1 Introduction

Shape and phase-tunable inorganic metal oxides with different morphologies have drawn much attention on both relevant theories and technologies in recent years because of their fascinating optical, electrochemical properties, and photocatalytic activities and widespread applications in solid-state lightning device, supercapacitor, photocatalyst, and thermoelectric material [1,2,3,4,5,6,7]. Generally, the shape and phase-tunable inorganic metal oxides strongly depend on the synthetic method, surface reconstruction and treatment, facet control, band-gap modulation, heterostructure construction, and so on [8,9,10,11,12,13,14,15,16,17]. Strontium titanate (SrTiO3) is an important photocatalyst, it has continually received significant attention due to its low recombination, high charge carrier mobility, and high charge transfer and separation efficiency [18,19,20]. However, it has two obvious disadvantages for pure SrTiO3 which are wide band-gap of 3.2 eV and low visible photocatalytic activity for the photocatalytic degradation of organic dyes [20, 21]. Therefore, a lot of work has been done around overcoming the above-mentioned two shortcomings of pure SrTiO3 [22,23,24,25]. In the past few years, much works have been devoted to the degradation activity of SrTiO3 with different morphologies for the degradation of organic pollutant or photocatalyzed hydrogen production under xenon lamp with emitting simulated sunlight irradiation [26, 27]. Ouyang et al. [28] reported that the nanoporous SrTiO3 photocatalysts synthesized by a nano-template-assisted sol–gel hydrothermal reaction method exhibit a high photocatalytic performance for the degradation of organic pollutant under xenon lamp irradiation. Kuang et al. [29] synthesized the single-crystal-like porous SrTiO3 nanocubes by a facile hydrothermal reaction method showing an enhanced photocatalytic activity in hydrogen evolution from water splitting under xenon lamp irradiation. Zheng et al. [30] prepared the SrTiO3 hollow microspheres by hydrothermal method showing a superior photocatalytic activity in photoreduction of Cr(VI) under xenon lamp irradiation. Li et al. [31] synthesized the cubic-perovskite SrTiO3 by a facile solvothermal route displaying a facet-dependent photocatalytic activity for the degradation of rhodamine B and methyl orange under xenon lamp irradiation. Wang et al. [9] reported the surface-reconstructed SrTiO3 nanocrystals enclosed with {023} and {001} facets by introducing oxygen vacancies showing a high photocatalytic activity for the hydrogen evolution under xenon lamp irradiation. Hsieh et al. [10] prepared the shape-tunable SrTiO3 crystals by mixing TiCl4, SrCl2, and LiOH in ethanol solution showing that the surface facet control enhances photocatalyzed hydrogen production under xenon lamp irradiation. Up to now, there are only a few reports on the adjust and control for the synthesis of SrTiO3 particles with different morphologies by introducing mineral acid and adjusting reaction time for the promoted photocatalytic degradation of organic dyes.

Zhou et al. [32] reported that the TiO2 nanoheterostructures with different morphologies synthesized by HCl or H2SO4 with assisted hydrothermal method exhibit enhanced photocatalytic activity and gas-sensitive performance. Zhou and co-workers have developed a facile acid-hydrothermal method to fabricate TiO2 microspheres, nanoflowers, nanotrees, and nanobelts by varying the HCl concentration and revealed the shape-dependent tuning of photoluminescence properties. [33] Shen et al. [34] synthesized the single-crystalline TiO2 nanostructures with different morphologies by a novel inorganic acid (HCl, HNO3, or H2SO4)-assisted hydrothermal route showing the morphology, phase purity, composition, crystallite size, and photocatalytic activity of TiO2 to have a high dependency in the concentration of inorganic acid. All these studies indicate that introduction of mineral acids to adjust the phase purity and surface morphology could increase oxygen vacancy concentration and enhance optical absorption coefficient and charge transfer ability of semiconductor photocatalyst, resulting in the improved photocatalytic activity for the photocatalytic degradation of organic dyes. Motivated by this, SrTiO3 photocatalyst with controllable phase purity and morphology can be prepared by adding mineral acid.

In this paper, we have successfully synthesized different shapes and phase-tunable SrTiO3 perovskite-type metal oxides with different morphologies using various mineral acids. Particle shape control from rhombus to nanoparticles to flowers-like and mixing of cubes and nanoparticles were achieved by adopting the strategy of mineral acid and reaction time. The five SrTiO3 samples were compared for the phase structure, crystallinity, purity, shape, optical absorption coefficient, oxygen vacancy concentration, charge carrier separation efficiency, and photocatalytic activity for the photocatalytic degradation of congo red dye under ultraviolet and simulated sunlight irradiation. SrTiO3 rhombus particles have been found to exhibit the best optical absorption coefficient, charge transfer and separation efficiency, and photocatalytic activity, demonstrating that shape control is a fascinating direction for improving physicochemical properties of metal oxide semiconductor materials. Based on these results, a possible photocatalytic mechanism involved in the photo-degradation of congo red dye using pure SrTiO3, O (oxygen vacancy)—SrTiO3, or I (impurity)—SrTiO3 was proposed.

2 Experimental procedure

2.1 Synthesis of SrTiO3 with different morphologies

Stoichiometric amounts of commercial P25 TiO2 and Sr(OH)2·8H2O powders (99%)) were mixed in 60 mL deionized water to obtain mixed solution with molar ratio of Sr:Ti = 1: 1. After that 3 mL mineral acid (HNO3, HCl, or H2SO4 with analytically pure grade) and 9.9993 g sodium hydroxide with analytically pure grade were added to the above solution. All the above processes are carried out under the action of magnetic stirring. After two hours, the solution was transferred to a high-pressure reactor with polytetrafluoroethylene lining, and the precursor products were obtained at 220 °C for different reaction times (24 and 40 h). The excess alkali was removed by cooling and cleaning with deionized water and ethyl alcohol to obtain the solid product. Finally, the solid product was dried at 60 °C for 5 h to obtain the final product. The samples prepared at 3 mL HNO3 and HCl and reaction time at 24 h were denoted as S1 and S3, respectively. Whereas the samples prepared at 3 mL HNO3, HCl, and H2SO4 and reaction time at 40 h were denoted as S2, S4, and S5, respectively. Figure 1 shows the chemical route for the preparation of SrTiO3.

Fig. 1
figure 1

Chemical route for the preparation of SrTiO3

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2.2 Material characterization

The phase structure, purity, and component of Samples S1, S2, S3, S4, and S5 were analyzed by a D8 advanced type X-ray diffractometer with CuKα radiation at a wavelength of 0.15406 nm and operated at 40 kV and 30 mA. The surface morphology of Samples S1, S2, S3, S4, and S5 were characterized by a JSM-6701F-type field-emission scanning electron microscopy (FE-SEM). Ultraviolet–Visible (UV–Vis) diffuse reflectance spectra of Samples S1, S2, S3, S4, and S5 were measured on a UV–Vis spectrophotometer and BaSO4 used as the reference. X-ray photoelectron spectroscopy (XPS) was used to measure the charge state, chemical component of Samples S1, S2, S3, S4, and S5 by a KRATOS X SAM 800 X-ray photoelectron spectrometer. The electrochemical properties of Samples S1, S2, S3, S4, and S5 were investigated on a CHI760E electrochemical workstation using a three-electrode cell configuration. The experiment was similar to the literature [11] except changing the working electrode and electrolyte as SrTiO3 and 1 mol L−1 KOH aqueous solution, respectively. The slurry was evenly dispersed on the nickel foam with area of 1 cm × 1 cm and transferred to the thermostat drying oven to dry at 60 °C for 5 h. The photocatalytic degradation experiments of Samples S1, S2, S3, S4, and S5 were described in literature [11]. 100 W/m2 high-pressure mercury lamp and 150 W xenon lamp are used to emit UV-light and simulated sunlight (290 < λ < 800 nm), respectively. According to literature [35], the hole (h+), hydroxyl radical (·OH), and superoxide radical (·O2−) can be detected by disodium ethylenediaminetetraacetic acid (EDTA-2Na), 2-propanol (IPA), and 1,4-benzoquinone (BQ), respectively. The EDTA-2Na, IPA, and BQ are used as scavengers. The trapping experiment was similar to above photo-degradation experiment of congo red dye except the addition of 1 mmol scavengers in the photocatalytic reaction system.

3 Results and discussion

3.1 Structural analysis

The phase structure, purity, and composition of all SrTiO3 samples have been identified on the basis of the respective XRD patterns as shown in Fig. 2. The XRD results were fitted by MDI Jade 6.0 package. The black curve, red curve, vertical line, and blue line represents the experiment value, calculated value, Bragg peak, and the difference between the experiment and calculated values, respectively. The results indicate that the calculated results are in good match with the observed XRD diffraction peaks. All XRD diffraction peaks can be assigned to (100), (110), (111), (200), (210), (211), (220), (300), (310), (311), and (222). For the Samples S1, S2, S3, and S4, they are all pure cubic SrTiO3 perovskite-type oxides with standard JCPDF card no. 35–0734. There was no significant difference in diffraction peak position among the four samples, despite changes in the mineral acid, and reaction time [Fig. 2a, b, c, and d]. In Fig. 2e, Sample S5 presents the mixed phases of SrTiO3 with standard JCPDF card no. 35-0734 and SrSO4 with standard JCPDF card no. 05-0593. As to strong sulfuric acid (H2SO4), it was stopped being used to synthesize the SrTiO3 fine particles as H2SO4 can react easily with strontium (Sr) ion and form SrSO4 precipitates. The mean crystallite sizes (D) of Samples S1, S2, S3, S4, and S5 were calculated on the basis of line broadening of (110), (111), and (200) peaks using the Scherrer’s Eq. (1) are about to be 135.7046, 93.1833, 83.2245, 100.5236, and 38.6325 nm, respectively.

Fig. 2
figure 2

XRD patterns and crystal structure of SrTiO3. a S1, b S2, c S3, d S4, e S5, and f crystal structure

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$$D = \frac{{k\lambda }}{{\beta \cos \theta }},$$
(1)

where k is the shape factor, the value is generally between 0.62 and 2.08 on the basis of different morphologies [36,37,38], β is the full-width at half maximum (FWHM) of x-ray peak, λ is the X-ray wavelength, and θ is the Bragg diffraction angle.

The cell parameter (a) and unit cell volume (V) of Samples S1, S2, S3, S4, and S5 were also calculated by Eqs. (2) and (3).

$$\frac{1}{{d^{2} }} = \frac{{\left( {h^{2} + k^{2} + l^{2} } \right)}}{{a^{2} }},$$
(2)

where d and (h, k, l) are the interplanar spacing and plane indices of SrTiO3 crystal, respectively.

$$V = a^{3}$$
(3)

The calculated mean crystallite size, cell parameter, and unit cell volume for different samples are shown in Table 1. As can be seen from Table 1, Sample S3 has the smallest mean crystallite size, largest cell parameter, and unit cell volume than that of other samples. As for HNO3, the crystallite size decreased with the increasing of reaction time, while HCl showed the opposite trend. Figure 2f shows the crystal structure of SrTiO3. The cubic SrTiO3 perovskite-type oxides possess a Pm3m symmetry. For the ABO3 structure, the oxygen (O) and titanium (Ti) atoms form a perfect octahedron due to the O atom located in the face-center positions of cubic unit cell, while the strontium (Sr) atom lying outside the oxygen octahedron [39].

Table 1 Unit cell volume, crystallite size, interplanar spacing value, and cell parameter of Samples S1, S2, S3, S4, and S5
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3.2 Morphology analysis

The morphologies of SrTiO3 perovskite-type oxides with different acids and reaction times are displayed in Fig. 3. Figure 3a shows the typical SEM images of SrTiO3 rhombus particles with HNO3 and reaction 24 h. The rhombus particles are non-uniform, with smooth and clean surfaces. The particle size ranges up to several tens to several hundreds of nanometer. Figure 3b illustrates the SEM image of cubic-like SrTiO3 aggregates with different size ranges treated by HNO3 and reaction 40 h. The hydrothermal synthesis conditions inhibited grain irregular growth remarkably, and the particle size of cubic-like SrTiO3 aggregates clearly became polarized. The SrTiO3 nanoparticles were synthesized by the HCl-assisted hydrothermal process at 220 °C for 24 h, without using other organic additives, which is shown in Fig. 3c. The mean particle size of SrTiO3 nanoparticles is around 80 nm as shown in Fig. 3f. As the reaction time increased to 40 h, the cubic-shaped SrTiO3 particles appeared in the sample as shown in Fig. 3d. It is worth noting that increasing the reaction time of either hydrochloric acid or nitric acid in assisted hydrothermal synthesis process can cause the SrTiO3 particles to be large and small. The particle size of SrTiO3 particles observed by SEM images is larger than that of crystallite size calculated by XRD results as the particle may contain a few crystals [40]. Figure 3f shows the SEM image of SrTiO3 particles with H2SO4 and reaction at 24 h. In Fig. 3f, the flowerlike nanostructures with an average particle size of 40 nm were confirmed. Acid type and reaction time are crucial for the formation of SrTiO3 particles with different morphologies.

Fig. 3
figure 3

SEM images and particle size distribution of SrTiO3. a S1, b S2, c S3, d S4, e S5, and f particle size distribution for the Sample S3

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3.3 Plausible formation mechanism of SrTiO3 particles with different morphologies

When the inorganic acid is absent from the precursor solution, Sr(OH)2·8H2O is easy to react with the metastable phase of anatase TiO2 in the P25 TiO2 to produce cubic shape of SrTiO3 particles [41,42,43]. When the inorganic acid is introduced into the precursor solution, Sr(OH)2·8H2O can react with the inorganic acid first, and its chemical reaction equations are shown as follows.

$${\text{Sr(OH)}}_{{\text{2}}} \cdot {\text{8H}}_{{\text{2}}} {\text{O + 2HNO}}_{{\text{3}}} {\text{ }} \to {\text{ Sr(NO}}_{{\text{3}}} {\text{)}}_{{\text{2}}} + 10{\text{H}}_{{\text{2}}} {\text{O}}$$
(4)
$${\text{Sr(OH)}}_{{\text{2}}} \cdot {\text{8H}}_{{\text{2}}} {\text{O + 2HCl }} \to {\text{ SrCl}}_{{\text{2}}} + 10{\text{H}}_{{\text{2}}} {\text{O}}$$
(5)
$${\text{Sr(OH)}}_{{\text{2}}} \cdot {\text{8H}}_{{\text{2}}} {\text{O + H}}_{{\text{2}}} {\text{SO}}_{{\text{4}}} \to {\text{SrSO}}_{{\text{4}}} + 10{\text{H}}_{{\text{2}}} {\text{O}}$$
(6)

Subsequently, the titanium dioxide can react with the sodium hydroxide and strontium salt to form strontium titanate. The relevant chemical equations can be described in detail as follows:

$${\text{TiO}}_{{\text{2}}} + 2{\text{NaOH + Sr(NO}}_{{\text{3}}} {\text{)}}_{{\text{2}}} \to {\text{SrTiO}}_{{\text{3}}} {\text{ + 2NaNO}}_{{\text{3}}} {\text{ + H}}_{{\text{2}}} {\text{O}}$$
(7)
$${\text{Ti}}{{\text{O}}_{\text{2}}} + 2{\text{NaOH + SrC}}{{\text{l}}_{\text{2}}} \to {\text{SrTi}}{{\text{O}}_{\text{3}}}{\text{ + 2NaCl + }}{{\text{H}}_{\text{2}}}{\text{O}}$$
(8)
$${\text{TiO}}_{{\text{2}}} + 2{\text{NaOH + SrSO}}_{{\text{4}}} \to {\text{SrTiO}}_{{\text{3}}} {\text{ + Na}}_{{\text{2}}} {\text{SO}}_{{\text{4}}} {\text{ + H}}_{{\text{2}}} {\text{O}}$$
(9)

SrSO4 is very stable, so it is very difficult to react with TiO2 and NaOH. Therefore, the presence of SrSO4 was observed in sample S5 as shown in Fig. 2e. It is worth noting that the inorganic acid was added very little throughout the experiment. This is just one part of reaction process. Other reaction processes can be described in literature [43]. Several reaction processes in the precursors lead to the unevenness of the resulting strontium titanate particles. According to the literatures [44, 45], the TiO2 consists of TiO62− octahedra with different coordination modes to form the metastable anatase and stable rutile phases. Under the action of OH groups, TiO62− octahedron in the TiO2 is easy to bridge with it. Several TiO62− octahedrons coordinate with Sr ions in strontium nitrate to form strontium titanate with a rhombus structure as shown in Fig. 3a. For the strontium chloride, only the vertex positions of two TiO62− octahedrons are bridged with strontium ions, resulting in the formation of SrTiO3 spherical particles as shown in Fig. 3c. As the reaction time increased, the strontium titanate particles gradually regained their cubic shape due to the steric repulsion and thermodynamically stable [45]. It can be seen that the morphology of SrTiO3 highly depends on the structure of strontium salt formed during the intermediate process and reaction time.

3.4 Optical properties

Figure 4a shows the UV–Visible diffuse reflection spectra of Samples S1, S2, S3, S4, and S5. In the wavelength range of 200–320 nm, the reflectance for each sample is almost constant. In the wavelength range of 320–400 nm, the reflectance for each sample is dramatically increase. Above 400 nm, the reflectance increased slowly with increasing of wavelength. Under careful identification of UV–Visible diffuse reflection spectra in Fig. 4a, the lower reflectance of Sample S5 in the ultraviolet region was found toward shorter wavelengths, indicating that the reflectance behavior might be affected by the SrSO4 impurity. The relative parameters including color parameters (CIE L*, a*, b*), hue angle (Ho), chroma (c*), and total color difference (ΔECIE*) of Samples S1, S2, S3, S4, and S5 can be estimated by literatures [46,47,48,49] and are shown in Table 2. The result of white color is mainly governed by the color parameter L*: the larger L* value represents the more intense white color of SrTiO3 perovskite-type oxides. At the same time, the largest L* value also represents the lightness of SrTiO3 perovskite-type oxides. As can be seen from Table 2, Sample S5 has a largest L* value implying that Sample S5 has the brightest white color. The real photos of Samples S1, S2, S3, S4, and S5 are shown in Fig. 4d. The color of SrTiO3 samples is consistent with the results in Table 2.

Fig. 4
figure 4

a UV–Visible diffuse reflection spectra, b UV–Vis absorption spectra, c optical band-gap (Eg) values, and d real photos of Samples S1, S2, S3, S4, and S5

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Table 2 Color coordinates and Eg values of Samples S1, S2, S3, S4, and S5
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The UV–Vis absorption spectra of Samples S1, S2, S3, S4, and S5 were calculated using the Kubelka–Munk (K–M) equation and UV–vis diffuse reflectance spectra. Figure 4b shows the UV–Vis absorption spectra of Samples S1, S2, S3, S4, and S5. Two obvious absorption bands at 200–400 (Fig. 4b) and 400–800 nm (enlargement inset in Fig. 4b) can be observed. Two absorption bands are assigned to the intrinsic band-gap adsorption [29, 43, 50] and oxygen vacancy [9] of SrTiO3, respectively. The existence of oxygen vacancy can be further confirmed by XPS spectra. In Fig. 4b, Sample S1 exhibits the strongest ultraviolet and visible absorption coefficient, while Sample S5 shows a lowest visible absorption coefficient due to the effect of SrSO4 impurity. It can be seen that the ultraviolet and visible absorption coefficient of SrTiO3 samples follows the order: S1 > S5 > S4 > S2 > S3 and S1 > S2 > S4 > S3 > S5, respectively.

The optical band-gap (Eg) values of Samples S1, S2, S3, S4, and S5 can be obtained from α as a function of wavelength from the Tauc relation [41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56]. The Eg values of Samples S1, S2, S3, S4, and S5 can be obtained from the intercept of the straight line portion of (αhν)1/2 versus hν plot to (αhν)1/2= 0. Figure 4c shows the Eg values of Samples S1, S2, S3, S4, and S5. The Eg values of Samples S1, S2, S3, S4, and S5 are found to be 3.046, 3.072, 3.039, 3.024, and 3.061 eV, respectively. The variation tendency is inconsistent with the results of SEM observation, XRD calculation, and previous reports [57] due to the effect of different morphologies.

3.5 XPS analysis

To confirm the chemical composition and oxygen vacancy of Samples S1, S2, S3, S4, and S5, XPS spectra of Samples S1, S2, S3, S4, and S5 are measured by a x-ray photoelectron spectrometer. The survey scan XPS spectra of Samples S1, S2, S3, S4, and S5 are shown in Fig. 5a. It can be confirmed that the major elemental compositions of Samples S1, S2, S3, S4, and S5 are Sr, Ti, O, and C. The C 1s characteristic peak at 284.2 eV can be ascribed to the calibration peak of XPS instruments [58].

Fig. 5
figure 5

a XPS of survey spectra of Samples S1, S2, S3, S4, and S5. O1s high-resolution XPS spectra of Samples b S1, c S2, d S3, e S4, and f S5

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Figure 5b–f shows the O1s high-resolution XPS spectra of Samples S1, S2, S3, S4, and S5. For Samples S1, S2, S3, and S4, the O1s peak in the wide range can be resolved into two peaks at 528.71–528.92 and 530.11–530.19 eV ascribed to lattice oxygen (OL) [59] and adsorbed oxygen (OA) [60, 61], respectively. The deconvoluted O 1s characteristic peaks of Sample S5 mainly comprised oxygen in lattice oxygen about at 528.83 eV, adsorbed oxygen at 529.92 eV, and the oxygen in the sulfate group (S-O) at 531.76 eV [62] as shown in Fig. 5f. The OA/OL molar ratios of Samples S1, S2, S3, S4, and S5 are summarized in Table 3. The result shows that Sample S5 has a minimum value, which exhibits a worst photocatalytic activity. The remaining samples can only identify the existence of oxygen vacancies in the samples, but it is difficult to distinguish the concentration of oxygen vacancies.

Table 3 XPS parameters of Samples S1, S2, S3, S4, and S5
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3.6 Electrochemical properties

3.6.1 Electrochemical impedance spectroscopy (EIS) analysis

EIS technology was applied to understand the charge transfer process of SrTiO3 samples. Figure 6 shows the EIS spectra of Samples S1, S2, S3, S4, and S5. Only a straight line can be observed for the Samples S1, S2, S3, S4, and S5 as the semicircle is barely visible. The straight line can be assigned to the Warburg impedance [63]. The angle between the straight line and X-axis for the Samples S1, S2, S3, S4, and S5 follows the order: S1 > S5 > S2 > S3 > S4. This large angle usually means that the radius of the semicircle is smaller. The smallest arc radius on EIS spectra of SrTiO3 sample means a fast interfacial charge transfer and high charge separation efficiency [64,65,66,67,68]. The results imply that the Sample S1 have a highest photocatalytic activity for the photocatalytic degradation of organic dyes.

Fig. 6
figure 6

EIS spectra of Samples S1, S2, S3, S4, and S5

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3.6.2 Cyclic voltammetry (CV) studies

Cyclic voltammetry (CV) is an excellent method to analyze electrochemical properties of electrodes. Figure 6a–e shows the CV curves of Samples S1, S2, S3, S4, and S5 measured at the scan rates of 3, 5, 10, 20, 30, 40, 50, and 100 mV·s− 1 in the potential range of 0–0.5 V. For the Samples S1, S2, S3, S4, and S5, the specific current increased with the increasing of scan rate. The anode and cathode peaks move towards higher and lower potentials with the increasing of scan rate, respectively. As can be seen from Fig. 7, the different morphologies of SrTiO3 result in a slight shift in the position of reduction peak. For the SrTiO3 rhombus particles (Fig. 7a), two redox peaks at 0.20 and 0.25 V can be observed. Sample S1 has a maximum integrated area in CV curve than that of other samples, indicating that the SrTiO3 rhombus particles possess a highest electrochemical performance among all samples. With the increase of reaction time, the particles grow into the shape of a cube in orientation, but the integrated area in CV curve of Sample S2 is reduced as shown in Fig. 7b.

Fig. 7
figure 7

Cyclic voltammograms and specific capacity vs. scan rate of SrTiO3. a S1, b S2, c S3, d S4, e S5, and f specific capacity vs. scan rate of Samples S1, S2, S3, S4, and S5

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It is noteworthy that the homogeneous SrTiO3 spherical nanoparticles (Fig. 7c) have a larger integrated area in CV curve than that of SrTiO3 particles with cubes and spherical nanoparticles (Fig. 7d). In addition, the influence of impurities on the electrochemical properties of SrTiO3 cannot be ignored. In Fig. 7e, the SrSO4 impurity resulted in relatively large integrated area in CV curve of Sample S5. Derikvandi et al. [69] reported that the semiconductor photocatalyst has the best CV response which means the photocatalyst has a lower electron/hole (e−/h+) recombination and high photocatalytic activity for the photocatalytic degradation of organic dyes. Therefore, the best CV curve for Sample S1 implies that Sample S1 exhibits a highest photocatalytic activity.

The specific capacity of Samples S1, S2, S3, S4, and S5 were estimated through CV curve using Eq. (10). [5]

$$Qs = \frac{1}{{vm}}\int\limits_{{vi}}^{{vf}} {I \times VdV},$$
(10)

where, Qs, v, m, and the integral term are the specific capacity (C·g−1), scan rate (mV·s−1), mass loading of active material (g), and anodic peak area in the CV curve, respectively. Figure 6f shows the specific capacity vs. scan rate of Samples S1, S2, S3, S4, and S5. As scan rate increases, Qs decreases due to the asynchronous movement of charges with respect to scan rate [5]. However, an abnormal phenomenon can be observed for the Sample S1 at the scan rates of 3 and 5 mV·s−1 due to the effect of high oxygen vacancy concentration. It clearly confirms that the Sample S4 shows the lowest electrochemical performance as compared to other SrTiO3 samples, which may be due to the heterogeneous particles, lower electrical conductivity, and lower facet-dependent, resulting in the reduced Qs value.

3.6.3 Galvanostatic charge–discharge analysis

Figure 8a–e presents the typical charge–discharge curves of Samples S1, S2, S3, S4, and S5 at current density of 1, 1.2, 1.4, 1.6, 1.8, and 2 A/g. For all samples, the charge–discharge curves were non-linear which confirms the battery-type behavior of SrTiO3 particles. The variation tendencies for different samples are consistent with the CV measurements, suggesting that the as-prepared SrTiO3 particles have a pseudocapacitive behaviors. For the Sample S1, the longest discharge timing confirmed that the SrTiO3 rhombus particles can potentially increase the energy dissipation time. The SrTiO3 rhombus particles providing larger interfacial area are favorable for hydroxide (OH−) ions to participate in the reaction of faradaic type. The result indicates that Sample S1 has a highest photocatalytic activity as the photo-generated electron–hole pairs can react with OH− to produce hydroxyl radical (·OH). OH attacks the organic dye to produce non-toxic and harmless products.

Fig. 8
figure 8

Charge–discharge curves and specific capacitances of SrTiO3. a S1, b S2, c S3, d S4, e S5, and f specific capacitance vs. current density of Samples S1, S2, S3, S4, and S5

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The specific capacitance of Samples S1, S2, S3, S4, and S5 can be calculated using Eq. (11) [5].

$$Q_{{sc}} (F \cdot g^{{ - 1}} ) = \frac{{I \cdot \Delta t}}{{\Delta V \cdot m}},$$
(11)

where Qsc, I, Δt, m, and ΔV are the specific capacitance (F·g−1), discharge current (A), discharge time (s), mass of active material (g) on the electrode, and working potential window (V), respectively. Figure 8f shows the specific capacitance vs. current density of Samples S1, S2, S3, S4, and S5. It can be seen that Sample S1 exhibits high rate capacitance retention. After careful analysis, it is found that the SrTiO3 rhombus particles have three main merits: (1) the unique polyhedral structure enables intimate contact between particles; (2) high crystallinity enhances the electrical conductivity and makes it ease for charge transfer or transport across the special structure; (3) high electrochemical stability provides high charge storage for the SrTiO3 rhombus particles. This work confirms that crystal surface control is a key factor to consider in synthesizing and designing of perovskite-type photocatalyst to improve the electrochemical performance and photocatalytic activity.

3.7 Photocatalytic activity

3.7.1 Ultraviolet photocatalytic activity

Figure 9a shows the time-dependent photocatalytic degradation of congo red dye in the presence of Samples S1, S2, S3, S4, and S5 under ultraviolet light irradiation. The initial dye concentration and catalyst content are 1 mg/L and 1 g/L, respectively. According to literature [43], congo red dye is a stable and non-biodegradable molecule. The adsorption experiment shows that the adsorption percentage of congo red dye is about 8.5% for 0.5 h. The degradation percentage of each sample increases with the increasing of irradiation time. The degradation percentage of SrTiO3 samples under ultraviolet light irradiation for 150 min follows the order: S1 > S5 > S4 > S2 > S3.

Fig. 9
figure 9

a Time-dependent photocatalytic degradation of congo red dye in the presence of Samples S1, S2, S3, S4, and S5 under ultraviolet light irradiation; b Plots of ln(Ct/C0) vs. irradiation time for Samples S1, S2, S3, S4, and S5

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The degradation curves of Samples S1, S2, S3, S4, and S5 are well fitted by a pseudo-first-order kinetics model. The pseudo-first-order kinetics rate (k) for the photocatalytic degradation of congo red dye over the Samples S1, S2, S3, S4, and S5 can be obtained by the literature [43]. A plot of ln(Ct/C0) versus t for the Samples S1, S2, S3, S4, and S5 with different irradiation times for the congo red dye are shown in Fig. 9b. The k values of Samples S1, S2, S3, S4, and S5 can be obtained directly on the basis of Origin 8.0 software from the regression analysis of linear fitting curve in Fig. 9b. The rate constant k for Samples S1, S2, S3, S4, and S5 is obtained as 0.02788, 0.00661, 0.00429, 0.01207, and 0.01603 min−1, respectively. This implies that Sample S1 for the photocatalytic degradation of congo red dye under ultraviolet light irradiation has a degradation percentage 6.50 times higher than that of Sample S3. The result confirms that Sample S1 exhibits a strongest photocatalytic activity for the photocatalytic degradation of congo red dye. The result further indicates that the crystal surface control plays a critical role in the aspect of improving photocatalytic activity. Sample S5 exhibits that a relatively high photocatalytic activity may be due to the effect of SrSO4 impurity. The result indicates that the rate constant is directly proportional to photocatalytic activity of Samples S1, S2, S3, S4, and S5.

3.7.2 Visible photocatalytic activity

Figure 10a shows the time-dependent photocatalytic degradation of congo red dye in the presence of Samples S1, S2, S3, S4, and S5 under simulated sunlight irradiation for 180 min. The degradation percentage of SrTiO3 samples follows the order: S1 > S2 > S4 > S3 > S5. The variation tendency is consistent with the optical absorption coefficient of visible light in UV–vis absorption spectrum. Figure 10b shows the first-order kinetic plots of Samples S1, S2, S3, S4, and S5 for the photocatalytic degradation of congo red dye. The rate constant k for Samples S1, S2, S3, S4, and S5 is obtained as 0.01368, 0.00715, 0.00288, 0.00553, and 0.00210 min−1, respectively. The result indicates that Sample S1 has a highest visible photocatalytic activity, while Sample S5 exhibits a worst visible photocatalytic activity. Visible photocatalytic activity may be related to the synergistic effects of optical absorption coefficient, oxygen vacancy concentration, impurity content, and charge utilization and separation efficiency for the SrTiO3 particles.

Fig. 10
figure 10

a Time-dependent photocatalytic degradation of congo red dye in the presence of Samples S1, S2, S3, S4, and S5 under simulated sunlight irradiation; b Plots of ln(Ct/C0) vs. irradiation time for Samples S1, S2, S3, S4, and S5

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3.8 Photocatalytic mechanism

Based on the above analysis, a possible photocatalytic mechanism and charge transfer process for the congo red dye degraded by pure SrTiO3, O (oxygen vacancy)—SrTiO3, or I (impurity) -SrTiO3 was proposed as illustrated in Fig. 11. Silva et al. [70] reported that the SrTiO3 nanoparticles exhibit a photocatalytic activity mainly due to the valence band (VB) hole attack directly in the organic dye.

Fig. 11
figure 11

Photocatalytic mechanism for the congo red dye degraded by pure SrTiO3, O (oxygen vacancy)—SrTiO3, and I (impurity)—SrTiO3 under simulated sunlight irradiation

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$$h^{ + } + {\text{dye}} \to {\text{degradation products}}$$
(12)

In the uniform SrTiO3 nanoparticles, the ·OH and ·O2− are not formed due to the larger Eg value and no defect or impurity level, which is why the photocatalytic activity of Sample S3 is poor [71]. The dominant active species for the Sample S3 can be determined by the trapping experiment as shown in Fig. 12. The degradation percentage of Sample S3, Sample S3 + EDTA-2Na, Sample S3 + IPA, and Sample S3 + BQ is 46.5%, 32.3%, 85.6%, and 88.7%, respectively. The result confirmed that the hole (h+) plays an important role in the photo-degradation of congo red dye over Sample S3.

Fig. 12
figure 12

Trapping experiments of active species for Sample S3 under simulated sunlight irradiation

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In the SrTiO3 photocatalytic system, Sr4d and Ti3d form the conduction band (CB) of SrTiO3, and O2p, Sr4p, O2s, S4s, and Ti3p form the VB of SrTiO3 [72, 73]. Since the energy of incident simulated sunlight is smaller than that of Eg value of SrTiO3 particles, it is difficult to excite the electron transition from VB to CB. However, the oxygen vacancy or impurity will provide a special energy level for SrTiO3 particles, which will promote the electron transition to CB. The charge carrier generation process can be recorded as follows:

$${\text{SrTiO}}_{{\text{3}}} + {\text{defect or impurity level}} + {\text{hn}} \to {\text{e}}^{ - } + {\text{ h}}^{ + }.$$
(13)

The valence band holes or conduction band electrons can react with OH− or O2/H2O2 to form ·OH as the redox potential of OH−/·OH is + 1.89 V versus NHE or the redox potential of O2/·O2− is − 0.33 V versus NHE [74,75,76,77,78,79,80]. Therefore, valence band holes easily react with hydroxide ions to form ·OH. The conduction band electrons react with oxygen to form ·O2−. ·O2− reacts with electrons and hydrogen ions again to produce hydrogen peroxide (H2O2). In addition, electrons can react similarly with oxygen and hydrogen ions. Under the action of electrons or ·O2−, hydrogen peroxide will produce ·OH. ·OH reacts with dye molecule to produce non-toxic and harmless products. The simple chemical reaction can be expressed as follows:

$${\text{h}}^{ + } + {\text{OH}}^{ - } \to \cdot {\text{OH}}$$
(14)
$$e^{ - } + {\text{O}}_{{\text{2}}} \to \cdot {\text{O}}^{{{\text{2}} - }}$$
(15)
$$\cdot {\text{O}}^{{{\text{2}} - }} + {\text{2H}}^{ + } + e^{ - } \to {\text{H}}_{{\text{2}}} {\text{O}}_{{\text{2}}}$$
(16)
$${\text{2}}e^{ - } + {\text{O}}_{{\text{2}}} + {\text{2H}}^{ + } \to {\text{H}}_{{\text{2}}} {\text{O}}_{{\text{2}}}$$
(17)
$$e^{ - } + {\text{H}}_{{\text{2}}} {\text{O}}_{{\text{2}}} \to \cdot {\text{OH}} + {\text{OH}}^{ - }$$
(18)
$$\cdot {\text{O}}^{{{\text{2}} - }} + {\text{H}}_{{\text{2}}} {\text{O}}_{{\text{2}}} \to \cdot {\text{OH}} + {\text{OH}}^{ - } + {\text{O}}_{{\text{2}}}$$
(19)
$$\cdot {\text{OH}} + {\text{dye}} \to {\text{degradation products}}$$
(20)

The SrTiO3 rhombus particles have more crystal surface exposure, which is favorable for the photocatalytic degradation of organic dyes. To further confirm the role of h+, ·OH, and ·O2− in Sample S1 in the whole photocatalytic reaction process, Fig. 13 shows the trapping experiments of active species for the Sample S1 under simulated sunlight irradiation. The result shows that h+, ·OH, and ·O2− play critical roles in the photocatalytic reaction process. This indicates that the experimental results are consistent with the proposed photocatalytic mechanism. Therefore, the photocatalytic activity of SrTiO3 rhombus particles is obviously enhanced due to the synergistic effects of low recombination of electron–hole pair, high oxygen vacancy concentration, charge separation, and crystal surface exposure. In addition, SrSO4 impurity also plays an important role in suppressing the visible light photocatalytic activity of SrTiO3.

Fig. 13
figure 13

Trapping experiments of active species for Sample S1 under simulated sunlight irradiation

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4 Conclusion

Shape and phase-tunable SrTiO3 perovskite-type metal oxide with high oxygen vacancy concentration can be synthesized by different mineral acids including HNO3, HCl, or H2SO4 and different reaction times. The effects of phase purity, surface morphology, optical absorption coefficient, and charge transfer and separation efficiency on the optical, electrochemical performances and photocatalytic activity of SrTiO3 perovskite-type metal oxide were systematically studied. The SrTiO3 rhombus particles exhibit highest charge transfer and separation efficiency and photocatalytic activity for the photocatalytic degradation of congo red dye under simulated sunlight irradiation than that of the other SrTiO3 samples. The high photocatalytic activity for the SrTiO3 rhombus particles can be assigned to the synergistic effects of band edge position, oxygen vacancy concentration, impurity content, optical absorption coefficient, charge utilization and separation efficiency, and crystal surface exposure. The facet-dependent technology for improving the charge transfer and separation efficiency and photocatalytic activity can be used to synthesize other metal oxide semiconductors.