1 Introduction

Amphiphilic diblock copolymer is formed by the junction of covalent bond between one hydrophilic polymer segment and the other hydrophobic polymer segment, which can form micelles or aggregates by self-assembly in selective solvent because of the thermodynamic incompatibility between two different polymer segments and phase separation characteristics. Therefore, it can form micron- or nano-scale micro-reactor in solution with controllable size by regulating the length and property of polymer segments [1]. Meanwhile, different shapes of micro-reactor such as ball, rod and vermicular, etc. can be obtained by changing the solvent composition, pH, temperature during the amphiphilic diblock copolymer’s self-assembly [2, 3].

Poly(ε-caprolactone) (PCL) is a kind of semicrystalline polymer with good biocompatibility, biodegradability and etc. [4]. In recent years, the polymer self-assembly technology has received considerable attention, and the amphiphilic diblock copolymer prepared from hydrophobic polycaprolactone and hydrophilic polymer is the focus of attention. Poly(acrylic acid) (PAA) is a pH sensitive weak electrolyte, it can unite with nonionic polymers electron acceptor and cationic polyelectrolyte, and interact with cations [5]. Therefore, the synthesis of amphiphilic copolymer contains PCL and PAA blocks make great significance, since it can not only improve the such polymer’s performance, but also expand the its application [6].

With the continuous development of nanotechnology, multifunctional nanocomposite draws more and more attention, especially that based on block copolymer templates [7]. The preparation of nanometer materials was optimized because of the block copolymers’ properties. Several synthetic methodologies have been widely developed to prepare TiO2 material with significant properties, such as sol–gel method [8,9,10,11], micelle and inverse micelle methods [12,13,14], sol method [15, 16], hydrothermal method [17,18,19,20,21], evaporation-induced self-assembly method [21], solvo-thermal method [22, 23], chemical vapor deposition [24], physical vapor deposition [25], direct oxidation method [26], and so on. The sol–gel process is one of the most widely used methodology for preparing nanoparticles due to several advantages, such as high purity, good uniformity, low processing temperature, stability and versatility of processing [27, 28]. And the sol–gel process with diblock copolymer as template has been applied in the preparation of thin film, ceramic fiber materials, microporous inorganic membrane, porous aerogel material and functional composite [29, 30]. Nanometer TiO2 has became one of research hotspots in the field of material due to its significant physical and chemical properties. It has good stability, high catalytic efficiency, no secondary pollution, non-toxic and low cost as photocatalyst, and has special advantages in the degradation of toxic and refractory organic matter and inorganic matter. TiO2 photocatalysis material has practical application in many areas, such as the self-cleaning materials, air purification, wastewater treatment, super hydrophilic coating, etc. [31].

While, with the exponential growth of research activities, modified doping have been widely progressed, such as nonmetallic doping [32,33,34,35,36,37,38], metallic doping [39,40,41,42] and semiconductor doping [43,44,45,46,47]. Among the variety of methods targeting improve the photocatalytic activities, the semiconductor doping attracted much attention. Since the semiconductor of different width forbidden band compounded with each other, urging the separation of electronic-hole, reducing electronic restructuring, widen the range of spectral response, further improve the photocatalytic efficiency in consequence. Carcel et al. [43] reported the photodegradation for methyl orange (MO) of TiO2, WO3 and TiO2/WO3 films in different pH, and get films of improved photocatalytic efficiency with the addition of H2O2. Yang et al. [44] obtained composite that shows higher acetaldehyde degradation efficiency than mono-TiO2 in the presence of WO3, and studied the effect on the different ratio of W/Ti compounds on the degradation efficiency. Ren et al. [45] reported TiO2–SiO2 catalyst that prepared using TiOSO4·2H2O and SiO2 as precursor, and show fine catalysis activity after calcinations. Fateh et al. [46] obtained TiO2–SiO2 films via dip-coating method on polycarbonate resin template showing improved photocatalytic efficiency after the combination of SiO2.

Herein, we report the synthesis of hybrid TiO2/WO3 nanoparticles via a sol–gel process by self-assembly amphiphilic diblock copolymer PCL-b-PAA as the template, toluene as solvent, using titanium tetra-isopropoxide (TTIP) and tungsten hexachloride (WCl6) as precursor. Transmission electron microscopy (TEM), scanning electron microscope (SEM), X-ray diffraction analyzer (XRD), X-ray photoelectron spectroscopy analyzer (XPS), UV–Visible spectroscopy (UV–vis), thermal gravity analysis (TGA), etc. had been used to characterize the morphology, structure and performance of hybrid TiO2/WO3 nanoparticles. The effects of different doping ratios on photocatalytic performance of TiO2/WO3 nanoparticles were investigated in terms of degradation of methylene blue (MB).

2 Experimental

2.1 Chemicals and materials

The amphiphilic diblock copolymer, poly(ε-caprolactone)-block-poly(acrylic acid) (PCL-b-PAA) [Mw/Mn = 1.18; Mn (PCL) = 8600 g mol−1; Mn (PAA) = 4200 g mol−1] prepared in a similar procedure reported in our previous work [6], acted as a template in sol–gel process. Toluene was refluxed over sodium and distilled under a nitrogen atmosphere before use. Concentrated hydrochloric acid (HCl, 37%) and methylene blue (MB) were purchased from Sinopharm Chemical Reagent Co. Ltd. Titanium tetra-isopropoxide (TTIP) and tungsten hexachloride (WCl6) were purchased from Aladdin Chemistry Co. Ltd.

2.2 Preparation of hybrid TiO2/WO3 nanospheres

All manipulations involving low moisture conditions were carried out using Schlenk techniques. In a typical preparation, 200 mg dried PCL-b-PAA was dissolved in 12 mL toluene under nitrogen atmosphere in a Schlenk tube with a magnetic stir bar. Then, 1 mL WCl6 solution in toluene (0.1 g mL−1) was injected into the reaction system. The solution was stirred for 1 h at room temperature. After that 0.4 mL TTIP was added to the solution and stirred for 2 h. Then, 0.1 mL HCl (37%) was added into the solution. The mixture was stirred for 2 h and aged for 24 h at room temperature. The different molar ratio of TTIP and WCl6 in Ti:W = 95:5, 90:10, 85:15, 80:20 and 75:25 were employed respectively. After that the sample was annealed at 100 °C for 1 h followed by calcination at 500 °C, 600 °C or 700 °C for 5 h.

To obtain the TiO2/WO3 precursor nanoparticles for TEM observation, the mixed precursor solution in the Schlenk tube was extracted by acetone several times to remove the polymer residue. The mixture was then added into 30 mL acetone and stirred for 30 min. After centrifuging the solution, the supernatant was removed by a dropper. This process was repeated 5 times. A suspension of TiO2/WO3 precursor nanoparticles was obtained and observed by TEM.

2.3 Instruments and measurements

The as-synthesized TiO2/WO3 nanoparticles before and after calcinations were characterized by SEM (JSM-6390LV, JEOL Ltd.), TEM (JEM-1400, 120 kV, JEOL Ltd.), powder X-ray diffraction (XRD) (Cu-Kα radiation (λ = 1.5418 Å), X’pert PRO, Panalytical Co.) and X-ray photoelectron spectroscopy (XPS) (Al-Kα radiation (hν = 1486.6 eV), Thermo ESCALAB 250Xi, USA). The UV–Visible absorption spectra were recorded with a UNICO UV-2102PC spectrophotometer. Nitrogen adsorption–desorption isotherms of TiO2/WO3 nanoparticles were measured by Micromeritics ASAP 2020 surface area and porosity analyser. The photocurrent measurements of TiO2/WO3 spin-coated film were carried out with a CHI 660C workstation. The photoluminescence (PL) analyses were measured at room temperature by a Fluorolog-3-P UV–VIS-NIR fluorescence spectrophotometer illuminated with a 360 nm He–Cd laser.

2.4 Photocatalytic activities

The photocatalytic activity of the hybrid TiO2/WO3 nanoparticles was tested under visible light. A 400 W high-pressure mercury lamp was placed 10 cm away from the reaction vessel, which was used to provide a full-spectrum emission without any filter to simulate the sunlight source. The illumination intensity was 5 W m−2. The photocatalytic activities of TiO2/WO3 nanoparticles were evaluated by photodegradation of MB. The selected photocatalyst was dispersed in the MB solution (10 mg mL−1) to achieve a concentration of 1 mg mL−1. The mixed suspension was first stirred in the dark for 1 h to reach the adsorption–desorption equilibrium of MB. The concentration of residual MB was determined by recording the decrease in the maximum absorbance of MB at 654 nm after various reaction times using UV–vis spectrophotometer every 20 min.

3 Results and discussion

TiO2/WO3 precursor nanoparticles were formed inside the PCL-b-PAA micelles with PCL segment as “shell” and the PAA segment as “core” [45] which coordinated with WCl6 and TTIP. Although extracted by acetone several times, TiO2/WO3 precursors particles with irregular morphology were still formed (Fig. 1a). However, spherical nanoparticles of TiO2/WO3 could be formed (Fig. 1b) after acetone extraction followed by calcinations at 600 °C for 5 h.

Fig. 1
figure 1

SEM images of hybrid TiO2/WO3 nanoparticles (Ti:W = 85:15): a after acetone extraction without calcination; b after acetone extraction and calcination at 600 °C for 5 h

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TEM observation (Fig. 2) showed that TiO2/WO3 precursor after acetone extraction several times appeared as regular spheres with diameter size from 100 to 200 nm. Such spherical morphology indicated that the hydrolysis of TTIP and WCl6 happened in the core of the PCL-b-PAA micelles as templating agent. The effects of the molar ratio of PCL/PAA segment, the molar ratio of TTIP/WCl6/PAA and etc. on the morphology and property of the obtained TiO2/WO3 nanoparticles are under investigation.

Fig. 2
figure 2

TEM image of hybrid TiO2/WO3 precursor nanoparticles (Ti:W = 85:15) after acetone extraction several times

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Energy dispersive spectroscopy (EDS) analysis was conducted to confirm the composition of the hybrid TiO2/WO3 nanoparticles as shown in Fig. 3. The characteristic peaks of Ti, W, O and C atoms can be clearly identified. The presence of C atom is probably attributed to residual C atom produced from the incomplete removal of PCL-b-PAA even after calcinations [48,49,50].

Fig. 3
figure 3

EDS spectrum of hybrid TiO2/WO3 nanoparticles (Ti:W = 85:15) after calcination at 600 °C for 5 h

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Figure 4 shows the typical XRD patterns of the TiO2/WO3 nanoparticles calcinated at various temperatures for 5 h. At 500 °C, the anatase peaks were mainly observed in the XRD patterns. At 700 °C, the rutile crystal peaks can be clearly identified, which reveals the temperature has a great impact on the transformation of crystal structure [51]. Also the peaks of orthorhombic WO3 were observed at 700 °C, compared with the weak peaks at 500 °C and 600 °C, which may caused by the formation of amorphous WO3. The anatase-to-rutile transformation temperature has been mostly reported from 600 to 900 °C for the initiation and finishing temperature, respectively [52].

Fig. 4
figure 4

The XRD patterns of hybrid TiO2/WO3 nanoparticles calcinated at 500 °C, 600 °C and 700 °C, respectively. A denotes anatase phase of TiO2, R denotes rutile phase of TiO2 and W denotes orthorhombic WO3, respectively

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XPS spectra of TiO2/WO3 nanoparticles displays two peaks at 459.1 eV and 464.8 eV (Fig. 5a) resulted by the spin orbit split of Ti2p, indicating the existence of the Ti–O bonds. Furthermore, the measured binding energy value is higher than the standard one is attribute to the transformation of Ti–O–Ti to Ti–O–W. Figure 5b showed the result of fitting peak separation curve. The peaks at 35.5 eV and 37.8 eV are attributed to the formation of W6+, existing in the form of WO3. While, the peak at 36.3 eV is corresponded to the Ti3p.

Fig. 5
figure 5

XPS spectra of a Ti2p and b W4f and Ti3p of hybrid TiO2/WO3 nanoparticles (Ti:W = 85:15) after calcination at 600 °C. The solid lines are experimental spectra and the others are fitting ones

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Figure 6 shows the cyclic voltammetry curve of TiO2/WO3 (Ti:W = 85:15) spin-coated film after calcinations at 600 °C. The closed circular curve demonstrates the coloring-fading process is reversible. There are two obvious anodic peak, corresponding to the different active points formed when Li+ and H+ are injected to the TiO2/WO3 film.

Fig. 6
figure 6

Cyclic voltammetry curve of TiO2/WO3 (Ti:W = 85:15) spin-coated film after calcination at 600 °C

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Photoluminescence emission spectra (PL) have been widely used to investigate the efficiency of charge trapping, immigration, transfer and to understand the fate of electron–hole pairs about the semiconductor particles [53]. Figure 7 shows the PL spectra of TiO2/WO3-600 (Ti:W = 85:15, calcinated at 600 °C) and P25. It can be seen that the PL intensity of TiO2/WO3 is much lower than that of P25. This indicated that WO3 doping can effectively inhibit the recombination of photo-generated electrons and holes, in consequence the separation of light carriers inside TiO2/WO3 film is much better, and the absorption of light intensity degree is higher than that of P25, thus TiO2/WO3 might show stronger photocatalytic activity than P25.

Fig. 7
figure 7

PL spectra of TiO2/WO3-600 nanoparticles (Ti:W = 85:15, calcinated at 600 °C) and P25

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Figure 8 displays the nitrogen adsorption–desorption isotherms and pore size distribution curve of TiO2/WO3-600 and P25. The TiO2/WO3-600 in this work exhibits a type IV isotherm and a type H2 hysteresis loop [54]. The pore diameters of TiO2/WO3-600 and P25 are 8.5 and 9.9 nm, respectively, indicating the formation of mesoporous structure. The smaller pore size reflects the inhibition of grain growth and aggregation of residual carbon due to polymer carbonization [54].

Fig. 8
figure 8

BET curves of TiO2/WO3-600 nanoparticles (Ti:W = 85:15, calcinated at 600 °C) and P25

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Figure 9 shows the UV–vis absorption spectra of TiO2/WO3-600 and P25. The enhanced absorption of TiO2/WO3-600 in the visible region can be attributed to the effect of WO3 doping. The fundamental absorption band edge of P25 stopped at around 400 nm and did not show any visible light absorption, however, TiO2/WO3-600 showed an absorption extending beyond 400 nm. The difference in absorption characteristics of P25 and TiO2/WO3-600 proves that WO3 was successfully doped into TiO2 nanostructures. The visible light absorbance of TiO2/WO3-600 is attributed to the existence of tungsten species such as Ti–O–W bonds.

Fig. 9
figure 9

UV–vis absorption spectra of TiO2/WO3-600 nanoparticles (Ti:W = 85:15, calcinated at 600 °C) and P25

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In order to investigate the photo catalytic behaviour of the as-synthesized TiO2/WO3 nanopartilces, the photodegradation of MB as a model pollutant was carried out. Firstly, the MB solution (10−3 g L−1) was scanned at 200–800 nm. It found that the maximum intensity of the characteristic peak of MB at 665 nm decreased with increasing the irradiation time. So the variation of absorbance at 665 nm was used to show the photodegradation curve of MB. The photodegradation of MB by TiO2/WO3-600 with different doping ratios and P25 are shown in Fig. 10a. When the molar ratio of Ti/W is 85:15, the sample shows the best photocatalytic activity as MB can be completely photodegradation in shorter time. Besides all the doping act better photocatalytic activity compared with P25 under the same condition, this is due to the semiconductor of different width forbidden band compounded with each other, urging the separation of electronic- hole, reducing electronic restructuring, widen the range of spectral response, further improve the photocatalytic efficiency [55]. Figure 10b shows the photodegradation curve of MB by TiO2/WO3-600 in doping ratio of Ti:W = 85:15 at different calcination temperature. It has been well understood that a higher activity can be achieved by improving many aspects of TiO2-based photocatalysts, including surface area, crystallinity, absorption ability, non-mental doping and water dispersity [56, 57]. From a photocatalytic point of view, anatase is generally considered to be the more active phase due to its higher reduction potential and low re-combination rate of electron–hole pairs compared to rutile [58]. Therefore, the photocatalytic activity of TiO2/WO3-500 is better than TiO2/WO3-700. While, due to the special electronic states, the two crystal structures allow for a semiconductor–semiconductor junction [59]. TiO2/WO3-600 exhibits a higher photocatalytic activity than other photocatalysts. The TiO2/WO3-600 with the doping ratio of Ti:W = 85:15 can nearly completely degrade the MB solution (10−3 g L−1) within 100 min.

Fig. 10
figure 10

Photodegradation curves of MB catalyzed by a TiO2/WO3-600 nanoparticles (Ti:W = 95:5, 90:10, 85:15, 80:20 and 75:25 respectively, calcinated at 600 °C) and P25; b TiO2/WO3 nanoparticles (Ti:W = 85:15) calcianted at 500 °C, 600 °C and 700 °C, respectively

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4 Conclusions

Visible-light active hybrid TiO2/WO3 spherical nanoparticles were successfully fabricated via a sol–gel process using an amphiphilic PCL-b-PAA diblock copolymer as template followed by calcination. In comparison with commercial catalyst P25, the hybrid TiO2/WO3 exhibited better catalytic behaviour in the photodegradation of MB. Doping ratio of Ti:W and calcination temperature, in our case, have a great significance on the catalytic activity of such hybrid TiO2/WO3 nanoparticles. The best degradation activity was present by using the hybrid TiO2/WO3 nanoparticles with a doping ratio of Ti:W = 85:15 calcinated at 600 °C.