Visible Light Responsive Strontium Carbonate Catalyst Derived from Solvothermal Synthesis

: A single crystalline phase of strontium carbonate (SrCO 3 ) was successfully obtained from solvothermal treatments of hydrated strontium hydroxide in ethanol (EtOH) at 100 ◦ C for 2 h, using speciﬁc Sr:EtOH mole ratios of 1:18 or 1:23. Other solvothermal treatment times (0.5, 1.0 and 3 h), temperatures (80 and 150 ◦ C) and di ﬀ erent Sr:EtOH mole ratios (1:13 and 1:27) led to formation of mixed phases of Sr-containing products, SrCO 3 and Sr(OH) 2 xH 2 O. The obtained products (denoted as 1:18 SrCO 3 and 1:23 SrCO 3 ), containing a single phase of SrCO 3 , were further characterized in comparison with commercial SrCO 3 , and each SrCO 3 material was employed as a photocatalyst for the degradation of methylene blue (MB) in water under visible light irradiation. Only the 1:23 SrCO 3 sample is visible light responsive (E g = 2.62 eV), possibly due to the presence of ethanol in the structure, as detected by thermogravimetric analysis. On the other hand, the band gap of 1:18 SrCO 3 and commercial SrCO 3 are 4.63 and 3.25 eV, respectively, and both samples are UV responsive. The highest decolourisation e ﬃ ciency of MB solutions was achieved using the 1:23 SrCO 3 catalyst, likely due to its narrow bandgap. The variation in colour removal results in the dark and under visible light irradiation, with radical scavenging tests, suggests that the high decolourisation e ﬃ ciency was mainly due to a generated hydroxyl-radical-related reaction pathway. Possible degradation products from MB oxidation under visible light illumination in the presence of SrCO 3 are aromatic sulfonic acids, dimethylamine and phenol, as implied by MS direct injection measurements. Key ﬁndings from this work could give more insight into alternative synthesis routes to tailor the bandgap of SrCO 3 materials and possible further development of cocatalysts and composites for environmental applications. This work investigated the e ﬀ ects of precursor concentrations (Sr:ethanol mole ratios), solvothermal temperatures and treatment times on the properties of SrCO 3 materials and their photocatalytic degradation of MB in water under visible light irradiation, as a function of pH and temperature. Kinetic and mechanistic studies of the MB degradation process were carried out through reaction rate determination and identiﬁcation of the end-products. The photocatalytic performance of synthesized SrCO 3 was compared with that of commercially available material, in order to derive insights into the relationships between properties and catalytic activity.


Introduction
Textile industries employ over 10,000 dyes and pigments in the manufacturing of cotton, leather, clothes, wool, silk and nylon products [1][2][3]. An estimated 700,000 tons or more of synthetic dyes are thought to be annually discharged into the environment [4], causing serious water pollution as many of these dyes are toxic, highly water soluble and highly stable against degradation by sunlight or increased temperature [5]. Therefore, effective treatments of dye-contaminated water have continuingly received great attention by academic and industrial sectors. Various wastewater treatment methods have been applied to remove toxic dyes from wastewater, such as coagulation-flocculation, adsorption, membrane separation, biodegradation and oxidation processes [6]. Among these methods, photocatalytic oxidation processes have been proven to be simple and effective at organic dye decomposition, forming relatively Table 1. Synthesis, key characteristics and bandgap energy of synthetic SrCO 3 .

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This work investigated the effects of precursor concentrations (Sr:ethanol mole ratios), solvothermal temperatures and treatment times on the properties of SrCO 3 materials and their photocatalytic degradation of MB in water under visible light irradiation, as a function of pH and temperature. Kinetic and mechanistic studies of the MB degradation process were carried out through reaction rate determination and identification of the end-products. The photocatalytic performance of synthesized SrCO 3 was compared with that of commercially available material, in order to derive insights into the relationships between properties and catalytic activity.

Effects of Synthesis Conditions
Solvothermal treatments of strontium nitrate in ethanol (EtOH) were carried out at various temperatures (80, 100 and 150 • C), treatment times (0.5, 1, 2 and 3 h) and Sr:EtOH mole ratios (1:13, 1:18, 1:23 and 1 :27). From powder X-ray diffraction (PXRD) results in Figure 1, a single phase of SrCO 3 was obtained from two conditions: 2 h solvothermal treatment at 100 • C using a Sr:EtOH mole ratio of 1:18 or 1:23. These samples are denoted as 1:18 SrCO 3 and 1:23 SrCO 3 in further discussions. Notably, mixed phases of SrCO 3 and hydrated strontium hydroxides (Sr(OH) 2 ·xH 2 O, where x is the number of molar coefficient of water in strontium hydroxide solid) were obtained from all other synthesis conditions (results shown in Supplementary Materials: Figures S1 and S2). Typical diffraction peaks correspond well with (110), (111), (021), (002), (012), (130), (220), (221), (132) and (113) orthorhombic SrCO 3 lattice planes [34,36], whereas other diffraction peaks match with those of previously reported Sr(OH) 2 ·H 2 O [37] and Sr(OH) 2 ·8H 2 O phases [38]. The formation of Sr(OH) 2 xH 2 O is possibly due to adsorbed alcohol, promoting the addition of OH functional groups on the solid surface [39], upon solvothermal crystallization of Sr-containing products. FTIR spectra of the prepared Sr-containing samples are shown in Figure 2. The absorption bands located within 1700-400 cm −1 regions were attributed to the vibrations in CO3 2− groups. The strong broad absorption at 1470 cm −1 was considered to be due to an asymmetric stretching vibration, and the sharp absorption bands at 800 cm −1 and 705 cm −1 can be specified to the bending out-of-plane vibration and in-plane vibration, respectively. The weak peak at 1770 cm −1 indicated a combination FTIR spectra of the prepared Sr-containing samples are shown in Figure 2. The absorption bands located within 1700-400 cm −1 regions were attributed to the vibrations in CO 3 2− groups. The strong broad absorption at 1470 cm −1 was considered to be due to an asymmetric stretching vibration, and the sharp absorption bands at 800 cm −1 and 705 cm −1 can be specified to the bending out-of-plane vibration and in-plane vibration, respectively. The weak peak at 1770 cm −1 indicated a combination of vibration modes of the CO 3 2− groups and Sr 2+ . The sharp peak at 3500 cm −1 was assigned to the stretching mode of -OH-in Sr(OH) 2 , and the broad absorption peak around 2800 cm − SEM images of the obtained SrCO3 materials (derived from Sr:EtOH mole ratios of 1:18 or 1:23) are compared with those of commercial SrCO3 in Figure 3. Whisker-like SrCO3 and spherical particles were obtained under these respective synthesis conditions. Figure 3c highlights the relatively large rod-like particles of commercial SrCO3. Variation in particle sizes was observed in solvothermally obtained SrCO3, with particle sizes being smaller for the 1:18 SrCO3 samples. Notably, commercial SrCO3 contains much larger particles than those of the synthesized material. From literature [26,27], SrCO3 production plants utilize two common methods, the black ash method and the soda method, in conversion of celestine ore (SrSO4) to SrCO3 ( Table 1). The black ash method involves hightemperature calcination of the ore to obtain SrS, with crystalline SrCO3 solid being formed after dissolving the SrS in aqueous Na2CO3, followed by precipitation. The soda method produces SrCO3 through the two-step decomposition reaction between celestine and aqueous Na2CO3, to obtain precipitated SrCO3. From this information, as the formation of commercial SrCO3 does not require high temperatures (>150 °C) for solvent evaporation and precipitation of SrCO3, the larger grain size of the commercial SrCO3 sample is probably due to the fast solvent evaporation during the precipitation processes. SEM images of the obtained SrCO 3 materials (derived from Sr:EtOH mole ratios of 1:18 or 1:23) are compared with those of commercial SrCO 3 in Figure 3. Whisker-like SrCO 3 and spherical particles were obtained under these respective synthesis conditions. Figure 3c highlights the relatively large rod-like particles of commercial SrCO 3 . Variation in particle sizes was observed in solvothermally obtained SrCO 3 , with particle sizes being smaller for the 1:18 SrCO 3 samples. Notably, commercial SrCO 3 contains much larger particles than those of the synthesized material. From literature [26,27], SrCO 3 production plants utilize two common methods, the black ash method and the soda method, in conversion of celestine ore (SrSO 4 ) to SrCO 3 ( Table 1). The black ash method involves high-temperature calcination of the ore to obtain SrS, with crystalline SrCO 3 solid being formed after dissolving the SrS in aqueous Na 2 CO 3 , followed by precipitation. The soda method produces SrCO 3 through the two-step decomposition reaction between celestine and aqueous Na 2 CO 3 , to obtain precipitated SrCO 3 . From this information, as the formation of commercial SrCO 3 does not require high temperatures (>150 • C) for solvent evaporation and precipitation of SrCO 3 , the larger grain size of the commercial SrCO 3 sample is probably due to the fast solvent evaporation during the precipitation processes. dissolving the SrS in aqueous Na2CO3, followed by precipitation. The soda method produces SrCO3 through the two-step decomposition reaction between celestine and aqueous Na2CO3, to obtain precipitated SrCO3. From this information, as the formation of commercial SrCO3 does not require high temperatures (>150 °C) for solvent evaporation and precipitation of SrCO3, the larger grain size of the commercial SrCO3 sample is probably due to the fast solvent evaporation during the precipitation processes.  Thermogravimetric analysis (TGA) plots ( Figure 4) suggest thermal stability of all SrCO 3 samples up to 600 • C. Slight weight loss (<1%) was likely due to moisture or solvent residue [40]. The 1:23 SrCO 3 sample gives a relatively high weight loss of 0.21%, which corresponds to the removal of surface adsorbed moisture and ethanol (weight loss upon heating up to 400 • C) and the loss of ethanol from the SrCO 3 lattice at ca. 450 • C. Decomposition of SrCO 3 takes place at temperatures above 800 • C as a result of conversion to SrO. Thermogravimetric analysis (TGA) plots ( Figure 4) suggest thermal stability of all SrCO3 samples up to 600 °C. Slight weight loss (<1%) was likely due to moisture or solvent residue [40]. The 1:23 SrCO3 sample gives a relatively high weight loss of 0.21%, which corresponds to the removal of surface adsorbed moisture and ethanol (weight loss upon heating up to 400 °C) and the loss of ethanol from the SrCO3 lattice at ca. 450 °C. Decomposition of SrCO3 takes place at temperatures above 800 °C as a result of conversion to SrO. Based on the PXRD and TGA results, chemical transformation of hydrated strontium hydroxide in the presence of ethanol under solvothermal treatments leads to the formation of SrCO3 and ethanol incorporated SrCO3 materials, as proposed by the reactions below. In general, CO2 in air can react with strontium hydroxide to form SrCO3, which precipitates after the sonication step and solvothermal treatments. Ethoxide could be formed under basic conditions, resulting in an CH3CH2O⋅⋅⋅Sr 2+ ⋅⋅⋅OCH2CH3 intermediate, which is subsequently transformed to ethanol incorporated in SrCO3. Note that the amount of ethanol incorporated within the SrCO3 is sufficiently low, such that a single phase of SrCO3 was observed in PXRD pattern of the 1:23 SrCO3 sample. Based on the PXRD and TGA results, chemical transformation of hydrated strontium hydroxide in the presence of ethanol under solvothermal treatments leads to the formation of SrCO 3 and ethanol incorporated SrCO 3 materials, as proposed by the reactions below. In general, CO 2 in air can react with strontium hydroxide to form SrCO 3 , which precipitates after the sonication step and solvothermal treatments. Ethoxide could be formed under basic conditions, resulting in an CH 3 CH 2 O···Sr 2+ ···OCH 2 CH 3 intermediate, which is subsequently transformed to ethanol incorporated in SrCO 3 . Note that the amount of ethanol incorporated within the SrCO 3 is sufficiently low, such that a single phase of SrCO 3 was observed in PXRD pattern of the 1:23 SrCO 3 sample.    Figure 6a illustrates the colour removal efficiencies of 10 ppm MB aqueous solutions in the dark and under visible light irradiation after 1 h treatment with SrCO3. Similar colour removal efficiencies from treatment of MB(aq) with 1:18 SrCO3 in the dark and under light illumination suggested major adsorption processes occurred due to the wide bandgap of the 1:18 SrCO3 sample. On the other hand, the visible responsive 1:23 SrCO3 and commercial SrCO3 gave higher colour removal efficiencies under irradiation conditions than those from dark experiments, implying both adsorption and photodegradation of MB are of importance. Therefore, from these catalyst screening tests, the colour removal efficiencies of aqueous MB solutions strongly depend on the bandgap energy of SrCO3 materials and that the 1:23 SrCO3 is the most active catalyst. Figure 6b demonstrates that only low colour removal efficiencies occur due to adsorption (in the dark) and photolysis (irradiation and no SrCO3). Treatments of dye solutions with 1:23 SrCO3 is much less effective (low colour removal efficiency) under dark conditions in comparison to decolourisation under visible light irradiation. These results suggest that the main process of MB colour removal is caused by photocatalytic treatment by using the SrCO3 photocatalyst rather than adsorption. On the other hand, the visible responsive 1:23 SrCO 3 and commercial SrCO 3 gave higher colour removal efficiencies under irradiation conditions than those from dark experiments, implying both adsorption and photodegradation of MB are of importance. Therefore, from these catalyst screening tests, the colour removal efficiencies of aqueous MB solutions strongly depend on the bandgap energy of SrCO 3 materials and that the 1:23 SrCO 3 is the most active catalyst. Figure 6b demonstrates that only low colour removal efficiencies occur due to adsorption (in the dark) and photolysis (irradiation and no SrCO 3 ). Treatments of dye solutions with 1:23 SrCO 3 is much less effective (low colour removal efficiency) under dark conditions in comparison to decolourisation under visible light irradiation. These results suggest that the main process of MB colour removal is caused by photocatalytic treatment by using the SrCO 3 photocatalyst rather than adsorption. materials and that the 1:23 SrCO3 is the most active catalyst. Figure 6b demonstrates that only low colour removal efficiencies occur due to adsorption (in the dark) and photolysis (irradiation and no SrCO3). Treatments of dye solutions with 1:23 SrCO3 is much less effective (low colour removal efficiency) under dark conditions in comparison to decolourisation under visible light irradiation. These results suggest that the main process of MB colour removal is caused by photocatalytic treatment by using the SrCO3 photocatalyst rather than adsorption.  The percentage of MB colour removal after treatment with SrCO 3 photocatalyst (sample 1:23) is shown in Figure 7a. When a suspension of SrCO 3 in 10 ppm fresh MB solution was kept in the dark for 3 h, the concentration of dye slightly decreased, while the colour of the dye solution remained unchanged. It was observed that the absorption capacity of MB on the SrCO 3 surface is negligible because the specific area of the prepared SrCO 3 photocatalyst is low (9.23 m 2 ·g −1 ). Upon visible irradiation, the prepared SrCO 3 gave a high percentage of MB colour removal (>99% after 3 h visible irradiation). The percentage of MB colour removal after treatment with SrCO3 photocatalyst (sample 1:23) is shown in Figure 7a. When a suspension of SrCO3 in 10 ppm fresh MB solution was kept in the dark for 3 h, the concentration of dye slightly decreased, while the colour of the dye solution remained unchanged. It was observed that the absorption capacity of MB on the SrCO3 surface is negligible because the specific area of the prepared SrCO3 photocatalyst is low (9.23 m 2 •g −1 ). Upon visible irradiation, the prepared SrCO3 gave a high percentage of MB colour removal (>99% after 3 h visible irradiation). In order to prove that hydroxyl radicals (•OH) are the active species in the photocatalytic degradation process, experiments were conducted in the presence of a radical scavenging reagent. One such reagent, tert-butyl alcohol (tert-BuOH), if present, should significantly inhibit the oxidation of MB [41]. The result in Figure 7b indicates that after treatment for 3 h, adding tert-BuOH resulted in poor colour removal efficiencies (6.90%), whereas in the absence of the reagent very high colour removal efficiencies (>99%) were achieved. The formation of a product arising from the reaction between tert-BuOH and •OH as ascribed through a radical pathway [41] thus resulted in the poor activity, confirming that hydroxyl radicals are the important active species assisting MB degradation.

Decolourisation of Methylene Blue (MB)
The effect of pH on the MB decolourisation under visible light irradiation was examined over a range of pH 3-9. The colour removal efficiency reached 73% after 1 h treatment at pH 3, while lower colour removal efficiencies were obtained at pH 5.5 (51%), pH 7 (42%) and pH 9 (29%) over the same time period, as shown in Figure 8a. In addition, the natural logarithm of the MB concentrations was In order to prove that hydroxyl radicals (•OH) are the active species in the photocatalytic degradation process, experiments were conducted in the presence of a radical scavenging reagent. One such reagent, tert-butyl alcohol (tert-BuOH), if present, should significantly inhibit the oxidation of MB [41]. The result in Figure 7b indicates that after treatment for 3 h, adding tert-BuOH resulted in poor colour removal efficiencies (6.90%), whereas in the absence of the reagent very high colour removal efficiencies (>99%) were achieved. The formation of a product arising from the reaction between tert-BuOH and •OH as ascribed through a radical pathway [41] thus resulted in the poor activity, confirming that hydroxyl radicals are the important active species assisting MB degradation.
The effect of pH on the MB decolourisation under visible light irradiation was examined over a range of pH 3-9. The colour removal efficiency reached 73% after 1 h treatment at pH 3, while lower colour removal efficiencies were obtained at pH 5.5 (51%), pH 7 (42%) and pH 9 (29%) over the same time period, as shown in Figure 8a. In addition, the natural logarithm of the MB concentrations was plotted as a function of irradiation time, affording a linear relationship, as presented in Figure 8b. Using the first-order model, the highest rate constant of MB colour removal was obtained at pH 3, with the degradation being slowest at pH 9. The decreasing rate constants of MB decolourisation with increasing pH may be the result of the presence of carbonate (CO 3 2− ) and hydroxide (OH − ) ions, which are radical scavengers [42,43]. At pH 5.5-10, the low colour removal efficiencies may be due to the following reactions. The effect of temperature on the degradation of MB as a function of time is discussed in Figure  9. From Figure 9a, it can be observed that higher temperatures result in higher MB colour removal efficiencies. Under visible light irradiation, the MB colour removal efficiency reached 100% after 1 h treatment at 70 °C. In all cases MB concentrations decrease with irradiation time. The linear plots between the natural logarithm of the MB concentration versus irradiation time are shown in Figure  9b, which indicate that the decolourisation process follows first-order kinetics. The rate constants of MB decolourisation increased with temperature, indicating that MB removal by 1:23 SrCO3 is overall endothermic. The 1:23 SrCO3 sample is rather stable during the photocatalytic MB degradation reaction, as only negligible concentrations of Sr (<10 ppm) were detected in the treated MB solution. The effect of temperature on the degradation of MB as a function of time is discussed in Figure 9. From Figure 9a, it can be observed that higher temperatures result in higher MB colour removal efficiencies. Under visible light irradiation, the MB colour removal efficiency reached 100% after 1 h treatment at 70 • C. In all cases MB concentrations decrease with irradiation time. The linear plots between the natural logarithm of the MB concentration versus irradiation time are shown in Figure 9b, which indicate that the decolourisation process follows first-order kinetics. The rate constants of MB decolourisation increased with temperature, indicating that MB removal by 1:23 SrCO 3 is overall endothermic. The 1:23 SrCO 3 sample is rather stable during the photocatalytic MB degradation reaction, as only negligible concentrations of Sr (<10 ppm) were detected in the treated MB solution.
between the natural logarithm of the MB concentration versus irradiation time are shown in Figure  9b, which indicate that the decolourisation process follows first-order kinetics. The rate constants of MB decolourisation increased with temperature, indicating that MB removal by 1:23 SrCO3 is overall endothermic. The 1:23 SrCO3 sample is rather stable during the photocatalytic MB degradation reaction, as only negligible concentrations of Sr (<10 ppm) were detected in the treated MB solution.    The proposed reaction pathway of MB photooxidation over SrCO3 photocatalyst is outlined in Figure 11. The detected degradation products, as identified from fragments based on m/z ratio, are illustrated in blue, while undetectable but expected intermediates [44,45] are presented in black. These results are in general agreement with previous works that report the generated intermediates during the MB photodegradation process [44,45]. The proposed reaction pathway of MB photooxidation over SrCO 3 photocatalyst is outlined in Figure 11. The detected degradation products, as identified from fragments based on m/z ratio, are illustrated in blue, while undetectable but expected intermediates [44,45] are presented in black. These results are in general agreement with previous works that report the generated intermediates during the MB photodegradation process [44,45].

Degradation Products
Catalysts 2020, 10, x FOR PEER REVIEW 14 of 15 Figure 11. Proposed photocatalytic degradation pathway of MB. Detected degradation products are illustrated in blue, while expected but undetectable [44] species are presented in black.

Synthesis of Strontium Carbonate (SrCO3)
Strontium carbonate (SrCO3) was synthesized by a solvothermal method modified from the procedure of Zhang et al. [34]. A suspension of 20 g Sr(OH)2.8H2O in ethanol (100 mL) was sonicated in an ultrasonic bath for 20 min, followed by solvothermal treatment in an autoclave at 80, 100, 120 or 150 °C for 2 h. The reaction mixtures were left at room temperature to cool down to room temperature. Then, the precipitates were washed with deionized water to remove Sr(OH)2 •xH2O, dried and kept in a dry condition at room temperature. After obtaining the optimal treatment temperature, the reaction time was investigated through the above procedure by fixing the treatment temperature at 100 °C and varying reaction time between 0.5, 1, 2 or 3 h. The strontium-based samples were prepared by varying the Sr(OH)2 •8H2O: ethanol mole ratio as either 1:13, 1:18, 1:23 or 1:27, and then the above procedures were followed using a treatment temperature of 100 °C for 2 h.

Materials Characterisation
The crystallinity and the phase structure of the samples were investigated using X-ray  Figure 11. Proposed photocatalytic degradation pathway of MB. Detected degradation products are illustrated in blue, while expected but undetectable [44] species are presented in black.

Synthesis of Strontium Carbonate (SrCO 3 )
Strontium carbonate (SrCO 3 ) was synthesized by a solvothermal method modified from the procedure of Zhang et al. [34]. A suspension of 20 g Sr(OH) 2 ·8H 2 O in ethanol (100 mL) was sonicated in an ultrasonic bath for 20 min, followed by solvothermal treatment in an autoclave at 80, 100, 120 or 150 • C for 2 h. The reaction mixtures were left at room temperature to cool down to room temperature. Then, the precipitates were washed with deionized water to remove Sr(OH) 2 ·xH 2 O, dried and kept in a dry condition at room temperature. After obtaining the optimal treatment temperature, the reaction time was investigated through the above procedure by fixing the treatment temperature at 100 • C and varying reaction time between 0.5, 1, 2 or 3 h. The strontium-based samples were prepared by varying the Sr(OH) 2 ·8H 2 O: ethanol mole ratio as either 1:13, 1:18, 1:23 or 1:27, and then the above procedures were followed using a treatment temperature of 100 • C for 2 h.

Materials Characterisation
The crystallinity and the phase structure of the samples were investigated using X-ray diffractometry (PXRD, Bruker AXS model D8 advance). The measurements were examined with CuKα radiation between 2θ values of 10-80 degrees, at a scan rate of 0.075 degree·min −1 using accelerating voltage and currents of 40 kV and 40 mA, respectively. Chemical composition and bonding information were probed using Fourier transform infrared spectrophotometry (FT-IR, Elmer model lamda 800). Diffusion reflectance spectra were measured on a UV-VIS spectrophotometer (Agilent Cary 5000) using a scanning rate of 200-1100 nm. Sample morphologies were investigated using scanning electron microscopy (SEM). The thermal decomposition of SrCO 3 was monitored using a thermogravimetric analyzer (TGA, TA instruments SDT 2960 Simultaneous DSC-TGA).

Catalyst Performance Examinations
SrCO 3 samples were dispersed in 10 mL of 10 ppm MB aqueous solution in order to observe the change in colour under dark and visible light irradiation conditions. Before illumination, the suspensions were stirred in the dark for 5 min. Then, suspensions were irradiated using an LED (16 × 12 V EnduraLED 10 W MR16 dimmable 4000 K with λ > 400 nm) [46]. The colour removal efficiency of MB was monitored as a function of degradation time by measuring the absorbance of the dye solution after treatment. In order to terminate the reaction, the photocatalyst was filtered off using a syringe filter (0.45 µm). The absorbance of the dye was then measured, and the concentration of remaining MB was quantified using the absorbance at maximum wavelength (around 664.5 nm) using the Beer Lambert law.
The colour removal efficiency of MB was calculated via Equation (1): where C 0 is the concentration of fresh MB solution, and C t is the concentration of dye residue after treatment at t minutes.
Leaching of strontium ions may be a major cause of photocatalyst deactivation. Therefore, the amount of strontium ions in the filtered MB solution was quantified by flame atomic absorption spectrometry (FAAS, Perkin Elmer, Waltham, MA, USA).
A mass spectrometer (micro TOF MS, Bruker, Billerica, MA, USA) equipped with electrospray ionization (ESI) source was employed to detect MB degradation products. For this, direct injection of the treated MB solution (with 1:23 SrCO 3 ) under visible light irradiation was carried out, with fragments examined over the range m/z 50-700.

Conclusions
In this work, a solvothermal method without any calcination step was employed to prepare a single crystalline phase of strontium carbonate (SrCO 3 ). Ethanol incorporated SrCO 3 , a visible light responsive SrCO 3 material having a bandgap energy of 2.62 eV, was obtained from the solvothermal treatment of hydrated strontium hydroxide in ethanol at Sr:EtOH of 1:23. Nevertheless, the synthesis conditions strongly influence the bandgap energy of SrCO 3 , as UV responsive SrCO 3 material can also be obtained by varying the precursor concentration. The narrow bandgap SrCO 3 material can be utilized as a photocatalyst for decolourisation of methylene blue in water under visible light irradiation. Effective decolourisation of 10 ppm methylene blue aqueous solutions was achieved with >99% colour removal efficiencies after 3 h treatment, under visible light irradiation over the 1:23 photocatalyst, using a catalyst loading of 4 g·L −1 . The decolourisation is mainly due to photocatalytic processes. The rate constant values showed a direct correlation with temperature, but decolourisation was most rapid at low pH. In addition to the conventional uses of SrCO 3 in pyrotechnics and frit manufacturing, synthesized SrCO 3 materials have their place as semiconductors and cocatalysts employed in energy and environmental applications. The key findings of this work highlight that incorporated ethanol in the SrCO 3 structure results in a narrowing of the energy bandgap in SrCO 3 , with the material being a visible light responsive semiconductor and active photocatalyst in dye degradation. Results from this work may suggest alternative synthesis routes to obtain visible responsive SrCO 3 materials, for further development of new composites and cocatalysts in broader applications.