Influence of acid type and concentration on the synthesis of nanostructured titanium dioxide photocatalysts from titanium-bearing electric arc furnace molten slag

Abstract

In this paper, nanostructured titanium dioxide (TiO₂) photocatalysts with different crystal structures and morphologies were successfully synthesized from titanium-bearing electric arc furnace molten slag (Ti-bearing EAF slag). The effects of different kinds of acid and acid concentrations during the acidolysis process on the crystal structures and morphologies of TiO₂ were systematically studied. The TiO₂ crystal phase transformation mechanism and morphology evolution in different acid systems and concentrations were discussed in detail. In addition, the photocatalytic properties of TiO₂ obtained in different acid systems were investigated. It is shown that rutile and anatase type TiO₂ were obtained in hydrochloric acid and sulfuric acid, respectively, while two types coexisted in nitric acid. With increasing sulfuric acid concentration, anatase would be apt to change to rutile. The growth mechanism of TiO₂ from Ti-bearing EAF slag was also discussed. The TiO₂ synthesized from sulfuric acid showed better photocatalytic activity than that from hydrochloric or nitric acid, and its photodegradation efficiency can reach 90.52% in 90 min for rhodamine B solutions.

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1. Introduction

TiO₂ is an important semiconductor material used in various fields such as coatings,¹ fillers for plastic and paper, photocatalysts,^2,3 gas sensors⁴ and solar energy conversion materials.^5,6 Owing to its relatively high redox potential, chemical stability and nontoxic nature, TiO₂ has been regard as one of the best photocatalysts for many years,³ and its photocatalytic properties strongly depend on crystalline phase and micromorphology. Till now, various methods had been developed to prepare nano TiO₂ with different morphologies (size, shape, etc.) and crystalline phases by using pure chemical reagents.^7–9 For instance, Wu et al.¹⁰ proposed a sol-hydrothermal method to synthesize pure anatase or rutile type TiO₂ nanocrystals with uniform size from titanium n-butoxide in various acidic mediums. They found that the acid type such as HCl, HNO₃, H₂SO₄, and CH₃COOH had significant influence on the crystalline phase of obtained nanoparticles. Rajesh et al.¹¹ also studied the effects of acid species and concentration on the obtained TiO₂ particles from TiCl₄ by a direct thermal hydrolysis approach. It was shown that the samples synthesized in HCl, HNO₃ and HClO₄ were rutile phase with a rhombus shape, while anatase TiO₂ with flake shape was obtained in the H₂SO₄ system. Kutty et al.¹² synthesized fine powders of TiO₂ with a high degree of crystallinity from TiOCl₂ solution using the hydrothermal method. They found that SO₄²⁻ anions could retard the formation of rutile. Later, in Cheng et al.’s work,¹³ rodlike nanoparticles of rutile were prepared by using TiCl₄ as the titanium source, and the formation of rutile was ascribed to a growth unit with negative charge and consisting of Cl⁻ anions. The study of Paola et al.¹⁴ indicated that the relative proportions of the TiO₂ polymorphic phases obtained by thermolysis of TiCl₄ in HCl solutions were mainly dependent on the total concentration of HCl in the precipitation solution. So, it is reasonable to conclude that the nature and concentration of acid played an important role in the synthesis of TiO₂. However, the preparation of TiO₂ photocatalysts by chemical reagents containing Ti is not suitable for real industrial application due to its high cost. Therefore, how to synthesize TiO₂ photocatalysts with different crystalline phases from low cost raw natural materials, especially solid wastes by using an effective method is still a challenge issue.

Ti-bearing slags such as Ti-bearing blast furnace slag (Ti-bearing BF slag) and Ti-bearing electric arc furnace molten slag (Ti-bearing EAF slag) etc., are considered as valuable secondary resources in China due to their relatively higher Ti contents. Considering the cost of the solid wastes, Li et al.^15,16 proposed a novel process which includes molten NaOH treatment, hydrolysis, acidolysis and calcination, to prepare nanostructured TiO₂ photocatalyst from Ti-bearing EAF slag. According to above analysis, it is known that the textural properties of obtained TiO₂ from chemical reagents determined as a function of the nature and concentration of the inorganic acid used for the synthesis. So, the effect of acidic medium used in the acidolysis process on the prepared TiO₂ should not be neglected in the novel system. However, the influence of acid species and concentration on crystalline phase and morphology of TiO₂ synthesized from Ti-bearing EAF slag was seldom investigated, especially the transformation mechanism of TiO₂ in different inorganic acids with different concentrations were not clear.

In this paper, nanostructured TiO₂ were synthesized from Ti-bearing EAF slag. The effects of acid species and concentrations on the crystalline phases and morphologies of TiO₂ were systemically investigated. And the TiO₂ crystal phase transformation mechanism and morphology evolution in different acid systems during the acidolysis process and concentrations were discussed in detail. In addition, the photocatalytic properties of as-prepared TiO₂ in different acid systems were also examined.

2. Experimental

2.1 Materials

The Ti-bearing EAF slag samples were provided by Panzhihua Steel Company (Sichuan Province, China), which were obtained from vanadium–titanium magnetite concentrate through direct reduction reaction by rotary hearth furnace and smelting separation by electric furnace. The slag's main chemical compositions and phase structure were shown in Table 1 and Fig. 1, respectively. As shown in Fig. 1, it is indicated that the Ti-bearing phases of the slag were anosovite solid solution (M_xTi_yO₅, x + y = 3, M = Mg, Fe, Ti) and Mg₂TiO₄, while the main impurity phases were MgAl₂O₄ and amorphous metasilicate.

Table 1 Main chemical compositions of the Ti-bearing EAF slag wt%

Composition	TiO2	Al2O3	MgO	SiO2	CaO	Fe2O3
Content	50.9	19.4	12.9	8.0	5.4	2.9

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Fig. 1 XRD pattern of the Ti-bearing EAF slag.

All the chemical regents employed were analytical grade (Sinopharm Chemical Reagent Co. Ltd) and the distilled water was used throughout the experiment.

2.2 Sample characterization

Chemical compositions were analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES, TELEDYNE Leeman Labs). The phase structure was investigated by X-ray diffraction using Cu Kα radiation (λ = 0.154056 nm) with 40 kV, 200 mA and a speed of 10° min⁻¹ (XRD, M21X, MAC SCIENCE Co. Ltd, Japan). The morphology and microstructure of the samples were examined by field emission scanning electron microscopy (SEM, Zeiss, Supra-55) and transmission electron microscopy (TEM, FEI, F20), the amount of ionic doping in products was analyzed by energy-dispersive X-ray spectroscopy attached with SEM (EDS, Zeiss, Supra-55). X-ray photoelectron spectroscopy (XPS, AXIS Ultra) was used to analyze the atomic surface state on each sample with Al Kα X-rays. The BET surface area was determined by the N₂ adsorption/desorption isotherms (Micrometrics, ASAP 2010). The absorption spectra of the samples were tested by a UV-vis spectrophotometer (Persee, TU-1901).

2.3 Experimental process

2.3.1 Synthesis of nanostructured TiO₂ and separation principle. The general flow sheet of the process for synthesis of nanostructured TiO₂ from Ti-bearing EAF slag was illustrated as Fig. 2. In detail, the Ti-bearing EAF slag was ground to about 120 mesh and mixed with NaOH homogeneously with Ti-bearing EAF slag/NaOH mass ratio (R_slag/NaOH) at 1 : 1.5 in the nickel crucible. Then, the nickel crucible was placed into a muffle furnace and heated to 700 °C, holding for 1 h with free access to air. After the molten NaOH treatment process, the nickel crucible was taken out rapidly and then cooled at room temperature. The obtained slag was called alkali fusion slag. According to our previous research, it is known that the main composition of the alkali fusion slag were Na₂TiO₃, NaMO₂ (M = Ti, Mg and Fe), NaAlO₂ and amorphous sodium silicate, and the corresponding reactions of Ti-bearing EAF slag with the molten NaOH can be described as follows:

M_xTi_yO₅ + 4NaOH → Na₂TiO₃ + 2NaMO₂ + 2H₂O, (x + y = 3, M = Ti, Mg, Fe) (1)

MgAl₂O₄ + 2NaOH → 2NaAlO₂ + MgO + H₂O (2)

MgSiO₃ + CaSiO₃ + 4NaOH → 2Na₂SiO₃ + CaO + MgO + 2H₂O (3)

Fig. 2 Flow sheet of the process for synthesis of nanostructured TiO₂ from Ti-bearing EAF slag.

Then the alkali fusion slag was ground to about 200 meshes and 3 g ground alkali fusion slag was leached in 300 mL distilled water with magnetic stirring for 1 h. After that, the slurry was filtered and the residue was dried in oven at 80 °C for 24 h. During this process, the water soluble sodium salts such as Na₂SiO₃ and NaAlO₂ could be separated from the water insoluble sodium titanate salts based on their different solubility in water. In the meantime, most of Na₂TiO₃ would be converted to Na_2−xH_xTiO₃ according to the following eqn (4)¹⁷ and the rest still existed as Na₂TiO₃:

Na₂TiO₃ + 2H₂O → Na_2−xH_xTiO₃ + 2NaOH (4)

As for the acidolysis process, 200 mL different acid solutions with different concentrations were mixed with the residue obtained from water leaching in flask and then heated to boiling by heating jacket with standard reflux apparatus for 6 h. After filtration, hydrated TiO₂ was obtained, and the related reactions during this procedure can be illustrated as follows:

Na₂TiO₃ + 4H⁺ → Ti(OH)₂²⁺ + 2Na⁺ + H₂O (5)

Na_2−xH_xTiO₃ + (4 − x)H⁺ → Ti(OH)₂²⁺ + (2 − x)Na⁺ + H₂O (6)

Finally, the obtained residue was dried in oven at 80 °C for 24 h and calcined in muffle furnace at 500 °C for 1 h to prepare nanostructured TiO₂. It should be noted that the acidolysis process was the key step to determine the type of crystalline phase and morphology of obtained nanostructured TiO₂ from Ti-bearing EAF slag. Therefore, this paper was mainly focus on the acidolysis process of sodium titanate with different types of acid at different concentrations in order to elucidate the transformation mechanism of nanostructured TiO₂ clearly.

2.3.2 Photocatalytic activity measurements. The photocatalytic activity of obtained nanostructured TiO₂ from Ti-bearing EAF slag with different acids was investigated by measuring the photodegradation efficiency of rhodamine B aqueous solution. Photocatalytic reactions under the visible light irradiation were carried out in a quartz glass reactor equipped with a magnetic stirrer and a collimated light source. 0.05 g as-prepared TiO₂ and 200 mL rhodamine B aqueous solution (5 mg L⁻¹) were placed into the reaction vessel. O₂ was continuously bubbled into the solution for 15 min before switching on the lamp and throughout the occurrence of the photocatalytic reactions. At the given intervals of parallel light irradiation, the samples of 5 mL were withdrawn from the solution and the catalyst was separated from the solution by filtration. The quantitative determination of rhodamine B solution was performed by measuring its absorption with a UV-vis spectrophotometer. In addition, the control experiments without photocatalysts and without light irradiation were also conducted (ESI,† 2).

The photodegradation efficiency (η) was evaluated as the follows:

where A₀ and A_t (mg L⁻¹) are the concentrations of rhodamine B solution at the beginning and after illumination for t (min) time, respectively.

3. Results and discussion

During the acidolysis process, the reactions occurred between the residue (Na₂TiO₃/Na_2−xH_xTiO₃) and acid solution can be regarded as liquid–solid reactions (eqn (5)–(7)). The residue obtained from water leaching process might be dissolved in acid solution and then Ti ions would form TiO₂ crystal nucleus on the surface of undissolved residue by heterogeneous nucleation, and in the meantime, ion exchange reaction would also occur between the rest undissolved residue and H⁺ to form hydrated TiO₂, which can be converted to TiO₂ by drying and calcination. The two competitive reactions, namely, dissolution reaction and H⁺ ion exchange reaction would happen simultaneously. According to the distribution of titanium species at different pH values (Fig. 3)¹⁸, it is shown that Ti element has three existence forms including Ti(OH)₂²⁺, Ti(OH)₃⁺ and Ti(OH)₄ when the solution pH value is below 7. Since the acidolysis process was conducted at pH value below 1, the Ti element would mainly exist as Ti(OH)₂²⁺ ions in the solution, which means that the dissolution reaction (eqn (5)–(7)) between Na₂TiO₃/Na_2−xH_xTiO₃ and acid solution would be the key reaction during the acidolysis process.

Fig. 3 The distribution of titanium species without anions in solutions with different pH values (reprinted with permission from (ref. 18); Copyright@2001 Elsevier).

Considering the anions existed in acid solution, the Ti(OH)₂²⁺ would combine with H₂O or anion such as Cl⁻, NO₃⁻ or HSO₄⁻ to form a complex six-fold coordinated monomer with octahedral coordinated structure, which has the formula [Ti(OH)₂R_n(OH₂)_m]²⁻ⁿ, where n + m = 4 and R = Cl⁻, NO₃⁻ or HSO₄⁻. Then, the presence of aquo and hydroxo ligands in the coordination sphere of this complex allows the condensation by olation, which involves the elimination of aquo ligands and formation of dimers and small chains. These dimmers or small chains would link together by further condensation to process oxolation reaction with HR elimination and formation oxo bridges between octahedrons, resulting in the precipitation of polymers with cross-linked networks. The polymers gave rise to, by dehydration, TiO₂ with different crystalline phases. In addition, since the anions existed in acid solution have different spatial radius and steric hindrance effect, they could play a role of template to help the formation of TiO₂ crystal with different morphologies. So, the acidolysis process was the key step to determine the type of crystalline phase and morphology of the final products.

3.1 Effect of acid species during the acidolysis process

In this part, different inorganic acids such as HCl, H₂SO₄ and HNO₃ were used to investigate the effect of acid species on the crystalline phase of the obtained nanostructured TiO₂. For the sake of identical H⁺ concentration, HCl, HNO₃, H₂SO₄ solutions with respective concentrations of 0.9, 0.9, and 0.45 mol L⁻¹ were used to examine the function of different anions. Fig. 4 gave the XRD patterns of samples obtained from different acids. It can be seen that the all products were TiO₂ with different crystalline phases. Specifically, anatase type TiO₂ was mainly obtained from H₂SO₄ solution while rutile type TiO₂ was gained from HCl solution. Both of the two types including anatase and rutile TiO₂ was achieved from HNO₃ solution and the amount of anatase was more than that of rutile.

Fig. 4 XRD patterns of TiO₂ synthesized from Ti-bearing EAF slag in different acid systems.

Generally, rutile, anatase and brookite are three polymorphs of TiO₂ existed in nature. However, the fundamental structure units in these crystals are all [TiO₆] octahedrons, only their modes of arrangement and link are different. In rutile, [TiO₆] octahedrons link by sharing an edge along the c axis to form chains and then corner-shared bonding among chains leads to a three-dimensional framework. In anatase, the formation of a three-dimensional framework is all with edge-shared bonding among [TiO₆] octahedrons. The structure of brookite is slightly complicated and contains either edge-shared bonding or corner-shared bonding.¹⁹

Bearing the structures of the titania polymorphs in mind, it is obvious that linear chains can only form rutile type nuclei, while skewed chains can only form anatase type nuclei.²⁰ The mechanism of the formation of anatase and rutile from [TiO₆] octahedrons can be illustrated by Fig. 5. It was reasonable to assume that R in [Ti(OH)₂R_n(OH₂)_m]²⁻ⁿ occupied a position while OH and H₂O occupied b position to illustrate the polymerization process between [Ti(OH)₂R_n(OH₂)_m]²⁻ⁿ monomers. Since the ion radius sorting was HSO₄⁻ > OH⁻ > NO₃⁻ > Cl⁻, the Cl⁻ would be eliminated after all the water molecular were removed (the chloride ions being more strongly bound groups than hydroxy ligands). In that case, when the concentrate of H⁺ and amount of Cl⁻ were identical in HCl system, Ti(OH)₂Cl₂(OH₂)₂ octahedrons would prior to link together by sharing equatorial edges with aquo ligands elimination to form linear chains. Then, a three-dimensional network was formed by sharing vertices with HCl elimination, resulting in the formation of rutile type TiO₂ (ref. 19 and 21) as indicated in Fig. 5a. However, the ion radius of HSO₄⁻ was larger than that of OH⁻, so, HSO₄⁻ in Ti(OH)₂(HSO₄)₂(OH₂)₂ monomers would be eliminated with aquo ligands elimination and formation of skewed chains.²² Then the skewed chains polymerized further by oxolation led to the formation of the anatase type TiO₂ as shown in Fig. 5b. Because the ion radius of NO₃⁻ was between that of HSO₄⁻ and Cl⁻, it means both of the linear chains or skewed chains might be formed in HNO₃ system. As a result, the sample obtained from HNO₃ system consisted of rutile and anatase TiO₂.

Fig. 5 The formation mechanism of anatase and rutile from octahedron formed in different acids systems.

Fig. 6 gave the SEM images of samples synthesized in different acid systems. It can be seen that the sample obtained in H₂SO₄ was spherical particles with many protuberances on their surfaces (Fig. 6a). The samples synthesized in HCl or HNO₃ system was nearly all needle-like nanorod clusters (Fig. 6b and c).

Fig. 6 SEM images of TiO₂ synthesized from Ti-bearing EAF slag in different acid systems (a) H₂SO₄; (b) HCl; (c) HNO₃.

Fig. 7 illustrated TEM images of samples obtained in different acid systems. It is shown that each spherical particle obtained in H₂SO₄ was composed of many nanoparticles with average size of 5 nm (inset in Fig. 7a). The lattice spacing (0.35 ± 0.005 nm) of each nanoparticle shown in Fig. 7a was assigned to the (101) planes in anatase type TiO₂, indicating the nanoparticle was single crystalline structure of anatase. In addition, the samples synthesized in HCl or HNO₃ systems both had needle-like structures with average length of 50 nm (shown as insets in Fig. 7b and c). From the HRTEM images (insets in Fig. 7b and c), it can be seen that the lattice spacing were about 0.32 ± 0.005, which belonged to the (110) planes in rutile type TiO₂, further confirming that the as-prepared nanorod structures were rutile type TiO₂.

Fig. 7 TEM and HRTEM images (insets) of TiO₂ synthesized from Ti-bearing EAF slag in different acid systems (a) H₂SO₄; (b) HCl; (c) HNO₃.

The products with different morphologies were determined by the structure of the polymorphs and their organization during the crystal nucleus growth process. Since HSO₄⁻ or SO₄²⁻ has huger spatial radius and larger steric hindrance effect compared with NO₃⁻ and Cl⁻ ions, which can adsorb on every crystal face resulting in the crystal growth without orientation.¹⁰ So, the sample obtained in H₂SO₄ system existed as spherical clusters. However, the ion radius of NO₃⁻ and Cl⁻ were smaller and the charge number of them were less compared to SO₄²⁻, indicating that NO₃⁻ and Cl⁻ prefer to adsorb on the (110) and (101) planes, leading to these ions have stronger ability to restrain the radial growth of the crystals.^16,23 As a result, the samples obtained in HCl or HNO₃ systems had needle like structures.

3.2 Effect of H₂SO₄ concentration during the acidolysis process

Fig. 8 gave the XRD patterns of samples prepared in H₂SO₄ system with different H₂SO₄ concentrations. It can be seen that only the diffraction peaks of anatase type TiO₂ appeared when the acid concentration was 0.3 mol L⁻¹, suggesting that the sample obtained in H₂SO₄ solution with concentration fixed at 0.3 mol L⁻¹ was single phase of anatase type TiO₂. With the increasing of H₂SO₄ concentration to 0.6 mol L⁻¹, a weak diffraction peak indexed to rutile type TiO₂ appeared at 2θ = 25.38°, indicating that mixtures of anatase and rutile type of TiO₂ with a higher fraction of anatase were obtained. Further increasing H₂SO₄ concentration to 0.75 and 1 mol L⁻¹, the diffraction peak intensity of rutile type TiO₂ at 2θ = 25.38° became strong, suggesting that the amount of rutile type TiO₂ was increased with the H₂SO₄ concentration increasing.

Fig. 8 XRD patterns of TiO₂ synthesized from Ti-bearing EAF slag with different H₂SO₄ concentrations.

Depending on the analysis of the XRD patterns, it is known that the product obtained in H₂SO₄ system would change from anatase to rutile type TiO₂ spontaneously with the increasing of H₂SO₄ concentration. On the basis of the crystal nucleus formation mechanism shown in Fig. 5, a possible transformation mechanism of anatase changed to rutile type TiO₂ was proposed, which could be illustrated as Fig. 9.

Fig. 9 The mechanism of phase transformation from Ti(OH)₂(HSO₄)₂ to anatase and rutile in different acid concentration solutions.

According to the (ref. 22), titanium would combine with SO₄²⁻ or HSO₄⁻ to form Ti(OH)₂SO₄(H₂O)₃ or Ti(OH)₂(HSO₄)₂(H₂O)₂ complex ions in H₂SO₄ system. In their opinion, this structure prefer to link together to form an infinite zigzag of –Ti–O–Ti–O– chains (Fig. 9a). Titanium is octahedrally coordinated by two bridging oxygen atoms, an oxygen atom from each of three sulphate ions, and one water molecule. Based on the aforementioned analysis, the zigzag of –Ti–O–Ti–O– chains were skewed chains, which was prior to form the anatase type TiO₂ by edge-shared polymerization (Fig. 9b). However, when the H₂SO₄ concentration increased, the formed zigzag chains of [TiO₆] octahedrons might be resolved into detached [TiO₆] octahedron or their small clusters (Fig. 9c). These detached [TiO₆] octahedrons or small clusters might link to each other by sharing vertices during the restacking process, resulting in three-dimensional lattices with corner-shared bonding (Fig. 9d). Thus, rutile type TiO₂ appeared in concentrated H₂SO₄ system. So, TiO₂ nanostructures obtained in H₂SO₄ system would be apt to change from anatase to rutile with the increasing of H₂SO₄ concentration.

Fig. 10 is SEM images of samples obtained in H₂SO₄ system with different acid concentrations. It can be seen that the sample existed as granular mosaic structure when the acid concentration was 0.3 mol L⁻¹ (Fig. 10a). With the acid concentration increasing to 0.6 and further to 1.0 mol L⁻¹, the morphologies of the obtained samples changed to spherical clusters with many protuberances on their surfaces (Fig. 10a–c). Associated with the XRD patterns shown in Fig. 8, it was reasonable to assume that the protuberances on the surfaces of spherical clusters were the morphologies of rutile type TiO₂, which transformed from anatase to rutile type TiO₂ with increasing H₂SO₄ concentration. The rutile has anisotropic characteristic and prefers to grow along c axis, forming nanorod structures,^20,24 which would result in protuberances on the surfaces of already formed spherical clusters. Fig. 11 illustrated TEM images of samples obtained in H₂SO₄ system with acid concentration at 0.3 and 1.0 mol L⁻¹, respectively. As shown in Fig. 11a, the prepared sample was composed of spherical nanoparticles when the H₂SO₄ solution concentration was 0.3 mol L⁻¹. The lattice spacing (0.35 ± 0.005 nm) of each nanoparticle shown in the HRTEM image (insets in Fig. 11a) was assigned to the (101) planes in anatase type TiO₂, indicating the nanoparticle was single crystalline structure of anatase. However, when the H₂SO₄ solution concentration increased to 1.0 mol L⁻¹, there were some nanorod structure appeared on the surfaces of obtained spherical nanoparticle clusters. From the HRTEM image (insets in Fig. 11b), it can be seen that the lattice spacing were about 0.32 ± 0.005, which belonged to the (110) planes in rutile type TiO₂, further confirming that the as-prepared nanorod structures were rutile type TiO₂.

Fig. 10 SEM images of samples obtained in sulfuric acid system with different acid concentrations (a) 0.3 mol L⁻¹; (b) 0.6 mol L⁻¹; (c) 0.9 mol L⁻¹; (d) 1.0 mol L⁻¹.

Fig. 11 TEM and HRTEM images (insets) of TiO₂ synthesized from Ti-bearing EAF slag in sulfuric acid system with different acid concentrations (a) 0.3 mol L⁻¹; (b) 1.0 mol L⁻¹.

Based on the aforementioned analysis, it is reasonable to conclude that both the acid species and concentration played important roles in the formation of different TiO₂.

3.3 Photocatalytic properties of samples obtained from different acid systems

Fig. 12a gives the UV-vis absorption spectra of samples obtained from different acid systems. The absorption spectra of the samples had some degree of red shift. And the corresponding absorption edges of samples obtained in H₂SO₄, HCl and HNO₃ system were 396, 430 and 404 nm, respectively. Their band gap energies can be calculated by the following equation:

αhν = A(hν − E_g)ⁿ (9)

where h is the Planck's constant, ν is the frequency of light, A is a constant, and n is equal to 2 for an allowed indirect transition or 1/2 for an allowed direct transition.²⁵ As for TiO₂, the transition is direct and therefore (αhν)² is plotted as a function of hν from which the band gap energy could be attained by Tauc Plot. The band gap energies estimated by extrapolating a straight line to the abscissa axis from the plots and the results were shown in Fig. 12b. The band gap energies of TiO₂ (3.06, 3.02 and 2.95 eV) were all nearly narrower than that of pure TiO₂ (3.20 eV), which might be ascribed to the doping of different ions in the solution. From the EDS analysis results in Table 2, it can be seen that Fe and Si were doped into the samples with different weight percentages, leading to the band gap energies of TiO₂ decreasing.

Fig. 12 (a) UV-vis absorption spectra of TiO₂ obtained from Ti-bearing EAF slag in different acid systems; (b) (αhν)² is ploted as a function of hν from which the band gap energy is obtained by Tauc Plot.

Table 2 EDS analysis of samples synthesized from Ti-bearing EAF slag in different acid systems

Element	Weight%
	HCl	HNO3	H2SO4
C K	26.54	23.70	23.91
O K	34.91	40.32	36.17
Ti K	35.12	32.47	38.01
Si K	0.50	0.67	—
Fe K	2.92	2.84	1.90
Total	100.00	100.00	100.00

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In order to confirm the doping of these of elements, XPS spectroscopy was utilized to detect elements and determine their chemical states in prepared samples. From the wide survey spectra of the samples synthesized from different acids, titanium and oxygen peaks were identified obviously, and the carbon peak was attributed to the adventitious hydrocarbon as shown in Fig. S1.† Fig. 13 showed the Ti 2p, O 1s, Fe 2p and Si 2p high resolution spectra of the as-prepared TiO₂. It should be noted from Fig. 13c and d that Fe and Si elements co-exist in the TiO₂ samples and the valence states were +3 (Fe) and +4 (Si), respectively. The binding energies of the double peaks were around 458.3 eV and 464.1 eV for Ti 2p_3/2 and Ti 2p_1/2. The energy position of this doublet only corresponded to the Ti⁴⁺ oxidation state, suggesting that none of Ti³⁺ existed in all the products.

Fig. 13 XPS spectra for (a) Ti 2p, (b) O 1s, (c) Fe 2p and (d) Si 2p of pure TiO₂ and as-prepared TiO₂ in different kinds of acids.

In addition, it can be clearly seen from Fig. 13a that the binding energy of Ti 2p_2/3 peak for as-prepared TiO₂ was 458.5–458.8 eV, which was 0.2–0.5 eV greater than that of pure TiO₂. The decrease of the electron density around Ti atom might be due to the greater electronegativity of Si via O acting on Ti.²⁶ The shielding effect was weakened, and then the binding energy was increased. This result proved the formation of Ti–O–Si bond in the as-prepared TiO₂. In addition, the substitution of Fe³⁺ for tetravalent Ti⁴⁺ in TiO₂ and formation of Ti–O–Fe bond would create oxygen deficiency, also leading to the gradual peak shift of Ti 2p.

As the O 1s spectra shown in Fig. 13b, the pure TiO₂ of O 1s was located at 529.5 eV, corresponding to lattice oxygen of TiO₂. The shift of O 1s spectra from 529.5 eV (for pure TiO₂) to 529.8–530.1 eV (for as-prepared TiO₂) suggested that Fe³⁺ or Si⁴⁺ ions were doped into TiO₂.

Based on the aforementioned analysis, it is reasonable to assume that Fe³⁺ and Si⁴⁺ ions were co-doped into TiO₂.

Fig. 14 gives the absorption spectra of rhodamine B solutions in the presence of TiO₂ obtained in different acid systems. It is shown that all the absorbances of rhodamine B solutions decreased obviously with prolonging the illumination time, suggesting that rhodamine B solutions can be photodegraded by the TiO₂ samples under the visible light illumination. Moreover, the maximum absorption wavelengths of the degraded solutions exhibited hypsochromic shifts to a certain degree, which might be ascribed to the stepwise formation of a series of N-deethylated intermediates.²⁷

Fig. 14 Absorbance spectra of rhodamine B solutions in the presence of TiO₂ obtained in different acid systems (a) H₂SO₄; (b) HNO₃; (c) HCl.

According to the formular (8), the corresponding photodegradation efficiencies of TiO₂ photocatalysts obtained in different acid systems were shown in Fig. 15. The relatively higher photodegradation efficiency of the TiO₂ obtained from H₂SO₄ system can reach 90.52%, while that of the other two samples synthesized from HNO₃ and HCl were 85.22% and 79.65%, respectively. This case might be attributed to the different crystal phases and specific surface areas of as-prepared TiO₂. The larger specific surface area may generate more active sites for adsorbing oxygen atoms to form active hydroxyl groups, which leads to the photocatalytic activity improved.²⁸ As shown in Fig. 6 and 7, the TiO₂ obtained from H₂SO₄ was spherical particle composed of many nanoparticles with average size of 5 nm, resulting in larger specific surface area of 162 m g⁻¹. Whereas, the samples synthesized in HCl and HNO₃ both were needle-like nanorod clusters with average length of 50 nm, leading to decreased specific surface area of 152 and 150 m g⁻¹, respectively. Moreover, the band gap energy of the sample obtained from H₂SO₄ was the largest (3.06 eV), implying it had higher redox potential than that of the others, which could also increase the photodegradation efficiency correspondingly.

Fig. 15 The photodegradation efficiencies of nanostructured TiO₂ obtained from different acid solutions.

In addition, the control experiments without photocatalysts and without light irradiation were carried out to confirm the photocatalysis property of as prepared TiO₂ and the results were shown in Fig. S2.† It can be seen from Fig. S2a† that the adsorption–desorption equilibrium between catalyst and rhodamine B can be reached in only 15 min and the adsorption rate was about 11.8%. As shown in Fig. S2b,† the photodegradation efficiency of rhodamine B under visible light irradiation without catalyst was only about 13% in 90 min. Therefore, the as-prepared TiO₂ photocatalysts played key role in degradation of rhodamine B.

4. Conclusions

Nanostructured TiO₂ photocatalysts with different crystalline phases and morphologies were synthesized from Ti-bearing EAF slag. The influences of acid species and H₂SO₄ concentrations during the acidolysis process on the crystal phases and morphologies of TiO₂ were studied. And the TiO₂ crystal phase transformation mechanism and morphology evolution in different acid species and concentrations were discussed in detail. It is shown that rutile type TiO₂ with needle-like nanorod structures and anatase type TiO₂ with spherical nanoparticle structures were preferred to be obtained from HCl and H₂SO₄, respectively, while two types coexisted with needle-like nanorod structures in HNO₃. Moreover, anatase type TiO₂ would be apt to change to rutile type TiO₂ with increasing of H₂SO₄ concentration. Moreover, the photocatalytic properties of nanostructured TiO₂ photocatalysts were evaluated by photodegradation of rhodamine B solutions. The nanostructured TiO₂ synthesized from H₂SO₄ showed better photocatalytic activity than that from HCl or HNO₃, and its photodegradation efficiency can reach 90.52% in 90 min for rhodamine B solution.

Acknowledgements

This study was supported by the National Basic Research Program of China (no. 2014CB643401, 2013AA032003), the National Nature Science Foundation of China (nos 51372019, 51277025 and 51072022).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra13942a

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