Novel Single-Crystal Hollandite K1.46Fe0.8Ti7.2O16 Microrods

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Novel Single-Crystal Hollandite K1.46Fe0.8Ti7.2O16 Microrods: Synthesis, Double Absorption, and Magnetism Qadeer-Ul Hassan,† Dou Yang,† Jian-Ping Zhou,*,† Yu-Xi Lei,† Jing-Zhou Wang,† and Saif Ullah Awan‡ †

School of Physics and Information Technology, Shaanxi Normal University, Xi’an 710119, People’s Republic of China Department of Electrical Engineering, National University of Sciences and Technology (NUST), Islamabad 44000, Pakistan

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‡

ABSTRACT: Novel multifeatured hollandite K1.46Fe0.8Ti7.2O16 (KFTO) was synthesized by a simple hydrothermal method. Magnetic KFTO microrods were well controlled to long rectangular rods with pyramid-shaped tops. A KFTO growth mechanism was proposed on the basis of examining phase and morphology of the samples acquired at different reaction times. The KFTO morphology was confirmed by the calculated surface energies. The UV−vis diffuse reflectance spectra of KFTO microrods showed double absorption with band gaps of 2.01 and 2.16 eV, which was further confirmed by photoluminescence. First-principles studies revealed that the double absorption and magnetic properties originate from the d−d transitions of Fe3+ under the crystal field. The magnetic property could be applied in ferromagnetic semiconductor devices and the double absorption could be applied in visible-light harvesting. This work highlights the multifunctional KFTO microrods with low cost and environmental friendliness.

1. INTRODUCTION Titanium dioxide (TiO2) semiconductor has been widely investigated because of its long-term chemical stability, high catalytic activity, nontoxicity, strong optical absorption, low cost, and excellent band alignment to many oxidation− reductions.1−3 Many researchers focus on TiO2 nanoparticles for their outstanding performance in photocatalysis, hydrogen production, chemical sensors, sensitized solar cells, and adsorbents due to the large surface area. However, the TiO2 photoelectrochemical efficiency is limited by some factors, such as large band gap and low charge carrier transfer.4 Especially, the TiO2 wide band gap limits its light-harvesting capability to the visible light range, wasting about 95% of the solar light energy. This puts forward a challenge to develop new materials with small band gap relative to TiO2 to utilize most sunlight energy. Many efforts have been made to extend the TiO2 harvesting light range with long photogenerated charge carrier lifetimes,5 including composites with noble metal nanoparticles6,7 and other compounds,8 nonstoichiometric ion-doped TiO2,9−12 dye sensitization,13,14 and complex compounds based on TiO2.15−18 Among these research studies, complex compounds based on TiO2 can adjust the electronic structure and transfer the light absorption from UV to visible light, preserving the reliability of the crystal structure.18−22 The absorption extension to the visible light region can be achieved by the charge-transfer evolution involving the d electrons in the transition metals. Such transition metals create a novel electronic state within the electronic band of TiO2, which could imprison the stimulated © XXXX American Chemical Society

electrons from the TiO2 valence band and avoid the recombination of charge carriers. A TiO2-based M2IO−M2IIIO3−TiO2 system, where the Roman superscript represents the valence state of metal M, crystallizes in four different structures: spinel, tridymite, hollandite, and freudenbergite.23,24 Hollandite compounds represented as a general formula AxB8O16, where A is alkali or alkaline-earth cations, for example,, K, Na, Ba, Sr, Rb, and Cs while B is usually a mixture of tetravalent and trivalent cations in the oxygen octahedron, for example,, Mn, Ti, Al, Cr, V, and Si, enjoy rich compositions. The ideal hollandite shares a tetragonal structure with the space group I4/m.25−28 Recently, we have synthesized freudenbergite Na2Fe2Ti6O16 and Na0.9Mg0.45Ti3.55O8 with novel potential applications in photochemistry by a simple hydrothermal method.15,18 Fe(III) ions easily incorporate into the crystal lattice because of their close ionic radius. What is more, Fe(III) can create shallow charge-trapping centers within the TiO2 environment to produce a fresh energy level inside the band gap of TiO2, expressing as a double absorption. The double absorption allows electronic excitation by photons with less energy18,19,29 to extend the light absorption to the visible range.2 The double absorption commonly exists in doped materials,30−32 LiBr− H2O solution33 and organic compounds34,35 but appears only in limited inorganic materials, including MoP2,36 WP2,37 FeS2,38,39 Cu2SnS3,40 and Bi2Fe4O9.41,42 In addition, the origin Received: August 31, 2018

A

DOI: 10.1021/acs.inorgchem.8b02481 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. XRD patterns of KFTO powders prepared via the hydrothermal method (a) at 1.0 M KOH for 80 min with various reaction temperatures, (b) at 300 °C for 80 min with different alkaline concentrations, (c) at 1.0 M KOH, 300 °C for 80 min with various Fe/Ti ratios, and (d) at 1.0 M KOH, 300 °C for different times. received without further purification. Fe(NO3)3·9H2O and TiO2 with K1.46Fe0.8Ti7.2O16 stoichiometry were added into 108 mL of aqueous solution with different KOH concentrations. After stirring for 30 min, the mixture was transferred into a 135 mL stainless steel autoclave, sealed, and heated up to different temperatures for hydrothermal treatment under mechanical stirring, and finally, the mixture was cooled down to room temperature naturally. KFTO was collected and washed three times with distilled water and ethanol. Finally, the sample was dried at 80 °C for 24 h in an oven. Detailed characterizations were carried out to gain an insight into KFTO. The crystal structure was determined by an X-ray diffractometer ( D/Max2550, Rigaku, Japan) equipped with Cu Kα radiation of wavelength λ = 0.1541 nm. All of the samples were measured at a scanning rate of 5°/min in the 2θ range of 5−70° under 40 kV and 100 mA. The morphologies of the samples were examined by field emission scanning electron microscopy (FESEM, Hitachi, S4800) at an accelerating voltage of 15 kV. Energy dispersive X-ray spectroscopy was used to determine the elemental ratio in the samples. The crystalline characteristics of the KFTO microrods were determined by high-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED, Tecnai G2F20 S-TWIN, FEI, USA) at an accelerating voltage of 200 kV. Xray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB MKII X-ray photoelectron spectrometer (VG40 Scientific, UK) with monochromated Al Kα radiation to examine the elements in the samples. The magnetic properties were investigated in the temperature range of 10−300 K by a vibrating sample magnetometer using a superconducting quantum interference device (Cryogen-Free Magnet System, Cryogenic Ltd, UK). The magnetic hysteresis loops of KFTO microrods were measured at 10 and 300 K. UV−vis diffuse reflectance spectra were obtained using a UV−vis− NIR spectrophotometer (LAMBDA 950, PerkinElmer, USA) under the wavelength range of 200−800 nm. The photoluminescence (PL) excitation spectra of the samples were measured by using a

of the double absorption is an ongoing topic of debate. K1.46Fe0.8Ti7.2O16 (KFTO) with the hollandite structure is also found to enjoy double absorption in this work. KFTO was synthesized first by a solid-state reaction in 1986,43 but its physical and chemical properties have rarely been explored because pure KFTO is difficult to be synthesized by other techniques due to its instability.44,45 On the other hand, the carrier transport and charge separation within the photoelectric materials are significant for device performance, greatly depending on the morphology and configuration of the oxide-based semiconductor. Thus, nanorods are in demand as one of photoelectrode composition, efficiently shortening the electron transport path for charge collection. The main challenge to synthesize materials with large-scale size is to control the particle nucleation and growth stages during the reaction. Herein, we explored a new manifestation of a TiO2-based lamellar structure of KFTO single crystallite with rectangular microrods. We synthesized rodlike KFTO after a series of experiments by a hydrothermal method. The effects of experimental parameters are researched in detail, including reaction temperature, alkaline concentration, reaction time, and Fe/Ti ratio. Also, the multifeatures of the KFTO compound were investigated by theoretical simulation and experimental studies, including the double absorption and magnetism.

2. EXPERIMENTAL METHODS AND MEASUREMENTS KFTO was prepared by a hydrothermal method under different experimental conditions. The chemicals titanium dioxide (98.8%), iron nitrate (98.5%), and potassium hydroxide (85%) were used as B


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Inorganic Chemistry fluorescence spectrometer (F-7000, Hitachi, Japan) with a xenon lamp at a detection wavelength of 699 nm. The Fourier transform infrared (FT-IR) spectra of the KFTO samples in a KBr pellet were recorded on a V70 FTIR spectrophotometer (Bruker).

peaks correspond to monoclinic K2Ti6O13 [space group: C2/ m(12), JCPDS no. 74-0257] and K2Ti4O9 [space group: C2/ m(12), JCPDS no. 32-0861]. The arising of peaks at 17.45°, 27.76°, 36.11°, 40.36°, and 62.48° indicates the formation of the KFTO phase. The incomplete reaction of TiO2 is verified by a small peak at 25.30°. When the Fe/Ti molar ratio increases to 1:4.5, the intensity of the peak at 27.76° increases. The crystallinity is significantly improved and the pure crystalline KFTO is obtained when Fe/Ti = 1:3, which suggests that the Fe ion solubility is limited and an excess amount of Fe(NO3)3·9H2O is needed. It can be seen that further increase in the Fe/Ti ratio to 1:2.25 brings the Fe2O3 (33.32°) phase (JCPDS no. 80-2377) in the product. Thus, Fe/Ti molar ratio of 1:3 is the optimal condition to synthesize pure KFTO phase. Therefore, we prepared samples at 300 °C, 1 M KOH, and Fe/Ti = 1:3 with different reaction times to probe the formation process of KFTO crystallinity. Figure 1d shows the XRD patterns of the samples synthesized at 300 °C and 1 M KOH for different reaction times. The main phases are K2Ti6O13 and K2Ti4O9 with small KFTO in the powder under a short reaction time of 10 min. Pure KFTO powder is obtained over 20 min. We selected part samples hydrothermally synthesized at 1 M KOH, 300 °C, and Fe/Ti = 1:3 reaction times of 20, 40, and 80 min for further detailed research, abbreviated as KFTO-20, KFTO-40, and KFTO-80, respectively. The FT-IR spectra are recorded to evaluate the Fe/Ti−O bonds in the hollandite KFTO powders. Figure 2 shows the

3. COMPUTATIONAL DETAILS The band structures, densities of states, and magnetic properties of KFTO were calculated with the Vienna ab initio simulation package program developed at the Institute für Materialphysik of the Universiät Wien46 within the MedeA software environment.47 The exchange−correlation energy of the Perdew−Burke−Ernzerhof functional48 with generalized gradient approximation (GGA) and the projector augmented wave potentials based on the density functional theory were adopted for the calculations.49 A Monkhorst−Pack k-point mesh of 3 × 3 × 8 was used with the k point separation of 0.3/ Å in the Brillouin zone of the reciprocal space. The kinetic cutoff energy of 500 eV was chosen for plane-wave expansion. The valence electronic configurations for K, Fe, Ti, and O are K 4s1, Fe 3d64s2, Ti 3d24s2, and O 2s22p4, respectively. The electronic iteration convergence for the structural optimization and energy calculation was 1 × 10−5 eV/atom for a selfconsistent field. The optimal atomic positions were obtained by relaxing them. Then, the simplified LDA + U (LDA plus onsite Coulomb repulsion term) approach was used in the calculations of electronic structure. The Coulomb repulsion applied to the Ti d orbital and Fe d orbital was 2 eV.50 4. RESULTS AND DISCUSSION 4.1. Structural Analysis. Figure 1a shows the X-ray diffraction (XRD) patterns of KFTO samples synthesized by hydrothermal treatment with 1 M KOH for 80 min at different temperatures. KFTO is a predominant phase with a little anatase TiO2 (JCPDS no. 78-2486) at 260 °C. The phase purity is significantly improved with the increase in temperature. Pure KFTO is obtained over 300 °C, indicating that temperature plays a decisive role in KFTO formation. The diffraction peaks well meet the pure KFTO (JCPDS no. 770990) with a tetragonal structure with lattice parameters of a = b = 10.147 Å and c = 2.962 Å, suggesting highly crystalline KFTO. The KFTO structure is composed of interlinked FeO68− and TiO68− octahedra to form a framework structure with square hollandite-like tunnels (H tunnels) and empty rutile-like tunnels (R tunnels) along the c-axis.27 These tunnels are occupied by K+ ions at 2a (K1) and 4e (K2) Wyckoff sites. The Fe3+, Ti4+, and O2− ions occupy 8h Wyckoff sites.23,51 The KOH concentration is also crucial in the synthesis of pure KFTO as shown in Figure 1b. The KFTO phase is less and the mixture cannot get a good crystallinity under a low alkali concentration of 0.5 M. The content of KFTO increases with the KOH concentration, and pure KFTO is obtained at 1 M KOH at 300 °C for 80 min. However, further increasing KOH concentration, other phases are formed, including monoclinic K2Ti6O13 and K2Ti4O9. Therefore, we consider that the optimal alkali concentration is 1 M for the synthesis of pure KFTO. It is interesting to note that an appropriate amount of Fe/Ti molar ratio is an important factor to affect the morphology, phase, and crystallinity in the hydrothermal synthesis of the pure KFTO products. Figure 1c shows the XRD patterns of the samples synthesized with different Fe/Ti molar ratios from 1:9 to 1:2.25 under the same reaction condition of 300 °C, 1 M KOH, and 80 min. When Fe/Ti = 1:9, the main diffraction

Figure 2. FT-IR spectra of KFTO samples.

FT-IR spectra of the KFTO samples. The characteristic vibration bands in the 800−400 cm−1 region are mainly assigned to the (Fe/Ti)O68− octahedral modes. The sharp band near 770 cm−1 is attributed to (Fe/Ti)O68− stretching modes, whereas the weak bands around 630−590 cm−1 are due to the bending vibrations of the (Fe/Ti)O68− octahedra.23,26 These results further confirm the formation of hollandite KFTO. 4.2. Microstructure and Morphology. Figure 3 shows the typical SEM images of the as-prepared KFTO microrods synthesized by the hydrothermal method at 300 °C, 1.0 M with different reaction times. Figure 3a−d illustrates the typical overall view of the KFTO-80 microrods with different magnifications. The pure KFTO exhibits a flower-like structure composed of rectangular microrods. The uniform microrods with smooth surfaces display large square columns with pyramidal tops, where the square length is about 500 nm and height is in the range of 5−10 μm. The microrods of KFTO-20 and KFTO-40 are slightly smaller than those of KFTO-80 as shown in Figure 3e,f. C


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Inorganic Chemistry

Figure 3. Representative SEM images of the KFTO products prepared at 300 °C, 1.0 M alkaline concentration with different reaction times. (a−d) KFTO-80 with different magnifications, (e) KFTO-20, and (f) KFTO-40. (g) SEM image of the KFTO microrod and corresponding EDXS mappings of the elements, demonstrating uniform element distributions in the KFTO rectangular microrod. (h) EDXS spectrum of the microrod.

Figure 4. (a) Typical TEM images of an individual KFTO microrod obtained at 1.0 M KOH, 300 °C for 80 min with Fe/Ti = 1:3; (b) partly magnified TEM image taken from the top area of the KFTO microrod; (c) schematic illustration of the crystal planes for the KFTO microrod; (d− f) corresponding HRTEM images taken from the red rectangular areas in (a); (g−i) corresponding HRTEM image taken from the red rectangular areas in (d−f); and (j) corresponding SAED pattern from the top area of the microrod in (a).

consistent with the SEM results. Figure 4d−f exhibits the corresponding enlarged images from the red rectangular areas in Figure 4a, recorded at the top, side edge, and rectangular edge, respectively. Figure 4g−i shows the corresponding HRTEM images taken from the red rectangular areas in Figure 4d−f, respectively. All of the figures show three sets of lattice fringes with spacings of 0.28, 0.28, and 0.50 nm, ascribed to (1 0 1), (1̅ 0 1), and (2 0 0) crystal planes, respectively. The angles 74°, 74°, and 32° between (1 0 1) and (2 0 0), (1̅ 0 1) and (2 0 0), and (1 0 1) and (1̅ 0 1) crystal planes are consistent with the theoretical values of crystalline KFTO. The corresponding SAED image in Figure 4j indicates

Energy dispersive X-ray spectrometry (EDXS) mappings in Figure 3g exhibit that K, Fe, Ti, and O elements are unambiguously distributed in the KFTO microrod, suggesting the uniform element distributions along the single-phase KFTO microrod. The atomic ratio of K/Fe/Ti/O (5.92:3.45:24.59:66.04) is close to the stoichiometric value in K1.46Fe0.8Ti7.2O16 as shown in Figure 3h, confirming that single-phase KFTO is obtained. The microstructural and morphological details of the KFTO samples were further evaluated by HRTEM as shown in Figure 4. Micrographs from the front of a KFTO crystallite in Figure 4a,b clearly show a long rectangular rodlike structure, which is D


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Inorganic Chemistry

Figure 5. (a) Formation illustration of lamellar KFTO microrods and (b) lamellar structure of KFTO formed by the alternation of two layers of (Fe, Ti)O68− octahedra.

Figure 6. High-resolution XPS spectra of (a) K 1s, (b) Fe 2p, (c) Ti 2p, and (d) O 1s for the pure KFTO samples. The chemical states are mainly K+, Fe3+, Ti4+, and O2−, close to that in crystalline K1.46Fe0.8Ti7.2O16.

hydrothermal process, providing potassium ion and providing an alkaline environment to dissolve the precursors. It is well known that TiO2 undergoes water-induced dissolution as structural units of TiO68− octahedra [eq 2] and then recrystallization, resulting in titanate formation under the high temperature and high alkali concentration of the hydrothermal environment. 52,53 Though K 2 Ti 6O 13 and K2Ti4O9 have been prepared by a more complex process,54,55 they are synthesized in a short reaction time and low Fe/Ti ratio under the hydrothermal environment of high temperature 300 °C in this work. The K2Ti6O13 and K2Ti4O9 nuclei are formed through strong electrostatic attraction with oriented aggregation between K+ ions and TiO68− octahedra [eq 4]. Then, the layered structure is formed as shown in Figure 5a. By increasing Fe concentration and reaction time, the Fe3+ (0.64 Å) ions, which are similar to the Ti4+ ions (0.61 Å), replace Ti4+ ions in the TiO68− octahedral framework through ion diffusion, keeping the octahedral symmetry of the original tetragonal crystal to form the pure KFTO phase [eq 5]. Thereupon, nucleation starts with the formation of primary particles, resulting in hollandite KFTO precipitation. This process needs 20 min to form the pure KFTO phase. Consequently, the Ostwald ripening process occurs through small particles in the suspension redissolving and then depositing on the surfaces of larger particles with different

that the as-prepared KFTO microrods grow along the [0 0 1] direction in length. The fine HRTEM crystallographic strips and clear periodic SAED patterns confirm the single-crystalline behavior of the KFTO microrods. Figure 4c presents the schematic illustration of the crystal orientation for the KFTO microrods based on the above discussion. It is notable that the {3 0 1} crystal planes are parallel to the short edges and the {2 0 0} planes are parallel to the long edge in the microrod. The long rectangular KFTO microrods with well-defined crystalline facets have been synthesized. 4.3. Crystal Growth Mechanism. In the last decade, many researchers have reported that the microstructures of metallic oxides can be hydrothermally controlled in alkaline solution. The OH− concentration in a hydrothermal environment plays a key role in the growth rate of crystal faces, leading to the formation of anisotropic particles, such as rodlike morphology in our case. Although the detailed information of the reaction mechanism in the hydrothermal process is still unclear, the phase and powder morphology acquired at different reaction times give an evidence of reactions from anatase TiO2 to KFTO with an intermediate reactant. The schematic growth process of the uniform KFTO microrods along their SEM images is illustrated in Figure 5 on the base of the above experimental results. KOH plays dual roles in this E


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Inorganic Chemistry

Figure 7. (a,b) Magnetic hysteresis loops at 10 and 300 K for KFTO-80 microrods and (c) magnetic moment as a function of temperature following FC and ZFC modes under a magnetic field of 0.4 T.

Figure 8. (a) UV−vis absorption spectra of the pure KFTO samples. (b) ln(αhν) vs ln(hν − 2.21) and ln(hν − 2.53), meaning KFTO is a direct gap semiconductor, and (c) (αhν)2 vs photon energy (hν) to determine the band gap.

growth rates according to their surface energies.56 Thus, a single phase of KFTO microrods was formed by transforming the anatase structure to KFTO without changing the parent TiO68− crystal units. Figure 5b illustrates the idealized structure of KFTO, constituting the alternation of two types of layers. Each layer is formed in turn by an assembly of two (Fe/Ti)O68− octahedrons represented by cyan color, bonded together by the potassium ions displayed as spheres inside the light purple cubic box. According to the XRD results, we generally speculate the hydrothermal reaction process starting with KOH, Fe(NO3)3· 9H2O, and TiO2 precursors as follows KOH → K+ + OH−

(1)

TiO2 (amorphous) + H 2O → TiO68 − + H+

(2)

Fe(NO3)3 ·9H 2O + OH− → Fe(OH)3 + HNO3

(3)

K+ + TiO68 − → K 2Ti6O13 /K 2Ti4O9

(4)

energy of 529.6 eV corresponds to the O2− state in the KFTO crystallite and the additional peak at 531.63 eV is attributed to the oxygen from CO2 and H2O adsorbed on the powder surface.57 It is concluded that the K and Fe ions have successfully incorporated into the TiO2 crystal lattice to form layered KFTO. However, a little change in the valence state of Fe and Ti ions may be possible according to the formula K1.46Fe0.8Ti7.2O16, in which the valence states of Fe and Ti ions are mainly 3+ and 4+, but a little 2+ and 3+ respectively. 4.5. Magnetic Properties. The hysteresis loops of KFTO80 measured at 10 and 300 K are presented in Figure 7a,b, respectively, indicating the coexistence of ferromagnetic and paramagnetic characters in the sample. We subtracted the background signals from the sample holder for getting more visible finding of ferromagnetism. The saturation magnetization (Ms) of the sample was calculated by subtracting the paramagnetic component from the M−H data. The saturation magnetization (Ms) values are approximately 0.016 and 0.09 emu/g, while the coercivity values (Hc) are 130 and 90 Oe at 300 K and at 10 K, respectively. Figure 7c shows the temperature dependence of magnetization under a magnetic field of 0.4 T from 10 to 300 K by zero-field cooling (ZFC) and field cooling (FC) for KFTO-80. The magnetization of the KFTO-80 microrods increases gradually with decreasing temperature. This is a general phenomenon at low temperature because of the decrease in thermal agitation. All unpaired spins are almost aligned and the net coupling enhances, and resultantly, the magnetic moment increases. A small divergence in ZFC and FC curves appears, and usually this bifurcation is a common phenomenon in the diluted magnetic semiconductor. 4.6. Optical Characterizations. Figure 8a shows the UV− visible diffuse reflectance spectra of the KFTO microrods, revealing obvious wide absorption in the visible light region (400−700 nm). Therefore, KFTO is active under visible light in comparison with TiO2, which absorbs less than 400 nm ultraviolet light. A significant aspect in Figure 8a is the two

Fe3 + + K 2Ti6O13 /K 2Ti4O9 → K1.46Fe0.8Ti 7.2O16 (microrods)

(5)

4.4. Chemical State. XPS analysis was carried out to examine the chemical state of elements in KFTO powders. Figure 6 shows the XPS spectra of K 1s, Fe 2p, Ti 2p, and O 1s, where the C 1s peak at 284.6 eV was used as a reference for the calibration of the binding energy scale. The survey XPS spectra (not shown here) confirm that KFTO samples consist of K, Fe, Ti, and O elements. K 2p signals show binding energies around 292.3 eV (K 2p1/2) and 284.6 eV (K 2p3/2) [Figure 6a], corresponding to the K+ cation. The peaks around 710.8 and 724.5 eV [Figure 6b] are assigned to Fe 2p3/2 and Fe 2p1/2, respectively, of a Fe3+ ion. The binding energies of Ti 2p in Figure 6c around 463.7 eV (Ti 2p1/2) and 458.0 eV (Ti 2p3/2) are identical to the Ti4+ chemical state. O 1s XPS spectra in Figure 6d show two binding peaks. The binding F


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Inorganic Chemistry

Figure 9. PL spectra of KFTO samples. The peaks are fitted with a Gauss function.

absorption edges, which only exist in some limited inorganic materials18,36,37,39−42,58 as an ongoing topic of debate, although common in composites, doped materials, and organic compounds. The optical absorption spectra are typical of semiconducting transition-metal oxide compounds, like Fe2O3 and Fe3O4, because of the Fe3+ ions. Fe2O3 and Fe3O4 exhibit direct and indirect semiconductors for the complex electronic behaviors.59−62 KFTO is determined as a direct band gap material though its complex crystal structure. The optical absorption near the band edge follows αhν = A(hν − Eg )n /2

(6)

where A is a proportionality constant and α, ν, h, and Eg are the absorption coefficient, light frequency, Planck’s constant, and band gap energy, respectively. The power index n is determined by the type of electronic transition in a semiconductor. KFTO is a direct band gap material because n = 1 for both absorption edges by linearly fitting ln(αhν) versus ln(hν − Eg) as shown in Figure 8b with the data near the inflection point in Figure 8a. Then, the plots of (αhν)2 dependence on the photon energy hν in Figure 8c show us that the Eg values of KFTO are 2.01 and 2.16 eV by the intercept of the extrapolated line to the hν axis. These values are much lower than the TiO2 band gap (3.2 eV), and the flat absorption between two absorption edges enhances the visible light harvesting.2,18 Therefore, it is suggested that other materials can be loaded on KFTO to boost the photochemical or photocatalytic properties. The double absorption edges are further confirmed by the PL spectra in Figure 9. All samples exhibit two peaks centered around 587.4 and 558.4 nm, which are consistent with the double band edges. 4.7. Band Characteristics. The electronic bands were calculated using an ab initio method to understand the origin of optical and magnetic properties of KFTO. Figure 10a shows the KFTO primitive cell with tetragonal parameters of a = b = 10.182 Å, c = 2.966 Å, whose molecular formula is K2FeTi7O16 for simple calculation. A K2FeTi7O16 unit cell was built by substituting one Ti atom for a Fe atom in the K2Ti8O16 crystal, where eight Ti atoms are equivalent. The atomic fractional coordinates for various elements are K at (0.0055 0.9948 0.5000), Fe at (0.8417 0.6806 0.5000), Ti at (0.3417 0.8423 0.5000), and O at (0.6459 0.6863 0.5000) and (0.3326 0.0401 0.5000) after atomic position relaxation. The Coulomb repulsion applied to the Ti d orbital and Fe d orbital was 2 eV to calculate the electronic structure.50 K2Ti8O16 has a framework structure built of (Fe/Ti)O68− octahedra that share edges to form double zigzag ribbons constructed by blocks linked through octahedral vertices as shown in Figure 10b. This linkage of polyhedra consists of two different tunnels: the first is the square hollandite-like tunnels (H tunnels), which

Figure 10. (a) Primitive cell and (b) polyhedral structure of KFTO. (c) Charge density difference between elements and (d) blown-up charge density difference of an Fe atom. The yellow and turquoise isosurfaces suggest electron density increase and decrease by 0.024 electrons/Å3, respectively. (e) Magnetization density of elements in the KFTO primitive cell. The turquoise and yellow isosurfaces correspond to negative and positive spin-charge densities of 0.0005 electrons/Å3, respectively.

contain alkali metal, and the second is empty rutile-like tunnels (R tunnels). The polyhedra of atoms in H type of tunnels construct cubes linked by face sharing and compressed along the c axis.27,43 The charge redistributions are shown in Figure 10c by plotting the charge density difference in a K2FeTi7O16 crystal. The electrons obviously transfer from the central titanium ions to the edge oxygen ions in the octahedra. This transfer of electrons results in the accumulation of charges around the oxygen atoms and consumption over the titanium atoms, suggesting the existence of an ionic component between O and Ti atoms. On the other hand, the electron density of Fe atoms piles up along the FeO68− octahedra directions between iron and oxygen atoms as shown in Figure 10d. The redistributions are from crystal field splitting, which results in the indirect exchange interaction between the magnetic iron ions through nonmagnetic oxygen ions. Figure 10e shows the spin densities of different elements in the K2FeTi7O16 primitive cell. It is established that the magnetization density is predominantly confined to the iron atoms and a little to the oxygen atoms. Thus, the magnetic nature of a K2FeTi7O16 crystal is assured G


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Inorganic Chemistry

Fe3+ → Fe4+ + Fe2+).32,63 However, this charge recombination center limits the photocatalytic efficiency, resulting in a low photocatalytic ability of K2FeTi7O16. As a consequence, the energy levels around the Fermi surface undergo a spin split because of the indirect exchange interaction between the magnetic iron ions through nonmagnetic oxygen ions, causing magnetism in K2FeTi7O16. The total magnetic moment produced in the K2FeTi7O16 primitive cell is 0.0107 μB, which gives a reasonable explanation for the magnetic behavior in Figure 7. The observed ferromagnetism in these semiconductor systems is intrinsic resulting from the incorporation of Fe3+ ions into the hollandite-type TiO2 lattice. The magnetic moment mainly originates from the d−d transitions in Fe3+ ions.41,42 4.8. Surface Energy. The final crystal morphology is determined by the crystal structure and surface energy during crystal growth in the Ostwald ripening process. Commonly, the growth rate of the (h k l) face is proportional to its attachment energy. The surface growth rate dominates the crystal morphology, where the face with a lower growth rate will be the most morphologically pronounced. The surface with the lowest surface energy diminishes rapidly for its high crystal growth rate. Then, we calculated the surface energies of the K2FeTi7O16 crystal to understand the formed microrods in Figure 3. The vacuum layer is established as 14 Å to eliminate the interaction between the two surfaces of every slab. All ideal K2Ti7FeO16 (1 0 0), (0 0 1), (1 1 0), and (3 0 1) surfaces have several different surface configurations under consideration of the surface atoms and terminational patterns as shown in Figure 12. There are four surface models for the (1 0 0) surface, three models for the (3 0 1) surface, and two models for the (0 0 1) and (1 1 0) surfaces. The two layers of atoms at the bottom of the slab are fixed to maintain the bulk structure during the calculation, allowing the other atoms to relax. The surface model of the K2Ti7FeO16 crystal is derived by cutting the bulk K2Ti7FeO16 crystal in a defined direction. When the ideal crystal is cut, two complementary surfaces are produced. The cutting energy is defined as the cleavage energy, that is,64 1 unrel unrel Eclunrel = [Eslab (A) + Eslab (B) − nE bulk ] (7) 4S

by the presence of yellow and turquoise isosurfaces at Fe and O atoms. Then, we calculate the band structure and densities of states of K2FeTi7O16 to understand the magnetism and double absorption deeply. The coordinates of high-symmetry K points are Z(0 0 0.5), A(0.5 0.5 0.5), M(0.5 0.5 0), G(0 0 0), R(0 0.5 0.5), X(0 0.5 0), N(0 −0.5 0), S(0 −0.5 0.5), C(−0.5 −0.5 0.5), and B(−0.5 −0.5 0) in the first Brillouin zone as shown in Figure 11a. Figure 11b,c shows the electronic band structures

Figure 11. (a) High-symmetry points in the first Brillouin zone for the band structure calculation, (b,c) spin-polarized band structures, and (d) total density of states and partial density of states of KFTO.

of K2FeTi7O16 near the Fermi levels. There are no clear difference between positive and negative directions (such as GM and GB directions and GX and GN directions, respectively) though the Fe ion reduces the I4/m(87) symmetry of K2Ti8O16. The multiband nature of K2FeTi7O16 is in good agreement with our aforementioned UV−visible absorption in Figure 8. The valence and conduction bands are divided by intermediate energy bands represented by red lines in Figure 11b,c. Figure 11d shows the total and partial densities of states, which gives clear evidence that the valence band top and conduction band bottom are mainly from the O 2p and Ti 3d orbitals, respectively. The middle energy bands are mainly from Fe 3d orbital state and a small part from O 2p orbital state, resulting in a coupling interaction between these two orbitals. The origin of double absorption is an ongoing topic of debate in inorganic materials. Double adsorption in monoclinic Cu2SnS3 was attributed to the structural defects.40 The second absorption edge in semimetallic MoP2 and WP2 is the result of transition among the sub-bands inside the band gap between the valence band and conduction band.36,37 The double absorption in Bi2Fe4O9 was assigned to the d−d transitions of Fe3+ ions.41,42 On the base of above discussion, there are three different types of possible optical transitions in K2FeTi7O16: (1) from the valence band to the conduction band (Ec); (2) from the middle band to the conduction band; and (3) from the valence band to the middle band. Therefore, it is suggested that the presence of a middle band in K2FeTi7O16 acts as a charge recombination center for electron−hole pairs, resulting from the d−d transition of Fe3+ (2T2g → 2A2g, 2T1g) or the charge-transfer transition between interacting iron ions (Fe3+ +

where Eunrel slab stands for the unrelaxed energy of the slab, Ebulk is the total energy of the bulk crystal, n is the number of bulk crystal that the slab contains, S is the surface area of the slab, 4 in the denominator refers to the four surfaces produced when the crystal is cut, and A and B represent two complementary surfaces. In this work, all surfaces are considered to have selfcompensating characteristics. The changed energy after slab optimization is defined as the relaxation energy, expressed as 1 unrel rel Erel(A) = [Eslab (A) + Eslab (A)] (8) 2S where Erel slab(A) represents the total energy after slab relaxation. The sum of cleavage energy and relaxation energy can give surface energy, that is, Esur(A) = Eclunrel(A) + Erel(A)

(9)

Ti2−O3−Fe3, Ti2−O2−O3−Fe2−K1, Ti1−Fe1, and Ti1−Fe1 terminated surfaces are selected for (1 0 0), (0 0 1), (1 1 0), and (3 0 1) surfaces because of their lower surface energy as H


Article

Inorganic Chemistry

0 1) surface because of its higher dangling bond density as shown in Figure 12m−p. Then, the crystal spontaneously transforms into a specific shape with exposed facets that minimize the total surface free energy.3 The surfaces with high reactivity usually diminish rapidly during the crystal growth under equilibrium conditions. The (1 1 0) facets with higher surface energy (1.569 J/m2) absorb more ions and disappear gradually. The particles deposit on the {3 0 1} surfaces rapidly and leave large {1 0 0} surfaces. Figure 12l shows the equilibrium shape of K2Ti7FeO16 simulated using the Hartman−Perdok method with the four surface energies. The simulated morphology exhibits a clear elongation along the c direction, showing a good agreement with the experimental results in Figures 3 and 4.

5. CONCLUSIONS In summary, we prepared single-crystal KFTO with a tetragonal structure belonging to the hollandite family by a facile hydrothermal method. Uniform KFTO microrods obtained after systematic experiments show long rectangular rods with pyramid-shaped tops. A growth mechanism was proposed for the KFTO microrods, which was simulated by calculating the surface energies. Semiconductor KFTO exhibits magnetic property and double absorption with band gaps of 2.01 and 2.16 eV. The former favors the magnetic separation to recycle KFTO and the latter extends the light adsorption range, which is rare in inorganic materials. First-principles studies reveal that the double absorption and magnetic properties originate from the d−d transitions of Fe3+ under the crystal field effect. The present work suggests that the magnetic semiconductor KFTO has potential applications in electric devices owing to its simple preparation, broad visibleregion absorption, low cost, and environmentally benign nature.

Figure 12. Side views of different surfaces of the K2Ti7FeO16 crystal structure, (a) (1 0 0)-Ti2−O3−Fe3, (b) (1 0 0)-O2−K1, (c) (1 0 0)Ti2−Ti3−O3−O4, (d) (1 0 0)-O1−O2−K1, (e) (0 0 1)-Ti2−O2−O3− Fe2−K1, (f) (0 0 1)-Ti2−O2−O3−K1, (g) (1 1 0)-Ti1−Fe1, (h) (1 1 0)-O1−O2−O3, (i) (3 0 1)-Ti1−Fe1, (j) (3 0 1)-Ti3−K2−O2−O3−O4, and (k) (3 0 1)-Ti1, where the surfaces signed with bold red alphabetical numbers have lower surface energy. (l) K2Ti7FeO16 equilibrium shape simulated with the Hartman−Perdok method. Top views of the equilibrium shape of K2Ti7FeO16 surface structures, (m) (100), (n) (001), (o) (110), and (p) (301) surfaces. The atoms were labeled with their coordination, such as 2-fold-coordinated oxygen (O2c), 3-fold-coordinated iron (Fe3c), 4-fold-coordinated titanium (Ti4c), and so on.

shown in Table 1. The calculated surface energies follow the order (1 0 0) < (3 0 1) < (0 0 1) < (1 1 0). The (3 0 1) surface

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Table 1. Cleavage Energy, Relaxation Energy, and Surface Energy of the (100), (001), (110), and (301) Surfaces with Different Terminationsa surface plane (100)

(001) (110) (301)

termination

Eunrel cl (J/m2)

Erel (J/m2)

Esur (J/m2)

Ti2−O3−Fe3 O2−K1 Ti2−Ti3−O3−O4 O1−O2−K1 Ti2−O2−O3−Fe2−K1 Ti2−O2−O3−K1 Ti1−Fe1 O1−O2−O3 Ti1−Fe1 Ti3−K2−O2−O3−O4 Ti1

0.855 0.788 1.492 1.239 1.013 1.013 3.221 2.123 1.150 1.153 1.037

−0.573 −0.460 −0.812 −0.663 −0.502 −0.490 −1.652 −0.549 −0.726 −0.563 −0.609

0.282 0.328 0.680 0.576 0.511 0.523 1.569 1.574 0.424 0.590 0.428

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jian-Ping Zhou: 0000-0003-0807-1404 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant no. 51672168).

■

REFERENCES

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a

The bold number indicates the corresponding terminal surface with a lower surface energy.

has higher surface energy than the (1 0 0) surface because both Ti’s coordination and Fe’s coordination on the surface of the former are lower than the latter. Although Ti’s coordination and Fe’s coordination on the surface of (3 0 1) are lower than those of (0 0 1), the latter has higher energy because of more dangling bond types such as twofold O, threefold O, and fourfold K. The (1 1 0) surface is much less stable than the (0 I


Article

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K


Novel Single-Crystal Hollandite K1.46Fe0.8Ti7.2O16 Microrods

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