Ferrites as Photocatalysts for Water Splitting and Degradation of

Chapter 3

Ferrites as Photocatalysts for Water Splitting and Degradation of Contaminants Bangxing Ren,1 Ying Huang,1 Changseok Han,1 Mallikarjuna N. Nadagouda,2 and Dionysios D. Dionysiou1,* 1Environmental

Engineering and Science Program, University of Cincinnati, Cincinnati, Ohio 45221-0012, United States 2Department of Mechanical and Materials Engineering, Wright State University, Dayton, Ohio 45324, United States *E-mail: [email protected]

Ferrites are a group of materials with great potentials in photocatalysis thanks to their excellent properties such as relatively small band gaps, stable structures, and low cost. In this book chapter, the use of ferrites and ferrite-based composites is reviewed as photocatalysts for water splitting and degradation of contaminants. Special attention is paid to the performance of these materials under visible light irradiation. The synergistic action of ferrites with common oxidants including hydrogen peroxide (H2O2), peroxymonosulfate (PMS), and peroxydisulfate (PDS) in decomposing pollutants is also addressed.

Introduction With sunlight being one of the most abundant renewable energy resources, there has been tremendous enthusiasm for the harvest, conversion, and utilization of solar energy in recent decades. Continuous efforts have been made in the fields of photovoltaic cells, photocatalytic water splitting, light-driven reduction of carbon dioxide, and photocatalytic decomposition of contaminants (1–5). Following the debut of TiO2 photocatalysis field, the development of photocatalysts with novel compositions and structures has led to substantial progress in the efficiency of photocatalytic reactions (6–8). The ideal photocatalyst requires a suitable band gap for harvesting light, excellent nanostructures for facile © 2016 American Chemical Society

transport and separation of charge carriers, and proper positions of conduction band (CB) and valence band (VB) for redox reactions. Particularly, photocatalysts with visible light response are favored since ultraviolet light comprises only a very small portion of the sunlight spectrum (4). Ferrites have been considered as a promising candidate for efficient photocatalysts because of their small band gaps, stable structures, and unique magnetic properties. Besides, they are widely available and can be produced at relatively low cost (9, 10). The majority of ferrites share the general formula of AB2O4. As shown in Figure 1, oxygen anions are arranged in the manner of cubic closed packing, while the tetrahedral sites (A) and octahedral sites (B) are occupied by metal cations. With negative charges of oxygen anions being balanced, this structure can offer various combination of metal cations. When all divalent metal cations take A sites and all iron(III) cations occupy B sites, the structure is defined as normal spinel ferrites with the formula of MeA[Fe2]BO4. When all divalent metal ions take B sites and equal number of iron(III) ions take both sites, the structure is written as FeA[MeFe]BO4 and called inverse spinel ferrites. In fact, some metal cations have strong preference for certain site, for instance, Zn for A site and Cu for B site (11). However, with great variations in the synthesis conditions, ferrite crystals may deviate largely from the perfect structure mentioned above, and instead show an empirical formula of [M1-xFex]A[MxFe2-x]BO4 (0 ≤ x ≤ 1). Moreover, the arrangement of the metal ions become more complex when a third metal is introduced, forming ternary ferrites (11). The difference in the composition and arrangement of metal ions is found to affect the electrical, optical, and magnetic properties of ferrites as well as their activities in certain catalytic reactions (9).

Figure 1. The crystal structure of AB2O4 with A and B occupying the tedrahedral sites and octahedral sites, respectively.

80

Ferrites for Photocatalytic Water Splitting In the process of photocatalytic water splitting, electron–hole ( ) pairs are generated when photocatalysts are irradiated with light which has greater energy than the band gap energy. Subsequently, water molecules are reduced by photogenerated electrons in CBs to hydrogen and oxidized to oxygen by holes in VBs. To make the reaction thermodynamically possible, the position of CB is required to be more negative than the redox potential of H+/H2 (0 V vs. NHE), while the position of VB should be more positive than the redox potential of O2/H2O (1.23 V vs. NHE) (3). As shown in Figure 2, the CB positions of ferrites are well above 0 V, which make them theoretically favorable for H2 evolution. However, the reaction rate of photocatalytic water decomposition can be low with ferrites acting alone. Several measures have been employed to promote the reaction efficiency. One of them is the introduction of co-catalyst such as Pt and RuO2, and another is the use of sacrificial agents such as alcohol to scavenge the photogenerated holes (5, 9). The application of ferrites in photocatalytic water splitting can be classified in three parts based on their roles in the photocatalytic system: ferrites as photocathodes, photoanodes, and aqueous suspensions.

Figure 2. The bandgap and positions of VB and CB of representative ferrites (n and p refer to the semiconductor type of ferrites, data obtained from reference (9)). Ferrites as Photocathodes The use of CoFe2O4-based materials as photocathodes was reported by Yang and coworkers (12). The electrodes were fabricated by electrochemical deposition of CoFe2O4 porous nanosheets on fluorine-doped tin oxide (FTO) coated glass followed by a heat treatment in air at 600 °C. The deposition process was conducted in an aqueous solution of cobalt and iron nitrates with the aid of urea. Under a visible light irradiation of 30 mW cm-2, the electrodes immersed in electrolytes with no bias voltage applied achieved only a small photocurrent of ~ −0.3 µA cm-2. 81

Cao et al. prepared CaFe2O4 thin films on FTO glass substrate and applied them as photocathodes for water splitting under visible light irradiation (13). The thin films were deposited on FTO glass by a pulsed laser deposition (PLD) machine with the starting CaFe2O4 pellet produced through heat treatment of CaCO3 and Fe2O3 at 1100 °C. The deposition was conducted at 550 °C, which is relatively low compared to the CaFe2O4 films prepared by conventional calcination at temperatures as high as 1100-1200 °C. At this relatively low temperature of 550 °C, CaFe2O4 thin films with a uniform thickness of 100 nm and smooth surface were obtained. In a three-electrode configuration with Pt as counter electrode, a negative photocurrent of −117 µA cm-2 at −0.06 V vs. NHE and a photocurrent of 0.6 µA cm-2 at zero bias voltage were observed. Following this work, Cao and coworkers also investigated the photoelectrochemical performances of multiple p-n junction p-CaFe2O4/n-ZnFe2O4 photo electrodes (14). The composite electrodes were fabricated using the PLD technique by electrodepositing CaFe2O4 and ZnFe2O4 layers on FTO glass alternatively. When a single layer of CaFe2O4 or ZnFe2O4 was deposited respectively, typical photocathode and photoanode properties were observed. A negative photocurrent and a positive open circuit voltage (+0.025 V, λ = 430 nm) were recorded on a single p-n junction of CaFe2O4/ZnFe2O4 electrode with CaFe2O4 layer contacting the electrolyte, which clearly proved that it performed as a photocathode. The effects of p-n junction number and single-layer thickness were also explored by comparing the performances of the electrodes with junction number of 10, 15, 20, and 25 as well as single-layer thickness of 12.5 nm, 25 nm, and 125 nm. The 20-junction photoelectrode exhibited a remarkable photocurrent of −25.23 μA cm-2 at 0.4 V vs. NHE and the highest open circuit potential of 0.97 V, which was about 5 times higher than that of the single junction electrode (0.13 V). The multi-junction electrode with 12.5 nm single-layer thickness was shown to perform better at more positive applied potential. Metal-doped CaFe2O4 photocathodes were also prepared in an attempt to improve photo electrochemical properties (15). The electrodes were fabricated by co-depositing CaFe2O4 and various metals on antinomy-doped tin oxide (ATO) glass with a radio frequency magnetron co-sputtering technique and then annealing at 650 °C in O2 flow. The Ag-doped CaFe2O4 exhibited a photocurrent of ~ −20 µA cm-2 with no applied bias under visible light illumination, which is about 23 times higher than that of the undoped CaFe2O4 electrodes. The dramatic enhancement of photocurrent by Ag doping was ascribed to the increase of carrier mobility induced by the improved symmetry around Fe atom as well as the red-shift of photoabsorption. Au-doped CaFe2O4 only showed a slight 2-fold increase of photocurrent compared to the pristine CaFe2O4 because Au doping exerted a negative effect on the crystallinity of CaFe2O4 even though it improved the absorption in the visible light region. Moreover, Ida and coworkers explored the performances of photo electrochemical cells (PECs), which consist of CaFe2O4-based photocathodes and n-type semiconductor photoanodes (16–18). CaFe2O4 powder was first prepared by calcining the precursors obtained through a sol-gel method. Then the powder was deposited on a Pt substrate and melted at 1200 °C which led to a smooth flat film with a thickness around 100 µm. Unfortunately, small cracks on the CaFe2O4 82

surface were observed after reaction, which showed the photocathode was slightly decomposed during the process and thus resulted in a decrease of photocurrent (16). The performance of the PECs with ferrites used as photocathodes in visible light induced water splitting are summarized in Table 1. Ferrites as Photoanodes The photoelectrochemical behavior of ZnFe2O4 as photoanodes were studied by Tahir and coauthors (19, 20). The ZnFe2O4 thin films were deposited on FTO glass substrates from a single source bimetallic precursor through an aerosol-assisted chemical vapor deposition process. The precursor was prepared in a mixed solution of methanol and ethanol with various ratios. The nanostructure and morphology of the films could be tuned by varying the composition of solvent, deposition duration, and deposition temperature. The presense of compact particles with hexagonal shape was observed on the surface of the films deposited with pure methanol. With an increasing proportion of ethanol in the solvent, nanorods began forming on the surface of the films. It was found that the ZnFe2O4 electrode fabricated using 0.1 M solution of precursor in ethanol at 450 °C for a duration of 35 min exhibited the highest photocurrent density of 350 µA cm-2 at 0.44 V vs. NHE. The vertical growth of ZnFe2O4 nanostructure is believed to promote the transport of photogenerated minority carrier from the electrode surface to the electrolyte (20). Meanwhile, Kim et al. presented an innovative approach to produce 1D ZnFe2O4 photoanode (21). In this study, zinc nitrate solution was dropped on the β-FeOOH nanorods which were first grown on FTO glass through a wet chemistry process. The mixture was then annealed at 550 °C for 3 h or 800 °C for 20 min. The ZeFe2O4 nonorods were finally obtained by dissolving the ZnO in NaOH solution. To improve the poor crystallinity and surface quality, the as-prepared nanorods were subjected to a hybrid microwave annealing (HMA) with graphite powders as susceptor for 5 min. I-V curves revealed that the nanorods annealed at 550 °C with HMA post treatment exhibited the highest photocurrent density of 240 µA cm-2 at 0.43 V vs NHE, which was a remarkable enhancement compared to the value (~ 15 µA cm-2) of the nanorods without the HMA treatment. In addition, evolution of hydrogen and oxygen gases was found to follow very closely the stoichiometric ratio with faradaic efficiency higher than 90% through a duration of 3 h. As the evidence indicated by electrochemical impedance spectroscopy, the authors believed the positive effect of HMA treatment is a result of the improved crystallinity and decreased surface defects sites of ZnFe2O4. The investigations of ZnFe2O4/Fe2O3 composite photoanodes have been reported by several groups as well (22–24). In the research of McDonald et al., the composite films were prepared in a similar approach to Kim’s study described above except the FeOOH films first went through a heat treatment before the zinc precursor was introduced (24). The best performing composite was found to be the sample with ZnFe2O4/Fe2O3 ratio of 1, because further increase of ZeFe2O4 significantly reduced the surface area, counteracting the positive effect of ZnFe2O4/Fe2O3 heterojunctions. The photoelectrochemical properties of the PECs with ferrite photoanodes are listed in Table 2. 83

Table 1. The performance of the PECs with ferrites used as photocathodes Short-circuit current density (µA cm-2)

Opencircuit voltage (V vs. NHE)

Onset potential (V vs. NHE)

Photoanode

Light source

Electrolyte

CoFe2O4

Pt

λ > 390 nm, 30 mW cm-2

0.1M Na2S

CaFe2O4

Pt

300 W Xe

0.1M Na2SO4

~ −100

CaFe2O4/ZnFe2O4

Pt

500 W Xe

0.1M Na2SO4

−15

20-CaFe2O4/ZnFe2O4

Pt

500 W Xe

0.1M Na2SO4

−40

Ag-doped CaFe2O4

Pt

300 W Xe (λ: 300-800 nm)

0.2 M K2SO4

~ −150

CaFe2O4

TiO2

500 W Xe

0.1 M NaOH

−220

+0.97

+0.51

CaFe2O4

ZnO

500 W Xe

0.1 M NaOH

−250

+0.82

+0.51

CaFe2O4/Ca2Fe2O5

TiO2

500 W Xe

0.1 M NaOH

−275

+1.09

+0.5

84

Photocathode

H2/O2 ratio

Ref.

Yang et al. (12) +0.66

Cao et al. (13)

+0.13

+0.7

Cao et al. (14)

+0.97

+1.3

Cao et al. (14)

~ +0.4

Sekizawa el al. (15) 10-20

Ida et al. (18) Ida et al. (17)

3.7

Ida et al. (16)

Table 2. The performance of the PECs with ferrites used as photoanodes

a

Photoanode

Light source

Electrolyte

Photocurrent density (µA cm-2) (V vs. NHE)a

Ref.

ZnFe2O4

AM 1.5

1 M NaOH

350 (0.44)

Tahir et al. (19, 20)

ZnFe2O4

AM 1.5 (100 mW cm-2)

1 M NaOH

240 (0.43)

Kim et al. (21)

ZnFe2O4/Fe2O3

AM 1.5 (100 mW cm-2)

1 M NaOH

100 (0.59)

Dom et al. (22)

ZnFe2O4/Fe2O3

300 W Xe

0.5 M NaOH and 0.1 M glucose

440 (0.40)

Guo et al. (23)

ZnFe2O4/Fe2O3

AM 1.5 (100 mW cm-2)

1 M NaOH

~ 500 (0.40)

McDonald et al. (24)

The voltage applied when the photocurrent density was recorded.

85

Ferrite Photocatalysts as Aqueous Suspensions Mangrulkar et al. prepared Fe3O4 nanoparticles with a size of 10-12 nm through a facile co-precipitation method and employed them for photocatalytic water splitting driven by visible light (25). The authors investigated the effect of the catalyst dosage, co-catalyst dosage, species and dosage of sacrificial agents, and irradiation intensity on the performance of Fe3O4 as photocatalyst. The results indicated that the highest hydrogen evolution was achieved at a catalyst dosage of 50 mg L-1, a co-catalyst dosage of 4.75 mg L-1, and an ethanol dosage of 50 g L-1. The reaction was found to be only likely to happen at an elevated temperature of 85-100 °C and the catalyst was only stable for up to 22 h at this temperature. Alkaline earth metal ferrites such as MgFe2O4, CaFe2O4, and BaFe2O4 and their composites with various semiconductors were widely studied as photocatalysts for water splitting (26–30). One good example of these is the composite of MgFe2O4 and g-C3N4 as photocatalysts for water decomposition under visible light (28). The MgFe2O4 was synthesized through a citric acid-aid sol-gel method and then was loaded onto g-C3N4 through a facile one-pot approach. Pt nanoparticles were deposited on the composite nanostructure during the reduction induced by the photogenerated electrons from g-C3N4. With triethanolamine applied as the hole sacrificial agent, the composite loaded with 150 mg MgFe2O4 showed the highest hydrogen evolution efficiency. The quantum yield (QY) and turnover number were recorded as 1.79% and 154.1 for this sample, which increased remarkably compared to those values of the pure g-C3N4 (0.63% and 49.4). The authors concluded that the promoted charge separation caused by MgFe2O4 and the high activity of Pt as hydrogen reduction sites contributed to the enhanced performance of the composite nanomaterial. In addition, Kim and coworkers synthesized the composite photocatalyst with p-CaFe2O4 nano-islands spread on the layer of n-PbBi2Nb1.9W0.1O9 aimed at expanding the photoabsorption in the visible light region and promoting the separation of photogenerated electrons and holes (27). CaFe2O4 was prepared by a sol-gel approach and PbBi2Nb1.9W0.1O9 was synthesized through a solid-state method. The composite was prepared through a hydrothermal reaction of the two components. It was observed that the composite exhibited a QY of 38% of O2 evolution with AgNO3 as electron scavenger under visible light irradiation. On the evidence of the post reaction analysis, the composite retained the structure well even after more than 100 h of water splitting reaction. The hybrid materials also showed the capability to photodegrade gaseous compound such as acetaldehyde. A number of studies have been conducted using NiFe2O4-based materials as photocatalysts for hydrogen evolution (31–34). Hong et al. introduced the mesoporous NiFe2O4 nanospheres made by self-assembly associated aerosol spray pyrolysis with Pluronic F127 as the structure-directing agent (31). The mesoporous properties such as surface areas, average pore sizes, and pore volumes of the nickel ferrite materials can be easily tuned by varying the concentration of surfactant and calcining the precursors at different heating rate. The precursor prepared with 10 g L-1 Pluronic F127 were heated at 300 °C with a heating ramp of 5 °C min-1, exhibiting the highest hydrogen evolution rate of 0.09 µmol h-1 because of its high surface area, wide absorption in the visible light 86

region, and higher crystallinity. The sample showed no noticeable deterioration in performance after repeated use. Another attempt was made to assemble the NiFe2O4@TiO2 composite with core-shell structure by Kim and coworkers (32). The TiO2 shell layer was deposited on the pre-synthesized NiFe2O4 core through the controlled hydrolysis of titanium precursor. The authors proposed that the ascorbic acid played an important role in the forming of TiO2 layer by coordinating titanium complexes and the surface of NiFe2O4. Hydrogen evolution was clearly observed from irradiated suspension of the core-shell composites. The applications of CuFe2O4-based ferrites for photocatalytic water decomposition were reported in several studies as well (35–38). CuFe2O4 ferrites were obtained though a heat treatment at 1000 °C of homogenized mixture of CuO and Fe2O3 in the research of Saadi et al (36). A QY of 1% was reported for the H2 evolution over a time span of 90 min. The performance of the copper ferrites started to degrade as the reaction continued more than 2h. It was found that the oxidation products of the hole scavengers K2S adsorbed on the surface of the photocatalyst and blocked the active sites for the reaction. Therefore, the activity can be restored by purging the suspension with N2 or replacing the electrolyte with fresh solution. Continuing their work on Cu-based ferrites, this research group explored the effect of Mn doping though the formation of a series of CuFe2-xMnxO4 (x = 0, 0.4, 0.6, 0.8, 1.2, 1.6, and 2.0) ternary nanomaterials (35). Introducing Mn into the CuFe2O4 nanostructure significantly influenced the electrochemical properties of the nanomaterials, for instance, the narrowing band gap. All samples demonstrated photocatalytic activity with the sample (x = 0.4) exhibiting the highest quantum efficiency of 1.59 %. We have also seen considerable attention being given to ZnFe2O4-based photocatalysts in recent years (39–48). Song and coworkers demonstrated the synthesis of hollow ZnFe2O4/ZnO nanospheres with hard template (44). The sphere templates were prepared through a hydrothermal approach with glucose as staring materials. Zinc and iron(III) ions can readily adsorbed on the surface of the carbon template thanks to the rich hydroxyl and carboxyl groups formed during the hydrothermal synthesis. Moreover, the molar ratio of ZnFe2O4 to ZnO in the photocatalyst can be easily tuned by varying the concentrations of the staring salt solution. The carbon template disappeared after calcination and left a hollow structure with uniform shell about 10 nm thick. Best photocatalytic performance was observed with the sample of ZnFe2O4 : ZnO = 7 : 3. The recorded quantum efficiency and hydrogen generation rate were 1.61 % and 2.15 mmol h-1 g-1, respectively, for this sample, which were about 7 times higher than those of the pure ZnFe2O4 sample. Two factors mainly contributed to the improved performance of ZnFe2O4/ZnO composite. One is the inhibited charge carriers recombination caused by proper band structure alignments of these two components, and the other is the largely shorten diffusion distance of charge carriers which resulted from the very thin hollow spheres. Similar promotion of photocatalytic activity induced by semiconductor heterojunctions were also demonstrated in the cases of ZnFe2O4/CdS (48) and ZnFe2O4/SrTiO3 (41). The photocatalytic performances of the ferrites mentioned above are summarized in Table 3. 87

Table 3. The photocatalytic hydrogen production of ferrites used as aqueous suspensions Photocatalyst

Cocatalyst

Sacrificial agent

Light source

Hydrogen evolution rate (mmol h-1 g-1)

Fe3O4

Pt

Ethanol

400 W Tungsten

8.23

NaNa2SO3 in 0.25 M NaOH

200 W Tungsten

MgFe2O4

QY(%)a

Ref.

Mangrulkar et al. (25) 0.5

Zazoua et al. (29)

88

MgFe2O4

Pt and RuO2

Methanol

450 W Hg-arc (λ ≥ 420 nm)

0.045

0.57

Kim et al. (30)

MgFe2O4/g-C3N4

Pt

Triethanolamine

300 W Xe (λ ≥ 430 nm)

0.175

1.79

Chen et al. (28)

MgFe2O4/CaFe2O4

Pt and RuO2

Methanol

450 W Hg-arc (λ ≥ 420 nm)

0.82

10.1

Kim et al. (30)

CaFe2O4

Pt and RuO2

Methanol

450 W Hg-arc (λ ≥ 420 nm)

0.014

0.16

Kim et al. (30)

CaFe2O4/PbBi2Nb1.9W0.1O9

Pt

0.05 M AgNO3

450 W Xe (λ ≥ 420 nm)

38b

Kim et al. (27)

CaFe2O4/PbBi2Nb1.9W0.1O9

Pt

Methanol

450 W Xe (λ ≥ 420 nm)

BaFe2O4

Pt and RuO2

Methanol

λ ≥ 420 nm

NiFe2O4

NaNa2S2O3

500 W Halogen

NiFe2O4

Methanol

250 W Xe (λ ≥ 420 nm)

Kim et al. (27) 13

1.44

1.73

Borse et al. (26)

0.53

Rekhila et al. (34)

0.52

Peng et al. (33)

Sacrificial agent

Light source


QY(%)a

Ref.

NiFe2O4

Methanol

500 W Xe (λ ≥ 420 nm)

0.046

0.0075

Hong et al. (31)

NiFe2O4/TiO2

Methanol

UV (λ = 365 nm)

CuFe2O4

0.025 M K2S in 1M KOH

600 W Tungsten

0.1

Saadi et al. (36)

CuFe2-xMnxO4

0.025 M K2S

500 W Tungsten

1.59 (x = 0.4)

Helaili et al. (35)

CuFe2O4

0.05 M oxalic acid

250 W Xe

CuFe2O4/TiO2

NaNa2S2O3 in KOH

600 W Tungsten

ZnFe2O4

Pt

Methanol

500 W Hg-arc (λ ≥ 420 nm)

ZnFe2-xTixO4

Pt

Methanol

ZnFe2O4

Photocatalyst

Cocatalyst

Kim et al. (32)

1.72

Yang et al. (37)

89

1.3

Kezzim et al. (38)

1.3

0.15

Borse et al. (39)

500 W Hg-arc (λ ≥ 420 nm)

6.7

0.77 (x = 0.06)

Borse et al. (40)

0.05 M Na2SO3

λ ≥ 420 nm and λ ≥ 250 nm

0.02 (λ ≥ 420 nm) and 0.86 (λ ≥ 250 nm)

ZnFe2O4/SrTiO3

0.025M Na2S2O3 in NaOH

600 W Tungsten

ZnFe2O4

Methanol

λ ≥ 420 nm and AM 1.5

Xu et al. (45)

Boumaza et al. (41) 0.13

0.19

Dom et al. (42) Continued on next page.

Table 3. (Continued). The photocatalytic hydrogen production of ferrites used as aqueous suspensions Sacrificial agent

Light source


QY(%)a

Ref.

ZnFe2O4/ZnO

Methanol

λ ≥ 420 nm and AM 1.5

2.15

1.61

Song et al. (44)

ZnFe2O4/CdS

0.175 M Na2S and 0.125 M NaSO3

λ ≥ 400 nm

2.44

Yu et al. (48)

ZnFe2O4/C

Methanol

300 W Xe (λ ≥ 420 nm)

1.14 (C 17 wt. %)

Zhu et al. (46)

ZnFe2O4

Methanol

250 W Xe (λ ≥ 420 nm)

0.048

Lv et al. (43)

Photocatalyst

90

a

Cocatalyst

Hydrogen evolution was observed for the photocatalyst but no data are available for calculating QY.

b

Calculated from the data of oxygen evolution.

Ferrites for Photocatalytic Degradation of Contaminants Recently, the use of ferrites as visible light active photocatalysts for the degradation of contaminants in water treatment has gained much interest. The band gaps of around 2 eV enable them to absorb visible light to generate ) pairs (10). However, pairs tend to recombine electron–hole ( quickly which reduces the efficiency to degrade contaminants. One solution to decrease the recombination is to introduce other compounds such as ZnO (49), TiO2 (50–52) and reduced graphene oxide (rGO) to ferrites to form composite photocatalysts. In this case, reactive oxygen species including superoxide anion radical (O2•−) and hydroxyl radical (•OH) yielded from the reaction between and adsorbed O2 are responsible for the degradation of contaminants. Another is to add electron acceptors such as H2O2 (53–55), PMS (56) or PDS (57) to the reaction matrix, which can form •OH (eq. 1-3) or sulfate radical (SO4•−) (eq. 4-8). In both cases, oxidation plays a significant role in the degradation of contaminants which includes direct oxidation of adsorbed substrate molecules and oxidation by •OH radical generated by the oxidation of hydroxide ions (eq. 9).

Composite photocatalysts are synthesized in order to take advantage of their different band gap positions which can cause greater separation of the pairs in order to improve the efficiency of degradation processes. The combination ways of composite photocatalysts vary a lot: (1) ferrites mixed with other nanoparticles to form composite powder such as ZnFe2O4/TiO2 (58); (2) ferrites doped on lattice structure such as CoFe2O4/rGO (59); (3) ferrites acting as a magnetic core and other materials as a shell such as ZnFe2O4/ZnO (49). When H2O2, PMS or PDS are added with ferrites under light irradiation, heterogeneous photo-assisted Fenton-like systems are formed which can 91

overcome some of the limitations of traditional homogeneous Fenton system such as the formation of iron sludge and acidic operation pH. In the structure of MFe2O4, M plays a significant role when catalyzing oxidants. When degrading Remazol Black 5 (RB5), catalytic activity of ferrites (M = Cu, Zn, Ni and Co) towards H2O2 and PMS under visible light followed the order CuFe2O4 > ZnFe2O4 > NiFe2O4 > CoFe2O4 and CoFe2O4 > CuFe2O4 > NiFe2O4 > ZnFe2O4 (60). The photocatalytic degradation of organic molecules is of great importance in water treatment and dyes are often selected as model molecules with few other potential organic contaminants. Their chemical formula, molecular weight and structure are listed in Table 4.

Table 4. Chemical information of selected dyes

92

Photodegradation of Dyes by Ferrites Methyl Orange (MO) MO dye is a common azo-dye consisting of two benzene rings connected by an azo group, in which one of the rings contains a dimethyl amine and the other contains a sulfonic acid group. MO dye is commonly used as target contaminant when studying the photocatalytic activity of ferrite; examples of results are summarized in Table 5. As shown in Table 5, MO dye can be directly decomposed by various ferrites under UV or visible light irradiation. Usually ferrite alone shows low efficiency to degrade the target contaminants, while its performance can be improved by forming doped ferrites or mixing with other photocatalysts. There are several factors affecting the photocatalytic activity of doped ferrites, such as the type and concentration of surfactant and metal precursors during the synthesis. For example, four common surfactants (cetyltrimethylammonium bromide (CTAB), polyvinylpyrrolidone (PVP), sodium dodecyl sulfate (SDS), and oleic acid) were used in the synthesis process of ZnFe2-xLaxO4 but only PVP worked well (61). Combining with other photocatalysts can improve the photocatalytic activity of ferrites, however there are many factors needed to be considered and optimized in the preparation process in order to achieve a desirable photocatalytic activity and degradation efficiency of MO. It was reported that varying the type or weight ratio of two or three catalysts produced different catalytic performances (51, 62–68). The structure is another important factor, for example, when ZnFe2O4/TiO2 was a hollow composite, the highest photocatalytic activity was achieved at TiO2 of 25 wt. % (62), while when ZnFe2O4/TiO2 was powder composite the optimum amount of TiO2 was at 95 wt. % (51). Additionally, the difference in performance between Pt/BiFeO3 and Ag/BiFeO3 emphasizes the type of additional element can play a role since Ag worked better than Pt to improve the photocatalytic activity of BiFeO3 (63, 64).

Methylene Blue (MB)

MB is another common dye that has been studied as the substrate to evaluate the photocatalytic activity of ferrites. The structure of MB is shown in Table 4, and it contains a phenothiazine with a dimethyl amine on each side. Table 6 summarizes the studies about degradation of MB dye by various ferrites under light irradiation. Although MO is an anionic dye and MB is a cationic dye, the results showed that similar to the case of MO, large amounts of MB dye could be removed by ferrites when used in combination with other photocatalysts. The performance of the ferrites was affected by many factors such as structure, synthesis method and the presence of additives (69–74). Yuan et al. reported an interesting 3D flower-like structure of SnS2–MgFe2O4/rGO which took advantage 93

of the synergistic effect of three different photocatalysts in charge separation to enhance photocatalytic activity (69). The SnS2–MgFe2O4/rGO showed the highest photocatalytic activity with a rate constant of 0.0085 min-1 compared to MgFe2O4 (0.0062 min-1), MgFe2O4/rGO (0.0054 min-1), SnS2 (0.0066 min-1) and SnS2–MgFe2O4 (0.0072 min-1). Another aspect that has been studied is the synthesis method. It can be seen that CaFe2O4 prepared by a solution combustion method (SCS–CaFe2O4) degraded 100% of the MB, outperforming CaFe2O4 prepared by a solid–state reaction method (SSR–CaFe2O4) which only degraded 25.9% of MB in the same time period (70). This performance was attributed to the smaller particle size and higher specific surface area of SCS–CaFe2O4 over SSR–CaFe2O4, which made it easier to transfer the photo-induced charge carriers to the surface and created more active sites for pollutants to react with (70). In addition to the modification of the photocatalysts, various additives were studied to promote photocatalytic activity. Liu et al. synthesized octahedral ZnFe2O4 by a simple solution combustion method which only removed 16.5% of MB in 60 min under visible light irradiation and the addition of KOH dramatically can react with OH− to improved the degradation of MB to 94% (71). Since • form OH through eq. 9, the increase of KOH concentration could increase the degradation efficiency of MB.

Rhodamine B (RhB) Rhodamine B is another cationic dye that has commonly been studied for photocatalytic activity of ferrites (75–82). Since RhB is potentially toxic and carcinogenic, it is banned in foods and cosmetics. Table 7 shows the degradation of RhB dye by various ferrites under light irradiation. Through comparing the degradation of MO, MB and RhB by CoFe2O4–rGO0.45–6h–25 under visible light, it was shown that the photocatalyst had higher degradation efficiency for cationic dyes than anionic dyes (59). The CoFe2O4-rGO contained oxygen groups, resulting in negatively charged surface, which caused electrostatic repulsion to anionic dyes and thus prevented the molecule to diffuse to the surface of catalyst to be decomposed. In addition to catalyst structure, catalyst morphology can also affect the photocatalytic activity. Qin et al. reported that 46.5% of RhB was degraded by PrFeO3 nanotubes while it was 29.5% and 15.7% for PrFeO3 porous fibers and PrFeO3 nanoparticles, respectively (75). The highest efficiency of nanotubes was due to their higher specific surface area (20.6 m2 g-1) compared to nanofibers (15.5 m2 g-1) and nanoparticles (6.4 m2 g-1). These results confirmed that the specific surface area is important for photocatalysts because it is one of the determinant factors that can affect the number of active sites on the surface of catalysts. The effect of pH on the degradation process was investigated in studies dealing with the photocatalytic degradation of RhB by BiFeO3 (76). The degradation of RhB decreased significantly when the pH was changed from 3.5 to 4.3 because the pKa value of RhB is about 4.0. When the pH is lower than 4.0, the aromatic 94

carboxylic groups are in the nondissociated form and this allows the dye molecules to diffuse more easily to the surface of the catalyst. When the pH value is higher than 4.0, RhB might aggregate which increased dimerization of RhB and makes the molecule too large to be adsorbed. Besides, pH also influences the surface charge of the photocatalyst and this affects the photocatalytic activity of the catalyst. Usually the catalyst would be negatively charged at pH > pHPZC (point of zero charge) and positively charged at pH < pHPZC. It may affect the adsorption of pollutants on the surface of catalysts, which is directly associated with the activity of the photocatalyst. Reusability is another aspect of ferrite-based photocatalysts because their magnetic property allows their easy separation from the reaction solution using an external magnetic field, at least in bench scale studies. It was reported that the degradation of RhB by MnxZn1−xFe2O4/β-Bi2O3 (15 wt.%) under sunlight irradiation (150 min) only decreased from 99.1% to 82.7% after five-time reuse of catalyst (77). After careful synthesis, most ferrite-based photocatalysts can retain their photocatalytic activity after reuse for at least 3 times with little leaching of metal ions (77).

Heterogeneous Photocatalytic Fenton and Fenton-like Processes Fe(II, III) ions are known as efficient catalytic components for H2O2 that generate highly active hydroxyl radicals (eq. 2-3, M=Fe) which can degrade organic pollutants (83–90). In ferrite-H2O2-visible light system, •OH can be generated via three pathways: reaction between photo-generated electrons and H2O2, reaction between surface metal active sites and H2O2, and reaction between photo-generated holes and OH−. Table 8 shows the photodegradation of various chemicals by H2O2/ferrite-based catalysts. The surface metal active sites can catalyze H2O2 independently or synergistically. For example, in the photodegradation of ammonia by rGO-MnFe2O4, X-ray photoelectron spectroscopy results reveled the coexistence of Mn(III) and Fe(II) after photocatalytic reaction, indicating that Mn and Fe components in the rGO-MnFe2O4 system perform independent catalytic functions (53). However in the photocatalysis of Orange II by core-shell CuFe2O4@C3N4 or both could catalyze H2O2 decomposition to hybrid, surface generate •OH, and the reduction by is thermodynamically favorable which is beneficial for the redox cycles of and in CuFe2O4 (eq. 10) (83).

Sharma et al. reported that during the the photocatalytic degradation of MB by H2O2, the catalytic activity followed the order: CuFe2O4 (k = 0.286 min-1) > ZnFe2O4 (k = 0.267 min-1) > NiFe2O4 (k = 0.138 min-1) > CoFe2O4 (k = 0.078 min1) (84). However, when combined with g-C3N4, CoFe2O4 showed higher activity than NiFe2O4 to catalyze H2O2 for degrading MB under visible light irradiation (λ > 400 nm) (85, 86). 95

Table 5. Degradation of MO dye by ferrite-based photocatalysts Dye (mg L-1)

Catalyst (g L-1)

Irradiation time (min)

Irradiation source

Degradation (%)

Ref.

ZnFe2-xLaxO4

33.33

0.33

60

400 W Hg UV

85

(61)

NiFe2-xLuxO4

10

2

60

400 W Hg UV

63

(65)

NiFe2-xDyxO4

10

2

60

400 W Hg UV

60

(66)

CuFe2-xSmxO4

10

2

60

400 W Hg UV

60

(67)

FeN-TiO2-CoFe2O4

10

1

300

125 W Hg (λ: 254−356 nm)

100

(52)

ZnFe2O4-Lactoferrin

20

0.5

180

175 W Hg UV-A (λ = 365 nm)

100

(68)

ZnFe2O4@ZnO

18.33

0.66

240

2×4 W UV-A (λ = 365 nm)

99

(49)

18.33

0.66

240

125 W Hg Visible light

63

ZnFe2O4

18.33

0.66

240

125 W Hg Visible light

16

ZnFe2O4/TiO2 (9.09 wt. %)

Not mentioned

0.5

720

3×8 W sunlight

78

ZnFe2O4/TiO2 (16.67 wt. %)

Not mentioned

0.5

720

3×8 W sunlight

83

ZnFe2O4/TiO2 (25 wt. %)

Not mentioned

0.5

720

3×8 W sunlight

92


Not mentioned

0.5

720

3×8 W sunlight

82


Not mentioned

0.5

720

3×8 W sunlight

88


10

4

120

300 W Xe (λ > 420 nm)

67


10

4

120

300 W Xe (λ > 420 nm)

75


10

4

120

300 W Xe (λ > 420 nm)

98

96

Photocatalyst

(62)

(51)

97

Photocatalyst

Dye (mg L-1)

Catalyst (g L-1)


Irradiation source

Degradation (%)


10

4

120

300 W Xe (λ > 420 nm)

87

BiFeO3

5

2.5

210

300 W Xe (λ > 420 nm)

15

Pt/BiFeO3 (0.5 wt. %)

5

2.5

210

300 W Xe (λ > 420 nm)

55


5

2.5

210

300 W Xe (λ > 420 nm)

70


5

2.5

210

300 W Xe (λ > 420 nm)

60

Pt/BiFeO3 (98 molar %)

20

1

120

450 W Xe (λ > 420 nm)

70

Ag/BiFeO3 (96.7 molar %)

20

1

120

450 W Xe (λ > 420 nm)

91

Ag/BiFeO3 (93.3 molar %)

20

1

105

450 W Xe (λ > 420 nm)

99

Ag/BiFeO3 (90 molar %)

20

1

90

450 W Xe (λ > 420 nm)

100

CoFe2O4/rGO (45 wt. %)

20

0.25

180

800 W Xe (λ > 420 nm)

37.5

Ref.

(63)

(64)

(59)

Table 6. Degradation of MB dye by ferrite-based photocatalysts

98

Photocatalyst

Dye (mg L-1)

Catalyst (g L-1)


Irradiation source

Degradation (%)

Ref.

TiO2/CoFe2O4 (35 wt. %)

10

0.1

750

2×8 W UV-A (λ = 365 nm)

76

(58)

TiO2/NiFe2O4 (35 wt. %)

10

0.1

750

2×8 W UV-A (λ = 365 nm)

68

TiO2/Fe3O4 (35 wt. %)

10

0.1

720

2×8 W UV-A (λ = 365 nm)

50

Graphene−ZnFe2O4

50

0.2

50

300 W Xe visible

100

Graphene− ZnFe2O4

50

0.2

160

Nature sunlight

87

CdS−ZnFe2O4

24

1

90

160 W Hg visible

97

CdS−CoFe2O4

24

1

180

160 W Hg visible

97

CdS-NiFe2O4

24

1

180

160 W Hg visible

98

CoFe2O4

20

0.25

180

800 W Xe (λ > 420 nm)

10


20

0.25

180

800 W Xe (λ > 420 nm)

55


20

0.25

180

800 W Xe (λ > 420 nm)

64


20

0.25

180

800 W Xe (λ > 420 nm)

71


20

0.25

180

800 W Xe (λ > 420 nm)

88


20

0.25

180

800 W Xe (λ > 420 nm)

97


20

0.25

180

800 W Xe (λ > 420 nm)

91


20

0.25

180

800 W Xe (λ > 420 nm)

91

(72)

(73)

(59)

99

Photocatalyst

Dye (mg L-1)

Catalyst (g L-1)


Irradiation source

Degradation (%)

Ref.

CaFe2O4 (solid-state reaction synthesis)

3.2

1

45

500 W Xe (λ > 420 nm)

29

(70)

CaFe2O4 (solution combustion synthesis)

3.2

1

45

500 W Xe (λ > 420 nm)

100

graphene−CoFe2O4

20

0.25

180

40 W daylight lamp (λ: 400-720 nm)

70

graphene−CoFe2O4/CdS

20

0.25

180

40 W daylight lamp (λ: 400-720 nm)

77

ZnFe2O4 (octahedral)

10

0.375

60

visible light

16.5

ZnFe2O4 (octahedral) + H2O2

10

0.375

60

Visible light

100

ZnFe2O4 (octahedral) + KOH

10

0.375

60

Visible light

94

(74)

(71)

Table 7. Degradation of RhB dye by ferrite-based photocatalysts

100

Photocatalyst

Dye (mg L-1)

Catalyst (g L-1)


Irradiation source

Degradation (%)

Ref.

LaFeO3/montmorillonite

9.58

1

90

150 W Hg (λ > 400 nm)

99.34

(78)

PrFeO3 (nanotubes)

23.95

0.5

1800

60 W Desk lamp (λ > 400 nm)

90

(75)

PrFeO3 (porous fibers)

23.95

0.5

1800


73

PrFeO3 (particles)

23.95

0.5

1800


40


20

0.25

180

800 W Xe (λ > 420 nm)

72.2

(59)

CdS−ZnFe2O4

10

0.5

60

500 W Xe (λ > 420 nm)

100

(79)

CdS−CoFe2O4

10

0.5

60

500 W Xe (λ > 420 nm)

100

Ca2Fe2O5

10

2.5

60

125 W Hg visible light

47

(80)

CdS−ZnFe2O4

24

1

150


95

(81)

CdS−CoFe2O4

24

1

180


95

CdS−NiFe2O4

24

1

180


95

BiFeO3 (pH = 1.5)

25

0.5

45

Nature sunlight

100

BiFeO3 (pH = 2.5)

25

0.5

35

Nature sunlight

100

BiFeO3 (pH = 3.5)

25

0.5

45

Nature sunlight

72

BiFeO3 (pH = 4.3)

25

0.5

45

Nature sunlight

6.7

BiFeO3 (pH = 10.0)

25

0.5

45

Nature sunlight

7


Not mentioned

0.5

720

3×8 w sunlight

91

(76)

(62)

101

Photocatalyst

Dye (mg L-1)

Catalyst (g L-1)


Irradiation source

Degradation (%)

Ref.

MnxZn1−xFe2O4/β−Bi2O3 cycle1

10

2

150

300 W Xe sunlight

99.1

(77)

MnxZn1−xFe2O4/β−Bi2O3 cycle 2

10

2

150

300 W Xe sunlight

94


10

2

150

300 W Xe sunlight

91


10

2

150

300 W Xe sunlight

84


10

2

150

300 W Xe sunlight

82.7

NixZn1−xFe2O4

1.44

16

300

Nature daylight

75

(82)

Table 8. Degradation of various chemicals by H2O2 and ferrite-based photocatalysts

102

Substrate

Photo-catalyst

C0,substrate (mg L-1)

Catalyst (g L-1)

Reaction time (min)

Irradiation source

Degradation (%)

Ref.

Ammonia

Graphene–MnFe2O4

50

4

600

300 W UV-vis (λ > 400 nm)

92

(53)

Methylene Blue

BaFe12O19

10

1

360

3 W LED (λ: 420–700 nm)

70.80

(54)

Orange II

CuFe2O4/C3N4 (core-shell)

9.81

0.1

150

500 W Xe (λ > 420 nm)

91

(83)

MB

CuFe2O4

10

0.5

15

150 W Xe visible light

k = 0.286 min-1

(84)

MB

ZnFe2O4

10

0.5

15


k = 0.267 min-1

MB

NiFe2O4

10

0.5

15


k = 0.138 min-1

MB

CoFe2O4

10

0.5

15


k = 0.078 min-1

MB

g-C3N4/NiFe2O4

10

1

240

300 W Xe (λ > 400 nm)

85

(85)

MB

g-C3N4/CoFe2O4

10

0.25

180

300 W Xe (λ ≥ 400 nm)

95

(86)

MB (cationic dye)

LaMnxFe1-xO3 (x = 0.2)

15

2

25


99.15

(87)

103

Substrate

Photo-catalyst

C0,substrate (mg L-1)

Catalyst (g L-1)

Reaction time (min)

Irradiation source

Degradation (%)

Safranine-O (cationic dye)


15

2

30


97.18

Remazol Turquoise Blue (anionic dye)


60

2

70


95.84

Remazol Brilliant Yellow (anionic dye)


60

2

40


95.14

Glycerol

CuFe2O4

6299

5

240


40

(88)

Phenol

BiFeO3

50

0.6

60

Nature daylight

91.3

(89)

Bisphenol A

Bi2Fe4O9

15

1.5

60

150 W Xe (λ: 420–700 nm)

54

(90)

Ref.

Besides, with the assistance of H2O2, both cationic and anionic dyes could be efficiently degraded by ferrites under visible light irradiation which is different from that of direct photodegradation by ferrites. As reported by Jauhar et al., LaMnxFe1−xO3 could efficiently catalyze the H2O2 to degrade both cationic dyes (MB and Safranine-O) and anionic dyes (Remazol Turquoise Blue and Remazol Brilliant Yellow) in the dark as well as under visible light, and the introduction of visible light enhanced the degradation process dramatically (87). In addition to dyes, other organic comtaninants such as phenol (89) and bisphenol A (90), could also be effecienty removed by photo-Fenton system with bismuth ferrite as catalyst. Additionally, there are two reports about sulfate radicals generated from ferrites/PMS (or PDS) under visible light. One report is about the degradation of Orange II via PMS/ZnFe2O4/visible light (56). Through electron paramagnetic resonance spectroscopy and classic quenching experiments, both •OH and SO4•− were confirmed to be active species in this system (56). The possible mechanism to generate SO4•−, reacts was proposed via three pathways: PMS captures with H2O/OH− to generate •OH or PMS to yield SO5•−, and directly oxidizes adsorbed substrate molecules (56). Another report is about the degradation of Orange II via PDS/ZnFe2O4/visible light (57). Similar to PMS/ZnFe2O4/visible light, both •OH and SO4•− were involved in the degradation process of Orange II according to the quenching agents experiments (57). It was proposed that -PDS, -Orange II, could also in addition to the reactions between activate PDS to generate SO4•− (57).

Conclusions and Outlook It is well known that various factors have impacts on the performance of photocatalysts in water splitting and photocatalytic degradation of contaminants. Since it is not feasible to directly compare the effect of these factors including, but not limited to, spectrum and intensity of irradiation, surface properties of catalysts, and reactor configuration, all results tabulated in the tables of this chapter only serve as a rough guide. Ferrite-based materials have been evidently proved to be effective in photocatalytic water decomposition. However, the reported quantum yields of ferrites acting alone are quite far away from the values that are desired for any practical application (91). As suggested by the results discussed in this chapter, forming heterojunctions by combining ferrites with either other ferrites or semiconductors leads to noticeable improvement in performance. So future research is necessary to systematically investigate the mechanism of such heterostructures and thus facilitate the design of novel ferrites-based composites with high activity. Besides, ferrite-based catalysts are confirmed to be effective photocatalysts when applied alone or with oxidants to utilize UV/visible light/sunlight to degrade various pollutants. Different preparation methods and synthesis parameters affect the size, morphology and structure of the materials, which will have influences 104

on the specific surface area, active sites on the surface and surface charges, which are the reasons for the different photocatalytic performance. The introduction of other photocatalysts into ferrites or the addition of oxidants (H2O2/PMS/PDS) pairs, allowing more of these species can inhibit the recombination of to be available for generations of reactive species such as •OH and SO4•− in order to improve the degradation of contaminants. Ferrite-based photocatalysts also exhibited good stability in photo-Fenton or photo-Fenton-like systems and showed that they can be reused for multiple times. Researchers demonstrated the easy separation of ferrite nanomaterials from aqueous suspension by attaching a magnet to a vial or beaker. Nevertheless, this is not sufficient to warrant the facile separation of these magnetic materials in actual industrial applications. Magnetic fields decrease significantly with increasing distance and thus cannot provide sufficient attraction to the particles in a real large scale reactor. Moreover, creating a strong magnetic field over such a large area seems not an economically attractive option. Therefore, this fact implies that it will be helpful to design a proper reactor system which can fully take advantage of the magnetic property of ferrites in future work.

Acknowledgments

D. D. Dionysiou would like to acknowledge funding from the US National Science Foundation (CBET 1236331) for support on his work on iron-based photocatalytic materials. B. Ren acknowledges the support from China Scholarship Council (CSC) for PhD student (No. 201206260144). Y. Huang also thanks CSC for PhD student scholarship (No. 201306270057).

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