Over the past few decades, perovskite materials owning a featured ABX₃ molecular formula and fascinating functional properties have received global research interest and have been widely investigated. Extensive efforts have been paid to improve the performances of these existed ABX₃ materials and to seek new members in perovskite family^[1,2,3,4,5]. In this way, the development of an efficient and accessible approach is of great importance for the nucleation design and crystallization control, as well as the rational tailoring of morphology, geometrical shape, surface characteristics and size, which have close relationship to the physicochemical properties and functional performances of these perovskite materials^[6,7].

As one of the most popular and versatile materials in perovskite family^[8], SrTiO₃ shows significant applications in diverse fields including solar cells^[9,10], photocatalysis^{[11,12,13,14]}, water splitting^[14,15,16], fuel cells^[17], superconducting^[18], etc. Recently, with the development of nanoscience and nanotechnology, nanostructured SrTiO₃ has also received tremendous attention due to its peculiar properties and versatile functions in above-mentioned fields. So far, various SrTiO₃ nanostructures including nanowires^[12], nanocubes^[19,20,21] and truncated octahedrons^[22] have been successfully obtained through different synthetic methods. However, the polycrystalline SrTiO₃ gives rise to the existence of massive grain boundaries, generating a natural energy barrier for the electron transport, and thus deteriorates the electric property of SrTiO₃. Therefore, SrTiO₃ nanostructures with excellent crystal quality are definitely required. On the other hand, most of the reported SrTiO₃ nanostructures are still in powder form, which will undoubtedly bring mass loss during cyclic utilization. Therefore, the fixing of SrTiO₃ nanostructure with strong substrate adherence is preferable.

From the point of crystallography, SrTiO₃ shows a simple cubic (s.c.) crystallographic structure and a space group Pm-3m, implying a minimum surface energy in (001) plane. For this reason, the crystallographic morphology of SrTiO₃ is favorable to form cube-like structure. However, the functional properties of SrTiO₃ nanostructures such as catalytic reactions are strongly dependent on its crystal facet. Considering all these points, an approach that can realize the in-situ growth of SrTiO₃ nanostructures for strong substrate adherence, the crystal facet tailoring for selective catalytic reaction and superior crystal quality is generally needed for further promoting their applications in diverse fields.

As a conventional surface treatment method for enhancing metal wear resistance, plasma electrolytic oxidation (PEO) technology owns many unique advantages. It has been regarded as an ideal method to prepare metallurgical bonded film with strong adherence for cyclic utilization in some harsh environment. Furthermore, the porous PEO film can provide sufficient nucleation sites for the further in-situ growth of SrTiO₃ nanostructures. In this paper, a two-step method by combining traditional PEO and hydrothermal method to in-situ synthesize SrTiO₃ nanostructures on metal substrate was developed based on our previous work^[23,24,25]. The PEO process is utilized to produce Sr/Ti contained porous film, while the hydrothermal process enables a rational tailoring of the morphology of SrTiO₃ from microcube to nanosheet. Through detailed structural characterizations, we confirmed that both the SrTiO₃ microcube and Sr_1-δTiO₃ nanosheet are single crystals, and the two nanostructures show (100) and (110) exposed facets, respectively. It is expected that this hybrid synthetic strategy will open up more opportunities for SrTiO₃ nanostructures to be used in diverse fields and can thus be further extended to the synthesis of a variety of metal oxide nanostructures with predominant advantages and promising applications.

The PEO treatment was applied to preparation of porous TiO₂ film in accordance with the experiment routes of our previous work^{[23,24,25,26]}. In a typical process, Ti substrate was used as anode, two pieces of high-purity graphite served as the counter electrode and the mixture of Na₂B₄O₇, Sr(AC)₂, NaOH and EDTA-2Na was used as electrolyte. During a normal PEO process, the Ti substrate was immersed into the electrolyte under pulse DC power supply and charged for 12 min. The temperature of the electrolyte was kept at around 20℃. The current density, duty cycle and frequency were maintained at 0.13 A/cm², 60% and 1000 Hz, respectively.

In this step, porous PEO film containing Sr source was used as the nucleation site of SrTiO₃ nanostructures. The film was vertically immersed into 15 mL NaOH solution (0.5 mol/L, 1 mol/L, 1.5 mol/L) in a Teflon-lined autoclave and heated at 180℃ for different hydrothermal durations (1 h, 2 h, 4 h, 6 h, 8 h).

The phases and crystal structures of SrTiO₃ samples were characterized by X-ray diffraction (XRD, Rigaku D/max 2400). 3D spatial profiles of PEO film was measured via a 3D X-ray microscope (Xradia Versa XRM 500). X-ray photoelectron spectroscopy (XPS, Thermal VG/ESCALAB250) was used to obtain the binding energy of SrTiO₃ samples. The morphology and composition of SrTiO₃ nanostructures were characterized by a field-emission scanning electron microscopy (FE-SEM, FEI Inspect F50) equipped with a Quanta 600 Energy Dispersed X-ray spectrometer (EDS) system. The microstructure and crystallinity of SrTiO₃ nanostructures were analyzed using a 200 kV transmission electron microscopy (TEM, Tecnai G2 F20). UV-Vis diffuse reflectance spectra (DRS) of SrTiO₃ nanostructures were obtained on a HITACHI U-3900 spectrophotometer.

Previous work has demonstrated the merits of PEO method in obtaining metal oxide nanostructures with strong substrate adherence^{[23,24,25,26,27]}. The porous film provides the nucleation sites and the precursor source for the formation of metal oxide nanostructures. In this work, the PEO film exhibits a typical porous structure comprising of numerous volcano-like channels formed during the instantaneous micro-arc discharge (Fig. S1). The porous structure is very uniform with the pore size varying from hundreds of nanometers to several micrometers. The depth of the pores can be up to 6 μm, which can be confirmed from the 3D X-Ray image (Fig. S2). In addition, the pores distribute homogeneously both outside and inside of the TiO₂ film, providing sufficient nucleation sites for the in-situ growth of SrTiO₃ nanostructures.

XRD measurement was carried out to investigate the structure/phase information of all PEO samples before (Fig. S3) and after (Fig. 1) hydrothermal treatment. The (110) (101) and (111) peaks of TiO₂ in rutile phase (JCPDS no. 21-1276; a=b=0.495 nm, c=0.296 nm) at 2θ=27.4°, 36.1° and 41.2° can be clearly distinguished. In addition, a slight trace of (101) peak at 2θ=25.3° can also be detected, which originates from anatase phase TiO₂ (JCPDS no. 21-1272; a=b=0.379 nm, c=0.951 nm). The coexistence of these two TiO₂ phases in the PEO film coincides with the results of other groups^[28]. The diffraction peaks at 22.4°, 40.0° and 46.5° for PEO films (Fig. S3) obtained under low concentration of Sr(AC)₂ (LSCE) match well with the (110) (111) and (200) lattice planes of SrTiO₃ (JCPDS no. 35-0734; a=b=c=0.391 nm), which means that SrTiO₃ has already been formed during the PEO process. However, these peaks of SrTiO₃ in PEO film disappear under high concentration of Sr(AC)₂ (HSCE). This can be understood that the excessive Sr²⁺ could lead to the sedimentation of phosphate radical and cause the weakening of micro-arc discharge. As a result, the crystallization of Sr species in PEO film is inhibited due to the lack of discharge energy. The Ti, Sr and O concentrations in PEO film are ascertained through EDS measurement (Table S1). That all samples contain a certain amount of Sr element although the SrTiO₃ phase is not detected in the case of HSCE. A reasonable reason is that SrTiO₃ exists in amorphous phase in PEO matrix. This assertion could be further verified from the diffraction dome in XRD pattern in the range of 23°-36°, which is in good agreement with the formation of amorphous tungstate fabricated by PEO method^[23,24].

Hydrothermal treatment is applied on PEO film for the in-situ growth of SrTiO₃ nanocrystal. Fig. 2 shows the typical morphology of SrTiO₃ nanocrystals in-situ nucleated on HSCE PEO film after hydrothermal treatment. It can be seen that the surface of the PEO film is fully covered with numerous cube-like nucleus with regular crystalline facets. Each micro-cube shows smooth surface and sharp edges, and the typical size of these microcubes is 1 µm -3 µm. In addition, it is also found that NaOH solution plays a key role in the formation of SrTiO₃ nanostructures. The addition of NaOH in hydrothermal reaction will promote the high density of nucleation and accelerate the growth rate of SrTiO₃ nanocrystals (Fig. S4)^[12], while few SrTiO₃ nanocrystals can be found without the participation of NaOH solution (Fig. S5). As we prolong the treating time, no evident difference in size and morphology of SrTiO₃ microcubes is observed (Fig. S4(a)-(d)), indicating that the PEO film has approximately reached the precipitation-dissolution equilibrium under 0.5 mol/L NaOH solution in only 1 h. The hydrothermal growth of SrTiO₃ nanocrystals under other NaOH concentrations (1.0 mol/L, 1.5 mol/L) was also implemented (Fig. S6-S7). Interestingly, the morphology of these samples is quite similar only except for some traces of etching at the edges and corners of microcubes (Figure S6(a)-(c)), which is caused by the dissolution of low coordinated Sr and Ti atoms. Compositional analysis on the microcube layer using EDS measurement shows that the atomic percentages of Sr, Ti and O elements are 20.77%, 24.16% and 55.06% (Table S2), matching well with the stoichiometric ratio of standard SrTiO₃. The cubic SrTiO₃ phase can also be confirmed by TEM analysis. Fig. 2(c) shows the low-magnification TEM image of SrTiO₃ microcubes assembled together with all exposed crystal facets of (100) planes and 90° included angle of two random adjacent planes. The HRTEM image and FFT pattern along the [110] zone axis (Fig. 2(d-e)) further indicates that the microcube is single crystal without obvious structure defect. The inter-planar spacing between neighbour lattices is 0.393 nm and 0.281 nm, respectively, which matches well with the distances of (100) and (011) planes of cubic SrTiO₃. All these compositional and structural results, together with XRD results (Fig. 1), have firmly demonstrated the formation of high-quality cubic SrTiO₃ on the surface of porous TiO₂ film and similar morphology has been observed in previous work^[12].

When the Sr content in initial electrolyte was reduced, the SrTiO₃ nanocrystals show quite different morphology as compared to SrTiO₃ microcubes. In SEM images (Fig. 3(a)) widespread ultra-thin nanosheets take the place of SrTiO₃ microcubes and fully cover the PEO film. Fig. S8 gives the SEM image of PEO sample treated in 1 mol/L NaOH for different durations. It can be seen that the size of nanosheets varies among 100 nm-200 nm as the reaction time is 0.5 h (Fig. S8(a)), and then grows to micron scale as the time extends to 1 h (Fig. S8(b)) and 2 h (Fig. S8(c)). Further increasing the treating time causes the edge curling and assembling of sheet structure (Fig. S8(d)-(e)), which is probably induced by the stress release. Adjusting the NaOH concentration also leads to the size evolution and morphology changing of nanosheets. Taking Fig. S8, Fig. S9 and Fig. S10 as comparisons, the sheet size shows an increasing tendency in pace with the increase of NaOH concentration. Through the HRTEM image (Fig. 3(d)) and corresponding FFT pattern (Fig. 3(e)), we can calculate the interlayer spacing of such nanosheet to be 0.371 nm along (001) plane and 0.278 nm along (110) plane of SrTiO₃.

The exposed facet is calculated to be (11¯0) crystalline plane. Additionally, it can be seen that the crystalline quality of nanosheet is not so good as microcube (Fig. 2(d)). EDS data (Table S2) shows that the Sr/Ti ratio of SrTiO₃ nanosheets is about 0.21, which is much smaller than the stoichiometric ratio of standard SrTiO₃. In order to get more convincing conclusion, XPS element component analysis is used to collect the signals of the topmost surface of SrTiO₃ nanosheets. The Sr/Ti ratio of nanosheet is found to be around 0.31 (Table S3), which is similar to the EDS result. Thus these nanosheets can be regarded more accurately as Sr_1-δTiO₃. The XPS result (Fig. 4) of Sr exhibits two peaks at around 133.7 eV and 135.4 eV which correspond to the 3d_5/2 and 3d_3/2 electron orbit of Sr²⁺. Two peaks of Ti2p located at 458.0 eV (2p_3/2) and 463.8 eV (2p_1/2) are observed, which belong to Ti⁴⁺ in SrTiO₃^{[12, 29]}, while no signal of Ti⁴⁺ from TiO₂ is detected due to the full covering of Sr_1-δTiO₃ nanosheets.

Based on all the results above, the tentative formation mechanism of SrTiO₃ microcubes and nanosheets isproposed (Fig. 5). The PEO film serves as both substrate and precursor for the nucleation of SrTiO₃ nanocrystals and the Sr/Ti source for hydrothermal reaction. During hydrothermal process, Sr/Ti species in the PEO film can quickly dissolve under elevated temperature in alkaline environment, then the released Sr²⁺ and Ti⁴⁺ species suffer from a hydrolysis process. The hydrolysis of Ti⁴⁺ is very quick^[30], leading to the rapid formation of [Ti(OH)_y]^4-y, which is further aggregated into colloid species {[Ti(OH)_y]^4-y}_n. [Sr(OH)_x]^2-x are also formed at the same time. In alkaline condition, [Sr(OH)_x]^2-x will break the Ti-O bond to incorporate into the Ti-O cluster to form SrTiO₃^[31,32,33]. Since PEO film under LSCE could only provide a relatively low Sr concentration (Table S1) during SrTiO₃ formation process, the [Sr(OH)_x]^2-x couldn’t penetrate into the whole Ti-O sol. As a result, Ti-O prefers to exhibit a sheet-like morphology with insufficient incorporation of Sr and thus leads to the formation of non-stoichiometric Sr_1-δTiO₃ nanosheets. The condition is contrary in HSCE sample with a Sr/Ti ratio of 0.79. The increase of Sr concentration around HSCE sample can also lead to faster growth rate of SrTiO₃ cube than that of Sr_1-δTiO₃ nanosheet, which is evidenced in SEM results (Fig. S4-S10).

The optical properties of SrTiO₃ microcube and Sr_1-δTiO₃ nanosheets were roughly examined through a UV-Vis spectrometer. Fig. 6 shows the UV-Vis absorption spectra of PEO film and SrTiO₃ samples synthesized under different conditions. It can be seen that the SrTiO₃ microcube has only strong absorption around 390 nm with smaller inclination, while the Sr_1-δTiO₃ nanosheets show two absorption edges at around 335 nm and 400 nm, respectively. Apparently, the 390 nm absorption is directly from SrTiO₃ microcube, which corresponds to a band gap of 3.18 eV. For Sr_1-δTiO₃ nanosheets, the absorption edge located at around 335 nm, which corresponds to a band gap of 3.71 eV, exhibits obvious blue-shift in comparison with the bulk SrTiO₃ (3.25 eV) due to size effect^[34,35], while the 400 nm absorption edge can be attributed to the electron transition of rutile TiO₂. In addition, the absorption intensity at 335 nm exhibts an increasing tendency along with the increase of NaOH concentration and soaking time, indicating more Sr_1-δTiO₃ nanosheets formed on the surface of PEO film. What’s more, from the SEM images (Fig. S8-S10) of nanosheet samples, it can also be seen that the amount of nanosheets shows an increasing tendency along with the increase of NaOH concentration and soaking time. As a result, it can be concluded that the SEM results (Fig. S8-S10) and the UV-Vis absorption spectra of nanosheet sample strongly support the assertion that the nanosheets are mainly composed of Sr_1-δTiO₃. The difference of absorption features between SrTiO₃ cubes and Sr_1-δTiO₃ nanosheets will provide more opportunities for promising applications in the fields of photocatalytic water-splitting, photocatalytic degradation and heterogeneous catalysis, etc.

SrTiO₃ microcubes and Sr_1-δTiO₃ nanosheets have been in-situ fabricated on PEO film through a combined technology. The SrTiO₃ microcubes obtained under HSCE show a regular cubic structure with (001) exposed crystal facets and superior crystalline quality without defects. Reducing the concentration of Sr source induces an obvious morphology evolution from microcubes to Sr_1-δTiO₃ nanosheets with a thickness of about several nanometers and (110) exposed facet. In addition, the two types of SrTiO₃ nanostructures show significant difference in composition and optical absorption properties. SrTiO₃ microcubes own a higher Sr concentration and a bulk-like optical absorption behavior, while Sr_1-δTiO₃ nanosheets with insufficient Sr content exhibit obvious blue-shift in optical absorption due to the size effect. It is believed that the initial Sr/Ti atomic ratio in PEO film is mainly responsible for the morphology evolution of SrTiO₃ nanostructures. This feasible in-situ synthetic strategy to SrTiO₃ nanostructures with modified morphology, tunable band gap and optical properties will pave a solid way toward their promising application in the fields of photocatalysis for clean energy and environmental processing.

LIU Xiao-Yuan^1,2, LIU Bao-Dan¹, JIANG Ya-Nan¹, WANG Ke^1,2, ZHOU Yang^1,2, YANG Bing¹, ZHANG Xing-Lai¹, JIANG Xin¹

(1. Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China; 2. School of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China)