al. [129] fabricated BiVO₄/Bi₂S₃/NiCoO₂ ternary heterostructured catalysts for PEC water splitting through the electrodeposition method with the maximum photocurrent density of 3.80 mA cm⁻² (at 1.23 V vs. RHE), higher than that for BiVO₄/Bi₂S₃ and BiVO₄, which achieved 3.39 mA cm⁻² and 2.49 mA cm⁻² (at 1.23 V vs. RHE) photocurrent density, respectively. NiCoO₂ is a p-type semiconductor. BiVO₄/Bi₂S₃/NiCoO₂ formed n–n–p type heterojunctions, as depicted in Figure 9. Fang et al. [130] discussed the advantages of NiCoO₂ as the efficient co-catalytic system in BiVO₄-led catalysis. Ni–Co oxides possess remarkable active surface area for an electrochemical reaction and show significant stability. The tetrahedral sites in NiCoO₂ are vacant and provide space for electronic transitions, create more active sites, avoid back recombination, and are cost-effective [130].

8. Bismuth Oxide (Bi₂O₃)-Based Heterostructured Nanocatalysts

Bi₂O₃ is considered an excellent semiconductor with a band gap of 2.6–2.8 eV, has high active surface area, and exhibits enhanced catalytic efficiency for OWS. Various heterostructures have been fabricated to improve the photocatalytic performances of Bi₂O₃. Bi₂O₃ forms effective heterojunctions with Bi₂SiO₅ [131], Bi₂WO₆ [107], Bi₂MoO₆ [132], BiVO₄ [133], and MoS₂ [134]. Goud et al. [134] fabricated MoS₂/Bi₂O₃ through the hydrothermal method. MoS₂ has a band gap of 1.93 eV and a HER efficiency of the order of platinum, along with exceptional optoelectronic properties [135]. As the resulted MoS₂/Bi₂O₃ heterojunction has a band gap of 1.61 eV, the decrement in band energy of heterostructured nanocatalysts escalates the photocatalytic activity by enhancing the spectrum of visible light absorption. The H₂ production rate reported for the MoS₂/Bi₂O₃ heterostructured catalyst (3075.21 μmol g⁻¹h⁻¹) was found to be nearly 7-fold higher than that of MoS₂ (428.14 μmol g⁻¹h⁻¹). The photocatalytic activity of MoS₂/Bi₂O₃ was also evaluated with a different hole scavenger for an efficient electron transfer, as shown in Figure 10a. The photocurrent response investigated for different compositions of MoS₂/Bi₂O₃ and pristine MoS₂ and Bi₂O₃ is illustrated in Figure 10b. Analogous to Bi₂WO₆, Bi₂MoO₆ is also an important Bi-based perovskite structure [136], with layers between [Bi₂O₂]²⁺ and [MoO₄]²⁻. Various modification strategies have been developed to improve the photocatalytic activity of Bi₂MoO₆, which has an optical band energy of 2.7 eV. Bi₂MoO₆ has a layered structure with enormous potential for CO₂ reduction, H₂ generation, O₂ generation, pollutant degradation, and many more environmental applications [137,138]. However, the efficiency of this single component photocatalyst is restricted due to rapid charge carrier recombination and slow charge transportation. To enhance its catalytic activity, Bi₂Mo₆ forms heterostructures with different semiconductors [139,140]. Fu et al. [132] synthesized Bi₂O₃/Bi₂MoO₆ Z-scheme heterojunctions yielding a 96.4% degradation efficiency, and the higher photocatalytic H₂ evolution rate of 12.97 μmol h⁻¹g⁻¹ in visible light irradiation. Chao et al. [107] synthesized Bi₂O₃-Bi₂WO₆ p–n heterostructured nanocatalysts through the solvothermal method. The results highlighted that Bi₂O₃-Bi₂WO₆ formed p–n heterojunctions as they have a more positive VB than needed for the oxidation potential and have more negative CB than needed for the reduction potential, as shown in Figure 11a, and it exhibited the highest photocatalytic efficiency of 878 μmol h⁻¹ g⁻¹, along with a marvelous durability, which was confirmed by performing three cycles, as revealed in Figure 11b. Ye et al. [141] synthesized Bi₂O₃/BiVO₄ p–n heterojunctions through the hydrothermal method with the improved photoelectrocatalytic activity. It yielded a maximum photocurrent density of 2.58 mA cm⁻² at 1.2 V vs. Ag/AgCl. Shafiq et al. [133] also fabricated Bi₂O₃/BiVO₄ heterojunctions and reported their photocatalytic activity for H₂ production, which was found to be as high as 171.13 μmol h⁻¹ g⁻¹. This response was markedly higher than the performance of individual Bi₂O₃ and BiVO₄ semiconductor nanocatalysts. Salomao et al. [142] fabricated ZnBi₃₈O₆₀/Bi₂O₃ with increased surface area and a photocurrent density of −1.55 mA cm⁻² with 100% H₂ generation efficiency. Syah et al. [143] synthesized Bi/Bi₂O₃ on Ni-foam electrocatalysts, exhibiting high surface area, higher mobility, and an effective transportation of electrons. Bi/Bi₂O₃ on Ni-foam yielded a high photocurrent density, low overpotential, and excellent robustness. Salomao et al. [144] designed ternary heterostructures of Bi₂O₃/Al₂Bi₂₄O₃₉/Al₂Bi₄₈O₇₅ (Bi₂O₃/BiAl) nanowires through a spray pyrolysis method with the suitable VB and CB for redox reactions towards water splitting. They show the highest photocurrent density of −4.85 mA cm⁻² at 0 V vs. RHE with H₂ evolution of 348 µmol/cm²/h and 93% faradaic efficiency.

9. Applications

The catalytic efficiency of Bi-based heterostructured nanocatalysts for varied applications in photocatalytic, photoelectrocatalytic, and electrocatalytic H₂ production via water splitting are discussed in this section.

10. Photocatalytic (PC) Water Splitting

The idea of photocatalysis comes from natural photosynthesis, which converts photonic energy into chemical energy [145]. In photocatalytic water splitting, a semiconductor photocatalyst with a narrow band gap and suitable redox potential is required. The principle of photocatalysis is band theory, which consists in continuous energy levels known as Fermi levels. The reduction potential and oxidation potential in electrons and holes depend on the band alignment. Photocatalysts are basically light-harvesting semiconducting materials. Upon visible light irradiation, the very initial step is the excitation of electron–hole pairs; then, the electrons and holes come to the surface of the semiconductor and participate in HER and OER reactions at respective positions, as shown in Figure 12.

Photocatalytic H₂ generation from water splitting is an environmentally friendly and effective pathway. Generally, in an efficient water splitting process for H₂ generation, there should be an effective interaction between light and catalyst [146]. However, the major challenge in this process is the back recombination of photogenerated charge carriers. Modified heterostructures are well accomplished in improving the photocatalytic efficiency of OWS and are the preferred choice of catalytic systems to deal with back recombination. Bi-based photocatalysts have been studied vastly in the last decade due to their narrow band energies falling in the visible light range, high efficiency, environmentally benign nature, and cost-effectiveness. Li et al. [147] synthesized Bi₂MoO₆/C₃N₄ heterojunctions with the H₂ generation of 563.4 µmol h⁻¹ g⁻¹ through the solvothermal method. Graphitic carbon nitride (g-C₃N₄) has attracted much attention due to its exceptional catalytic property, low toxicity, high stability, and narrow band gap of 2.7 eV; on the other hand, Bi₂Mo₆ also possesses a narrow band gap of ~2.8 eV. The prepared heterostructures have a high photocatalytic efficiency for H₂ generation as well as for bacterial disinfection. Mei et al. [148] synthesized Bi₂MoO₆/CdSe-diethylenetriamine (DETA) forming Z-scheme heterojunctions through the solvothermal method, with an H₂ production of 5.92 mmol g⁻¹ h⁻¹, ascribed to the efficient separation of photogenerated charge carriers. CdSe has a band gap of 1.7 eV, a suitable band for H₂ production. It has adequate active sites for photocatalytic H₂ generation but poor photo-stability, attributed to photo-corrosiveness. Forming Z-scheme heterostructures with Bi₂MoO₆ can improve the stability of CdSe, as well as enhance its photocatalytic activity for H₂ generation. Zhao et al. [149] fabricated Bi₂Ti₂O₇/g-C₃N₄/Bi₄Ti₃O₁₂, a ternary heterostructured photocatalyst, through an electrospinning method with the H₂ evolution of 638 mmol h⁻¹ g⁻¹. Samsudin et al. [83] synthesized TiO₂/BiVO₄ via the wet-impregnation method. The heterostructure formation remarkably improved the H₂ production efficiency due to pronounced interfacial charge transfer. Wan et al. [62] fabricated ZnIn₂S₄, a CuOx/Bi₂MoO₆ photocatalyst via the hydrothermal method. It is a Z-scheme photocatalyst for efficient H₂ and O₂ production via PC water splitting. Pan et al. [26] synthesized BiPO₄/RGO through the hydrothermal route, yielding the H₂ production rate of 306 μmol/h/g, which turned out to be nearly 20-fold higher than that of pristine BiPO₄ (14.9 mmol h⁻¹). Wu et al. [106] synthesized a Bi₂WO₆-BiO nanocomposite via the hydrothermal method. It showed a desirable H₂ generation efficiency of 73.12 μmol h⁻¹ g⁻¹ with appropriate VB and CB. Zhao et al. [29] synthesized Bi₂MoO₆/RGO through the electrospinning method. It is a p–n heterojunction with an enhanced H₂ evolution efficiency of 794.7 μmol h⁻¹. Chen et al. [76] fabricated Cr-doped Bi₄Ti₃O₁₂ nanosheets via the hydrothermal method. They exhibited an H₂ generation efficiency of 116.75 μmol h⁻¹ g⁻¹, which was 2.85 times higher than that of Bi₄Ti₃O₁₂. Fu et al. [78] synthesized NiO/Carbon Dots/BiVO₄ through the hydrothermal method with an H₂ evolution of 1.21 μmol/h. Guo et al. [28] constructed Sn-doped ZnO/BiOCl via the hydrothermal method. It is an effective Z- scheme heterostructured photocatalyst displaying an H₂ evolution rate of 4146.77 μmol h⁻¹ g⁻¹. After going through the aforementioned results, it can be concluded that Bi-based photocatalysts have received an immense interest due to their whopping H₂ production efficiency. Among all the reported photocatalysts, Z-scheme plays an effective role in enhancing the activity of semiconductor photocatalysts. Some of the most important Bi-based heterostructured photocatalysts are demonstrated in Table 1 with their H₂ evolution performance.

11. Photoelectrocatalytic (PEC) Water Splitting

PEC cells consist of a working electrode, reference electrode, and counter electrode and contain an electrode/electrolyte interface, as shown in Figure 13. Water splitting occurs at the surface of the catalyst or at the active sites. Platinum has long been used as the most efficient photoelectrocatalyst for HER due to its binding energy and Gibbs free energy [150]. PEC cells directly convert solar energy to electrical energy. 1.23 eV of energy is required for water splitting. Generally, n-type semiconductors are preferred for photoanode for their better catalytic activity [151]. PEC is developed from PC and EC. Band energy is a very important parameter for selecting a semiconductor photocatalyst for higher efficiency. PEC cells must have a suitable band potential for water splitting; that is, the CB should have more negative reduction potential than H₂. Similarly, VB should have more positive oxidation potential than O₂ [152].

In a photo-electrolysis reaction, H₂ and O₂ evolve at the respective photo-electrode by water splitting. PEC water splitting operation does not produce greenhouse gases, and it is a very standard approach to replace H₂ production from fossil derivatives [153]. The various important Bi-based nanocatalysts for PEC are reported in Table 2, with enhanced efficiency towards water splitting. The heterojunction formation enhances the PEC efficiency of the nanocatalysts. It is one of the effective approaches for the separation of charges, and extends the visible absorption region by forming different mechanisms, as discussed above.

Sitaaraman et al. [125] synthesized BiVO₄ heterojunctions with tungsten oxide (WO₃). The band gap of WO₃ is ~2.8 eV, and that of BiVO₄ is ~2.4 eV. The photocurrent density of the heterostructure was found to be 0.64 mA cm⁻² at 1.23 V RHE, which was higher than the photocurrent densities of BiVO₄ and WO₃. Zeng et al. [154] synthesized cobalt phosphate on WO₃/Mo-BiVO₄ through the conventional deposition annealing method, yielding a photocurrent density of 5.38 mA cm⁻², with the generation of H₂ and O₂ in the order of 94.7 mmol cm⁻² h⁻¹ and 46.5 mmol cm⁻² h⁻¹, respectively, suggesting it is a remarkable PEC catalyst for H₂ production, with an IPCE of 81% at 400 nm. Vinoth et al. [155] synthesized Co/S-gC₃N₄/BiOCl heterojunctions through the hydrothermal method. Three-dimensional g-C₃N₄ nanosheets split water into H₂ and O₂ with the higher HER rate of up to 101.4 μmol g⁻¹ h⁻¹, which turned out to be much higher than that of bare g-C₃N₄ [156]. Co-doped, S-gC₃N₄/BiOCl type II heterostructured photoanodes exhibited the photocurrent density of 393 µA cm⁻² at 1.23 V vs. RHE, with enhanced charge separation and electron migration. Wang et al. [157] fabricated BiVO₄/FeOOH/NiOOH via the hydrothermal method. It showed a photocurrent density of 5.87 mA cm⁻² at 1.23 V vs. RHE. Pilli et al. [158] engineered WO₃/BiVO₄/Co-Pi via the electrodeposition method, with a photocurrent density of 6.72 mA cm⁻² at 1.23 V vs. RHE, and an IPCE of 90% at 450 nm.

12. Electrocatalytic (EC) Water Splitting

This is a very promising method for green H₂ production. Electrocatalytic water splitting comprises of two reactions: H₂ evolution reaction (HER) at cathode and O₂ evolution reaction (OER) at anode. HER is a two-electron process in an acidic medium, as depicted in Figure 14, whereas OER is a four-electron process. In the electrolysis of water for H₂ production, HER is a key half reaction, whereas OER is a bottleneck for enhancing the OWS due to its sluggish kinetic reactions. Higher potential is needed to overcome the kinetic barrier known as overpotential (ƞ). The major concern for electrocatalytic operation is the high ƞ and large kinetic barriers [159]. For an applicable water splitting reaction, an electrocatalyst with higher activity and desirable stability is required [160]. A current density of 10 mA cm⁻² is needed to compare the activities of the electrocatalysts, which corresponds to almost 12% of solar energy [161].

Electrolysis of water in acidic medium;

Reduction at cathode: 2H⁺ + 2e⁻ → 2H₂

Oxidation at anode: 2H₂O → O₂ + 4H⁺ +4e⁻

Overall reaction: 2H₂O → H₂ + O₂

For the above water splitting reaction, 1.23 V electrode potential is required at standard conditions.

Electrocatalysts are important in carrying out effective water splitting applications. The challenge is to develop a catalyst with high durability, low cost, high efficiency, and maximum stability. Electrocatalysts play a significant role in lowering kinetic reactions, and in overcoming kinetic energy barriers. For enhanced activity in an electrocatalyst, a few things need to be addressed; one is conductivity for the better transfer of electrons and the other is favorable electrodes for convenient active sites for better reactions [162,163]. Yang et al. [164] synthesized MoSe₂/Bi₂Se₃ through the hot injection method, as depicted in Figure 15c. MoSe₂ is a metal dichalcogenide with a narrow band gap of ~1.5 eV, but with poor conductivity. It is used as an energy storage material. On the other hand, Bi₂Se₃ enhances electrochemical efficiency by forming heterojunctions for HER performance [165]. MoSe₂/Bi₂Se₃ has a current density of 85 mA cm⁻², a low Tafel slope of 44 mV dec⁻¹, and an ƞ of 300 mV, as shown in Figure 15a,b. Khatun et al. [166] synthesized bismuth iron molybdenum oxide (BFMO) composed of Bi₂MoO₆ and Bi₃(FeO₄) (MoO₄)₂, which acts as an effective catalyst for both OER and HER reactions. The synthesized electrocatalyst had an excellent electrocatalytic activity towards HER, with an ƞ of 207 mV at current density 10 mA cm⁻², and OER, with an ƞ of 266 mV at 10 mA cm⁻². It exhibited a potential of 1.67 eV and possessed higher stability, suggesting an excellent electrocatalytic system for OWS.

Yang et al. [167] synthesized Bi₂S₃ @CoS₂/MoS₂ with a low Tafel slope of 56.2 mV dec⁻¹ and an ƞ of 94.7 mV at 10 mA cm⁻² of current density. Stability of the electrocatalyst is the basic requirement for water splitting. Razzaque et al. [168] fabricated Bi₂Se₃ nanosheets via the electrochemical method. It showed a Tafel slope of 178 mV dec⁻¹ and an ƞ of 220 mV at 10 mA cm⁻² of current density. Li et al. [169] synthesized Au@Bi₂Se₃ NFs through the hot injection method; it exhibited a Tafel slope of 78 mV dec⁻¹ and an ƞ of 375 mV at 10 mA cm⁻² of current density with an excellent EC activity. The electrochemical systems whose yield accelerate cathodic current density at very low overpotential value are idealized for HER applications [170]. Dileep et al. [171] synthesized BiSbSe₃ nanostructure electrocatalysts with a Tafel slope of 178 mV dec⁻¹ and an ƞ of 220 mV at 10 mA cm⁻² of current density. They showed excellent HER activity. The various Bi-based electrocatalysts are listed in Table 3, showing excellent EC activities for HER. There are still challenges left for the construction of an effective electrocatalyst to carry out the fundamental application of water splitting.

13. Conclusions and Perspectives

Conclusions

This review discusses the various Bismuth-based nanocatalysts used for PC, EC, and PEC applications for H₂ generation from water splitting. Bi-based nanocatalysts have gained a lot of interest due to their potential for OWS applications, having a suitable band alignment, crystal structure, optoelectronic properties, non-toxicity, cost-effectiveness, and light harvesting ability, due to which they have been discussed in this review. Electrochemical impedance (EI) and current measurements are studied for the efficiency of the nanocatalyst. However, there are certain limitations due to their poor catalytic efficiencies, back recombination, instability, lower photogenerated charge mobilization, and photo-corrosion. Thus, heterojunctions are a very promising method for reducing the recombination of photogenerated charge carriers and have been fabricated with other semiconductors to improve water splitting efficiency. This review summarizes the various Bi-based heterostructured nanocatalysts that have been exploited for PC, EC, and PEC with excellent efficiency in H₂ generation, helping to develop a nanocatalyst for enhancement of the application of H₂ generation in the future. Various Bi-based heterostructured nanocatalysts, such as TiO₂/Bi₂WO₆, Bi₂WO₆/BiVO₄, Bi₂O₃/Bi₂WO₆, ZnIn₂S₄/BiVO₄, Bi₂O₃/Bi₂MoO₆, Bi₂MoO₆/RGO, Bi₂WO₆/Bi₂S₃, V₂O₅/RGO/BiVO₄, etc., have been discussed in this review, which helps in the development of water splitting for H₂ generation. The exclusive research progress on HER performances of the heterostructured nanocatalysts is due to their enriched active sites, structural modulation, upgraded intrinsic activity, higher charge separation ability via interfacial charge transfer, and better mobility. As a result, the synergistic effect between the two or more semiconducting materials on the rate of H₂ production have been discussed in detail. Primarily, Z-scheme heterostructures are effective catalytic systems to increase the efficiency of PC and PEC for an effective H₂ production. Ternary heterostructured nanocatalysts show higher efficiency in H₂ generation than their binary counterparts.

14. Future Perspectives

Environmentally clean and efficient energy is needed immediately to stop the usage of non-renewable energy sources. H₂ is considered a clean energy that can substitute non-renewable energy sources in the future. H₂ generation from PC, EC, and PEC is envisioned to solve the energy crisis and environmental hazards [172,173,174,175,176,177,178]. Although a few noteworthy achievements have been realized in Bi-based heterostructured nanocatalysts, there are still some challenges that remain, such as efficiency, applicability, and reusability as an innovative nanocatalyst for OWS. Over the past decades, there has been much research to find efficient and stable Bi-based photocatalysts for PC water splitting. Heterojunctions enhance the visible light absorption ability and charge separability, and increase the rate of H₂ generation. The challenges of Bi-based photocatalysts are thermodynamic barriers in the redox process and band alignment. Z-scheme heterostructures play a major role in increasing the efficiencies of nanocatalysts for H₂ generation. Upgradation and structural modifications are still needed in the area of PC application to make it available for industrial production. In PEC water splitting, Bismuth oxides-based heterostructured photoanodes have received much attention. Though various Bi-based catalysts are exploited for PEC applications, still they are left with a few challenges, such as cost, photo-corrosion, photostability, efficiency, etc., for the development of photoelectrodes. Experimental and theoretical strategies are important aspects for understanding the activity enhancement of the improved performance of electrocatalysts for HER and OER reactions. Bi-based heterostructures are also reviewed for EC water splitting, but in comparison to PC and PEC water splitting, there are fewer numbers of studies reported on Bi-based electrocatalysts. Electrocatalysts with high efficiency and stability need to be developed in the future with an extensive characterization for large scale applicability. The perspective of multi-functional applicability of Bi-based heterostructured catalysts towards a sustainable H₂ economy can be seen in Figure 16, which highlights the scope of these catalytic systems for scalable usage.

Author Contributions

S.S. is responsible for the preparation of the rough manuscript by surveying different reports and literature. I.S. collected the data and statics of the recent report. S.A.A. conducted the refinement of the manuscript. T.A. is responsible for the conceptualization, supervision, accruing funding, analysis, and finalization of the manuscript. All authors have read and agreed to the published version of the manuscript.

Acknowledgments

T.A. thanks UGC, New Delhi, Govt. of India for Research Grant for In-service Faculty Members. Authors also thank SERB, CSIR and MoE (SPARC/2018-2019/P843/SL) for financial support to Nano Chemistry and Nano Energy Labs. S.S., I.S. and S.A.A. thank UGC, New Delhi for the research fellowships. Authors also acknowledge the support provided through DST PURSE program at CIF, JMI.

Conflicts of Interest

The authors declare no financial competing interest.

Footnotes

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Figures and Tables

Figure 1. No. of publications “Bismuth based heterostructure” (data obtained from Scopus on 30 September 2022).

Figure 2. Schematic diagram of two different semiconductors showing heterojunction formation.

Figure 3. Schematic diagram of different types of heterojunctions.

Figure 4. Schematic charge-transfer process in the Bi2S3@rGO composite photoanode under light illumination. [Reprinted with permission from reference [101]. Copyright 2021, American Chemical Society].

Figure 5. (a,b) Schematics for band gap match and electron–hole pair separation in AgInS2 QDs/Bi2WO6 composite (before/after contact), (c) Reusability, (d) XRD pattern of 1 wt% AgInS2-modified Bi2WO6 before/after three cycles of PC-H2 generation, (e) Stability during prolonged storage of 1 wt% AgInS2-modified Bi2WO6, and (f) Visible light irradiation in presence of AgInS2/Bi2WO6. [Reprinted with permission from reference [109]. Copyright 2020, Elsevier].

Figure 5. (a,b) Schematics for band gap match and electron–hole pair separation in AgInS2 QDs/Bi2WO6 composite (before/after contact), (c) Reusability, (d) XRD pattern of 1 wt% AgInS2-modified Bi2WO6 before/after three cycles of PC-H2 generation, (e) Stability during prolonged storage of 1 wt% AgInS2-modified Bi2WO6, and (f) Visible light irradiation in presence of AgInS2/Bi2WO6. [Reprinted with permission from reference [109]. Copyright 2020, Elsevier].

Figure 6. (a) Photocatalytic H2 production of Bi2WO6/ZnIn2S4 with different amount of Bi2WO6 in comparison with the Bi2WO6 and ZnIn2S4 photocatalysts, (b) Cyclic H2 production on 12.5%wt Bi2WO6/ZnIn2S4, and (c) Charge transfer mechanism for H2 production by Bi2WO6/ZnIn2S4 heterojunction. [Reprinted with permission from reference [34]. Copyright 2021, Elsevier].

Figure 6. (a) Photocatalytic H2 production of Bi2WO6/ZnIn2S4 with different amount of Bi2WO6 in comparison with the Bi2WO6 and ZnIn2S4 photocatalysts, (b) Cyclic H2 production on 12.5%wt Bi2WO6/ZnIn2S4, and (c) Charge transfer mechanism for H2 production by Bi2WO6/ZnIn2S4 heterojunction. [Reprinted with permission from reference [34]. Copyright 2021, Elsevier].

Figure 7. The proposed mechanism of photocatalytic H2 evolution over ZIS/BVO under visible light irradiation, (a) Before contact, (b) Type II heterojunction, (c) Z-scheme system, and (d) Schematic diagram showing the synthesis of ZIS/BVO heterojunction. [Reprinted with permission from reference [35]. Copyright 2020, John Wiley and Sons].

Figure 8. (a,b) Photocatalytic H2 evolution performance of the prepared samples, (c) Wavelength dependence of quantum H2 evolution efficiency of ZIS/BVO-1.6, and (d) Multiple cycles of H2 production over ZIS/BVO-1.6. [Reprinted with permission from reference [35]. Copyright 2020, John Wiley and Sons].

Figure 9. (a) Estimation of the band alignment calculated from UPS analysis with regard to vacuum and converted to E (V) vs. RHE, whereas (b) shows the schematic of n–n–p junction ladder in the staggered band structure of BiVO4/Bi2S3/NiCoO2 photoanode. The terms krec and kct refer to the surface recombination and charge transfer through surface states, respectively, and (c) shows the schematic of the water oxidation of BiVO4/Bi2S3/NiCoO2 photoanode. [Reprinted with permission from reference [129]. Copyright 2022, Elsevier].

Figure 10. (a) Photocatalytic H2 evolution rate over various scavengers, (b) Photocurrent response of the MoS2, Bi2O3, 1-BO-MS, 2-BO-MS, and 3-BO-MS catalysts under visible light. [Reprinted with permission from reference [134]. Copyright 2021, Elsevier].

Figure 11. (a) Proposed band structures and photocatalysis mechanism of the Bi2O3-Bi2WO6 photocatalyst and (b) cycling tests of photocatalytic H2 evolution through the BWA400 photocatalyst. [Reprinted with permission from reference [107]. Copyright 2021, Elsevier].

Figure 12. Mechanism of photocatalytic water splitting.

Figure 13. Schematic diagram of photoelectrocatalytic (PEC) water splitting.

Figure 14. Schematic diagram of electrocatalytic (EC) water splitting.

Figure 15. (a) Polarization curves, (b) Tafel slope, and (c) Schematic illustration for synthesis of MoSe2/Bi2Se3 hybrids. [Reprinted with permission from reference [164]. Copyright 2017, John Wiley and Sons].

Figure 16. Schematic representation of future perspectives for hydrogen energy.

Table 1

Photocatalytic efficiency of Bi-based heterostructured nanocatalysts for HER performance.

Nanocatalysts	Type of Heterojunction	Synthesis Method	Photocatalytic H2 Generation	Ref.
BiPO4/RGO	Type II	Hydrothermal	306 μmol/h/g	[26]
Sn-doped ZnO/BiOCl	Z-scheme	Hydrothermal	4146.77 μmol/h/g	[28]
Bi2MoO6/RGO	p–n	Electrospinning	794.7 μmol/h	[29]
Bi2WO6/ZnIn2S4	Z-scheme	Hydrothermal	131.8 μmol/h/g	[34]
ZnIn2S4/BiVO4	Z-scheme	Self-assembly	5944 μmol/h/g	[35]
ZnIn2S4, CuOx/Bi2MoO6	Z-scheme	Hydrothermal, impregnation method	740.4 μmol/h/g	[62]
Cr-doped Bi4Ti3O12	Schottky	Sol–gel, hydrothermal	116.75 μmol/h/g	[76]
NiO/CDs/BiVO4	Z-scheme	Hydrothermal	1.21 μmol/h	[78]
TiO2/BiVO4	Type II	Wet-impregnation method	173 μmol/h/cm2	[83]
Bi2S3/CdS	Type I	Sonochemical method	5.5 mmol/h/g	[96]
Bi2WO6-BiO	Type II	Hydrothermal	73.12 μmol/h/g	[106]
Bi2O3-Bi2WO6	p–n	Solvothermal	878 μmol/g/h	[107]
AgInS2/Bi2WO6	p–n	Hydrothermal, hot-injection, deposition treatment	152.75 µmol/g	[109]
TiO2/Bi2WO6	p–n	Electrospinning	11580 μmol/h/g	[110]
Bi2WO6-graphene	Type II	Sonochemical method	159.20 mmol/h	[111]
BP/Bi2WO6	Z-scheme	Hydrothermal	21042 µmol/g	[113]
Bi2WO6/RGO	Type II	Electrospinning	935 µmol/h	[118]
Pt loaded Bi2WO6/CdS	Z-scheme	Hydrothermal	1223 mmol/h	[119]
Bi2WO6/CdS	Z-scheme	Hydrothermal	167 mmol/h	[119]
BP/BiVO4	Z-scheme	Hydrothermal	160 μmol/h/g	[124]
Bi2O3/Bi2MoO6	Z-scheme	In situ alkali treatment	12.97 μmol/h/g	[132]
Bi2O3/BiVO4	p–n	Electrodeposition	171.13 μmol/h/g	[133]
MoS2/Bi2O3	Z-scheme	Hydrothermal	3075.21 μmol/h/g	[134]
Bi2MoO6/g-C3N4	p–n	In situ solvothermal	563.4 µmol/h/g	[147]
Bi2MoO6/CdSe-DETA	Z-scheme	Solvothermal	5920 μmol/h/g	[148]
Bi2Ti2O7/g-C3N4/Bi4Ti3O12	Type II	Electrospinning/calcination	638 μmol/h/g	[149]

Table 2

PEC efficiency of Bi-based heterostructured nanocatalysts for HER performances.

Nanocatalyst	Synthetic Route	Photocurrent Density	H2 Production	Ref.
BiVO4/WO3/SnO2	Sol–gel	3.1 mA cm−2 at 1.23 V vs. RHE	-	[38]
V2O5/BiVO4	Facile electrodeposition	1.53 mAcm−2 at 1.5 V vs. Ag/AgCl	8.2 mmol/h	[53]
Bi2S3/BiVO4	PEC transformation	3.3 mAcm−2 at 1.23 V vs. Ag/AgCl	417 μmol/cm2/h	[94]
3-D rosette–rod TiO2/Bi2S3	Hydrothermal	3.98 mA cm−2 at 1.23 V vs. RHE	-	[97]
Bi2S3/ZnS/ZnO	Hydrothermal	220 mAcm−2 at 0.2 V vs. Ag/AgCl	-	[98]
BiVO4/Bi2S3/NiOOH	Photo-assisted electrodeposition	0.91 mA cm−2 at 1.23 V vs. RHE	-	[99]
WO3/Bi2S3	Chemical bath deposition	5.95 mAcm−2 at 1.23 V vs. RHE	-	[100]
Bi2S3@rGO	Facile-synthetic method	6.06 mA cm−2 at 1.23 V vs. RHE, IPCE 15.5% at 475 nm	68.5 µmol/h	[101]
Fe2O3/BiVO4	SILAR method	1.63 mA cm−2 at 1.23 V vs. Ag/AgCl, IPCE 90% at 450 nm	27.34 μmol/h/cm2	[123]
WO3/BiVO4	Spin coating	0.64 mAcm−2 at 1.23 V vs. RHE	-	[125]
V2O5/RGO/BiVO4	Electrodeposition	2.1 mAcm−2 at 1.5 V vs. Ag/AgCl	32.7 mmol/h	[128]
BiVO4/Bi2S3/NiCoO2	Electrodeposition	3.80 mA cm−2 at 1.23 V vs. RHE	-	[129]
Bi2O3/BiVO4	Hydrothermal	2.58 mA cm−2 at 1.2 V vs. Ag/AgCl		[141]
ZnBi38O60/Bi2O3	Electrodeposition	−1.55 mA cm−2 at 0 V vs. RHE	-	[142]

Table 3

Electrochemical efficiency of Bi-based heterostructures for HER performance.

Electrochemical Systems	Fabrication Route	Current Density/mA cm−2	Overpotential (ƞ)/mV	Tafel Slope/mV dec−1	Ref.
Inert 2D Bi2O3	Electrochemical	10	174.2	80	[25]
Bi2S3/Ni3S2/NF	Solvothermal	10	268	82	[37]
Bi2S3/MoS2	Hydrothermal	10	124	123	[95]
Bi/Bi2O3 on Ni-foam	Hydrothermal, deposition	10	250	55	[143]
MoSe2/Bi2Se3	Hot injection	85	300	44	[164]
Bi3(FeO4) (MoO4)2	Solvothermal	10	207	33	[166]
Bi2S3@CoS2/MoS2	Solvothermal	10	94.7	56.2	[167]
Bi2Se3	Electrochemical	10	220	178	[168]
Au@Bi2Se3 NFs	Hot injection	10	375	78	[169]
Au@MoS2/Bi2S3	Hydrothermal	10	72	40	[170]
BiSbSe3NPs	Electrochemical	10	72	31.6	[171]

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