A review on the visible light active BiFeO₃ nanostructures as suitable photocatalyst in the degradation of different textile dyes

Highlights

• Need for water purification to meet the growing demands of portable water.

• Techniques of water treatment and advantages of photocatalysis.

• BiFeO₃ as a potential candidate for water treatment.

• Improvising the photocatalytic property of BiFeO₃ nanostructures by doping and by different heterostructures.

Abstract

Different effluents from textile industries which are dumped into the water bodies have been a major concern of the modern world. The uncontrolled discharge of the synthetic textile dyes in water bodies especially have led to serious environmental problems and health hazards. Among the different existing approaches to treat water, photocatalysis is an attractive approach as it uses the inexhaustible and clean solar energy. Considering its ability to absorb in the visible light of solar spectrum, its multiferroic property and its crystal structure, researchers feel BiFeO₃ can provide a breakthrough in the treatment of water. In this article, a thorough review is presented on the various attempts done so far on degradation of different synthetic textile dyes solution using BiFeO₃ nanostructures.

Introduction

Of the 1.39 billion cu.km of water available on earth, only 2.5% is fresh water and a mere amount of 0.29–0.49% is available for humankind (Tripathi et al., 2015). Hence the treatment and recyclability of water is very important. Water pollution is one of the greatest challenges faced by the developing countries and researchers across the globe are carrying out different approaches to completely eliminate it (Rauf et al., 2011, Akpan and Hameed, 2009, Konstantinou and Albanis, 2004). Effluents from different industries like textile and pharma have become a great threat to the environment (Borker and Salker, 2006). Some effluents are toxic to aquatic life and some dyes from the textile industries can become carcinogens by undergoing anaerobic decolourisation (Zubair Alam et al., 2010, O’Neill et al., 1999). This is a clear indication for the need of treating effluents before they are discharged into the water bodies. The usual approaches used for the treatment of water are 1) Adsorption 2) Sedimentation 3) Coagulation and 4) Ion-floatating (Rauf et al., 2009, Chong et al., 2010a, Colmenares and Luque, 2014, Madhavan et al., 2008). A technique called advanced oxidation process (AOP) is a good alternate for the waste water treatment. AOP technique employs a method of oxidizing the effluents by the hydroxyl radical OH (Al-Kdasi et al., 2004, Elmolla and Chaudhuri, 2010). This process includes the following steps:

• Formation of OH⁻ (hydroxyl ions)

• Initial attack on target molecules by OH radicals and subsequently breaking them down into smaller and less harmful substances until complete mineralization.

The ability to destroy almost all toxic organic effluents by not transferring pollution to another form gives this technique an upper hand over different existing techniques (Mota et al., 2008). This method is very adaptable in small scales and is suitable for developing countries which are major victims of water pollution. Some of the AOP techniques are 1) UV- Photolytic technique 2) Fenton process 3) Photo-Fenton process 4) Ozonation process 5) Sonolysis 6) Photocatalysis 7) Bio-degradation (Ni et al., 2007, Baruah et al., 2012, Chong et al., 2010b). Of all these techniques – photocatalysis is a very promising tool because of its use in harvesting solar energy to treat the effluents in the waste water. The process involved in photocatalysis is as follows – when light of energy equal to or greater than the band gap of the semiconductor falls on it there is a generation of e⁻/hole pair and they move onto the surface of the catalyst and undergo redox reactions with the compounds bound on to the surface. The water (H₂O) molecules are oxidised by the holes to produce OH radicals and the electrons reduce the dissolved oxygen to produce O₂⁻ (i.e.) superoxide radical anion of oxygen.

Photocatalyst + hγ → e⁻ + h⁺

H₂O + h⁺ → OH + H⁺

O₂ + e⁻ → O₂⁻

The OH radicals and O₂⁻ ions produced, oxidize and reduce the dye molecules to produce other simpler species thus leading to decolourisation of the dye. The superoxide radicals anions reacts with H⁺ ions and leads to the formation of more OH radicals which aid in the oxidation of the dye.

OH + dye → dye  (oxidation)

e⁻ + dye → dye  (reduction)

Without the presence of dissolved oxygen and water molecule, there will be no formation of OH radicals and as a result there would be no degradation of the dye molecule through photocatalysis (Rauf et al., 2011).

Fig. 1 shows the general process involved in photocatalysis. Whenever light falls on the semiconductor there is a formation of electron and hole pair. Photosensitive semiconductors like TiO₂, ZnO, CdS, ZnS have been studied extensively for photocatalysis. But most of them are wide-band gap semiconductors which absorb only in the UV region of the solar spectrum which form only 4% of it. The efficiency of this process is greatly reduced by the recombination of the e⁻/hole pairs (i.e) indirectly decreasing the efficiency of the photocatalyst itself (Rajeshwar et al., 2008, Herrmann, 1999). However, in recent days a new group of materials which falls under the class of pervoskite structure has become materials of high interest for photocatalytic applications (Kanhere and Chen, 2014). The pervoskite crystals have the general formula of ABX₃ (Anon., 2015), where A and B are two cations of different sizes. A cation is larger than B cation. The properties of the crystals are determined by the cations. X is the anion which bonds to the cations. X can be a halide or an oxide. The ideal ABX₃ pervoskite has cubic symmetry with space group Pm3m, where B cation is 6 fold co-ordinated and A cation is 12 fold cubo octohedral co-ordinated surrounded by an octahedron of X-anions. This study has been restricted to ABO₃ pervoskites Fig. 2 which can be easily synthesized and mostly studied.

The ABO₃ pervoskite exhibits lattice distortions which results in transfer of crystal phases in following sequence- orthogonal, tetragonal, monoclinic and triclinic phases. The different degree of tilting results in different electronic and optical properties. Compared to the other photosensitive semiconductors, ABO₃ materials have several advantages in photocatalysis because of the large scope to design and alter band gap and other photophysical properties by the A and B cations (Shi and Guo, 2015). Some of the reported ABO₃ pervoskites for photocatalysis application fall into different groups like (i) Titanates: SrTiO₃, BaTiO₃, CaTiO₃, CoTiO₃, NiTiO₃, FeTiO₃, CdTiO₃ (ii) Tantalates: NaTaO₃, AgTaO₃, KTaO₃ (iii) Ferrites: BiFeO₃, LaFeO₃,GdFeO₃ (iv) Others like LaCaO₃, LaNiO₃ (Konta et al., 2004, Maeda, 2014, Qu. et al., 2014, Qu. et al., 2012, Kim. et al., 2009, Hassan et al., 2014, Zheng et al., 2011, Li et al., 2013a, Liu et al., 2007, Parida et al., 2010, Tang et al., 2011a, Sun et al., 2010, Tang et al., 2011b). BiFeO₃ is a widely studied photocatalyst since (i) its band gap lies in the visible region of solar spectrum (ii) exhibits multiferroic behaviour at room temperature which aids in efficient separation of charge carriers and (iii) high chemical stability of the sample (Wang et al., 2008, Godara et al., 2014, Tang et al., 2015, Han et al., 2016, Yu and An, 2009, Wang et al., 2014).

In this review, the different works carried out on BiFeO₃ based nanostructures on the photocatalytic degradation of various organic molecules are presented.

BiFeO₃ (BFO) material exhibits room temperature multiferroic behaviour. BFO shows a Neel temperature of 370 °C and a curie temperature of 830 °C. Though BiFeO₃ materials can be easily prepared at room temperature, these materials often exhibit impure phases like Bi₂Fe₄O₉ and Bi₂₅FeO₃₉ (Yang et al., 2011, Layek and Verma, 2012). The multiferroic BiFeO₃ exhibits a rhombohedral distorted pervoskite structure with space group R3c at room temperature. The magnetoelectric coupling property of BiFeO₃ opens up many degrees of freedom due to which these multiferroics can be used in fields of non-volatile memory, spintronics, sensors and piezoelectric devices (Sando et al., 2013, Jang et al., 2010, Bibes and Barthélémy, 2008, Comyn et al., 2005, Zeches et al., 2009, Bowen et al., 2014, Yu et al., 2009). In addition, band gap of BiFeO₃ materials lying in the visible light region of the solar spectrum extends its horizons to the field of photovoltaics and photocatalysis (Tiwari et al., 2015, Lotey and Verma, 2013, Deng et al., 2011, Joshi et al., 2008, Gao et al., 2015a).

The photocatalysis mechanism involved in the dye degradation using BiFeO₃ nanoparticle as the photocatalyst is shown in Fig. 3

BiFeO₃ nanoparticles are found to have good catalytic property in the visible solar spectrum. The effect of BiFeO₃ nanoparticles on different dyes like Methyl orange (MO), Rhodamine B (RhB), Methylene Blue (MB), Congo red (CR), Reactive Black-5 (RB-5) are summarized

Various works carried out using BiFeO₃ nanoparticles on Methyl orange, an azo-dye used widely in the textile industry has been shown in Table 1. Despite their wide applications they have been reported to show mutagenic property (Ventura-Camargo and Marin-Morales, 2013).

Rhodamine B (RhB) is a cationic dye used in many industrial applications. This dye added to its wonderful application is also identified to have some carcinogenic properties (Jaina et al., 2007). Table 2 gives the details of the work carried out on the dye-Rhodamine B.

Methylene blue is a blue cationic thiazine dye used widely in the field of medicine to treat various illness and disorders. It’s chemical formula is C₁₆H₁₈N₃SCl (Rafatullah et al., 2010). Reactive black-5 (RB-5)is an azo dye which when reacting with textile fibers forms a covalent dye-fiber bond (El-Zawahry Fatm et al., 2016). Reactive azo dyes released from textile dyeing industries seem to be highly recalcitrant to conventional wastewater treatment processes. It is observed that almost about 90% of reactive dyes even after sludge treatment would remain unaffected. Congo Red (CR) is one of the most frequently used secondary diazo dye. Benzidine is a toxic metabolite of Congo red, which causes cancer of the bladder in humans. However, these dyes are popularly used in the textile industry (Sakkas et al., 2010) The details of work done using BFO nanoparticles on Methylene blue, Reactive black-5, Congo Red are given in Table 3.

The work carried out on degradation of different textile dyes using BiFeO₃ nanoparticles as a photo catalyst is explained here (Gao et al., 2015b, Lu et al., 2010, Xian et al., 2011, Gao et al., 2014, Gao et al., 2007, Wang et al., 2016a, Niu et al., 2015a, Wei et al., 2012, Huang et al., 2014, Soltani and Entezari, 2013a, Wang et al., 2011, Bharathkumar et al., 2016, Hengky et al., 2012, Soltani and Entezari, 2013b, Huo et al., 2010, Soltani and Entezar, 2013). From the studies it can be inferred that the synthesis procedure plays an important role in the formation of (i) smaller particle size and (ii) morphology with higher specific surface area. Since photocatalysis is a surface phenomenon, particles with smaller size and higher surface area show a greater photocatalytic activity than their bulk counterparts. Secondly, it is observed that the pH of the solution has a great influence on the dye decolourisation by BiFeO₃ nanoparticles. The isoelectric point of BFO nanoparticles is found to be around pH 2, which means that at lower pH the number of co-ions surrounding the BFO nanoparticles are less, hence paving a way for greater adsorption of the dye molecules on to the surface of the catalyst. This is verified by the studies done using zeta potential measurements by Hengky et al. (2012). BiFeO₃ nanoparticles showed recyclability and stability even after 5 cycles, although photocorrosion on BiFeO₃ nanoparticles was reported using X-ray Photoelectron Spectroscopy (XPS) studies (Hengky et al., 2012, Huo et al., 2010, Soltani and Entezar, 2013). For the treatment of water laden with organic, inorganic and microbial pollutants, semiconductor photocatalysis is a promising technology, however, separation and recycling of the photocatalysts in powdered form after treatment is extremely difficult. This not only leads to loss of the photocatalyst but also to secondary pollution by the photocatalyst particles (Mamba and Mishra, 2016). This difficulty is easily overcome by using BiFeO₃ photoctalysts because of its magnetic property which helps in the easy removal of the particles by using magnets once the treatment is over (Wang et al., 2011, Huo et al., 2010). Apart from the higher surface area, lower particle size creating oxygen vacancies on the surface of BiFeO₃ nanoparticles seems to show a great enhancement in the photocatalytic property as these defects helps in the efficient charge separation of the photo generated charges as well as an enhancement in the optical absorption of the BFO photocatalysts (Wang et al., 2016a). The enhancements in the properties were confirmed by Photoluminescence (PL) and Electron Impedance Spectroscopy studies (EIS). However it should be noted that increasing the defects on to surface beyond the optimal level make the defects act as recombination centres. Almost all the photocatalytic degradation studies of BiFeO₃ reported by the researchers obey the Pseudo-first order kinetics in terms of modified Langmuir-Hinshelwood model (Soltani and Entezari, 2013b, Soltani and Entezari, 2013a, Wang et al., 2011)

One more interesting observation was that BFO nanoparticles showed higher photocatalytic activity in UV light rather than visible light. The reason for this might be because of the source being completely an UV source, for which the energy of the UV light (E = hγ) is greater than the visible light since they have shorter wavelength, but in reality, the solar spectrum comprises of only 3–4% UV light.

But till date, the commercialization of BiFeO₃ nanoparticles as a photocatalytic oxide technology is still not possible because of the low photocatalytic activity of BiFeO₃ nanoparticles in spite of it displaying a great potential towards dye decolourisation (Dhanalakshmi et al., 2016). There are different steps taken by different researchers to overcome and increase the efficiency of the photocatalysis of BiFeO₃.

Introducing foreign atoms at the A, B or both sites (A,B) of BiFeO₃ as dopants, seems to influence the multiferroic property and it’s reasonable to observe some positive effects on the photocatalytic property of BiFeO₃ as well.

Rare earth (RE) elements are a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides plus scandium and yttrium. Their unique magnetic, luminescent, and electrochemical properties have made these elements become irreplaceable in our world of technology. From the previous studies done on rare earth doped BiFeO₃ in the Bismuth site, one can infer that it helps in (i) eliminating the formation of secondary phases, (ii) structural phase transformation and (iii) influencing the ferroelectric and ferromagnetic properties (Sati et al., 2014, Zhao et al., 2013, Mohan et al., 2014). Work carried out on rare earth doped BFO nanoparticles are highlighted in Table 4.

Most of the rare earth dopants have ionic radii almost similar or less than Bi³⁺ and hence could easily fit into the BiFeO₃ matrix. The band gap of the rare earth doped BiFeO₃ showed a marginal increase or decrease but they still are found to lie within the visible light range of the solar spectrum (Dhanalakshmi et al., 2016, Mohan et al., 2014, Mukherjee et al., 2012, Madhu et al., 2010, Kaur et al., 2015, Wu et al., 2013, Vanga et al., 2015a, Sakar et al., 2015, Chen et al., 2015, Guo et al., 2010, Zhang et al., 2016). The BiFeO₃ nanoparticle doped with rare earth elements showed an enhancement of photocatalytic behaviour upto certain concentration of rare earths and doping them above certain limit decreases the property. The increase in photocatalytic behaviour could be understood by the following reasons, (i) RE doped BiFeO₃ shows an increase in dielectric property of the multiferroic material as the concentration of the dopant increases. Due to this enhancement, there is an efficient separation of charge carriers which are produced by the photo excitation. As a result, there is an increase in space charge region and it is proportional to the Debye length

$$L_D=\sqrt{\frac{{{}_o}_{r}KT}{e^2{N}_D}}$$

where ε_o is the permittivity of free space and ε_r is the dielectric constant of the dielectric material, N_D is the donor density, K is the Boltzmann constant and T is the Temperature. From the expression, it can be verified that increase in dopant concentration increases Debye length and therefore recombination rate is decreased effectively (Chen et al., 2015, Guo et al., 2010). (ii)A universal behaviour shown by rare earth dopants at the transition composition are that they have higher dielectric constant than other compositions, leading to a higher space charge width and enhanced photocatalytic behaviour. Similar phase change has been reported for BiFeO₃ from rhombohedral to orthorhombic phase with the mixed space group of R3c as well as Pn21a, which is also polar and allows the polar displacements in the BFO system and is also verified with different dopants of Nd, La, Gd and Dy (Kaur et al., 2015, Sakar et al., 2015, Chen et al., 2015, Guo et al., 2010). Decrease in photocatlytic behaviour when the concentration of the dopant increases beyond an optimum concentration can be justified by two reasons, one is a due to the formation of non-polar phase of REFeO₃ phase which gives rise to decrease in charge separation. Moreover it is believed that at higher concentration, there might be a transition from ferroelectric to paraelectric phase (Sakar et al., 2015, Chen et al., 2015). Secondly, dopant concentration and space charge barrier are inversely related,

$$W=\sqrt{\left(\frac{2_0{V}_s}{e{N}_d}\right)}$$

where W is the thickness of the space charge layer, ε and ε₀ are the static dielectric constants of the semiconductor and of the vacuum respectively, V_s is the surface potential, N_d is the number of dopant donor atoms, e is the electronic charge. Hence increasing the dopant concentration beyond the optimal level, leads to a decrease in space charge width and thereby increasing the probability of recombination, leading to a decrease in efficiency of the photocatalyst (Zhang et al., 2016). Balakumar et al. tried to explain the efficient charge separation in rare earth doped BiFeO₃ by taking into picture the ferroelectric nature of BiFeO₃ which has its own polarisation field with the dye medium (Sakar et al., 2015).

Metal dopants like Al, Mg, Ca, Mn were doped in the Bi³⁺ site of BiFeO₃. In some cases, both the Bi³⁺ and Fe³⁺ sites were doped with a rare earth and other dopant to see the enhancement in the photocatalytic studies. Table 5 summarizes the work done on co-doped and other dopants doped BFO for dye degradation.

Doping of metals does not seem to enhance the photocatalytic effect of BiFeO₃ nanoparticles to that great extent. Strontium and manganese doped BFO showed a little higher photocatalytic effect than the undoped BiFeO₃ (Bhushan et al., 2010, Pei and Zhang, 2013). but the effect of Ca, Al and Mg (Madhu et al., 2010, Pei and Zhang, 2013) on BFO did not show positive results when compared to undoped ones. Increase in photocatalytic activity in Sr-doped BiFeO₃ (Bhushan et al., 2010) is attributed to the formation of defections in the surface of the nanoparticle which is useful in the adsorption of oxygen which was also reported by Wang et al. (2016a). Co-doping with different dopants also failed to enhance the photocatalytic behaviour. Vanga et al. (2015b) in their studies used photo-fenton process to study the degradation of Methylene blue which seemed to be more efficient than just using the catalyst. The decrease in the photocatalytic property may be due to the increase in recombination rate of photo generated electrons and holes due to the variations in the band gaps. It can be inferred that, type of dopant greatly influences the degradation of organic pollutants.

Addition of co-catalysts on BiFeO₃ nanoparticles have shown manifold increase in the photocatalytic property compared to the bare BiFeO₃ nano particles as reported in Table 6 (Niu et al., 2015b, Di et al., 2014, Lu et al., 2015, Wang et al., 2016b).

The enhancement can be attributed to efficient charge separation brought about by the energy band of the heterostructure. Niu et al. (2015b) studied the Pt/BiFeO₃ heterostructure where Pt nanoparticles were used as co-catalyst. Platinum is a metal with a large work function which means they have lower Fermi energy level and therefore there is formation of schottky barrier (Fig. 4), due to the metal semiconductor junction which facilitates an efficient charge separation. The formation of Schottky barrier between an n-type semiconductor and a metal can be explained as follows. This barrier formation helps in efficient charge separation paving a way to increase the photocatalytic property of the photocatalyst. Within the system, the energy needed to bring an electron from the metallic fermi energy to the vacuum is the work function of the metal Φ_M and the energy difference between the minimum conduction band (CB) and the vacuum (Vac) energy is the electron affinity X_SM. When the metal and semiconductor are brought together, based on the stability, the direction of electron flow will be from the semiconductor to the metal because in most cases metal will have lower Fermi energies (E_Fermi) compared to semiconductors leading to formation of a potential barrier. Until the E_Fermi of the semiconductor reaches equilibrium with that of the metal, there would be a continued flow of electrons leading to a constant value of E_Fermi for both of the devices. The electrons (of the semiconductor) must overcome that barrier in order to flow from the semiconductor to the metal. This will cause a depletion of electrons at the semiconductor interface. In other words, there is an accumulation of negative electrons from the metal at the interface and a formation of the positive space charge layer or the depletion layer below the surface of the semiconductor in order to maintain electrical neutrality at the interface. This result influences the band bending in the conduction band. This bending or the Schottky barrier (Φ_B) can be expressed by the following equation

Φ_B = Φ_M − X_SM

Based on the stability concept, the Schottky barrier can serve as an effective electron trap where the electron is unable to flow back to the semiconductor. This makes the metal act as an electron sink for the photo-induced electron. The Schottky barrier prevents the recombination of the electron–hole and prolongs the lifespan of the electron for the photoreaction. Thus Pt/BFO heterostructure showed enhanced photocatalytic behaviour. They also confirmed the enhancement in photocatalytic behaviour by photoluminescense studies (Khan et al., 2015). In their studies they have reported that predominatiant active species for the dye degradation was O₂⁻ rather than OH⁻ ions. Similar effects are seen in g-C₃N₄/BiFeO₃ (Fan et al., 2015) photocatalyst. Adding upto 1.0 wt% of Pt on BiFeO₃ showed 5 times enhancement in photocatalytic activity when compared to bare BiFeO₃ and similar enhancement was also reported in Silver (Ag) particles on BFO but increasing the Platinum (Pt) concentration above this showed a decrease in photocatalytic activity. This might be due to the overcrowding of the co-catalyst on surface which tends to block the light absorption by BiFeO₃ nanoparticles.

Wang et al. (2016b) studied the enhancement brought about in coupling the effect of doping with the optimum concentration of rare earth element and cocatalyst on BFO photocatalyst. They have reported a significant improvement in the photocatalytic property of BFO.

Table 7 reports the work carried out by BiFeO₃ heterostructures composites on various pollutants.

These heterostuctures of BFO also showed a good increase in the dye degradation capability (Humayun et al., 2016, Li et al., 2009b, Gong et al., 2013, Fan et al, 2016, Niu et al., 2015c, Fan et al., 2015, Dai et al., 2013, Li et al., 2013b, Wang et al., 2016c) as compared to BiFeO₃ particles. Forming a core-shell, composites, heterojunctions hybrids with BFO were the different heterostructures used in studying the Photocatalytic property. Core-shell structure of BiFeO₃/TiO₂ as reported by Li et al. (2009b) involves two semiconductors. When two semiconductors like BiFeO₃ which has higher conduction band than TiO₂ are brought together the excitation of electron in the BiFeO₃ causes the electrons to be injected into the low lying conduction band of the other semiconductor, TiO₂ and form Ti³⁺ electron centre whereas the holes accumulate in the valence band of BiFeO₃ to form the hole centre. Thereby the recombination of photo generated electrons and holes is decreased by increasing the life time of the charge carriers and achieving an enhanced interfacial charge transfer to the adsorbed organic molecule and decomposing them. Dipolar fields of TiO₂ molecule acts as a driving force in helping the charge carriers to migrate to the surface. Still the exact mechanism of efficient photocatalytic property of this core-shell structure is yet to be explored and understood clearly. Through these heterostructures, the generation of hydrogen (H₂) by water splitting is also made possible. M.Humayum et al. studied photocatalytic effect of TiO₂ and Porous −BiFeO₃ nanocomposites (Humayun et al., 2016). Using these nanocomposites they have produced about 0.8 μmol/h of hydrogen by water splitting. This will not otherwise be possible if only BiFeO₃ is used because BiFeO₃ is a photoxidant and has its Conduction Band (CB) edge position slightly more positive than the Standard Hydrogen Electrode (SHE) as shown in Fig. 5.

Hybrid structures of BFO formed with optimum concentration of graphene and Multi Walled Carbon Nano Tubes (MWCNT) resulted in an increase in the photocatalytic property. This is because of the efficient charge separation and higher surface area (Li et al., 2013b, Wang et al., 2016c). TiO₂ being an efficient and extensively studied photocatalyst, Xiao Fei Wang et al. compared the efficiency of 10%BFO/MWCNT to N doped TiO₂ for the complete removal of 1 mM RhB. It has been reported that 10% BiFeO₃/MWCNT took 130 min, which includes 10 min of physical adsorption and 120 min of photocatalytic degradation whereas the N doped TiO₂ took 310 min under the same conditions. From the results, one can conclude that forming suitable heterostructures with BFO, we can achieve super enhancements in photocatalytic properties as compared to the already existing counterparts (Wang et al., 2016c).

Nanofibers are one dimensional structures with high aspect ratio and these structures are being confined in two dimensions. Their high aspect ratio aids in decreasing the recombination rate and thereby increasing the efficiency of the photocatalysts. The different works carried out using BiFeO₃ nanofibers are highlighted in Table 8.

The photocatalytic behaviour of fibers showed an increasing trend due to their one dimensional structure which inhibits the recombination possibilities due to an increased delocalization of electrons and enhanced recombination resistance. Added to it, surface of fibers appears to be like inter connected crystallite structures which make it possible for the photogenerated carriers to reach the surface easily and act on the dye molecules efficiently. Balakumar et al. studied on Dysprosium (Dy) doped BiFeO₃ particles and fibers (Sakar et al., 2015) and has reported enhanced photocatalytic property of fibers than the particles. The BiFeO₃ mesh and BiFeO₃ mat as photocatalyst were studied on the methylene blue dye. BiFeO₃ mesh showed much enhanced activity because of its larger surface interaction with the dye compared to the mat. These 1-D structures with closely packed inter connected structure facilates large charge interaction and restricts charge recombination (Feng et al., 2013). One of the advantages of these structures is the removal of the extra filtration stage of photocatalyst once the degradation process is complete (Liu et al., 2013). Mesoporous and hollow spherical BiFeO₃ nanoparticles reported by Yuning Huo et al. (Huo et al., 2011) also showed an enhanced degradation. BiFeO₃ nanoparticle with surface area 27m²g ⁻¹ and pore volume of 0.064 m³g⁻¹ showed the highest degradation. This can be attributed to the greater light absorbance by the hollow spheres via multiple light reflections. These hollow spheres also exhibited high recyclability and stability.

BiFeO₃, a room temperature multiferroic material is widely used as photocatalyst in decolourisation of different dyes both in the UV and visible light region because of its band gap, which lies in the visible region of solar spectrum. BiFeO₃ particles which have smaller size and higher surface area showed higher photocatalytic effect. To further increase the BiFeO₃ efficiency as photocatalyst, researchers tried out different techniques like (i) doping, (ii) heterostructures like core-shell, co-catalysts and composites and (iii) 1-D structures. All of them seemed to enhance the photocatalytic behaviour of BiFeO₃ greatly although the exact mechanism has not been exactly understood very well. But in all these, modified BFO seems to offer a good separation of charges which influences the efficiency of the photocatalyst. Apart from these properties BiFeO₃ nanoparticles have also showed recyclability and stability for at least five cycles. The problem of nano toxicity because of using nanomaterials in water treatment can completely be eliminated because of its ability to get separated easily from the treated water by using a magnet. From the above studies, it can be inferred that efficient charge separation is attained in all cases of the BFO nanostructures. In some heterostructures the efficiency of BFO catalysts was reported to be comparable with TiO₂, one of the most efficient photocatalysts.

Therefore now the challenge lies in moving the BiFeO₃ photocatalyst from lab scale to practical use. The effect of multiferroic behaviour and crystal structure on the photocatalyst has to be reviewed and studied. The scope of BFO photocatalyts can be extended further by studying the photocatalytic behaviour of BFO on different pollutants like pesticides, antibiotics and so on. Though there is a great progress in using BiFeO₃ based nanostructures as photocatalyst but still there is a greater scope for development in this technology.