From Titanium Sesquioxide to Titanium Dioxide: Oxidation-Induced

Article Cite This: Chem. Mater. 2018, 30, 4383−4392

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From Titanium Sesquioxide to Titanium Dioxide: Oxidation-Induced Structural, Phase, and Property Evolution Yangyang Li,† Yang Yang,‡ Xinyu Shu,† Dongyang Wan,§ Nini Wei,∥ Xiaojiang Yu,⊥ Mark B. H. Breese,⊥ Thirumalai Venkatesan,§ Jun Min Xue,† Yichen Liu,# Sean Li,# Tom Wu,*,# and Jingsheng Chen*,† †

Department of Materials Science and Engineering, National University of Singapore, Singapore 117575 Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China § NUSNNI-NanoCore, National University of Singapore, Singapore 117411 ∥ Imaging and Characterization Core Lab, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia ⊥ Singapore Synchrotron Light Source, National University of Singapore, 5 Research Link, Singapore 117603 # School of Materials Science and Engineering, University of New South Wales (UNSW), Sydney, NSW 2052, Australia

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‡

S Supporting Information *

ABSTRACT: In contrast to Ti4+-containing titanium dioxide (TiO2), which has a wide bandgap (∼3.0 eV) and has been widely explored for catalysis and energy applications, titanium sesquioxide (Ti2O3) with an intermediate valence state (Ti3+) possesses an ultranarrow bandgap (∼0.1 eV) and has been much less investigated. Although the importance of Ti3+ to the applications of TiO2 is widely recognized, the connection between TiO2 and Ti2O3 and the transformation pathway remain unknown. Herein, we investigate the oxidation-induced structural, phase, and property evolution of Ti2O3 using a complementary suite of microscopic and spectroscopic tools. Interestingly, transformation pathways to both rutile and anatase TiO2 are identified, which sensitively depend on oxidation conditions. Unique Ti2O3/TiO2 core−shell structures with annealing-controlled surface nanostructure formation are observed for the first time. The compositional and structural evolution of Ti2O3/TiO2 particles is accompanied by continuously tuned optical and electrical properties. Overall, our work reveals the connection between narrowbandgap Ti3+-containing Ti2O3 and wide-bandgap Ti4+-containing TiO2, providing a versatile platform for exploring photoelectrocatalytic applications in valence- and structure-tailored oxide materials.

I. INTRODUCTION

Although having been overlooked to date, titanium sesquioxide (Ti2O3) with Ti3+ ions is another important member of the titanium oxide family. In stark contrast to TiO2, Ti2O3 has an ultranarrow bandgap (∼0.1 eV),22,23 probably the most narrow intrinsic one among those of the oxide semiconductors. Leveraging the remarkable light absorption covering the full solar spectrum, we recently demonstrated that Ti2O3 exhibits an excellent photothermal effect, which makes it promising for desalination applications.24 Furthermore, Ti2O3 is also considered as a promising candidate for long-wavelength mid-infrared (8−12 μm) photodetection. However, until now, Ti2O3 has received a very limited amount of attention. Although the phase transformation from TiO2 to Ti2O3 has been reported by reduction of TiO2 via spark-plasma synthesis,25 carbothermal reduction,26 and CaH2,27 the reversed pathway from Ti2O3 to TiO2 remains unclear.

Titanium oxides are among the most abundant materials in the Earth’s crust. Because of the multiple valence states of Ti ions, titanium oxides include titanium monoxide (Ti2+O),1,2 titanium sesquioxide (Ti3+2O 3),3,4 the Magnéli phases (TinO2n−1, where n = 3, 4, 5, etc.),5 and titanium dioxide (Ti4+O2).6 In the family, TiO2 is the most famous member because of its great importance in fundamental research and practical applications.7 In particular, TiO2 has been extensively explored for environmental and energy technologies such as photocatalytic water splitting,8−12 photocatalytic reduction of CO2,13−15 photovoltaic solar cells,16−18 etc. Since the discovery of water photolysis on the TiO2 surface in 1972,8 TiO2 has been the most widely used oxide for photocatalytic applications because of its low cost, long-term stability, and high activity.19,20 However, the photocatalytic effect of TiO2 is limited by its light absorption because its wide bandgap (∼3.0 eV) corresponds to ultraviolet (UV) wavelengths (λ < 400 nm).19,21 © 2018 American Chemical Society

Received: April 26, 2018 Revised: June 13, 2018 Published: June 15, 2018 4383

DOI: 10.1021/acs.chemmater.8b01739 Chem. Mater. 2018, 30, 4383−4392

Article

Chemistry of Materials

Figure 1. (a) X-ray diffraction pattern and (b) Raman spectrum collected from Ti2O3 particles. (c) High-resolution STEM image of a Ti2O3 particle, showing the crystalline core and a disordered shell. The scale bar is 10 nm. (d) Enlarged high-resolution STEM image of the marked area in panel c. The scale bar is 2 nm. (e) Corresponding EDX spectrum taken from the Ti2O3 particle in panel c. (f) Ti L-edge EELS spectra collected from the Ti2O3 particle: areas 1 and 2 in panel c.

evolves to Ti2O3/TiO2 core−shell structures with surface nanostructures at high annealing temperatures. Our experiments carefully varied the oxidation conditions and identified the transformation pathways from Ti2O3 to anatase/rutile TiO2 along with the intermediate products. Furthermore, the evolution of the optical and electrical properties was investigated during the oxidation from Ti2O3 to TiO2. As opposed to that seen with black TiO2, our work demonstrated a conceptually different approach to achieving potential efficient photocatalysts based on Ti2O3/TiO2 oxide nanomaterials.

The most striking differences between TiO2 and Ti2O3 are their bandgap and ability to absorb sunlight. In photocatalytic research, bandgap narrowing of TiO2 to enhance the utilization of solar energy has been an important topic.21 An improved photocatalytic efficiency was widely reported in the reduced or doped TiO2 with enhanced light absorption.28−37 Processed TiO 2 , appearing red, 38 yellow, 3 9, 4 0 blue, 4 1− 43 and black,29−31,33,34 has been reported. Among them, black TiO2 nanocrystals exhibited photocatalytic activity dramatically higher than that of ordinary white TiO2, which was attributed to disordered Ti3+-containing shells.29,44 A general consensus for most of the reduced/doped TiO2 is that Ti3+ plays an important role in narrowing the bandgap and enhancing the photocatalytic activity of TiO2.29,43,44 As a stoichiometric compound of Ti3+, Ti2O3 (nanoparticles) shows a total absorption capacity of ∼92.5% over the full solar spectrum.24 Because of its ultranarrow bandgap of ∼0.1 eV, Ti2O3 may have great potential in photocatalytic applications. Ti2O3 is a stable compound with a trigonal-corundum structure, the same as that of Al2O3.45−47 In fact, the melting point of Ti2O3 (2130 °C) is even higher than that (2072 °C) of Al2O3 (one of the most stable oxides).48 The characteristics of structural and oxidation stabilities of black Ti2O3 particles have remained unexplored until now, which hinders the practical applications of titanium oxides. In this work, we systematically investigated the structural, phase, and property evolution of Ti2O3 particles during oxidation and their conversion to TiO2. A complementary suite of spectroscopic techniques, including scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS), and synchrotron-based X-ray absorption spectroscopy (XAS), were used to study the microstructure and electronic structure of the Ti2O3 particles. Even at room temperature, a thin (∼4−5 nm) Ti4+-containing shell layer was observed surrounding the crystalline Ti2O3 core, which further

II. EXPERIMENTAL PROCEDURES Structural Characterizations. The investigated Ti2O3 particles are commercial Ti2O3 powders (Sigma-Aldrich, 99.99%), with a size of ∼100 mesh. The ex situ powder X-ray diffraction (XRD) pattern was recorded on a Bruker D8 ADVANCE diffractometer with a Cu Kα radiation source (λ = 1.5406 Å) and LynxEye detector. Raman measurements were taken with a confocal micro-Raman system (Horiba Aramis), using a solid-state laser (λ = 473 nm). The laser beam was focused on a single isolated Ti2O3 particle using a 50× objective lens, with a numeric aperture (NA) of 0.50. The spot size was ∼2−3 μm, which is small enough to focus on a single Ti2O3 particle (Figure S1). Before every first Raman measurement, the spectrometer was calibrated using a standard silicon sample. Highresolution STEM, energy dispersive X-ray spectroscopy (EDX), and EELS were performed in a commercial aberration-corrected MonoProbe Cs scanning transmission electron microscope (Titan, FEI), operated at 300 kV. Low-resolution TEM images of annealed samples were collected with a JEOL 2010F transmission electron microscope, operated at 200 kV. Synchrotron-Based XAS Measurements. Synchrotron-based XAS was performed at the Surface, Interface and Nanostructure Science (SINS) beamline at the Singapore Synchrotron Light Source (SSLS).49 Ti2O3 particles were compressed on a conductive carbon tap. The sample was kept in an ultrahigh vacuum (1.5 × 10−10 mbar) chamber during the experiments. Linear polarized X-rays was used to 4384


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Figure 2. (a) Schematic of the experimental setup for XAS. (b) Sketch of the electronic transitions at the Ti L2,3 edge. (c) XAS spectra of Ti L edge obtained from Ti3+ (Ti2O3 bulk, left), Ti2O3 particles (middle), and Ti4+ (Nb-doped SrTiO3 substrate, right). collect the XAS spectra with an incidence angle of 90° from the sample surface in the total electron yield (TEY) mode at room temperature. The reference spectra were collected from pure Ti3+ (Ti2O3) bulk50 and Ti4+ (0.7 wt % Nb-doped SrTiO3 substrate) bulk with the same configuration. Ex Situ and in Situ Measurements. All annealing measurements were performed in air using a box furnace. Ti2O3 particles (2 g) and a duration of 2 h were used during the annealing experiments without specific clarification. The crystallographic structure of annealed particles was characterized by a Bruker D8 ADVANCE diffractometer, as described above. In situ micro-Raman measurements were taken using the confocal micro-Raman system (Horiba Aramis) with an isolated heating chamber. During the measurements, an Olympus 50× objective lens was used in a Linkam THMS600 stage. Physical Property Measurements. The diffuse reflectance values of all samples were measured by an ultraviolet−visible−near infrared spectrophotometer (Shimadzu SolidSpec-3700) in the integrating-sphere mode. The electrical properties of the particles were measured using the Quantum Design physical property measurement system (PPMS). Before the measurements, all samples were compressed into a pallet. Then, they were cut and polished to a rodlike shape to be measured by the PPMS. The four-probe method with gold wires was used for the measurements. The resistances of the samples annealed at ≥500 °C were all above 100 MΩ, verified by the Keithley 4200-SCS semiconductor characterization system.

respectively. The EDX result confirmed that only Ti and O elements existed in Ti2O3 particles. The negligible Cu signal originates from the copper TEM grid used to support the Ti2O3 particles during the TEM measurement. As shown in Figure 1c and Figure S2, a shell layer was observed surrounding the crystalline Ti2O3 core. Figure 1d shows enlarged STEM image of the marked area in Figure 1c. There is a surface layer of approximately 4.5 nm that appears disordered with inhomogeneous contrast to the crystalline Ti2O3 core. Background-subtracted EELS spectra at the Ti L2,3 edge collected from the shell area (point 2 in Figure 1c) and the core area (point 1 in Figure 1c) are shown in Figure 1f. Pronounced peak splitting was observed in the EELS spectrum collected from the shell area, which was attributed to the presence of Ti4+.52 Formation of this Ti4+-containing shell should result from the oxidation of the surface of the Ti2O3 particle in air. Because the shell layer is very thin, it was not detected by XRD or Raman measurements. This Ti4+containing shell acts as a protecting layer to prevent further oxidation of Ti2O3 particles. As shown in Figure S3, the main structure of Ti2O3 particles remained the same after exposure to outdoor ambient conditions for one month. Furthermore, we used synchrotron-based XAS to study the surfaces of Ti2O3 particles. XAS is a widely used technique for determining the local electronic structure of samples with a detection depth of 5−10 nm.53 Figure 2a shows the schematic of the XAS experimental configuration. The XAS spectrum at the Ti L2,3 edge (Figure 2b) was collected. As references, commercial pure Ti2O3 bulk50 and a Nb-doped SrTiO3 (STO) substrate were also investigated (Figure 2c). The XAS spectrum collected from the Ti2O3 particle surface exhibits four well-split peaks, similar to that collected from the STO (Ti4+) substrate. Thus, the surfaces of Ti2O3 particles could be confirmed to be Ti4+-rich, consistent with the EELS result. To clarify the transformation behaviors of Ti2O3 particles during oxidation at high temperatures, a series of annealing experiments were performed in air. Figure 3 shows typical power XRD patterns and corresponding photographs of the particle samples annealed from 400 to 900 °C. Interestingly, the color of annealed particles progressively changed from black (25 °C) to dark green (500 °C), orange (600 °C), yellow (700 °C), and white (≥800 °C). Meanwhile, the crystal structure of the particles was transformed from Ti2O3 to rutile TiO2 (≥600 °C). The orange and yellow samples, obtained after annealing at 600 and 700 °C, should be oxygen-deficient rutile TiO2.39,40 However, as opposed to the previous reports,

III. RESULTS AND DISCUSSION Ti2O3 has a trigonal R3̅c corundum structure, with the following unit cell parameters: a = b = 5.15 Å, and c = 13.64 Å.51 Figure 1a shows the typical powder XRD pattern collected from Ti2O3 particles at room temperature. Main peaks that appeared at 2θ = 23.90°, 33.15°, 34.86°, 40.30°, 48.84°, 53.87°, 61.36°, and 62.45° were indexed as (012), (104), (110), (113), (024), (116), (214), and (300) respectively, demonstrating the typical corundum structure of Ti2O3. In addition, no impurity phases were detected from the XRD results, indicating a phase-pure Ti2O3. A Raman scattering investigation was also performed on the Ti2O3 particles at room temperature. As shown in Figure 1b, seven Raman active modes were observed at 221.90, 267.38, 299.51, 341.74, 450.22, 501.32, and 557.81 cm−1, whose polarization properties are consistent with the D34 point group symmetry of the corundum structure.3 The second and sixth Raman bands were indexed to the A1g modes, while the other five were all Eg modes. To confirm the microstructure and composition of Ti2O3 particles, high-resolution STEM images and an EDX spectrum were collected and are shown in panels c and e of Figure 1, 4385


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prolonged (≤6 h) annealing experiments revealed that at 400 °C the anatase phase was always the dominant TiO2 phase (Figure S5). When the temperature surpassed 500 °C, the intensity of the anatase TiO2 (011) peak started to decrease, indicating that the anatase TiO2 was transformed into rutile TiO2. It is important to note that the coexistence of TiO2 polymorphs persists up to 800 °C, and the complete conversion to the rutile phase occurs at only 900 °C. In situ temperature-dependent micro-Raman spectroscopy was used to characterize the transformations of Ti2O3 particles. The capability of focusing on a micrometer scale allows the close inspection of individual particles and helps elucidate the transformation pathways.54,55 Because the coexistence of three phases, i.e., Ti2O3, rutile TiO2, and anatase TiO2, occurs around 500 °C, we designed two sets of experiments. In one set, the Ti2O3 particles were directly heated to 545 °C, while in the other set, the samples were heated to 465 °C. In general, as shown in Figures 4 and 5, softening and red shifts of Raman peaks were observed with an increase in temperature. Besides, the signal-to-noise ratio (SNR) of the Raman spectrum gradually became worser at higher temperatures. These effects can be attributed to lattice expansion and phonon−phonon interactions.56−60 As shown in Figure 4a, the Raman spectra of a single Ti2O3 particle as a function of temperature from 25 to 545 °C are dominated by the transformation from Ti2O3 to rutile TiO2. Distinct variations in the bandwidth, shape, and position of Raman modes were observed at 425 °C (Figure 4a,b), indicating the occurrence of the transformation. At 545 °C, almost pure rutile TiO261−63 characteristics were observed. Rutile TiO2 is tetragonal and belongs to the D4h14 (P42/mnm) space group with four active Raman modes (Figure S6): B1g (144 cm−1), multiphoton process (MPP, 237 cm−1), Eg (445 cm−1), and A1g (612 cm−1).62,63 Three Raman peaks, located at 227, 415, and 605 cm−1, were observed at 545 °C, which are consistent with the MPP, Eg, and A1g modes of rutile TiO2. The red shift of the Raman modes and the absence of the B1g

Figure 3. XRD patterns collected from Ti2O3 particles after annealing at different temperatures in air with a duration of 2 h. Corresponding photos of annealed Ti2O3 particles are also shown. Asterisks mark the (011) diffraction peak of anatase TiO2.

our work provided a new route for fabricating oxygen-deficient TiO2 by partial oxidation of Ti2O3 instead of reduction of TiO2. Close inspection of the XRD patterns reveals that 500 °C is the starting point for the transformation from Ti2O3 to rutile TiO2. It is important to note that the small peak located at 25.30°, marked by black stars in Figure 3, could be indexed to anatase TiO2 (011) (the simulated powder XRD pattern of anatase TiO2 is shown in Figure S4). Thus, anatase TiO2 was also formed during the annealing process, indicating another transformation pathway from Ti2O3 to anatase TiO2. In fact, the formation of anatase TiO2 occurred at 400 °C, and

Figure 4. (a) Evolution of the Raman spectra of a single Ti2O3 particle from 25 to 545 °C. (b) Temperature dependence of the shift in the Raman mode during the in situ micro-Raman experiment. 4386


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Figure 5. (a) Evolution of the Raman spectra of a single Ti2O3 particle from 25 to 465 °C, revealing the transformation from Ti2O3 to anatase TiO2. (b) Temperature dependence of the shift in the Raman mode during the in situ micro-Raman experiment.

Figure 6. TEM images of Ti2O3/TiO2 core−shell particles formed at 400 °C with different duration times: (a) 2, (b) 4, and (c) 6 h. (d−f) TEM images collected from three different Ti2O3/TiO2 core−shell particles formed at 500 °C, showing toothlike TiO2 shells.

Anatase TiO2 is tetragonal with D4h19 (I41/amd) space group symmetry. The spectrum collected at 465 °C is composed of four Raman modes located at 151, 392, 507, and 627 cm−1, which are in good agreement with those of anatase TiO2.67 Because of the temperature effect, red shifts were also observed in the peak positions. Because the XRD data suggest a dominating Ti2O3 phase in this temperature range, most likely a Ti2O3/A-TiO2 core−shell structure (inset of Figure 5b) was formed. To verify the assumptions about the Ti2O3 core−TiO2 shell structures, TEM images were collected from the samples annealed at 400 and 500 °C. Panels a−c of Figure 6 show the TEM images of the samples annealed at 400 °C with different duration times from 2 to 6 h. As expected, clear core−shell structures were observed. Moreover, the thickness of shells increased from ∼10 to ∼230 nm with a prolonged annealing

mode can be attributed to the high-temperature effect. As shown in Figure 4b, the region between 425 and 525 °C is featured with the coexistence of Ti2O3 (Eg) and rutile TiO2 (MPP, Eg, and A1g). This phase coexistence is consistent with the XRD results (Figure 3) and indicates that a Ti2O3/R-TiO2 core−shell structure (inset of Figure 4b) is formed in the intermediate temperature regime. In another set of experiments, Raman spectra were taken while the sample was heated to a lower temperature of 465 °C. In this case, anatase TiO2 was the dominating phase. Because anatase TiO2 is more photocatalytic than rutile TiO2,15 this finding may have important implications for catalysis applications. As shown in Figure 5a, the transformation starts at 365 °C (Figure 5a,b) with the coexistence of Ti2O3 and anatase TiO2 (Eg).64,65 When the temperature surpassed 405 °C, characteristics of pure anatase TiO266 were observed. 4387


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On the basis of the evolution of the structural and optical properties, the origin of the bandgap variation during phase transformation could be uncovered. As described above, the phase transformation pathway from Ti2O3 to TiO2 is Ti2O3 → TiO2−x → TiO2. It is well-known that the bandgap of TiO2 (3d0) is between the filled O 2p orbitals and the empty Ti 3d orbitals with a value of ∼3.0 eV (R-TiO2 for reference).19 Ti2O3 (3d1) is a well-investigated Mott insulator, which results from the strongly correlated 3d1 electrons at the Ti orbitals.68,69 Because of the strong Coulomb repulsion among these 3d1 electrons, the bandgap of Ti2O3 arises between the Ti 3d orbitals.70 As shown in Figure 8a, its

time. The surfaces of these core−shell structures, formed at 400 °C and indicated by red arrows, are smooth. Surprisingly, when the annealing temperature reached 500 °C, toothlike nanostructures, indicated by blue arrows, were observed at the sample surfaces, as shown in Figure 6d−f. More low-resolution TEM images are shown in Figure S7. On the basis of the XRD results (Figure 3), the formation of such a toothlike morphology of the TiO2 shell should be due to the drastic phase transformation from Ti2O3 to rutile TiO2 at 500 °C. Such toothlike surface nanostructures may drastically increase the overall TiO2 surface areas and further impact the potential catalysis capacity. After examining the structural evolution from Ti2O3 to TiO2, we turn to the corresponding physical properties tailored by high-temperature oxidation. Figure 7 shows the annealing

Figure 8. Schematic band diagrams of Ti2O3, TiO2−x, and TiO2. Abbreviations: CB, conduction band; VB, valence band; OV, oxygen vacancies.

bandgap is between a lower Ti 3d band, filled by the 3d1 electrons, and an upper empty Ti 3d band.23 When Ti2O3 was transformed into TiO2, Ti3+ was increased to Ti4+ with the 3d1 electron-filled band vanishing. As a result, the bandgap of TiO2 is between the empty Ti 3d band and the O 2p band, as shown in Figure 8c. It is noteworthy that before the stoichiometric TiO2 formed at a high temperature (900 °C), oxygen-deficient TiO2−x was formed at lower temperatures (600−800 °C). Thus, oxygen vacancy states were formed between the valence and conduction band of TiO2,71,72 as shown in Figure 8b. The electrons may be excited to the oxygen vacancy states from the valence band even with the energy of visible light, narrowing the intrinsic bandgap of TiO2.71 In our case, oxygen-deficient TiO2−x shows a yellow color with an optical reflection edge at ∼570 nm, corresponding to a bandgap of ∼2.2 eV. That is, the oxygen vacancy states formed ∼0.8 eV below the conduction band, which is consistent with the previous report.71 As a result, the bandgaps varied from 0.1 to 2.2 to 3.0 eV with the phase transformation from Ti2O3 to TiO2−x to TiO2. For many applications, electrical transport properties are important. We found that the resistance values of the samples annealed above 400 °C are very high (>100 MΩ) and exceed the resistance measurement limit of PPMS. This indicates that the annealing-induced TiO2 shell is effective at suppressing the carrier transport between conducting Ti2O3 particle cores. As shown in Figure 9, all samples show semiconducting behavior below 400 K, and higher resistivity was observed in the samples annealed with a longer duration time because of thicker TiO2 shells. The resistivity of Ti2O3 particles at 300 K is ∼0.009 Ω cm, consistent with the previous reports.4,73 For the Ti2O3/TiO2 core−shell samples, the resistivity was increased by ∼2−3 orders of magnitude. Specifically, the resistivities of those core−shell samples were at the range of

Figure 7. Diffuse reflectance spectra of the samples annealed at different temperatures.

temperature-dependent diffuse reflectance spectra of the samples in the wavelength range from the ultraviolet (UV) to the near infrared (NIR). For the RT sample, Ti2O3 exhibits strong broad-range light absorption, consistent with the previous report.24 As the temperature increased to 400 °C, the sample shows obvious reduction of reflectance compared to that of pure Ti2O3 in the NIR range from 720 to 1600 nm. (The result for prolonged annealing at 400 °C is shown in Figure S8.) This decrease in reflectance is supposed to be caused by the oxygen-deficient TiO2 shell. A significant increase in reflectance was observed when the annealing temperatures surpassed 600 °C, with the structure transformed to rutile TiO2. An optical reflection edge at ∼570 nm, corresponding to a bandgap of ∼2.2 eV, was consistently observed at high temperatures from 600 to 800 °C, in good agreement with the color variation (Figure 3), which reveals the products at this temperature range are oxygen-deficient rutile TiO2 (TiO2−x). Furthermore, as the temperature increased to 900 °C, another reflection edge at ∼410 nm became dominant, which corresponds to the intrinsic bandgap of rutile TiO2. 4388


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Figure 10. Schematic of the oxidation pathways from Ti2O3 to TiO2 particles. Corresponding lattice structures of Ti2O3, A-TiO2, and RTiO2 are also shown. Big blue spheres are Ti atoms. The small red sphere is the oxygen atom.

configuration, revealing the coordination number (CNo.) of titanium is 6. The bond lengths and angles of the octahedrally (or 6-fold) coordinated titanium atoms are indicated at the right side of the corresponding unit cells. As shown in Figure 11, the bonds between titanium and the oxygen atoms at the apexes of the octahedron in Ti2O3 are slightly longer than those in TiO2. Moreover, the octahedral configurations change with varied O−Ti−O bond angles, when Ti2O3 is transformed to A-TiO2 and R-TiO2. Importantly, the main variation during the transformation is that the coordination conditions of oxygen have been changed from 4- to 3-fold, which results in the oxidation of Ti3+ to Ti4+. More details about the parameters of the unit cells are listed in Table 1. As one of the most important findings of this work, the Ti2O3/TiO2 core−shell structures observed here are unique and distinct from that of reduced black TiO2 previously reported in the literature.29,44 Specifically, reduced TiO2 usually possesses a TiO2/TiO2−x core−shell structure (Figure S9),80,81 while a reversed Ti2O3/TiO2 core−shell structure was observed and proposed to be an efficient photocatalyst in this work. On the other hand, such mixed Ti3+−Ti4+ materials with tailored core−shell and surface structures were similar to Ti3+ self-doped TiO2 (mixed Ti3+−Ti4+), which have been reported to be efficient photocatalysts.41,43,82 Because no exotic impurity elements were introduced into self-doped TiO2, the intrinsic crystal structure of TiO2 was highly preserved. However, the bandgap of TiO2 was narrowed by the oxygen vacancies, which leads to the enhanced visible light absorption and photocatalytic performance.37,41 In the case of Ti2O3/TiO2 core− shell structures (without any other elements), the Ti2O3 core would act as the light-absorbing material because of its ultranarrow bandgap, while the TiO2 shell would act as the photocatalytic functional material because of its proper band alignment. A similar scenario was reported for the semiconductor/TiO2 core−shell structures, such as Si/TiO2,83 Cu2O/TiO2,84 and CdS/TiO2.85

Figure 9. Temperature-dependent resistivity of Ti2O3 particles and those annealed at 400 °C with different durations. The inset shows the ln(ρ) vs 1/T plot.

∼0.5−7.3 Ω cm (300 K), and the continuous tunability is advantageous for device applications. Furthermore, the ln(ρ) versus 1/T plot (inset of Figure 9) clearly shows thermal activation character for all samples. By assuming the complete carrier density contribution to the electrical resistivity,74 we can determine the activation energy from the equation ρ = ρ0 exp(Ea/kT),74,75 where ρ0 is the constant, Ea is the activation energy, k is Boltzmann’s constant, and T is the temperature. As shown in the inset of Figure 9, the activation energies in the range of 200−330 K were estimated to be 0.068−0.132 eV by extraction of the Arrhenius plots. The bandgap (Eg = 2Ea) of pure Ti2O3 was estimated to be 0.136 eV, which is consistent with the previous reports.4,75,76 As expected, the activation energy is higher with a thicker TiO2 shell, and the effective “bandgap” of Ti2O3/TiO2 core−shell samples could be manipulated up to 0.264 eV by tuning the annealing time. On the basis of the experiments described above, the transformation behavior of Ti2O3 particles during oxidation was clarified, which is schematically shown in Figure 10. As revealed by the XRD (Figure 3) and in situ Raman results (Figures 4 and 5), Ti2O3 particles start to be transformed to anatase TiO2 and rutile TiO2 at ∼400 and ∼500 °C, respectively, forming Ti2O3/TiO2 core−shell structures as the intermediate phases. TEM images (Figure 6) indicate that the thickness and morphology of the TiO2 shell could be controlled by tuning the annealing temperature and time, which further leads to the manipulation of the optical and transport properties of the Ti2O3/TiO2 core−shell structures. At annealing temperatures above 600 °C, the final product of oxidation transformation is rutile TiO2. As a result, Ti2O3 was transformed to anatase TiO2 and rutile TiO2 at different temperatures with the oxidation of Ti3+ to Ti4+. To shed light on the atomic structural variation during the oxidation, the details of the unit cells for Ti2O3,77 A-TiO2,78 and R-TiO279 are shown in Figure 11 and Table 1. In all structures, the basic building block consists of a titanium atom surrounded by six oxygen atoms in a distorted octahedral

IV. CONCLUSION In summary, we explored the transformation pathways between Ti3+-containing Ti2O3 and Ti4+-containing TiO2 using a complementary suite of experimental tools, revealing the connections between these two important members in the family of titanium oxides. Structural, phase, and property evolution during the oxidation process were systematically investigated as a function of annealing temperature and 4389


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Figure 11. Bulk structures of (a) Ti2O3, (b) A-TiO2, and (c) R-TiO2. In all structures, distorted octahedra are the basic building unites with Ti(CNo.) = 6. The bond lengths and angles of the octahedrally coordinated Ti atoms are indicated. CNo. is the coordination number.

Table 1. Parameters of the Unit Cells of Ti2O3, A-TiO2, and R-TiO2 space group

Z

lattice parameters

R3̅c

6

tetragonal

I41/amd

4

tetragonal

P42/mnm

2

a = b = 5.1490 Å c = 13.6420 Å α = β = 90°, γ = 120° a = b = 3.7850 Å c = 9.5196 Å α = β = γ = 90° a = b = 4.5937 Å c = 2.9587 Å α = β = γ = 90°

material

structure

Ti2O3

trigonal

A-TiO2

R-TiO2

unit cell volume (Å3)

y

z

313.22

Ti O

0.00000 0.31700

0.00000 0.00000

0.34500 0.25000

1.00 1.00

6 4

3+

0.1

136.38

Ti O

0.00000 0.00000

0.00000 0.00000

0.00000 0.21017

1.00 1.00

6 3

4+

3.2

62.43

Ti O

0.00000 0.30478

0.00000 0.30478

0.00000 0.00000

1.00 1.00

6 3

4+

3.0

■

ACKNOWLEDGMENTS This work is supported by the Singapore National Research Foundation under CRP Award NRF-CRP10-2012-02. The authors acknowledge the Singapore Synchrotron Light Source (SSLS) for providing the facility necessary for conducting the research. The laboratory is part of the National Research Infrastructure under the Singapore National Research Foundation.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b01739.

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Eg (eV)

x

duration. Importantly, unique Ti2O3/TiO2 core−shell structures with annealing-controlled surface nanostructures were observed for the first time. Along with the stoichiometric and structural evolution, optical and electrical properties were progressively tuned by annealing-controlled growth of Ti4+containing shells on the Ti3+-containing particle core. Connecting the multifunctional wide-bandgap TiO2 with the light-absorbing/conductive narrow-bandgap Ti2O3, Ti2O3 core−TiO2 shell structures may have important implications for the applications of titanium oxides in environmental and energy technologies.

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Ti occupancy CNo. state

structural parameters

SEM and TEM images of Ti2O3 particles; structural stability of Ti2O3 particles (outdoor); simulated XRD patterns for TiO2; XRD, reflectance, and low-resolution TEM images and schematics for core−shell structures; and the Raman spectrum of rutile TiO2 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Tom Wu: 0000-0003-0845-4827 Jingsheng Chen: 0000-0003-3188-2803 Notes

The authors declare no competing financial interest. 4390


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