**2. α-Hematite (Fe2O3)**

Hematite is the most thermodynamically stable phase of iron oxide and is formed by a hexagonal crystalline structure. It is used in catalysis processes and as pigments, in the lithium ion batteries and for the water treatment as an adsorbent [29, 30]. The strong adsorption capacity of organic and inorganic species by the hematite is related to its intrinsic chemical and electronic properties [31, 32]. The pH ZPC of the hematite is approximately 8.0, which implies that in aqueous media with pH below 8.0, the surface is positively charged and the interaction between anionic molecules is stronger [30, 31].

This semiconductor is also employed in photocatalysis due to its reduced band gap energy (*E*<sup>0</sup> = 2.2 eV), which allows the visible light absorption and with an energy level higher than water oxidation potential. Moreover, the α-Fe2O3 phase is a material with low cost and environment friendly and has good chemical and corrosion stability [33]. It is found that because of these characteristics, the hematite nanoparticles are potential candidates to good adsorbents for the removal of contaminants from aqueous systems.

Many methods have been researched and applied to produce hematite nanoparticles, but the most common techniques are the thermal decomposition, sol-gel, solvothermal and hydrothermal [34–36]. These different methodologies are used to control parameters like size, shape and crystal structure of the particles produced, with the aim to ensure interesting adsorption capacity and photocatalysis [37].

Thermal decomposition usually applies inorganic salts, such as nitrates, oxalates and chlorides, with low thermal stability to produce metal oxide nanoparticles in air atmosphere [38]. However, this process can be accomplished by calcination of magnetite (Fe3O4) or maghemite (γ-Fe2O3) at 500°C [39]. Al-Gaashani et al. [40] synthesized hematite by thermal decomposition of iron (III) nitrate 9-hydrate at different temperatures (300, 400, 500 and 600°C) under ambient conditions within a short time (20 min). The nanostructures showed crystallite sizes in a range to 10–30 nm with high surface area and optical band gap near 3.0 eV. The iron oxide α-Fe2O3 nanoparticles with different crystalline structure were prepared by [41], which calcinated the iron precursor ferric ammonium citrate at different temperatures (300, 400 and 550°C) under air atmosphere. The mean particle sizes of the samples were near 20 nm with excellent dye photocalytic degradation under visible light after 10 h. Yang et al. [42] applied direct thermal decomposition of goethite

(α-FeOOH, calcinated at 300°C, to produce hematite with a high adsorption capacity for arsenic (14.5 mg•g<sup>−</sup><sup>1</sup> ).

In the sol-gel method, a metal salt, usually chloride and nitrate, is dispersed in alcohol solution to form the "sol" phase by hydrolysis, condensation and polymerization. The "sol" phase is heated to evaporate the solvent and this mixture is converted to "gel" phase, which is composed for regular nanoparticles [43, 44]. Xiao et al. [45] evaluated the adsorption of chromium (VI) on hematite nanoparticles produced by sol-gel technique. The precursors used to form hematite were FeCl3•6H2O and (NH4)2CO3 dissolved in water and ethanol. The crystalline size of α-Fe2O3 nanoparticles samples showed values between 15 and 30 nm, with a removal efficiency of chromium near 50% to smaller particles. Sol-gel method was applied by [46] to form hematite nanoparticles from Fe(NO3)3•9H2O dispersed in ethanol. The particles produced by this method possess sizes below 10 nm and excellent optical properties, with a band gap energy near 2 eV. Raja et al. [47] employed ferric nitrate dissolved in ethylene glycol to produce hexagonal nanoparticles with interesting structural, optical and magnetic properties. The photocatalytic properties of hematite produce by sol-gel technique were evaluated by [44], that used Fe(NO3)3•9H2O dispersed in ethylene glycol to form nanoparticles with a crystalline size smaller than 31 nm and specific surface area between 37 and 57 m2 •g<sup>−</sup><sup>1</sup> . The best photocatalytic activity for H2 production was obtained on α-Fe2O3 calcined at 500°C, with an average evolution rate of 0.015 cm3 •h<sup>−</sup><sup>1</sup> •(mg catalyst)<sup>−</sup><sup>1</sup> .

Solvothermal methodology is a versatile process to synthesize uniform-sized nanoparticles, which are formed from iron salts dissolved in non-aqueous solvents, usually alcohol. This mixture often is transferred to autoclave to achieve reaction temperature near 180°C [38, 43]. Preparation of hematite nanoparticles through alcoholysis of ferric chloride under solvothermal condition has been carried out by [48]. FeCl3•6H2O was dissolved in ethanol and transferred into Teflon autoclave to achieve 180°C. This synthetic method applied created core/shell structures formed from α-Fe2O3 nanoparticles, which were used as catalysts in the oxidation of benzyl alcohol to benzaldehyde with 42% conversion and 95% selectivity. Sun et al. [49] developed a combined solvothermal/microwave method to prepare α-Fe2O3 nanosheet-assembled hierarchical hollow mesoporous microspheres from chloride ferric precursors dissolved in ethylene glycol. The reaction occurred into autoclave at 180°C for 180 min and the nanoparticles formed were applied to salicylic acid degradation, with removal efficiencies near 50%.

Hydrothermal is the most applied process in the synthesis of hematite due to low cost and energy consumption, high purity and short time to preparation. In this method, the iron (III) chloride or nitrate solutions are dispersed in aqueous media that contains ammonia to adjust the pH value above 9.0. Later, the solution is heated at high temperatures (≈ 150°C) in an autoclave to decompose the precursors and combine their ions to form new compounds with high homogeneity [50, 51]. Hematite nanoparticles were synthesized by [52] by a simple hydrothermal synthesis method using only Fe(NO3)3•9H2O and NH3•H2O as raw materials into a Teflon autoclave at different temperatures (80, 100, 120 and 150°C) for 10 h. This research demonstrated that nanoparticles with diameter of 30–100 nm only were produced at temperatures above 120°C in a reaction time within 5 h, occurring an increase in the particles size when longer times were evaluated. The methylene blue adsorption on surface of nanostructured t-ZrO2-modified α-Fe2O3 composite synthesized in a mesoporous structure by a hydrothermal route was studied by [53]. The composite was prepared from FeCl3•6H2O and ZrOCl2•8H2O dissolved in an aqueous media with cetyl trimethyl ammonium bromide (CTAB). Thereafter, the reaction mixture was transferred to a Teflon autoclave and heated at 180°C for 48 h. The

**67**

*Photocatalytic Adsorbents Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.79954*

with 75 nm and a specific surface area near 25 m2

with a degradation above 90% in the presence of H2O2.

due to the synergistic effect of adsorption and photocatalysis.

was applied, as well as when the photocatalysis is incorporated.

**3. Metal-organic frameworks (MOFs)**

capacity (84 mg•g<sup>−</sup><sup>1</sup>

area (31.57 m2

•g<sup>−</sup><sup>1</sup>

nanoparticles synthesized showed excellent adsorption capacity with an efficient upper 95% in the removal of chromium, cobalt, nickel, cadmium, lead, copper and mercury. Dong et al. [54] obtained grain-like hematite by hydrothermal approach using FeCl3•6H2O and urea dissolved in a water-ethanol solution heated at 180°C for 8 h in a Teflon autoclave. The researchers determined that in a volume ratio of ethanol/water equal to 1:1, the grain-like nanoparticle formation is favored, with no regular structures, when only water or alcohol were used. The photocatalyst response achieved approximately 99% of dye removal after 24 h under visible light. The researches above-mentioned investigated the adsorption capacity and photocatalysis separately, however the hematite is interesting to organic and inorganic compound removal in water due to the join between both systems, since this photocatalytic adsorbent has a system that captures and degrades the contaminants efficiently. The conjugated response of hematite was evaluated by [55] that synthesized nanoparticles

•g<sup>−</sup><sup>1</sup>

) were shown to exhibit adsorption efficient near 80% of organic

) and limited photocatalytic degradation (≈ 20%). Cheng et al. [56] also analyze the conjugated adsorption and photocatalytic removal of organic dye rhodamine B for cauliflower-like α-Fe2O3 microstructures constructed by nanoparticle-based buds. The hematite nanoparticles were synthesized from aqueous solution containing ammonia and polyvinylpyrrolidone (PVP) that was mixed with Fe(acetylacetonate)3 dispersed in a toluene solution and transferred to autoclave at 150°C for 24 h. The structure formed with high surface

dye in wastewater and structurally enhanced visible light photocatalytic activity,

Liu et al. [57] prepared α-Fe2O3 hollow spheres with novel multiple porous shells by solvothermal treatment of FeCl3•6H2O dissolved in ethylene glycol and CH3COONa. Carbon spheres were added into the mixture and the resultant suspension was transferred to an autoclave at 200°C for 10 h. The carbon was removed by calcination at 500°C during 3 h. The nanostructures formed are composed of particles from 20 to 40 nm in size that reduced the dye concentration below 90%

Hematite nanoparticles were synthesized by [58] applying hydrothermal method to react FeCl3•6H2O dissolved in aqueous solution containing NH4OH and pectin into a Teflon autoclave. The size of the nanoparticles was achieved as 42 nm when pectin was added probably due to less agglomeration. The experimental data demonstrated that dye removal reached values upper 80% when only adsorption

Metal-organic frameworks (MOFs) are a class of functional materials synthesized by the assembly of the metal ions/clusters and organic linkers like cyano and pyridyl, carboxylates, phosphonates and crown ethers, which are connected to metal ions/clusters through coordination bonds of moderate strength [59, 60]. The MOFs have been studied since 1990s with more than 20,000 structures synthesized and evaluated for applications such as adsorption, catalysis, drug delivery, sensing, separation, gas storage, bioimaging and so on [61–64]. These structures have shown huge potential in these areas due to distinctive features, such as high porosity and surface area, chemically adjustable pore, uniform structures, tunable surface properties (functional groups), good thermal stability, unsaturated metal centers and even the catalytically active organic linkers [65–68]. The most typical metal-organic

, which showed a high adsorption

#### *Photocatalytic Adsorbents Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.79954*

*Advanced Sorption Process Applications*

specific surface area between 37 and 57 m2

•(mg catalyst)<sup>−</sup><sup>1</sup>

degradation, with removal efficiencies near 50%.

•h<sup>−</sup><sup>1</sup>

rate of 0.015 cm3

).

ity for arsenic (14.5 mg•g<sup>−</sup><sup>1</sup>

(α-FeOOH, calcinated at 300°C, to produce hematite with a high adsorption capac-

In the sol-gel method, a metal salt, usually chloride and nitrate, is dispersed in alcohol solution to form the "sol" phase by hydrolysis, condensation and polymerization. The "sol" phase is heated to evaporate the solvent and this mixture is converted to "gel" phase, which is composed for regular nanoparticles [43, 44]. Xiao et al. [45] evaluated the adsorption of chromium (VI) on hematite nanoparticles produced by sol-gel technique. The precursors used to form hematite were FeCl3•6H2O and (NH4)2CO3 dissolved in water and ethanol. The crystalline size of α-Fe2O3 nanoparticles samples showed values between 15 and 30 nm, with a removal efficiency of chromium near 50% to smaller particles. Sol-gel method was applied by [46] to form hematite nanoparticles from Fe(NO3)3•9H2O dispersed in ethanol. The particles produced by this method possess sizes below 10 nm and excellent optical properties, with a band gap energy near 2 eV. Raja et al. [47] employed ferric nitrate dissolved in ethylene glycol to produce hexagonal nanoparticles with interesting structural, optical and magnetic properties. The photocatalytic properties of hematite produce by sol-gel technique were evaluated by [44], that used Fe(NO3)3•9H2O dispersed in ethylene glycol to form nanoparticles with a crystalline size smaller than 31 nm and

•g<sup>−</sup><sup>1</sup>

H2 production was obtained on α-Fe2O3 calcined at 500°C, with an average evolution

Solvothermal methodology is a versatile process to synthesize uniform-sized nanoparticles, which are formed from iron salts dissolved in non-aqueous solvents, usually alcohol. This mixture often is transferred to autoclave to achieve reaction temperature near 180°C [38, 43]. Preparation of hematite nanoparticles through alcoholysis of ferric chloride under solvothermal condition has been carried out by [48]. FeCl3•6H2O was dissolved in ethanol and transferred into Teflon autoclave to achieve 180°C. This synthetic method applied created core/shell structures formed from α-Fe2O3 nanoparticles, which were used as catalysts in the oxidation of benzyl alcohol to benzaldehyde with 42% conversion and 95% selectivity. Sun et al. [49] developed a combined solvothermal/microwave method to prepare α-Fe2O3 nanosheet-assembled hierarchical hollow mesoporous microspheres from chloride ferric precursors dissolved in ethylene glycol. The reaction occurred into autoclave at 180°C for 180 min and the nanoparticles formed were applied to salicylic acid

Hydrothermal is the most applied process in the synthesis of hematite due to low cost and energy consumption, high purity and short time to preparation. In this method, the iron (III) chloride or nitrate solutions are dispersed in aqueous media that contains ammonia to adjust the pH value above 9.0. Later, the solution is heated at high temperatures (≈ 150°C) in an autoclave to decompose the precursors and combine their ions to form new compounds with high homogeneity [50, 51]. Hematite nanoparticles were synthesized by [52] by a simple hydrothermal synthesis method using only Fe(NO3)3•9H2O and NH3•H2O as raw materials into a Teflon autoclave at different temperatures (80, 100, 120 and 150°C) for 10 h. This research demonstrated that nanoparticles with diameter of 30–100 nm only were produced at temperatures above 120°C in a reaction time within 5 h, occurring an increase in the particles size when longer times were evaluated. The methylene blue adsorption on surface of nanostructured t-ZrO2-modified α-Fe2O3 composite synthesized in a mesoporous structure by a hydrothermal route was studied by [53]. The composite was prepared from FeCl3•6H2O and ZrOCl2•8H2O dissolved in an aqueous media with cetyl trimethyl ammonium bromide (CTAB). Thereafter, the reaction mixture was transferred to a Teflon autoclave and heated at 180°C for 48 h. The

.

. The best photocatalytic activity for

**66**

nanoparticles synthesized showed excellent adsorption capacity with an efficient upper 95% in the removal of chromium, cobalt, nickel, cadmium, lead, copper and mercury. Dong et al. [54] obtained grain-like hematite by hydrothermal approach using FeCl3•6H2O and urea dissolved in a water-ethanol solution heated at 180°C for 8 h in a Teflon autoclave. The researchers determined that in a volume ratio of ethanol/water equal to 1:1, the grain-like nanoparticle formation is favored, with no regular structures, when only water or alcohol were used. The photocatalyst response achieved approximately 99% of dye removal after 24 h under visible light.

The researches above-mentioned investigated the adsorption capacity and photocatalysis separately, however the hematite is interesting to organic and inorganic compound removal in water due to the join between both systems, since this photocatalytic adsorbent has a system that captures and degrades the contaminants efficiently. The conjugated response of hematite was evaluated by [55] that synthesized nanoparticles with 75 nm and a specific surface area near 25 m2 •g<sup>−</sup><sup>1</sup> , which showed a high adsorption capacity (84 mg•g<sup>−</sup><sup>1</sup> ) and limited photocatalytic degradation (≈ 20%).

Cheng et al. [56] also analyze the conjugated adsorption and photocatalytic removal of organic dye rhodamine B for cauliflower-like α-Fe2O3 microstructures constructed by nanoparticle-based buds. The hematite nanoparticles were synthesized from aqueous solution containing ammonia and polyvinylpyrrolidone (PVP) that was mixed with Fe(acetylacetonate)3 dispersed in a toluene solution and transferred to autoclave at 150°C for 24 h. The structure formed with high surface area (31.57 m2 •g<sup>−</sup><sup>1</sup> ) were shown to exhibit adsorption efficient near 80% of organic dye in wastewater and structurally enhanced visible light photocatalytic activity, with a degradation above 90% in the presence of H2O2.

Liu et al. [57] prepared α-Fe2O3 hollow spheres with novel multiple porous shells by solvothermal treatment of FeCl3•6H2O dissolved in ethylene glycol and CH3COONa. Carbon spheres were added into the mixture and the resultant suspension was transferred to an autoclave at 200°C for 10 h. The carbon was removed by calcination at 500°C during 3 h. The nanostructures formed are composed of particles from 20 to 40 nm in size that reduced the dye concentration below 90% due to the synergistic effect of adsorption and photocatalysis.

Hematite nanoparticles were synthesized by [58] applying hydrothermal method to react FeCl3•6H2O dissolved in aqueous solution containing NH4OH and pectin into a Teflon autoclave. The size of the nanoparticles was achieved as 42 nm when pectin was added probably due to less agglomeration. The experimental data demonstrated that dye removal reached values upper 80% when only adsorption was applied, as well as when the photocatalysis is incorporated.
