Introduction

The azo dye contaminated wastewater is produced in the dyeing and printing industries. The wastewater needs to be purified to remove the hazardous contaminates. If the wastewater contains toxic compounds, the bio-chemical technique cannot be applied to treat the wastewater [1,2,3,4]. The advanced oxidation techniques, e.g. photocatalytic oxidation, were applied to remove various organic pollutants in the wastewater [5,6,7,8]. Among the various kinds of photocatalytic materials, titanium dioxide was believed to be the most promising photocatalyst [9,10,11,12,13].

The expensive photocatalyst cannot be used only once in the wastewater treatment plant, so that the collection of the materials from the water is essential to the application of the photocatalytic technique. The collection of the fine photocatalyst powders from the water is difficult and inefficient [14, 15]. The titanium dioxide was deposited on different substrates or supports to facilitate the separation process. Nevertheless, the supported photocatalyst was normally not as powerful as the small TiO2 powders [16,17,18]. The interaction between the photocatalyst and the support was worthy of investigation, and the influences of the support on the activity of the TiO2 must be clarified.

The TiO2 in the supported form could be reused for water treatment due to the prompt separation of the photocatalyst and the water. Furthermore, if the activity of the supported TiO2 can be enhanced too, the supported TiO2 will inevitably find the wide application in the large-scale water treatment plant. The TiO2 nano-crystals were synthesized on the HZSM-5 zeolite to facilitate the photocatalyst-water separation in this work. The materials were characterized and were used to degrade the Acid Red 1 (AR1) dye. An interesting finding of this work was the promising activity of the supported photocatalyst, i.e. TiO2-HZSM-5 (TOHZ).

Experimental details

TOHZ preparation

The preparation of the TOHZ followed the general sol–gel procedure. The first solution was an ethanol aqueous solution (8 mL anhydrous ethanol + 3 g cetyltrimethyl ammonium bromide + 2 mL tetrabutyl titanate + 0.1 mL hydrochloric acid). The second solution was the mixture of anhydrous ethanol (4 mL) and water (0.9 mL). The HZSM-5 (Nankai Catalyst Corporation, China; SiO2/Al2O3 ratio = 25) was added in the solution A, and the mixture was stirred for 60 min. The solution B was mixed with the former mixture under stirring to form a gel. The gel was dried at 110 °C for 12 h, and then the gel was calcined for 3 h at 450 °C. The powders were gently ground before use. The TOHZ composite contained 30% titanium dioxide.

Characterization methods

The SEM observation was taken on a FEI QUANTA 250 scanning electron microscope. A transmittance electron microscope (FEI G2 20 TEM) was applied for the high-resolution observations. The FT-IR spectra were recorded using a Frontier spectrometer. The elemental analyses were obtained on an ESCALAB X-ray photoelectron spectroscopy (250Xi). A UV–Vis spectrometer (LAMBDA 35) was used to study the material’s band positions.

Photocatalytic AR1 degradation

The AR1 (C18H13N3NaO8S2) was used to study the degradation efficiency on the TOHZ. The material was mixed with 50 mL AR1 solution (40 mg/L) in a quartz beaker. The material and AR1 solution were stirred in the dark until the dye concentration was stable. After that, the UV lamp irradiated at 253.7 nm to initiate the reaction. The dye concentration was determined by the absorption spectrum (721E).

Results and discussions

SEM and TEM morphologies

Fig. 1 gives the surface morphologies of the TiO2, TOHZ and HZSM-5. The nano-sized anatase TiO2 crystals aggregated to form large particles in the size more than 12 μm, as shown in Fig. 1a. Some small particle were also in the material as the result of grinding. The HZSM-5 was composed of regular-shaped particles and small fragments (Fig. 1b). The typical HZSM-5 particle was in the size of 2 × 3 × 6 μm3. As presented in Fig. 1c, the TiO2 and HZSM-5 were combined in the TOHZ. Although the zeolite particles could still be seen in the image, the surface of the zeolite particles was not as smooth as the surface in Fig. 1b. When the HZSM-5 particles were mixed in the sol, the tetrabutyl titanate was adhered on the zeolite particles. The hydrolysis of the tetrabutyl titanate took place on the HZSM-5 surface. Therefore, the zeolite provided the surface as the nucleation center for the sol–gel process. The formation of TiO2 crystals also originated on the zeolite’s surface, and there was a restrained aggregation of the TiO2 nano crystals [19].

Fig. 1
figure 1

SEM photos of a TiO2, b HZSM-5, c TOHZ. (SEM condition: FEI QUANTA 250, HV 25 kV, WD 10 mm, Mag 2500 ×)

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The cross-section TEM image of TOHZ is shown in Fig. 2. The HZSM-5 surface was tightly coated with a TiO2 layer in the thickness of approximately 100 nm. The distinct boundaries of the TiO2 layer and the HZSM-5 surface demonstrated the strong combination of the two materials. The Ti–OH network was constructed on the HZSM-5 surface, and the crystallization of the anatase TiO2 crystals occurred on the HZSM-5 surface too. The nano-sized TiO2 crystals could be distinguished in the image, while the crystallite size was approximate the value calculated using the X-ray diffraction results.

Fig. 2
figure 2

TEM image of the cross section of TOHZ. (TEM condition: FEI Tecnai G2 20, 200 kV)

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Material composition

Fig. 3 presents the infrared absorptions of the HZSM-5, TOHZ and TiO2 in the medium and far infrared spectra. The absorption of the stretching vibration of the hydroxyl group at 3429 cm−1 and the absorption of the bending vibration of the hydroxyl group at 1631 cm−1 were attributed to the surface adsorbed water. These two absorptions of the HZSM-5 and TOHZ had higher intensity than the absorptions of the TiO2, since the porous HZSM-5 zeolite was a good adsorbate of water. The HZSM-5 zeolite was composed of tetrahedral Al-O and Si–O, which had the antisymmetrical stretching vibrations at 796 and 1232 cm−1. The absorptions at 1064 and 439 cm−1 were also related to the stretching vibration of the Si(Al)-O bonds. The antisymmetrical stretching vibration of the penta-ring in the HZSM-5 led to the absorption at 545 cm−1. The strong absorption centered at 336 cm−1 was related to the stretching vibration of the Ti–O bond. The surface water was important to the photocatalyst since the photogenerated hydroxyl radicals were transformed from hydroxyl groups. The infrared spectra did not provide any proof for the interaction between the TiO2 and the HZSM-5.

Fig. 3
figure 3

Infrared spectra of HZSM-5, TOHZ and TiO2. (Condition: PE Frontier FT-IR/FIR spectrometer, wavenumber step 1 cm−1)

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The chemical environments of the TOHZ are presented in the XPS spectra, as shown in Fig. 4. Fig. 4a gives the XPS spectrum of titanium, including the Ti2p1/2 orbital and Ti2p3/2 orbital. The electron binding energies of these two orbitals were 464.4 and 458.7 eV. The titanium XPS spectrum represented the typical Ti4+ oxidation state in the TOHZ [20]. Fig. 4b gives the XPS spectrum of oxygen, including both the oxygen in the HZSM-5 and TiO2. The maximum absorption peak at 529.8 eV was attributed to the Ti–O bond in the TiO2. The binding energies of 530.5 and 531.9 eV were for the hydroxyl group. The Si(Al)-O bonds in the HZSM-5 were represented by the binding energy peaks at 532.6 and 533.2 eV. The chemical environments of TiO2 were not influenced by the HZSM-5.

Fig. 4
figure 4

XPS spectra of TOHZ. a Ti2p, b O1s. (Condition: ESCALAB 250Xi, Al Kα, pass energy 100.0 eV, energy step 1.000 eV)

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Band positions

Fig. 5 schematically illustrates of the band positions of the TOHZ. The band positions were calculated using the equations, ECB = X—EC − 1/2Eg and EVB = ECB + Eg [21, 22]. The band gap Eg of the TOHZ was 3.08 eV, which was obtained using the UV–Vis diffuse reflectance spectrum. The ECB (conduction band edge) and the EVB (valence band edge) were − 0.63 and 2.45 V, respectively. Since the OH−/·OH oxidation potential was 1.89 V and the O2/O2−· reduction potential was O2 − 0.13 V, the ·OH and O2−· were produced on the excited TOHZ.

Fig. 5
figure 5

Schematic illustration of the band positions of the TOHZ. The band gap Eg of the TOHZ was 3.08 eV (UV–Vis diffuse reflectance spectrum: LAMBDA 35 UV–vis spectrometer, scanning speed 480 nm/min)

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AR1 removal.

Fig. 6 illustrates the influences of the TOHZ dosage on AR1 decoloration, including both adsorption and degradation. The adsorption of the dye constantly increased with rising TOHZ dosage. Nearly 20% of the AR1 molecules could be removed by adsorption in the presence of 500 mg/L TOHZ. The adsorption of the dye was the initial step for the subsequent degradation reaction, and the adsorbed AR1 molecules could eventually be degraded as well. Besides the dye removal by adsorption, the degradation of the AR1 molecules was the key character of the photocatalyst. The AR1 degradation efficiency steadily increased when the TOHZ dosage was less than 300 mg/L. After 30 min of reaction, 45.4% of the AR1 molecules were degraded in the solution using 300 mg/L of TOHZ. If the TOHZ dosage was raised further, the AR1 degradation efficiency increased quite slowly.

Fig. 6
figure 6

Effects of TOHZ dosage on AR1 decoloration (AR1 solution 50 mL, 40 mg/L; reaction time 30 min; UV light 253.7 nm)

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Fig. 7 shows the reusability of the TOHZ on AR1 degradation. The reaction time was 30 min and the TOHZ dosage was 300 mg/L. 5 mL of the AR1 solution was taken to determine the AR1 concentration after each treatment cycle. Subsequently, another 5 mL AR1 stock solution was added to restore the initial concentration. The degradation efficiency slightly decreased after each cycle, and 73.7% of the TOHZ’s activity was retained after five cycles.

Fig. 7
figure 7

Reusability of the TOHZ on AR1 degradation (AR1 solution 50 mL, 40 mg/L; reaction time 30 min; TOHZ dosage 300 mg/L; UV light 253.7 nm)

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Conclusions

The TiO2 nano-crystals were synthesized on the HZSM-5 zeolite to prepare the TOHZ. The distinct boundaries of the TiO2 layer and the HZSM-5 surface demonstrated the strong combination of the two materials. The size of the crystallites of titanium oxide in TOHZ decreased in comparison with pure titanium oxide. The infrared spectra and the XPS spectra did not provide any proof for the chemical interaction between the TiO2 and the HZSM-5. The •OH and O2−• could be produced on the excited TOHZ due to the conduction band and valence band positions. The AR1 degradation efficiency steadily increased with rising TOHZ dosage below 300 mg/L. The degradation efficiency slightly decreased after each treating cycle, and 73.7% of the TOHZ’s activity was retained after five cycles.