Bismuth-Based Compounds as Visible Light Photocatalyst for Remediation and Water Splitting

Abstract

Enhancing demand for environmental protection has become an urgent need more than ever. For this purpose, water the most known indispensable essences for survivorship of aboveground organisms should be specifically considered. Today, quality of water as dominant source influence of the animate systems has been endangered by various harmful contamination levels. Accordingly, rescuing approaches and cleaning compounds in safe manner demanding for improvement of the quality of potable and industrial utilizing waters are daily pursued. Different materials of bismuth having layered structures, hybridized orbitals, low band gap, and band positions can be attended because of significant ability of water remediation. At this book chapter, we reviewed the photocatalytic efficiency of Bi−compounds, the heterojunction and Z−scheme composites of them, and the synthetization method. Heterojunction or Z−scheme combinations led to obtain high separation photogenerated electrons−hole and reduction of the recombination rate. Furthermore, type II of heterojunction and Z−scheme connections with other Bi- or non-Bi compounds was applied as an effective solution to enhance photocatalytic performance. The major points are related to the activity about (1) water remediation and (2) photoelectrochemical water splitting. The presented review tries to demonstrate the high potential of Bi−compounds and Bi−composites for water remediation and hydrogen and oxygen production through redox reactions of water activated by solar light irradiation, respectively.

10.1 Introduction

Worldwide problem of harmful pollutants has been known as the most challenging issue in view of environmental hygiene, health of human, and living organisms on earth (Schwarzenbach et al. 2010; Mahlambi et al. 2015). Human and living organisms need pure and healthy water and air for surviving. Hence, useful technology of photocatalysis process was done in versatile arena like elimination of organic polluters and produce sustainable energy (Aziz and Sopyan 2010; Patil et al. 2015; Kumar 2017). Photocatalyst process started under light irradiation as excitation source for produce active oxidant species on the surface catalyst for proceeding pollutant degradation. Nowadays, attention of researchers for gaining high degradation efficiency with cost-effective has been drawn to use available, non-expensive, and renewable energies such as light source of natural sunlight. It is well clear that sunlight as most available global source could be applied for photocatalysis process. Sunlight spectrum composed of three regions: UV − region about 5%, visible − region about 53%, and infrared region about 42%. The percentage of sunlight constituents can answer to photocatalysts necessity to get light energies and electron stimulation and move them up from conduction band to valence band (Fig. 10.1). From this view, photocatalysts can be categorized to UV-activated and visible light-activated photocatalysts. UV photocatalysts are composed from the semiconductors with wide band gaps more than 3 eV such as ZrO₂, TiO₂, and ZnO but visible ones own lower band gaps between 2 and 3 eV (Akueus 2012; Alahiane et al. 2014; Reddy et al. 2018). Today, researchers have been focused on the synthesize of visible light photocatalysts to benefit from optimal using of main region of natural sunlight. For this matter, we tried to introduce and describe Bi−compounds as visible light photocatalyst within remediation and splitting of water.

Fig. 10.1

Mechanism of photocatalytic degradation of pollutants over the semiconductor surface. (Reprinted with permission of Springer from Li et al. 2018)

10.2 Photocatalyst Semiconductors

A good photocatalyst should have special properties including (a) sensitive to light, (b) activation under visible or UV light, (c) biological and chemical inert, (d) stable under light without photocorrosion phenomena, (e) affordable, and (f) environmentally safety. To achieve this purpose, vast ranges of semiconductors have been used in photocatalysis process. Effective photocatalysts should produce capability of active radicals (OH^•, O₂^•) for oxidation of pollutants which shown in Fig. 10.1. Redox potential of photogenerated valence band holes is positive as enough value (H₂O/OH^• = 2.23 eV) to react with adsorbed water molecules to generate hydroxyl radicals. Position of conduction band is sufficient negative for reduction of adsorbed oxygen molecules to produce superoxide radicals. Figure 10.2 shows the various semiconductors including of oxide metals such as TiO₂, ZnO, CuO, SnO₂, WO₃, MnO₂, Bi₂O₃, and Fe₂O₃; chalcogenide metals such as ZnS, MoS₂, WS₂, Bi₂S₃, and FeS; non-metallic such as GO, g-C3N4, and rGO; and mixed metals such as Cu-TiO₂ and Bi-TiO₂ (Opoku et al. 2017). There are different proposed processes to enhance the photocatalytic efficiency, such as preparation of the composites through the elemental modification, heterojunctions with the other semiconductors, and doping with the other elements. Ag and Au metals form could produce plasmonic electrons acting as electron donor for plasmonic nanocomposites (Myung et al. 2014; Alarfaj 2016). Photocatalysts could be interacted with other narrow band gap semiconductors in order to obtain effective heterojunction. Heterojunction helped transferring of charged species between two semiconductors which led to reduce recombination rate of photogenerated electron−hole. Band positions of semiconductors have a key effect at various heterojunctions (Wang et al. 2014; Ge et al. 2019). Photocatalysts were used in two forms of powder and film within degradation reactions with some advantages and disadvantages for each of them.

Fig. 10.2

Positions of conduction and valence bands and potentials of typical semiconductors for environmental purifications and capability of them in generation of reactive oxygen species. (Reprinted with permission of Springer from Li et al. 2018)

10.2.1 Bi−compounds

Bi−compounds considered as bold visible light-active photocatalysts and have recently drawn rapidly great attention from photocatalyst researchers. Bi³⁺ shows remarkable stability in the different compounds such as Bi₂S₃ (Jin and He 2017), Bi₂WO₆ (Chen et al. 2010), BiFeO₃ (Ponraj et al. 2017), BiVO₄ (Yin et al. 2010b), Bi₄Ti₃O₁₂ (Buscaglia et al. 2011), BiPO₄ (Li et al. 2011), Bi₂O₂CO₃ (Huang et al. 2015b), and BiOX (X = Cl, Br, I) (Zhang et al. 2008) highly noticed owing to the respected tight band gap, high stability, cost-effective, and environment friendly. Almost all of them have layered structure and sheet like from the view of shape. Although Bi⁵⁺−compounds, such as KBiO₃ and NaBiO_3, can also be activated by visible light, Bi⁵⁺−compounds are less considered due to the instability of Bi⁵⁺ ions. In Bi³⁺ compounds, hybridization of O 2p and Bi 6s orbitals leads to move valence bands to upward states which favor for photocatalytic applications. It can be highlighted that high mobility predicted for photo-induced charge carriers on the Bi−compounds surface due to dispersion of 6s orbitals of bismuth. On the other hand, Bi−compounds have band gaps <3.0 eV that indicate the high activity in visible region. Bi−photocatalysts have interesting capabilities within the environmental issues for removing the organic pollutants such of azo dyes (Zhang et al. 2007; Qin et al. 2012), redox treatments of toxic gases such as NO and CO₂ (Ai et al. 2011a; Jin and He 2017), photoactivated water splitting for H₂ and O₂ evolution reaction. A diverse scientific studies about photocatalytic performance of Bi−compounds and many other reviewing literature about the photocatalyst field were done (Zhao et al. 2014; Meng and Zhang 2016; He et al. 2018). The present review focused on the photocatalytic activity of various Bi−compounds.

10.2.1.1 Bi₂X₃

Bi₂X₃ (X = O, S, Se, Te) compounds including Bi and other elements of group VI are generally named bismuth chalcogenides with nomenclature of Bi₂O₃, Bi₂S₃, Bi₂Se₃, and Bi₂T₃. Bi₂O₃, based on the phase structures has different band gap values in the range of 2.1–2.8 eV, causing for consideration as a durable photoactivated by the white light. Bi₂O₃ formed from different polymorph phases which include α, β, δ, γ, and ω with crystal network of monoclinic, tetragonal, body-centered cubic, face-centered cubic, and triclinic, respectively. Various Bi₂O₃ known phases have low stability which lead to quick interphase conversion through switching of the temperature condition. Bi₂O₃ nanostructures have remarkable physicochemical characters such as band gap having low energy, dielectric permittivity, ion conductivity, and photoconductivity which the highlighted properties make Bi₂O₃ as a stable visible light photocatalytic candidates within water splitting and remediation of organic polluters. Recently, controlled synthetization of Bi₂O₃ with specified morphology and certain phase has become a hotspot for photocatalysis researchers. For instance, uniform hierarchical bismuth oxide structures were synthesized and demonstrated excellent visible light activity about degradation of rhodamine B (Zhou et al. 2009). Monoclinic phase of Bi₂O₃ was prepared via calcination of hydrothermal production from (BiO)₂CO₃ precursor and indicated excellent photoactivated degradation of NO gas and formaldehyde via visible light radiance (Ai et al. 2011b). Bi₂S₃ was exhibited as a wonderful light-harvesting photocatalyst because of having tight band gap ~1.7 eV and excited in visible and near-IR regions. Bi₂S₃ nanocatalysts have been synthesized in a variety of dimensions of one-directional, e.g., rode in Fig. (10.3a), two-dimensional, e.g., sheet in Fig. 10.3b, and three-dimensional, e.g., urchin-like in Fig. 10.3c by standard oxygen-free, hot injection, and solvothermal methods, respectively (Wu et al. 2010; Zhang et al. 2011). The photogenerated holes on the Bi₂S₃ semiconductor have efficient energy about 1.62 eV for oxidation of adsorbed water molecules to produce high oxidants such as OH^• for degradation of dye contaminants (Zhang et al. 2011). Wu et al. (2010) reported that Bi₂S₃ nanodots and nanorods were synthesized by hot injection method. Uniform Bi₂S₃ nonodots show high photocatalytic degradation for rhodamine B due to the presence of high surface area.

Fig. 10.3

Transmittance electron microscopy images of (a) nanorods, (b) nanosheets, and (c) scanning electron microscopy image of nanospheres of Bi₂S₃. (Reprinted with permission of Springer from Meng and Zhang 2016)

Bi₂Se₃ semiconductor with the layered structure is composed from several monolayers with 0.96 nm thickness that bonded around z−axis with the following configuration Se-Bi-Se-Bi-Se (Sun et al. 2012). Bi₂Se₃ has great potential in photoelectrochemical, optical, and thermoelectrically devices and photocatalysis applications owing to the small band gap and high mobility of charge species (Sun et al. 2012). Bismuth telluride (Bi₂Te₃) also has very narrow band gap about 0.15 eV with trigonal structure and high melting point. Bi₂Te₃ applied in thermoelectric generators and refrigeration due to the thermoelectric properties at 25 °C (Teweldebrhan et al. 2010). Big problem for Bi₂Se₃ and Bi₂Te₃ arrived from the great probability of recombination rate of photogenerated electron−hole pairs that deprives them of the eventual photocatalytic activity.

10.2.1.2 BiOX

Bismuth oxyhalides represented by BiOX (X = Cl, Br, I) can be considered as the most famous bismuth compounds due to appropriate optical properties and high applications in environment treatment. BiOXs have layered standings similar to other Bi−compounds which characterized by segments of Bi₂O₂ interleaved by double segments of halogens. Layered structures suggested promising large space for polarizing orbitals and created dipoles which could led to separate charge carriers (Lei et al. 2009).

Density functional theory calculation method simulated electrical structures of Bi−oxyhalides (Huang and Zhu 2008). Both the valence band and conduction band of BiOX composed of X np (n = 2–5 for X = F, Cl, Br, and I, respectively), O 2p, and Bi 6p orbitals. The observed band gaps based on computations have resulted as 2.79 eV, 2.34 eV, 1.99, and 1.38 eV for BiOF, BiOCl, BiOBr, and BiOI, respectively (Zhang et al. 2008; Su et al. 2010). Results exhibited that heavy halogen has smaller band gap. So, BiOF as photocatalyst could be excited by UV light, while BiOI activated by visible and near-IR light. It can be stated that BiOBr and BiOCl are repeatedly applied because of the desired amounts of band gaps. The conduction band orbital density isosurfaces are illustrated in Fig. 10.4 for BiOX with the involving of Bi 5d states.

Fig. 10.4

The conduction band orbital density isosurfaces of BiOX (a) X = F, (b) X = Cl, (c) X = Br, and (d) X = I with the adoption of Bi 5d states. (Reprinted with permission of Elsevier from Huang and Zhu 2008)

BiOCl is a UV-sensitive photocatalyst with experimental band gap with range of 3.1–3.5 eV and computational calculated band gap of 2.8 eV (Zhang et al. 2006, 2016; Lei et al. 2009). Excited-BiOCl indicated eminent photocatalytic efficiency for pollutant elimination. For instance, Zhang et al. (2006) synthesized durable BiOCl nanoplates via simple hydrolysis method which shown high efficiency about photodegradation of methyl orange activated by UV light. BiOCl nanosheets with {100} facet could exhibit high photocatalytic activity due to produce oxygen vacancies under UV illumination. In order to make optimum usage of solar energy, there is a remarkable tendency for evaluation of photocatalytic activity of BiOCl under visible light irradiation. If BiOCl coupled with some dyes which have intrinsic physicochemical properties could have visible light activity. For example, Xiong et al. (2011) reported that synthesis of square-like BiOCl nanoplates by hydrothermal method, which has [Cl–Bi–O–Bi–Cl] layered structure and demonstrated high photocatalyst performance for rhodamine B compared to commercial TiO₂ (P25). In this work, diffraction reflectance UV spectroscopy studies for BiOCl nanoplates reported a wide band gap 2.9 eV, so photosensitization process overcome on the photocatalytic process for rhodamine B degradation. At other work, Ye et al. (2012) fabricated marvelous BiOCl with oxygen vacancies under Ar purging, which exhibited 20 times photocatalytic activity than conventional BiOCl for photodegradation of rhodamine B induced via visible light. Porous BiOCl nanosheets also demonstrated photosensitized removal of rhodamine B. Also, three-dimensional hierarchical BiOCl nanoplates having remarkable photocatalysis efficiency have been successfully prepared. For example, solvothermal with polyol mediator technique was applied for synthesis of specific morphologies BiOCl hierarchical nanostructures. Hierarchical BiOCl (see Fig. 10.5) showed high photoremediation of rhodamine B activated by visible light compared with nanosheets or nanoplates of BiOCl and P25 (Xiong et al. 2013). Unlike BiOCl, BiOBr were introduced as visible light-sensitive semiconductor with inherently appropriate band gap for utilization of sunlight and suggested as a powerful catalyst about photodegradation of organic polluters under white light illumination. Recently, considerable researches have been done to evaluate photocatalytic activity of BiOBr in environmental treatment and photocatalytic water splitting fields. Lamellar and plate-based BiOBr structures were prepared that showed great photocatalyst performance for pollutant degradation (Shang et al. 2009).

Fig. 10.5

Scanning electron microscopy images (a–i) and transmittance electron microscopy images (j–l) of BiOCl nanostructures synthesized via solvothermal method in the presence of polyols: ethylene glycol, diethylene glycol, and triethylene glycol. EG, DEG, and TEG stand for ethylene glycol, diethylene glycol, and triethylene glycol, respectively. (Reprinted with permission of Elsevier from Xiong et al. 2013)

BiOBr nanosheets used for photoreduction of Cr(VI) induced by visible light and the reusability indicated high efficiency for reduction process. Researchers have also attracted to synthesize three-dimensional hierarchical BiOBr to enhance the photocatalytic properties which has more advantages in comparison with one-dimensional or two-dimensional structures (Shi et al. 2013). BiOBr with mesoporous structure showed higher visible light photocatalytic efficiency for harmful tetrabromobisphenol A compared to commercial TiO₂. High ranges of pollutants such as dyes, e.g., rhodamine B, methyl orange, methylene blue, and organic, e.g., phenol and toluene have been proposed as mannequin pollutants to exhibit the photocatalyst activity of BiOBr compounds under visible light irradiation (Zhao et al. 2014). Among BiOX compounds, BiOI has narrowest band gap besides highest utilization of solar source. BiOI is a semiconductor with intrinsic rapid recombination of charge carriers singly, so BiOI cannot show acceptable photocatalytic performance. Therefore, a lot of strategies were proposed for combination/synthetization of BiOI with other semiconductors to improve the related photocatalytic activity.

10.2.1.3 Bi₂MO₆

Bi₂MO₆ are known as the famous triplet oxygen − bismuth compounds with Aurivillius¹ structure depicted by (Bi₂O₂)²⁺(A_n − 1B_nO_3n + 1)²⁻ (A = Ba, Bi, Pb, so on., B = Ti, Nb, W, Mo, so on.) which has intercalated structures with sheets of perovskite-bearing octahedral (A_n−1B_nO_3n + 1)²⁻ sandwiched array between (Bi₂O₂)²⁺ layers. Until now, a variety of bismuth Aurivillius oxides containing bismuth tungstate, bismuth molybdate, and bismuth subcarbonate have been fabricated, which has excellent potential for photocatalysis usages such as water treatment and photocatalytic water splitting (Zhao et al. 2014; Meng and Zhang 2016).

Bismuth tungstate (Bi₂WO₆) is known as one of the easiest structures of the Aurivillius group (n = 1) having a layered standing with WO₆ sheets. The perovskite block in Bi₂WO₆ composited of 2D array of WO₆ octahedral linked corner, with thick octahedral layer. Bi₂WO₆ has great potential for oxygen evolution reaction within hydrolysis and oxidation of toxic polluters under white light. Zhang et al. (2007) reported that various morphologies of Bi₂WO₆ nano and microstructures, including flower-, tire-, and spiral-like shapes, showed excellent solar light photo-activated catalytic efficiency for remediation of rhodamine B that could be related to the presented morphology, size, and structure. Furthermore, the pH value of the solution contained of pollutant also defines the photocatalytic performance of photocatalyst. Zhu et al. (2016) proved the pH effect of initial solution on the photocatalysis performance of nanosheets Bi₂WO₆ for degradation of rhodamine B which could be related to mode and adsorption−desorption of rhodamine B on the semiconductor surface. Bi₂WO₆ also exhibited high performance for air treatment and water splitting applications (Larson and Zhao 2016). Yu et al. suggested that well-crystalized bismuth tungstate with high surface area which could perform photocatalytic degradation of formaldehyde gas in air (Yu et al. 2005).

Bi₂MoO₆ is also another layered member of Aurivillius compounds which has recently drawn enormous scientific attentions due to the photocatalytic properties within hydrolysis and photooxidation of contaminants. The layered structure Bi₂MoO₆ is synthesized via refluxing method which exhibited high photocatalytic efficiency for oxygen liberation from an aqueous solution of AgNO₃ induced by solar light (Shimodaira et al. 2006). The obtained results suggested that photocatalytic activity attributed to crystallinity and high rate of charge transfer in layered structure Bi₂MoO₆. Zhang et al. (Huang et al. 2018) suggested that nanosheets and microrods of Bi₂MoO₆ were selectively synthesized via change of pH of precursor solution in hydrothermal method and demonstrated efficient visible light photocatalytic degradation of methylene blue. Bi₂MoO₆ has been synthesized via solid state and solvothermal or hydrothermal methods similar to Bi₂WO₆ (Yin et al. 2010a; Zhang et al. 2010). Comparison studies for synthesis method of Bi₂MoO₆ showed that smaller size, large surface area, and efficient photocatalytic performance obtained from samples which fabricated via hydrothermal and solvothermal methods not solid-state reaction. Furthermore, microwave method was also applied to synthesize of Bi₂MoO₆ in short time with good photocatalytic performance (Xie et al. 2008). Different work, thin film of Bi₂MoO₆ (200 nm thickness) fabricated via thermal evaporation deposition process (see Fig. 10.6) which showed high visible light-responsive photocatalyst property for rhodamine B degradation (Cuéllar et al. 2011).

Fig. 10.6

Evaluation of color changing of adsorbed rhodamine B on the Bi₂MoO₆ thin film at different interval time (a) and an experiment done on the bare glass (b and c). (Reprinted with permission of Elsevier from Cuéllar et al. 2011)

Bismuth titanate, also one important member of Aurivillius oxide family, introduced by a variety of compositions and showed high visible light-sensitive or UV-sensitive photocatalytic for pollutants. Sillenite Bi₁₂TiO₂₀ nanowires (Hou et al. 2009) and perovskite Bi₄Ti₃O₁₂ (Li et al. 2016) demonstrated high photocatalytic efficiency for methyl orange under light (<400 nm). Cubic phase − Bismuth titanates (Bi₁₂TiO₂₀) with a variety of morphological structures such as flower-looking like, belt-looking like, and tetrahedral-looking-like shapes prepared by easy approach showed in Fig. 10.7, which exhibited high photocatalytic degradation performance for methylene orange and p-nitrophenol (Guo et al. 2013).

Fig. 10.7

Schematic for fabrication of different morphology for Bi₁₂TiO₂₀ by hydrothermal approach. TTIP rephrases for Ti(OC₃H₇)₄. (Reprinted with permission of Royal Society of Chemistry from Guo et al. 2013)

10.2.1.4 BiVO₄

Bismuth vanadate (BiVO₄) is known as the most versatile member of Bi−compounds which has three crystallite phases: tetragonal zircon, monoclinic, and tetragonal scheelite structures. BiVO₄ with monoclinic crystalline phase became white light photoactivated because of low required energy band gap about 2.4 eV compared to other phases. Hence, BiVO₄ with high adsorption in visible light region and narrow band gap was considered as new materials for photocatalytic applications and other related researches. Due to special physicochemical features of BiVO₄ such as Ferro elasticity and theoretical band gap about 2.047 eV obtained from density functional theory method, it has been considered as photocatalytic activity, recently (Wang et al. 2019a). Multi shell hollow spheres of BiVO₄ synthesized via carbonate template under thermal conditions are depicted in Fig. 10.8. Hollow shapers bear great photo-induced performance within elimination of methylene blue under solar light. Figure 10.9 also confirmed the claimed morphology for BiVO₄ with scanning electron microscopy and transmittance electron microscopy images (Zong et al. 2017).

Fig. 10.8

Schematic depicts the fabrication approach of hollow spheres BiVO₄. (Reprinted with permission of Elsevier from Zong et al. 2017)

Fig. 10.9

(a) Scanning electron microscopy image of Bi–V–O single-shell hollow spheres, (b) scanning electron microscopy image of Bi–V–O double−shell hollow spheres, (c) transmittance electron microscopy image of Bi–V–O single−shell hollow spheres, (d) transmittance electron microscopy image of Bi–V–O double−shell hollow spheres, (e) transmittance electron microscopy image of an individual Bi–V–O double−shell hollow spheres, (f) high-resolution transmittance electron microscopy image of an individual Bi–V–O double−shell hollow spheres. (Reprinted with permission of Elsevier from Zong et al. 2017)

Recently, variety ranges of BiVO₄ structures have been synthesized and applied in photocatalytic performances such as elimination of polluters and H₂ or O₂ liberation from water splitting process (Wang et al. 2019b). Hollow microspheres of BiVO₄ showed considerable solar light photocatalysis efficiency for remediation of rhodamine B and 2−propanol (Sun et al. 2013a). At 1998, Kudo’s team reported a great candidate with high potential for photocatalytic water splitting named BiVO₄ for the first time (Huang et al. 2017). Years later, Kudo et al. demonstrated photoactivated efficiency of bismuth vanadate for O₂ liberation from AgNO₃ solution under visible light (Kudo et al. 1999).

10.2.1.5 BiFeO₃

BiFeO₃ compound shows simultaneous multiferroic and magnetoelectric behaviors at ambient conditions that led to widely employing of BiFeO₃ in the arena of nonvolatile memory, spintronic, sensors, and piezoelectric apparatus (Lam et al. 2017; Ponraj et al. 2017). BiFeO₃ photocatalyst having rhombohedral disordered perovskite is a new kind of reliable solar light-activated photocatalyst within the organic polluter remediation because of its small band gap and great chemical stability. Last year, BiFeO₃ attracted considerable attention in photocatalytic environmental applications (particular degradation dye pollutants such as methylene blue and rhodamine B due to its weak ferromagnetic feature led to recycling from treated solution (Ponraj et al. 2017). Optical band gap of BiFeO₃ reported between 2.2 and 2.8 eV in literatures. Mesoporous BiFeO₃ hollow sphere was synthesized and used for degradation of rhodamine B and 4−chlorophenol under 500 W Xe-lamp irradiation (Gao et al. 2015). Soltani et al. (Soltani and Entezari 2013a) demonstrated that reactive black 5 bears three main UV/visible peaks at wavelengths of 620, 312, and 254 nm. The generation of some new intermediates such as sulfone, sulfonate, and amine groups prepared in the UV/visible regions is the reason for observation of three main peaks. The residual of small organic intermediates can explain the changing of color solution as well as the decrease of pH.

10.2.2 Modification of Bi−compounds

Although Bi−compounds were introduced as visible light-responsive photocatalyts in water treatment, some compounds such as BiFeO₃ and BiOXs have weak adsorption ability which led to poor performance for photocatalytic degradation of pollutants. Relatively poor efficiency can attribute to (i) high recombination rate of electron−hole in bulk or surface, (ii) positions of conduction band or valence band related to O₂ reduction or H₂O oxidation, respectively, and (iii) small surface area for photocatalytic process. So far, many researches were devised for resolving the problematic issues such as morphology modifications, doping, and heterojunctions (Chen et al. 2016a) with other semiconductors and generation of vacancies over the surface. Applying the solutions, either the recombination rate or light harvesting can be effectively decreased or increased, respectively, which led to high performance. For more clearance, follow the more detailed discussion below.

10.2.2.1 Morphology Control

The chemophysical properties of semiconductors could be changed by main structural factors, size, morphology, and defects, respectively. Subsequently, photocatalytic properties of catalysts can improve by the structural parameters. In this section, we are focused on morphology control of Bi−compounds and investigated photocatalytic performance. Morphology studies were shown improved photocatalytic efficiency because of produce low-dimensional or hierarchical structures, which could create reactive sites, high rate of mass transfer, and more harvesting amount of visible light. In the following, some more innovative producting techniques of low-dimensional and hierarchical structures of Bi−compounds were discussed.

10.2.2.1.1 Bi-compounds with Low-Dimensional Structure

Nanomaterials have multifarious dimensions which could be classified to four categories: zero-dimension such as nanoparticles, one-dimension such as nanorods and nanowires, two-dimension such as nanoplates or nanosheets, and three-dimension such as nanospheres or nanoflowers (Jeevanandam et al. 2018). At the recent years, Bi−compounds were synthesized by control of synthetization parameters to obtain special size including nanoparticles, nanobelts, and nanoflowers for photocatalytic applications. Soltani et al. (Soltani and Entezari 2013b, c) reported that BiFeO₃ nanoparticles synthesized via ultrasound with narrow size distribution as visible light photocatalyst which exhibited higher photocatalytic performance for methylene blue and rhodamine B compared to BiFeO₃ synthesized by sol−gel method. Nanodots, nanorods, and nanosheets of Bi₂S₃ nanostructures synthesized and used for degradation of rhodamine B, methylene blue, and methyl orange which results pointed to photocatalytic activity depends to dimension (see Fig. 10.10) (Wu et al. 2010).

Fig. 10.10

Transmittance electron microscopy images of synthesized Bi₂S₃ nanostructures with various concentrations of Bi (a)1:0.5, (b) 1:1, (c) 1:1.5, and (d) 1:1.7. (Zong et al. 2017). (Modified)

The properties of size and porosity of snow-like Bi₂WO₆ particles depicted in Fig. 10.11 resulted in high white light photoactivated performance for degradation of rhodamine B (Zhuo et al. 2013). Spherical Bi₂WO₆ nanoparticles were fabricated via hydrothermal route with average size 85 nm bear great photoactivity for elimination of rhodamine B under solar light (Wang et al. 2015). Bi₂WO₆ with nanoplate two-dimensional structure with 30 nm length size exhibited high performance for photoactivated remediation of aquatic solution of rhodamine B under solar light which could be related to small particle size and high surface area (Zhang and Zhu 2005). Another work reported the hydrothermal preparation of nanoplate Bi₂WO_6-x with high surface oxygen vacancy with 2.1 times higher photocatalytic degradation of 2–4-dichlorophenol than pristine Bi₂WO₆ (Lv et al. 2016). High photocatalytic performance can attribute to high surface oxygen vacancy states.

Fig. 10.11

Scanning electron microscopy images of Bi₂WO₆ with different morphologies synthesized at pH = 1 and pH = 5. (Reprinted with permission of Elsevier from Zhuo et al. 2013)

One-dimensional Bi₂MoO₆ nanosheets were fabricated via electrospinning which demonstrated remarkable photocatalytic efficiency within remediation of rhodamine B and methylene blue under simulated visible light (Sun et al. 2013b).

10.2.2.1.2 Hierarchical Bi−compounds

Hierarchical structures can be highlighted as ordered architectures assembled from low- dimensional building blocks, such as nanofibers, nanorods, nanoribbons, nanosheets, and nanoplates. Recently, great attention has been attracted to hierarchical assemblies due to the proper electronic, optical properties, layered structure, and catalytic efficiency which has bold difference with low-dimensional sub-component (Luo et al. 2019; Song et al. 2019). Herein, the synthesis of hierarchical Bi−compounds has attracted more efforts from the view of specific morphologies to gain high photocatalytic performance. The hierarchical Bi₂WO₆ hollow tubes demonstrated high photo-induced catalytic efficiency for elimination of rhodamine B under simulated visible light, which was related to Bi₂WO₆ structure, tight band gap, and gross surface area (Yafei et al. 2013). Zargazi et al. indicated the high simultaneous photocatalytic and sonophotocatalytic performances of Bi₂WO₆ nanoflowers for binary mixture (methylene blue and rhodamine B) synthesized by ultrasonic-assisted hydrothermal which attributed effect of morphology in adsorption of pollutants from binary mixtures (Zargazi and Entezari 2019c). Both dyes were decomposed on the catalyst surface and bulk solution by sonophotocatalytic process which is observed in Fig. 10.12. Hierarchical flowerlike Bi₂MoO₆ crystals synthesized by simple hydrothermal method show permanent photo-induced reduction of CO₂ into methanol and ethanol (Dai et al. 2016). Morphology of Bi₂MoO₆ flower showed high influence in separation of photogenerated electron−hole and adsorption of light. Sharma et al. reported the preparation of Bi₂S₃ nanoflowers which exhibited the high photocatalytic degradation of two different binary mixtures of rhodamine B and methylene blue and 4-nitrophenol and 4-chlorophenol from suspension (Sharma and Khare 2018). Novel nanoflower structures of BiOCl with small band gap about 2.87 eV but huge average size about 1.5 μm were routinely prepared at 25 °C using the L−Lysine template. The interesting structure indicated high photocatalytic remediation of rhodamine B under solar light. The observed perfect photocatalytic performance can be appropriated to phase purity, high exposure of {110} planes, thin nano-petals structure, tight band gap, and the relatively large surface area.

Fig. 10.12

Adsorption behaviors (dark) and sonophotocatalytic (light and US) degradation of rhodamine B/methylene B: Sono-BWO sample (a and a′), Hydro-BWO sample (b and b′). (Reprinted with permission of Elsevier from Zargazi and Entezari 2019c)

10.2.2.1.3 Nano Bi Films

Recently, nano thin films of Bi−compounds attracted great attentions due to special applications in multiple fields such as water splitting, solar cell, and remediation environmental (Patil et al. 2015; Lee and Ebong 2017). Generally, powder compounds have serious problems including recollection and reusing, agglomeration effect, and respiration problems for human. To resolve problem’s powders, immobilized films introduced as new solution for photocatalytic applications. Until now, Bi−films prepared by various methods such as chemical bath deposition (Gao et al. 2011), liquid phase deposition (Song et al. 2004), spin coating (Tyagi et al. 2015), sol−gel (Zargazi and Entezari 2019a), chemical vapor deposition (Brack et al. 2015), electrochemical deposition (Chahkandi and Zargazi 2019), electrophoretic deposition (Zargazi and Entezari 2019b), and so on.

Using the abovementioned methods, Bi−thin films deposited on conductive and non-conductive substrate were applied for degradation of various pollutants. For instance, Bi₂WO₆ deposited over the surface of stainless steel mesh using the anodic electrophoretic method and applied for remediation of binary mixture of 4−nitrophenol and 4−chlorophenol (Zargazi and Entezari 2019b). High photocatalytic degradation for film could be attributed to the effect of film thickness and substrate in separation of electron−hole. Alfaifi et al. reported the preparation of Bi₂WO₆ electrodes with nanoplates and Bucky ball-shaped microsphere morphologies by aerosol-assisted chemical vapor deposition which was applied for degradation of methylene blue and rhodamine B (Alfaifi and Bayahia 2019). Alfaifi and Bayahia suggested the energetic and interfacial features of Bi₂WO₆ film to increase solar energy and photocatalytic activity of film. BiFeO₃ film also deposited on the same substrate by anodic electrophoretic deposition method which exhibited high photocatalytic efficiency for decomposition of rhodamine B dye (Zargazi and Entezari 2018). BiFeO₃ film demonstrated higher photocatalytic degradation than BiFeO₃ powder due to substrate effect in decreasing of recombination rate of photo-induced charge pairs. At another work, forestlike BiFeO₃ films are fabricated by using cathodic electrophoretic deposition on the stainless steel mesh which indicated high photocatalytic performance for phenol compounds. Forestlike morphology of BiFeO₃ film depicts in Fig. 10.13 shows key effect in harvesting and multi-scattering of visible light which led to high degradation efficiency (Zargazi and Entezari 2019a). Venkatesan et al. (2018) shown the preparation of stable monoclinic − BiVO4 film by radio frequency − sputtering on the fluoride tin oxide and the degradation application of rhodamine 6G. Photocatalytic reduction of Cr hexavalent is conducted by Bi₂S₃ films in single and binary mixtures. Chahkandi et al. reported the novel deposition square wave voltammetry method of Bi₂S₃ film on the stainless steel mesh which exhibited high reduction rate for conversion toxic Cr(VI) to non-toxic Cr(III) (Chahkandi and Zargazi 2019).

Fig. 10.13

BiFO₃ film coated on substrate (stainless steel mesh) (a), BiFO₃ film on wire surface (b), treelike structure (c) and nanobranches of BiFO₃ (d). (Reprinted with permission of Elsevier from Zargazi and Entezari 2019a)

10.2.2.2 Heterojunction

Over the past decades, designing of heterojunctions was introduced as a best route to reduce the recombination rate of electron−hole produced under light irradiation (Wang et al. 2014; Huang et al. 2015a). The Bi-based photocatalysts, with an appropriate band gap, have capability to produce electrons under solar light. However, the excited electron and holes potentially recombined very fast together. From this view, heterojunction construction can have a great role in enhancing photocatalytic efficiencies of Bi-based photocatalysts. Bi-based heterojunctions include conventional and Z−scheme heterojunctions. Among the conventional heterojunctions, the type II junction is the most usual one, while in Z−scheme type, the newly merged direct Z−scheme heterojunction appears to be the most effective junction structure used for exploring the capacity of photo-generated carriers (Wang et al. 2014; Low et al. 2017). Figure 10.14 depicted schematics for heterojunctions (Type II) and Z−scheme heterojunction.

Fig. 10.14

Charge separation of (a) heterojunction (Type II) and (b) direct Z-scheme heterojunction. CB, VB, and PS stand for conductive band, valence band, and photocatalyst semiconductors, respectively. (Modified)

Binary Bi are heterojunctions with two kinds of Bi−compounds, and also Binary Bi compounds can produce heterojunctions with non-Bi−compound.

10.2.2.2.1 Type II: Conventional Heterojunction

In comparison with the trinary types of heterojunctions, the second junction type is the most suitable one. Bi-compounds and semiconductors with small band gap formed heterojunction (type II) as conduction band and valence band levels of semiconductors should be lower than the Bi−compound portion. For example, Fan et al. (2016) fabricated a binary Bi−compounds Bi₂MoO₆-BiOI heterojunction (Fig. 10.15a, b) by anion exchange method which exhibited high photocatalytic degradation efficiency for rhodamine B in comparison with BiOI or Bi₂MoO₆ alone (Fig. 10.15c). Optimal molar ratio of Mo/I is 50% made heterojunction (Type II) between two components having highest efficiency under white light (Fig. 10.15d). It is notable that three matched Bi−compounds can be combined together to produce a ternary heterojunction such as Bi₂S₃/Bi₂O₃/MoS₂ (Ke et al. 2017). Improved photocatalytic activity of Bi₂S₃/Bi₂O₃/MoS₂ ternary Bi−compounds can be attributed to enhancing of light adsorption and high separation of electron−hole by double heterojunction (Type II) (Fig. 10.16). Moreover, some other heterojunctions with low band gap semiconductors such as non-Bi−compounds were performed for improving photocatalytic degradation of different pollutants.

Fig. 10.15

(a) Photocatalytic degradation of rhodamine B. (b) The rate constants for Rhodamine B degradation on BOI, Bi₂MO_6, and Bi₂MO₆/BOI composites. (c) Recycling. (d) Total organic carbon changes for photocatalytic degradation of rhodamine B by using Bi₂MO₆/BOI = 50% as photocatalyst. (Reprinted with permission of World Scientific from Fan et al. 2016)

Fig. 10.16

Diagram for (a) energy band of Bi₂O₃, MoS₂, and Bi₂S₃ and (b) the formation of the three-phase p−n heterojunction and the possible charge separation. (Reprinted with permission of Elsevier from Ke et al. 2017)

For instance, g−C₃N₄ compounds could be coupled with Bi₂WO₆, BiVO₄, Bi₂S₃, Bi₂O₃, and Bi₂MoO₆ which exhibited improved photocatalytic properties in degradation of pollutants. Numerous synthesis strategies for heterojunctions (Type II) have been introduced, and most of Bi-based heterojunctions led to improve photocatalytic efficiency (Table 10.1).

Table 10.1 Some heterojunctions of Bi−based with Bi and non−Bi semiconductors

10.2.2.2.2 Direct Z-Scheme Heterojunctions

Yu et al. (2013) introduced a direct Z−scheme heterojunction to clarify the improvement of photocatalytic property of a TiO₂/g−C₃N₄ composite. The reported type of Z−scheme heterojunction does not need electron medium unlike other Z−scheme heterojunctions such as liquid phase. Built-in electric field between the interface of two semiconductors (I, II) acted as cite for charge transfer. The Z−scheme heterojunction has the same structure to a conventional heterojunction (type II), while charge transfer is different for two heterojunctions (Fig. 10.14b). In a Z−scheme heterojunction, charge transferring occurred by the built-in field at the interface of two semiconductors, while spatial separation conducted in heterojunction (type II). Cost-effective and high redox ability are the most prominent features for direct Z−scheme heterojunction. Numerous Bi-based Z−scheme heterojunctions have been fabricated and suggested. For example, BiOBr/g−C₃N₄ direct Z−scheme heterojunction was prepared via simple reflux method. The resulted BiOBr/g−C₃N₄ indicated more photocatalytic efficiency for remediation of rhodamine B, levofloxacin in comparison with BiOBr, or g−C₃N₄ alone. Meanwhile, BiVO₄ and Ag₃VO₄ composited together under hydrothermal treatment to form direct Z−scheme. The obtained composite has high photocatalytic activity for degradation and reduction of bisphenol and Cr(VI), respectively (Jing et al. 2019). Ternary Z−scheme heterojunctions synthesized for Bi-based compounds such as Bi₂WO₆/g−C₃N₄/rGO show enhanced efficiency by transferring of electrons between double Z−scheme heterojunctions. Some of Z−scheme heterojunctions including Bi-compounds are summarized in Table 10.2.

Table 10.2 Some of synthesized Z-scheme heterojunctions for Bi-based materials

10.3 Application

10.3.1 Water Remediation

According to previous section, the function of Bi-based photocatalysts through water remediation was extended. Bi-based nanocomposites, plasmonic composites, and carbon-based are the most famous composites which are applied for degradation of pollutants existed in water [134–167]. Existed pollutants include dyes, pharmaceutical, phenolic compounds, and toxic heavy metals. Table 10.3 shown some of Bi−compounds and composites in two forms of powder and film which applied for degradation of pollutants.

Table 10.3 Some of Bi−based photocatalyts in powder and film forms used for water remediation

10.3.2 Water Splitting

Energy frugality in the near future will be a major challenge around the world. Scientists are focused on researches providing clean and sustainable energy sources to decrease probability of complete disappear the unrenewable energies and to manage the pollutants. Burning of hydrogen in the presence of oxygen is not emitted any contaminants. Hence, hydrogen can be considered as a promising renewable fuel which is applied in vehicles, aircrafts, and electrical devices. Water splitting is a promising way to produce H₂. Different techniques for water splitting have been applied such as photoelectrochemical systems (Chen et al. 2016b), photocatalytic (Ni et al. 2007), photobiological (Poudyal et al. 2015), and thermal decomposition (Lapicque 1983). Among them, photoelectrochemical and photocatalytic water splitting are known as simplest, cost-effective, and efficient methods for hydrogen production which mechanism of H₂ production depicted in Fig. 10.17 (Abe 2011).

Fig. 10.17

(a) Mechanism of water splitting over semiconductor photocatalyst and (b) levels of conduction and valence bands for photocatalyst with overall water splitting efficiency. C.B. and V.B. stand for conductive band and valence band, respectively. (Reprinted with permission of Elsevier from Abe 2011)

Photoelectrochemical water splitting manners are categorized in three types which are depicted in Fig. 10.18. The solar light is considered as effective source by Z−scheme compared to the conversional process. Therefore, the hydrogen evolution occurred under proton reduction by electrons of conduction band and oxygen evolution take place by holes of valence band. It can be concluded that the water hydrolysis progressed through the event of cyclic redox pair. Figure 10.18a, b shows n- and p-type semiconductors involved in water splitting. Figure 10.18c illustrates the combination of two various photo electrodes, as oxidation and reduction reactions can be simultaneously done and can more effectively employ solar energy. Over the surface of nanomaterials having high ratio of surface to volume, the charge carriers are generated because of the reduced size with high surface area, different shapes, and controlled morphology. Therefore, nanomaterials can be applied in water splitting process established at the nanomaterials surface. Many researches demonstrate the 50–90% increment in the efficiency of photoelectrochemical water splitting.

Fig. 10.18

Photoelectrochemical water splitting systems using n-type semiconductor (a), p-type semiconductor (b), and tandem system (c), C.B., V.B., and B.G. stand for conductive band, valence band, and band gap, respectively. (Reprinted with permission of Elsevier from Abe 2011)

The structural and electronic features of applied photo-anodes/cathodes in nanomaterials are the main factors affecting the photoelectrochemical water splitting mechanism. Various visible light materials were applied in photoelectrochemical water splitting as photo-electrodes. Recently, Bi-based materials have been widely used in the manufacturing of photo-electrodes in visible light materials systems. For instance, BiFeO₃ photo-anodes were synthesized by using dual-source low-pressure chemical vapor deposition and used in photocatalytic and photoelectrochemical water splitting induced by solar light. Results of incident photon-to-electron conversion efficiency suggested 23% efficiency for photoelectrochemical water splitting activated by light illumination (400 nm) (Moniz et al. 2015). Another work has reported high efficient nanoporous Bi₂WO₆ photo-anodes which synthesized by facile drop-casting method (Dong et al. 2017). The Bi₂WO₆ photo-electrode showed highly significant efficiency for photoelectrochemical water splitting which exhibited photocurrent almost ten times higher than traditional photo-electrodes. Some of Bi-based compounds and nanocomposites synthesized by various methods and applied in photoelectrochemical and photocatalytic water splitting are reported in Table. 10.4.

Table 10.4 Some of Bi-compounds applied for water splitting

10.4 Conclusions and Prospects

Some of special properties of Bi-based semiconductors, such as narrow band gap, layered structures, and controllable morphologies, have attracted more attentions from researchers in photocatalyst field. Almost all of Bi-based photocatalysts type and the catalytic applications have been discussed in this chapter. According to some challenges about Bi-based photocatalytic compounds, noted as fast recombination rate of electrons−hole and low light adsorption and practical approaches suggested to defeat the related challenges. Furthermore, main accomplished works until now have been summarized within morphology control and heterojunctions. However, probable problems for using Bi-based semiconductors can be disappeared, but further studies are still needed to improve the related progresses. Future works could be focused on below issues:

• 1. Until now, significant applications of Bi−nanomaterials can be highlighted as destruction of organic polluters and bacteria of wastewater and purification of air through denitration. Preparation of Z−scheme structures can be nominated as an applicable method for increasing the H₂ generation via photoactivated water splitting under solar light. Further works try to develop advanced Bi−nanomaterials to improve the applicable arena such as photocatalytic synthetization of organic compounds and photoactivated reduction for elimination of heavy metals.

• 2. Pragmatic applications of photocatalysts based on bismuth compounds are rarely storied. Designing the new applicable photocatalytic reactor can permanently precipitate the scale-up process. It can represent the potential industrialization capability of the advanced Bi−nanomaterial. Moreover, establishing of experiments by a solar simulator instead of a bulb shows the more reality of solar-activated photocatalysis performance of mentioned compounds.

• 3. The applicable fields along with further advancements can be propagated through consolidation of different useful techniques such as electrochemistry, membrane technique, and biotechnology. Despite many of bismuth-based semiconductors establish remarkable photoactivity efficiency induced by solar/visible light, they are far from full-fledged commercialization of the advanced nanomaterial. The perfect promised and interesting properties of Bi-based compounds can gift a bright future within environmental aspects and renewable energy sources.