Enhanced Photodecomposition of Methylene Blue in Water with Sr 1 − x K x TiO 3 − δ @ PC-polyHIPEs under UV and Visible Light

Photocatalytic method was investigated to remove water pollutant methylene blue (MB) produced in textile, plastic, and dye industries. PC-polyHIPEs were prepared by light-induced polymerization of dopamine in transparent polyHIPEs which were synthesized by polymerization within high internal phase emulsions. Sr1−xKxTiO3−δ (x = 0–0.5) nanoparticles were incorporated and adhered to PC-polyHIPEs to form Sr1−xKxTiO3−δ@PC-polyHIPEs for the first time. The catalysts were characterized by XRD, FTIR, TGA, UV-Vis DRS, and SEM and their photocatalytic properties for MB decomposition were measured over UV-Vis spectrometer. The PC-polyHIPEs were of interconnected porous structure with around 100 μm pores and 30 μm windows. Sr1−xKxTiO3−δ@PC-polyHIPEs showed excellent MB decomposition activity under either UV or visible light although Sr1−xKxTiO3−δ alone worked only under UV light.When x = 0.3, Sr1−xKxTiO3−δ@PC-polyHIPEs showed the highest photocatalytic performance due to the existence of more oxygen vacancies. When the water solution with 50mg LMB and 1.6 gcat. L −1 Sr0.7K0.3TiO3−δ@PC-polyHIPEs was exposed to visible light for 160min at room temperature, 88.3% of MB was decomposed. After being used for eight cycles, 87.6% activity of fresh Sr0.7K0.3TiO3−δ@PC-polyHIPEs still remained. The influences of salinity, temperature, and catalyst concentration on the catalytic activity were studied. For MB decomposition under visible light, the activation energy of Sr0.7K0.3TiO3−δ@PC-polyHIPEs was calculated to be 12.3 kJmol −1 and the kinetics analysis revealed that the photocatalysis followed the second-order reaction. These findings demonstrated that Sr1−xKxTiO3−δ@PC-polyHIPEs were an effective candidate for real application in decomposition of MB in water.


Introduction
Methylene blue (MB) dye is produced as a water pollutant from textiles, cosmetics, printing, dyeing, food processing, and paper-making industries [1,2].Discharge of this colored effluent presents a major environmental problem for developing countries because of its toxic and carcinogenic effects on living beings [1].There are several technologies adopted to remove MB in water like biological degradation, adsorption, Fenton's reagent method, coagulation processes, photocatalytic decomposition, and so on [3].Among these, photocatalytic method to degrade these pollutants is the most desirable as no second pollutants are generated in this process [4].SrTiO 3 has been an attractive catalyst for organics degradation under UV light [5][6][7][8].However, there are some disadvantages hindering its real application, such as easy precipitation and agglomeration [9], low photocatalytic efficiency under visible light [10][11][12], and the difficulty of collection and separation [13].To overcome such disadvantages, incorporation of SrTiO 3 into a catalyst support and substitution of potassium ion (K + ) for part of Sr 2+ in SrTiO 3 to generate oxygen vacancy (Sr 1− K  TiO 3− ) may favour the improvement of its photocatalytic activity.
Up to now, different kinds of inorganic porous catalyst supports have been reported [14], but little porous polymer catalyst support has been used.Compared to inorganic supports, polymer owns the following advantages and is more suitable for water treatment: low density, big pores, inertia to acids and bases, flexibility, low cost, and high viscosity [15][16][17][18].Polydopamine coated polyHIPEs (PC-polyHIPEs) are a type of macroporous polymer, which has been confirmed to be an excellent catalyst support and has large attachment to inorganic catalyst particles.It is synthesized by light inducing polymerization of dopamine in transparent polyHIPEs obtained via polymerization within high internal phase emulsions (HIPEs) [19][20][21][22][23].The transparent substrate and macropores facilitate the adsorption of light, the mass transfer of substances like gases or liquids, and the host-guest interactions [24].
In this paper, with incorporation of Sr 1− K  TiO 3− nanoparticles into PC-polyHIPEs, we obtained an efficient and durable material for photodecomposition of MB in model wastewater under visible light.

Preparation of PC-polyHIPEs.
PC-polyHIPEs were prepared according to the method described in our previous paper [22].In a word, it was obtained by light inducing polymerization of dopamine in transparent polyHIPEs which was made via polymerization within high internal phase emulsions.

Preparation of Sr 1−x K
x TiO 3− Nanoparticles.Sr 1− K  TiO 3− nanoparticles were prepared by means of sol-gel.Typically, 1 mL of Ti(OBu) 4 was mixed with 3 mL of ethanol and 0.3 mL of anhydrous acetic acid to form a titanium precursor solution.Sr(NO 3 ) 2 and KNO 3 aqueous solution (1 M) were added dropwise to the as-prepared titanium precursor solution with the expected stoichiometry at room temperature.The resulting solution was stirred to turn into milky sol and finally transformed to gel.Afterwards, the wet gel was dried at 120 ∘ C overnight, and then the foamy solid was heated to 200 ∘ C in air in a tube furnace at a rate of 5 ∘ C min −1 to remove the organic ligands.Successively, the precursor was heated to 450 ∘ C and kept for 3 h to decompose the nitrates and calcined at 850 ∘ C for 4 h in air.Finally, it was ground to obtain Sr 1− K  TiO 3− nanoparticles.

Preparation of Sr 1−x K
x TiO 3− @PC-polyHIPEs.The photocatalyst Sr 1− K  TiO 3− @PC-polyHIPEs was fabricated by chemical impregnation method [25].In short, 100 mg of Sr 1− K  TiO 3− nanoparticles was dispersed into 500 mL of mixed solvents (ethylene glycol : ethanol = 7 : 3, V/V) under sonication for 1 h, and then 80 mg of PC-polyHIPEs was immersed into the as-obtained solution.After 24 h, the asformed Sr 1− K  TiO 3− @PC-polyHIPEs were collected and washed repeatedly followed by drying at 110 ∘ C for 12 h.The dried Sr 1− K  TiO 3− @PC-polyHIPEs were then centrifuged in water with 7000 rpm to make sure that only the stabilized imbedded Sr 1− K  TiO 3− remained onto the PC-polyHIPEs.
2.5.Characterization.X-ray diffraction test was recorded on X'pert Powder diffractometer (XRD, PANalytical Company, Holland), using Cu  ( = 0.1541 nm) radiation at 40 kV and 30 mA with a step scan of 0.065 ∘ .The Fourier transform infrared (FTIR) spectra were carried out on a Bruker Vertex 70 FTIR spectrometer from 4000 to 600 cm −1 with a resolution of 4 cm −1 .The morphologies were observed with scanning electron microscopy (SEM, JSM-7800F, JEOL) at 3 kV.The Brunauer-Emmett-Teller (BET) specific surface area, pore volume, and pore width were operated on BET Tristar 3000 analyzer (Micromeritics, United States).TGA analyzer (Q50, TA instrument) was used to measure the weight loss and differential TG curves of the samples.Each time, about 5 mg of the sample was loaded in the platinum crucible and heated in air from room temperature to 800 ∘ C at a heating rate of 10 ∘ C min −1 .The UV-Vis diffuse reflection spectroscopy (UV-Vis DRS) of samples was measured with a UV-Visible system (Agilent Varian Cary 100) equipped with a labsphere diffuse reflectance accessory.
2.6.Photocatalytic Activity Evaluation.The photocatalytic activities of Sr 1− K  TiO 3− nanoparticles and Sr 1− K  TiO 3− @ PC-polyHIPEs were evaluated by decomposition of MB.In each run, Sr 1− K  TiO 3− nanoparticles or Sr 1− K  TiO 3− @ PC-polyHIPEs were, respectively, dispersed into 100 mL MB aqueous solutions and then kept at dark for 2 h to establish the adsorption-desorption equilibrium.The initial concentration  0 of MB was recorded after the equilibrium was established.Then all the experiments were conducted at room temperature with pH = 7 at various irradiation time under UV or visible light with an initial MB concentration of 50 mg L −1 .The photocatalytic process was made over a homemade light exposure system under UV at wavelength of 365 nm or visible light at 450 nm.The average light intensity is 3.0 mW cm −2 .The photocatalytic activity was obtained by measuring the concentration of MB solution recorded on a UV-Vis spectrometer (Cary 3 UV-Visible, Agilent, United States) at  = 664 nm.Before MB concentration measurement, for Sr 1− K  TiO 3− @PC-polyHIPEs, the sponge-like catalyst was readily removed, while, for Sr 1− K  TiO 3− catalyst, the nanoparticle suspension solution was centrifuged at 7000 rpm for 30 min to precipitate the particles to make sure the measurement is accurate.The MB degradation efficiency (%) can be calculated with the following: In the equation,  0 and  correspond to the initial concentration and the concentration of MB at time , respectively.For comparison, the amount of Sr 1− K  TiO 3− was kept the same in Sr 1− K  TiO 3− and Sr 1− K  TiO 3− @PC-polyHIPEs catalysts during the test (the loading concentration of Sr 1− K  TiO 3− would be confirmed by TGA).Absorbance (a.u.) observed that the PC-polyHIPEs owns ca. 100 m pores and 30 m windows as shown in the magnified micrograph of PC-polyHIPEs in Figure 2(b).Such kind of stable regular large scaled porous structure of PC-polyHIPEs is hard to be obtained for inorganic catalyst supports.Figure 2(c) indicates that when Sr 0.7 K 0.3 TiO 3− nanoparticles are incorporated, the interconnected macroporous structure of PC-polyHIPEs is well preserved.The higher magnification micrograph demonstrated in Figure 2(d) confirms that welldefined cubic nanoparticles structure has been successfully incorporated into the PC-polyHIPEs.The average size of these cubic primary particles is 100 nm.However, some smaller nanoparticles with diameter about 10 nm coexist.Moreover, these nanoparticles are dispersed evenly without aggregation, which indicates Sr 0.7 K 0.3 TiO 3− has strongly adhered to PC-polyHIPEs.The strong adherence between PC-polyHIPEs and Sr 0.7 K 0.3 TiO 3− nanoparticles ensures the durability of catalyst when reacting with organic compounds in water.These results demonstrate that the high viscosity of PC-polyHIPEs plays an important role in producing an inorganic/polymer composite porous catalyst.The formation of the hybrid Sr 1− K  TiO 3− @PC-polyHIPEs catalyst is confirmed by FTIR.The spectra of Sr 1− K  TiO 3− @PC-polyHIPEs, PC-polyHIPEs, and SrTiO 3 are displayed in Figure 3. Pure SrTiO 3 has an obvious peak at 1433 cm −1 .PC-polyHIPEs showed three distinct peaks at 2962, 1726, and 1159 cm −1 .Characteristic peaks appeared at 1433, 2962, 1726, and 1159 cm −1 in Sr 1− K  TiO 3− @PC-polyHIPEs, indicating that potassium is successfully incorporated into SrTiO 3 and is adhered to PC-polyHIPEs.

Results and Discussion
The BET specific surface areas (SSA) of the Sr 1− K  TiO 3− photocatalyst after K-doping are shown in Table 1.It can be seen that all the nanocatalysts Sr 1− K  TiO 3− have larger SSA after calcination at 850 ∘ C. With the increasing amount of K-doping, the SSA of Sr 1− K  TiO 3− gradually increases, and the nanocubic Sr 0.7 K 0.3 TiO 3− has the largest SSA of 31.09m 2 g −1 .However, when the amount of K substitution exceeds 30%, the SSA of the catalyst Sr 1− K  TiO 3− decreases gradually.The SSA of Sr 0.6 K 0.4 TiO 3− and Sr 0.5 K 0.5 TiO 3− is 28.16 and 25.24 m 2 g −1 , respectively.When the doping amount is more than 30%, two phases of perovskite structure Sr 1− K  TiO 3− and TiO 2 are formed, which may cause the SSA to decrease.
Taking Sr 0.7 K 0.3 TiO 3− as an example, the N 2 adsorptiondesorption curves of Sr 0.7 K 0.3 TiO 3− nanoparticles and Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs are shown in Figure 4 after cooperating with the PC-polyHIPEs.
According to IUPAC classification, the N 2 adsorptiondesorption curves of these three samples belong to the IV-type curve with hysteresis loop.The hysteresis loop of Sr 0.7 K 0.3 TiO 3− is located at 0.5∼0.96(/ 0 ), indicating that there are micropores and mesopores in Sr 0.7 K 0.3 TiO 3− .
Based on this result, the average pore width of Sr 0.7 K 0.3 TiO 3− is 20.89 nm.And the hysteresis loop position (/ 0 = 0.96) of the support PC-polyHIPEs and the composite material Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs are almost the same, indicating that the samples are of macroporous structure [26].As the pores in PC-polyHIPEs are 30∼100 m in diameter, over such material, the N 2 adsorption-desorption isothermal curve does not exactly reflect the pore situation.
The SSA of the pure support PC-polyHIPEs is 0.113 m 2 g −1 .
After adding Sr 0.7 K 0.3 TiO 3− , the SSA of the composite Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs is increased to 2.041 m 2 g −1 , indicating that the large surface of Sr 0.7 K 0.3 TiO 3− is successfully incorporated into the PC-polyHIPEs.
Figure 5 shows the TG-DTG plot of the pure Sr 0.7 K 0.3 TiO 3− nanoparticles, PC-polyHIPEs, and the composite Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs.The weight loss of pure Sr 0.7 K 0.3 TiO 3− (Figure 5(a)) is very tiny with 0.8 wt.% during the heating process from 20 to 800 ∘ C, suggesting that the as-fabricated Sr 0.7 K 0.3 TiO 3− is extremely steady.By contrast, the weight loss of PC-polyHIPEs is almost 100% (Figure 5(b)), indicating the total combustion in air.The PC-polyHIPEs exhibits marked mass loss peaks at around 264 ∘ C, 369 ∘ C, and 455 ∘ C, while the composite material Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs also has significant mass loss at the above-mentioned temperatures.After incorporating Sr 0.7 K 0.3 TiO 3− into PC-polyHIPEs, the residual weight is 45.4 wt.%, demonstrating Sr 1− K  TiO 3− constitutes 45.4 wt.% of Sr 1− K  TiO 3− @PC-polyHIPEs.

The Photocatalytic Property
of Sr 1−x K x TiO 3 @PC-polyHIPEs 3.2.1.The Effect of K-Doping.The catalytic performance of Sr 1− K  TiO 3− (x = 0∼0.5)under UV light is shown in Figure 6.The catalytic performance of Sr 1− K  TiO 3− (x = 0.1∼0.5) is better than pure SrTiO 3 .Moreover, the catalytic activity of Sr 1− K  TiO 3− increases with the addition of potassium till x = 0.3.The catalyst Sr 0.7 K 0.3 TiO 3− was the best, over which the degradation rate of MB reached 42% (/  = 0.58) when the UV-irradiation time was 20 min.However, when  > 0.3, the photodegradation activity of According to the results of Figure 6, the optimum Kdoping amount is 30% for Sr 1− K  TiO 3− catalysts.Therefore, we choose Sr 0.7 K 0.3 TiO 3− catalyst and catalyst support PC-polyHIPEs to form new composite Sr 0.7 K 0.3 TiO 3 @PC-polyHIPEs and then study its photodecomposition activity for MB under both UV and visible light.Its photodegradation performance is shown in Figure 7.When PC-polyHIPEs are added, the new photocatalyst Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs has a very high MB catalytic activity, and the degradation rate can be achieved about 71.9%only under 20 min UV-irradiation.In contrast, Sr 0.7 K 0.3 TiO 3− has a degradation rate of only 42% when irradiated under UV light for 20 min, showing that the degradation efficiency of Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs under UV light was increased by 71.4%.After incorporation, the higher efficiency of Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs under UV light can be ascribed to a better dispersion of Sr 0.7 K 0.3 TiO 3− onto PC-polyHIPEs, which increases the active sites of catalyst exposed to MB.Some small particles around 10 nm attached inside the pores of PC-polyHIPEs may cause quantum efficiency enhancement [27].
It can be observed from Figure 7 that pure Sr 0.7 K 0.3 TiO 3− has no photocatalytic activity under visible light, so do PC-polyHIPEs.However, after incorporation, Sr 0.7 K 0.3 TiO 3− @ PC-polyHIPEs have excellent catalytic efficiency for MB degradation under visible light.The MB degradation rate can reach 56.5% after visible light irradiation for 20 min, which is impossible for pure Sr 0.7 K 0.3 TiO 3− alone.When the aqueous suspension containing 50 mg L −1 MB and 1.6 g cat.L −1 Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs is exposed to visible light for 160 min at room temperature, 88.3% of MB is decomposed.
To further explore the photocatalytic activity of Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs under visible light irradiation, UV-Vis DRS spectra of Sr 0.7 K 0.3 TiO 3− @PC-polyHI-PEs and neat SrTiO 3 were recorded (Figure 8).The light absorption of SrTiO 3 ranges from 200 to 400 nm, confirming that it does not absorb visible light.However, the absorption of Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs ranges from 200 to 640 nm, covering both UV and visible range.Without addition of noble metal dopants in the Sr 0.7 K 0.3 TiO 3− perovskite, the uncommon visible light response of Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs may be explained by the dye-sensitization or ligand-to-metal charge transfer that occurs between Sr 0.7 K 0.3 TiO 3− nanoparticles and PC-polyHIPEs [28][29][30].

The Effect of Catalyst Concentration.
Since the catalyst concentration has a significant effect on the catalyst activity, the effect of the concentration of Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs on the activity of photocatalytic degradation of MB is investigated.The concentration of catalyst Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs is set as 0.4, 0.8, 1.6, and 2.4 g cat.L −1 , and the light source is visible light.The experimental results show that the degradation efficiency of the catalyst Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs increases with the increase of the catalyst concentration in the range 0.4∼1.6 g cat.L −1 , as the number of reactive sites increases with rising catalyst concentration [28].However, when the catalyst concentration reaches 2.4 g cat.L −1 , the degradation efficiency starts to drop.

The Effect of Salinity.
Nowadays, a lot of waste water from dyeing has been discharged into the sea.However, the sea water is different from the freshwater resources.It has certain salinity which can affect the activity of catalyst.Therefore, the effect of salinity on the catalytic activity of the composite catalyst is investigated in order to provide a theoretical basis for further industrial applications of the catalyst.Figure 9 shows the photocatalytic degradation properties of Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs at different salinity ( NaCl : 10, 20, 40 g L −1 ).It can be seen that the salinity of seawater affects the degradation activity of MB at different levels under visible light compared with the deionized water.
When the salinity is increased, the catalytic activity turns low.Therefore, the effect order of salinity on the degradation rate of MB over Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs under visible light from high to low is 40 g L −1 > 20 g L −1 > 10 g L −1 .It has been reported that chloride ions may combine with the photocatalyst to affect the excited electron-hole pairing, which has a hindrance effect on the photocatalytic reaction rate [31].

The Effect of Temperature.
In actual water purification process, the water temperature between summer and winter varies.In order to explore the optimum operating conditions of Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs, the effect of temperature (0∼40 ∘ C) on the photocatalytic degradation for MB under visible light is studied and its result is shown in Figure 10.
The degradation rates of Sr 0.7 K 0.3 TiO 3 @PC-polyHIPEs are 52.9%,56.5%, and 59.3% at 0 ∘ C, 20 ∘ C, and 40 ∘ C, respectively, under visible light for 20 min.It can be seen that, with the increase of temperature, the photocatalytic degradation rate is increasing.This is due to the fact that the effective collisions between Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs and MB are enhanced as the molecules become more active at a higher temperature.
The activation energy () and reaction order of catalytic reaction are extremely important to studying the process and mechanism of catalytic reactions.Accordingly, the activation energy of Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs was estimated according to the Ozawa plots.Based on the above results, the activation energy of Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs for MB decomposition under visible light is calculated to be 12.3 kJ mol −1 .The concentration of MB in the photocatalytic reaction with Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs under visible light is measured at different time with results shown in Table 2.According to the rate equation using the integral method, if the reaction followed the first-order kinetics, this  means that ln  0 / versus t (Time) should give a straight line; if the reaction followed the second-order kinetics, this means that 1/ versus t should give a straight line.As shown in Figure 11, the 1/ versus  gives a straight line; therefore, it can be concluded that the photocatalysis between Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs and MB under visible light follows the second-order reaction.

The Effect of Cycle Times.
Figure 12 displays the photocatalytic degradation of MB for 160 min reaction under visible light when Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs are utilized eight times, yet the photocatalytic activity is well preserved with 87.6% of fresh catalyst degradation ratio even after the eighth cycle.It can be concluded that Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs have excellent durability and reusability under visible light.Moreover, Sr 1− K  TiO 3 @PC-polyHIPEs own many other merits.Firstly, they are flexible and can be separated and collected easily from MB solution.Secondly, the novel material is of low density so it can be floated and easily fixed in water solution, causing high contacting area between catalyst and MB.Thirdly, it is stable either in acid or in base environment and could be suitable for various water conditions.As a kind of low-cost and environmentfriendly hybrid material, Sr 1− K  TiO 3 @PC-polyHIPEs can Cycle numbers (times) be a promising photocatalyst for water purification under visible light.

Conclusions
PC-polyHIPEs with large and interconnected pores were fabricated by light-induced polymerization of dopamine in transparent polyHIPEs which were prepared by polymerization within high internal phase emulsions.Sr 1− K  TiO 3− nanoparticles were successfully incorporated and adhered to PC-polyHIPEs to form a novel catalyst compound Sr 1− K  TiO 3− @PC-polyHIPEs.The catalyst showed excellent MB decomposition performance under either UV or visible light although Sr 1− K  TiO 3− alone worked only under UV light.When  = 0.3, Sr 1− K  TiO 3− @PC-polyHIPEs showed the highest photocatalytic performance due to the existence of more oxygen vacancies.When the aqueous suspension with 50 mg L −1 MB and 1.6 g cat.L −1 Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs is exposed to visible light for 160 min at room temperature, 88.3% of MB was decomposed.After being used for eight cycles, 87.6% activity of fresh Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs still remained.The influences of salinity, temperature, and catalyst concentration on the photocatalytic activity were studied.For MB decomposition under visible light, the activation energy of Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs was calculated to be 12.3 kJ mol −1 and the kinetics analysis revealed that the photocatalyst was the second-order reaction.These findings demonstrated that Sr 1− K  TiO 3− @PC-polyHIPEs were an effective catalyst in real application for MB degradation.

Figure 12 :
Figure 12: Degradation ratio of MB solutions in the presence of Sr 0.7 K 0.3 TiO 3− @PC-polyHIPEs at eight cycle times.