Intermediate Band Material of Titanium-Doped Tin Disulfide for Wide

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Intermediate Band Material of Titanium-Doped Tin Disulfide for Wide Spectrum Solar Absorption Keyan Hu,†,‡ Dong Wang,† Wei Zhao,† Yuhao Gu,§ Kejun Bu,† Jie Pan,† Peng Qin,† Xian Zhang,*,†,§ and Fuqiang Huang*,†,§ †

CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China ‡ School of Mechanical and Electrical Engineering, Jingdezhen Ceramic Institute, Jingdezhen 333403, PR China § Beijing National Laboratory for Molecular Sciences and State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China ABSTRACT: Intermediate band (IB) materials are of great significance due to their superior solar absorption properties. Here, two IBs peaking at 0.88 and 1.33 eV are reported to be present in the forbidden gap of semiconducting SnS2 (Eg = 2.21 eV) by doping titanium up to 6 atom % into the Sn site via a solidstate reaction at 923 K. The solid solution of Sn1−xTixS2 is able to be formed, which is attributed to the isostructural structure of SnS2 and TiS2. These two IBs were detected in the UV−vis−NIR absorption spectra with the appearance of two additional absorption responses at the respective regions, which in good agreement with the conclusion of first-principles calculations. The valence band maximum (VBM) consists mostly of the S 3p state, and the conduction band minimum (CBM) is the hybrid state composing of Ti 3d (eg), S 3p, and Sn 5s, and the IBs are mainly the nondegenerate t2g states of Ti 3d orbitals. The electronic states of Ti 3d reveal a good ability to transfer electrons between metal and S atoms. These wide-spectrum absorption IBs bring about more solar energy utilization to enhance solar thermal collection and photocatalytic degradation of methyl orange.

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materials.20,21 Titanium sulfide TiS2 is isostructural to SnS2. More importantly, the bandgap of TiS2 semiconductor is 0.53 eV,22 which is smaller than that of SnS2 (Eg = 2.0−2.6 eV). Since the Ti4+ and Sn4+ ions are of similar ionic radii (Ti4+ 0.74 Å, Sn4+ 0.81 Å), TiS2−SnS2 system may form a solid solution.23 In the electronic structure of Sn1−xTixS2, Ti 3d orbitals can be located between the forbidden gap of SnS2, which is the configuration of IBs. As seen in Figure 1a, the Sn1−xTixS2 compound can have three absorption peaks, which including the S 3p → Sn 5s (Eg1), S 3p → Ti 3d (Eg2), and Ti 3d → Sn 5s (Eg3) transitions.24,25 The IBs of this Sn1−xTixS2 may result in strong absorption in the domain of UV−vis−NIR of solar spectra.26−30 Supposing that Eg1, Eg2, and Eg3 are corresponding to the solar spectrum λ1, λ2, and λ3, respectively, the solar spectral absorptivity of Sn1−xTixS2 IB materials theoretically reaches 95% (Figure 1b). In this paper, Ti-doped SnS2 samples were synthesized by solid-state reactions at 923 K. First-principles calculations and solar absorption spectrum measurements revealed the existence of IBs for the Sn1−xTixS2 materials. Furthermore, photocatalytic investigation confirmed that the extra absorption from the IBs can photogenerate more electrons and holes to decompose

INTRODUCTION Metal sulfides, including CdS,1,2 SnS,3 Sn(S,Se)2,4 In2S3,5,6 CuAl(S,Se)2,7 CuInS2,8 and so on, have gained considerable attention for their excellent photoelectric physical and chemical properties, which have applications in photovoltaic cells, photocatalysis, photoelectricity, and so on. Tin disulfide crystallizes in the typical layered CdI2-type hexagonal primitive lattice. It is a semiconductor possessing an energy gap of Eg = 2.0−2.6 eV.9 The Sn atom sits at the center of an S6 octahedral. The octahedron layer is sandwiched between two hcp sulfur slabs. These sulfur slabs are held together by weak van der Waals attraction.10−12 Because of its importance as a semiconductor, SnS2 has attracted extensive interests in the fields of photocatalysis,10,13 photodetection14 and optoelectronic devices.11 However, the most challenging work is to design its optical bandgap and enhance the solar absorption efficiency of SnS2 in the whole solar range. Many efforts have been performed to harvest wide-spectrum absorption of SnS2 by introducing various dopants (In,15 Fe,16 V,17 W,18 Zn,19 etc.), but few successful experimental works with effective photoelectric effects are able to improve photovoltaics. Elemental Ti as an early 3d-block transition metal possesses a more effective ability than the others to induce the charge transfer between metal and ligand, which has been used in ferroelectric, photoelectric perovskite-type titanates and IB © XXXX American Chemical Society

Received: January 16, 2018

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DOI: 10.1021/acs.inorgchem.8b00143 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Proposed scheme of (a) Schematic band structures of SnS2, TiS2 and Sn1−xTixS2 potential absorption bands, showing the bandgap (Eg1) and sub-bandgaps (Eg2, Eg3) represent photon absorptions. (b) Distribution of solar energy density.

Figure 2. Structural analysis of as-prepared Sn1−xTixS2 (x = 0.00−0.06) powders. (a) PXRD patterns of Sn1−xTixS2 and (b) Approximately linear relationship between lattice constants (a, c) and x for Sn1−xTixS2. (c) Raman patterns of Sn1−xTixS2; inset: partial enlargement patterns of Sn1.96Ti0.04S2. (d) XPS spectrum of Ti 2p for the obtained Sn0.96Ti0.04S2. spectrum (EDS) and high-resolution transmission electronmicroscopy (HRTEM) elemental mapping were conducted using a JEOL 2100F microscope. The optical absorption spectra of samples were obtained at room temperature, and were carried out on the UV−vis−NIR spectrometer (Hitachi U4100) equipped with an integrating sphere. XPS experiments were carried out on a RBD upgraded PHI-5000C ESCA system (PerkinElmer) with Mg Kα radiation (hv = 51253.6 eV). Binding energies were calibrated by using the containment carbon (C1s = 284.6 eV). First-Principles Calculations. The electronic structure calculations were implemented based on Vienna ab initio simulation package (VASP) within the framework of density-functional theory (DFT).31 We constructed a 4 × 4 × 4 supercell containing 192 atoms, and then substituted Ti in one cell corresponding to the experimental doping content of 1.5625%. In the assumed pseudopotential, the 4s, 4p states of Sn, the 3s, 3p states of S, and the 3d, 4s states of Ti are treated as valence electrons. Moreover, the cutoff energy of the plane wave was chosen at 300 eV. For the Perdew−Burke−Ernzerhof (PBE) structure optimization, 2 × 2 × 1 Γ-centered Monkhorst−Pack grids were used,

methyl orange (MO). With the wide-spectrum solar absorption properties, these IB materials can be promising candidates for efficient photovoltaic cells, solar absorbers or environmental materials.

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EXPERIMENTAL SECTION

Preparation of Sn1−xTixS2 Powders. A series of Sn1−xTixS2 samples (x = 0.00, 0.02, 0.04, and 0.06) were synthesized by solidstate reaction at 923 K from the high-purity elements (Sn (4N, Aladdin), S (4N, Aladdin), and nanometer titanium powders (4N, Aladdin). The stoichiometric reactants were mixed, ground, and sealed in quartz tubes, followed by slowly heating to 923 at 2 K/min. The reactions were held at this temperature for 12 h and then ended by turning off the furnace. The obtained powders were ground, resealed in quartz tubes and heated to 923 at 10 K/min for 24 h before cooling to room temperature. Characterizations. The powder X-ray diffraction (PXRD) measurements were performed on Bruker D8 Focus (at 40 kV and 40 mA) using Cu Kα radiation (λ = 1.5418 Å). Energy-dispersive B

DOI: 10.1021/acs.inorgchem.8b00143 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry and the Hellmann−Feynman force on all atomic sites was less than 0.01 eV/Å. Photocatalytic Degradation. The photocatalytic efficiency of the Sn1−xTixS2 (x = 0.00, 0.02, 0.04, and 0.06) samples was measured by evaluating the degradation of MO solution under a AM 1.5G xenon lamp of 300 W cut by a 400 nm light filter. A Pyrex glass vessel was used as the photoreactor. In the procedure of decomposing MO solution, 100 mg of Sn1−xTixS2 samples were poured into 100 mL of a 10 mg/L MO solution. Afterward, the mixture was stirred in the dark for 30 min to reach the adsorption equilibrium and was then irradiated with the lamp. In order to avoid thermal decomposition and evaporation of the solvent, the solution was cooled by a jacket of circulating water. The MO concentration in the solution was determined by measuring the peak intensity at 465 nm with a UV− vis spectrophotometer.

for Sn0.96Ti0.04S2 are presented in Figure 2d. The Ti 2p3/2 and Ti 2p1/2 XPS peaks at the energies of 458.5 and 464.5 eV are characteristic of the Ti4+−S bonds.34 In addition, the Ti3+ 2p3/2 and Ti3+ 2p1/2 satellite peaks at 456.2 and 462.5 eV do not appear in the XPS spectrum, suggesting the absence of the Ti3+ in our synthesized sample. Hence, Ti atoms in the Sn4+ sites carry a +4 oxidation state, which balance the negative charge of two sulfur anions, as inTiS2. The result is in agreement with the PXRD data and Raman spectra. The Sn1−xTixS2 (x = 0.00−0.06) powders have similar morphologies, hence, only that of the Sn0.96Ti0.04S2 powders were illustrated. As shown in Figure 3a, the size of the sheet-like

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RESULTS AND DISCUSSION The compositions and phase purity of the as-prepared Sn1−xTixS2 (x = 0.00, 0.02, 0.04, and 0.06) powders were confirmed by PXRD, as illustrated in Figure 2a. The PXRD pattern of the undoped SnS2 and Ti-doped SnS2 fit perfectly to the hexagonal space group (P-3m1). Diffraction peaks of SnS2 at 2θ = 15.055, 32.208, 41.999, and 50.108° correspond to (001), (011), (012), and (1̅20) planes of the hexagonal SnS2 structure (Powder Diffraction File (PDF) no. 23−0677, International Centre for Diffraction Data (ICDD)), respectively.32 With Ti doping, the diffraction peaks shift continuously to high angle, indicating the decrease of lattice constants, as depicted in Figure 2b. Accordingly, the lattice constants (a, c) of Sn1−xTixS2 powders (x = 0.00, 0.02, 0.04, and 0.06) were refined using Jade 6.5 software, respectively, as shown in Table 1. This reductions of lattice constants results from the Table 1. Lattice Constants (a, c) Variation of Sn1−xTixS2 with the Doping Content of Ti Atoms composition

a (Å)

c (Å)

V (Å3)

SnS2 Sn0.98Ti0.02S2 Sn0.96Ti0.04S2 Sn0.94Ti0.06S2

3.6471 3.6452 3.6431 3.6422

5.8962 5.8942 5.8921 5.8902

67.918 67.824 67.722 67.667

Figure 3. TEM, EDS and HRTEM of as-prepared Sn0.96Ti004S2. (a) TEM image of well-dispersed nanostructured Sn0.96Ti004S2; inset: nanosheet structure of Sn0.96Ti004S2. (b) SEM and elemental mapping of S, Sn, and Ti of sample. (c and d) HRTEM image and the corresponding FFT pattern.

Sn0.96Ti0.04S2 ranges from 100 to 200 nm, and the layered structure can be easily observed from the zoomed TEM image (inset of the Figure 3a). Elemental mapping of the Sn0.96Ti0.04S2 powders is illustrated in Figure 3b. The uniform distribution of Ti reveals the homogeneity of the Sn0.96Ti0.04S2 powders. The elemental ratio of Sn/Ti/S is 1.047/0.038/2, which is close to the nominal ratio of Sn0.96Ti0.04S2. The high degree of crystallinity of Sn0.96Ti0.04S2, consistent with the PXRD result, is demonstrated by the clear lattice fringes from the HRTEM photo (Figure 3c) and the corresponding fast Fourier transformation (FFT) pattern (Figure 3d). The two sets of lattice fringes, with the same lattice distances of 3.2 Å and an angle of 60°, are attributed to the (100) and (010) planes of the hexagonal SnS2, respectively. The UV−vis−NIR absorption spectra of Sn1−xTixS2 and the AM1.5G solar spectrum are shown in Figure 4a. Only one absorption edge appears in the absorption spectrum of SnS2, corresponding to an energy gap of 2.21 eV, which is similar to the previous value (2.20 eV).33 After doping with Ti, the solar absorption of Sn1−xTixS2 is significantly enhanced. Except for the original absorption edge response at 2.21 eV (Eg1), an additional absorption edge and absorption peak occur remarkably at 1.33 eV (Eg2) and 0.88 eV (Eg3), respectively,

replacement of Sn4+ (0.81 Å) in the crystal lattice by smaller Ti4+ (0.74 Å) ions, confirming the successful doping of Ti. Besides, no detectable secondary phases can be found, such as SnTi2S5 (PDF No. 41−0984, Joint Committee on Powder Diffraction Standards (JCPDS)), Sn1.2Ti0.8S3 (PDF Nos. 39− 0431 and 40−1192, JCPDS) or TiS2 (PDF No. 15−0853, JCPDS). In order to further confirm the formation of single phase of Ti-doped SnS2 powders, the Raman spectrum and XPS measurements were performed. Figure 2c demonstrates the Raman spectra of the as-prepared Ti-doped SnS2 samples. The vibrational modes of the SnS2 hexagonal structures with P3̅m1 symmetry are expressed as Γ = A1g + Eg + 2A2u + 2Eu. SnS2 and Ti-doped SnS2 have similar Raman spectra which are dominated by a band of the frequency near 315 cm−1. These peaks are ascribed to the A1g mode, which is the high-frequency band caused by the vibration of the Sn−S bond.33 The additional Raman peak at 204 cm−1 corresponds to the Eg mode,12 as shown in the inset of Figure 2c. Because of the electronegativity difference, the Raman band moves to lower frequency as Sn atoms were partially substituted by the more electropositive Ti atoms. The XPS spectra of the Ti 2p orbital C

DOI: 10.1021/acs.inorgchem.8b00143 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. IB-induced wide-spectrum absorption of Ti-doped SnS2. (a) UV−vis−NIR absorption spectra of Sn1−xTixS2, in comparison with the standard AM1.5 sunlight irradiation in the shaded area. (b) Photographs of Sn1−xTixS2 powders. (c) Thermal image map of all samples after an AM 1.5G Xe lamp irradiation in a solar simulator for different durations.

Figure 5. Band structure and DOS calculated by VASP. (a and b) Band structure and partial DOS of undoped SnS2. (c and d) Band structure and partial DOS of Ti-doped SnS2. (e) Partial DOS of Ti 3d orbitals and splitting states of Ti 3d orbitals (Inset). (f) Total DOS of Ti-doped SnS2.

Ti-doped SnS2 is almost the same as the IB semiconductors reported previously, such as Sn-doped CuGaS2 and Sn-doped CuAlS2.35,36 Therefore, anticipating from the electronic structure of the compounds, the Ti doping can introduce

for all the doped samples (Figure 4a). Correspondingly, the color of the Sn1−xTixS2 samples varies from orange (x = 0) or deep yellow (x = 0.02) to light brown (x = 0.04) and brown (x = 0.06) (Figure 4b). The behavior of optical absorption of the D

DOI: 10.1021/acs.inorgchem.8b00143 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. (a) UV−vis absorption spectra of photocatalytic methyl orange degradation under λ > 400 nm of Sn1−xTixS2 (x = 0.00, 0.02, 0.04, and 0.06) after different illumination time. (b) Concentration evolution of MO. (c) Cycling photocatalytic degradations of Sn0.96Ti0.04S2 (compared with SnS2 black line). (d) Schematic photocatalysis of Sn1−xTixS2.

the Ti 3d orbitals. In an octahedral environment, Ti 3d states split into a high energy level of doubly degenerated eg (dz2, dx2−y2) orbitals and a low energy level of triply degenerated t2g (dxy, dxz, and dyz) set,38 as depicted in the inset of Figure 5e. The t2g orbitals located in the 0.78−1.02 eV energy range are the IBs, while the eg orbitals (energy range of 1.13−1.28 eV) are embedded in the high-energy region in the conduction band, as shown in Figure 5f. Therefore, the Eg1 (2.21 eV), Eg2 (1.33 eV), and Eg3 (0.88 eV) are mainly attributed to the S 3p → eg of Ti 3d, S 3p → t2g of Ti 3d, and t2g → eg transitions, respectively. The wide spectrum solar absorption of the Sn1−xTixS2 powders implies the generation of more electron−hole pairs under illumination, which is beneficial for photoelectric related processes. The photocatalytic efficiencies of the Sn1−xTixS2 powders were measured by decomposing MO solutions under the irradiation wavelength of λ > 400 nm. The UV−vis absorption spectra of photocatalytic degradation of MO solutions at different reaction time are shown in Figure 6a, and the concentration evolution of MO is shown in Figure 6b. The undoped SnS2 can degrade all the MO molecules after irradiation for 120 min. Accelerated degradation rates are observed for all the doped samples, compared with SnS2. The respective MO removals by SnS2, Sn0.98Ti0.02S2, Sn0.96Ti0.04S2, and Sn0.94Ti0.06S2, are 21.83, 36.07, 41.07, and 27.24% after 30 min of illumination. Therefore, the maximum initial reaction rate is achieved by the Sn0.96Ti0.04S2 sample. After irradiation for 90 min, 100% of MO are degraded. The cycling photocatalytic degradations of Sn0.96Ti0.04S2 have nearly the same features in five cycles (Figure 6c). For the semiconductor photocatalysis, electrons and holes generated by irradiation fall into the LUMO and HOMO levels of MO molecule and decompose the MO solution, as depicted in Figure 6d. In this case, the increased photocatalytic efficiencies of Ti-doped SnS2 powders, derived from the irradiation wavelength of λ > 400 nm, obviously result from their extended spectral response. SnS2 only absorbs

sub-bands into the VBM and CBM. These sub-bands act as the IBs which can efficiently extend the absorption ranges. The enhancement on the solar absorption of the doped samples is further confirmed by their photothermic effects. The thermal images of Sn1−xTixS2 disks, which are pressed from powder samples and irradiated with an AM 1.5G Xe lamp in a solar simulator for different durations, are shown in Figure 4c. After irradiation for 60 s, the temperature is 25 °C (SnS2), 34 °C (Sn0.98Ti0.02S2), 37 °C (Sn0.96Ti0.04S2), and 39 °C (Sn0.94Ti0.06S2), respectively. The accelerated heating rates of Ti-doped SnS2 samples are attributed to the intense solar absorption and the extended absorption ranges. Consequently, the Ti doped SnS2 are good solar absorbers with wide spectrum solar absorption. First-principles calculations are performed to further confirm the IB character of the Ti-doped SnS2. The electronic band structure and density of states (DOS) of undoped SnS2 are depicted in Figure 5a,b, the VBM of SnS2, which is composed of S 3p states, located at the K point. However, the CBM, which consists predominantly of the mixed orbitals between Sn 5s and S 3p states, located near the G point. Therefore, SnS2 is a typical semiconductor having a calculated indirect band gap of 1.28 eV. The theoretical bandgap is smaller than the experimental one (Eg1 = 2.21 eV), due to the S−Sn−S layer depending on the weak van der Waals force connection. Besides, the use of DFT always results in underestimated band gaps of semiconductors.37 By analyzing the electronic band structure and DOS of Tidoped SnS2 (Figure 5c,d), we can conclude that the VBM of Ti-doped SnS2 is mainly composed of S 3p orbitals, and the CBM is consisted by the hybridized states of the Ti 3d, S 3p, and Sn 5s orbitals. Remarkably, three IBs appear between the VBM and the CBM, as shown in Figure 5c which is similar to the previous works.30 Hence, the electron transition may occur from VBM → IB (Eg2), from IB → CBM (Eg3), and from VBM to CBM (Eg1), respectively. Figure 5e shows the partial DOS of E

DOI: 10.1021/acs.inorgchem.8b00143 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry photons with high energy (hv1 ≥ 2.21 eV). However, the Tidoped SnS2 samples with IBs can absorb the extra photons in the energy range of hv2 (1.33 eV) and hv3 (0.88 eV) and hence can generate more electron−hole pairs (Figure 6d), while the photocatalytic activity of Sn1−xTixS2 is decreased when Ti content increases up to x = 0.06 (Sn0.94Ti0.06S2), as shown in Figure 6b. According to the literature,39 energy levels of IBs in semiconductors form the deep energy traps and function as active recombination centers for electrons and holes. However, the recombination can be reduced if the dopant concentration (NT) is no more than NT = 1021 cm3 (or even less). Since the Ti concentration of the as-prepared Sn0.94Ti0.06S2 (NT ≈ 8.87 × 1020 cm−3) has exceeded the optimal concentration, the energy levels of IBs can serve as the recombination centers. The increased centers can lead to a decrease in photocatalytic activity.

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CONCLUSIONS In summary, a new series of IB materials, namely, the Ti-doped SnS2, were successfully synthesized by solid-state reactions. The formation of IBs between VBM and the CBM of Ti-doped SnS2 were confirmed by first-principles calculations and UV−vis− NIR measurements. In addition to the intrinsic absorption at 2.21 eV, a sub-bandgap absorption edges (VBM → IB, 1.33 eV) and a weak absorption peak (IB → CBM, 0.88 eV) were observed. Due to the wide spectrum solar absorption, these new IB materials possess excellent photocatalytic and solar thermal collection properties. The enhanced light absorption indicates that Ti-doped SnS2 may be promising candidates for solar absorbers.

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AUTHOR INFORMATION

Corresponding Authors

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

Fuqiang Huang: 0000-0001-7727-0488 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Key Research and Development Program (grant 2016YFB0901600), CAS Center for Excellence in Superconducting Electronics, the Key Research Program of Chinese Academy of Sciences (grants QYZDJ-SSW-JSC013, and KGZD-EW-T06), Science and Technology Commission of Shanghai (grants 16JC1401700 and16ZR1440500), and NSF of Jiangxi Province (grant nos. 20143ACB21004, 20151BDH80031, 20151BAB212008, and GJJ150912).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b00143 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b00143 Inorg. Chem. XXXX, XXX, XXX−XXX