2.2. Characterization
Powder X-ray diffraction (XRD) measurements were performed on a Bruker D8 Focus diffractometer with Cu Kα radiation and a Lynx Eye detector at a scanning rate of 2°/min. Field emission scanning electron microscopy (FESEM) images and scanning electron microscopy with an energy dispersive X-ray analytical system (SEM-EDX) were taken using Hitachi S-4800 scanning electron microscope. Transmission electron microscopy (TEM) images were taken using an FEI Tecnai F20 transmission electron microscopy operated at an accelerating voltage of 200 kV. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out using a SDTQ-600 thermal analyzer from room temperature to 800 °C at a heating rate of 10 °C/min under air flow. The nitrogen adsorption-desorption isotherm was obtained on AutoSorb iQ-C TCD analyzer (BET). The UV–vis spectra were recorded by using a Hitachi U–3900 H UV–visible spectrophotometer.
2.3. Photocatalytic measurements
For the photocatalytic measurement, 20 mg TiO2 nanoparticles were completely dispersed into 200 mL of rhodamine B (RhB) aqueous solution (10 mg/L). The sample was firstly magnetically stirred in the dark for 1 h to ensure absorption equilibrium of MB onto the surface of TiO2 nanoparticles. A 300 W xenon lamp providing UV–vis light with intensity 90 mW/cm2 was used as the UV light source. After given time intervals, 3 mL solution were taken out and centrifuged to remove the photocatalyst. The RhB concentration in the irradiated solution was monitored by measuring the absorption intensity of the supernatant at 553 nm. The degradation degree was evaluated according to C/C0 × 100%, where C0 and C represent the absorbance of the RhB solution before and after irradiation, respectively.
3. Results and discussion
3.1. Characterization results
The preparation of hierarchically mesoporous TiO2 nanomaterials can be divided into three steps as shown in Fig. 1. The first step is the formation of a gel through crosslinking of the polymer by addition of the NaAlg solution to the Ti(SO4)2 aqueous solution. It is worthwhile to mention that the addition method has a great impact on the macroscopic shape of the gel. The dropwise addition of the NaAlg solution forms spheres of Ti-alginate gel with a diameter of about 5 mm, while extruding the NaAlg solution continuously results in noodle shapes of Ti-alginate gel. The resultant Ti-alginate gel is translucent because of the permeation of water or air in it (Fig. 1a). The following step is to dry the gel in a freeze dryer, and then obtain the substance with a volume shrinkage of 60% (Fig. 1b). Finally, hierarchically mesoporous TiO2 materials are prepared after calcination in muffle furnace. Moreover, this sintering step induces an additional 50% volume shrinkage of Ti-alginate substance (Fig. 1c).
Fig. 1