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Synthesis and electrical properties of antimony–doped tin oxide–coated TiO2 by polymeric precursor method
Materials Science in Semiconductor Processing 98 (2019) 70–76
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Synthesis and electrical properties of antimony–doped tin oxide–coated TiO2 by polymeric precursor method
T
Xue Lia, Jianhua Qianb,∗, Jiasheng Xuc, Yudong Sunc, Lin Liuc a
Department of Materials Physics and Chemistry, School of Materials Science and Engineering, and Key Laboratory for Anisotropy and Texture of Materials, Ministry of Education, Northeastern University, Shenyang, 110819, Liaoning, China b College of Chemistry, Chemical Engineering and Environmental Engineering, Liaoning University of Petroleum and Chemical Technology, Fushun, 113001, Liaoning, China c Provincial Key Laboratory for Functional Compounds Synthesis and Application, Bohai University, Jinzhou, 121013, Liaoning, China
A R T I C LE I N FO
A B S T R A C T
Keywords: TiO2 Antimony–doped tin oxide N–type semiconductor Polymeric precursor method Composite
Antimony–doped tin oxide–coated TiO2 (TiO2@SbxSn1-xO2 or TiO2@ATO) composite was fabricated by a facile polymeric precursor (Pechini) method, with the percent yield up to 90%. Different doping amount of antimony was used to investigate the effect on conductivity of the TiO2@ATO composite. Various characterization methods including XRD, SEM, HRTEM, XPS, UV–vis, FTIR and TG–DSC, have been applied to get deep insights into the structure, morphology and the enhanced conductivity. The composite with the lowest resistivity (4.2 Ω cm) shows a core–shell structure with the shell thickness of ∼18 nm and the ATO average nanoparticle size of 9.63 nm. Finally, a possible formation mechanism of the composite was proposed.
1. Introduction Titanium dioxide (TiO2) has attracted considerable attention for years because of its intrinsic properties, including abundant reserves, low cost, physical and chemical stability, nontoxicity and high brightness [1–3]. The use of TiO2 as a white pigment in polymer materials has inspires a variety of interest for researchers [4]. However, some composite materials are prone to generate static electricity arising from collision and friction, such as plastics, rubber and coatings. When electrostatic charges accumulate to a certain extent, electrostatic discharge would occur, and even leads to breakdowns, fires, radio radar interference and other malignant events. In order to prevent or eliminate the electrostatic impact, the electrical conductivity of TiO2 needs to be improved. The strategy of TiO2 surface modified with antimony–doped tin oxide (ATO) can be adopted [5,6]. ATO as a typical n–type transparent conductive oxides material (TCO) possesses many superior properties, such as the high conductivity and optical transparency in the visible range, thermal stability and high reflectivity for infrared radiation [7,8]. Liu et al. have successfully prepared core–shell TiO2@ATO nanofibers, which exhibited better electroconductivity [9]. Wang et al. reported on the synthesis of TiO2@ATO and demonstrated that the obtained sample possessed light color and a good electrical conductivity [10]. The conventional method for the coating of ATO layer on the TiO2
∗
surface is chemical co–precipitation [10–12], in which hydrochloric acid is adopted to prevent hydrolysis of stannic chloride and antimony trichloride, and sodium hydroxide usually serves as the precipitant and pH adjusting agent. The pH value of the reaction system must be controlled during the fabrication of TiO2@ATO composite. However, the introduction of chloride and the sodium ions can destroy the photoelectric effect of the product. Moreover, a large amount of water is needed to remove chloride ions in the washing process, producing a large amount of acid waste water and causing environmental pollution. In addition, the chemical homogeneity is not easily to be achieved due to the differences in the solubility between the precipitates of antimony and tin ions. Pechini route (polymerized complex method) is one of the most widely used method to synthesis various nanopowders with controllable size and low agglomeration [13–15]. The method is based on the chelate compound generated from cations and weak organic acids (citric acid is preferred), which further polymerizes with polyols to form polymer resin. The mixing of different metal ions at atomic level can be achieved due to the homogeneous dispersion of metal ions in the polymer resin. During the calcination process, the organic groups decompose into carbon dioxide and water. The crystallization of the oxide is determined by the calcination temperature. In this work, we describe the preparation of TiO2@ATO (TA) core–shell composite by the polymeric precursor (Pechini) method, which is available to a large scale production and without adjusting the
Corresponding author. E-mail address: [email protected] (J. Qian).
https://doi.org/10.1016/j.mssp.2019.03.024 Received 15 December 2018; Received in revised form 26 February 2019; Accepted 19 March 2019 1369-8001/ © 2019 Elsevier Ltd. All rights reserved.
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pH during the reaction. The method is environmentally friendly as the water based and the materials of citric acid and ethylene glycol are nontoxic. The as–obtained TA composites show high purity, high homogeneity and controlled stoichiometry. Various characterization methods including XRD, SEM, HRTEM, XPS, UV–vis, FTIR and TG–DSC, have been applied to getting deep insights into the enhanced conductivity.
2. Experimental TiO2, obtained from Jinzhou titanium industry Co. Ltd., China. Anhydrous ethanol (≥99.7%, AR), tannic chloride hydrate (SnCl4·5H2O, ≥99.0%, AR), antimony trichloride (SbCl3, ≥98.0%, AR), citric acid (≥99.5%, AR) and ethylene glycol (AR) were used without further processing or purification. Typically, 2.1 g of SnCl4·5H2O and a certain amount of SbCl3 was dissolved in 20 mL of anhydrous ethanol. The antimony doping concentration was labeled as x%, x was the molar ratios of [Sb]/[Sn], which were 0, 5%, 15%, 25%, 35% and 50%, their corresponding conductive composites were marked as TA0, TA5, TA15, TA25, TA35, TA50, respectively. Citric acid (CA) and ethylene glycol (EG) were added to the resulting solution after reflux under stirring for 2 h. The mixture was heated up to 60 °C and maintained reflux for additional 3 h. At this stage, the solution was completely homogenous and formed a polymetric precursor solution. 3 g of TiO2 powders were added into deionized water. The suspension was ultrasonic for 40 min and reflux under stirring at 90 °C. Then, the polymetric precursor solution was dropwise added into the TiO2 suspension by a pump for about 1.5 h. The mixture was reflux after the titration of the polymetric precursor solution, where the hydrolysis of most Sn4+ and Sb3+ ions could occur in the hydrous solution. The suspension became yellow and aged for 12 h. The resulting solution was filtered and rinsed by anhydrous ethanol, dried at 100 °C for 3 h and calcined at 200 °C, 400 °C, 550 °C, 650 °C and 800 °C for 2 h. The percent yield of TA composite was calculated as follows:
The yield = (Actual yield/theoretical yield) × 100% The resistivity was measured by the FT–300 electrical resistivity meter. 1.5 g of the conductive composite was putted into the cylindrical model of the stainless steel, a slice was obtained after pressed with 20 Mpa pressure. The resistivity was calculated from the equation: ρ = RA/L , where, ρ is the resistivity of the conductive composite (Ω·cm), R is the resistance of testing (Ω), and A is the area of the sample pole (cm2), L represents the height of the sample pole (cm). X–ray diffraction (XRD, Germany Bruker AXS−D8) was employed to identify the characteristic crystal phases of the TA composites at a scanning rate of 2deg/min, data were collected in the range of 10–80° (2θ). The surface morphology was revealed by a scanning electron microscope (SEM, HITACHI S–4800) together with the energy dispersive spectroscopy (EDS) detection system. The transmission electron microscopy (TEM, JEM−2100F) was carried out to observe the further information of the lattice of ATO particles. The binding energy of TA composites was obtained via X−ray photoelectron spectroscopy (XPS) measurement (USA Thermo ESCALAB−250) at room temperature. Ultraviolet and visible spectrophotometer (Japan Shimadzu UV−2550) was conducted to acquire the optical absorbance spectra. To investigate the formation mechanism, the thermogravimetric (TG) analysis and differential scanning calorimetric (DSC) analysis of the TA precursors were performed (USA TA DSC–Q20), a heating rate of 10 °C/min to 800 °C in the atmosphere of air. Fourier Transform Infrared (FTIR) spectra were recorded at room temperature (Japan Shimadzu IRTrancer–100), using KBr pellets in the range of 400–4000 cm−1.
Fig. 1. XRD patterns of (a) TiO2 and TA35 precursors calcinated under different temperatures and (b) TA composites with various Sb doping concentration (calcinated at 550 °C for 2 h), (c) average crystal size of ATO nanoparticles calcinated at different temperatures.
3. Results and discussion 3.1. Effects of calcination temperature and Sb doping content The percent yield by the polymeric precursor (Pechini) method was
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up to 90%. Calcination temperature and Sb doping content have an effect on the microstructure of TA composite. Fig. 1 shows the XRD patterns of pure TiO2, TA precursors calcinated under different temperatures and TA composites with various Sb doping content. All of the diffraction peaks can be indexed into the rutile TiO2 (PDF#21–1276) and cassiterite SnO2 (PDF#41–1445) [16]. The peaks at 2θ = 26.5°, 33.9°, 37.9°, 51.8°, 61.9°, 65.9° correspond to (110), (101), (200), (211), (310) and (301) crystallographic planes of high–purity of SnO2, respectively [17]. Nevertheless, there is no extra peaks corresponding to antimony oxide, indicating that Sb5+ or Sb3+ ions replace the Sn4+ ions of the SnO2 crystal lattice and forms the doped semiconductor [18]. The increase of calcination temperature results in the more intense and narrower characteristic peaks of ATO particles, suggesting the better crystalline quality and the growth of crystal particles (Fig. 1a). In Fig. 1b, the intensity of ATO diffraction peaks are reduced and the peaks widths become broader with the increasing of Sb doping concentration (calcinated at 550 °C for 2 h), indicating the decreased grain particle size. Furthermore, the average crystalline size was calculated from the peaks (101) and (211) by the Debye–Scherrer formula for spherical particles [19].
D=
Kλ β cos θ
(1)
where, D is the crystallite size, K is Scherer constant, λ is the wavelength (0.15405 nm), β is the full width at half maximum (FWHM), and θ is the Bragg reflection angle. The calculated crystallite sizes increase from 4.7 to 9.2 nm with the increasing of heat temperature (Fig. 1c). However, the crystallite sizes of ATO particles decreases sharply from 8.9 to 6.7 nm as the Sb doping concentration increases from 0 to 50 mol %, as shown in Table 1. This phenomenon can be attributed to the segregation of Sb on the surface of ATO particles, inhibiting the growth of grain [18]. In addition, the lattice parameters (a, c) and unit cell volume (Å3) of TA composites were calculated from the (110) and (101) peaks by the following formula [20].
1 h2 + k2 l2 = + 2 2 2 d a c
Fig. 2. (a) Absorbance and (b) (αhν)2 plots of TA composites with various Sb dopant and their Eg (inset).
(2) decrease of resistivity. However, the Sb3+ component appears and acts as the acceptor states (holes), which shows a compensation behavior between the Sb3+ and Sb5+, leading to the resistivity increase [22]. The resistivity of the TA composite is dependent on the ratio of Sb5+ and Sb3+ [23,24]. Another reason is the increasing of charge scattering on the grain–boundary contacts between the particles due to the reduction grain size [25]. Fig. 2 shows the absorbance spectra and (αhν)2 plots of TiO2 and the TA composites with different Sb concentration. The absorbance intensity of the TA composite become stronger in the range of 200–400 nm in comparison with the pure TiO2, which may be ascribed to the UV absorption properties of the ATO particles. However, the transmittance of the composite is obviously decreased in the visible range. It can be attributed to the blue–darkening color and the increased photon scattering from crystal defects of ATO particles [26]. In addition, a shift of the absorption edges observed with the increase of Sb concentration is related to the band gap change. The optical band gap (Eg) of TA composite is estimated from Tauc's relationship [27].
where d is the interplanar distance, (h k l) represent the Miller indices. The calculated values and corresponding unit cell volume (V = a2c) are presented in Table 1. The unit cell volumes of the ATO particles are smaller than that of undoped SnO2 sample, which is attributed to the Sn4+ ions substitution by the Sb5+ ions or the number of Sb5+ ions are more than the Sb3+ ions, Since the ionic radius of Sn4+(0.69 Å) is smaller than that of Sb3+ (0.76 Å) and larger than that of Sb5+ (0.61 Å) [21]. As for the electrical properties, the undoped sample possesses a very high resistivity in the order of 106 Ω cm. However, the resistivity decreased sharply to 4.2 Ω cm with enhancement of Sb doping content up to 0.35, which may be ascribed to the extra carrier from the substitution of Sb5+ at Sn4+ sites. The resistivity increased as the doping content increased above 35%. There are two reasons to explain the phenomenon. When the doping content is low, Sb5+ is the dominating component and introduces donor states (electrons), resulting in the Table 1 Structural parameters of TA composites with different Sb doping concentrations. Sb doping level
Grain size (nm)
Lattice parameters (Å)
Unit cell volume 3
αhv = C(hv − Eg )1 / 2
(3)
Resistivity
(%)
D(101)
D(211)
a–axis
c–axis
(Å )
(Ω·cm)
Undoped 5 15 25 35 50
8.9 8.7 8.4 7.7 7.6 6.7
8.7 8.5 8.2 7.8 7.7 6.3
4.7348 4.7291 4.7243 4.7206 4.7205 4.7318
3.1779 3.1714 3.1823 3.1893 3.1794 3.1807
71.2275 71.0576 71.0258 71.0706 70.8470 71.2157
4.7 × 106 3.5 × 104 32.0 4.8 4.2 6.8
where C is a constant, α is the absorption coefficient, hν is the photon energy. The optical band gap can be obtained by the intercept of the tangent to the (αhν)2 against hν plot, as shown in Fig. 2b, and the resulted values are listed in the inset of Fig. 2b. The calculated band gaps are 3.077, 3.076, 3.072, 3.069, 3.070, 3.068, and 3.066 eV for the TiO2 and composites with 0%, 5%, 15%, 25%, 35% and 50% Sb doping content, respectively. The absorption edges of the TA composites are slight red–shifted with comparison to the pure TiO2, indicating the modification of the electronic structure and the optical properties of the 72
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original TiO2. It probably results from the trace doping of Sn4+ on the interface between TiO2 and ATO particles during the calcination, or the coupling of SnO2 and TiO2 [28]. The similar red shifts have been reported in the previous literature [29,30]. The band gap energy shows a decrease trend with increasing Sb doping concentration. The phenomenon is in contrast to the Burstein–Moss (B–M) effect. According to the Burstein–Moss theory, the incorporation of Sb ions leads to the increase of carrier concentration, which results in a bandgap broaden [31]. The narrowing effect in the band gap can be explained by the sp–d exchange interaction between the band electrons and the localized d electrons of the Sb ions replaced the Sn4+ ions. In addition, the Sb doping may cause the stoichiometry defects of Sb–doped SnO2, such as oxygen vacancies. The structural disorder in SnO2 leads the valance band tail to unoccupied states of conduction band edge, resulting in the bandgap decreasing [32]. Similar bandgap shrinkage has been reported in the doping model [33]. The UV absorbance spectra variation combined with the little decrease in band gap values are further confirmed the successfully developed of ATO particles on TiO2 surface.
and O can be observed in the TA35 composite, further confirming the successfully coating of ATO nanoparticles. The Sn 3d5/2 and Sn 3d3/2 peaks for the TA35 precursor, TA15, TA35, and TA50 locate at the binding energy of 486.82, 486.73, 486.73, 486.84 eV and 495.22, 495.13, 495.13, 495.24 eV, respectively (Fig. 5b). The peaks separation is 8.4 eV, which indicates the lattice Sn4+ ion in SnO2 [35]. Chemical shift observed for the various Sb doping concentration are 0.09–0.11 eV, resulting from the change in oxidation state of atom [36]. The O 1s and Sb 3d5/2 are overlapped as shown in Fig. 5c, which can be separated into four Gaussian peaks located at 530.4, 530.7, 531.0, and 532.8 eV. The peak at 530.7 eV can be assigned to the Sb 3d 5/2. The peak centered at 530.4 eV can be attributed to TieO bond. The peak located at 531.0 eV is responding to the Sn(Sb)eO bond. Moreover, the peak at binding energy of 532.8 eV is associated with H2O or hydroxyl group on the surface. The stronger intensity of the peak in the precursor than that in TA35, indicating the more presence of H2O. The Sb3d3/2 can be employed to analyze the oxidation state of Sb, which is decomposed into two characteristic peaks at 539.7 and 540.2 eV corresponding to the Sb3+ and Sb5+, respectively (Fig. 5d). The ratio of Sb5+/Sb3+ in the TA35 precursor, TA15, TA35, and TA50 estimated by the area are 0.60, 1.96, 4.79 and 3.46, respectively. The Sb5+ predominated in the TA samples, resulting in the decrease of resistivity, implying the oxidation of antimony in the calcination process. The Sb5+/Sb3+ ratio is larger in the TA35 sample than that in the other TA samples, possible signifying the better conductivity resulted from the high ratio of Sb5+/Sb3+. The higher carrier concentration generates due to the relative larger amount of Sb5+ in the TA composites [17,22]. The result is good agreement with the XRD analysis.
3.2. Surface morphology and composition Fig. 3 illustrates the SEM images of TiO2, precipitate, and TA35 composite and the corresponding EDS analysis. In Fig. 3a, the original TiO2 particles are of sphere shape with a smooth surface, and the grain sizes are around 250 nm. The rough surface and some granules can be observed in the precipitate sample (Fig. 3b). Moreover, the compact and homogenous grains are shown on the surface of TiO2, as presented in Fig. 3c, which can be derived that the ATO particles were successfully coated on the TiO2 surface. The EDS reveals the existence of Ti, O, Sn and Sb elements, further confirming the ATO particles on the TiO2 cores (Fig. 3d). The chemical composition (wt.%) of Ti, O, Sn, Sb in the TA35 composite is 44.51%, 38.86%, 12.86%, 3.77%, respectively. The molar ratio of [Sb]/[Sn] is 0.286, which is close to the experimental ratio of 0.35. The TEM images with different magnifications and high–resolution TEM (HRTEM) images of TA35 composite are shown in Fig. 4. The TiO2 core and ATO shell structure can be observed, and the shell thickness is about 18 nm (Fig. 4a and b). In Fig. 4c, the ATO particles is regular sphere and the mean grain size is of 9.63 nm estimated by the HRTEM. The lattice spacing of 0.334 nm is corresponding to the growth orientation in the direction of rutile SnO2 (110) plane, which is in good accordance with the XRD results (Fig. 4d) [34]. In order to obtain the further information of chemical composition and valence state of the elements, XPS analysis were conducted, as presented in Fig. 5. In Fig. 5a, the peaks corresponding to C, Sn, Sb, Ti
3.3. Formation mechanism Fig. 6 shows the FTIR spectra of citric acid, precursor solution, TiO2 and the TA35 precursor powders with various treatment temperatures. In Fig. 6a, the peak at 3376 cm−1 and 3421 cm−1 are related to OeH stretching vibrations of structural OH groups. The new bands at 1641 cm−1 and 1382 cm−1 are observed for the precursor sample, which can be assigned to the asymmetric and symmetric stretching vibration of the carboxylate ions (COO−) respectively. It indicates the formation of metal–citrate complexes. The band at 1738 cm−1 is associated with the CeO stretching vibration of uncoordinated eCOOH group, suggesting that there is some amount of residue from citric acid. Ethylene glycol is characterization by two bands at 1085 cm−1 and 1048 cm−1 which are the CeCeO stretching vibrations peaks. In Fig. 8b, all of the samples show a broad band at around 647 cm−1, which belong to TieOeTi stretching and bending vibrational modes [37]. The absorption peaks around 3421 cm−1 and 1643 cm−1 can be ascribed to the stretching vibrations of the surface hydroxyl groups (OH) and the physical absorbed water on the surface of the oxides [37,38]. The peak centered at 1718 cm−1 in precipitate is assigned to the carboxyl groups (C]O) of the citric acid. Moreover, a new band at 1234 cm−1 associate with the δ(SnOH) is recorded in the samples of precipitate [39]. It disappears after calcination due to the water elimination and the development of SneOeSn bonds which can be observed at 1390 cm−1 [40]. The TG and DSC characteristics of TA35 precursor were employed to analyze the reaction mechanism, as presented in Fig. 7. Two weight losses can be observed at 20–150 °C, 150–400 °C in TG curve. The first step of weight loss (2.4%) corresponding to an endothermal peak in the DSC curve can be ascribed to the evaporation of the residual ethyl alcohol and water. The second weight loss (3.6%) accompanied by a wide exothermic peak is attributed to the evaporation of the crystal water and the combustion of the organic groups, such as ethylene glycol and polymer, also the removal of the un–polymerized citric acid [41]. Moreover, the process is accompanied by the crystallization of the amorphous SbeSn precursors into ATO nanoparticles with the water release [18]. The weight remains constant above 400 °C in the TG
Fig. 3. SEM images of (a) TiO2, (b) precipitate, (c) TA35 composite and (d) the corresponding EDS of TA35 composite. 73
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Fig. 4. TEM and HRTEM of TA35 composite (inset) corresponding particle size distribution.
SnCl4 and SbCl3 were dissolved by the ethanol solution. SnCl4 and SbCl3 react with ethanol to form tetraethoxy tin and antimony triethoxide [42]. Citric acid was used as the complexing agents, which could chelate Sn4+ and Sb3+ ions with the carboxyl groups or hydroxy groups to
curve, indicating that the organic residues has been completely removed and the final product of TA composite is obtained. According to the above analysis, a possible formation mechanism of ATO particles coated on TiO2 surface is presented. In the first step,
Fig. 5. (a) XPS spectra of TiO2 and TA composites with various Sb concentration and (b)–(d) the respective high resolution spectra of the Sn 3d, O 1s and Sb 3d peaks with fitted line. 74
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Fig. 8. FTIR spectra of (a) citric acid and precursor solution, (b) TiO2, the dried TA35 precursor and the TA35 precursors calcined at 200 °C and 550 °C.
Fig. 6. FTIR spectra of (a) citric acid and precursor solution, (b) TiO2, the dried TA35 precursor and the TA35 precursors calcined at 200 °C and 550 °C.
the TiO2 aqueous solution, citrate polyester accompanied by the hydrolysis absorbed on the surface of TiO2 by the hydrogen bond with the surface hydroxyl groups. During the reaction, the intermolecular polycondensation of the hydrolyzates with the dehydration and dealcoholization reaction and particles growth took place. Most hydrolyzates coated on the surface. The absorbed water and ethyl alcohol were removed, amorphous SnO2 and Sb2O3 were formed during the drying stage. In the calcination process, amorphous metal oxide crystallized and Sb3+ ions were gradually oxidized into Sb5+ ions. Sb5+ ions mainly substituted Sn4+ in the lattice of SnO2, which generated n–type semiconductor (ATO) with the average particle size of 9.63 nm bonded on the TiO2 surface. The uniform coating of the excellent conductive ATO particles endows the TiO2@ATO composite with a better conductivity. 4. Conclusions Fig. 7. TG/DSC curves of TA35 precursor.
The TiO2@ATO conductive composite has been synthesized by an environment friendly and simple synthesis method. The composite possessed a typical core–shell structure and the ATO average particle size is 9.63 nm. The increase of antimony content reduced the grains of ATO particles. The UV absorbance intensity of the TiO2@ATO composite became stronger in the range of 200–400 nm in comparison with the pure TiO2. The resistivity as low as 4.2 Ω cm was achieved for the composite when the optimum antimony doping concentration was 35 mol% (Sb/Sn molar ratio). The coating mechanism of ATO nanoparticle film has been proposed, and this method can be extended to
prevent the hydrolysis of the cations in the ethanol solution. Subsequently, the titration of ethylene glycol resulted in the generation of esterification and polycondensation reaction between the uncoordinated eCOOH and eOH and formed the SbeSn polymer precursor. The cations incorporated to the polymer favor the uniform distribution of metallic ions throughout the organic matrix, which led to a precise control of cations concentration in the particles. The second stage, when the metal polymer precursor was added into 75
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facile synthesis of the other core–shell composite with doped semiconductor.
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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Synthesis and electrical properties of antimony–doped tin oxide–coated TiO2 by polymeric precursor method
T
Xue Lia, Jianhua Qianb,∗, Jiasheng Xuc, Yudong Sunc, Lin Liuc a
Department of Materials Physics and Chemistry, School of Materials Science and Engineering, and Key Laboratory for Anisotropy and Texture of Materials, Ministry of Education, Northeastern University, Shenyang, 110819, Liaoning, China b College of Chemistry, Chemical Engineering and Environmental Engineering, Liaoning University of Petroleum and Chemical Technology, Fushun, 113001, Liaoning, China c Provincial Key Laboratory for Functional Compounds Synthesis and Application, Bohai University, Jinzhou, 121013, Liaoning, China
A R T I C LE I N FO
A B S T R A C T
Keywords: TiO2 Antimony–doped tin oxide N–type semiconductor Polymeric precursor method Composite
Antimony–doped tin oxide–coated TiO2 (TiO2@SbxSn1-xO2 or TiO2@ATO) composite was fabricated by a facile polymeric precursor (Pechini) method, with the percent yield up to 90%. Different doping amount of antimony was used to investigate the effect on conductivity of the TiO2@ATO composite. Various characterization methods including XRD, SEM, HRTEM, XPS, UV–vis, FTIR and TG–DSC, have been applied to get deep insights into the structure, morphology and the enhanced conductivity. The composite with the lowest resistivity (4.2 Ω cm) shows a core–shell structure with the shell thickness of ∼18 nm and the ATO average nanoparticle size of 9.63 nm. Finally, a possible formation mechanism of the composite was proposed.
1. Introduction Titanium dioxide (TiO2) has attracted considerable attention for years because of its intrinsic properties, including abundant reserves, low cost, physical and chemical stability, nontoxicity and high brightness [1–3]. The use of TiO2 as a white pigment in polymer materials has inspires a variety of interest for researchers [4]. However, some composite materials are prone to generate static electricity arising from collision and friction, such as plastics, rubber and coatings. When electrostatic charges accumulate to a certain extent, electrostatic discharge would occur, and even leads to breakdowns, fires, radio radar interference and other malignant events. In order to prevent or eliminate the electrostatic impact, the electrical conductivity of TiO2 needs to be improved. The strategy of TiO2 surface modified with antimony–doped tin oxide (ATO) can be adopted [5,6]. ATO as a typical n–type transparent conductive oxides material (TCO) possesses many superior properties, such as the high conductivity and optical transparency in the visible range, thermal stability and high reflectivity for infrared radiation [7,8]. Liu et al. have successfully prepared core–shell TiO2@ATO nanofibers, which exhibited better electroconductivity [9]. Wang et al. reported on the synthesis of TiO2@ATO and demonstrated that the obtained sample possessed light color and a good electrical conductivity [10]. The conventional method for the coating of ATO layer on the TiO2
∗
surface is chemical co–precipitation [10–12], in which hydrochloric acid is adopted to prevent hydrolysis of stannic chloride and antimony trichloride, and sodium hydroxide usually serves as the precipitant and pH adjusting agent. The pH value of the reaction system must be controlled during the fabrication of TiO2@ATO composite. However, the introduction of chloride and the sodium ions can destroy the photoelectric effect of the product. Moreover, a large amount of water is needed to remove chloride ions in the washing process, producing a large amount of acid waste water and causing environmental pollution. In addition, the chemical homogeneity is not easily to be achieved due to the differences in the solubility between the precipitates of antimony and tin ions. Pechini route (polymerized complex method) is one of the most widely used method to synthesis various nanopowders with controllable size and low agglomeration [13–15]. The method is based on the chelate compound generated from cations and weak organic acids (citric acid is preferred), which further polymerizes with polyols to form polymer resin. The mixing of different metal ions at atomic level can be achieved due to the homogeneous dispersion of metal ions in the polymer resin. During the calcination process, the organic groups decompose into carbon dioxide and water. The crystallization of the oxide is determined by the calcination temperature. In this work, we describe the preparation of TiO2@ATO (TA) core–shell composite by the polymeric precursor (Pechini) method, which is available to a large scale production and without adjusting the
Corresponding author. E-mail address: [email protected] (J. Qian).
https://doi.org/10.1016/j.mssp.2019.03.024 Received 15 December 2018; Received in revised form 26 February 2019; Accepted 19 March 2019 1369-8001/ © 2019 Elsevier Ltd. All rights reserved.
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pH during the reaction. The method is environmentally friendly as the water based and the materials of citric acid and ethylene glycol are nontoxic. The as–obtained TA composites show high purity, high homogeneity and controlled stoichiometry. Various characterization methods including XRD, SEM, HRTEM, XPS, UV–vis, FTIR and TG–DSC, have been applied to getting deep insights into the enhanced conductivity.
2. Experimental TiO2, obtained from Jinzhou titanium industry Co. Ltd., China. Anhydrous ethanol (≥99.7%, AR), tannic chloride hydrate (SnCl4·5H2O, ≥99.0%, AR), antimony trichloride (SbCl3, ≥98.0%, AR), citric acid (≥99.5%, AR) and ethylene glycol (AR) were used without further processing or purification. Typically, 2.1 g of SnCl4·5H2O and a certain amount of SbCl3 was dissolved in 20 mL of anhydrous ethanol. The antimony doping concentration was labeled as x%, x was the molar ratios of [Sb]/[Sn], which were 0, 5%, 15%, 25%, 35% and 50%, their corresponding conductive composites were marked as TA0, TA5, TA15, TA25, TA35, TA50, respectively. Citric acid (CA) and ethylene glycol (EG) were added to the resulting solution after reflux under stirring for 2 h. The mixture was heated up to 60 °C and maintained reflux for additional 3 h. At this stage, the solution was completely homogenous and formed a polymetric precursor solution. 3 g of TiO2 powders were added into deionized water. The suspension was ultrasonic for 40 min and reflux under stirring at 90 °C. Then, the polymetric precursor solution was dropwise added into the TiO2 suspension by a pump for about 1.5 h. The mixture was reflux after the titration of the polymetric precursor solution, where the hydrolysis of most Sn4+ and Sb3+ ions could occur in the hydrous solution. The suspension became yellow and aged for 12 h. The resulting solution was filtered and rinsed by anhydrous ethanol, dried at 100 °C for 3 h and calcined at 200 °C, 400 °C, 550 °C, 650 °C and 800 °C for 2 h. The percent yield of TA composite was calculated as follows:
The yield = (Actual yield/theoretical yield) × 100% The resistivity was measured by the FT–300 electrical resistivity meter. 1.5 g of the conductive composite was putted into the cylindrical model of the stainless steel, a slice was obtained after pressed with 20 Mpa pressure. The resistivity was calculated from the equation: ρ = RA/L , where, ρ is the resistivity of the conductive composite (Ω·cm), R is the resistance of testing (Ω), and A is the area of the sample pole (cm2), L represents the height of the sample pole (cm). X–ray diffraction (XRD, Germany Bruker AXS−D8) was employed to identify the characteristic crystal phases of the TA composites at a scanning rate of 2deg/min, data were collected in the range of 10–80° (2θ). The surface morphology was revealed by a scanning electron microscope (SEM, HITACHI S–4800) together with the energy dispersive spectroscopy (EDS) detection system. The transmission electron microscopy (TEM, JEM−2100F) was carried out to observe the further information of the lattice of ATO particles. The binding energy of TA composites was obtained via X−ray photoelectron spectroscopy (XPS) measurement (USA Thermo ESCALAB−250) at room temperature. Ultraviolet and visible spectrophotometer (Japan Shimadzu UV−2550) was conducted to acquire the optical absorbance spectra. To investigate the formation mechanism, the thermogravimetric (TG) analysis and differential scanning calorimetric (DSC) analysis of the TA precursors were performed (USA TA DSC–Q20), a heating rate of 10 °C/min to 800 °C in the atmosphere of air. Fourier Transform Infrared (FTIR) spectra were recorded at room temperature (Japan Shimadzu IRTrancer–100), using KBr pellets in the range of 400–4000 cm−1.
Fig. 1. XRD patterns of (a) TiO2 and TA35 precursors calcinated under different temperatures and (b) TA composites with various Sb doping concentration (calcinated at 550 °C for 2 h), (c) average crystal size of ATO nanoparticles calcinated at different temperatures.
3. Results and discussion 3.1. Effects of calcination temperature and Sb doping content The percent yield by the polymeric precursor (Pechini) method was
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up to 90%. Calcination temperature and Sb doping content have an effect on the microstructure of TA composite. Fig. 1 shows the XRD patterns of pure TiO2, TA precursors calcinated under different temperatures and TA composites with various Sb doping content. All of the diffraction peaks can be indexed into the rutile TiO2 (PDF#21–1276) and cassiterite SnO2 (PDF#41–1445) [16]. The peaks at 2θ = 26.5°, 33.9°, 37.9°, 51.8°, 61.9°, 65.9° correspond to (110), (101), (200), (211), (310) and (301) crystallographic planes of high–purity of SnO2, respectively [17]. Nevertheless, there is no extra peaks corresponding to antimony oxide, indicating that Sb5+ or Sb3+ ions replace the Sn4+ ions of the SnO2 crystal lattice and forms the doped semiconductor [18]. The increase of calcination temperature results in the more intense and narrower characteristic peaks of ATO particles, suggesting the better crystalline quality and the growth of crystal particles (Fig. 1a). In Fig. 1b, the intensity of ATO diffraction peaks are reduced and the peaks widths become broader with the increasing of Sb doping concentration (calcinated at 550 °C for 2 h), indicating the decreased grain particle size. Furthermore, the average crystalline size was calculated from the peaks (101) and (211) by the Debye–Scherrer formula for spherical particles [19].
D=
Kλ β cos θ
(1)
where, D is the crystallite size, K is Scherer constant, λ is the wavelength (0.15405 nm), β is the full width at half maximum (FWHM), and θ is the Bragg reflection angle. The calculated crystallite sizes increase from 4.7 to 9.2 nm with the increasing of heat temperature (Fig. 1c). However, the crystallite sizes of ATO particles decreases sharply from 8.9 to 6.7 nm as the Sb doping concentration increases from 0 to 50 mol %, as shown in Table 1. This phenomenon can be attributed to the segregation of Sb on the surface of ATO particles, inhibiting the growth of grain [18]. In addition, the lattice parameters (a, c) and unit cell volume (Å3) of TA composites were calculated from the (110) and (101) peaks by the following formula [20].
1 h2 + k2 l2 = + 2 2 2 d a c
Fig. 2. (a) Absorbance and (b) (αhν)2 plots of TA composites with various Sb dopant and their Eg (inset).
(2) decrease of resistivity. However, the Sb3+ component appears and acts as the acceptor states (holes), which shows a compensation behavior between the Sb3+ and Sb5+, leading to the resistivity increase [22]. The resistivity of the TA composite is dependent on the ratio of Sb5+ and Sb3+ [23,24]. Another reason is the increasing of charge scattering on the grain–boundary contacts between the particles due to the reduction grain size [25]. Fig. 2 shows the absorbance spectra and (αhν)2 plots of TiO2 and the TA composites with different Sb concentration. The absorbance intensity of the TA composite become stronger in the range of 200–400 nm in comparison with the pure TiO2, which may be ascribed to the UV absorption properties of the ATO particles. However, the transmittance of the composite is obviously decreased in the visible range. It can be attributed to the blue–darkening color and the increased photon scattering from crystal defects of ATO particles [26]. In addition, a shift of the absorption edges observed with the increase of Sb concentration is related to the band gap change. The optical band gap (Eg) of TA composite is estimated from Tauc's relationship [27].
where d is the interplanar distance, (h k l) represent the Miller indices. The calculated values and corresponding unit cell volume (V = a2c) are presented in Table 1. The unit cell volumes of the ATO particles are smaller than that of undoped SnO2 sample, which is attributed to the Sn4+ ions substitution by the Sb5+ ions or the number of Sb5+ ions are more than the Sb3+ ions, Since the ionic radius of Sn4+(0.69 Å) is smaller than that of Sb3+ (0.76 Å) and larger than that of Sb5+ (0.61 Å) [21]. As for the electrical properties, the undoped sample possesses a very high resistivity in the order of 106 Ω cm. However, the resistivity decreased sharply to 4.2 Ω cm with enhancement of Sb doping content up to 0.35, which may be ascribed to the extra carrier from the substitution of Sb5+ at Sn4+ sites. The resistivity increased as the doping content increased above 35%. There are two reasons to explain the phenomenon. When the doping content is low, Sb5+ is the dominating component and introduces donor states (electrons), resulting in the Table 1 Structural parameters of TA composites with different Sb doping concentrations. Sb doping level
Grain size (nm)
Lattice parameters (Å)
Unit cell volume 3
αhv = C(hv − Eg )1 / 2
(3)
Resistivity
(%)
D(101)
D(211)
a–axis
c–axis
(Å )
(Ω·cm)
Undoped 5 15 25 35 50
8.9 8.7 8.4 7.7 7.6 6.7
8.7 8.5 8.2 7.8 7.7 6.3
4.7348 4.7291 4.7243 4.7206 4.7205 4.7318
3.1779 3.1714 3.1823 3.1893 3.1794 3.1807
71.2275 71.0576 71.0258 71.0706 70.8470 71.2157
4.7 × 106 3.5 × 104 32.0 4.8 4.2 6.8
where C is a constant, α is the absorption coefficient, hν is the photon energy. The optical band gap can be obtained by the intercept of the tangent to the (αhν)2 against hν plot, as shown in Fig. 2b, and the resulted values are listed in the inset of Fig. 2b. The calculated band gaps are 3.077, 3.076, 3.072, 3.069, 3.070, 3.068, and 3.066 eV for the TiO2 and composites with 0%, 5%, 15%, 25%, 35% and 50% Sb doping content, respectively. The absorption edges of the TA composites are slight red–shifted with comparison to the pure TiO2, indicating the modification of the electronic structure and the optical properties of the 72
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original TiO2. It probably results from the trace doping of Sn4+ on the interface between TiO2 and ATO particles during the calcination, or the coupling of SnO2 and TiO2 [28]. The similar red shifts have been reported in the previous literature [29,30]. The band gap energy shows a decrease trend with increasing Sb doping concentration. The phenomenon is in contrast to the Burstein–Moss (B–M) effect. According to the Burstein–Moss theory, the incorporation of Sb ions leads to the increase of carrier concentration, which results in a bandgap broaden [31]. The narrowing effect in the band gap can be explained by the sp–d exchange interaction between the band electrons and the localized d electrons of the Sb ions replaced the Sn4+ ions. In addition, the Sb doping may cause the stoichiometry defects of Sb–doped SnO2, such as oxygen vacancies. The structural disorder in SnO2 leads the valance band tail to unoccupied states of conduction band edge, resulting in the bandgap decreasing [32]. Similar bandgap shrinkage has been reported in the doping model [33]. The UV absorbance spectra variation combined with the little decrease in band gap values are further confirmed the successfully developed of ATO particles on TiO2 surface.
and O can be observed in the TA35 composite, further confirming the successfully coating of ATO nanoparticles. The Sn 3d5/2 and Sn 3d3/2 peaks for the TA35 precursor, TA15, TA35, and TA50 locate at the binding energy of 486.82, 486.73, 486.73, 486.84 eV and 495.22, 495.13, 495.13, 495.24 eV, respectively (Fig. 5b). The peaks separation is 8.4 eV, which indicates the lattice Sn4+ ion in SnO2 [35]. Chemical shift observed for the various Sb doping concentration are 0.09–0.11 eV, resulting from the change in oxidation state of atom [36]. The O 1s and Sb 3d5/2 are overlapped as shown in Fig. 5c, which can be separated into four Gaussian peaks located at 530.4, 530.7, 531.0, and 532.8 eV. The peak at 530.7 eV can be assigned to the Sb 3d 5/2. The peak centered at 530.4 eV can be attributed to TieO bond. The peak located at 531.0 eV is responding to the Sn(Sb)eO bond. Moreover, the peak at binding energy of 532.8 eV is associated with H2O or hydroxyl group on the surface. The stronger intensity of the peak in the precursor than that in TA35, indicating the more presence of H2O. The Sb3d3/2 can be employed to analyze the oxidation state of Sb, which is decomposed into two characteristic peaks at 539.7 and 540.2 eV corresponding to the Sb3+ and Sb5+, respectively (Fig. 5d). The ratio of Sb5+/Sb3+ in the TA35 precursor, TA15, TA35, and TA50 estimated by the area are 0.60, 1.96, 4.79 and 3.46, respectively. The Sb5+ predominated in the TA samples, resulting in the decrease of resistivity, implying the oxidation of antimony in the calcination process. The Sb5+/Sb3+ ratio is larger in the TA35 sample than that in the other TA samples, possible signifying the better conductivity resulted from the high ratio of Sb5+/Sb3+. The higher carrier concentration generates due to the relative larger amount of Sb5+ in the TA composites [17,22]. The result is good agreement with the XRD analysis.
3.2. Surface morphology and composition Fig. 3 illustrates the SEM images of TiO2, precipitate, and TA35 composite and the corresponding EDS analysis. In Fig. 3a, the original TiO2 particles are of sphere shape with a smooth surface, and the grain sizes are around 250 nm. The rough surface and some granules can be observed in the precipitate sample (Fig. 3b). Moreover, the compact and homogenous grains are shown on the surface of TiO2, as presented in Fig. 3c, which can be derived that the ATO particles were successfully coated on the TiO2 surface. The EDS reveals the existence of Ti, O, Sn and Sb elements, further confirming the ATO particles on the TiO2 cores (Fig. 3d). The chemical composition (wt.%) of Ti, O, Sn, Sb in the TA35 composite is 44.51%, 38.86%, 12.86%, 3.77%, respectively. The molar ratio of [Sb]/[Sn] is 0.286, which is close to the experimental ratio of 0.35. The TEM images with different magnifications and high–resolution TEM (HRTEM) images of TA35 composite are shown in Fig. 4. The TiO2 core and ATO shell structure can be observed, and the shell thickness is about 18 nm (Fig. 4a and b). In Fig. 4c, the ATO particles is regular sphere and the mean grain size is of 9.63 nm estimated by the HRTEM. The lattice spacing of 0.334 nm is corresponding to the growth orientation in the direction of rutile SnO2 (110) plane, which is in good accordance with the XRD results (Fig. 4d) [34]. In order to obtain the further information of chemical composition and valence state of the elements, XPS analysis were conducted, as presented in Fig. 5. In Fig. 5a, the peaks corresponding to C, Sn, Sb, Ti
3.3. Formation mechanism Fig. 6 shows the FTIR spectra of citric acid, precursor solution, TiO2 and the TA35 precursor powders with various treatment temperatures. In Fig. 6a, the peak at 3376 cm−1 and 3421 cm−1 are related to OeH stretching vibrations of structural OH groups. The new bands at 1641 cm−1 and 1382 cm−1 are observed for the precursor sample, which can be assigned to the asymmetric and symmetric stretching vibration of the carboxylate ions (COO−) respectively. It indicates the formation of metal–citrate complexes. The band at 1738 cm−1 is associated with the CeO stretching vibration of uncoordinated eCOOH group, suggesting that there is some amount of residue from citric acid. Ethylene glycol is characterization by two bands at 1085 cm−1 and 1048 cm−1 which are the CeCeO stretching vibrations peaks. In Fig. 8b, all of the samples show a broad band at around 647 cm−1, which belong to TieOeTi stretching and bending vibrational modes [37]. The absorption peaks around 3421 cm−1 and 1643 cm−1 can be ascribed to the stretching vibrations of the surface hydroxyl groups (OH) and the physical absorbed water on the surface of the oxides [37,38]. The peak centered at 1718 cm−1 in precipitate is assigned to the carboxyl groups (C]O) of the citric acid. Moreover, a new band at 1234 cm−1 associate with the δ(SnOH) is recorded in the samples of precipitate [39]. It disappears after calcination due to the water elimination and the development of SneOeSn bonds which can be observed at 1390 cm−1 [40]. The TG and DSC characteristics of TA35 precursor were employed to analyze the reaction mechanism, as presented in Fig. 7. Two weight losses can be observed at 20–150 °C, 150–400 °C in TG curve. The first step of weight loss (2.4%) corresponding to an endothermal peak in the DSC curve can be ascribed to the evaporation of the residual ethyl alcohol and water. The second weight loss (3.6%) accompanied by a wide exothermic peak is attributed to the evaporation of the crystal water and the combustion of the organic groups, such as ethylene glycol and polymer, also the removal of the un–polymerized citric acid [41]. Moreover, the process is accompanied by the crystallization of the amorphous SbeSn precursors into ATO nanoparticles with the water release [18]. The weight remains constant above 400 °C in the TG
Fig. 3. SEM images of (a) TiO2, (b) precipitate, (c) TA35 composite and (d) the corresponding EDS of TA35 composite. 73
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Fig. 4. TEM and HRTEM of TA35 composite (inset) corresponding particle size distribution.
SnCl4 and SbCl3 were dissolved by the ethanol solution. SnCl4 and SbCl3 react with ethanol to form tetraethoxy tin and antimony triethoxide [42]. Citric acid was used as the complexing agents, which could chelate Sn4+ and Sb3+ ions with the carboxyl groups or hydroxy groups to
curve, indicating that the organic residues has been completely removed and the final product of TA composite is obtained. According to the above analysis, a possible formation mechanism of ATO particles coated on TiO2 surface is presented. In the first step,
Fig. 5. (a) XPS spectra of TiO2 and TA composites with various Sb concentration and (b)–(d) the respective high resolution spectra of the Sn 3d, O 1s and Sb 3d peaks with fitted line. 74
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Fig. 8. FTIR spectra of (a) citric acid and precursor solution, (b) TiO2, the dried TA35 precursor and the TA35 precursors calcined at 200 °C and 550 °C.
Fig. 6. FTIR spectra of (a) citric acid and precursor solution, (b) TiO2, the dried TA35 precursor and the TA35 precursors calcined at 200 °C and 550 °C.
the TiO2 aqueous solution, citrate polyester accompanied by the hydrolysis absorbed on the surface of TiO2 by the hydrogen bond with the surface hydroxyl groups. During the reaction, the intermolecular polycondensation of the hydrolyzates with the dehydration and dealcoholization reaction and particles growth took place. Most hydrolyzates coated on the surface. The absorbed water and ethyl alcohol were removed, amorphous SnO2 and Sb2O3 were formed during the drying stage. In the calcination process, amorphous metal oxide crystallized and Sb3+ ions were gradually oxidized into Sb5+ ions. Sb5+ ions mainly substituted Sn4+ in the lattice of SnO2, which generated n–type semiconductor (ATO) with the average particle size of 9.63 nm bonded on the TiO2 surface. The uniform coating of the excellent conductive ATO particles endows the TiO2@ATO composite with a better conductivity. 4. Conclusions Fig. 7. TG/DSC curves of TA35 precursor.
The TiO2@ATO conductive composite has been synthesized by an environment friendly and simple synthesis method. The composite possessed a typical core–shell structure and the ATO average particle size is 9.63 nm. The increase of antimony content reduced the grains of ATO particles. The UV absorbance intensity of the TiO2@ATO composite became stronger in the range of 200–400 nm in comparison with the pure TiO2. The resistivity as low as 4.2 Ω cm was achieved for the composite when the optimum antimony doping concentration was 35 mol% (Sb/Sn molar ratio). The coating mechanism of ATO nanoparticle film has been proposed, and this method can be extended to
prevent the hydrolysis of the cations in the ethanol solution. Subsequently, the titration of ethylene glycol resulted in the generation of esterification and polycondensation reaction between the uncoordinated eCOOH and eOH and formed the SbeSn polymer precursor. The cations incorporated to the polymer favor the uniform distribution of metallic ions throughout the organic matrix, which led to a precise control of cations concentration in the particles. The second stage, when the metal polymer precursor was added into 75
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facile synthesis of the other core–shell composite with doped semiconductor.
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