Introduction

Titanium dioxide (TiO2), as one of the most important transition-metal oxides, has attracted extensive attention for its superior physical and chemical properties and exhibits wide potential applications in various fields such as solar energy conversion [1, 2], water purification and treatment [3–5]. TiO2 is a common choice for photocatalyst candidates owing to its high photocatalytic activity, excellent chemical stability, low cost, and nontoxicity. However, due to its wide band gap energy of 3.0–3.2 eV, TiO2 is active only under near-ultraviolet irradiation [6–8]. From the viewpoint of utilizing solar light, visible light responsive TiO2-based photocatalysts are highly desired [9–12]. Moreover, TiO2 has a relatively high electron–hole recombination rate, which will decrease its photoactivity [13–15].

To solve these issues and further improve the photocatalytic performance of TiO2 catalysts in the visible region, the optical absorption of TiO2 catalysts must be adjusted to shift from the UV region to the visible-light region, enabling more photons to be absorbed and utilized in decomposing the pollutants, and the photogenerated electrons and holes must be separated into different positions on the catalysts. The advancement of the visible light response of TiO2 and decrease of the recombination rate of the photogenerated electrons and holes can be achieved by doping transition metal [16–18], inorganic dye-sensitizing [19, 20], valuable metal deposition [21–25], and coupling titania with other semiconductors [26–31].

Recently, core–shell nanocomposites have received significant attention owing to their importance in diverse fields of dye-sensitized solar cells [32], composite catalysts [33], energy storage and conversion [34, 35], and gas sensors [36]. The synthesis methods of core–shell nanocomposites include simultaneous fabrication [37, 38], sequential fabrication [39, 40], and displacement reaction fabrication [41, 42]. Inspiringly, coupling titania with other semiconductors core–shell architecture nanocomposites afford a possible solution as a result of the synergy to improve the photocatalytic performance of TiO2 catalysts. TiO2-based photocatalytic heterostructures or nanocomposites such as noble metal/TiO2 [21–25, 32], carbon/TiO2 [43, 44], metal oxide/TiO2 [45–47], and metal chalcogenide/TiO2 [48] have been proposed and studied. The previously reported TiO2-based nanocomposites display a greatly enhanced photocatalytic performance in organic pollutant degradation in the visible light region. Among these semiconductor nanomaterials, hematite (α-Fe2O3), an n-type semiconductor (E g = 2.1 eV), is the most thermodynamic stable species of iron oxides. Hematite has been intensively researched in Li-ion batteries [35], water splitting [49], environmental treatment [50, 51], and optical devices because of its favorable band gap, abundance, stability, biodegradability, low cost, and nontoxicity. α-Fe2O3/metal oxide heterostructures, such as α-Fe2O3/Ag/SnO2 [52], α-Fe2O3/ZnO [53], α-Fe2O3/CdS [54], and α-Fe2O3/TiO2 [55–58] can decrease the recombination rate of photogenerated electrons and holes [52–54]. Furthermore, the α-Fe2O3/metal oxide heterostructures can be used to improve the visible light response of nanocomposites [52–58].

Herein, we reported a new method to synthesize α-Fe2O3@TiO2 composites based on hard template-assisted method. In this paper, α-FeOOH nanorods act as hard templates to fabricate core–shell α-Fe2O3@TiO2 composite nanorods with different shell thicknesses. Then, the photocatalytic degradation Rhodamine B (RhB) is chosen as a model reaction to evaluate the catalytic performance of the as-prepared α-Fe2O3@TiO2 composites under the irradiation of UV and visible light. The present core–shell α-Fe2O3@TiO2 composite nanorods exhibit good photocatalytic activity.

Experimental section

Materials

FeCl3·6H2O, tetrabutyl titanate (TBOT), RhB, NaOH, and concentrated ammonia solution (28 wt%) were of analytical grade and purchased from Sinopharm Co. Other chemicals and solvents are reagent grade and commercially available. Deionized water was used for all experiments.

Characterization

Transmission electron microscopy (TEM) observation was conducted on a Philips TECNAI-12 instrument. Scanning electron microscopy (SEM) was applied to investigate the morphologies of samples, which were carried out with Hitachi S-4800 (Japan). X-ray diffraction (XRD) data were obtained with a graphite monochromator and Cu Kα radiation (λ = 0.1541 nm) on a D8 advance superspeed powder diffractometer (Bruker). The energy-dispersive X-ray (EDX) analysis was performed on a KEVEX X-ray energy detector. Fourier transform infrared (FTIR) spectra for the various samples were recorded on a Bruker Tensor 27 spectrometer operated at a resolution of 4 cm−1. Carefully weighed quantities of the different samples were subjected to thermogravimetric analysis (TGA) on a STA409PC (NETZSCH) TGA instrument at a heating rate of 10 °C min−1 under vacuum from 30 to 600 °C. Ultraviolet–Visible (UV–Vis) diffuse reflectance spectra (DRS) were recorded with an UV–Vis spectrophotometer using BaSO4 as the reference (Shimadzu, UV-3600). UV–Vis absorption spectra were taken by a Shimazu UV-2501 double-beam spectra photometer.

Preparation of core–shell α-Fe2O3@TiO2 composites

Synthesis of α-FeOOH nanorods

In a typical process, FeCl3·6H2O (0.675 g) and NaOH (0.5 g) were dissolved in 50 mL H2O under mechanical stirring. Then the mixture was loaded into a Teflon-lined stainless steel autoclave of 50 mL capacity. The autoclave was sealed and maintained at 180 °C for 24 h and then cooled to room temperature naturally. Then, the yellow precipitates were separated and collected by centrifugation, followed by washing with deionized water and ethanol for 3 times, respectively.

Synthesis of α-Fe2O3@TiO2 core–shell composite nanorods

The uniform TiO2 shell core–shell structures were prepared via the versatile kinetics-controlled coating method [59]. The α-FeOOH nanorods (0.075 g) were dispersed in absolute ethanol (100 mL), and mixed with concentrated ammonia solution (0.30 mL, 28 wt%) under ultrasound for 15 min. Then 1.0 mL of TBOT was added dropwise in 5 min, and the reaction was allowed to proceed for 24 h at 45 °C under continuous mechanical stirring. The resultant products were separated and collected, followed by washing with deionized water and ethanol for 3 times, respectively. Then, the obtained powders were dried at 65 °C for 24 h. The final product was obtained after calcination at 500 °C for 3 h.

Photocatalytic activity test

The photocatalytic activities of the samples were evaluated by the degradation of RhB in an aqueous solution under the irradiation of ultraviolet (UV) light and visible light. The reactions were conducted at ambient temperature and pressure. A 300 W Hg lamp (wavelength <400 nm) was employed as UV light source to trigger the photocatalytic reactions. A 250 W Xe lamp (wavelength >420 nm) was employed as simulated sunlight source. Typically, the Hg lamp was positioned 10 cm away from the reactor. The prepared α-Fe2O3@TiO2 composites (0.05 g) were dispersed in a 100 mL of RhB aqueous solution (c 0 = 1.5 × 10−5 mol L−1) in a reaction cell. Before irradiation, the suspensions were magnetically stirred in the dark for 40 min to achieve an adsorption–desorption equilibrium between the dye molecules and catalysts. The suspension was magnetically stirred before and during the illumination. At given time intervals, 3 mL solution was drawn from the reaction system. After centrifugation, the residual concentration of RhB was detected at the maximum absorption wavelength (553 nm) of RhB in the UV–Vis spectrum based on Lambert–Beer’s law.

Results and discussion

Characterization of core–shell α-Fe2O3@TiO2 composite nanorods

The morphologies of α-FeOOH nanorods, core–shell α-FeOOH@TiO2 nanorods, and core–shell α-Fe2O3@TiO2 nanorods were characterized using TEM. α-FeOOH nanorods were first synthesized with a diameter of ~75 nm and length of` ~1 μm as the hard templates (Fig. 1a). Then, via a sol–gel reaction of TBOT (added 0.75 mL) in ethanol/ammonia mixtures, core–shell α-FeOOH@TiO2 nanorods (labeled as α-FeOOH@TiO2-0.75) were obtained with a diameter of ~140 nm (Fig. 1b). The thickness of amorphous TiO2 shell is ~30 nm (inset of Fig. 1b). Last, after calcination of α-FeOOH@TiO2-0.75 at 500 °C, a new style core–shell structure of the α-Fe2O3@TiO2 composite (labeled as α-Fe2O3@TiO2-0.75) is shown in Fig. 1c. After calcination, the dehydration of α-FeOOH nanorods led to some hollows on the surface of nanorods. On the other hand, hydrous titanium oxide loses H2O molecules and forms TiO2 nanoparticles (average diameter is about 20 nm), which are coated on the surface of α-Fe2O3 nanorods.

Fig. 1
figure 1

TEM and SEM images of a, d α-FeOOH, b, e α-FeOOH@TiO2-0.75, c, f α-Fe2O3@TiO2-0.75

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The morphologies of α-FeOOH nanorods, core–shell α-FeOOH@TiO2 nanorods, and core–shell α-Fe2O3@TiO2 nanorods are also confirmed by SEM. In Fig. 1d, α-FeOOH shows rod-shape structure with the smooth surface. The core–shell α-FeOOH@TiO2 displays some degree of adhesion among these nanorods (Fig. 1e). Compared with α-FeOOH@TiO2, TiO2 nanoparticles are coated on the surface of α-Fe2O3 nanorods and the surface roughness increases (Fig. 1f).

The chemical composition of these samples was analyzed by XRD. In Fig. 2a, all the identified diffraction peaks are assigned to the phase pure α-FeOOH (JCPDS Card No. 29-0713). When the amorphous TiO2 is coated on α-FeOOH (Fig. 2b), XRD pattern still shows the diffraction peaks of α-FeOOH, indicating the thin layer of TiO2 coating. After calcinations (Fig. 2c), except 25.2° diffraction peak, the identified diffraction peaks are attributed to the phase pure rhombohedral Fe2O3 (JCPDS Card No. 33-0664). The peak at 25.2° corresponds well with the (101) diffraction of anatase TiO2 (JCPDS Card No. 21-1272).

Fig. 2
figure 2

XRD patterns of a α-FeOOH, b α-FeOOH@TiO2-0.75, c α-Fe2O3@TiO2-0.75. The vertical lines are the standard XRD patterns of α-FeOOH (JCPDS Card No. 29-0713) (black line), hematite α-Fe2O3 (JCPDS Card No. 33-0664) (red line), and anatase TiO2 (JCPDS Card No. 21-1272) (Color figure online)

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Figure 3 shows the FTIR spectra of (a) α-FeOOH, (b) α-FeOOH@TiO2-0.75, (c) α-Fe2O3@TiO2-0.75 composites. For α-FeOOH nanorods (Fig. 3a), the broad peak at 3125 cm−1 is attributable to the stretching mode of the bulk hydroxyl groups in the goethite structure (ν OH). The two peaks at 896 and 795 cm−1 are assigned to the bending modes of the bulk hydroxyl groups in (δ OH) and out (γ OH) of the (001) plane. In curve b, the typical FTIR spectrum of α-FeOOH@TiO2-0.75 is similar to the spectrum of α-FeOOH nanorods. However, after calcinations of α-FeOOH@TiO2-0.75, the strong absorption peak at 529 cm−1 corresponds to the Fe–O vibrations, confirming the forming of hematite α-Fe2O3.

Fig. 3
figure 3

a FTIR of a α-FeOOH, b α-FeOOH@TiO2-0.75, c α-Fe2O3@TiO2-0.75. b TG curves of a α-FeOOH and b α-FeOOH@TiO2-0.75

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TGA was used to determine the dehydration process of (a) α-FeOOH and (b) α-FeOOH@TiO2-0.75. In Fig. 3b, curve a displays the weight loss of α-FeOOH. Only one weight loss step ranging from 200 to 315 °C is due to the intramolecular dehydration. However, for α-FeOOH@TiO2-0.75 (curve b), two main steps of weight loss shown in curve b are observed. The first step ranging from 25 to 305.9 °C corresponds to the loss of adsorption moisture and intramolecular dehydration of α-FeOOH, and the condensation of residual –OH groups on the hydrous amorphous TiO2. The second weight loss ranging from 305.9 to 416 °C is assigned to the crystallization of anatase [60].

Tunable thickness of the TiO2 coating of core–shell α-Fe2O3@TiO2 nanorods

The shell thickness might be adjusted by changing the amount of TBOT (0.5 mL, 1.0 mL and 1.5 mL TBOT, labeled as α-FeOOH@TiO2-0.5, α-FeOOH@TiO2-1 and α-FeOOH@TiO2-1.5, respectively). In Fig. 4b and c, core–shell α-FeOOH@TiO2-1 and α-FeOOH@TiO2-1.5 show a certain degree of conglutination and aggregation. In Fig. 5a–c, TEM images of α-FeOOH@TiO2 composites display the increased coatings on the surface of α-FeOOH and the shells’ thicknesses are about 10, 50, and 100 nm, respectively. After the calcination of α-FeOOH@TiO2 composites, TEM images of α-Fe2O3@TiO2 composites (labeled as α-Fe2O3@TiO2-0.5, α-Fe2O3@TiO2-1, α-Fe2O3@TiO2-1.5) are shown in Fig. 4d–f. After the calcination, the conglutination and aggregation become heavier (Fig. 4e, f). In Fig. 5d–f, it is clearly shown that the shell thickness of TiO2 equals to the corresponding thickness of amorphous TiO2 coating. Moreover, with the increase of thickness of amorphous TiO2 coating, α-Fe2O3 nanorods are packed by large amounts of TiO2 nanoparticles to form the core–shell structure. Compared with the α-Fe2O3@TiO2-0.75 (Fig. 1c), the thicknesses of the TiO2 shells increase markedly and the shells of TiO2 become much more compact (Fig. 5e, f). Therefore, the shell’s thickness of core–shell α-Fe2O3@TiO2 nanorods could be effectively controlled by varying the amount of TBOT.

Fig. 4
figure 4

TEM images of a α-FeOOH@TiO2-0.5, b α-FeOOH@TiO2-1, c α-FeOOH@TiO2-1.5, d α-Fe2O3@TiO2-0.5, e α-Fe2O3@TiO2-1, f α-Fe2O3@TiO2-1.5

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Fig. 5
figure 5

TEM images with larger amplification of a α-FeOOH@TiO2-0.5, b α-FeOOH@TiO2-1, c α-FeOOH@TiO2-1.5, d α-Fe2O3@TiO2-0.5, e α-Fe2O3@TiO2-1, f α-Fe2O3@TiO2-1.5

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XRD data further demonstrate that the chemical compositions of these samples are α-Fe2O3@TiO2 composites (Fig. 6). Evidently, with the increase of the thickness of the TiO2 layer, the intensity of these TiO2 peaks enhance gradually. Additionally, in curve c and d, the peak at 27.4° is assigned to the (110) diffraction of rutile TiO2 (JCPDS Card No. 21-1276) and the peak intensity increases with the increase of the content of Ti. To confirm the composition of α-Fe2O3@TiO2 composites and the content of TiO2 in the composites, EDX is used to characterize α-Fe2O3@TiO2 composites and the result is shown in Fig. 7. EDX spectra showed that the main elemental compositions of nanocomposites are Fe, Ti, and O. The atom ratios Ti:Fe of α-Fe2O3@TiO2-0.5, α-Fe2O3@TiO2-0.75, α-Fe2O3@TiO2-1, and α-Fe2O3@TiO2-1.5 are determined to be 1:9.9, 1:2.8, 1:1.8, and 1:0.72, respectively, which are consistent with the increase of the shells thickness. The corresponding weight fractions of TiO2 in four composites are 9.1, 26.3, 35.7, and 58.1 %, respectively.

Fig. 6
figure 6

XRD patterns for α-Fe2O3@TiO2 obtained with different thickness TiO2 shells. a α-Fe2O3@TiO2-0.5, b α-Fe2O3@TiO2-0.75, c α-Fe2O3@TiO2-1, d α-Fe2O3@TiO2-1.5. The vertical lines are the standard XRD patterns of hematite α-Fe2O3 (JCPDS Card No. 33-0664) (red line), anatase TiO2 (JCPDS Card No. 21-1272) (blue line), and rutile TiO2 (JCPDS Card No. 21-1276) (magenta line) (Color figure online)

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Fig. 7
figure 7

EDX spectra of samples a α-Fe2O3@TiO2-0.5, b α-Fe2O3@TiO2-0.75, c α-Fe2O3@TiO2-1, d α-Fe2O3@TiO2-1.5

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Figure 8a shows the diffuse reflectance spectra (DRS) of (a) TiO2, (b) α-FeOOH, (c) α-Fe2O3, (d) α-Fe2O3@TiO2-0.5, (e) α-Fe2O3@TiO2-0.75, (f) α-Fe2O3@TiO2-1, and (g) α-Fe2O3@TiO2-1.5. In curve a, pure TiO2 only absorbs ultraviolet radiation of less than 400 nm. Pure α-FeOOH shows the visible light absorption property (curve b). Compared with TiO2 and α-FeOOH, α-Fe2O3 displays not only stronger absorption in the ultraviolet region of less than 400 nm but also adequate and strong absorption in the visible light region of 400–900 nm (curve c). From curve d to curve g, when TiO2 was coated on the surface of α-Fe2O3, α-Fe2O3@TiO2 composites show the similar the absorption properties as to α-Fe2O3. With the increase of TiO2 amount, the absorption in the visible light region is decreased. The band gap energy of the samples can be confirmed by roughly comparing to the plot in Fig. 8b, which is obtained via the transformation based on the Kubelka–Munk function [58]. In Fig. 8b, A is the absorbance, h is the Planck constant, and ν is the light frequency. According to the function, the estimated band gap values of TiO2, α-FeOOH, α-Fe2O3, and α-Fe2O3@TiO2-1.5 are 3.30 eV, 2.26 eV, 2.05 eV, and 2.10 eV, respectively. The band gap values of other α-Fe2O3@TiO2 nanocomposites are between α-Fe2O3 and α-Fe2O3@TiO2-1.5, which indicate that α-Fe2O3@TiO2 nanocomposites may be a better candidate for photocatalysts in terms of utilization of visible solar light.

Fig. 8
figure 8

a The UV–Vis diffuse reflectance spectra (DRS) of the a TiO2, b α-FeOOH, c α-Fe2O3, d α-Fe2O3@TiO2-0.5, e α-Fe2O3@TiO2-0.75, f α-Fe2O3@TiO2-1, and g α-Fe2O3@TiO2-1.5. b, c The plot of (Ahv)2 versus hv

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Effect of calcination temperature

The effect of calcination temperature was studied. In typical reaction, α-FeOOH@TiO2-0.75 composites were calcinated at 500 °C for 3 h. Then, the calcination process proceeded at 400, 600, 700, and 800 °C, respectively. The morphologies of these products are shown in Fig. 8. It is obvious that a lot of TiO2 nanoparticles adsorb on the surface of α-Fe2O3 at 400 °C (in Fig. 9a). Furthermore, some hollows appear on the surface of nanorods. At 600 °C, TiO2 nanoparticles are coated on the surface of α-Fe2O3 nanorods, forming compact TiO2 nanoparticles shell (in Fig. 9b). At higher temperature (700 °C), the number of TiO2 nanoparticles decreases and TiO2 nanoparticles are congregated and fused to form TiO2 shell (in Fig. 9c). When the calcination temperature is 800 °C, TiO2 nanoparticles disappear from the surface of α-Fe2O3 nanorods owing to the fusion of TiO2 nanoparticles to fabricate the intact and smooth TiO2 shell. The crystal structure of the samples obtained at different calcination temperature was examined using XRD and the results are shown in Fig. 10. With the increase of calcination temperature, the crystalline structure of TiO2 have changed from anatase to rutile titania. XRD patterns show that the position and relative intensity of diffraction peaks are in accordance with rhombohedral Fe2O3 and anatase or rutile titania, which confirms that the products are α-Fe2O3@TiO2 composites.

Fig. 9
figure 9

TEM images of the α-FeOOH@TiO2-0.75 after calcination at a 400 °C, b 600 °C, c 700 °C, and d 800 °C for 3 h

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Fig. 10
figure 10

XRD patterns of α-FeOOH@TiO2-0.75 calcined at different temperatures for 3 h, a 400 °C, b 500 °C, c 600 °C, d 700 °C, e 800 °C. The vertical lines are the standard XRD patterns of hematite α-Fe2O3 (JCPDS Card No. 33-0664) (red line), anatase TiO2 (JCPDS Card No. 21-1272) (blue line), and rutile TiO2 (JCPDS Card No. 21-1276) (magenta line) (Color figure online)

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Photocatalytic performance of core–shell α-Fe2O3@TiO2 nanorods

To evaluate the photocatalytic property of the as-prepared core–shell α-Fe2O3@TiO2 nanorods, the photocatalytic degradation of RhB under the irradiation of UV light and visible light is chosen as a model reaction. Pure α-Fe2O3 nanorods have no photocatalytic activity under the irradiation of UV light. From Fig. 11, it is clearly found that the concentration of RB decreased rapidly under the irradiation of UV light, in accordance with the photos of sample with different irradiation time (insert of Fig. 11a, b). The whole decolorization process required 150 min in the presence of two α-Fe2O3@TiO2 composites. The linear relations between ln(c t/c 0) and reaction time by different catalysts under irradiation of UV light are displayed in Fig. 12a, from which the reaction rate constants are calculated and listed in Table 1. As displayed in Table 1, the α-Fe2O3@TiO2-1 under the conditions displays the highest rate constant (ca. 0.030 min−1) among all the samples. It is found that the thickness of TiO2 shell affects the photocatalytic properties of α-Fe2O3@TiO2 composites. The increase of rate constant is not proportional to the shell thickness. With the increase of TiO2 thickness, the rate constants reach the saturation.

Fig. 11
figure 11

Absorption spectra of a solution of RB (1.5 × 10−5 M, 100 mL) in the presence of 50 mg of a α-Fe2O3@TiO2-0.75, b α-Fe2O3@TiO2-1.5 under exposure to UV light

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Fig. 12
figure 12

First-order kinetic plot of RhB degradation catalyzed by different catalysts under irradiation of a UV light and b visible light

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Table 1 Comparison of Photocatalytic performances of various catalysts for photocatalytic degradation of RhB
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The photocatalytic activity for the α-Fe2O3@TiO2 core–shell nanorods was also evaluated under the irradiation of visible light. The linear relations between ln(c t/c 0) and reaction time are displayed in Fig. 12b. The reaction rate constants are calculated and listed in Table 1. Pure α-Fe2O3 nanorods also have no photocatalytic activity under the irradiation of visible light. Pure TiO2 powder shows slow reaction rate under the irradiation of visible light due to its narrow absorption region. However, the reaction constant of α-Fe2O3@TiO2-1 is about 2.5 times that of pure TiO2 powder. In addition, the reaction rate constants obtained under the irradiation of visible light are smaller than those under the irradiation of UV light, which is attributed to rapid generation electron–hole pairs of TiO2 shell under the irradiation of UV light. α-Fe2O3 core cannot affect the UV light absorption of TiO2 shell. On the contrary, in core–shell structure of TiO2@α-Fe2O3, the α-Fe2O3 shell will adsorb some UV light and block rapid generation electron–hole pairs of TiO2 core [50]. The main charge-transfer processes between TiO2 shell and α-Fe2O3 core, as shown in Fig. 13, might clarify the improved visible light photocatalytic activity of α-Fe2O3@TiO2 composites. Under the visible light irradiation, TiO2 shell cannot absorb the visible light (Fig. 8a, curve a), resulting in no generation of electron–hole pairs. However, α-Fe2O3 core can be easily excited and generate charge carriers. Then, the photogenerated electrons transfer from the conduction band (CB) of α-Fe2O3 to CB of TiO2 under the effect of built-in electric field and the concentration gradient of electrons, while photogenerated holes accumulate in the valence band (VB) of α-Fe2O3 [55, 56, 58]. The negative electrons in CB of TiO2 will further react with molecular oxygen O2 dissolved in the RhB solution to generate the superoxide anion O2 −. On the other hand, the accumulated holes in VB of α-Fe2O3 will react with OH− species, generating reactive hydroxyl radicals (·OH) [55, 56, 58]. Therefore, core–shell α-Fe2O3@TiO2 composites show the efficient photocatalytic activity in visible light region. The reaction constants of core–shell α-Fe2O3@TiO2 composites are smaller than those reported in recent literatures [61–65], which might be attributed to the different reaction conditions, such as the initial concentration, catalysts dosage.

Fig. 13
figure 13

Schematic illustrating the charge-transfer process between TiO2 shell and α-Fe2O3 core under visible light irradiation

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The photocatalytic activities for the core–shell α-Fe2O3@TiO2-0.75 nanorods obtained at different calcination temperature were also evaluated under the irradiation of UV light. The linear relations between ln(c t/c 0) and reaction time are displayed in Fig. 14. From 400 °C to 800 °C, the kinetic rate constants are 11.4, 28.5, 25.4, 8.5, and 5.6 min−1, respectively. Obviously, α-Fe2O3@TiO2-0.75 nanorods obtained at 500 °C exhibit the best photocatalytic activity, because anatase titania has higher photocatalytic activity than rutile titania [66]. With the increase of the calcination temperature, the transformation of crystalline structure of TiO2 from anatase to rutile titania leads to the decline of photocatalytic activities (in Fig. 14). Furthermore, the morphology change of TiO2 shell affects photocatalytic property. At high calcination temperature, TiO2 nanoparticles are fused to form the intact and compact TiO2 shell (Fig. 9), causing the fast electron–hole recombination rate. On the contrary, at relative low calcination temperature, TiO2 shells are made up of small nanoparticles, which have a large number of grain boundaries and interfaces on the surface of α-Fe2O3 core, slowing down electron–hole recombination rate. Therefore, through the catalytic performance test, the photocatalytic performances of core–shell α-Fe2O3@TiO2 composites heavily depend on the thickness, density, and crystal structure of TiO2 coating shells.

Fig. 14
figure 14

First-order kinetic plot of RhB degradation catalyzed by α-Fe2O3@TiO2-0.75 nanocomposites prepared at different calcination temperature under irradiation of UV light

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Conclusions

In conclusion, novel core–shell α-Fe2O3@TiO2 nanorods with different shell thicknesses were synthesized using α-FeOOH nanorods as hard templates. The core–shell α-Fe2O3@TiO2 composites exhibited the excellent optical response and photocatalytic activity from the ultraviolet to the visible region. The tunable photocatalytic activities are attributed to the adjustable TiO2 shells of core–shell α-Fe2O3@TiO2 composites. The α-Fe2O3@TiO2-1 displayed the best photocatalytic activity under both UV light and visible light. The anatase titania has the higher photocatalytic activity. The proposed synthesis strategy might provide a facile and effective method for developing noble semiconductors core–shell heterojunction nanocomposites.