**2.2. α-Fe2O3**

Hematite (α-Fe2O3) is the most thermodynamically stable form of iron oxide under ambient conditions and it is also the most common form of crystalline iron oxide. The iron and oxygen atoms are naturally arranged in the corundum structure, which is trigonal−hexagonal scale‐ nohedral (3 2/m) with space group R−3c, lattice parameters *a* = 5.0356 Å, *c* = 13.7489 Å, and six formula units per unit cell [135,136]. It is easy to understand hematite's structure based on the packing of the anions, O2-, which are arranged in a hexagonal closed−packed lattice along the [001] direction. The cations (Fe3+) occupy the two−thirds of the octahedral interstices (regularly, with two filled followed by one vacant) in the (001) basal planes, and the tetrahedral sites remain unoccupied. The arrangement of cations can also be considered as producing pairs of FeO6 octahedra that share edges with three neighboring octahedra in the same plane and one face with an octahedron in an adjacent plane in the [001] direction (Fig. 6). The face−sharing is responsible for a trigonal distortion of the octahedra as the proximal iron atoms are repelled to optimize the crystal's Madelung energy. As a result, hematite exhibits a C3v symmetry and there are two different Fe−O bond lengths (Figure 6). However, the electronic structures of the distorted FeO6 octahedral are thought to be similar to undistorted clusters [133,136]. Hematite is antiferromagnetic at temperatures below 260 K and is a weak (parasitic) ferromagnet at room temperatures. While the magnetic properties of hemitate are not particularly dependent on its photo electrochemical performance, the iron spin configuration does influence the optoelec‐ tronic and carrier transport properties of hematite. The absorption of photons by hematite starts from the near−infrared spectral region where weak absorption bands (with absorption coefficients, a, of the order 103 cm−1) are due to transition states electrons between two d orbital energy levels of the Fe3+ ion, which are split by an intrinsic crystal field [136,138]. Analysis by means of a Tauc plot shows the indirect nature of the band gap for the α-Fe2O3 involving d orbital to d orbital transition sand a direct transitions from O (2p) to Fe (3d), which occurs only for band gaps > 3.2 eV [139-141].

the post−transformation from anatase/brookite phase via thermal treatment (phase transfor‐ mation temperature required depends on the particle size of TiO2) [110] and (iii) mechanical processing [111]. Although rutile is considered to be less active in photocatalytic reactions compared to anatase, nanostructured rutile has also been used photocatalysis applications and in some cases show even higher activity than anatase. Band gap of rutile TiO2 is 0.2 eV smaller than anatase one and further results in a wider absorption range, which may be the advantage

178 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

Afterwards, various morphologies of rutile have been developed [112-119], with the nanorod being is a common morphology. The synthesis routes of such rutile nanorods with a high aspect ratio have been well documented in the literature [114,119,120-129]. Generally speaking, the presence of Cl ions as mineralizer in the synthesis system is favourable for rutile TiO2, regardless of the source of Cl. In the case of the specific synthesis routes of controlling morphology of rutile, there are two representative examples demonstrate the formation of faceted rutile crystals. One is the rapid formation of self-assembled microspheres with rutile nanorods by microwave heating of TiCl3 at 200°C for only 1 min [115]. The nanorods are exposed with {110} and {111} facets, but because of the extremely rapid growth rate, the surface is not smooth. Interestingly, the synthetic rutile nanorods have a smaller bandgap of 2.8 eV compared with the conventional 3.0 eV, which may facilitate the photocatalysis ability under visible light irradiation. The other one is reported by Kakiuchi et al. [116], who observed the dependence of degree of perfection of facets on hydrothermal temperature, where TiCl3 was also used as a precursor together with NaCl additive. For example, at low temperature (80°C), only needle−like nanorods without well−recognized facets were formed. However, when elevating the temperature to 200°C, well−developed lateral {110} and top {111} facets can be observed. Apparently, this result indicates that a higher temperature is favorable for growing

Compared with anatase and rutile, brookite phase TiO2 has attracted little interest due to the generally considered lack of photocatalytic activity. However, increasing literatures have shown that brookite is also photocatalytically active and even has unique photocatalytic properties in some cases [130-133]. However, among the synthetic brookites, crystal facets are usually non-recognizable. Interestingly, Buonsanti et al. [134] developed a nonhydrolytic synthesis route to successfully prepare high−quality anisotropically shaped brookite nanorods with a length of 30−200 nm. These rods are determined to be dominantly enclosed with the longitudinal {210}/{100} and basal {001} facet, which is in agreement with the equilibrium shape

Hematite (α-Fe2O3) is the most thermodynamically stable form of iron oxide under ambient conditions and it is also the most common form of crystalline iron oxide. The iron and oxygen atoms are naturally arranged in the corundum structure, which is trigonal−hexagonal scale‐ nohedral (3 2/m) with space group R−3c, lattice parameters *a* = 5.0356 Å, *c* = 13.7489 Å, and six formula units per unit cell [135,136]. It is easy to understand hematite's structure based on the packing of the anions, O2-, which are arranged in a hexagonal closed−packed lattice along the

of this phase.

crystals with well-developed facets.

**2.2. α-Fe2O3**

of brookite crystals predicted from the Wulff construction.

**Figure 6.** The unit cell (left) of hematite shows the octahedral face−sharing Fe2O9 dimers forming chains in the c direc‐ tion. A detailed view (right) of one Fe2O9 dimer shows how the electrostatic repulsion of the Fe3+ cations produce long (light grey) and short (dark grey) Fe−O bonds [136,137].

Hematite (α-Fe2O3), an environmental friendly n-type semiconductor (*E*g = 2.1 eV), has been widely used in many fields such as lithium−ion batteries [142], gas sensors [143–145], photo‐ catalysis [146,147], water treatment [148] and water splitting for generating H2. Hematite is one promising candidate for photocatalytic applications due to its narrow band gap of about 2.0−2.2 eV. Further, hematite absorbs light up to 600 nm, collects up to 40% of the solar spectrum energy, is stable in most aqueous solutions (pH > 3), and is one of the cheapest semiconductor materials available. Due to the band gap value α-Fe2O3 and the fact that it,s valence band edge is substantially lower than the water oxidation potential, it is a promising photoanode material for photoelectrochemical (PEC) water splitting. Photochemical water splitting involves a dispersed material in pure water and accordingly produces hydrogenand oxygen homogene‐ ously throughout the solution [149]. The theoretical photocurrent density of α-Fe2O3 is ~12.6 mA/cm2 under AM 1.5 G solar irradiation, and the solar energy conversion efficiency is ~15.5% in an ideal tandem PEC cell [150,151]. However, the photocatalytic performance of α-Fe2O3 is limited by certain factors such as high recombination rate of electrons and holes, low diffusion lengths of holes (2−4 nm), and poor conductivity, which led to both low efficiencies and a larger requisite over potential for photo-assisted water oxidation [152-156]. Many attempts have been made by researchers to overcome these anomalies of α-Fe2O3 such as lowering the recombi‐ nation rate by forming nanostructures, enhancement in conductivity by doping with suitable metals and improving the charge transfer ability [157,158]. Apart from water-splitting applications, the photocatalytic activity of hematite can be used for the elimination of organic compounds in water treatment applications.

As the surface area plays an important role in determining the photocatalytic activity of materials, researchers have attempted to reduce the size of photocatalytic materials and enhance the photocatalytic properties of these materials by producing hematite in a nanoscale powder form. Many methods have been followed to synthesize α-Fe2O3 in a nanocrystalline form and in different shapes including hydrolysis [159], co-precipitation [160,161], hydrother‐ mal methods [162– 164], solvothermal methods [165,166], ionic liquid-assisted synthesis [167], thermal decomposition [168], combustion methods [169], and a combination reflex condensa‐ tion and hydrothermal method [170].

Hosseinian et al. [171] synthesized nanostructured iron oxide of different morphologies and different phase compositions (α-Fe2O3 and Fe3O4) by a solid−state reaction (SSR) route. The photocatalytic activity was checked with respect to degradation of rhodamine B(RhB), and it was observed that the samples containing a mixture of α-Fe2O3 and Fe3O4 showed better photocatalytic activity than that of the pure α-Fe2O3. The higher photocatalytic activity observed for a mixed-phase sample was attributed to the higher transfer of electrons and holes generated during the photoreaction of α-Fe2O3 to the valence band of Fe3O4, which limits the recombination rates [172]. Yang et al. [173] synthesized α-Fe2O3 nanoparticles of uniform size (170nm to 2μm) by a hydrothermal route to study both magnetic as well as photocatalytic properties. The α-Fe2O3 powders with the smaller crystallite sizes show the highest photoca‐ talytic degradation efficiency than that of the powders with larger crystallite sizes. Further, all the samples showed higher efficiency for degradation of the dye than that of the commercially available Degussa P25. Apte et al. [169] synthesized nano structured α-Fe2O3 powders in size ranging 25−55nm and their photocatalytic activity was analyzed with respect to the decom‐ position of hydrogen sulfide (H2S) gas. α-Fe2O3 (necked structures) showed good photocata‐ lytic properties and production of H2. Zhou et al. [174] synthesized nanorods of α-Fe2O3 by thermal dehydration and compared the photocatalytic activity with microrods. The authors reported a higher degradation rate for rhodamine B(RhB) for nanodimensional α-Fe2O3 than that of the corresponding micron-sized rods. Higher Fe−O bond stretching frequencies were proposed as one of the key factors behind the enhanced photocatalytic activity. Particle size, composition, porosity, and the local structures are also the key factors that affect the photo‐ catalytic properties of materials. Townsend et al. [159] compared the photocatalytic activity of three forms of Fe2O3 including bulk (crystallite size 120 nm), ultrasonicated bulk (crystallite size 40 nm), and nanopowders of α-Fe2O3 (crystallite size 5.4 nm). They found that the rate of oxygen evolution is higher when the crystallite size becomes smaller, and the highest rate was reported for α-Fe2O3 nanopowders (1072 μmol/h g). In the case of α-Fe2O3 nanopowders, the hole diffusion length is comparable to the crystallite size, which results in more availability of holes to react with water. Dang et al. [160] reported the effects of calcination temperature, reaction temperature, amount of catalyst, and duration of reaction on the catalytic properties. They reported an increase in photocatalytic activity with increasing calcination temperature, reaction temperature, and catalytic amount up to a certain extent, after which the activity decreases. Similar effects were also reported by Pawar et al. [175] for α-Fe2O3 nanoparticles synthesized by a sol-gel technique followed by the heat treatment at different calcination temperatures. The efficiency of the catalyst was analyzed with respect to various experimental variables such as calcination temperature, pH, light intensity, and concentration of dye and catalyst. Samples calcined at 600°C show the highest photocatalytic activity because of the formation of the more dominant α-Fe2O3 phase. The photocatalytic properties were analyzed for the 3−10 pH range, and the reactions at higher pH conditions showed better photocatalytic properties. In basic pH conditions, formation of OH⋅ radical is more favored and electrostatic abstractive effects between cationic malachite green dye and negatively charged surface of α-Fe2O3 increases, which results in a higher probability of dye degradation. Light intensity shows a linear effect on the photocatalytic properties of α-Fe2O3 due to the increased availability of photons for the reaction. Similar effects were also reported by Liu et al. [176] for α-Fe2O3 nanorods. These authors examined the effect of the amount of catalyst and initial dye concen‐ tration on the photocatalytic properties. The optimum catalyst amount was reported to be 50 mg/L to achieve the highest photocatalytic activity. However, the photocatalytic activity degrades with increasing dye concentration. This effect was justified in terms of a decrease in transparency with an increase in dye and catalyst concentration after a particular value.

energy, is stable in most aqueous solutions (pH > 3), and is one of the cheapest semiconductor materials available. Due to the band gap value α-Fe2O3 and the fact that it,s valence band edge is substantially lower than the water oxidation potential, it is a promising photoanode material for photoelectrochemical (PEC) water splitting. Photochemical water splitting involves a dispersed material in pure water and accordingly produces hydrogenand oxygen homogene‐ ously throughout the solution [149]. The theoretical photocurrent density of α-Fe2O3 is ~12.6

in an ideal tandem PEC cell [150,151]. However, the photocatalytic performance of α-Fe2O3 is limited by certain factors such as high recombination rate of electrons and holes, low diffusion lengths of holes (2−4 nm), and poor conductivity, which led to both low efficiencies and a larger requisite over potential for photo-assisted water oxidation [152-156]. Many attempts have been made by researchers to overcome these anomalies of α-Fe2O3 such as lowering the recombi‐ nation rate by forming nanostructures, enhancement in conductivity by doping with suitable metals and improving the charge transfer ability [157,158]. Apart from water-splitting applications, the photocatalytic activity of hematite can be used for the elimination of organic

As the surface area plays an important role in determining the photocatalytic activity of materials, researchers have attempted to reduce the size of photocatalytic materials and enhance the photocatalytic properties of these materials by producing hematite in a nanoscale powder form. Many methods have been followed to synthesize α-Fe2O3 in a nanocrystalline form and in different shapes including hydrolysis [159], co-precipitation [160,161], hydrother‐

thermal decomposition [168], combustion methods [169], and a combination reflex condensa‐

Hosseinian et al. [171] synthesized nanostructured iron oxide of different morphologies and different phase compositions (α-Fe2O3 and Fe3O4) by a solid−state reaction (SSR) route. The photocatalytic activity was checked with respect to degradation of rhodamine B(RhB), and it was observed that the samples containing a mixture of α-Fe2O3 and Fe3O4 showed better photocatalytic activity than that of the pure α-Fe2O3. The higher photocatalytic activity observed for a mixed-phase sample was attributed to the higher transfer of electrons and holes generated during the photoreaction of α-Fe2O3 to the valence band of Fe3O4, which limits the recombination rates [172]. Yang et al. [173] synthesized α-Fe2O3 nanoparticles of uniform size (170nm to 2μm) by a hydrothermal route to study both magnetic as well as photocatalytic properties. The α-Fe2O3 powders with the smaller crystallite sizes show the highest photoca‐ talytic degradation efficiency than that of the powders with larger crystallite sizes. Further, all the samples showed higher efficiency for degradation of the dye than that of the commercially available Degussa P25. Apte et al. [169] synthesized nano structured α-Fe2O3 powders in size ranging 25−55nm and their photocatalytic activity was analyzed with respect to the decom‐ position of hydrogen sulfide (H2S) gas. α-Fe2O3 (necked structures) showed good photocata‐ lytic properties and production of H2. Zhou et al. [174] synthesized nanorods of α-Fe2O3 by thermal dehydration and compared the photocatalytic activity with microrods. The authors reported a higher degradation rate for rhodamine B(RhB) for nanodimensional α-Fe2O3 than

164], solvothermal methods [165,166], ionic liquid-assisted synthesis [167],

under AM 1.5 G solar irradiation, and the solar energy conversion efficiency is ~15.5%

mA/cm2

mal methods [162–

compounds in water treatment applications.

180 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

tion and hydrothermal method [170].

In a photoreaction, the porosity of the catalyst plays a major role in enhancing the photocata‐ lytic properties. Sundarmurthy et al. [177] synthesized 1D α-Fe2O3 nanobraids and nanoporous structures by electrospinning to analyze the photocatalytic properties. The nanostructures show superior photocatalytic activity for the degradation of Congo red dye (CR) in a small fraction of time due to the porous surface and nanosized crystallites of α-Fe2O3, which provide more active catalytic centers and allow effective interaction between organic dye and α-Fe2O3, thereby enhancing photocatalytic degradation performance. α-Fe2O3 porous structures were prepared by Zhang et al. [162] and the photocatalytic activity was analyzed by the degradation of methylene blue(MB). They analyzed the effect of porosity and the amount of catalyst on photocatalytic activity. It has been reported that an optimized amount of catalyst (20 mg) is required for getting the highest rate of degradation of MB, less or more than this amount leads to lower photocatalytic activity. Large amounts of catalyst result in lesser illumination, and when the amount of catalyst is insufficient, the active sites are not sufficient to degrade the

organic dye. Geng et al. [178] followed a number of Ni2+/surfactant system routes for synthe‐ sizing α-Fe2O3 with a porous structure and rough surface which shows better photocatalytic properties than that of the α-Fe2O3 nanoparticles in the degradation of MB as a result of higher surface area. Gang et al. [147] prepared α-Fe2O3 micro/nano spheres synthesized by hydro‐ thermal synthesis followed by the thermal treatment. The micro/nano spheres show a better dye degradation efficiency than that of the nanopowders. The calculated reaction rate for spherical structures is more than twice than that of the reaction rate of nanopowders and 12 times the reaction rate of the micron-sized powders. The better photocatalytic activityis the result of the higher specific surface area and porous structures. properties than that of the α‐Fe2O3 nanoparticles in the degradation of MB as a result of higher surface area. Gang et al.[147] prepared α‐Fe2O3 micro/nano spheres synthesized by hydrothermal synthesis followed by the thermal treatment. The micro/nano spheres show a better dye degradation efficiency than that of the nanopowders. The calculated reaction rate for spherical structures is more than twice than that of the reaction rate of nanopowders and 12 times the reaction rate of the micron‐sized powders. The better photocatalytic activityis the result of the higher specific surface area and porous structures.

optimized amount of catalyst (20 mg) is required for getting the highest rate of degradation of MB, less

or more than this amount leads to lower photocatalytic activity. Large amounts of catalyst result in

lesser illumination, and when the amount of catalyst is insufficient, the active sites are not sufficient to

synthesizing α‐Fe2O3 with a porous structure and rough surface which shows better photocatalytic

**Fig 7**. Photocatalytic degradation rate of RhB over the α‐Fe2O3 nanostructures under visible light illumination in the presence of H2O2 additive (a),and SEM/TEM images of the α‐Fe2O3 **Figure 7.** Photocatalytic degradation rate of RhB over the α-Fe2O3 nanostructures under visible light illumination in the presence of H2O2 additive (a),and SEM/TEM images of the α-Fe2O3 nanostructures: (b,c)S1;(d,e)S2;(f,g)S3;and(h,i)S4 [179].

nanostructures: (b,c)S1;(d,e)S2;(f,g)S3;and(h,i)S4[179]. Xu et al.[179], Zhou et al.[180] and Bharathi et al.[170] reported the effect of the surface morphology of α‐Fe2O3 on its photocatalytic activity. α‐Fe2O3 nanostructures with different morphologies such as microflowers, nanospindles, nanoparticles and nanorhombohedra were synthesized (Fig. 7)[179]. The photocatalytic activity was analyzed by monitoring the degradation of RhB in the presence of the catalyst. The best photocatalytic activity was observed for the samples with highest surface area and Xu et al. [179], Zhou et al. [180] and Bharathi et al. [170] reported the effect of the surface morphology of α-Fe2O3 on its photocatalytic activity. α-Fe2O3 nanostructures with different morphologies such as microflowers, nanospindles, nanoparticles and nanorhombohedra were synthesized (Fig. 7) [179]. The photocatalytic activity was analyzed by monitoring the degra‐ dation of RhB in the presence of the catalyst. The best photocatalytic activity was observed for the samples with highest surface area and porosity. Similar surface area effects were also reported by Cheng et al. [181] for flower-like α-Fe2O3 nanostructures synthesized by a biphasic interfacial reaction route. The photocatalytic properties of α-Fe2O3 were evaluated by meas‐ uring the degradation of RhB. The results were compared with the commercial α-Fe2O3

porosity. Similar surface area effects were also reported by Cheng et al.[181] for flower‐like α‐Fe2O3

nanostructures synthesized by a biphasic interfacial reaction route. The photocatalytic properties of

α‐Fe2O3 were evaluated by measuring the degradation of RhB. The results were compared with the

commercial α‐Fe2O3 powders and nanoflowers were found to have a better photocatalytic property than

the commercial powders. The enhancement was related to the increase in crystallinity and increase in

the surface area, which is also supported by results of other authors for TiO2[182] and Fe2O3[183]. Similar

surface area effects were also reported by Cao et al. [184], Xu et al.[166], and Li et al.[163] for α‐Fe2O3 hollow

microspheres prepared by solvothermal and hydrothermal methods. The photocatalytic activity was

analyzed by the degradation of salicylic acid. The hollow spheres associated with nanosheets show

better photocatalytic activity than that of then anoparticles of α‐Fe2O3. Similar results were also reported

powders and nanoflowers were found to have a better photocatalytic property than the commercial powders. The enhancement was related to the increase in crystallinity and increase in the surface area, which is also supported by results of other authors for TiO2 [182] and Fe2O3 [183]. Similar surface area effects were also reported by Cao et al. [184], Xu et al. [166], and Li et al. [163] for α-Fe2O3 hollow microspheres prepared by solvothermal and hydrother‐ mal methods. The photocatalytic activity was analyzed by the degradation of salicylic acid. The hollow spheres associated with nanosheets show better photocatalytic activity than that of then anoparticles of α-Fe2O3. Similar results were also reported by Majiet al. [168], where α-Fe2O3 powders prepared at 500°C show better photocatalytic activity for the degradation of rose Bengal dye than that of the powders prepared at 600°C and commercially available TiO2(Degussa−25) as a result of higher surface area. α-Fe2O3 hollow spindles and spheres were prepared by Li et al. [164] and Xu et al. [167], respectively. These authors reported an en‐ hancement in photocatalytic degradation efficiency as a result of the enhancement in specific surface area, which results in more unsaturated surface coordination sites exposed to the solution. The hollow microsphere facilitates more electron-hole transport and lowers the recombination rate. Hollow microspheres allow multiple reflections of visible light within the interior that encourage a more efficient use of the light source and enhance light−harvesting, leading to an increased quantity of ⋅OH available to participate in the photo-catalytic reaction. Along with this, the hollow spheres also provide ideal channels for the dye molecules and increase the probability of interaction.

organic dye. Geng et al. [178] followed a number of Ni2+/surfactant system routes for synthe‐ sizing α-Fe2O3 with a porous structure and rough surface which shows better photocatalytic properties than that of the α-Fe2O3 nanoparticles in the degradation of MB as a result of higher surface area. Gang et al. [147] prepared α-Fe2O3 micro/nano spheres synthesized by hydro‐ thermal synthesis followed by the thermal treatment. The micro/nano spheres show a better dye degradation efficiency than that of the nanopowders. The calculated reaction rate for spherical structures is more than twice than that of the reaction rate of nanopowders and 12 times the reaction rate of the micron-sized powders. The better photocatalytic activityis the

optimized amount of catalyst (20 mg) is required for getting the highest rate of degradation of MB, less

or more than this amount leads to lower photocatalytic activity. Large amounts of catalyst result in

lesser illumination, and when the amount of catalyst is insufficient, the active sites are not sufficient to

degrade the organic dye. Geng et al.[178] followed a number of Ni2+/surfactant system routes for

synthesizing α‐Fe2O3 with a porous structure and rough surface which shows better photocatalytic

properties than that of the α‐Fe2O3 nanoparticles in the degradation of MB as a result of higher surface

area. Gang et al.[147] prepared α‐Fe2O3 micro/nano spheres synthesized by hydrothermal synthesis

followed by the thermal treatment. The micro/nano spheres show a better dye degradation efficiency

than that of the nanopowders. The calculated reaction rate for spherical structures is more than twice

than that of the reaction rate of nanopowders and 12 times the reaction rate of the micron‐sized

powders. The better photocatalytic activityis the result of the higher specific surface area and porous

**Fig 7**. Photocatalytic degradation rate of RhB over the α‐Fe2O3 nanostructures under visible light

**Figure 7.** Photocatalytic degradation rate of RhB over the α-Fe2O3 nanostructures under visible light illumination in the presence of H2O2 additive (a),and SEM/TEM images of the α-Fe2O3 nanostructures: (b,c)S1;(d,e)S2;(f,g)S3;and(h,i)S4

Xu et al. [179], Zhou et al. [180] and Bharathi et al. [170] reported the effect of the surface morphology of α-Fe2O3 on its photocatalytic activity. α-Fe2O3 nanostructures with different morphologies such as microflowers, nanospindles, nanoparticles and nanorhombohedra were synthesized (Fig. 7) [179]. The photocatalytic activity was analyzed by monitoring the degra‐ dation of RhB in the presence of the catalyst. The best photocatalytic activity was observed for the samples with highest surface area and porosity. Similar surface area effects were also reported by Cheng et al. [181] for flower-like α-Fe2O3 nanostructures synthesized by a biphasic interfacial reaction route. The photocatalytic properties of α-Fe2O3 were evaluated by meas‐ uring the degradation of RhB. The results were compared with the commercial α-Fe2O3

illumination in the presence of H2O2 additive (a),and SEM/TEM images of the α‐Fe2O3

nanostructures: (b,c)S1;(d,e)S2;(f,g)S3;and(h,i)S4[179].

Xu et al.[179], Zhou et al.[180] and Bharathi et al.[170] reported the effect of the surface morphology of

α‐Fe2O3 on its photocatalytic activity. α‐Fe2O3 nanostructures with different morphologies such as

microflowers, nanospindles, nanoparticles and nanorhombohedra were synthesized (Fig. 7)[179]. The

photocatalytic activity was analyzed by monitoring the degradation of RhB in the presence of the

catalyst. The best photocatalytic activity was observed for the samples with highest surface area and

porosity. Similar surface area effects were also reported by Cheng et al.[181] for flower‐like α‐Fe2O3

nanostructures synthesized by a biphasic interfacial reaction route. The photocatalytic properties of

α‐Fe2O3 were evaluated by measuring the degradation of RhB. The results were compared with the

commercial α‐Fe2O3 powders and nanoflowers were found to have a better photocatalytic property than

the commercial powders. The enhancement was related to the increase in crystallinity and increase in

the surface area, which is also supported by results of other authors for TiO2[182] and Fe2O3[183]. Similar

surface area effects were also reported by Cao et al. [184], Xu et al.[166], and Li et al.[163] for α‐Fe2O3 hollow

microspheres prepared by solvothermal and hydrothermal methods. The photocatalytic activity was

analyzed by the degradation of salicylic acid. The hollow spheres associated with nanosheets show

better photocatalytic activity than that of then anoparticles of α‐Fe2O3. Similar results were also reported

result of the higher specific surface area and porous structures.

182 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

structures.

(a)

[179].

Apart from crystallite size, the orientation of crystallites also plays a major role in enhanc‐ ing the photocatalytic properties. This effect has been reported by Wu et al. [185], in which the authors prepared α-Fe2O3 nanocubes by a solvothermal method and reported a higher photocatalytic property for the {104} planes than that of the samples with {012} planes. The photocatalytic properties involve Fenton's reaction. The amount of Fe3+ on the surfaces of the catalyst play a very important role in the Fenton reaction in which the reduction of Fe3+ to Fe2+ generates hydroxyl radicals (⋅OH) [186]. It has been reported by Lv et al. [187] that {104} planes of α-Fe2O3 contain 10.3 atoms/nm2 of exposed Fe3+ ions, whereas the {012} planes contain 7.33 atoms/nm2 of exposed Fe3+ ions. This explains the higher reactivity of {104} planes than that of the {012} planes. Along with the surface morphology, oxygen pressure and amount of the catalyst also play a major role in enhancing the photocatalytic properties. Isaev et al. [188] reported an enhancement in the photocatalytic activity with an increase in the quantity of Fe2O3 up to a certain point, after that, the photocatalytic activity is decreased. Similarly, the authors reported an enhancement in dye degradation with increased oxygen content. The reason behind the enhancement in photocatalytic behavior is due to the formation of more oxygen-containing active species such as HO⋅, O2⋅, and HO2⋅ oxidizing species. Zhou et al. [189] investigate visible−light−induced photodegradation of model dye rhodamine B (RhB) in the presence of hydrogen peroxide (H2O2) over hematite architec‐ tures, namely 1D nanorods, 2D nanoplates, and 3D nanocubes (Fig. 8), and the reactivity trend can be rationalized as exposed facets in the order {110} > {012} >> {001}. This photoca‐ talytic activity order can be well explained by different facets of α-Fe2O3 surface atomic and electronic structures.

molecules and increase the probability of interaction.

Apart from crystallite size, the orientation of crystallites also plays a major role in enhancing the photocatalytic properties. This effect has been reported by Wu et al.[185], in which the authors prepared α‐Fe2O3 nanocubes by a solvothermal method and reported a higher photocatalytic property for the {104}

reaction. The amount of Fe3+ on the surfaces of the catalyst play a very important role in the Fenton

**Figure 8**. Representative morphologies and structures of α-Fe2O3 architectures.(a) TEM image and (b) HRTEM image of 2D α-Fe2O3 nanoplates. Insets: FFT pattern and drawing of a plate. (c) TEM image and (d) HRTEM image of 3D α-Fe2O3 nanocubes. Insets: FFT pattern and drawing of a cube. (e) TEM image and f) HRTEM image of 1D α-Fe2O3 nanorods. Insets: FFT pattern and drawing of a rod. Side views of surface terminations of α-Fe2O3. **Figure 8.** Representative morphologies and structures of α-Fe2O3 architectures.(a) TEM image and (b) HRTEM image of 2D α-Fe2O3 nanoplates. Insets: FFT pattern and drawing of a plate. (c) TEM image and (d) HRTEM image of 3D α-Fe2O3 nanocubes. Insets: FFT pattern and drawing of a cube. (e) TEM image and f) HRTEM image of 1D α-Fe2O3 nano‐ rods. Insets: FFT pattern and drawing of a rod. Side views of surface terminations of α-Fe2O3. (g) {001}, (h) {012}, and (i) {110}. Large black spheres are oxygen and small gray spheres are iron. The coordinatively unsaturated iron atoms on the {012} and {110} surfaces are shown by arrows [189].

(g) {001}, (h) {012}, and (i) {110}. Large black spheres are oxygen and small gray spheres are iron. The coordinatively unsaturated iron atoms on the {012} and {110} surfaces are shown by arrows[189].

reported by Lv et al.[187] that {104} planes of α‐Fe2O3 contain 10.3 atoms/nm2 of exposed Fe3+ ions, whereas

#### reaction in which the reduction of Fe3+ to Fe2+ generates hydroxyl radicals (•OH) [186]. It has been **3. Ternary oxide system**

#### the {012} planes contain 7.33 atoms/nm2 of exposed Fe3+ ions. This explains the higher reactivity of {104} **3.1. BiVO4**

planes than that of the {012} planes. Along with the surface morphology, oxygen pressure and amount of the catalyst also play a major role in enhancing the photocatalytic properties. Isaev et al. [188] reported an enhancement in the photocatalytic activity with an increase in the quantity of Fe2O3 up to a certain point, after that, the photocatalytic activity is decreased. Similarly, the authors reported an enhancement in dye degradation with increased oxygen content. The reason behind the enhancement in photocatalytic behavior is due to the formation of more oxygen‐containing active species such as HO•, O2•, and HO2• oxidizing species. Zhou et al.[189] investigate visiblelightinduced photodegradation of model dye rhodamine B (RhB) in the presence of hydrogen peroxide (H2O2) over hematite architectures, namely 1D nanorods, 2D nanoplates, and 3D nanocubes (Fig. 8), and the reactivity trend can be rationalized as exposed facets in the order {110} > {012} >> {001}. This photocatalytic activity order can be Bismuth vanadate (BiVO4), which is an n−type semiconductor, has been identified as one of the most promising photocatalytic materials. As it is well known, BiVO4 exists in three polymorphs of monoclinic scheelite, tetragonal scheelite, and tetragonal zircon structures, with bandgaps of 2.4, 2.34, and 2.9 eV, respectively. BiVO4 exists naturally as the mineral pucherite with an orthorhombic crystal structure [190]. However, BiVO<sup>4</sup> prepared in the laboratory does not adopt the pucherite structure but crystallizes either in a scheelite or a zircon-type structure (Fig. 9) [191,192]. The scheelite structure can have a tetragonal crystal system (space group: *I*41/*a* with *a* = *b* = 5.1470 Å, *c* = 11.7216 Å) or a monoclinic crystal system (space group: *I*2/b with *a* = 5.1935 Å, *b* = 5.0898 Å, *c* = 11.6972 Å, and *b* = 90.3871) [192,193] while the zircon-type structure has a tetragonal crystal system (space group: *I*41/a with *a* = *b* = 7.303 Å and *c* = 6.584 Å) [192,194].

well explained by different facets of α‐Fe2O3 surface atomic and electronic structures.

molecules and increase the probability of interaction.

184 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

the {012} and {110} surfaces are shown by arrows [189].

**3. Ternary oxide system**

Å and *c* = 6.584 Å) [192,194].

**3.1. BiVO4**

Apart from crystallite size, the orientation of crystallites also plays a major role in enhancing the photocatalytic properties. This effect has been reported by Wu et al.[185], in which the authors prepared α‐Fe2O3 nanocubes by a solvothermal method and reported a higher photocatalytic property for the {104} planes than that of the samples with {012} planes. The photocatalytic properties involve Fenton's reaction. The amount of Fe3+ on the surfaces of the catalyst play a very important role in the Fenton

(g)

**(h )** 

**(I)** 

**Figure 8**. Representative morphologies and structures of α-Fe2O3 architectures.(a) TEM image and (b) HRTEM image of 2D α-Fe2O3 nanoplates. Insets: FFT pattern and drawing of a plate. (c) TEM image and (d) HRTEM image of 3D α-Fe2O3 nanocubes. Insets: FFT pattern and drawing of a cube. (e) TEM image and f) HRTEM image of 1D α-Fe2O3 nanorods. Insets: FFT pattern and drawing of a rod. Side views of surface terminations of α-Fe2O3. (g) {001}, (h) {012}, and (i) {110}. Large black spheres are oxygen and small gray spheres are iron. The coordinatively unsaturated iron atoms on the {012} and {110} surfaces are shown by arrows[189].

**Figure 8.** Representative morphologies and structures of α-Fe2O3 architectures.(a) TEM image and (b) HRTEM image of 2D α-Fe2O3 nanoplates. Insets: FFT pattern and drawing of a plate. (c) TEM image and (d) HRTEM image of 3D α-Fe2O3 nanocubes. Insets: FFT pattern and drawing of a cube. (e) TEM image and f) HRTEM image of 1D α-Fe2O3 nano‐ rods. Insets: FFT pattern and drawing of a rod. Side views of surface terminations of α-Fe2O3. (g) {001}, (h) {012}, and (i) {110}. Large black spheres are oxygen and small gray spheres are iron. The coordinatively unsaturated iron atoms on

reaction in which the reduction of Fe3+ to Fe2+ generates hydroxyl radicals (•OH) [186]. It has been reported by Lv et al.[187] that {104} planes of α‐Fe2O3 contain 10.3 atoms/nm2 of exposed Fe3+ ions, whereas the {012} planes contain 7.33 atoms/nm2 of exposed Fe3+ ions. This explains the higher reactivity of {104} planes than that of the {012} planes. Along with the surface morphology, oxygen pressure and amount of the catalyst also play a major role in enhancing the photocatalytic properties. Isaev et al. [188] reported an enhancement in the photocatalytic activity with an increase in the quantity of Fe2O3 up to a certain point, after that, the photocatalytic activity is decreased. Similarly, the authors reported an enhancement in dye degradation with increased oxygen content. The reason behind the enhancement in photocatalytic behavior is due to the formation of more oxygen‐containing active species such as HO•, O2•, and HO2• oxidizing species. Zhou et al.[189] investigate visiblelightinduced photodegradation of model dye rhodamine B (RhB) in the presence of hydrogen peroxide (H2O2) over hematite architectures, namely 1D nanorods, 2D nanoplates, and 3D nanocubes (Fig. 8), and the reactivity trend can be rationalized as exposed facets in the order {110} > {012} >> {001}. This photocatalytic activity order can be

Bismuth vanadate (BiVO4), which is an n−type semiconductor, has been identified as one of the most promising photocatalytic materials. As it is well known, BiVO4 exists in three polymorphs of monoclinic scheelite, tetragonal scheelite, and tetragonal zircon structures, with bandgaps of 2.4, 2.34, and 2.9 eV, respectively. BiVO4 exists naturally as the mineral pucherite with an orthorhombic crystal structure [190]. However, BiVO<sup>4</sup> prepared in the laboratory does not adopt the pucherite structure but crystallizes either in a scheelite or a zircon-type structure (Fig. 9) [191,192]. The scheelite structure can have a tetragonal crystal system (space group: *I*41/*a* with *a* = *b* = 5.1470 Å, *c* = 11.7216 Å) or a monoclinic crystal system (space group: *I*2/b with *a* = 5.1935 Å, *b* = 5.0898 Å, *c* = 11.6972 Å, and *b* = 90.3871) [192,193] while the zircon-type structure has a tetragonal crystal system (space group: *I*41/a with *a* = *b* = 7.303

well explained by different facets of α‐Fe2O3 surface atomic and electronic structures.

**Figure 9.** Crystal structures of (a) tetragonal scheelite and (b) zircon-type BiVO4 (red: V, purple: Bi, and gray: O). The crystal structure of monoclinic scheelite is very similar to what is shown in (a) with the exception being the subtle changes in atomic positions of Bi, V, and O. Local coordination of V and Bi ions in (c) tetragonal scheelite, (d) mono‐ clinic scheelite, (e) and zircon−type BiVO4 structure with bond lengths shown in Å [192,194].

In the scheelite structure, four O atoms coordinate each V ion within a tetrahedral site and eight O atoms from eight different VO4 tetrahedral units coordinate each Bi ion [192,193]. Fig. 9(a) shows the four−coordinated V center and the eight−coordinated Bi center alternating along the [001] direction. Two Bi centers and one V center coordinate each O atom in this structure, and a three−dimensional structure was formed by holding the Bi and V centers. The only difference between the tetragonal and monoclinic scheelite structure is that the local environ‐ ments of V and Bi ions are more significantly distorted in the monoclinic structure, which removes the fourfold symmetry necessary for a tetragonal system. For example, in the tetragonal scheelite, all four V−O bond lengths were equal (1.72 Å), while in a monoclinic scheelite structure, there are two different V−O bond lengths(1.77 Å and 1.69 Å). In the same manner, in the tetragonal scheelite structure, only two very similar Bi−O distances exist (2.453 Å and 2.499 Å), while in the monoclinic scheelite structure, the Bi−O distances change significantly (2.354 Å, 2.372 Å, 2.516 Å and 2.628 Å) [192,193]. The significant distortion of the Bi−polyhedra indicates that the Bi 6s alone is more sterically expressed in the monoclinic scheelite structure.

It should be noted that the monoclinic scheelite structure of BiVO4 was originally reported with the space group *I*2/b, which is a nonstandard space group [192,193]. Some recent studies of BiVO4 have used a standard space group *C*2/*c*, which is converted from *I*2/b. Changes in the crystallographic axes via the conversion of a monoclinic *I*−centered (body-centered) cell to a monoclinic C-centered cell are shown in Fig. 10 [195]. With this cell conversion, the new cell parameters for *C*2/*c* are *a*' = 7.2472 Å, *b*' = 11.6972 Å, *c*' = 5.0898 Å, and *β*' = 134.225°. The choice of the *I*−centered monoclinic cell has the advantage of easily showing its structural relationship to the tetragonal scheelite structure that was reported in a body−centered space group, *I*41/a, using the identical unit cell choice and crystallographic axes. Since both *I*2/*b* and *C*2/*c* space groups, which have different unit cell choices and crystallographic axes, are commonly used to describe the monoclinic scheelite structure of BiVO4, it is necessary to clarify the space group used when referring to specific atomic planes or crystal directions as well as the *hkl* indices of X-ray diffraction peaks in order to prevent any possible confusion [192].

**Figure 10.** Cell conversion of (a) *I*−centered monoclinic to (b) *C*−centered monoclinic. *a*, *b*, *c*, and *β* represent the unit cell parameters for the *I*−centered cell and a, b0, c0, and b0 for the C−centered cell [195].

In the zircon−type structure, V is still stabilized by four O atoms and Bi is coordinated by eight O atoms. However, since two VO4 units provide two O atoms to Bi, each Bi is surrounded by only six VO4 units, as shown in Fig. 9 (e). To form a 3D structure, two Bi centers and one V center are connected by all oxygen atoms, which holds the V and Bi centers together [192].

It was reported that the low temperature synthesis (e.g., precipitation at room temperature) can form a zircon−type structure [192,196,197]. However, in this process, kinetics plays a critical role in the determination of final products, so the structure type obtained at low temperatures may change with different synthesis methods used and detailed conditions. A phase transition from tetragonal zircon to monoclinic scheelite was reported to occur irrever‐ sibly at 670−770 K [192,196]. Among scheelite structures, the tetragonal phase is a high temperature phase and the phase transition between monoclinic scheelite BiVO4 and tetrago‐ nal scheelite BiVO4 was observed to occur reversibly at 528 K [192,196].

In the zircon−type BiVO4, the charge-transfer transition from O 2p orbitals to empty V 3d is mainly responsible for the bandgap transition. In the scheelite structure, the bandgap is reduced because the 6s state of Bi3+ appears above the O 2p and the transition from Bi 6s2 (or hybrid Bi 6s2 −O 2p orbitals) to the V 3d becomes possible. Among scheelite BiVO4 structures, Tokunaga et al. reported that monoclinic scheelite structure shows much higher photocatalysis activity for the photocatalytic water oxidation compared with tetragonal scheelite structure [192,198]. The bandgap energies of the tetragonal and monoclinic scheelite BiVO4 shows little difference and the more severe distortion of the metal polyhedra present in the monoclinic scheelite BiVO4is the reason why the photocatalytic performance is different [192,198]. As discussed earlier, the local environment of Bi in the monoclinic scheelite structure is much more distorted than that in the tetragonal scheelite structure (Fig. 9 (c) and (d)).

of BiVO4 have used a standard space group *C*2/*c*, which is converted from *I*2/b. Changes in the crystallographic axes via the conversion of a monoclinic *I*−centered (body-centered) cell to a monoclinic C-centered cell are shown in Fig. 10 [195]. With this cell conversion, the new cell parameters for *C*2/*c* are *a*' = 7.2472 Å, *b*' = 11.6972 Å, *c*' = 5.0898 Å, and *β*' = 134.225°. The choice of the *I*−centered monoclinic cell has the advantage of easily showing its structural relationship to the tetragonal scheelite structure that was reported in a body−centered space group, *I*41/a, using the identical unit cell choice and crystallographic axes. Since both *I*2/*b* and *C*2/*c* space groups, which have different unit cell choices and crystallographic axes, are commonly used to describe the monoclinic scheelite structure of BiVO4, it is necessary to clarify the space group used when referring to specific atomic planes or crystal directions as well as the *hkl* indices of

**Figure 10.** Cell conversion of (a) *I*−centered monoclinic to (b) *C*−centered monoclinic. *a*, *b*, *c*, and *β* represent the unit

In the zircon−type structure, V is still stabilized by four O atoms and Bi is coordinated by eight O atoms. However, since two VO4 units provide two O atoms to Bi, each Bi is surrounded by only six VO4 units, as shown in Fig. 9 (e). To form a 3D structure, two Bi centers and one V center are connected by all oxygen atoms, which holds the V and Bi centers together [192].

It was reported that the low temperature synthesis (e.g., precipitation at room temperature) can form a zircon−type structure [192,196,197]. However, in this process, kinetics plays a critical role in the determination of final products, so the structure type obtained at low temperatures may change with different synthesis methods used and detailed conditions. A phase transition from tetragonal zircon to monoclinic scheelite was reported to occur irrever‐ sibly at 670−770 K [192,196]. Among scheelite structures, the tetragonal phase is a high temperature phase and the phase transition between monoclinic scheelite BiVO4 and tetrago‐

In the zircon−type BiVO4, the charge-transfer transition from O 2p orbitals to empty V 3d is mainly responsible for the bandgap transition. In the scheelite structure, the bandgap is reduced because the 6s state of Bi3+ appears above the O 2p and the transition from Bi 6s2

Tokunaga et al. reported that monoclinic scheelite structure shows much higher photocatalysis activity for the photocatalytic water oxidation compared with tetragonal scheelite structure

−O 2p orbitals) to the V 3d becomes possible. Among scheelite BiVO4 structures,

(or

X-ray diffraction peaks in order to prevent any possible confusion [192].

186 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

cell parameters for the *I*−centered cell and a, b0, c0, and b0 for the C−centered cell [195].

nal scheelite BiVO4 was observed to occur reversibly at 528 K [192,196].

hybrid Bi 6s2

**Figure 11.** (a) Mechanism for the formation of BiVO4 quantum tubes. (b) Optical absorption edge of BiVO4 quantum tubes (top and bottom insets:photodegradation of RhB vs. irradiation time under visible light). (c) TEM image of BiVO4 quantum tubes after the photodegradation [205]

Morphology control is an efficient method to facilitate carrier transportation and light harvesting, accelerate charge movement within the material structure and assist the collec‐ tion and separation of electron-hole pairs at the interface of the materials [199-202]. Control‐ lable synthesis of BiVO4 with controlled morphologies has been notable developed (such as nanorods, nanowires, nanotubes (NTs), nanobelts, nanoellipsoids, hollow spheres, and even some hierarchical architectures) and corresponding morphology-dependent photocatalytic properties have also been extensively studied [203-211]. For example, Tada et al. first fabricated BiVO4 nanorods (NRs) using polyethylene glycol (PEG) as a shape−directing agent [203]. Yu et al. developed a template−free solvothermal method to synthesize BiVO<sup>4</sup> nanotubes (NTs) [203]. Xie et al. reported a novel assembly−fusion strategy for the synthe‐ sis of BiVO4 quantum tubes with an ultra-narrow diameter of 5 nm, ultrathin wall thick‐ ness down to 1 nm, and exposed {010} facets (Fig. 11 (a), (c)) [206]. As the increase of the reaction time, optical absorption edge and band energy of the BiVO4 quantum tubes are significantly blue−shifted compared with bulk BiVO4, which is due to the well-known quantum size confinement effect (Fig. 11 (b)).

Nanosized building blocks, such as nanowires, nanobelts, nanosheets, and nanotubes possess interesting properties, and the self-assembling of them into hierarchical architectures is much more interesting and has attracted great attention [199]. Liu et al. and Chen et al. synthesized BiVO4 porous hollow microspheres composed of single−crystalline nanosheets using a solvothermal−induced self-assembling method (Fig. 12 (a), (b)) [207,209]. These hollow microspheres exhibited excellent photocatalytic activity due to the increased specific surface area and light harvesting ability. Xie et al. also reported the multi-responsive function of ellipsoidal BiVO4 assembled from many small nanoparticles with major exposed {101} facets [210]. Similarly, Zhao et al. synthesized uniform hyperbranched BiVO4 via a surfactant−free hydrothermal route (Fig. 12 (c)) [211]. The crystal consists of four trunks with branches distributed on opposite sides, this unique structure is beneficial from the different growth rates along a, b, and c axes: preferentially along the [100] direction at the beginning and subsequently along the [010] and [001] directions. The loosely packed building units of the hyperbranched structure exhibits excellent photocatalytic activity, because (i) the small crystal size allows the inside generated electron-hole pairs efficiently transporting from inside out to the surface and (ii) the large surface area provides abundant active sites for the photocatalytic reaction and promotes light harvesting as well as reactant adsorption.

One of the main reasons for the charge recombination in BiVO4 is the long diffusion length of the photo−induced electrons [212-214]. Tailoring porous BiVO4, especially ordered porous structures, can shorten the diffusion length and thus facilitate charge migration, providing a readily accessible channel and increasing the adsorption of reactants and the supply of more surface active sites [214,210]. Yu et al. reported that ordered mesoporous BiVO4 shows a higher photoactivity than conventional BiVO4, and this mesoporous BiVO4 was fabricated by nanocasting using mesoporous silica KIT−6 as the replica parent template (Fig. 13 (a)) [210]. Ordered macroporous BiVO4 with controllable dual porosity was synthesized by Xie et al. for efficient solar water splitting and the relationship between the geometrical characteristics and the charge migration was also demonstrated (Fig. 13 (b)) [215]. There are mainly two factors that determined by the geometrical characteristics of periodically ordered macroporous structures (Fig. 13 (b), (i)): the diameter of the macropores surrounded by the final skeletal walls (denoted as D1) and the diameter of the pores between neighboring macropores (denoted as D2). Previously, Lee et al. observed an efficient photo-induced charge drift mobility within the proper D1 size [216]. Based on this, Xie et al. further synthesized ordered macroporous BiVO4 architectures with controllable dual porosity (aforementioned as D1 and D2) via a modified colloidal crystal templating method (Fig. 13 (b), (i) and (ii)), and verified that charge migration in periodically ordered macroporous architectures has a strong dependence on D1 and D2 (Fig. 13 (b), (iii)) [215]. On the one hand, no matter in the bulk and on the surface, it is believed that a smaller D2 is favorable for charge migration. On the other hand, a smaller D1 blocks bulk charge migration but facilitates surface charge migration.

The specific crystal facet determines the surface active sites and even the electronic structure, as a result crystal facets play a critical role in photocatalysis. [198,217], and consequently, it is of great importance to develop the crystals exposed with highly reactive facets [217-223]. Xi et al. synthesized well-defined BiVO4 nanosheets exposed with {001} facets using a straightfor‐

Nanosized building blocks, such as nanowires, nanobelts, nanosheets, and nanotubes possess interesting properties, and the self-assembling of them into hierarchical architectures is much more interesting and has attracted great attention [199]. Liu et al. and Chen et al. synthesized BiVO4 porous hollow microspheres composed of single−crystalline nanosheets using a solvothermal−induced self-assembling method (Fig. 12 (a), (b)) [207,209]. These hollow microspheres exhibited excellent photocatalytic activity due to the increased specific surface area and light harvesting ability. Xie et al. also reported the multi-responsive function of ellipsoidal BiVO4 assembled from many small nanoparticles with major exposed {101} facets [210]. Similarly, Zhao et al. synthesized uniform hyperbranched BiVO4 via a surfactant−free hydrothermal route (Fig. 12 (c)) [211]. The crystal consists of four trunks with branches distributed on opposite sides, this unique structure is beneficial from the different growth rates along a, b, and c axes: preferentially along the [100] direction at the beginning and subsequently along the [010] and [001] directions. The loosely packed building units of the hyperbranched structure exhibits excellent photocatalytic activity, because (i) the small crystal size allows the inside generated electron-hole pairs efficiently transporting from inside out to the surface and (ii) the large surface area provides abundant active sites for the photocatalytic reaction and

One of the main reasons for the charge recombination in BiVO4 is the long diffusion length of the photo−induced electrons [212-214]. Tailoring porous BiVO4, especially ordered porous structures, can shorten the diffusion length and thus facilitate charge migration, providing a readily accessible channel and increasing the adsorption of reactants and the supply of more surface active sites [214,210]. Yu et al. reported that ordered mesoporous BiVO4 shows a higher photoactivity than conventional BiVO4, and this mesoporous BiVO4 was fabricated by nanocasting using mesoporous silica KIT−6 as the replica parent template (Fig. 13 (a)) [210]. Ordered macroporous BiVO4 with controllable dual porosity was synthesized by Xie et al. for efficient solar water splitting and the relationship between the geometrical characteristics and the charge migration was also demonstrated (Fig. 13 (b)) [215]. There are mainly two factors that determined by the geometrical characteristics of periodically ordered macroporous structures (Fig. 13 (b), (i)): the diameter of the macropores surrounded by the final skeletal walls (denoted as D1) and the diameter of the pores between neighboring macropores (denoted as D2). Previously, Lee et al. observed an efficient photo-induced charge drift mobility within the proper D1 size [216]. Based on this, Xie et al. further synthesized ordered macroporous BiVO4 architectures with controllable dual porosity (aforementioned as D1 and D2) via a modified colloidal crystal templating method (Fig. 13 (b), (i) and (ii)), and verified that charge migration in periodically ordered macroporous architectures has a strong dependence on D1 and D2 (Fig. 13 (b), (iii)) [215]. On the one hand, no matter in the bulk and on the surface, it is believed that a smaller D2 is favorable for charge migration. On the other hand, a smaller D1

promotes light harvesting as well as reactant adsorption.

188 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

blocks bulk charge migration but facilitates surface charge migration.

The specific crystal facet determines the surface active sites and even the electronic structure, as a result crystal facets play a critical role in photocatalysis. [198,217], and consequently, it is of great importance to develop the crystals exposed with highly reactive facets [217-223]. Xi et al. synthesized well-defined BiVO4 nanosheets exposed with {001} facets using a straightfor‐

**Figure 12.** (a) Formation mechanism, UV−Vis absorption, and RhB photodegradation of hollow BiVO4 microspheres [207]. (b) Morphology evolution of BiVO4 hollow spheres via a hydrothermal method using urea as the guiding surfac‐ tant (I: 2 h; II: 4 h; III: 8 h; IV: 12 h; V: 24 h; scale bar is 2 μm) [208]. (c) Morphology evolution of hyperbranched BiVO4 at intervals of 10 min (I), 20 min (II), 30 min (III), 45 min (IV), 1 h (V), and 3 h (VI), respectively (the scale bars are 100, 200, 200, 200, 400 and 500 nm, respectively) [210].

ward hydrothermal route without any template or organic surfactant (Fig. 15 (a)) [218]. Typically, BiVO4 crystals show a regular decahedron shape with controllable exposed facets of {010}, {011}, {110} and {111}, as shown in Fig. 14. Li et al. Synthesized BiVO4 with a highly exposed (010) facet using TiCl3 as a directing agent, and correlated this to the high activity in O2 evolution on BiVO4 (Fig. 15 (b)) [219]. Inspired by this work, facet-dependent photocatalytic activity for water oxidization on BiVO4 was investigated by density functional theory (DFT) calculations, particularly between the (010) and (011) facets (Fig. 14 (c)) [220]. The (010) facet has a higher activity compared with the (011) facet due to its higher charge carriers mobility, easier adsorption of water, and lower overall potential energy of O2 evolution.

Recently, many studies have reported that photo−induced electrons and holes may be drifted to different crystal facets [217,221-223], which means photo-reduction and oxidation may

**Figure 13.** (a) Proposed process for the fabrication of ordered mesoporous BiVO<sup>4</sup> [211]. (b) (i)Schematic representation of dual porosity in periodically ordered porous BiVO4; (ii) typical SEM images of corresponding BiVO4; (iii) relation‐ ship between PEC performance and dual porosity [216].

**Figure 14.** Typical crystal of BiVO4 exposed with the {010}, {011}, {110} and {111} facets.

happen preferentially on different facets. Therefore, the cooperation of different facets is very important to obtain high quantum efficiency. Using photochemical labeling, Li et al. discov‐ ered that photo-excited electrons-driven reduction reaction (Pt−photodeposition) and photoexcited holes-driven oxidation reaction (MnOx-photodeposition) take place on the {010} and {110} facets, respectively (Fig 16a) [222], which implies that the photo-induced electrons and holes move to the {010} and {110} facets, respectively. Notably, it provides a very useful inspiration to selectively deposited co-catalyst on specific facets via photodeposition (Fig. 16 (b)) [223]. Using this concept, the photocatalyst with Pt on the {010} facets and MnOx on the {110} facets exhibits a much higher activity in both photocatalytic and PEC water oxidation, compared with the counterparts with randomly distributed Pt and PbO2 co−catalysts (Fig. 16 (c), (d)). The coupling of co−catalysts on selected semiconductor facets may open up a new strategy for developing highly efficient photocatalysts.

**Figure 15.** (a) SEM and HRTEM images of BiVO4 nanoplates exposed with the {001} facets [213]. (b) Facet(010/110) dependent photoactivity of oxygen evolution on BiVO4 [214].

## **3.2. Bi2WO6**

happen preferentially on different facets. Therefore, the cooperation of different facets is very important to obtain high quantum efficiency. Using photochemical labeling, Li et al. discov‐ ered that photo-excited electrons-driven reduction reaction (Pt−photodeposition) and photoexcited holes-driven oxidation reaction (MnOx-photodeposition) take place on the {010} and {110} facets, respectively (Fig 16a) [222], which implies that the photo-induced electrons and holes move to the {010} and {110} facets, respectively. Notably, it provides a very useful inspiration to selectively deposited co-catalyst on specific facets via photodeposition (Fig. 16 (b)) [223]. Using this concept, the photocatalyst with Pt on the {010} facets and MnOx on the {110} facets exhibits a much higher activity in both photocatalytic and PEC water oxidation, compared with the counterparts with randomly distributed Pt and PbO2 co−catalysts (Fig. 16 (c), (d)). The coupling of co−catalysts on selected semiconductor facets may open up a new

**Figure 14.** Typical crystal of BiVO4 exposed with the {010}, {011}, {110} and {111} facets.

**Figure 13.** (a) Proposed process for the fabrication of ordered mesoporous BiVO<sup>4</sup> [211]. (b) (i)Schematic representation of dual porosity in periodically ordered porous BiVO4; (ii) typical SEM images of corresponding BiVO4; (iii) relation‐

strategy for developing highly efficient photocatalysts.

ship between PEC performance and dual porosity [216].

190 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

The Aurivillius family have a general formula of Bi2An−1BnO3n+3 (A = Ca, Sr, Ba, Pb, Bi, Na, K and B = Ti, Nb, Ta, Mo, W, Fe), and Bi2WO6 is the simplest member of this family (where n = 1) and usually have the layer structures and unique properties [224]. Fig. 17 shows a schematic structure of the Bi2WO6 crystalline with orthorhombic structures constructed by alternating (Bi2O2)n 2n+ layers and perovskite−like(WO4)n 2n- layers [225]. More recently, many Aurivilliusbased compounds have been reported which exhibit interesting properties suitable for photocatalytic applications. Of these, Bi2WO6 is the simplest and probably the most studied example within this family. In this bismuth tungstate, the perovskite−like structure is defined by WO6 units which form a layer perpendicular to the (100) direction and sandwiched between the (Bi2O2) 2+ units. Layers sandwiched structure favors the efficient separation of photogener‐ ated electron-hole pairs and then improves the photocatalytic activity, which can be ascribed to the formed internal electric fields between the slabs [226,227]. Due to its preferable band composition and unique layered structure, Bi2WO6 possesses several advantages as photoca‐ talysts over the competing materials, especially in the view of practical applications, including its desirable visible− light absorption, relatively high photocatalytic activity and good stability.

Bi2WO6 consists of accumulated layers of corner-sharing WO6 octahedral sheets and bismuth oxide sheets [228,229]. The conduction band of Bi2WO6 is composed of the W5d orbital; its valence band is formed by the hybridization of the O2p and Bi6s orbitals, which not only makes

**Figure 16.** (a) Charge separation between the {010} and {110} facets confirmed by Pt and PbO2 photodeposition on Bi‐ VO4 [218]. (b) Selective deposition of dual redox co-catalysts on specific facets of BiVO4 [219]. (c, d) Photoelectrocatalyt‐ ic and photocatalytic water oxidation activity of BiVO4 with selectively deposited co-catalysts on specific facets and randomly distributed co-catalysts [220].

the VB largely dispersed and thus results in a narrowed band gap of Bi2WO6 (2.8 eV) capable of absorbing visible light (*λ* > 400 nm), but also favors the mobility of photogenerated holes for specific oxidation reactions [230]. Such a band structure indicates that charge transfer in Bi2WO6 upon photoexcitation occurs from the O2p + Bi6s hybrid orbitals to the empty W5d orbitals, as illustrated in Fig. 18 [231].

As early as in 1999, the solid−state method was first used by Kudo et al. to synthesize Bi2WO6 photocatalyst [232], but the particle sizes of the product are in micrometers and the specific surface area is very small, which greatly limit its application in the photocatalysis. In the aim of obtaining micro or nanosized Bi2WO6 structures with enhanced photocatalytic activity, several groups have developed many advanced synthetic methods including sol−gel method [233], combustion synthesis method [234], ultrasonic method [235], co-precipitation method [236], sol-gel method/calcining method [237], and hydro/solvothermal method [238-245].

Bi2WO6 micro/nano−structures with diverse shapes exhibit different photocatalytic activities, and currently some of them have been used not only for the photodegradation of other organic pollutants but also for the photocatalytic disinfection. In 2005, Zhu's group have developed a Bi2WO6 nanoplates [246,247] applied in the photodegradation of rhodamine B (RhB) under visible−light irradiation. Notably, the photocatalytic reaction constant (*k*) of the best quality Bi2WO6 nanoplates is three times higher than that of the sample prepared by solid−state reaction [246]. In addition, they found a significantly pH-dependence of the photo-assisted

**Figure 17.** Structure of Bi2WO6 showing the WO4 2- and Bi2O2 2+ layers [225].

**Figure 18.** Band structure of the Bi2WO6 photocatalyst. [231]

the VB largely dispersed and thus results in a narrowed band gap of Bi2WO6 (2.8 eV) capable of absorbing visible light (*λ* > 400 nm), but also favors the mobility of photogenerated holes for specific oxidation reactions [230]. Such a band structure indicates that charge transfer in Bi2WO6 upon photoexcitation occurs from the O2p + Bi6s hybrid orbitals to the empty W5d

**Figure 16.** (a) Charge separation between the {010} and {110} facets confirmed by Pt and PbO2 photodeposition on Bi‐ VO4 [218]. (b) Selective deposition of dual redox co-catalysts on specific facets of BiVO4 [219]. (c, d) Photoelectrocatalyt‐ ic and photocatalytic water oxidation activity of BiVO4 with selectively deposited co-catalysts on specific facets and

As early as in 1999, the solid−state method was first used by Kudo et al. to synthesize Bi2WO6 photocatalyst [232], but the particle sizes of the product are in micrometers and the specific surface area is very small, which greatly limit its application in the photocatalysis. In the aim of obtaining micro or nanosized Bi2WO6 structures with enhanced photocatalytic activity, several groups have developed many advanced synthetic methods including sol−gel method [233], combustion synthesis method [234], ultrasonic method [235], co-precipitation method [236], sol-gel method/calcining method [237], and hydro/solvothermal method [238-245].

Bi2WO6 micro/nano−structures with diverse shapes exhibit different photocatalytic activities, and currently some of them have been used not only for the photodegradation of other organic pollutants but also for the photocatalytic disinfection. In 2005, Zhu's group have developed a Bi2WO6 nanoplates [246,247] applied in the photodegradation of rhodamine B (RhB) under visible−light irradiation. Notably, the photocatalytic reaction constant (*k*) of the best quality Bi2WO6 nanoplates is three times higher than that of the sample prepared by solid−state reaction [246]. In addition, they found a significantly pH-dependence of the photo-assisted

orbitals, as illustrated in Fig. 18 [231].

randomly distributed co-catalysts [220].

192 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

degradation of RhB in aqueous Bi2WO6 as the pH varies from 5.03 to 9.89, where the highest degradation rate was achieved at pH 6.53. It is proposed that the pH of the solutions can affect the mode and extent of adsorption of RhB on the Bi2WO6 surface and further the transformation rate of RhB indirectly. They further used the total organic carbon measurement to determine the high mineralized degree of RhB [247]. Further studies on the mechanism shows that a photocatalytic process and a photosensitized process is involved in the the Bi2WO6−assisted photodegradation of RhB [247]. However, the contribution of RhB photodegradation driven by the light−excited RhB was much slower than by the light−excited Bi2WO6. The experimental results show that only 19% of RhB was degraded by photosensitized action, while 81% of RhB was degraded by a photocatalytic process [247].

In order to further improve the photocatalytic activity of Bi2WO6, some groups have developed Bi2WO6 nanoplates superstructures [248,249-253]. Zhang group also prepared Bi2WO6 micro/ nanostructures, including nanoplates, tyre/helixlike, disintegrated−flower−like and flower −like superstructures [238,239]. The photodegradation results of RhB show that these Bi2WO6 micro/nanostructures exhibit different photocatalytic activities under visible-light (*λ* > 400 nm) irradiation, as shown in Fig. 19 (a). Among these photocatalysts, the uncalcined flower-like Bi2WO6 superstructure prepared with pH = 1 processes an improved photocatalytic perform‐ ance, which can degrade 84% of RhB in 60 min [238]. Besides, the photocatalytic performance can be further improved by the calcination process, and the result calcined flower−like Bi2WO6 superstructure has a higher photocatalytic activity, which can degrade 97% of RhB in 60 min (Fig. 19 (b)). This performance is also superior to other traditional photocatalysts such as TiO2 (P25) and bulk SSR-Bi2WO6 powder prepared by solid−state reaction [239].

**Figure 19.** (a) The photodegradation efficiencies of RhB as a function of irradiation time by different Bi2WO6 nano/ micro−structures: (A) the uncalcined flower−like Bi2WO6 superstructure prepared with pH = 1, (B) the uncalcined dis‐ integrated flower-like Bi2WO6 superstructure prepared with pH = 2.5, (C) the calcined tyre/helix−like Bi2WO6 super‐ structures prepared with pH = 1 and P123, (D) the uncalcined Bi2WO6 nanoplates prepared with pH = 7.5; [239] (b) the photodegradation efficiencies of RhB as a function of irradiation time by photocatalyst samples: (A) the calcined flow‐ er-like Bi2WO6 superstructure, (B) the uncalcined flower-like Bi2WO6 superstructure, (C) TiO2 (P25), (D)bulk SSR-Bi2WO6 powder, and (E) blank [238].

The novel flower-like superstructures of the uncalcined or calcined Bi2WO6 is mainly respon‐ sible for the highly improved photocatalytic activity. At the same time, as shown in SEM images (Fig. 20), there are plenty of meso− or macro−diameter sized pores in the flower-like superstructures, which can be considered as electron transport paths that also contributes to the photocatalysis process. [254]. It is generally believed that it is an integral part of the architectural design if the reactant molecules can easily move in or out of the nanostructured materials, the efficiency of the photocatalysis can be improved, and here, meso− or macro −diameter sized pores provides the important transport paths [254,255]. They also believe that the introduction of textural transport paths in the uncalcined or calcined Bi2WO6 superstruc‐ tures facilitate the reactant molecules to easily incorporate with the reactive sites on the framework walls of photocatalysts, which leads to excellent photocatalytic performance for the degradation of RhB [239]. On the other hand, fewer defects, which acting as electron-hole recombination centers, can be significantly reduced by improved crystallinity of Bi2WO6 through the calcination process of Bi2WO6 [238]. This has been proved by Amano et al. [256] who experimentally investigated the influence of crystallization on the lifetime of photoexcited electrons from Bi2WO6. The recombination rate of electrons decay with holes can be charac‐ terized by the intensity of transient IR absorption after a 355 nm laser pulse [256]. If an appreciable absorbance at 100 μs in Bi2WO6 crystalline can be observed, it means a slow recombination rate and a long lifetime of photogenerated carriers, which is beneficial for driving appreciable photocatalytic reactions. However, no transient absorption for amorphous Bi2WO6 samples was observed implying a fast recombination of electron-hole pairs, leading to negligible photocatalytic activity. Therefore, the higher photocatalytic activity of the calcined Bi2WO6 is explicable in several cases [238,251,256].

In order to further improve the photocatalytic activity of Bi2WO6, some groups have developed Bi2WO6 nanoplates superstructures [248,249-253]. Zhang group also prepared Bi2WO6 micro/ nanostructures, including nanoplates, tyre/helixlike, disintegrated−flower−like and flower −like superstructures [238,239]. The photodegradation results of RhB show that these Bi2WO6 micro/nanostructures exhibit different photocatalytic activities under visible-light (*λ* > 400 nm) irradiation, as shown in Fig. 19 (a). Among these photocatalysts, the uncalcined flower-like Bi2WO6 superstructure prepared with pH = 1 processes an improved photocatalytic perform‐ ance, which can degrade 84% of RhB in 60 min [238]. Besides, the photocatalytic performance can be further improved by the calcination process, and the result calcined flower−like Bi2WO6 superstructure has a higher photocatalytic activity, which can degrade 97% of RhB in 60 min (Fig. 19 (b)). This performance is also superior to other traditional photocatalysts such

194 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

as TiO2 (P25) and bulk SSR-Bi2WO6 powder prepared by solid−state reaction [239].

**Figure 19.** (a) The photodegradation efficiencies of RhB as a function of irradiation time by different Bi2WO6 nano/ micro−structures: (A) the uncalcined flower−like Bi2WO6 superstructure prepared with pH = 1, (B) the uncalcined dis‐ integrated flower-like Bi2WO6 superstructure prepared with pH = 2.5, (C) the calcined tyre/helix−like Bi2WO6 super‐ structures prepared with pH = 1 and P123, (D) the uncalcined Bi2WO6 nanoplates prepared with pH = 7.5; [239] (b) the photodegradation efficiencies of RhB as a function of irradiation time by photocatalyst samples: (A) the calcined flow‐ er-like Bi2WO6 superstructure, (B) the uncalcined flower-like Bi2WO6 superstructure, (C) TiO2 (P25), (D)bulk SSR-

The novel flower-like superstructures of the uncalcined or calcined Bi2WO6 is mainly respon‐ sible for the highly improved photocatalytic activity. At the same time, as shown in SEM images (Fig. 20), there are plenty of meso− or macro−diameter sized pores in the flower-like superstructures, which can be considered as electron transport paths that also contributes to the photocatalysis process. [254]. It is generally believed that it is an integral part of the architectural design if the reactant molecules can easily move in or out of the nanostructured materials, the efficiency of the photocatalysis can be improved, and here, meso− or macro −diameter sized pores provides the important transport paths [254,255]. They also believe that the introduction of textural transport paths in the uncalcined or calcined Bi2WO6 superstruc‐ tures facilitate the reactant molecules to easily incorporate with the reactive sites on the framework walls of photocatalysts, which leads to excellent photocatalytic performance for the degradation of RhB [239]. On the other hand, fewer defects, which acting as electron-hole

Bi2WO6 powder, and (E) blank [238].

**Figure 20.** SEM (A) and TEM (B) images of an individual flower−like Bi2WO6 superstructure (inset: SEAD pattern re‐ corded at the corner of this individual sphere); (C) SEM image of a broken Bi2WO6 sample; (D) TEM image of a peeled fragment (inset: SEAD pattern recorded at this individual nanoplate) (conditions: pH = 1, hydrothermally treated at 160 °C for 20 h, no surfactant, uncalcined) [238].

It is worth noting that Amano et al. [256] have demonstrated a high photocatalytic activity of crystalline Bi2WO6 under visible-light (*λ* > 400 nm) irradiation for oxidative decomposi‐ tion of gaseous acetaldehyde (AcH) to produce CO2, however, amorphous Bi2WO6 sample exhibits negligible photocatalytic activity under same condition. Because the mineralization of colorless AcH does not involve a dye−sensitized process, this result provides an solid conclusion that crystalline Bi2WO6 has excellent visible−light−driven photocatalytic activity. Furthermore, the photocatalytic activity of crystalline Bi2WO6 has been finely evidenced by its diffuse reflectance photoabsorption spectrum and action spectrum, that is, 8% apparent quantum efficiency at wavelength of 400 nm [256]. In the future, much more clear evidence needs to be provided to explore the mechanism of visible−light−driven photocatalytic activity of Bi2WO6. [256].

Recently, Amano et al. [252] reported the preparation of Bi2WO6 superstructures with similar hierarchical architecture, secondary particle size, crystalline shape, exposed crystalline lattice planes, and crystalline content. The only difference is that as the increasing hydrothermal reaction temperature, their specific surface areas of the products were different due to the increase thickness of crystalline rectangular platelets. The specific surface area of the product is very important, because when levels of crystalline content of Bi2WO6 flake-ball particles is similar, the higher the specific surface area is, the better the photocatalysis ability it shows. This proportional relation could be explained by the fact that the initial rate of AcH decom‐ position was expressed by first-order kinetics with respect to the amount of surface-adsorbed AcH, which is proportional to the specific surface area of Bi2WO6 samples.

**Figure 21.** (a) TEM, (b) HRTEM, and (c) structural model of Bi2WO6 square nanoplates. (d) CH4 generation over nano‐ plates and the SSR sample under visible−light irradiation (*λ* > 420 nm) [257].

In 2011, Zou and coworkers reported a remarkable increase in the CO2 reduction with water to yield CH4 over Bi2WO6 square nanoplates (Fig. 21 (a)−(c)) under visible-light irradiation as compared with yield over Bi2WO6 made by solid-state reaction (SSR) [257]. In detail, the CH4 production rate increases from 0.045 mmol g -1 h -1 for the SSR sample to 1.1 mmol g -1 h -1 for the nanoplate catalyst (Fig. 21 (d)) [257]. Considering that the band gap of B2WO6 nanoplates and SSR sample is very close, geometrical factors of the photocatalyst is mainly responsible for the photoactivity enhancement. Firstly, reducing lateral dimension of the nanoplate to the nanometer scale offers a higher specific surface area. Secondly, the ultrathin geometry of the nanoplate facilitates the transfer of the charge carriers from the bulk onto the surface, where they participate in the photoreduction reaction. Thirdly, the preferentially exposed (001) crystal plane of the nanoplates is more effective than other crystal planes [257].

## **3.3. BiOX (X = Cl, Br, and I)**

Another new class of interesting layered materials, Bismuth oxyhalides (BiOX; X = Cl, Br, and I), shows promising photocatalytic energy conversion and environment remediation ability, because of their unique layered−structure−mediated fascinating physicochemical properties and suitable band structures, along with their high chemical and optical stability, nontoxicity, low cost, and corrosion resistance [258-260]. The layered BiOX (X = Cl, Br and I) semiconductor materials, as members of the Sillen−Aurivillius family, have a tetragonal PbFCl−type structure (space group *P*4/*nmm*), which consists of [X−Bi−O−Bi−X] slices stacked together by the nonbonding (van der Waals) interactions through halogen atoms along the *c*-axis [261]. In each [X−Bi−O−Bi−X] layer, a bismuth center is surrounded by four oxygen and four halogen atoms, creating an asymmetric decahedral geometry. The covalent bonds is the interaction bond within the [Bi2O2] layers, whereas the [X] layers are stacked together by van der Waals, forces (nonbonding interactions) between the X atoms along the *c*-axis. The strong intralayer covalent bonding and the weak interlayer van der Waals interaction can induce the formation of layered structures. For BiOX crystals, the valance band maximum mainly comprises of O2p and X np states (n = 3, 4, and 5 for X = Cl, Br and I, respectively) and the Bi 6p states dominate the conduction band minimum [262-266]. As the atomic numbers of X increases, the contribution of X ns states increases remarkably, and the dispersive characteristic of band energy level becomes more and more striking, thereby narrowing the band gap. Taking BiOCl, for example, as illustrated in Fig. 22, BiOX (X = Cl, Br, I) are characterized by the layered structure that are composed of [Bi2O2] slabs interleaved with double halogen atom slabs along the [001] direction. A highly anisotropic structural, electrical, optical, and mechanical properties of this material origin from the nature of its strong intralayer covalent bonding and the weak interlayer van der Waals, interaction, this unique structure allows BiOX to apply in many promising potential applications including photocatalytic wastewater and indoor-gas purification, water splitting, organic synthesis, and selective oxidation of alcohol [267-277].

This proportional relation could be explained by the fact that the initial rate of AcH decom‐ position was expressed by first-order kinetics with respect to the amount of surface-adsorbed

**Figure 21.** (a) TEM, (b) HRTEM, and (c) structural model of Bi2WO6 square nanoplates. (d) CH4 generation over nano‐

In 2011, Zou and coworkers reported a remarkable increase in the CO2 reduction with water to yield CH4 over Bi2WO6 square nanoplates (Fig. 21 (a)−(c)) under visible-light irradiation as compared with yield over Bi2WO6 made by solid-state reaction (SSR) [257]. In detail, the CH4 production rate increases from 0.045 mmol g -1 h -1 for the SSR sample to 1.1 mmol g -1 h -1 for the nanoplate catalyst (Fig. 21 (d)) [257]. Considering that the band gap of B2WO6 nanoplates and SSR sample is very close, geometrical factors of the photocatalyst is mainly responsible for the photoactivity enhancement. Firstly, reducing lateral dimension of the nanoplate to the nanometer scale offers a higher specific surface area. Secondly, the ultrathin geometry of the nanoplate facilitates the transfer of the charge carriers from the bulk onto the surface, where they participate in the photoreduction reaction. Thirdly, the preferentially exposed (001)

Another new class of interesting layered materials, Bismuth oxyhalides (BiOX; X = Cl, Br, and I), shows promising photocatalytic energy conversion and environment remediation ability, because of their unique layered−structure−mediated fascinating physicochemical properties and suitable band structures, along with their high chemical and optical stability, nontoxicity, low cost, and corrosion resistance [258-260]. The layered BiOX (X = Cl, Br and I) semiconductor materials, as members of the Sillen−Aurivillius family, have a tetragonal PbFCl−type structure (space group *P*4/*nmm*), which consists of [X−Bi−O−Bi−X] slices stacked together by the nonbonding (van der Waals) interactions through halogen atoms along the *c*-axis [261]. In each

crystal plane of the nanoplates is more effective than other crystal planes [257].

plates and the SSR sample under visible−light irradiation (*λ* > 420 nm) [257].

**3.3. BiOX (X = Cl, Br, and I)**

AcH, which is proportional to the specific surface area of Bi2WO6 samples.

196 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**Figure 22.** The schematic diagram of crystal structure of BiOCl (green, Cl atoms;yellow, Bi atoms; red, O atoms) [278].

One−dimensional nanostructures (1D), which refers to the materials with nanoscaled thickness and width, while the length can be several micrometers or longer, is considered to be promising in photocatalysis application. The prolonged length scale may allow the 1D nanomaterials to contact the macroscopic world for various measurements [279,280]. Besides, the high aspect ratio of 1D nanostructured semiconductors also facilitates the fast photoexcited electron-hole separation, which is favorable for highly efficient photocatalytic reactions.

**Figure 23.** SEM images of the prepared (A) PAN/BiCl3 nanofibers and (B) BiOCl nanofibers [281].

BiOX (X = Cl, Br and I) material is naturally preferentially grow into nanoplates/sheets with 2D features due to its highly anisotropic layered structures. As a result, hard templates for the synthesis of the 1D bismuth oxyhalide nanostructures is commonly used, becaused the template can be easily removed by subsequent thermal or chemical treatments [281-283]. For example, Liu et al. [281] developed the electrospinning method to synthesize BiOCl nanofibers, as shown in Fig. 23. After thermal removal of the polyacrylonitrile (PAN) template at 500°C for 10 h, they can obtain the BiOCl nanofibers with diameters ranging from 80 to 140 nm. Interestingly, the as-prepared BiOCl nanofibers showed high activity towards rhodamine B (RhB) degradation under the UV irradiation, and the photodegradation rate was found to be about three times faster than that of Bi2O3 nanofibers obtained in the same way. In addition to PAN, some other templates involving activated carbon fibers (ACFs) [282] and anodic aluminium oxide (AAO) [283] have also been used to prepare BiOCl nanofibers/nanowire arrays, which displayed efficient photocatalytic performance in the degradation of organic dyes.

In the past few years, 2D nanomaterials, such as graphene, transition metal dichalcogenides and layered double hydroxides (LDHs), have gained great attention for their extraordinary physical/chemical features and promising applications in a great deal of applications [284-287]. Intrinsically, van der Waals bonds or electrostatic forces between the layer structure in such 2D nanostructures is the origin of its lamellar structure. Similarly, the layered structure makes BiOX (X = Cl, Br and I) tend to the intrinsic 2D nanostructures, such as nanoplates, nanosheets and nanoflakes. The formed intra-electric field between [Bi2O2] layers and halogen atom layers could accelerate the transfer of the photo-induced carriers and enhance the photocatalytic activity of BiOX (X = Cl, Br and I) [288].

To date, numerous synthetic methodologies have been exploited for the preparation of 2D BiOX nanomaterials, such as hydrolysis, [288-292] hydrothermal/solvothermal synthesis, [293-295] and thermal annealing [296]. For instance, recently, Zhang's group [293] has synthe‐ sized 2D BiOCl nanosheets with predominantly exposed {001} and {010} facets by selective addition of the mineralizing agent NaOH. Interestingly, BiOCl nanosheets with exposed {001} facets displayed higher UV−induced photocatalytic degradation of MO dye, while the counterpart with exposed {010} facets exhibited higher degradation activity under visible light. On the one hand, the generated internal electric field along the [001] direction is more favorable for direct semiconductor photoexcitation under UV irradiation as shown in Fig. 24, which was also confirmed by the higher photocurrent of {001} facets than that of {010} facets from the transient photocurrent responses. On the other hand, compared with {001} facets, the larger surface area and open channel feature of {010} facets facilitate the adsorption of dye molecules, which further results in its better indirect dye photosensitization performance under visible light irradiation.

in photocatalysis application. The prolonged length scale may allow the 1D nanomaterials to contact the macroscopic world for various measurements [279,280]. Besides, the high aspect ratio of 1D nanostructured semiconductors also facilitates the fast photoexcited electron-hole

BiOX (X = Cl, Br and I) material is naturally preferentially grow into nanoplates/sheets with 2D features due to its highly anisotropic layered structures. As a result, hard templates for the synthesis of the 1D bismuth oxyhalide nanostructures is commonly used, becaused the template can be easily removed by subsequent thermal or chemical treatments [281-283]. For example, Liu et al. [281] developed the electrospinning method to synthesize BiOCl nanofibers, as shown in Fig. 23. After thermal removal of the polyacrylonitrile (PAN) template at 500°C for 10 h, they can obtain the BiOCl nanofibers with diameters ranging from 80 to 140 nm. Interestingly, the as-prepared BiOCl nanofibers showed high activity towards rhodamine B (RhB) degradation under the UV irradiation, and the photodegradation rate was found to be about three times faster than that of Bi2O3 nanofibers obtained in the same way. In addition to PAN, some other templates involving activated carbon fibers (ACFs) [282] and anodic aluminium oxide (AAO) [283] have also been used to prepare BiOCl nanofibers/nanowire arrays, which displayed efficient photocatalytic performance in the degradation of organic

In the past few years, 2D nanomaterials, such as graphene, transition metal dichalcogenides and layered double hydroxides (LDHs), have gained great attention for their extraordinary physical/chemical features and promising applications in a great deal of applications [284-287]. Intrinsically, van der Waals bonds or electrostatic forces between the layer structure in such 2D nanostructures is the origin of its lamellar structure. Similarly, the layered structure makes BiOX (X = Cl, Br and I) tend to the intrinsic 2D nanostructures, such as nanoplates, nanosheets and nanoflakes. The formed intra-electric field between [Bi2O2] layers and halogen atom layers could accelerate the transfer of the photo-induced carriers and enhance the photocatalytic

To date, numerous synthetic methodologies have been exploited for the preparation of 2D BiOX nanomaterials, such as hydrolysis, [288-292] hydrothermal/solvothermal synthesis, [293-295] and thermal annealing [296]. For instance, recently, Zhang's group [293] has synthe‐ sized 2D BiOCl nanosheets with predominantly exposed {001} and {010} facets by selective addition of the mineralizing agent NaOH. Interestingly, BiOCl nanosheets with exposed {001}

separation, which is favorable for highly efficient photocatalytic reactions.

198 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**Figure 23.** SEM images of the prepared (A) PAN/BiCl3 nanofibers and (B) BiOCl nanofibers [281].

dyes.

activity of BiOX (X = Cl, Br and I) [288].

**Figure 24.** (a) Crystal structure of BiOCl. (b) Model showing the direction of the internal electric field in each of the BiOCl nanosheets. (c) Photocurrent responses of the BiOCl nanosheets in 0.5 M Na2SO4 aqueous solutions under UVvis irradiation [293].

Researches have also using density functional theory (DFT) computations to reveal the nature of such a facet-dependent photocatalytic property in BiOX (X = Cl, Br and I) [297]. The halogen X−terminated {001} facets shows great thermodynamic stability and could efficiently separate photo-generated electron−hole pairs, whereas the formation of deep defect levels in the band gap of BiX−terminated {110} and other facets with surface O vacancies are bad for the carrier separation. This finding reveals the insight into the fundamental facet determined photoca‐ talysis of BiOX (X = Cl, Br and I), which explains the superior photocatalytic performance of BiOX (X = Cl, Br and I) nanosheets with higher percentage of {001} facets than those with lower ones [289,290,296].

Self-assembly is a strong tool in nanotechnology fabrication of making low dimensional (e.g., 1D nanorods, 2D nanosheets, etc.) materials into their higher−dimension (3D) multifunctional superstructures, which plays a major role in material synthesis and device engineering and has been paid much attention recently [298-302]. Comparison with 1D and 2D nanostructures, 3D hierarchical nano/microstructures, which integrate the features of the nanoscale building units and their assembled architectures, are more attractive for solar energy storage and conversion [300-302]. Furthermore, 3D architectures could endow the BiOX (X = Cl, Br, I) semiconductors with improved light harvesting, shortened diffusion pathways, faster interfacial charge separation and more reactive sites, thus enhancing their photocatalytic efficiencies.

Hydro/solvothermal routes are definitely the most robust method among the methodological synthesis of the 3D BiOX (X = Cl, Br and I) hierarchical assemblies, [303-322] which are usually carried out at critical conditions of water or other organic solvents. In 2008, a generalized solvothermal process has been developed by Zhang et al. [303], who use ethylene glycol (EG) to prepare BiOX (X = Cl, Br, and I) hierarchical microspheres from 2D nanoplates. The band gaps of the resulting BiOX (X = Cl, Br and I) samples are calculated to be 3.22, 2.64, and 1.77 eV for BiOCl, BiOBr, and BiOI, respectively. Under visible-light irradiation, the BiOI sample exhibited the best photocatalytic performance with the order of BiOI > BiOBr > BiOCl evaluat‐ ed by MO dye solution degradation. Almost at the same time, Tang et al. [304] also prepared 3D microspherical BiOBr architectures assembled by nanosheets through EG−assisted solvo‐ thermal synthesis. The band gap of the BiOBr architectures is 2.54 eV, so it shows higher photocatalytic activity for MO decomposition under visible−light irradiation than the BiOBr bulk plates.

How to realize the hierarchical architectures in the microstructure modulation of BiOX (X = Cl, Br and I) nano/microstructures with hollow voids draws much attention due to their better penetrability and higher light utilization. Recently, Huang group [313] has developed a method to synthesize uniform BiOBr hollow microspheres in the presence of 2-methoxyethanol solvent a mini-emulsion-mediated solvothermal route. The size of the BiOBr hollow microspheres is in the in the range of 1-2 mm and shell thickness of about 100 nm, which are composed of numerous interlaced 2D nanosheets. As demonstrated in Fig 25, by observing a Tyndall effect of the precursor suspension, the author confirmed that the 1 −hexadecyl−3−methylimidazolium bromide ionic liquid ([C16Mim]BrIL) can not only serve as a Br source but also create a colloidal mini-emulsions. The diameter of the BiOBr hollow microsphere is determined by the size of the of the emulsion because the reaction takes place at the phase interface edge of the mini−emulsion rather than in the itself. Under visible-light irradiation, such BiOBr hollow microspheres displayed superior photocatalytic activity in degradation of RhB dye and reduction of CrVI ions to the samples with micro-flower shape. Xia and co-workers [319] prepared BiOI hollow microspheres by the EG−assisted solvothermal method using 1-butyl-3-methylimidazoliumio‐ dine ([Bmim]I) IL as the reactive templates and I source. Under visible-light irradiation, such 3D BiOI hollow microspheres exhibited higher photocatalytic activity toward MO degrada‐ tion than that of 2D BiOI nanoplates. Besides the halide ion-containing ILs [304,304,312,314,317,322], surfactants such as poly(vinylpyrrolidone) (PVP) [308] and hexadecyl‐ trimethylammonium bromide (CTAB) [311,312,314,316,318,319] have been used to tailor the self-assembly process of the BiOX (X = Cl, Br, I) hierarchical architectures, and in particular CTAB could act as reactive template to provide Br - ions for BiOBr.

superstructures, which plays a major role in material synthesis and device engineering and has been paid much attention recently [298-302]. Comparison with 1D and 2D nanostructures, 3D hierarchical nano/microstructures, which integrate the features of the nanoscale building units and their assembled architectures, are more attractive for solar energy storage and conversion [300-302]. Furthermore, 3D architectures could endow the BiOX (X = Cl, Br, I) semiconductors with improved light harvesting, shortened diffusion pathways, faster interfacial charge separation and more reactive sites, thus enhancing their photocatalytic

200 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

Hydro/solvothermal routes are definitely the most robust method among the methodological synthesis of the 3D BiOX (X = Cl, Br and I) hierarchical assemblies, [303-322] which are usually carried out at critical conditions of water or other organic solvents. In 2008, a generalized solvothermal process has been developed by Zhang et al. [303], who use ethylene glycol (EG) to prepare BiOX (X = Cl, Br, and I) hierarchical microspheres from 2D nanoplates. The band gaps of the resulting BiOX (X = Cl, Br and I) samples are calculated to be 3.22, 2.64, and 1.77 eV for BiOCl, BiOBr, and BiOI, respectively. Under visible-light irradiation, the BiOI sample exhibited the best photocatalytic performance with the order of BiOI > BiOBr > BiOCl evaluat‐ ed by MO dye solution degradation. Almost at the same time, Tang et al. [304] also prepared 3D microspherical BiOBr architectures assembled by nanosheets through EG−assisted solvo‐ thermal synthesis. The band gap of the BiOBr architectures is 2.54 eV, so it shows higher photocatalytic activity for MO decomposition under visible−light irradiation than the BiOBr

How to realize the hierarchical architectures in the microstructure modulation of BiOX (X = Cl, Br and I) nano/microstructures with hollow voids draws much attention due to their better penetrability and higher light utilization. Recently, Huang group [313] has developed a method to synthesize uniform BiOBr hollow microspheres in the presence of 2-methoxyethanol solvent a mini-emulsion-mediated solvothermal route. The size of the BiOBr hollow microspheres is in the in the range of 1-2 mm and shell thickness of about 100 nm, which are composed of numerous interlaced 2D nanosheets. As demonstrated in Fig 25, by observing a Tyndall effect of the precursor suspension, the author confirmed that the 1 −hexadecyl−3−methylimidazolium bromide ionic liquid ([C16Mim]BrIL) can not only serve as a Br source but also create a colloidal mini-emulsions. The diameter of the BiOBr hollow microsphere is determined by the size of the of the emulsion because the reaction takes place at the phase interface edge of the mini−emulsion rather than in the itself. Under visible-light irradiation, such BiOBr hollow microspheres displayed superior photocatalytic activity in degradation of RhB dye and reduction of CrVI ions to the samples with micro-flower shape. Xia and co-workers [319] prepared BiOI hollow microspheres by the EG−assisted solvothermal method using 1-butyl-3-methylimidazoliumio‐ dine ([Bmim]I) IL as the reactive templates and I source. Under visible-light irradiation, such 3D BiOI hollow microspheres exhibited higher photocatalytic activity toward MO degrada‐ tion than that of 2D BiOI nanoplates. Besides the halide ion-containing ILs [304,304,312,314,317,322], surfactants such as poly(vinylpyrrolidone) (PVP) [308] and hexadecyl‐ trimethylammonium bromide (CTAB) [311,312,314,316,318,319] have been used to tailor the self-assembly process of the BiOX (X = Cl, Br, I) hierarchical architectures, and in particular

CTAB could act as reactive template to provide Br - ions for BiOBr.

efficiencies.

bulk plates.

**Figure 25.** The schematic formation process of the BiOBr hollow microspheres by the mini-emulsionmediated solvo‐ thermal route [313].

Apart from the hydro/solvothermal syntheses, other synthetic procedures are also used to synthesize the ordered superstructures of BiOX (X = Cl, Br and I) semiconductors, such as hydrolysis [323,324], direct precipitation [325,326], sonochemical route [327,328], refluxing method [329], chemical bath [330] and solution oxidation process [331]. For example, Xiong and co-workers [331] reported a rapid in situ oxidation process to fabricate 3D flower-like BiOCl hierarchical nanostructures by reacting metallic Bi nanospheres and FeCl3 aqueous solution at room temperature. As illustrated in Fig. 26, in the presence of Cl- ions, the redox potential of Bi species could be reduced from +0.308 V (Bi3+/Bi vs. SHE) to +0.16 V (BiOCl/Bi). Therefore, the high redox potential of Fe3+ (E(Fe3+/Fe2+) = +0.771 V) could oxidize the surface of Bi nanospheres into the final 3D BiOCl hierarchical nanostructures. Compared with the commercial BiOCl sample, such flower-like BiOCl nanostructures obtained displayed much better RhB photodegradation activity and higher photoelectric conversion performances.

**Figure 26.** Schematic illustration of the fabrication of flower−like BiOCl hierarchical nanostructures by an in situ oxida‐ tion process [331].

Interestingly, Zhang [332] found that the photoactivity of BiOCl nanosheets shows a highly exposed facet-dependent effcet. The BiOCl nanosheets with exposed {001} facets showed higher direct semiconductor photoexcitation activity towards pollutant degradation due to both the surface atomic structure and suitable internal electric fields under UV light irradiation. Under visible light, highly exposed {010} facet BiOCl nanosheets shows superior indirect dye photosensitization activity for methyl orange degradation, which is due to the larger surface area and open channel characteristic of BiOCl nanosheets. It is belived that the enlarged surface area and open channel could enhance the adsorption capacity of methyl orange molecules as well as provide more contact sites between the photocatalyst and dye molecules, thereby facilitating the indirect dye photosensitization process because more efficient electron injection from the photoexcited dye into the conduction band of the catalyst happened (Fig. 27). These findings not only clarified the origin of facet-dependent photoreactivity of BiOCl nanosheets but also provided effective guidance for the design and fabrication of highly efficient bismuth oxyhalide photocatalyst.

**Figure 27.** Schematic illustration of facet-dependent photoreactivity of BiOCl single-crystalline nanosheets [332].

**Figure 28.** Schematic diagram of three types of semiconductor heterostructures [338].

It is speculated by Ye et al. that oxygen vacancy induced by UV light could yield an inter‐ mediate state between the valence and conduction bands to narrow the band gap, which may make oxygen−deficient BiOCl a promising alternative for the visible−light−driven photocata‐ lytic reaction [333,334]. Zhang group recently found that oxygen vacancies of BiOCl can be created by the reductive ethylene glycol because it could easily react with the oxygen −terminated (001) surface at 160°C, which is evidenced by the electron spin resonance (ESR) spectra [335]. The resulting oxygen vacancies not only extended the light-response edge up to 650 nm but also enabled the effective capture of photoinduced electrons and molecular oxygen to generate superoxide anion radicals, both of which are of great important for realizing high photocatalysis efficiency of the photocatalyst. Recently, Xie et al. demonstrated that with the reduced thickness of the {001} facet−dominant BiOCl nanosheets to the atomic scale, the defects mainly change from isolated defects to triple vacancy, which could significantly promote the sunlight−driven photocatalytic activity of BiOCl nanosheets. The enhanced adsorption capability, the separation of electron-hole pairs and the generated reductive photoexcited electrons is mainly responsible for this improvement [261].
