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Shape-Tunable SrTiO3 Crystals Revealing Facet-Dependent Optical and Photocatalytic Properties Pei-Lun Hsieh,† Gollapally Naresh,‡,∥ Yu-Sheng Huang,† Chun-Wen Tsao,⊥ Yung-Jung Hsu,⊥ Lih-Juann Chen,*,†,§ and Michael H. Huang*,‡,§ Department of Materials Science and Engineering, ‡Department of Chemistry, and §Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing Hua University, Hsinchu 30013, Taiwan ∥ Department of Chemical and Biological Engineering, University of Sheffield, Sheffield S10 2TN, U.K. ⊥ Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan Downloaded via KEAN UNIV on July 17, 2019 at 11:43:20 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: SrTiO3 cubes with tunable sizes of 160−290 nm have been synthesized by mixing TiCl4, SrCl2, and LiOH in pure ethanol or a water/ethanol mixed solution at just 70 °C for 3 h. Replacing water/ethanol with water/hexanol and water/ethylene glycol, and fine tuning the amounts of other reagents, resulted in the formation of edge-truncated cubes and {100}-truncated rhombic dodecahedra, respectively. X-ray diffraction and transmission electron microscopy characterization, supported by Rietveld refinement analysis, have revealed shape-dependent tuning in lattice parameters. The cubes display slight size-related optical band shifts, and they show clearly more blue-shifted light absorption than the other particles exposing significant {110} faces. The {100}-truncated rhombic dodecahedra are far more efficient than cubes at photodegradation of methylene blue and photocatalyzed hydrogen evolution from water in the presence of methanol. The photocatalytic activity variation should arise from different degrees of surface band bending for the {100} and {110} faces of SrTiO3, suggesting surface facet control as a strategy for enhancing photocatalyzed hydrogen production.

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INTRODUCTION Formation of various semiconductor crystals such as Cu2O, Ag2O, PbS, and Ag3PO4 particles with tunable shapes has enabled the observations of their facet-dependent electrical conductivity, photocatalytic activity, and optical properties.1−7 In particular, it has been found that the light absorption and emission properties of Cu2O crystals exhibit size and facet dependence, showing that the band diagram of Cu2O should have size and facet components.2,8,9 As particle size increases, the band gap decreases gradually for Cu2O crystals, reaching several hundreds of nanometers. For large Cu2O cubes, octahedra, and rhombic dodecahedra having the same particle volume, they still display discernibly different colors and thus appreciable band-gap variation.8,9 Such observations can be understood from the presence of an ultrathin surface layer with different band structures for various planes.10 Density functional theory calculations on Si and Ge further reveal that the bond length, bond geometry, and frontier orbital electron distribution within the thin surface layer differ from those of the bulk interior, resulting in the observed facet-dependent electrical conductivity behaviors of Si and Ge wafers.11−14 Hence, the face-related optical, photocatalytic, and electrical properties of semiconductors are quantum mechanical in origin. To expand the examination of optical and photocatalytic facet effects of other semiconductor materials, strontium titanate (SrTiO3) is a good choice to study. It has a perovskite © 2019 American Chemical Society

crystal structure with an indirect band gap of 3.25 eV and a direct band gap of 3.75 eV.15 Its band energy positions are similar to those of TiO2, so photocatalytic water splitting is possible.16 SrTiO3 crystals with cubic to rhombic dodecahedral structures have been reported by hydrothermal treatment of a mixture of TiCl4 and SrCl2 in various alcohols including ethanol, 1,2-propanediol, and ethylene glycol or by varying the volume of 1,3-propanediol used, at 180 °C for 48 h.17−19 The acidity of alcohols (or pKa value) is believed to cause particle shape transformation. Hydrothermal synthesis of a mixture of TiCl4, SrCl2, and LiOH (or KOH) has produced SrTiO3 cubes.20,21 It is desirable to prepare SrTiO3 crystals with tunable shapes and sizes using a much shorter reaction time to save energy. Although SrTiO3 microstructures with different morphologies have been shown to give shape-related band-gap variation, it is difficult to correlate absorption shifts to specific crystal facets.22 Facet-dependent optical properties of SrTiO3 still need to be confirmed using crystals with sharp faces. The particles can then be tested for facet-dependent dye photodegradation and photoassisted hydrogen evolution properties. Interestingly, while SrTiO3 has been coupled with other materials to enhance photocatalytic water splitting to generate hydrogen, examples of facet effects of pure SrTiO3 crystals for Received: March 5, 2019 Revised: May 15, 2019 Published: May 16, 2019 13664

DOI: 10.1021/acs.jpcc.9b02081 J. Phys. Chem. C 2019, 123, 13664−13671

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The Journal of Physical Chemistry C this reaction seem unavailable.23−25 However, SrTiO3 nanocubes, nanoflakes, and mesocrystals have been used for photocatalytic hydrogen evolution.26−28 SrTiO3 triangular prisms and nanocubes have also been tested for facet effects on the photodegradation of Rhodamine B and methyl orange.29 In this work, we have synthesized size-tunable SrTiO3 cubes at 70 °C in just 3 h using a water/ethanol mixture. Particle shape evolution from cubic to truncated cubic and truncated rhombic dodecahedral structures was achieved by adopting the strategy of alcohol variation. The crystals show unusual lattice constant decrease from cubes to truncated rhombic dodecahedra. Rietveld refinement has been carried out to obtain shape-specific lattice parameters and various bond lengths.30 Despite the fairly large particle sizes, slight size- and shapedependent light absorption band shifts have been observed, showing that semiconductor crystals generally possess optical facet effects. The three particle samples were compared for photodegradation activity of methylene blue and photocatalytic hydrogen evolution. In both cases, truncated rhombic dodecahedra exposing significant {110} faces have been found to exhibit the best photocatalytic activity, demonstrating that facet control is a promising direction for improving watersplitting activity. A modified band diagram is drawn to explain the facet-dependent optical and photocatalytic behaviors.

with zero shift, and cell parameters were refined. Later, the shape and full-width-at-half-maximum parameters were refined one by one. The thermal parameters and occupancy were refined in the later stages of the refinement by systematically starting with heavier atoms first and lighter atoms in the last. Photodegradation Experiment. For fair photocatalytic activity comparison, the amounts of various catalysts used should possess the same total particle surface area. Knowing the dimensions of SrTiO3 cubes (164 nm), edge-truncated cubes, and {100}-truncated rhombic dodecahedra, their particle surface areas and volumes were calculated (see Table S3). From the surface-to-volume ratios (1:0.84:0.99), which correspond to surface-area-to-weight ratios, 10.0, 11.9, and 10.1 mg of cubes, edge-truncated cubes, and {100}-truncated rhombic dodecahedra were used, respectively. First, SrTiO3 crystals were added to a sample vial. Methylene blue in deionized water was added with a final concentration of 6 ppm. The total solution volume is 15 mL. The solution was stirred for 30 min in a dark environment for molecular adsorption− desorption equilibrium. Next, the solution was placed 30 cm away from a 500 W Xe lamp and started illumination. The light intensity reaching the vial was 475 mW/cm2. A volume of 0.8 mL was withdrawn at regular time intervals and centrifuged immediately for spectral measurements. Photocatalytic Hydrogen Evolution. The same amounts of SrTiO3 crystals as catalysts were prepared and added to 25 mL of 20% methanol in water for hydrogen evolution reaction. The solution was placed in a sealed steel reaction cell, which was connected to a gas chromatography system. The cells were irradiated with light from a 300 W Xe lamp placed 13 cm apart. Light intensity reaching the cell was 455 mW/cm2. The gas samples were recorded periodically to determine the hydrogen output. Instrumentation. Scanning electron microscopy (SEM) images were taken using a JEOL JSM-7000F scanning electron microscope. X-ray diffraction (XRD) patterns were collected using a Bruker D2 PHASER desktop diffractometer with Cu Kα radiation. Scanning transmission electron microscopy (STEM) images were taken on a Cs-corrected high-resolution transmission electron microscope (JEM-ARM200F). UV−vis diffuse reflectance spectra were obtained by using a JASCO V670 spectrophotometer equipped with a solid sample holder. An X500 xenon lamp from Blue Sky Technologies was used as the light source in the photodegradation experiments. The Mott−Schottky measurements were performed on an Autolab PGSTAT204 potentiostat at a fixed frequency of 50 Hz. The electrochemical cell consists of an indium tin oxide working electrode fully coated with SrTiO3 particles over an area of 1 cm2, a Pt counter electrode, a Ag/AgCl reference electrode (3 M KCl), and the Na2SO4 electrolyte (0.5 M, pH = 6.72). Shimadzu GC-2014 gas chromatography was used to measure the amount of hydrogen evolution.

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EXPERIMENTAL METHODS Synthesis of SrTiO3 Crystals with Tunable Sizes and Shapes. Different proportions of deionized water and ethanol with a total volume of 2.5 mL in vials were cooled in an ice bath to make SrTiO3 cubes with approximate edge lengths of 290, 250, 200, and 160 nm. Next, 26 μL of titanium(IV) chloride was added dropwise with stirring for at least 5 min. To make the smallest cubes, 31 μL of TiCl4 solution was used. Afterward, 1 mL of aqueous solution containing 84 mg of strontium chloride (SrCl2·6H2O) and 3.7 mL of 3 M aqueous LiOH solution were added dropwise sequentially. See Table S1 in the Supporting Information for the exact reagent amounts used. After further stirring for 30 min, the resulting solutions showed slightly different degrees of transparency. Nitrogen gas was then purged into the solution for 30 s, and vials were sealed tightly with caps. The solutions were heated at 70 °C in an oven for 3 h. The resulting white precipitates were centrifuged three times using deionized water and then three times with ethanol. The particles were then dried in the oven at 50 °C for ∼ 20 h. To make SrTiO3 edge-truncated cubes and {100}-truncated rhombic dodecahedra with sufficiently small sizes, ethanol was replaced with 1-hexanol and ethylene glycol, respectively. In addition to solvent variation, most of the reagent amounts have been tuned to achieve these particle shapes. See Table S2 for the full reagent amounts used. After brief purging of the vial with nitrogen gas, the final reaction mixture was transferred to a Teflon-lined stainless steel autoclave and then heated at 200 °C for 20 h. The precipitate was cleaned using the conditions described above. Rietveld Refinement. Rietveld refinements of the X-ray diffraction (XRD) data of SrTiO3 cubes, edge-truncated cubes, and {100}-truncated rhombic dodecahedra were carried out using the FULLPROF program suite.29 For refinements, the XRD data were collected in a 2θ range of 10−90° with step size of 0.01° and scan speed of 0.1°/min. In the initial refinement runs, scale factor, background coefficients along

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RESULTS AND DISCUSSION

In this study, SrTiO3 cubes have been synthesized by mixing TiCl4, SrCl2·6H2O, and LiOH in a water/ethanol solution and heating the mixture in an autoclave at 70 °C for 3 h (see Table S1). SrTiO3 is likely produced from the following reactions:31 TiCl4 + H 2O → TiOCl 2· 2HCl 13665

DOI: 10.1021/acs.jpcc.9b02081 J. Phys. Chem. C 2019, 123, 13664−13671

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Figure 1. (a−d) SEM images of the synthesized SrTiO3 cubes with tunable edge lengths of (a) 290 nm, (b) 250 nm, (c) 200 nm, and (d) 160 nm. (e, f) SEM images of the synthesized (e) edge-truncated cubes and (f) {100}-truncated rhombic dodecahedra.

conditions as the 160 nm cubes but were found to have a larger average particle size. For facet-dependent property investigation, it is necessary to prepare SrTiO3 crystals of different morphologies. Employing the strategy of introducing different alcohols, and adjusting the amounts of TiCl4 and SrCl2 used, edge-, or {110}-truncated, cubes were produced at 200 °C for 20 h when ethanol was replaced with hexanol as a cosolvent. Use of ethylene glycol in place of ethanol resulted in the formation of {100}-truncated rhombic dodecahedra. See Table S2 for the exact reagent amounts used. Figure 1e,f gives the SEM image of the products. As the degree of {110} edge truncation increases, {100}-truncated rhombic dodecahedra exposing largely {110} faces were obtained. Size distribution histograms for the two truncated particles are also available in Figure S3. The average opposite face distances of edgetruncated cubes and {100}-truncated rhombic dodecahedra are 181 and 168 nm, respectively. Although introduction of different alcohols is effective in tuning the particle shape, which may be related to the rate of crystal growth and the typical addition of alcohol in sol−gel chemistry, it is nevertheless not a systematic approach to crystal shape evolution. XRD patterns of the synthesized particles are shown in Figure 2. All of the peaks match with the standard pattern of cubic SrTiO3. However, only the peak positions of SrTiO3 cubes essentially match with the standard pattern. Appreciable shifts to higher 2θ angles have been recorded for the truncated cubes and rhombic dodecahedra, indicating that they have smaller lattice parameters. Table S4 offers the exact peak

TiOCl 2·2HCl + 4LiOH → Ti(OH)4 + 4Li+ + 4Cl− + H 2O

Ti(OH)4 + SrCl 2 → SrTiO3 + 2HCl + H 2O

Ti(OH)4 is a gel-like suspension, as observed after adding the reagents and stirring for some time to give a turbid or translucent solution (Figure S1). SEM characterization confirms the formation of a gel-like material (data not shown). After heat treatment, white SrTiO3 particles were obtained. Using LiOH as the source of hydroxyl ions was found to give cubic particles with sharp faces; replacing LiOH with NaOH and KOH, however, yielded less perfect cubes (Figure S2). It is unclear why LiOH is more effective at producing sharp cubes, as it is also a strong base. Initially, relatively large SrTiO3 cubes with an average edge length of ∼290 nm were prepared. For optical property characterization, smaller particle sizes are more desirable. By progressively increasing the volume of ethanol added relative to that of water, and raising the amount of TiCl4 and SrCl2 used for making the smallest cubes, SrTiO3 cubes with continuously shorter edge lengths of ∼250, 200, and 160 nm were obtained. Figure 1a−d presents SEM images of the synthesized SrTiO3 cubes with tunable sizes. Spread of particle size is expected, considering particle synthesis via sol−gel chemistry. Figure S3a provides a particle size distribution histogram of the smallest cube sample with an average edge length of 164 nm prepared following the same reaction 13666

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unlikely that the Li+ cation dopant is a cause because TEM characterization shows uniform lattice images without evidence of impure atoms. Furthermore, SrTiO3 cubes synthesized at 180 °C for 6 h using KOH and LiOH have been reported to give identical XRD peak positions.20 It is also worthy to note that 5% Cr-doped SrTiO3 powder shows nearly identical XRD peak positions.32 Also, Li cations cannot occupy Sr cation sites because of charge imbalance, and Li occupancy in substitutional sites should lead to lattice expansion instead of contraction. Still off-center Li ion substitution for Sr ions in SrTiO3 formed by a conventional ceramic procedure has been suggested despite unchanged lattice parameters.33 Figure S5 provides SEM images of SrTiO3 truncated rhombic dodecahedra synthesized by introducing NaOH or LiOH and their XRD patterns. The particles look identical, and their XRD peak positions are also identical, confirming that the systematic peak shifts cannot be attributed to Li ion incorporation into the crystal lattice. Energy-dispersive X-ray spectroscopy (EDS) analysis on all of the SrTiO3 samples also indicates no presence of Li ions (Figure S6), recognizing that lithium is generally not detectable with EDS. Recent demonstration of optical size and facet effects of Cu2O nanocrystals has prompted the need to examine other semiconductor polyhedra for similar optical properties.2,8,9 Large Ag3PO4 crystals also exhibit facet-dependent light absorption shifts.6,7 Figure 4a presents UV−vis absorption spectra of the SrTiO3 cubes with tunable sizes. Despite the large particle sizes, continuous absorption band red shifts are still observed. Figure 4b is the corresponding Tauc plot for the four cube samples. The determined band gaps increase slightly from 3.227 eV for the 290 nm cubes to 3.258 eV for the smallest 160 nm cubes. Again, variation in particle size causes a minor but detectable band-gap shift, showing that band-gap size is still tunable for relatively large crystals, and our general understanding of quantum confinement effects with practically constant band-gap magnitude for large particles is not quite correct. Next, the UV−vis absorption spectra of SrTiO3 cubes (164 nm), edge-truncated cubes, and {100}-truncated rhombic dodecahedra are compared (Figure 4c). With similar particle volumes (Table S3), cubes show the most blue-shifted light absorption, whereas truncated rhombic dodecahedra having the largest proportion of {110} faces absorb light of longest wavelengths. In fact, the truncated rhombic dodecahedra have the smallest particle volume, yet they show the most redshifted absorption band edge. Such unusual behavior is believed to result from optical facet effects. Tauc plots reveal slight band-gap variations of 3.254, 3.158, and 3.151 eV for SrTiO3 cubes, edge-truncated cubes, and {100}-truncated rhombic dodecahedra, respectively (Figure 4d). Cubes exposing entirely {100} faces have the largest band gap. The results support the presence of facet-dependent optical properties in these large SrTiO3 crystals and suggest that optical size and facet effects are broadly observable in many semiconductor materials reaching fairly large dimensions. For facet-dependent photocatalytic activity comparison of 164 nm SrTiO3 cubes, 181 nm edge-truncated cubes, and 168 nm {100}-truncated rhombic dodecahedra, the amounts of crystals having the same total surface area need to be calculated first. From the single particle surface-area-to-volume ratios, the weights of different particle shapes needed were determined (Table S3). Figure 5 shows the results of methylene blue photodegradation using the three SrTiO3 samples. The recorded UV−vis absorption spectra of methylene blue as a

Figure 2. XRD patterns of the synthesized SrTiO3 (a) cubes, (b) edge-truncated cubes, and (c) {100}-truncated rhombic dodecahedra. (d) XRD pattern of standard SrTiO3.

positions for the three particle shapes. Rietveld refinement of the measured XRD patterns was performed to determine the exact lattice parameters and the various bond lengths in the crystal structure (Figure S4 and Table S5). Lattice constant a is 3.936 Å for SrTiO3 cubes, but it decreases to 3.912 and 3.909 Å for truncated cubes and truncated rhombic dodecahedra, respectively. The XRD results provide an unusual case of crystal shape effect to tuning of lattice parameters that has not been reported in other SrTiO3 studies.17 TEM characterization of a single SrTiO3 cube, edgetruncated cube, and {100}-truncated rhombic dodecahedron is displayed in Figure 3, revealing that the particles possess very

Figure 3. HAADF STEM images, SAED patterns, and high-resolution lattice images of (a) a single SrTiO3 cube, (b) edge-truncated cubes, and (c) {100}-truncated rhombic dodecahedron. Viewing direction is indicated.

sharp faces. The recorded selected-area electron diffraction (SAED) patterns give the same square arrangement of spots, indicating single crystallinity for all of the particle shapes. High-angle annular dark-field (HAADF) STEM images show clearly resolvable (100) and (110) lattice planes of SrTiO3. Remarkably, d-spacings for the (100) and (110) planes of SrTiO3 also decrease from cubes to truncated cubes and truncated rhombic dodecahedra, matching with XRD results that slight lattice constant changes accompany particle shape variation. The origin of this lattice contraction is unclear. It is 13667

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Figure 4. (a, b) UV−vis absorption spectra of size-tunable SrTiO3 cubes and their corresponding Tauc plots for the determination of indirect band gaps. (c, d) UV−vis absorption spectra of the synthesized SrTiO3 cubes, edge-truncated cubes, {100}-truncated rhombic dodecahedra, and their corresponding Tauc plots.

Figure 5. Extents of photodegradation of methylene blue using different SrTiO3 crystals as the catalysts.

function of the photodegradation time are available in Figure S7. The truncated rhombic dodecahedra were found to be far more active than cubes. Edge-truncated cubes also exhibited good photocatalytic activity, but not as active as the truncated rhombic dodecahedra. The relative photocatalytic activities can also be presented in terms of their average degradation rates, as also shown in Figure 5. The truncated rhombic dodecahedra are 5.3 times more active than cubes, showing the superior photocatalytic activity of the {110} faces of SrTiO3. Both the facet-dependent optical and photocatalytic behaviors of SrTiO3 crystals can be understood by using a hypothetical modified band diagram (Figure 6). The known valence and conduction band energies are used to represent the band structure for the bulk.34 However, the crystal must have different degrees of surface band bending. The {100} face is drawn to bend up to a greater extent to represent a significantly larger barrier to photoexcited electrons reaching to the {100} crystal surface. The {110} surface, having a small surface band bending, facilitates charge migration to the crystal exterior. If the degree of upward band bending for the conduction band of the {100} face is slightly larger than that for the valence band, this scenario leads to a larger surface band gap at the {100} face, explaining the more blue-shifted absorption of SrTiO3 cubes. In the Cu2O case, different orders

Figure 6. Adjusted band diagram of SrTiO3 accounting for the observed facet-dependent optical and photocatalytic behaviors. qX is the semiconductor electron affinity, qϕ is the semiconductor work function, Eg is the semiconductor band gap, Ec is the conduction band energy, Ev is the valence band energy, and Ef is the Fermi level.

of surface band bending have been drawn to account for the observed facet-dependent photocatalytic and optical properties.2 Instead of showing two different band diagrams, Figure 6 considers both optical and photocatalytic results to construct a single band structure that can be used to explain both properties. Another useful demonstration of facet-dependent photocatalytic properties of these SrTiO3 crystals is the activity comparison of photoinduced hydrogen evolution from water. While photogenerated excited electrons and holes migrating to the crystal surfaces can react with water and oxygen to produce 13668

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Figure 7. (a) Amounts of hydrogen gas produced from surface area-normalized SrTiO3 cubes, edge-truncated cubes, and {100}-truncated rhombic dodecahedra. (b) Amounts of hydrogen evolved per gram of catalyst used.

radical species for photodegradation of dye molecules, the electrons and holes can also split water to produce hydrogen and oxygen if the photocatalyst has appropriately located valence and conduction band potentials. Alternatively, hydrogen is evolved through the reduction of H+ by the excited electrons, and oxidation proceeds with the addition of methanol (CH3OH) to react with holes, forming CO2 and H2.35 Another role of methanol as a sacrificial reagent has also been suggested.36 Figure 7a presents results of photocatalyzed hydrogen evolution in the presence of methanol using the three SrTiO3 samples. Despite the fairly moderate hydrogen yield due to the large band gap of SrTiO3, the purpose of this experiment is to observe possible facet effects to this important reaction. Improvement in hydrogen gas production may be achieved knowing how crystal surfaces of SrTiO3 are strongly linked to its activity toward this reaction. Again, the truncated rhombic dodecahedra showed the best hydrogen evolution efficiency over 3 h of reaction, whereas cubes are the least efficient. In terms of the hydrogen evolution rate, the truncated rhombic dodecahedra are 5 times more efficient than the cubes. When expressing the data with normalized particle weight (Figure 7b), truncated rhombic dodecahedra produced 4.9 times more hydrogen than cubes, and about 2.3 times more hydrogen than that collected from the edge-truncated cubes. The results imply that the {110} faces of SrTiO3 are much more efficient for this reaction. This makes sense, considering the {110} faces are more efficient at photodegradation reaction through facile transport of excited electrons to the crystal surface. Figure 8 gives a possible band energy diagram to explain the experimental observations. Because of the different degrees of conduction band bending at the crystal surfaces, the {110} face, having a lesser degree of surface band bending, facilitates migration of photoexcited electrons to this surface and promotes H+ reduction to form hydrogen gas. This work demonstrates that crystal surface control is an important parameter to consider in the design of photocatalysts to greatly enhance water-splitting activity or hydrogen production in the presence of a sacrificial agent.

Figure 8. Band diagram for photocatalytic hydrogen evolution on SrTiO3 crystals in the presence of methanol with consideration of surface facet effects.

To validate the necessity of incorporating tunable degrees of surface-dependent band bending as seen in Figures 6 and 8, Mott−Schottky measurements were performed to obtain the band structures for the three SrTiO3 crystal shapes. According to the Mott−Schottky equation, the flat-band potential can be obtained from the extrapolated intercept at the x-axis of the linear fit line from the Mott−Schottky data.37,38 By assuming that the conduction band potential of SrTiO3, an n-type semiconductor, is empirically equal to the determined flat-band potential, the valence band potential can be obtained with the subtraction from the apparent band-gap value. Using the Mott−Schottky data (Figure S8), the valence and conduction band positions for the three SrTiO3 crystal shapes can be derived. Figure S9 shows that other than the notable optical band-gap difference between cubes and the other particle shapes, the conduction band potentials are fairly close for all of the samples, and the edge-truncated cubes actually present greater valence band and conduction band deviations from those of the other two crystal shapes. Because the edgetruncated cubes have an intermediate photodegradation and photocatalyzed hydrogen evolution efficiency, such conventional Mott−Schottky-derived band structures cannot properly explain the observed photocatalytic activity differences. Introduction of tunable surface band bending is necessary to understand the photocatalysis results. 13669

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CONCLUSIONS Size-tunable SrTiO3 cubes have been synthesized at just 70 °C for 3 h in pure ethanol or a water/ethanol mixed solution, whereas edge-truncated cubes and {100}-truncated rhombic dodecahedra were obtained at 200 °C for 20 h with the use of other alcohols. XRD and TEM characterization has revealed slightly different d-spacings of lattice planes for these particle morphologies, and Rietveld refinement was used to obtain their cell parameters and various bond lengths. Despite the large particle sizes, the optical band gap still increases slightly but steadily with decreasing cube size from 3.23 to 3.26 eV. Cubes exposing only {100} faces show clearly more blueshifted light absorption than other particles possessing significant {110} surfaces, demonstrating the existence of optical facet effects. {100}-truncated rhombic dodecahedra exhibit notably better activity than cubes toward photodegradation of methylene blue and photocatalyzed hydrogen evolution from water in the presence of methanol. Different degrees of surface band bending may explain the observed facet-dependent photocatalytic properties. This work further shows that facet-dependent optical and photocatalytic responses are general semiconductor properties. Exploration of semiconductor facet effects may lead to fabrication of efficient photocatalysts for hydrogen production.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b02081.

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Photographs taken in the synthesis of SrTiO3 crystals, SEM images, particle size distribution histograms, XRD peak positions and Rietveld refinement results, UV−vis absorption spectra of methylene blue as a function of reaction time, Mott−Schottky plots, and the derived band structures of SrTiO3 crystals (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.H.H.). ORCID

Yung-Jung Hsu: 0000-0003-3243-2644 Lih-Juann Chen: 0000-0002-0826-8680 Michael H. Huang: 0000-0002-5648-4345 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the Ministry of Science and Technology of Taiwan (MOST 104-2221-E-007-030-MY3, 104-2119-M-007-013-MY3, 106-2811-M-007-028, 107-3017F-007-002, and 107-2221-E-007-055-MY3).

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